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Expensive Lesson in MFI Tuning
I'm posting this in an effort to save some of you MFI freaks from the same kind of expensive suffering I am now enduring. I just hurt my new 3.0 liter MFI motor through a very stupid mistake on my part. Here is a photo of the damage, just to kind of get started:
http://forums.pelicanparts.com/uploa...1209408461.jpg This is the aft side of the #1 cylinder, eroded away after the head gasket let go. The head is similarly damaged. It was actually blowing hot cumbustion gasses out forcefully enough to blow a hole through the adjoining chain box as well. So, what caused this? In a word, "detonation". I believe the A/F ratio was going lean enough to cause severe enough detonation to seperate the head from the cylinder, on this, the least supported side of #1. So how did it run so lean, after all of my work setting the A/F ratio with the LM1? Simple, in retrospect. I changed mufflers. Not just a little change, but a wholesale change, from an old Leisritz sport to a pair of M&K "track" mufflers. My old Leisritz was (unbeknownst to me) slowly giving up the ghost. A baffle or two finally let go a couple of days before an Alfa club track day I was to attend, and it was sounding like a Jake brake on a diesel truck under acceleration. So, wanting to run that track day, I put on the only mufflers on hand - the M&K's. What I failed to do was to re-tune the MFI pump for the now much freer flowing exhaust. I had changed from a stock single out to a sport muffler years ago and did not have to re-tune, but this was a far more dramatic change in flow. A crapped out (probably enough to reduce its flow) old sport muffler to the very free flowing track mufflers. I should have known better. I should have hooked the LM1 back up and remeasured the A/F ratio. But, it was going to be "just for one day..." Expensive day... Anyway, I've got the whole thing torn down and have inspected for "collateral damage" (pistons, cylinders, and heads, rod bearings, etc.). Everything else is just fine. I already have a replacement head and cylinder on the way. So, while I sit back and lick my wounds, I hope you guys can learn from my mistake. MFI is a "dumb" system. Unlike CIS or Motronic, there is no mass airflow sensor, no feedback loop; nothing at all to "tell" the pump how much air is going in. That part is up to us, the tuners. Hell, even the lowly carburator is "smarter" than MFI; increasing air flow draws more fuel. Not so with MFI. And I knew that. I'm pretty upset with myself. Damn good thing I'm in good with my engine builder... |
Ouch! That's a hard lesson, Jeff.
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Jeff - sorry to see that. It's been recommended to me to have a real-time Lamda meter on my MFI car, so one can monitor A/F while driving (more accurate than tuning without load), so hopefully this can be avoided. It's not cheap, but it looks more reasonable now, knowing that this type of damage is a potential cost of not having one.
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That's a bad event, but i'm surprised it would have been only caused by a bad AFR / MFI tuning...
I'm maybe wrong, let's the gurus talk about that Philippe (on the MFI tuning way) |
Horrible news - sorry to hear.
It's worth mentioning that the early CIS (pre-'78 I believe) also do not have any sort of feedback system. There's no O2 sensor or anything - just the air flapper that the system uses to meter out the fuel in proportion to the amount the flapper is open. Kind of like a big carburetor in some respects, although from the sound of it, more sophisticated than the MFI. How does the MFI system determine how much fuel to send to the cylinders then? Just curious - I know nothing about this system. |
Jeff,
I was just about to write about the subject Philippe brings up. First I want to hear about the condition of the pistons (specifically the ring lands) and the rings. Next, there should be some very careful examination of the rod bearings. You are looking for any hint of ‘spider web’ like cracks in the ‘copper’ layer under the ‘babbet’ bearing material. You probably will need a 10X microscope and look at an angle. The place to look is in the center of the bearing shell closest from the piston. The problem is the soft bearing material can mask the fractured harder ‘copper’ material. Is the bearing darker in that area? Please tell us more about the engine configuration; type of piston, measured CR, cams, twin plug, sparkplug type, etc. What do the sparkplugs look like? What was your cranking compression? What fuel were you using? Octane? Did you have a head temperature reading? What was the weather that day? How was the torque on all the head nuts? How are the other five head gaskets? You say the mixture was too lean and there was detonation. Yes, there was the contemporary change in exhaust but what is the evidence of too lean and detonation (aside from a blown head gasket)? I’m concerned you only focus on mixture and detonation and not investigate all possible causes. In fact there may be the confluence of multiple causes. I think it unusual that the gasket failure is on the side. Most failures are just below the exhaust valve. I have a similar setup in my ‘street’ 914-6; 2808 cc with 92 mm RSR Mahle Nikasil, 11.5:1 CR, S-cams, 2.4S MFI modified for proper mixture, 112 octane VP race gas, etc. I would regularly change from my original, stock 914-6 muffler to a Bursch 2-into-1 straight pipe. Yes it would lean slightly but never enough to be an issue. With either exhaust I had a nice light gray tail pipe and good looking plugs (NGK BP8ES or BP9ES depending on conditions). One thing I do differently is use the “Rubbermaid Solution” with water into the cooling fan. This tends to keep the heads and cylinders much cooler that just air and huge oil coolers. Jeff, I am very sorry to see this and I feel for you. Let’s make sure it is not likely to happen again. Best, Grady |
