my three thoughts:

-head nut torque could be a serious issue with raceware or ARP head studs, especially. What kind of head studs?

-too much CR, as stated by others...though I have not generally seen any pistons for a 3.0L that are much over 11:1, if that

-the main thing, i think, is too much distributor advance. I would not run over 28* max. on pump 93 octane...maybe on a smaller bore engine or an engine that runs CIS, but definitely not a 3.0L hot rod with MFI!

The head studs are stock steel, top and bottom.

JE has sold these pistons for years. I verified the part number when I called them yesterday. My heads are stock save for the conversion to MFI. The deck height was towards the high side of the range. The piston/cylinder set came from EBS, a very reputable supplier. Folks have used their reconditioned sets with the JE's for years. I will mock up a cylinder and check the compression ratio per Grady's suggestion, but I would be surprised to find anything there. Maybe if my heads had been flycut to significantly reduce the combustion chamber volume, but they have not.

Advance was 33* total; right between the 30* and 35* marks. Maybe I will back that off a bit when I get it going again. It's my understanding, however, that MFI will tolerate more advance (than CIS or carbs) on a given motor because of its tendency to run better when on the rich side.

Anyway, lots to consider here. I've got a couple of weeks to do so until I will be able to put it back together. Lots of things to check before and as I reassemble it. I'll be sharing what I find as I go along. Maybe one of you will see something that jumps out from that. This is all pretty basic stuff; I believe something very basic went awry. I need to know what that was...

Jeff, you might be right about the torque wrench. Waiting for your test results.

My pistons/cylinders came from EBS also. By the way, they stood by their merchandise, and offered to either exchange the pistons or machine material from the domes at their expense. I opted for the latter, as it allowed me to tailor the CR exactly to my heads, deck height, etc.

Here is the complete story:

http://forums.pelicanparts.com/911-engine-rebuilding-forum/294214-9-8-10-1-a.html?highlight=piston

Make sure you read as far as post #14.

The torque wrench is fine. It consistently torques, as near as I can tell with my admittedly crude tests, within a pound or two of my beam style. Certainly not far enough off to be an issue. Scratch one variable off the list.

That little diversion out of the way, here is the big news of the day. I set out to calculate my compression ratio. The plan was to use the flat plate trick to block off the combustion chamber on one of the heads and measure its volume. Armed with that number, the deck height I used, cylinder displacement, and piston dome volume, I figured it would be easy enough to calculate. The only thing I didn't have was the dome volume, or a simple way to measure it. While pondering this, I did the first sensible thing I think I've done in the course of this investigation. I went to the JE website to see if they had full specs on my pistons.

Sure as hell, they are the 10.5:1 set. I truly thought, after speaking with a salesman to verify the number yesterday (while ordering new pin clips), that I had confirmed they are the 9.5:1's. That's exactly what he told me when I asked him. Why didn't I just check the website to begin with... why didn't I verify the part number before I put the damn thing together? Why should I have to? Rhetorical question, I know. My frustration is showing...

So, I have the 10.5:1's, part number 212436. Cross checked with the part numbers listed right here on Pelican. Un-freakin-believable. So, that's water under the bridge. Now for a recovery plan.

Scott, I'll study your thread about machining the domes. Beyond that, what are my options? New pistons are one option, obviously, albeit an expensive one. Twin plugging is another expensive one. Suggestions? Which would you guys choose?

Grady, could this be "it", the definitive answer we are looking for? I hope so. It seems to make sense now. The mix itself was undoubtedly leaner with the track mufflers, but if the compression was really 9.5:1, probably not a big enough change to cause this. Combined with the higher compression, in a motor that must have been on the ragged edge of detonation even when running rich, it makes sense that it was a critical enough shift to spell its demise. Hell, it was probably detonating before the muffler change. Setting the stage for this. The too lean condition was enough to push it over the edge it was teetering on anyway.

Edit: The dome is only .340" thick. Scott, I see you had to remove 7.5mm (about .300) from yours to drop from 10.9: down to 9.8:1. The bottoms of the domes on my pistons are anything but flat; they follow the angles of the valve pockets up to the top flat of the dome. I have a photo of the bottom of the piston a page or two back in this thread. So, scratch one option.

First, EBS and or J&E must have some liability here and should help.

A 10.5/1 (10.25/1) with twin plug is a very street able combo and will add to the responsive of the MFI set up.

Twin plugging burns the mix faster, requires less timing, and with the later combustion actually creates less back pressure on the compression stroke for a net gain in HP.

Cost is the only issue.

I like the old style twin plug distributor. Our host sells the caps & rotors pretty cheap. I would have an adapter made for your distributor unless someone else makes a kit.

I had a 2.8 twin plug I converted to MFI and that was the style I had. Someone just modified the stock distributor with an adapter place. It had an aftermarket trigger to replace the points, I just wired it to trigger both CDI's. Worked great.

911st,

10.5:1 is probably OK under good conditions (temperature, humidity, barometric) with a 2.4, 2.7, etc. When you get to the large bore size of a 3.0L, 10.25:1 (even on MFI) is too much for a single plug. The flame propagation rate is FAR less complete than even a 2.8.. don't forget, the air is expanding at a rate of roughly 1/(cube root x). inversely proportional to the volume x cubed.

Combine that with cams that generate high dynamic compression in a slightly lean condition, not to mention a radically advanced ignition curve, and you have a recipe for some crap happening. as unfortunate as it is, Porsche engines aren't alchemy. They act in accordance with scientific principles.

Jeff, I want to help, but I don't want to talk down to anyone, so if I am being infantile, don't hesitate to let me know ;). Have you read much about ignition advance curves, how they work, why someone would want to change them? I just want to help explain this. I know I'm only 21 years old and some of you might just think I am some random kid, but please bear with me and hear me out.

Basically, if you think of the piston going around, you want to just time it so you get the maximum forward 'BANG!' on the piston. Igniting the mixture significantly before the piston reaches TDC is making the 'BANG!' go the wrong way. When your mixture ignites before the piston has gone past TDC, you are pre-igniting. This is what happened to you. Severe detonation as a result of pre-ignition.

CIS cars generally run more advance than MFI cars because the camshafts that they have dictate that the peak Volumetric Efficiency occurs at lower rpm's than with MFI cars (generally speaking). As Volumetric Efficiency increases, advance decreases, but because flame speed is finite, it must be initiated much earlier with increasing speed. BUT, igniting it TOO SOON IS FAR WORSE than igniting it too late. At least, if you want to try pushing/optimizing your ignition advance, err on the side of less advance as opposed to more advance, and work your way up...slowly.

Carbureted cars have crappy fuel atomization. Crappy fuel atomization=less fuel molecule surface area=more advance required to ignite it in the same amount of time.

MFI is superb on fuel atomization. With excellent fuel atomization, you are looking to *MINIMIZE* your ignition advance, not increase it.

Increasing ignition advance CAN have positive effects on performance, to be sure, but you do not want to run more than is necessary/optimal.

More radical cams, such as MFI cams, produce higher peak cylinder pressures at higher rpm than CIS cars generally do. Therefore, you would want to tailor your ignition advance to have more advance at lower RPM, and less at higher rpm.

I am wondering why you set your advance at a fixed 33 deg.? Any reason not to build an adjustable advance curve into it? Henry Schmidt at Supertec built me a 911SC distributor (single plug) for my engine, did a wonderful job with it, and basically pushed that distributor to do things it was never intended to do. It is probably about 3* @ idle, 14-18* @ 3000rpm, all in at 27-28 deg. by 4000. It will be going into my motor, which is either going to be a 3.0 or a 2.8 SS single plug, with S cams, MFI, big port SC heads, 9.7:1 CR.

When you add a freer-flowing exhaust, this is again a reason to reduce ignition advance. (Opposite of increased back pressure)

When you increase compression, this is again a reason to reduce ignition advance. (ALWAYS measure the CR in an engine you build.)

Live and learn...SmileWavy

Regards,

Scott

Thanks Scott; that is great information and very well explained. Please don't feel like you are making me feel as though you are "talking down to me"; let 'er rip. When I stand to learn something, I try not to let my feelings or ego get in the way. So don't worry about it.

Anyway, my distributor, while having a very fast advance curve, is not really "fixed". It has the full advance range of a stock SC distributor, it just comes in much faster and is strictly mechanically governed. I guess functionally, it is essentially "fixed", as it is running at full advance in the entire rpm range in which I would ever drive it, especially on the track. But, then again, so is yours if it reaches full advance at 4,000 rpm. My motor rarely spins lower than that, except pulling out of the odd second gear corner.

The advance curve in my distributor was set up after a great deal of discussion with the guy who rebuilt and recurved it, Barry Hershon. We covered everything in our conversations; displacement, cams, induction, exhaust, and compression. Intended use, driving style; everything. Surprisingly, his recommendation pretty well paralleled what I felt I wanted when I called him. I was ready to be talked out of it and defer to his knowledge, most certainly.

