Personally I have never given it much credence. It was hard enough just to find one set of pistons to keep me under 11:1cr, then small base circle cam etc etc. In the end it would have cost me few grand more and even worse a big headache. So I ended up with about .050" or possibly a even a bit more in my 427ci.

I have done one dyno pull with no tuning and I had these stupid Car Chemistry baffles in there to quiet it down which didn't help too much and now I have taken them out and I have bigger headers to put on and I figure I should be near 500RWHP next time. That doesn't sound like big numbers but in a C3 but it is, considering we lose over 20% in drivetrain and other losses when the motor is bolted in to our beloved C3.

I have read some of the theory involved etc. but I like to see things in person or at least a reliable source. So the question is how much do you figure you would gain with the same motor, dynoed with .040" quench and say .060" quench like 10 million of them had from the factory.

Now the first question can be argued to the end of time but is there any documentation anywhere that shows the gains, if any, on a real test on a real motor with 2 different quenches similar to what I stated above. I would love to see that

 

I wish that i took a picture of piston tops that I have seen in the last 4-5 years in motors with minimal quenches. Like .030 - .042

The tops of the pistons have bare like new clean aluminum on 25-30%

The reason is turbulance caused by the squish keeping the fue so atomized that you don't get droplet formation and the wet greasy looking carbon buildup on the piston top. So you can run a motor leaner.

tighter quench is more detonation resistant and fuel efficient.

 

What is quench? Are you trying to cool something?

 

So what would be the power difference between an engine running a .040 quench and the same engine running a .050 quench?
I got mine with in .050ish.

 

In the motorcycle world we call it squish, same thing different moniker.

I've done a lot of experimentation with piston to head clearance on all types of engines, car, Harley, Japanese bikes, snowmobile, and gas and nitromethane. I have always found that the tighter the squish, the stronger the engine. They sound crisper, get better mileage, have less detonation problems, and make more power. Like George said, they have less carbon too.

Even the race motors ran better with the squish as tight as I could get it without slamming the head. I used to keep tightening the squish until the piston and head were just showing signs of touching after being run hard. I would the back off about .002". This was even in the nitromethane burners.

Smokey Yunick used to said to run them where the piston and head are just starting to get acquainted. I run my 406 at .034". I ran a snowmobile engine at .022". The snow race gurus said you couldn't run a 2 stroke that close. It would detonate and blow wholes in the pistons. I proved them wrong and with a sled I was trail riding on pump gas I was making within 5 horse of their all out race motors and I had a mild build. They all ran .070 to .090" squish. You should have seen their faces when we dynoed mine!

 

so if i am reading some of this correctly, wich i am prob not. a larger piston-bore size with reasonable compresion with .02 clearance will produce more horse power than say multiple pistons with same displacement value and .04 quench?

 

http://forum.grumpysperformance.com/...35c85ed29b0052

 

I say this subject get much more attention than it produces. Yea you run it as tight as you can in a race motor for max compression, after that there's not much if any. I run .090 on my race motor but I need that much cause the pistons hit the heads with anything less. Another thought is to look at the 426 Hemi, there is no quench there! But some will say chevys are faster! Not in a maxumum effort there not, each to their own cause there is always someone faster even if you don't believe thats possible.

 

Hemi's are faster in blown nitro applications. You can't get a wedge head motor down low enough in compression for blown fuel. The only other advantage a Hemi has is the valve layout gives a straighter shot at the intake valve, so they flow air well. In gas motors they need more timing, richer mixtures, and have more problems with detonation!

I started running nitromethane with hemispherical combustion chambers. We all had a lot of trouble with detonation, meaning myself and other racers. They would flatten rod rollers and break ring lands all the time. I'm talking NA Top Fuel Harleys. It wasn't until I welded up my chambers and bathtubbed them that I got control of detonation and started to rewrite the record books, and I didn't have to lean on the motor as hard. The bathtub had a 30 degree squish with a matching dome on the piston. I ran them at .035" squish.

I still build Top Fuel heads this way, although they are my own design built from billet, not welded up stock like the old days. Ask George (gkull), he has seen them. The motors now need more piston to head clearance because they have aluminum rods and heavy pistons, but you still set them up so they are close to hitting, somewhere around .055" is where I set them.