Jeff, the MFI pump delivers fuel based on two criteria - throttle position and pump rpm. It's own throttle position, not the throttle bodies that admit air into the intake ports. They are two different things, and one does not "see" the other. In other words, they operate totally independantly. The amount of delivery based upon throttle position is adjustable accross a very wide range. The amount of delivery based upon rpm is also adjustable, but through a much narrower range. There are a pair of flyweights that fling apart with rising rpm's; how far apart they are allowed to spread is governed by a series of three progressively heavier springs, that control idle, mid range, and high speed delivery. All three are adjustable, but through a somewhat narrow range.
The MFI pump is connected to the throttle bodies through one single rod. Disconnect that rod, and the pump will still deliver fuel. It doesn't "know" if the rod is there or not. Twist its throttle arm, and it will deliver more fuel, come hell of high water. The throttle bodies could be sitting on their idle stops and it wouldn't matter. Conversely, you could open the throttle bodies with the pump on its idle stop, just pumping away on an idle delivery (increased a bit by the rpm compensation). Totally independent systems. One of the keys to tuning this system is establishing a "correlation" between the fuel delivery supplied by the pump and the air being drawn in through the throttle bodies. It's a delicate balance. It's up to us, as the only "brains of the operation" to establish that correlation and provide a useable A/F ratio accross the operating range. |
Jeff, sorry to hear about your loss and thank you for warning the rest of us. I now have this safely filed away in my brain under "please don't repeat".
I am interested in what folks think about the LM-1? I have a 70S with MFI and tend to be a bit of a "data freak". Seems like something I could use. I have a Gunnison GasTester but frankly, the set up process drives me crazy. I just want to drive! cheers, Michael |
Ouch. Thats scary.
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My $.02:
Yes, MFI is a "reference based system". CIS, Motronic EFI, and carb's (in there own way) are "air flow sensing based" systems. If there is an change in air flow, they send more fuel. As noted, all the MFI knows is that if the throttle is open a given amount, and the RPM's are a given level, a predetermined amount of fuel is delivered. There are some modifications for altitude and tempeture on some systems. Increase air flow 10% the A/F ratio leans, 10%. Ones 13/1 quickly becomes 14.3/1 A/F. An EGT (exaust gas temp) gage or a wide band O2 is a good idea with any proformance system, but especially a reference type system. O2 on each side might even be helpful as if one injector fails, you could see some differance from side to side. An o2 on just one side can not tell one if there is a bad injector. Best practice would be an EGT on each exhaust primary tube. It might be noted that the 3.2 Carrera EFI in effect becomes a reference based system after about 5K rpms. The AFM reaches a point where it can not effectively measure changes in air flow. At that point it is just changes in rpm and if it is in WOT that determines how much fuel it will receive. The stock chip I believe runs the system rich as a protection. If one adds a freer flowing exaust, there is some safety built in. Chip tuners, I believe, like to lean this area to closer to an ideal to gain a bit of power. If one's mods flow better than the expectation there is a risk for the same failure to occur. I was lucky with my 3.2. I tested my chip on the dyno and found it was running lean above 5k rpms and was able to have it corrected. Please think to dyno test your car to verify your A/F's. Especially with changes in exaust... An off the shelf chip, even if made for your exact mods, may not be making enough fuel. A Mass Air Flow conversion helps. Some people run Motec in a vacuum sensing mode with a multi butterfly system and no air flow meter. This type of system is a reference based system and can run a risk of running lean. It uses throtle postion, rpm, and manifold vacunm along with intake air temp and other inputs to send a given amount of fuel. http://forums.pelicanparts.com/suppo...leys/smash.gif |
Hi Grady,
