Expensive Lesson in MFI Tuning

[Jeff Higgins]

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:



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...

[Jim Richards]

Ouch! That's a hard lesson, Jeff.

[GrantG]

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.

[philippeF]

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)

[Porsche-O-Phile]

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.

[Grady Clay]

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 Higgins]

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.

[mhackney]

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

[declang]

Ouch. Thats scary.

[911st]

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.

[Jeff Higgins]

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...

[snbush67]

Sorry for your loss, thank you for sharing this! Now I know better.

[Grady Clay]

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

[Flieger]

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.

[911quest]

If it was me the first thing I would test is the injector for cylinder #1 to see what its spray pattern looks like.

[pwd72s]

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...

[Grady Clay]

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

[Jeff Higgins]

More photos.

[pwd72s]

Originally Posted by Grady Clay

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

Grady, I'm really trying to not be a car guy these days, but I have difficulty pulling it off. Best to you...and Larry. I'll never forget he and his "Real Carrera"...

[Jeff Higgins]

Even more photos: