I was really looking forward to a good thread on this important topic. Grady has been a great help to me, as I was having pinging issues on my 78SC (rebuilt with higher cr pistons) even after every manner of adjustment and different fuel grades. Then I swapped in a larger fan with improved pulley ratios, as advised by Grady, and the problem is solved because the heads now run cooler. Can we please start again with a fresh thread and refrain from bickering?

"as I was having pinging issues on my 78SC (rebuilt with higher cr pistons) even after every manner of adjustment and different fuel grades." - rpiper -

The cooling fix only solves the problem for YOUR tested driving conditions and not a worst
case engine temperature one might "see" elsewhere. A better solution for the higher compression
would be twin plugs or a reduced timing advance curve, given that the engine has no knock sensors.
Furthermore, even though the above provides a solution, the margin of safety is unknown.

Here's a good thread indicating what was unknown until the engine was opened.

http://forums.rennlist.com/rennforums/showthread.php?t=205168&highlight=broken+rings

This probably took some time while the owner was unaware of what was happening.
This thread perfectly exemplifies that one never really knowns what's happening in the
engine until it's "opened", and now the owner has unforeseen damages. Since using
a performance chip that "pushes" the timing and increasing the compression ratio
result in very similar problematic issues, this thread is very applicable to those who use
performance chips. The analogy should be clear to all who understand engine basics.

Why run ANY engine without some margin of safety & risk what this thread infers?
Performance chips & some other engines mods reduce the margin of safety.
THAT'S THE POINT! Like I've said, some just don't get it or are just incapable
of thinking logically!

"I think that should serve well as a solid warning to those thinking about chipping their SC 3.0 w/ CIS!" - island911 -

Having a reading comprehension problem relating issues?

per your link "82-sc motor having a knocking noise . . ."

Exactly! Thank you for that well-reasoned and insightful comment (seriously). Of course it has nothing to do with detonation, but then Loren has a way of redirecting (poisoning) threads. Thanks, Island.

http://forums.pelicanparts.com/uploa...1131641284.jpg

:D :D

Before we focus on detonation (and pre-ignition) let’s step back and look at where we are going. There are two basic systems to control engines; “dumb” ones without feedback and “smart” ones with feedback. Don’t misinterpret the word “dumb” as some of these work great. Perhaps the dumbest is MFI where the engine “chip” characteristics are ground onto a mechanical space cam. Even so it works just fine but it leaves a lot of performance on the table. Feedback allows the control of the engine closer to the limit without damage.

MFI only knows the throttle position, RPM and ambient barometric pressure. It doesn’t know the load on the engine, the temperature (other than it isn’t in cold start or warm-up mode), exhaust gas composition, detonation, ignition timing or anything else. Everything is assumed to be perfect and it blithely goes about pumping fuel based on what was ground on a 2-dimension surface (the 3rd dimension is the output.) Even a carburetor is smarter as it measures mass air flow or intake manifold depression and meters the fuel accordingly.

What MFI gives up is emissions, fuel economy, drivability and performance. The highest compression ratio used with MFI is 9.9:1, not the 11.7:1 used by a GT-3 with Motronic ME 7.8 with feedback. Not only is the fuel mixture controlled using feedback but so is ignition timing, intake cam profile and intake resonance. I suspect throttle butterfly position also.

Porsche claims that the ’05 GT-3 knock control when “Using fuel of octane number 95 RON/85 MON results in a significant [up to10-12°] retardation of the ignition timing under load and the consequential reduction in power output.” Without feedback the consequences can be mechanical damage.

What does this have to do with a discussion about fuels and detonation?

Detonation is one of those lines you can’t step over without reducing performance or damaging the engine. On the other hand you want to be as close to the limit as possible. Without feedback you must necessarily leave a larger margin for error.

Using unnecessarily high octane fuel has unintended negative consequences. The throttle response is diminished, exhaust emissions are increased, fuel economy is reduced, performance is reduced and the fuel cost is higher.

The things that raise the threshold for detonation and pre-ignition are; lower combustion chamber temperatures, clean surfaces, twin ignition, better “swirl” of the incoming air/fuel charge, better atomization of the fuel leading to better vaporization, proper “squish”, combustion chamber/piston shape and more.