 

Well as far as cars are concerned the Hemi does run and bet the best, look at the times in the stock classes, hell for that manner ask A. Johnson and tell him how the Chevy's are faster. OK he runs a Pro Stock car with the Hemi and runs real tough. Now I know I sound like I like Hemi's but I don't, I run a Blown Big Block Chevy on alcohol in my dragster, the car that on my avatar.

 

As I said, the Hemi has an advantage on the flow through the heads, the chamber does not help it though. If they could keep the near central spark plug, the port configuration, and have a better chamber, they would run even better. But, at very high RPM's the squish becomes less important than at lower RPM's. In street motors it is very important, as it is in low rpm Harley motors on nitro, not that means anything here. The OP was talking street motors I'm sure.

 

Now that I think about it, the current Hemi run in Pro Stock has bathtub style chambers, as do the heads on Chrysler's OEM offerings. It is not the old 426 Hemi based motor.

 

IMHO, the new one ain't a hemi at all.
Their chamber is a bathtub with two quench pads, not really hemispherical at all.

All the hemis I've seen built and run are great with forced induction and/or 15:1 compression or so. Back it down to N/A and 9:1 and it's a completely different animal, a similarly built quench engine will own it. That's part of why they disappeared when Uncle Sam decided those old 60's muscle cars were dirty and introduced the smogger motor.
I'm sure Jim will chime in here. He's a fan of running 'em tight, says it keeps the rods from stretching

 

To the original question...you won't really be able to tell the difference on the dyno or in the car. i also like them as tight as I can get them. I've been known to have some piston/head rubbing going on too! It's just one of those *little things* that adds up in the overall combo.

My 555" has .009" of piston OUT of the hole. My old gaskets when it was a 540 were .037" for a .028" quench clearance. Today I have a .041" gasket for a total of .032".

Here's a pic of the 540...you can see the clean areas on the piston and also where pistons contact the head just a hair.



As mentioned, Hemi's have zilch for quench..as do many other Mopars. Dick Landy some pistons at one time with quench pads for Hemi's and swore they added 20 HP in a serious application.

If the chamber and head design were perfect it wouldn't be an issue, but the typical wedge design has a lot of surface area that can be cone stagnant without some mixture motion.

JIM

 

Jim's pistons are what I was referring to about being 25-30% clean.

Motorhead, on your last dyno run day did the dyno sheet also have a column giving "BSFC" ? When we do dyno tuning runs we adjust the A/F ratio to achive .48 or less BSFC levels. Generally a 4 cycle above 2000cc motor can only attain 30 - 37% efficientcy. .45 BSFC is equal to 30% efficient. .37 BSFC can only be achived in modern computer controlled fuel injection with lean burn technology and it is equal to about 36% efficient.

I was trying to find an article on squish or quench for chevy type wedge heads. The chamber design has allot to do with it, but anywhere from .030 - .050 is good .050 - .060 is gray and .060 and over is a negative. Very short skirt pistons like your 4 inch stroker small block might not be able to run a small squish number because of piston rock in the bore. forged pistons have to have more cylinder wall clearance

Modern aftermarket forged pistons and heads are designed to have a bigger squish area.

My advice is: When you pull off the head and the pistons looks like Jims. it is a sure sign that you motor is running good regardless of the actual squish number

 

As 427hotrod says above you won't see the difference on the dyno which what I said it's not worth all this hype. Something to think about also look at a closed chamber big block and then an open chamber one! Almost no quench in comparison to the closed chamber head, and they run the same ports and valve sizes for the most part in stock GM heads anyway. Which one do you think runs better?

 

Neither Both of them are gas sucking pigs. I would bet that a 427 small block making 600 hp can get higher MPG than either big block. They were designed in an era where mpg was not of any concern. Modern chamber designs are what you need

 

One thing of note is that when GM built this stuff they used .020-.025 or so deck height and steel shim head gaskets that were only .018"-.021" or so. So that means total quench was only around .038" to .046". They weren't using .038"-.041" composition gaskets like most folks use during rebuilds.

Maybe The General DID know a little something??

JIM

 

Minor aside. Is there a way to predict piston rock? The idea of reducing the quench until the piston hits the head makes me uncomfortable.

 

This is a subject for sure that will never have people come together on, so be it. Some have good reasons behind an opinion others just talk without any good knowledge, some of that in this thread, I don't mean you either. Your right maybe GM does know something or maybe just cheap in parts used.