Beyond that, I simply set total advance to the stock specs. It sounds like that was a mistake, even if it had the lower compression I thought it had. With the higher compression it was actually running, well, we see what happened. When it goes back together, if I stick with the single plug and change pistons to lower compression, I will certainly follow your recommendation and back total timing off a bit. I'm not looking to squeeze every last ounce of power out of it, so there is no reason to take undue risks.

What you say about total advance relative to atomization makes a lot of sense. Opinions seem to vary a bit on this one, however. Henry Schmidt (of SuperTec fame) has stated that MFI likes a lot of advance and a rich mixture. Granted, this may be apples and oranges, as his comments were made in the context of race motors on race gas. Would the philosophy hold in principle, if not actual numbers, on a street motor running pump gas? I don't know. I do know what will be safer, and like I said, I am not looking for every possible pony. Better safe than sorry...

Jeff-
My pistons were really "pointy" on top, so the first few millimeters didn't result in very much volume. I can't tell by your photos, but they look flatter on top. Perhaps it would require much less thickness to reduce the volume of the dome considerably.

To measure the volume of the piston dome, you just need a cylinder, piston, rings, a 1/4" plexiglass disk with a hole in it, a burette, and some solvent. Place the piston in the cylinder. Move the piston until the top of the dome is exactly flush with the top of the cylinder. Use a little grease to form a seal between the piston and cylinder. Seal the plexiglass to the top of the cylinder. Fill the resulting cavity with solvent, measuring the amount required. Remove plexiglass, and measure distance from the cylinder sealing surface to that nice, flat edge at the perimeter of the piston. Use this distance and the bore dimension to calculate the volume of a cylinder of those dimensions. Subtract the volume of the solvent you used, and you have the volume of the piston dome.

I bet EBS will get you a new set of pistons. It sounds like you didn't get what you ordered.

YTNUKLR

I thought I was talking about twin plug motor, not single. Yes that would be to high for a single plug street fuel motor.

However, factory 3.6 twin plug motors run near this compression. Granted, they have a knock sensor but with tame cams and leaner mix.

More aggressive cams, lower the effective compression level. A richer mix makes the mix less volatile and burns cooler.

It is not only the distance of the flame travel but what it has to go over or around. Sometimes smaller motor needs twin plugging more than a big displacement motor. 2.0's are an example.

930's are also an example of this. The effective compression on these with the boost turned up is closer to 11/1 and work well with single plug because of the open chamber design and rich mix. 3.6 turbos dropped the twin plug for a single plug set up.

I am far from an expert and and sory if I sould like I play one on the Internet.http://forums.pelicanparts.com/suppo...s/beerchug.gif

Measuring is a good idea.

Someone noted above, mocking up a test P&C. This is much easer. Just grease up the piston so fluid can not leak past it. Put the cylinder at tdc and poor in a measured amount of fluid till full to the bottom of the spark hole.

Then, just do the math. I used a big medical syringe and I transmission fluid if I recall.

I see now on the distributor. I thought you had it rebuilt to lock it at full advance all the time. That makes more sense. Your distributor sounds fine.

Had the mufflers been the ones you tuned it with, and the compression actually 9.5:1, I doubt 33 degrees of advance would have caused any problems at all. It's not that you were running too much advance by itself, it was just wayyy too much advance for 10.5:1 that caused problems.

Yes, MFI likes to run a bit rich, and it likes advance, of course. With race gas and twin plugs you just get a LOT more safety cushion to work with AFA timing compared to street gas and single plugs. 93 octane is a lot more explosive than 110, or even 100. With the conditions what they were, unfortunately you just 'ran out of safety cushion'.

The distributor I have is basically built with a lot of attention on the low end of the curve, for really nice low-speed transition/throttle response. I like a little safety margin, like you , and I am not going to try to get every last pony out of it, so my max advance will be a little lower. Also, I had in mind some hotter DC60 cams at the time, so a little less advance with more cam up top.

I am sorry that your engine blew and you have to deal with it. Even though it is just a long weekend and a box of parts from re-doing, it's something to deal with. On that long Saturday, just make sure you have enough beer and helpers, in that order.


2.0s are a special case...those high-domed heads are really only for those engines...anything larger than 2.0 and you probably have the flatter heads from 2.2, etc. I built a 2.0L race motor with single plugs for historical accuracy (could not run twin plugs for visual reasons) and running with similar twin plugged 2.0s, it was getting raped..

3.6s are out of this discussion for two little words: knock sensor .

With 930s you are talking dynamic compression ratio, which is not static compression. you are right, 930s with the boost turned up are making dynamic compression in the 11-13:1 range. It would be a mistake to think that single plugs is better than twin plugs for any application involving more boost and/or compression and/or cams..even if the factory did that with the 3.6T...