I'll answer as much as I can, but I don't have all of the information you are asking for. Hopefully with my answers, you can provide some more insight into what happened here. The rings and lands look as new as the day I put the motor together. No evidence of cracking on the top land, or closure of the gaps between the lands. All rings look brand new, with crisp edges and no cracks. I examined the rod bearings and saw no evidence of cracking or wear. They did not look any darker on the "top" side, closest to the piston. I will, however, at this point replace them. It will take me half an hour to do that, and will provide immeasurable peace of mind. The pistons are JE's with a nominal compression ratio of 9.5:1, riding in reconditioned cylinders from EBS (Kobelschmidts re-plated in nickasil). Heads are stock '83 SC save for welding up the CIS scallop and drilling the MFI port, and matching the intake ports to 36mm S throttle bodies. I don't remember the deck height offhand, but I do remember using two base gaskets to get it into the acceptable range. Cams are from John Dougherty; .485"/.470" lift and 254/238 degrees duration at 1mm lift, timed to 4.5mm. It's running a single plug (NGK BP7ES) sparked by a Barry Hershon recurved SC distributor and an MSD 6AL box. Plug gap is .045". Total timing is 33 degrees at 6,000 rpm. Two electrodes on other plugs were burned back almost to their bends. All head studs appeared to be properly torqued. I'm not sure what the cranking compression was; I'll have to wait to get it back together to check that. Fuel was locally available 93 octane from Chevron. I'm not sure if there is any ethanol content. The oil temperature did get unusually high (250 degrees), and I was actually getting ready to come in on the next lap. I was going to finish out the one I was on, do a cool-down, and go in. I was a little puzzled as to why temps got that high, as at my previous lapping day (with the sport muffler installed) they never went over 210 or so. I have an Elephant Racing "widemouth" front cooler and their finned lines. I do not have a head temperature sensor. Conditions at the time of failure were an ambient temp of around 50 degrees, overcast with some light drizzle. Pretty typical Northwest early spring day. No one else was having overheating issues. Other than the damage shown in the photo and the burned plugs, there was one other indication of excessive combustion temperatures. My relative new SSI's, and the stainless steel track mufflers, both turned a light straw color. The motor just turned over 5,000 miles and had been out to a couple of other track days, and the pipes looked as silver as new. They must have gotten fairly hot to discolor like that. Anyway, that's what I know at this point. These appear to be the classic symptoms of a lean condition, to me, anyway. I would really appreciate further insight. This is kind of an unusual motor, especially in regards to the short duration / high lift cams; maybe that increases low rpm dynamic compression too much or something. I don't know, but I would like to be able to put it back together without lingering doubts about the combination. Replacement parts are already on their way... |
Sorry for your loss, thank you for sharing this! Now I know better.
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Jeff,
Thanks for the great report. It may take me a day or so to digest. As you find more, please post. Nothing is insignificant. The first thing that jumps out at me is the weather was cool AND there wasn’t any issue the day before (with everything configured the same). It sounds like possibly something changed. Perhaps there was some failure. Did you re-fuel? Could the ignition timing have changed? Was the MFI pump in time? Were the cams still in time? When it was that hot was the front cooler hot? How about an image of the side of the #1 piston in the ring area and the skirt (and #4 for comparison). What does the underside of the piston crown look like? Any discoloration? What oil are you using? On the other five pistons and heads, what do they look like (color and any sign of detonation)? Is there any sign of a ‘pinched’ head gasket on another cylinder? Can you reassemble one cylinder and measure the piston-to-head clearance (deck)? Were the two cylinder base gaskets stock 0.25 mm or are they thicker? What I’m getting at here is the phenomenon of a high compression piston, CR lowered with cylinder base gaskets leaving a ‘detonation pocket’ at the perimeter. If this were the case, I would expect to see the ring land collapsed captivating the top ring. That is unless the piston has a lot of ‘meat’ between the crown and top ring. In that case the detonation might lift the head, leading to the gasket failure. Even if the above lame theory is correct, there probably is a ‘farther upstream’ issue that lead to this. I’ll keep putting out theories so you can see why I’m asking the questions I do. I agree about the change in color of the exhaust pipes. That seems to indicate very high exhaust gas temperatures. Are they the same L-to-R? One question we need to try and answer is; was the high temperature (exhaust and oil) a cause or an effect of the basic problem? The end result is the blown head gasket. WHY? The answer may be as simple as your original theory – change of exhaust = too lean = overheating = detonation = blown head gasket. Could be. I wouldn’t bet my pocketbook without investigating all possibilities. Best, Grady |
If the ambient air temp went down, it would be still leaner. Just throwing that out here.
