[Formulajones]

Originally Posted by scott70

Isn’t an ideal quench like around .040 ish? If I’m .035 in the hole and use a .045 head gasket isn’t that like .080 quench?

Yes

[Formulajones]

Originally Posted by b-man

I’ve taken a few all original 455 engines apart and all were between zero and..005” in the hole. Not saying they all were but that’s what I’ve seen.

My 421 HO was built with .045” Butler head gaskets and the pistons .007” to .008 above the deck.

Currently waiting for my 2276cc air cooled VW bus engine to be delivered, pistons are between .038” and .041” below the top of the cylinder, they don’t have a head gasket.

You should be shooting for .040” no matter what, it makes a difference in power and efficiency.

My son and I just finished his 2276. His was .020 above deck which ends up being a good thing. I can shim it where I wanted setting quench and compression, and I typically end up using a head gasket in these engines because of the compression and HP we shoot for. Gasket is just a copper ring. Shims are steel

[slowbird]

Just conversation sake, GM LS7 stock is around .060 with a compression ratio of around 11:1. Why wouldn't GM tighten it up more?

[65 Lamnas]

I don't know a lot about quench/squish other than what I've read.

My understanding is:

A relatively small (.032-.040") quench distance causes a turbulent "mixture motion" which in turn has a cooling effect to the combustion chamber. This deters/eliminates "hot spots" in the chamber and results in a more constant temperature. I think that's where the octane-tolerance comes from....no pre-ignition from said "hot spots". The turbulent motion also causes the fuel mixture to be more homogenous = more efficient burn.

Someone mentioned factory head gasket crushed thickness. Here's a photo of the old HO-Racing parts and specs book related to the gaskets.

I don't know if Royal Pontiac used the steel shim head gaskets as part of their Bobcat package to simply raise compression or if they actually knew about the benefits of a tight quench distance, but the gasket caused a positive effect for both.

I've taken apart MORE THAN 1 original '69 RA-IV engine that had the shim gaskets. Those engines appeared to have never been apart.

Don't forget how different fuel was back in the day. Benzene, Toluene, TEL were all a part of the old cocktail called fuel.

[Jay S]

What I have noticed is when the compression get up around 11:1 the less difference quench makes. It is probably dependent on the engine platform, but most times if compression is 11 or higher for a pump gas engine it is better choosing a thicker head gasket and loosing quench than shooting for the tight quench and ending up with the compression at 12:1.

Part of it is the compression ratio changes faster with small changes in combustion chamber sizes as compression increases. But the higher compression engine is also more efficient than a lower compression engine, and may not need quench as much. I think the quench is more important on lower compressions 10 and below than 10 and up.

With Aluminum rods the quench can change .050” or more cold versus hot. Blurr’s the lines defining quench.

["QUICK-SILVER"]

Originally Posted by slowbird

Just conversation sake, GM LS7 stock is around .060 with a compression ratio of around 11:1. Why wouldn't GM tighten it up more?

Think you need to look at compression figured from stroke at intake valve close on these engines.

Clay

[slowbird]

Originally Posted by "QUICK-SILVER"

Think you need to look at compression figured from stroke at intake valve close on these engines.

Clay

Dynamic compression I assume you're talking about?

[Jay S]

Originally Posted by slowbird

Dynamic compression I assume you're talking about?

Modern engines have VVT, with VVT Dynamic compression is a range for the load, RPM and what ever sensor data is used to change it. Knock sensors..ect…

[b-man]

Originally Posted by slowbird

Just conversation sake, GM LS7 stock is around .060 with a compression ratio of around 11:1. Why wouldn't GM tighten it up more?

My guess would be because of production tolerances.

They’re not custom built engines, they’re production line engines that have less control over tolerances in some areas than you’d have with your street or race engine builds.

The greater deck clearance than what many would consider optimum in the LS engine (and our Pontiac V8s) is the way it is for safety. It keeps things from running on the edge of being assembly friendly on an engine production line.

[tom s]

After being out of the hobby for a lot of years,when getting back in I hired a shop in Orange County CA to build my SD engine.I used a 428 4 bolt block with a cast 4in stroke crank that had been knife edged and hardened.Told them I wanted 9.5 CR with a set of 63 980 SD heads.Used a solid roller cam and ran it on a local engine dyno and made like 550HP.Drove it and raced it a little and after a couple years decided to freshen it up.It ran fine on the street with pump gas with no issues.After getting it apart we found out the shop had cut the TRW pistons and were 100 in the hole to get the 9.5 CR.It never pinged,rattled or anything unusual!Ended up getting a set of custom forged Ross dished pistons for it.Maybe when the quench get way to far like a 409 or hemi its not that important?Since that day all my builds have been what we think is proper!That engine is still running fine in my 63 Lemans after what must be about 25 years.FWIW,Tom

[steve25]

Crap!
.100” of the top does not leave much meat.

I guess if nothing else they where light!

[Half-Inch Stud]

The Stock 428 slugs have plenty of top for cutting to deck, like for 6.7 or 6.8" rods. Dish assures the middle is thick enough after the perimeter cut.

400 slug probably also, but no dish.

A mini-mill for pistons would be a nice capability.

[steve25]

Paul, physically to look at I have never had them both in my hands at the same time.

[lust4speed]

The stock engines I've torn down have all been .005" to .008" in the hole, and I haven't come across deeper ones. Maybe just luck of the draw.

Anyone want to tell the Mopar hemi guys that quench is necessary for horsepower?

The couple tech papers that I've read indicate that there is a no-man's zone between .050" and .080" that is very prone to detonation, and staying below or above the zone is necessary. This would explain why hemispherical heads are fine. The math also says that with a standard .039" thick head gasket then the piston should be no more than .011" down in the hole before additional risk of detonation occurs. Something gets nasty with flame travel in that zone that doesn't happen with less or more clearance.