Oh, one last thing...don't be sorry. I am too sensitive when people use this word, but I have had enough of self-deprecation. Everyone on the planet has something valuable to add, and they should say their piece with no apologies (if they do believe what they say). They can always get corrected later, but nobody should be sorry for anything here! We are all just learning, even if we think we are experts . :p


:)

Jeff:
That sucks!
If the bill from EBS clearly shows the intended CR (as mine does), I believe EBS would come through on this issue.
Options:
Stick with the intended 9.5 and have EBS send you the right pistons or, have them machine the wrong ones to get 9.5?
I'd prefer new ones with new rings.
The other issue is: How many thou will have to be machined off the heads to clean them up?
And how will that change the CR-picture?
The tops of the SC cylinders don't look that clean in the picture. I would prefer to have a very clean surface there to help with sealing and not just rely on the gasket.
Re: Cylinders:
Wouldn't you much rather have the 95mm Carrera cylinders without the head gaskets?
I believe they are compatible for height?

To do a twin-plug is much more expensive and it sounds to me that with 9.5 CR P/C's, and your present set-up, you can have a lot of fun and less worry.

Add a HT sensor?

The invoice from EBS has the part number for the 10.5:1's, Gunter. The big question is at what point was this miss-communicated, or miss-handled? I'll probably never know. I did not personally order the piston/cylinder set; they were ordered by the machince shop to which I had brought all of the pieces for cleaning, inspection, machining, and whatnot. They declared my original cylinders too far out of round and tapered to reuse, so offered to order up this set from EBS. Same shop did the welding and machining on the heads, so it was pretty obvious to them it was remaining a single plug motor. We even talked about compression ratios. I'm sure we were on the same page. Anyway, water under the bridge. Time to weigh options. Oh, and I do have a couple of new cylinders and heads on the way. I don't trust the rough ones either.

So I had a very pleasant, informative discussion with a JE tech rep this morning. I was hoping I could face off the tops of the domes (like Scott did) to lower compression. I figure I need to lose about 6.5 cc's of dome volume to drop one whole point of compression. He told me not to do it. While the 10.5:1's and 9.5:1's do in fact share the same forging, the differences in dome volume are not achieved through a difference in height. Their heights are identical. The 10.5:1's have a more filled out, bulbuous dome. Think "D" cup vs. "B" cup. The squish band is visibly wider on the 9.5:1's, as the dome is actually smaller in diameter.

He suggested shimming the cylinder bases. It turns out that a 1 mm shim would add my 6.5 cc's to the combustion chamber volume. I'll have to refine my calculations a bit when I get some time, as this does not account for volume of the dome at BDC, but it's close. I did a cursory search here on Pelican, and it appears base shims are, in fact, available in 1 mm thickness.

This solution would, however, raise my deck height to 2.4 mm. That would concern me after reading Grady's and Steve Weiner's comments about excessive deck height and detonation. Thoughts, anyone?

I've just read this thread, from the first all the way through. I'm glad that Jeff checked the torque wrench. There still can be issues here. Most of these are best used, when critical specs are required, in the upper half or upper quarter of the range. Most click type require to have the setting reduced to zero when stored. Most need to be recalibrated periodically, so choose a brand that can be serviced. When Jeff rechecked his torque after running a while, it was in an awkward position, that may have some influence. Of greater concern is that his check did not reveal any change. A problem that arises is that the break-free torque is higher than a dynamic torque. This is easy to see with a beam type wrench, and as an example try to torque a nut in stages say 10, 20, 30 ft lbs. When you stop at 10 ft lbs, then start again to go up to 20 you will not start turning the fastener unit 12 to 15 ft lbs and then it drops to say 11 and starts climbing. Final torque must be done in a smooth progression without stopping. So when you go to recheck the torque later, you must try to overcome the starting friction without over-torquing. Click type units are bad for this application since you cannot see the torque indication building up; they are more like a go / no-go guage. Critical applications require the correct tool, the correct technique, and often some relevant experience.

Twin plugs, by the time you are all said and done, is an additional $3k, easily

DON'T use a cylinder base gasket to achieve your intended compression ratio. It increases your deck height too much.

Hey Jeff,

Interesting thread, and I hope educational for all who read this.

While I won't tell you what to do here, I would vigorously suggest that you do everything in your power to maintain that 1mm deck height.

Stacking base gaskets to reduce CR is NOT the right way to fix this unless correct deck heights can be maintained,..:)

I will second this admonition.

I think both Steve, I and many others have written about this.

Best,
Grady

Jeff, somehow this is your fault - dumbass.

Glad I won't be making the same mistakes ;P