Your pictures have me worried. I have stock (and rusty) heat exchangers with most of the heater portion cut off. I have a 2 in 2 out sport muffler with a common "gutting" modification detailed by Panorama for increased flow. My induction, however, is not stock. I removed the plastic plenum and put in some aluminum framed screens. There is hardly any restriction. Here in Southern California, the temperatures for its operation are not too extreme. No heater or windows makes it unpleasant in cold or extreme heat. Top is probably 85 degrees and low is 60 degrees, possibly a wider spread. Humidity is almost always indectectable on driving days. Should I be worried about detonation? I don't think I have detonation as the car pulls strongly but I do not know the signs to look for. My muffler is painted a black color, sort of like carbon graphite over steel. I would see no "straw" color. http://forums.pelicanparts.com/uploa...1209429615.jpg |
If it was me the first thing I would test is the injector for cylinder #1 to see what its spray pattern looks like.
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Jeff...FWIW, don't be too hard on yourself. Your bank account is suffering enough. Back in '69, an owner of the first new "S" in our PCA region suffered a holed piston, at less than 10,000 miles, because the system was set too lean for the altitude he lived at. But then, he was still under warranty, did the appropriate screaming.
(edit) Your experience bears out the theory that MFI was designed with certain levels of muffler back pressure in mind... |
Paul,
Good to see you still lurking. Larry Skoglund was visiting this past weekend and I'm sure he says Hi! Give Cindy a hug for me. Give me a call. Best, Grady |
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Jeff...I'm sure Grady can weigh in here...but maybe 3.o displacement weighs in as well? A mechanic I trusted completely once told me that he thought 2.8 was about max. for an MFI system with "hot cams". The MFI on race cars had different pumps than those on street cars...just my understanding, anyway...larger pistons in the race car pumps, different pump cam profiles, etc.
Now, back to Grady, who surely knows... Just one more thought...Oregon and Washington have alcohol in pump fuel these days. |
I pulled the rings out of the grooves to get a better look, and sure enough the top compression ring was broken on #1. It actually spun freely all the way around the piston, not binding anywhere in the groove, and did not separate into two pieces until I pulled gently on the end to start it out of the groove. Anyway, all of the ring lands and grooves look fine. The skirts look fine, with some light scuffing top and bottom where one would expect.
The rod bearings also look fine, but will be replaced anyway. That's a no-brainer at this point. I'll upgrade to Raceware bolts at the same time, since I now have to toss the stock bolts anyway. The top of cylinder #4 is typical of the other 4 remaining good ones. No visible damage. The heads (#2 and #3 shown) look fine as well. I had already pulled the gaskets from the grooves, but they all came out freely in one piece. Well, except for one... Oil is Brad Penn 20/50. The base gaskets are the stock .025, for a total of .050 of gasket on each cylinder. I found my notes, and they say I set the deck height at 1.45mm. Towards the high end (lower compression), but removing one of the shims would have set it too low. The cam bolts were still tight, the pins were still in the cam gears, and the valve lash checked o.k. at TDC for #1 before I tore it down. I'm pretty sure the MFI pump was properly timed. It was the last time I put it back on, and the belt was still tight, so with no way of checking it now, I'll just hope it was in time. No reason why it wouldn't be. The oil lines and front cooler were very hot. They were actually steaming from a puddle I ran through on my way through the pits. So, I'm assuming the thermostat in the filter console was opening. It's a "T" console opened up to take the stock thermostat ('72). That's what I've got for now. Please keep the ideas coming. Paul, thanks for the words of encouragement. I'm actually not even all that "down" about all of this; it's costing me more time than money. A cylinder, a head, some rings, rod bearing and bolts, a handful of gaskets and a long Saturday puts me back on the road. I'm just curious at this point, and hoping to spare some one else this trouble. And next time, I probably wouldn't be quite so philosophical about it... |
With all respect, could it be time for 2 plugs per cylinder? FWIW
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Hi Jeff,
