[Matt Meaney]

from a mathematical stand point, a measured distance is a measured distance.

for a basic street/strip, iron or aluminum head, N/A engine, (no power adders)

Is there any advantage to having the piston at zero deck (to .005 in the hole) to achieve the desired quench distance?

or,

Is there any disadvantage to having the piston .015, .020, .025 below the deck and using a thin head gasket to get the same measurement?

[moontower69]

Yes, BIG disadvantage to being in the hole 25 or 30 thousandths is detonation, especially if you're on pump gas pushing 9.25 or 9.5:1 and higher on 91 octane (all we get in some states like here in CA). Stock motors were in the hole about 20 to 25 but gas was much better then. I'd have machine shop shoot for zero deck or .005, that will make the motor run much better and avoid detonation, ASSUMING the rest of the combo is balanced.

Cam must match compression and heads (look up posts from Cliff on this, extensive info on this forum). I know from experience that one of the worst setups is to put a smallish cam with high compression and have pistons way down in the whole. The stock cast pistons or those cast 8 valve relief pistons are in the hole 25-30...that might be okay for a pure stock build but not a mild cam set up.

I'm not an expert so others will chime in of course (just my experience over last 15 years with about 4 or 5 engine builds).

[b-man]

Originally Posted by Matt Meaney

from a mathematical stand point, a measured distance is a measured distance.

for a basic street/strip, iron or aluminum head, N/A engine, (no power adders)

Is there any advantage to having the piston at zero deck (to .005 in the hole) to achieve the desired quench distance?

or,

Is there any disadvantage to having the piston .015, .020, .025 below the deck and using a thin head gasket to get the same measurement?

No expert here, but I think as long as you strive for a quench distance somewhere between .035" and .060" on a N/A street/strip engine it really doesn't matter how you get there.

My latest street engine has the slugs .007" above the deck with a .045" thick gasket.

If you're pretty much stuck with pistons sitting .020" in the hole there's nothing wrong with running a .027" thick Cometic gasket other than the price of the gaskets.

[steve25]

The tighter the quench the better for Octane tollerence!
Even if you not after every last scrap of power it pays to deck the block , or get your Pistons made if doing so to give zero to +.005" deck, then the standard .039" crush of a head gasket will serve well to keep the motor out of detonation even if your on the high side of the cylinder pressure to octane game!

[scott70]

Originally Posted by Matt Meaney

from a mathematical stand point, a measured distance is a measured distance.

for a basic street/strip, iron or aluminum head, N/A engine, (no power adders)

Is there any advantage to having the piston at zero deck (to .005 in the hole) to achieve the desired quench distance?

or,

Is there any disadvantage to having the piston .015, .020, .025 below the deck and using a thin head gasket to get the same measurement?

Im kind of curious about that question too,lets see if someone chimes in with an answer and a explanation of the answer to your specific question.

[scott70]

Originally Posted by steve25

The tighter the quench the better for Octane tollerence!
Even if you not after every last scrap of power it pays to deck the block , or get your Pistons made if doing so to give zero to +.005" deck, then the standard .039" crush of a head gasket will serve well to keep the motor out of detonation even if your on the high side of the cylinder pressure to octane game!

Sounds like he might have the engine together already and a simple head gasket change is easier than machine work as long as its not detrimental to power making or detonation,etc.

[track73]

Yes, what about using a .027 Cometic gasket to get better quench.?

[steve25]

Anything you can do while you are in process of rebuilding or changing head gaskets to get the tightest quench that will work with your Rods, pistons and needed valve to piston clearance is the way to go, even if it means grinding the chamber out some to make up for the reduction in CC's!

[Blued and Painted]

In a perfect world, The 40 year old abused decks need squaring and re-surfacing any way.
The price difference between felpro and cometic's will just about cover the cost of machine work if the engine is already disassembled. If its a re-due, the thin gaskets would be cost effective if .012" will get you in the sweet spot.

[track73]

Originally Posted by Blued and Painted

In a perfect world, The 40 year old abused decks need squaring and re-surfacing any way.
The price difference between felpro and cometic's will just about cover the cost of machine work if the engine is already disassembled. If its a re-due, the thin gaskets would be cost effective if .012" will get you in the sweet spot.

I realize that but this engine doesn't have many mile after a rebuild (rings, bearings and a valve job) maybe 2000. I don't want to take the whole thing apart just to put on some no. 62 heads and a bullet RA4 copy. The stock pistons are the champhered one so the compression will be around 9.5.