The things that lower the threshold for detonation, but are desirable for performance, are; higher compression ratio, larger intake charge (“on” the cam or higher boost) and greater ignition advance (to a point.)

What are more things that affect the detonation limit and what can be done to raise that limit?

Best,
Grady


PS: You know it is possible to delete your own posts. Hint, hint.

Edit to complete a sentence.

Mr Grady:

Nicely done, Sir. My compliments to the "Chef". :)

Please don't misunderstand. I realize that twin plugging would certainly be a better solution but that's not in the budget. The engine was originally built for PCA stock class racing, so it is optimized within the rules (i.e. no twin plugging or internal mods) and has CIS. The engine makes 200 at the wheels so it is certainly operating at its maximum inherent limits. In the race car at speed with the fan pumping huge volumes of air due to sustained high RPMs and using a 930S cooler in the front bumper, there were no cooling issues. Now that the engine is in my daily driver, I was seeing detonation, regardless of settings, fuel or different advance curve (tried everything) especially on hot days. As the pinging was audible, it was severe, so I needed a solution. The larger fan and improved pulley ratio solved the problem the best that it could be solved outside of twin plugging. The margin of safety is not very wide, but there are always tradeoffs and this is one I'm willing to make.

Grady, thanks for your treatise, very informative. Here's what I'm not understanding.

I raced a Honda 500cc 4-cylinder GP bike in the mid-70's. That engine was air/oil-cooled, carbureted, and we built it with a 13:1 CR. The combustion chambers were partially hemispherical and the pistons were mildly wedged, sort of a semi-hemi. We achieved the high CR through a combination of the piston dome shape, a slightly lower wristpin location, and by milling the cylinder base. We used seriously advanced ignition timing (don't remember the actual number) and we ran Sunoco 96 octane race fuel at the time.

We ran Honda 750 carbs and jetted slightly on the rich side to keep the temperatures down to provide the margin of safety we are all discussing now. We NEVER had a problem with detonation, confirmed by visual inspection as we frequently pulled the head and cylinders to re-ring or clean up the valve seats. The only time we had any problem was when one main jet got plugged with a piece of grit, causing a lean condition and rapidly holing a piston. Otherwise we easily got a season out of one set of pistons.

So, what I'm not understanding is why can't a 911 engine be run at 10:5:1 or even 11:1, on 93 octane fuel, without concerns about detonation? Both engines are air/oil-cooled, both have similar combustion chambers (granted, the 911 is more hemispherical, but not by a lot), and the 911 has a much more sophisticated fuel management system.

Is it just the difference in combustion chamber shape and/or the quality of the fuel?

I bet it's the diameter of the cylinder and how far the flame front has to travel from the sparkplug. My car has 9.77:1 measured CR and it's probably too high for single plug and CIS. I had a carbureted short stroke 3.2 with twin plugs that had 12:1 and made 275 at the wheels. No detonation issues but had to run 110 and it was tired after 100 hrs. Sure was fun while it lasted though.....

I wish there was a clear cut answer to “Why can’t a 911 run high compression ratio?” The best answer I can give you is the temperatures are too high and there is poor intake mixing (swirl). I think the intake mixing issue is probably due to the nice symmetrical hemispherical design. The mixing is improved with the bump & dish CIS piston. It is improved by running very tight piston to head (squish) clearance – tighter than reasonable for most street cars.

The cooling issue is reflected by the improvements in performance when using the high ratio engine fan, keeping the engine at relatively high revs and the “Rubbermaid Solution”. It still isn’t as good as a water cooled head.

The 2.0 single plug vintage racers regularly use 12-13:1 but have the big piston squirters, all the cooling air is going to the heads & cylinders, the biggest oil pump, huge oil coolers, perfectly clean head & cylinder fins, carefully selected leaded race fuel and never see below 4500 RPM. Basically all the things you would do for a 3.5 but on a 2.0.

In the early ‘70s Alan Fritze and I modified some heads for water cooling (I think he was a University of Denver undergrad ME at the time). I might try that again. This time I’ll only do one head (#3 or #6) and plumb the single cylinder float chamber for variable octane fuel and use knock sensing. That way I only hole one piston at a time. HeHe.