Damn, sorry that happened to your engine. However, I find your analysis along with all the other guys interpreting very fascinating and education. In medicine we would call this an "M&M" or morbidity and mortality report. But you have included the "autopsy" photos which are very cool. Thanks for sharing this with all of us. I have missed about 6 months of the Saturday breakfast clubs (because I am training for a marathon and our group runs are every Sat. morning.) My race is this weekend and I was hoping to see your engine at the next Sat. meeting, but now this happened to you. Damn! James |
Jeff-
You said that "nominal" CR was 9.5:1. Did you measure this, or are you relying on what the pistons were supposed to be? I ask because, when I built my motor (JE pistons, see avatar for "before" configuration) I did measure, and found that the pistons that were supposed to be 9.8:1 were more like 11:1. This was in contradiction to what was marked with felt pen on the box! I sent them back to have a bunch of material milled from the domes. I'm glad I checked! |
Jeff,
The mystery deepens. I don’t see obvious signs of overheating or detonation. It may be there but I can’t see it from your images. I do see some other issues. Most important is the failure of #1 head gasket, the damaged head sealing surface on #2 yet the still perfect sealing surface on #3. That sorta indicates the entire 1-2-3 head/cam housing assembly was firmly attached at the #3 cylinder and loose at the #1 cylinder (with the blown gasket). Is this a cause or result? Lets discuss the images individually. 1336a http://forums.pelicanparts.com/uploa...1209435665.jpg Here is one of the rod bearings (all three look the same). The marks here (red arrow) probably is from construction debris in the oil system. Not germane here – I think. The area (yellow ovel) doesn’t show any evidence of damage but I think this needs very careful investigation on all six bearings. 1494a http://forums.pelicanparts.com/uploa...1209435736.jpg The #4 cylinder shows compression leakage past the head gasket on the intake side. That is very unusual. It appears there is some starting on the bottom (exhaust) side also. 1525a http://forums.pelicanparts.com/uploa...1209435795.jpg This appears to be #2 head. Note the damage to the sealing surfaces. This damage appears to be from ‘pounding’, meaning the mechanical beating between the head and cylinder as if the head(s) were loose. 1555a http://forums.pelicanparts.com/uploa...1209435896.jpg Here are both #3 and partially #2 heads. Note the perfect head-to-cylinder sealing of #3 (teal arrow) and the beating of #2 (red arrow). 1229a http://forums.pelicanparts.com/uploa...1209435963.jpg This is the underside of the #1 piston I assume. If there had been serious overheating I would expect discolored oil deposits here (brown oval). This is where knowing the type of oil is important. This appears way too clean for heat to be an issue. That said, the compression leak (blown head gasket) may have prevented this piston from getting hot enough to show any heat. What do the other pistons look like on the underside? 1262a http://forums.pelicanparts.com/uploa...1209436031.jpg I assume this is the #1 piston. Is this (red arrow) a crack or a simple scratch? The color on the piston above the top ring indicates to me too much piston-to-cylinder clearance. Clearly the top ring is doing its job as there isn’t any indication of combustion gasses below the ring. If you re-install the top ring and go around with a feeler gauge, I suspect you will find equal clearance and no ‘tight spots’. OK, so where does this lead? I’ll propose another theory: The heads got loose starting with #1 cylinder. WHY? Were they not torque properly? Is there something else at work here? After the engine was built and run, were the heads re-torqued? If so, how (sequence)? What other intervening maintenance? Do you have the 4-5-6 side apart yet? If not STOP and lets plan some tests. Best, Grady |
Your original conclusion seems the highest probability.
How did you set the A/F's originally? Did you just accept what the pump builder set it at on with the bench test? With a factory set up, it is adjusted at cruse. With a factory space cam that is designed to match the motor, this is adequate. However, that may not be enough with a custom set up. Did you verify your A/F's on a dyno, with a EGT, or wide band? I second the injector test. Any diesel shop can do this. |
Sorry Grady, too late - the other side is apart already. It has some of the same marks on the sealing surfaces, but not nearly as pronounced. #5 and #6 look pretty much like the top of the #4 cylinder shown above. The heads on #4 through #6 show evidence of some pounding as well. Thanks for pointing this out; your practiced eye is seeing things in my photos that I'm not seeing in real life. I'm assuming this needs to be cleaned up before it goes back together.
