L71 L72 427 Running on Pump Gas

[John M.]

Re: L71 L72 427 Running on Pump Gas

Michael,
That's a little more involved than what I had in mind. My effort is to eliminate the ping, hopefully by just changing head gasket thickness. However, I can make that measurement after I choose the head gasket using these other methods. In the meantime I could get the dome volume by placing the piston at an exact distance down the bore, filling the piston gap with the white grease and then CCing the space to the deck, right??
John

[Michael H.]

Re: L71 L72 427 Running on Pump Gas

I could get the dome volume by placing the piston at an exact distance down the bore, filling the piston gap with the white grease and then CCing the space to the deck, right??
John

Yes, that works very well, as long as the piston is down a precise distance for the calculation. Give us the exact bore size and the distance the piston is down and we can provice the number of CC's for a space that size without the dome/valve relief values.

It will be interesting to see how close the volume is to advertised specs for the piston.

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

John, An 11.2 SCR is getting pretty close to where is should run on 93 octane pump. Any luck on cc'ing the dome to make sure the 11.2 is accurate?

My thoughts at this point if it were mine. I'd work the chambers on the heads to ensure they were the same size and to smooth them. Secondly I'd work the pistons to smooth the sharp edges on the domes. This will lower the SCR and it will help with detonation. I'd use the .039 head gasket since the pistons are at the deck. I know Duke disagrees with the importance of quench but the research on high performance pump gas engines indicate quench is one of the key components. The research indicates that a lower SCR and larger quench is more prone to detonation than a tighter quench with a higher SCR. Also I'd run a 170 - 180* thermostat.

If you decide you want to go for lowering the SCR via a thicker head gasket, I noticed there is an .072 gasket as well as others listed on Summit Racing's web site.

http://store.summitracing.com/egnsearch.asp?N=700+4294925237+4294891421+4294908216+4294925082+4294840120+115+4294791081

Hope this helps,

Ted

[Duke W.]

Re: L71 L72 427 Running on Pump Gas

John, An 11.2 SCR is getting pretty close to where is should run on 93 octane pump. Any luck on cc'ing the dome to make sure the 11.2 is accurate?

What's the source of this statement?

Based on the evidence I have for BBs, which is primarily owner reports and as much information about the actual CR as is available, 93 PON might support 10.5 without severely compromising the spark advance map to avoid detonation, which is why I recommend a maximum true SCR of 10.25. As built by Tonawanda, most SHP big blocks are in the range of 10.5-10.75, which is why some OE built SHP BBs detonate and some don't on current unleaded premium.

As far as research on quench is concerned, I go by Taylor who says that the effects of tight quench are negated once quench exceeds .005 times cylinder bore, which is greater than GM's recommended .035-.040" minimum.

Can you list any verifiable references for your statements /about quench clearance?

Have you read Taylor?

Duke

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

Duke, you and I went down this path once and I don't know that we're going to come to agreement. John can decide which path he wants to take. I tried to help on both paths including pointing him to where he could obtain a .072 head gasket instead of having one custom built. I assume the .070 gasket was your suggestion.

There are articles out there on high performance pump gas engines in virtually every hot rod rag, there's articles from Lingenfelter, there's user experience on the various blogs, I have my personal experience with big blocks, etc. Taylor is clearly the engineering father on internal combustion engines and did specific experimentation on quench, spark plug placement, etc. I don't know that I've ever seen where he expiemented with BB or SB Chevy's. As you've stated before there are alot of variables in engine design. I generally like to go with real world examples on a given engine. I tried to put a poll out to see if we could get some more info from other L71 L72 owners. I haven't worked as much with the 427s but have worked alot with 396s up to .090 over and some 454s and 496s. The cam in question has very slow ramps and a very late intake closing event. The effective cylinder volume is much less than the SCR would indicate.

[Duke W.]

Re: L71 L72 427 Running on Pump Gas

I don't put much faith (and neither do most engineering professionals) in hot rod magazines or blogs - mostly amateurs who don't understand the basic science behind IC engines, so you end up with a lot of myths and misinformation.

I rely on the the classic IC engine textbooks and professional technical papers.

I also rely on working with Corvette owners, but, unfortunately, few have the skills or discipline to make all the measurements that are required to accurately compute the CR of each cylinder, and "engine builders" rarely do this, or if they do I never see that actual data -just some vague statement that the CR is... whatever, and this is reflected in the few responses to your poll.

Few really know what the true CR of their engine is, and most believe that it is equal to the value advertised by GM, but given observed production tolerance, especially deck height, it can easily vary up to half a point, and is usually less than the "ideal" advertised value.

If every Corvette owner was as skilled and disciplined as John McRae, we would be able to target, with a high degree of confidence, the exact maximum CR that each OE configuration can tolerate without detonation on available pump premium.

Duke

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

"Dyno testing has proven that improving the piston-to-head quench area from .060 to .045 will improve power even when the compression ratio remains the same. It is even possible to optimize the quench and increase compression without suffering detonation problems! This is because improving homogenization of the mixture in the chamber reduces the tendency for lean areas in the chamber to promote detonation." John Lingenfelter - John Lingenfelter on Modifying Small Block Chevy Engines - Page 48

And I guess John Lingenfelter doesn't know what he's talking about either.

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

"The suggested .040" static quench height is recommended as a good usable dimension for stock rod engines up to 6500 RPM. Above 6500 RPM, rod selection becomes important. Since it is the close collision between the piston and the cylinder head that reduces the prospect of detonation, never add a shim or head gasket to lower compression on a quench head engine. If you have 10:1 with a proper quench and then add an extra .040" gasket to give 9.5:1 and .080" quench, you will create more ping at 9.5:1 than you had at 10:1. The suitable way to lower the compression is to use a KB dish piston." Keith Black Pistons

Again I guess they don't know what they're talking about either

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

John,

I don't know if you've seen the exchanges between Duke and myself but we obviously are after different results when it comes to engine building. I'm more on the performance side and Duke appears by the exchange and past posts to take a more conservative approach.

