Putting My 3.0 MFI Motor Back Together

Jeff Higgins · 05-25-2008, 12:02 PM

I'm starting to reassemble my motor after my little hiccup (see my "expensive lesson in MFI tuning" thread). I figured I run this past you guys as a sanity check before I button her back up.

To recap, the pistons I was supplied provided a whole point more compression than what I had actually ordered. On top of that, I was running a great deal more ignition timing than I should have been, especially in light of the higher compression (33 degrees total). Finally, an exhaust change leaned it out and, well the rest is history. One blown CE ring and the commensurate head and cylinder damage. I think I've learned something from all of that, so here is the plan:

I'm keeping the 10.5:1's and actually lowering the deck height. I had it at 1.4 mm with two .25 mm base gaskets. I'm going to one gasket to lower deck height to 1.15 mm. Steve Weiner and Grady Clay both explained the concept of detonation caused by excessive deck height in their replies on my other thread. Essentially, if the squish band isn't "squishing" as designed, and forcing the charge towards the center of the combustion chamber and creating turbulence, they tell me a ring of very high compression is formed over the squish band. So, what I'm thinking is that I'll get a more effective "squish", and even though I'm raising compression just a wee bit, I'm actually decreasing the likelihood of detonation. Make sense, or am I off base?

I'll be reducing total advance to somewhere in the neighborhood of 24 degrees. I'm a little worried about combustion efficiency on a single plug at that timing. Thoughts?

When at the track for my DE days, I'll be running it on race gas as an added safety measure. We have three different octane ratings available at Pacific Raceways. I can't remember offhand what they are, but I'm sure I'll have no trouble getting suitable gas. The big question is with regards to my street driving. We have 92 and 93 octane available around here. Now I won't be running it anywhere near as hard on the street, so will I be able to get away with that low octane?

I'm thinking the MFI may be an advantage in that department. I can run it richer than carbs and still maintain decent combustion because of its much better atomization. I think that will provide the safety margin I need. Not rich enough to cause cylinder washing or anything like that; I'm thinking 11.5 to 12:1 A/F. As a side note, it did make it's best mid range power at 11.5:1 on the dyno. Kinda stinky, though...

I was clearly right on the edge prior to the damage. I had over 6,000 street miles on the motor before the fateful track day. That's kind of what has me thinking that with some minor tweaks on the tuning, I will have a safe, viable combination. The other choice is to switch to the 9.5:1 pistons; it's still not too late for me to do that. I really don't want to, but if I'm all wet on this stuff, I will. What would you guys do?

Avery · 05-26-2008, 03:52 AM

Hi Jeff: I've been reading with great interest, your lessons learned on playing with the MFI adjustments...thanks! Regarding your question, I think the planned change in deck height is a good one. I've had no experience with running motors with as high a CR as yours with no twin plug. Have you calc'd the new CR after lowering the deck height? I think dropping the total advance will help and even cure the detonation. Actual required total advance will however vary with the gas you use. You have to find the right advance by trial and error and correct exhaust gas monitoring and of course listening for pinging. If you optimize the total advance for pump gas, you'll be leaving some HP on the table when you go to race gas. If you run an electronic ignition system, you could program it to run on street gas (less total advance) and race gas (higher total advance).

Steve@Rennsport · 05-26-2008, 09:24 AM

Jeff,

Some thoughts,......

1) I would strongly recommend having the lower plug holes machined in the heads while its apart. We call that "planning",....

2) 10.5:1, single-ignition, and today's pump gasolines are simply not compatible,.......they are mutually exclusive, especially in hot weather. Rich fuel mixtures help, but they are not a panacea for the correct configuration.

3) 24 degrees of total timing will help the engine live with the combination, but thats REALLY retarded and that makes for very high cylinder head and EGT's.

Bottom line,.....you truly need twin-ignition to run pump gas at 10.5:1 with these larger bore engines.