All of the pistons look like #1 on the underside and the top side (yes, that is #1 in the photo, and that is only a scratch from the end of the ring). No discoloration on the bottoms, and pretty brown down to the top ring groove. According to the paperwork that came with them, piston to cylinder clearance was set at .015". The heads were re-torqued at 1,000 miles, following the same sequence used in assembly that is outlined in Wayne's rebuild book. Quite the pain in the rear end while the motor is in the car, by the way. Anyway, they were initially torqued to 24 ft lbs, and re-torqued to the same. Actually, more like "checked" - none needed re-torquing. The other intervening maintenance was the usual valve adjustment and oil change, first done at 100, then 500, then 1,000 miles. Oil changes are at 3,000 after those first three. I keep going back to the burned plugs and discolored exhaust; along with the high oil temps. It was clearly running hot, but the whole cooling system appears to have been functioning properly (front oil cooler and engine cooling fan). Those JE pistons do have a very wide top land, and a very thick dome. I wonder if that was enough to eliminate the other symptoms of detonation and high heat - the pinched down top groove or the oil burnt to the bottom. |
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Jeff,
You did everything ‘right’. Something went wrong. We really need to find ‘what’. I generally agree about the indications of over-temperature. IF that is the case, then why? Is the distributor still in place? Can we determine the ignition timing advance? Once the distributor is out, we can check its function on a test bench. The other issue is the MFI system. Could something failed there somewhere? I agree with checking the nozzles. That is easy. Could something have stuck in the pump? Looking at the head images you posted, they all appear to have the same ‘gun metal gray’ color. That is usually characteristic of detonation but not definitive. Do all six appear the same color? Tell me more about the sequence of overheating. Was there any indication on the first day? Did it occur first thing on the second morning or later in the event? {{VERY IMPORTANT}} what was done between when it ran properly on the first day and the temperature problem appeared on the second day? Nothing is insignificant! What fuel? What else? Was the 911 secure? I want you to think of everything. Lets assume your original theory is correct. How do we confirm that? Clearly we need to have the pump and nozzles inspected. I suspect we can enlist one of the primer pump specialists to do a quick test. Nozzles are easy, even I still have a tester. Any diesel shop can do this. A clue that it isn’t one nozzle or MFI pump cylinder is the #1 cylinder appears as the others. This can just as well be an ignition issue. Do we absolutely know what the ignition timing was during this time? Is it possible to have multiple firings before normal ignition timing? Ignition advance of 40-90º can have similar effects. I have a holed 2.0T piston to prove it. Lets discuss gasoline. Please take several gas samples in sealed containers. We may want to submit them for testing if all else fails. What do you know about the trail and source of the fuel? Is it possible that the fuel you probably added for the second day was low octane or contaminated? We need to carefully examine the contents of the fuel tank. Water and gas can separate out. The MFI system stirs everything up. I’m still concerned with the apparent compression leakage from many cylinders. We need to confirm with measurement that your pistons are really 9.5:1 CR as installed. If they are somehow 13:1, that would be a possible explanation. I think these are the major clues: The engine ran hot. There was a change between the first day and the second. It appears the heads came loose. All six cylinders appear the same. Please add others…. I feel we need Sherlock Holmes and Dr. Watson here. Without absolutely, positively knowing what happened here, we can’t be sure it won’t happen again. Best, Grady |
Do they still sell leaded race fuel. If you ran that you would get a white exhaust.
Maybe some one has the injector specs. I do not have my old MFI stuff anymore. More important is that they all flow the same. They operate at about 230 psi and I think they first open around 180 psi or so but I do not remember. The diesel shop is used to this. They cam test for opening presure if you want. If they are close, no problem. I would mark the one injector on the cylinder that went bad and let them know you want a quanity of flow test. You can mark each one if you want. Have them do a test first. Then a cleaning and retest if you want to know if you had a bad injector and get them ready for next time. Your note of going from a failed old muffler to what is basically a straight through pipe should be significant. Another thought, did you put your O2 on one side or both. I'd bet you are correct in your initial assessment about a change in muffler. MFI is known to be sensitive to muffler choice. |
First, sorry to see that you're having a problem, especially after that outstanding journey of discovery on the MFI pump. Also, I want to acknowledge that you all have much better MFI know-how than I do since I'm an MFI noob.