[Tom Vaught]

Stock street engine will have .015" to .023" down in the hole and a gasket around .042" crushed thickness so the total quench distance is about .065" from the piston deck to the head surface: closed chamber or open chamber.

So if you reduce that distance to .005" now you are talking about a .047" quench distance with more of the "non cooled" block surface deck removed. The heads will have the same cooling be it with a .010" head gasket or a .080" thick head gasket.

Head Gaskets don't do any cooling so a thinner head gasket or a multi-layer head gasket doesn't absorb/retain the heat like a copper head gasket would. The head gasket has to be able to seal the two surfaces (and still pass water between the two parts) unless you have a dry deck engine.

So dropping the piston down a bit in the hole transfers more head to the solid part of the block and makes that area even hotter. At some point if you drop the piston down in the hole far enough you made that volume part of the combustion chamber.

There is a dimension in that distance where the engine will be very unhappy and want to have combustion/ignition issues.

Better to be 1/2 point too low in compression ratio and have the engine be happy vs 1/4 of a point too high and have combustion issues. Fuel quality today sucks on average but the fuel octane is still higher vs World War II specifications. Course they were running around with 5 to 7.5 to 1 compression ratios too.

So back to the #1 (OP's ) question: thicker head gasket and less piston to deck clearance is better vs opening up the piston to deck clearance .010" to .040" and using a thin head gasket.

Tom V.

[scott70]

Well I kind of asked the same question a while ago if you want to look it up. After assembly I asked how far down in the hole were they. I got the spec sheets on the rebuild and noticed they were in the hole around .035 average,,,not happy.At that point,,I threw on some .027 cometics and ended up with 9.3 to 1 comp. The car runs very strong and i use 93 oct and some av fuel for the lead. I was told by Cliff I wouldnt see huge gains but if I got some pistons with a lower wrist pin location to get closer to zero deck, I could see significant hp/trq increase and end up with around 9.8 to 1. But keeping everything the same and just moving most of the quench up into the gasket instead of having it in the hole,,,what would that do to power or detonation problems?? Good question.

[GRX]

x2 on squaring and re-surfacing 40+ year old abused blocks. Not all were perfectly square from the factory to begin with.

Along those lines ... be sure your machine shop aligns their fixture off of the main saddles, not the oil pan deck.

[Tom Vaught]

SCOTT70,
1 POINT IN COMPRESSION IS WORTH 4% INCREASE IN TORQUE/HORSEPOWER.

So going from 9.3 to 9.8, is a 1/2 point increase in compression ratio or 2% more torque and Horsepower. You have a UD 255/263 camshaft (a great camshaft) so lets say the engine makes 500 hp (numbers are easy to play with). 500 times 2% = 10 more Horsepower at the same rpm points.

So you played around with pistons, head gaskets, etc and got 2% more.

The boost guys have it a lot better.

Say you drop the compression ratio from 9.3 to 8.75 compression ratio and add 8 psi of boost. You lost 2% on your 500 number so you are now at 490 hp but your are adding 50% more air mass to the engine. New HP is 735 HP and a very durable combination when off boost as far as getting gas anywhere.

Sometimes messing with a 2% change is really not worth much at the end of the day.
It is a change and the engine will like it but not like other changes.

Tom V.

[scott70]

Originally Posted by Tom Vaught

SCOTT70,
1 POINT IN COMPRESSION IS WORTH 4% INCREASE IN TORQUE/HORSEPOWER.

So going from 9.3 to 9.8, is a 1/2 point increase in compression ratio or 2% more torque and Horsepower. You have a UD 255/263 camshaft (a great camshaft) so lets say the engine makes 500 hp (numbers are easy to play with). 500 times 2% = 10 more Horsepower at the same rpm points.

So you played around with pistons, head gaskets, etc and got 2% more.

The boost guys have it a lot better.

Say you drop the compression ratio from 9.3 to 8.75 compression ratio and add 8 psi of boost. You lost 2% on your 500 number so you are now at 490 hp but your are adding 50% more air mass to the engine. New HP is 735 HP and a very durable combination when off boost as far as getting gas anywhere.

Sometimes messing with a 2% change is really not worth much at the end of the day.
It is a change and the engine will like it but not like other changes.

Tom V.