Jeff Higgins · 05-26-2008, 06:10 PM

Thanks for the input, guys. Steve, thanks for confirming what I should have already known. It's nice to hear it from some one with your experience, as opposed to hearing it from that nagging little voice in the back of my head. At this point, it will be very little more work to machine the second plug hole. I think I'll have Monty go ahead and do that. Time to start saving my allowance for something to light that second plug with...

Jeff Higgins · 05-28-2008, 09:04 AM

Well, the second spark plug holes are going in as we speak. Now it's time to decide how to light them.

I see our host carries the JB Racing twin plug distributor and the Electromotive setup. I'm leaning towards the JB Racing distributor. Comments? Pros and cons of the two different systems? Help me decide...

rs911t · 05-28-2008, 11:58 AM

3.0 MFI engine in a '72T? No question - twin plug dizzy. Steve sells one, too.

Steve@Rennsport · 05-28-2008, 11:58 AM

Quote:

Originally Posted by rs911t

3.0 MFI engine in a '72T? No question - twin plug dizzy. Steve sells one, too.

Indeed, I do,...

304065 · 05-30-2008, 07:39 AM

Jeff,

I agree with the Most Erudite Guru from The Pacific Northwest, Dr. Ing. Steve Weiner-- maybe not with regard to everything (like which wine to serve with fish)- but where matters of Engine Rebuilding are concerned, I'm in his corner. So let me echo that reducing deck height IS a good idea AND that twin-plugging is required in order to cope with the Reformulated Panther Pee (with 10% Ethanol, maybe they get the Panther drunk) that is passed off on an unsuspecting public as Gasoline.

Also, I wish to publicly throw down the gauntlet on the observation that you made best midrange power at 11.5:1 AFR. Are you absolutely certain, was the AFR equipment recently calibrated? Chemistry would suggest that something in the low thirteens is more likely. If you ARE absolutely certain, that raises other issues, such as what was going on to require SO much fuel that the power peaked there instead of leaner. That would send me in the direction of checking AFR from bank-to-bank, or if the dyno is equipped with the instrumentation, on an individual cylinder basis. NOT trying to be controversial, just thought I would raise the question and maybe have a fun discussion about it.

We know that too-rich mixtures can CAUSE detonation-- this is one of the observations of George Braly and the GAMI crew-- so I would shoot for tuning for best power in the low 13 range under load. From a chemistry standpoint the extra fuel at 11 AFR is just being wasted, so there must be a PHYSICAL thing going on, charge cooling from the excess fuel or something, or different flow rates among injectors, that is responsible for that observation.

Finally. . . heretic that I might be. . . despite the aesthetic RIGHTNESS of a 12-plug cap in your longhood. . . I have to say that crank triggered ignition IS more accurate and coil saturation a thing of the past. . . I would look at what Pelican offers and also surf on over to the Clewett engineering catalog, he makes some cool stuff. If I ever get around to building a GT motor I'm going to look hard at coil-on-plug, once you commit to direct ignition it makes sense to eliminate plug wires.

Good luck!

Steve@Rennsport · 05-30-2008, 07:55 AM

If I may add a few thoughts to Professor Von Cramer's treatise above,........

Max HP is around 13.1-13.2:1. This is THE target in a water-cooled engine but too lean for extended running in an air-cooled one. 11.5:1 is close for a turbocharged motor, but FAR too rich for a N/A one and all that does is wash down cylinder walls and dilute the oil (read: rod bearing issues). Shoot for 12.6-12.8:1 at full load, WOT.

Crank-triggered ignitions are not all the same, despite what they may look like. Some work OK on race engines that never idle and are operated above 6K all the time. Some cannot keep the plugs clean for street use and one struggles with all the associated problems with that. Some run OK when the AFR's are perfect,..........

You get what you pay for and I'd strongly encourage anyone considering such systems to do their due diligence and homework by talking to people with extensive experience with all these components. Coil-on-plug, used with good Engine Management, is the way to go, but don't overlook a good distributor/MSD setup when using carbs or MFI. If the distributor is in excellent condition, the timing signal is VERY steady as these are driven directly off the crank (which is very stiff), unlike other engines where they are driven from cam or other auxiliary drives.