However, one bit that I haven't seen mentioned is the fuel boost pump. If you are driving the engine hard - track - with a boost pump that is marginal, is it not possible that your flow demands exceeded the capacity of the boost pump and thereby leaned out an otherwise perfectly functioning MFI pump? Combine that with the leaning effects of a different exhaust and you could create a very lean condition. Ditto for the tank sock (do you have one?) and MFI filter. Eh? |
Hello Jeff
would you tell us more about the assembly : were the cylinders the same height, heads too ? were the head mating surface with cam tower right aligned (top), a head gasket couldn't have been crushed during assembly ? Hope this helps with my non academic technical english, i've seen a similar event with unaccurate top end assembly, hot gazes ruined the head mating surface through a bad gasket... I think your AFR's were right if this are the numbers you said (at least rich enough) and congrats to M. Clay analyses... |
Grady, this happened at a single day event. I'm sorry; I probably wasn't very clear about the time line on this. The previus track event was three weeks prior, under similar weather conditions. The car ran consistantly at about 210 degrees that whole day with no issues whatsoever. It had the failing Leisritz on it that day. All plugs came out with that kind of sooty MFI rich fouling on them, so all appeared normal. The only change made between the two events was the mufflers. That's why I keep going back to that.
The distibutor is still in place. I'll check the timing when I get it back together, but it looks like the body has at least not moved. The rotor can be advanced by hand, and springs back just fine. It was totally rebuilt, with all new bearings and whatnot, by Barry Hershon just prior to assembling the motor. He has been at it for a long, long time, and certainly knows his stuff. I would be surprised if anything failed on that end. Even if it did, and stuck at full advance or something, that shouldn't matter. I asked him to set it up for full mechanical advance, and to have full advance come in very early - like 2,000-2,500 rpm. The motor never sees below about 3,800-4,000 even in our tightest 2nd gear corner, so it's running on full advance all the time. I set that at 33 degrees, and I'll check to make sure it is still there when it's running again. Jim, you might be on to something. One of my friends that runs a local vintage series with a carburated 2.0 had similar symptoms occur last summer, on a fresh rebuild. It burned the ends off of several spark plugs and lost compression in a couple of cylinders. His compression loss was through failed rings, and did not damage the heads or cylinders. His did show significant pounding on the outer rod bearings (piston side of rod). He traced it down to a failing electric fuel pump. It delivered full volume until it heated up, then slowed to a trickle. So, it would pass the "beaker test" on the first beaker, and fool all of us. By about the fifth beaker in rapid succession, it was only running at about 10%. So, I'll look at the supply side and check my fuel pump flow. The filter itself was brand new with the new motor. The car only sat for a week between dropping the 2.4 and installing the 3.0. It had been driven several times a week for the five years I have owned it, so I don't believe it's a plugged screen (from sitting for an extended period of time). All six heads do appear about the same color. The pump is about a five year old rebuild from Gus that was never installed on anything. It's spotlessly clean inside, and everything functions very smoothly. It responds exactly as one would expect to the various mixture adjustments. All of the hard lines had pressure in them when I took them off. The injectors themselves were an eBay find; advertised as freshly cleaned and tested. While I've purchased other items from this seller, and they have always been as good or better than represented, these are looking like a possible "wild card". The motor did, however, run exceptionally well for 5,000 miles and a couple of track days with them. Plugs always looked identical from all cylinders; that kind of sooty black characteristic of slightly rich MFI. So I believe they were all functioning properly, but I will get them tested. I've got a couple weeks to kill waiting for parts anyway. A couple of us drove down to the track together and filled at the same Chevron station. Nothing in the other car indicated a problem with the gas. It was just a normal single day club lapping event, so it was never left unnattended between filling and running. Couple hundred folks milling around the pits all day, so it's unlikely some one would have fooled with it. Anyway, it's just straight pump unleaded premium; 93 octane in our area. If my compression is higher than the 9.5:1 advertised, that may be a problem. I'll call JE this morning (I need some wire lock rings anyway) and verify the part number with them. When it's back together I'll get a cranking compression number. The other thing I think I'll do is to go ahead and tune it with the new M&K sport muffler that's on it's way. Then I'll put the failed Leisritz back on, and see if it's plugged up enough to make it go super rich. Then I'll put the little track mufflers on and see how much leaner they make it run, if at all. That will either validate or invalidate my assumption about the muffler change. |
Jeff-
To reiterate, I wouldn't trust JE about the CR. I'm sure that if I had called them, they would have said that I had the right thing (what was indicated clearly on the outside of the box). You can measure your CR by purchasing a calibrated burette and making a plexiglass window to attach to your heads. I don't know much about SC heads, but I do know that others can vary significantly in combustion chamber volume, so it would be tough for a piston maker to determine what your CR might be, particularly when the variable of deck height is added. |
Just getting it on the table.