Well it wasnt the compression gain that was main question when i threw it out there. It was how much power am I giving up having the pistons that far in the hole and would it be worth it to get different pistons to get close to the 0 deck. Kind of the same question the OP put up here..035 in the hole and a .027 head gasket is .062 quench. You always hear go 0 deck with the head gasket you want for best power. The compression boost would just be a bonus. Pistons that far in the hole supposed not to be optimal.In other words the biggest gain would be from getting the piston out of the hole.The OP first question nails it on the head.His question ,,with the quench #'s staying the same,,does it matter power wise,,to have it in the hole with a thin head gasket or 0 deck with a thicker gasket.Also Im using the crower 60919 cam in my GTO.

[Matt Meaney]

Originally Posted by Tom Vaught

Stock street engine will have .015" to .023" down in the hole and a gasket around .042" crushed thickness so the total quench distance is about .065" from the piston deck to the head surface: closed chamber or open chamber.

So if you reduce that distance to .005" now you are talking about a .047" quench distance with more of the "non cooled" block surface deck removed. The heads will have the same cooling be it with a .010" head gasket or a .080" thick head gasket.

Head Gaskets don't do any cooling so a thinner head gasket or a multi-layer head gasket doesn't absorb/retain the heat like a copper head gasket would. The head gasket has to be able to seal the two surfaces (and still pass water between the two parts) unless you have a dry deck engine.

So dropping the piston down a bit in the hole transfers more head to the solid part of the block and makes that area even hotter. At some point if you drop the piston down in the hole far enough you made that volume part of the combustion chamber.

There is a dimension in that distance where the engine will be very unhappy and want to have combustion/ignition issues.

Better to be 1/2 point too low in compression ratio and have the engine be happy vs 1/4 of a point too high and have combustion issues. Fuel quality today sucks on average but the fuel octane is still higher vs World War II specifications. Course they were running around with 5 to 7.5 to 1 compression ratios too.

So back to the #1 (OP's ) question: thicker head gasket and less piston to deck clearance is better vs opening up the piston to deck clearance .010" to .040" and using a thin head gasket.

Tom V.

thank you, for your very informative post. it gave me a better perspective of just what's going on in there.

thanks to all who have contributed. the knowledge and helpfulness of the people on this site is great.

[Pete67FB400]

Originally Posted by Tom Vaught

. . .

So dropping the piston down a bit in the hole transfers more head to the solid part of the block and makes that area even hotter. At some point if you drop the piston down in the hole far enough you made that volume part of the combustion chamber.

There is a dimension in that distance where the engine will be very unhappy and want to have combustion/ignition issues.

. . .

Tom V.

Good topic with a lot of good information. I'd like to explore how using chamfered/beveled pistons affects quench.

Per Tom's post above, having the piston in the hole transfers more heat to the uncooled upper part of the block. But with a beveled piston, there is already, what - 0.25" or so?, of the upper cylinder wall exposed. Would adding another 0.020" from a piston down in the hole make much of a difference?

A few posters mentioned a tighter quench being better. With beveled pistons, you are opening up some of that quench area around the outer edge of the pistion. The bevel adds an outer ring of combustion chamber volume with the quench area between that outer ring and the main volume in the head. How does that affect the remaining quench area, if at all? Does having a tight quench distance become any more or less critical?

[Tom Vaught]

The whole idea of a quench head with a tighter (smaller clearance) is to move combustible fuel and air out of the quench area and force it towards the other parts of the combustion chamber creating a more complete mixture mixing and more power without detonation.

You can read more about that here: http://www.theoldone.com/archive/quench-area.htm

So basically when you add a bevel on the edge of the piston you create a dead spot where the mixture is not forced out of the quench area and does not create complete mixture mixing and move more towards a engine that wants to detonate.

So if you deck the engine but run a beveled piston you basically still have a potential issue with detonation because you are effectively moving the piston closer to the head but also trapping gases (mixture) that should be forced to mix in the other parts of the mixture in the combustion chamber.

Tom V.

A boosted engine has a larger cup in the center of the piston and a flat ring around the area above the ring lands to create a quench area.

[Pete67FB400]

Interesting reading at the T.O.O site.

I assume most people that use a beveled piston do it to lower CR in order to reduce the chance of detonation. Sounds like a better approach is to use a dished piston instead so that the quench area is maintained.

[gtofreek]

Exactly! You need the quench pad area to be flat for any benefit to be had. It's just like some of the factory built engines that have the piston down in the hole .100"-.150", to lower the compression, but then at 8:1, they still detonate under a heavy load. Much better off to have 9:1 with a near zero deck, as it will be much less likely to detonate.