The race shop I used told me that they have switched to fully dished pistons for short track engines because their testing found additional power over the currently favored D-shaped top. My guess would be that lazy low RPM engines could use the "puff" of mixture being swirled while high RPM max effort engines there is enough mixture turbulence that any additional swirl doesn't help.

Even with fully dished pistons there is about a 3/8" flat around the circumference of the piston, and when you come down to it there really isn't much more being offered by the D-shaped dished pistons.

[Jay S]

Usually a TRW/Speed Pro replacement OEM piston is .005” shorter than the original OEM cast piston it replaces, cast are usually .015” shorter. Some casts are .020” shorter. I am not sure if .005” is true on the factory forged pistons. Usually can work backwards from factory deck heights and get a decent range estimate for piston to deck height. That seems to apply for all brands. My OEM 71 455 had a piston to deck height of .003”-.004”, compression height measured 1.502” instead of 1.497” TRW/ Speed pros have. Typically a 400 OEM cast pistons I checked are 1.719” instead of 1.714” on a TRW/Speed pro forged. I don’t check them all, but it generally seems to line up with what others are reporting.

I have a 400 that the deck height is .013” taller on one bank versus the other. The taller bank was what you would expect, the shorter side is the anomaly. OEM mfg tolerances are not always as tight as you would expect them to be.

[performerrpm]

not all stock parts but so far for me..

467, eagle h beams, eagle forged 4 250 crank and mahle pistons....020 below.

455, stock crank, eagle h beam and ross 8cc flat .020 in the hole.

400, stock crank, eagle h beams, ross 8cc flat tops .020 on one bank and .015 on the other ?.

the 400 got zero decked, the 455 will get .027 cometics...but ive been reading tons...wondered about running it with edelbrocks .038 gaskets n calling it a day.

the 467 along with the other 400/434 which i expect the same .015/.020 below deck are both gonna be blown which opend another ball of worms on deck height. apparentmy u do want to be below deck with boost to control the heat on the piston tops with boost but not NA.


guess ill be following this one now

[69gtocv]

[QUOTE=lust4speed;6595621]The stock engines I've torn down have all been .005" to .008" in the hole, and I haven't come across deeper ones. Maybe just luck of the draw.

I just tore down my 72 400 that had never been apart. I got .007 in the hole for the two cylinders I checked. I was surprised they were that close to the top

[Schurkey]

Originally Posted by slowbird

everyone says you need a tight quench for good octane tolerance and power. But many engines from the factory came with somewhat large quench distance.

Or open combustion chambers with no quench pad at all.

Some of this is PURE CORPORATE POLITICS, along with management greed. The engineers intend for a certain quench distance, plus-or-minus some reasonable mass-production tolerance. This means they expect a certain level of uniformity from the production departments.

Then you've got the guy in charge of the machine tooling that takes a raw casting and turns it into an engine block. He gets a bonus for production speed. He gets a bonus for "holding down costs" on the machine tooling. And he tells Engineering that his machines, people, and processes require production variations of--pick a number--let's say .060 deck-height variation from one block to the next (but he secretly knows he can hold 1/3 that distance.) NOW he's going to make sure that EVERY BLOCK comes out as tall as possible--minimizing quench, but requiring less time to remove less metal, and faster speed from both removing less metal AND with less metal removed, the cutters last longer so they don't have to be changed as often.

This guy gets weekly/monthly/quarterly/year-end bonuses because he's "saving the Company money" when the real effect is that he's sabotaging compression ratio, sabotaging quench distance, sabotaging the actual horsepower produced, and the (computer-controlled) engines are running with retarded timing because they're running in detonation and the knock-sensor is going crazy.

The story I was told was that this actually happened at Chevrolet. Engines were not making the rated horsepower, which becomes a "deceptive advertising" issue. Engineers took a few engines picked at random...and measured them. The blocks were all TOO TALL, and wonderfully uniform. The Production Manager had to admit that he could hold much-closer tolerances than he was previously letting-on, and Engineers re-wrote the blueprint specs to force him to cut the blocks to what they were supposed to be all along.

Originally Posted by slowbird

So is it really as important as people think? If there are 2 engines both 11:1, one has .040 quench and the other has .075, will there really be a difference in octane tolerance and power?

"Everything depends on everything else."

How big are the quench pads on the head? How does the piston top mate with the quench pad? Is the spark plug biased toward the hot exhaust valve so the hottest gasses get burned before they can detonate? Or is the spark plug biased towards the intake valve, so the flame front has to cross more of the chamber before it can burn the already-hot mixture near the exhaust valve?

My understanding is that the WORST situation is piston-to-quench-pad distance from about .050 to about .100--.120. Tighter than .050 gives reasonable turbulence and a quick burn with a low volume of end-gasses to self-ignite. Greater than .100--.120 is essentially an open chamber, the flame front moves freely through the gap. There may be little turbulence, a slow, lazy burn that needs heaps of ignition advance...but it doesn't detonate because the flame front cleans-out the end-gasses.

But from about .050 to .100--.120-ish, you have this danger zone where the flame front can't reach the end-gasses, there's a relatively large amount of end-gasses, and the turbulence is weak and the burn is slow--lots of time for the end-gasses to absorb even more heat--so that's where the big detonation problem is.

[Dragncar]

I seen this Ed Smith interview where he was having the pistons come WAY out of the deck but the heads were machined back, sort of like a extension of the bore right into the head itself.
They did it for head gasket reasons. Said it helped. Kinda strange though.

[slowbird]

For everyone saying .050 to .100 is the danger zone, why would GM set up their LS engines in that danger zone? Wouldn't it be wiser of them to stay away from it if it really is an issue?