I think after a rebuild, one is supposed to re-torque the heads with some miles. Fuel in oxygenated here in California. Might be good to add a little high-octane at the track for some insurance. Were the heads cleaned up or fly cut? Did you cc the heads to check compression with the assembly? Still vote mufflers. Add an EGT for insurance. |
Hmm....no scruf on the piston crown yet hole was almost torch-welded trough head seal? Weird.
We were discussing this on 930 forum. Some knocked piston pictures coming: 930 piston, knocked dead: http://i139.photobucket.com/albums/q...ltedPiston.jpg Waterpumper knock with JE pistons, head looks like this: http://forums.pelicanparts.com/uploa...1209408040.jpg But then, piston looks like this: http://forums.pelicanparts.com/uploa...1209407353.jpg Hard to say but you might have had some sort of mechanical malfunction which let piston/head clearance grow and subsequent leak severely disturbed AFR's inside combustion chamber and then torched trough the crack in the seal. If piston look healthy then there might be another explanation... This is of course, pure speculation from my point of view. Let the experts decide. |
Jeff,
Before you start on a ‘final assembly”, it would be wise to mock-up a single piston, cylinder and head and measure the combustion chamber at TDC. You don’t need the cam housing. You need to position the engine so the sparkplug hole is up. This is somewhat ungainly but actually pretty easy. I grease the valve contact area and grease the cylinder before I bring the piston up to the top. I also grease the head gasket. Wipe off any excess. I use a 100 ml burette and regular Stoddard solvent. I’m beginning to suspect that you may have much higher CR than 9.5:1. Actual measurement is the only way to tell. Too high CR might agree with many of the clues. The ‘gun metal grey’ color in the combustion chamber. All six look the same. The burned electrodes on the sparkplugs. The problem happened after refueling (you may have had just enough octane prior) and your friends were probably dealing with their 8.5:1 CR. The lean effect of changing the exhaust. I didn’t say so before but I tend to use ignition advance in the 30-32º range for an engine like yours. I Don’t see any clues that are counter to too high CR. I recommend you measure the CR asap. If you find you have 11:1, you haven’t wasted any time on assembly and these pistons. This probably should have been done during the original assembly but that is water under the bridge. If it turns out this is detonation from too high CR, we need to look at every other component that could have been damaged. The rods come to mind. You are going to need to repair (at least) the damaged #2 cylinder and head. This will give you time to get new pistons fitted to cylinders (talk to Steve Werner at Rennsport) and measure the CR for them. Best, Grady |
Jeff, Sorry to here about your problems. Thanks for sharing the info.
Grady, You are amazing. I have learned so much from you and the members on this board. Thank You :) |
Something has been nagging at me since we started this analysis. Something Grady has mentioned several times. Something that should be a "no brainer", and one of the simplest operations in engine assembly. Head nut torque. In my efforts to consider every detail, no matter how seemingly insignificant, I had one of those cold chill, "oh crap" moments this afternoon at work. Now I don't know what to think.
Now just about any numbskull, including me, can read a torque wrench and torque a nut. But what if the torque wrench is off? In an effort to "do this right", I borrowed a clicker type 1/2" drive torque wrench, forsaking my beam style I've used for years. The 23.4 ft lbs called out for the head nuts is way at the low end of the adjustment range for this wrench, which I thought nothing of at the time. My "oh crap" epiphany came today when I was shopping on line for one of my own. I saw a lot of click type torque wrenches that said right in their product descriptions that they would not guarantee accuracy below a certain percentage of their adjustment range. You've got to be kidding - why even put numbers in a range where it is acknowledged to be inaccurate?? And if a brand new one isn't accurate in its lower range, what about one with some miles on it? So I'll be retrieving that very wrench from my buddy tomorrow. I need to test it to put my mind at ease. I'll torque a few different nuts and bolts throughout its range and see how the values compare to my beam type. There is probably nothing to it, but, what if in my quest to be as precise as possible, I actually used the inferior tool? I almost didn't post this, feeling kind of silly about such wild speculation. Maybe I'm more just "thinking out loud", where no idea is a stupid idea. Or, as Grady put it, no detail is insignificant. We call this "thinking outside the box" at work. If the components look correct, if there is no obvious answer jumping out in front of you during their inspection, then well, maybe they really are just fine. Maybe one of the very simplest process steps failed for some reason. Boy will I feel stupid if there turns out to be anything here. Maybe the real lesson will be "stick with the tools you know". We'll see. I'll report on this tomorrow. I'm just waiting for parts now anyway, so this will give me something productive to do. It will answer that nagging voice, anyway. |
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