JMHO, so YMMV,

Jeff Higgins · 05-30-2008, 08:03 AM

John, the A/F was measured at the muffler outlet by the dyno shop's instrumentation and by my LM1 that was laying in the front seat at the time (connected to the bung on the left hand SSI). They agreed within about .10 of a point or so.

My only thoughts concerning this relate to the high compression and advanced ignition. I'm thinking that with the spark lighting so early on a correct mixture, it may have been seeing peak cylinder pressure occuring too soon. In effect, "pushing back" on the compression stroke. The overly rich mix may have been slowing combustion enough to reduce or eliminate that affect. That's my purely wild ass guess, anyway. I would love to hear from some one who knows what he's talking about with this.

I'm with you on the ignition. I just picked up a twin plug Electromotive setup from a local R Gruppe buddy who had been running it on his 2.8 short stroke. As an aside, I just can't say enough about the camaraderie within the club - another member, who had first dibs on the Electromotive, alerted me to its availability and graciously stepped aside and let me have it. Big, big thanks to Brooke for giving me a great deal on it, and Kenik for letting me know about it. These guys saved the day (and a great deal of money) for me.

Anyway, so I've now got the Electromotive. Anyone have any suggestions on low speed, mid range, and high speed advance settings? 3.0 liter, MFI, 10.5:1 compression running on our locally available 93 octane Reformulated Cougar Pee (we call 'em "cougars" in this neck o' the woods...).

kenikh · 05-30-2008, 12:34 PM

Can't wait to see her run! The sad clown face you've been showing around lately was getting old.

Jeff Higgins · 05-30-2008, 07:00 PM

Well, I can't do anything about the clown face, but I'm working on the sad...

kent olsen · kent olsen's Garage

jeff

What kind of HP did you get before the fateful incident? Just got my heads off the other day. Now to remove and send my pistons/cylinders to EBS while I disassemble the heads.

V12man · 05-31-2008, 01:56 PM

Sorry John, but that should read:

Herr Prof. Ing. Steve Weiner

Jeff Higgins · 05-31-2008, 06:10 PM

I got about 198-199 at the rear wheels, Kent. I was hoping for a bit more. In retrospect (funny how much clearer things get under 20/20 hindsight) that may have been due to the undetected detonation, too advanced timing, or whatever. I'm scheduled to run it again on the 14th. I'll report back after that.

Flieger · Flieger's Garage

What's that translate to at the crank roughly? My 911/83 spec 2.7 MFI is supposed to be rated at 210 hp 188 lb-ft like the RS. You have more displacement, so you must have more power or maybe it is tuned for torque?

Jeff Higgins · 05-31-2008, 08:57 PM

The conversion factor I see used most often is +15% for crank horsepower. So, somewhere just shy of 230 at the crank. I'm hoping for better with the twin plugs.

911st · 06-03-2008, 01:54 PM

I have read. A 2.7RS w sport muffler put closer to 230fwhp.

A 2.8 twin plug w mod S cams, Webber 40's. and sport muff is 260FWHP. (Old porsche artical on a 914 build.)

I would hope for about 280 FWHP on a 3.0 twin plug MFI depending on cams, fuel ...

That is about 238 rwhp.

Compression and twin-plugging both add HP. I think the Twin Plugging adds about 3% on its own.

I am far from an expert. I did have a 2.8 twin plug once w specs as above except w RS MFI, but never dynoed it.

However, in a striped 72 911T body w LSD and 23" tall sticky 225/50/15 R1's's, it was wicked fast. 992TT's did not seem as fast to freeway speeds.

kenikh · 06-09-2008, 10:28 AM

How we doin' Higgy? We gonna see the blue car tomorrow night?

Jeff Higgins · 06-09-2008, 12:57 PM

Nope, not tomorrow. There were some problems in finding a good replacement head that delayed things a bit. The first one had been modified for a 98 mm bore (which I didn't notice) so I couldn't use it. I finally got a good one to Jeff so he could drill it up for the second plug, but that wasn't until Thursday evening. I'm hoping if all goes well that I might get everything back this week. If so, it might make an appearance on Saturday. We'll see.