Bobby, I understand about meeting the rules and the cost of twin plugs. What fuel are you using? What is your actual advance curve? What is your cranking compression and cylinder leak? What is your oil consumption (the oil dramatically reduces octane)?

Yes, the piston diameter makes a large difference. As I understand it, the flame front travels about the same speed in all engines. With a large cylinder and the spark plug off to one side there can be more combustion/detonation mischief on the far side.

With CIS cams (no overlap) the combustion chamber is sealed up much earlier in the compression cycle. I would think this would cause the cylinder pressures to rise sooner (before ignition) than with a high overlap cam. There is also the issue of not having the air/fuel mixture go completely through the cylinder and out the exhaust port during overlap leaving a cooler mixture in the combustion chamber. Everything else equal, you can’t have as high compression with a mild or no overlap cam as you can with a very radical race cam. Your 9.77:1 CR and no overlap may be equivalent to 14:1 with a high overlap cam. Worse because it can generate high cylinder pressures at low RPM.

You might give the Rubbermaid Solution a try on the street. That will significantly cool the heads on a hot & dry day for <$100. Do this as an experiment, using it on the street all the time is a pita.

Best,
Grady

Wavey-
My $0.02 is far less valuable than Grady's, either Steve's, Loren's but I have a guess. I don't think these things scale exactly linearly. I believe there are many ineficiencies as all of the parts grow in size. Cooling efficiencies have a lot to do with the size and thickness of the parts, temperature gradients and thicknesses of pistons, cylinder walls, as well as exposure to cooling air.

Good question - I'm curious about the other's response.?

Doug

As usual, Grady contributes big time, you are an asset to this board sir.

One thing I hate tho, is reading part of Loren's posts as quotes in the posts of others. Kind of defeats the purpose of an ignore list, doesn't it? ;)

I'd like us to stick to detonation issues and not personal issues...

Now, the fundamental problem is that the 911 engine is air cooled. It just doesn't have the cooling to 'extend' the detonation limit. Of course, the Rubbermaid solution is really just a water cooling solution when you think about it.

Now, what to do:
I'm an advocate of twin-plugging. Tho expensive, it reduces the flame travel distance.

Something else is the presence of debris buildup in the combustion chamber - this reduces the volume (hence increases the CR), and the rough, particulate like surface of these contaminants serves as nuclei to initiate detonation events.
- Coating the piston surface may help some, and is cheap - I paid about $100 or a tad more to put 3 different coatings on the 3 different working surfaces of all 6 of my pistons.

Like it or not a big problem w/ a 911/964/993 is the aircooled heads. H₂O is a much more efficient medium for keeping head temps under control, and if temps are controlled it helps an awful lot in detonation/preignition control.

* I have edited out a couple of non productive responses in this thread, if you don't like it, fire me

Thanks Bill.

I read that an aircraft flat 6, 275hp, FI, @ 75% max power w/350F head temps can have 100-150F difference around the top-outside of the cyl measured at 4 equal positions. Even those temps are unstable. The difference in these temps dropped significantly maintaining the same 75% with a CHT temp drop of 20F.

The author inferred wear & tear, friction, heat, etc.

Detonation was never an issue to even start. So connecting that to a land 911 eng means the temp differentials would be greater at the same 75%. It seems to me that CHT is life.

I wish my 3.0 had bosses for the 964 knock sensor bridge so I could hook up one of those basic knock indicator circuits based on a 555 timer and do some testing.

Though the thought of doing knock testing on a 911 engine strikes fear into the heart of my wallet.

Where's my daily-driver test mule? I had been meaning to buy a RWD volvo turbo, which would be perfect to play around with megasquirt, programmable ignition, knock sensing, etc. forged mahle pistons and a cast iron block. Trouble is finding a manual transmission one, that isn't too trashed.

Those two points make a great deal of sense. The 500 cc race engine, while very much on the oversquare side (large bore/short stroke), still had a much smaller piston diameter than any 911 engine. And - we ran fairly radical cams with lots of overlap. It's all coming together...

I was thinking that the (comparatively) very short intake and exhaust tracts could be a contributing factor too - plenty of cool air straight into the cylinder, not much time to hang out as exhaust.