If you want to ensure that the engine will run on 93 (or below) octane, be able to use the exact timing specs from GM, etc., then Duke's approach is the safest choice. At 10.25:1 SCR (per Duke) with the GM spec cam it probably doesn't matter on quench, chamber shape/surface, piston shape/surface, etc. because you're not pushing the envelope on running the engine on pump gas. As you move from 10.25 up to 11:1 SCR with the GM cam, quench, chamber shape/surface, piston shape/surface, etc play a more important role in preventing detonation.

It would have been interesting to have seen what octane was required to run the engine in it's existing configuration. By the mixes you indicated it looks like 96.5 was running it fine. Part of the challenge with big blocks is the piston dome. Running the upper limits of DCR is easier with flat tops than with domes due to the dome creating an obstruction for the chamber flow. Some years back a friend of mine was running a 465 Rodek (aka an aluminum BB Chevy) in an alcohol dragster. When you looked at the piston top you could see by the discoloration that the combustion wasn't occuring over the full surface of the piston. He reworked the piston domes to smooth them (which lowered the SCR) and gained .2 second in his 1/4 mile times. Also you could see a clear difference in the completeness of the discoloration from the combution process across the piston surface.

There are competitions out there where they see who can build the highest performance pump gas engine. They have had engines pushing over 13:1 SCR running on 91 octane pump gas. In that case they used a flat top piston that was dished in the chamber area. Naturally it was a specific purpose built engine and it wouldn't work well in the real world due to it's sensitivity to conditions nor am I suggesting that anyone follow that path.

John best of luck with your engine. At this point I'm signing off on the topic of pump gas engines before I get pointed with Duke again.

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

"Our entry's dimensions and chamber design were pretty much finalized after a consult with "W.J." My Pro Stock mentor from nearby Sugar Hill confirmed what I believed to be true: a small chamber with lots of quench and turbulence was the way to go. 'Detonation Prevention 101' soon began, as the whole engine was built on that premise" Jon Kaase - Winner of the Jeg's 2008 Engine Masters Challenge - Competitive event focused on building the highest performance engine on 91 octane pump gas

Obviously Warren Johnson and Jon Kaase haven't relied on Talyor's quench statement either.

[Ted S.]

Re: L71 L72 427 Running on Pump Gas

Seems like alot of "amateurs" are singing a similar song.

And finally from Warren Johnson of NHRA prostock fame - "If the facts don't agree with your theory...believe the facts, and make up a new theory."

Best of luck in your endeavors Duke.

[Duke W.]

Re: L71 L72 427 Running on Pump Gas

My interest is helping Corvette owners restore their engines to OE appearance and operating characteristics while improving performance across the entire operating range and increasing usable revs.

Those you quote are race engine builders, and as I have said many times, race engines are entirely different animals than road engines, yet many try to apply race engine theory and practice to road engines and often end up with a poor or cantankerous road engine.

For example, detonation on a road engine is primarily a low rev phenomenon. A race engine may detonate in this regime (but not at high revs), but it doesn't matter because a race engine is rarely operated at high load/low revs, so compression ratio can be pushed to the max.

An engine may not detonate on a dyno when induction air is at ambient temperature, but there could significant detonation in the car when underhood temperature and induction air temperature is 150 degrees F.

For a road engine one should take into account worse case operating conditions and leave a little margin, and the torque/power difference in a half point or so compression is only 1-2 percent which is the equivalent of 10-20 degrees induction temperature. The gain is not worth the hassle of having to blend in high octane fuel or constantly diddle with the timing.

GM dropped the SHP/FI CR half a point with an extra head gasket in '62-'63 to mitigate detonation, and it worked despite the increase in quench clearance. This should also work for John, and is the most expedient path since he does not want to disassemble the block to cut down the pistons.

In the case of OE engine restoration, you can manage CR or manage deck clearance, not both without decking the block or removing material from the pistons, which is difficult to calculate, so my advice is to follow my recommended maximum recommmended CRs and let deck clearance fall where it will, which is usually somewhere between .050-.070" on an undecked block with OE pistons, and so far this has proved successful with only minor initial timing variations required to eliminate detonation.

Duke

[Michael H.]

Re: L71 L72 427 Running on Pump Gas

My 68 L71 with an unopened engine from the factory ran just fine on the 91 octane gas we get here in AZ. It would occasionally diesel on at shut down.

I had the cam sprocket strip a few months back and I an now having it rebuilt as close to stock as possible using 11 to 1 pistons and a original blueprint cam. It will be done in the next week or two. It will be interesting to see the results.

John

John,

These cars experienced dieseling when they were new, even using good fuel of the era. I don't think dieseling today is any different. To eliminate that problem, leave the car in gear and allow the clutch to partially engage a bit while shutting down the engine.
I agree, stay with the original compression ratio. No need for any adjustments of any kind on the engine, including the head gaskets. Don't try to fix something that isn't broken.

[Michael H.]

Re: L71 L72 427 Running on Pump Gas

my 68 L71 runs perfect on (European) 91 octane (plus some occasional lead additive).

Erik

Yup, same here. L72 in a 66 and it the engine is built to exact factory specs, including the 11.0-1 C/R. No problem on 92-93 fuel. (no vacuum advance in operation though)

[Duke W.]

Re: L71 L72 427 Running on Pump Gas

Did you make all the measurements to compute the true as-built SCR, or are you going by the factory advertised value?

Duke