Running .035" Squish In 8V Cylinder Head

[glhs613]

Does anyone have any experience with running a "Tight" 0.035-0.040" squish clearance in the 8V Chrysler turbo motors? In specific, I'm curious how much additional static compression ratio might be possible with the 0.035" squish versus the standard 0.075-0.090" clearance? (Or whatever is typical) I've seen more and more articles where n/a engine builders were able to gain a few tenths of a compression point with setting a 0.035 squish. The tight squish clearance supposedly promotes better turbulence in the combustion chamber and gives some additional detonation resistance. My question is, how much additional detonation resistance in a 8V turbo motor? Would it be enough to go up 0.5 on static compression ratio?

[turbovanman²]

Subscribed.

[glhs875]

I think that is something well worth pursuing!

[5DIGITS]

..."Tight" 0.035-0.040" squish clearance in the 8V Chrysler turbo motors?

Excellent topic.
The benefits of doing this (decking the block approx. .040") can be well worth it.
Keep in mind that the engine to engine variations should have you measuring and determining how much needs to be cut, for YOUR engine.
I personally do not focus on the compression gain as much as I concentrate on squish and knock suppression.
The 2.2 has older attributes and the compression/ring-land locations are two key items that are 'Yester-tech'!!
Therefore, decking the block (not milling the head) and adding squish to a currently 'dead' chamber will breathe new life into the engine while the added squish will suppress knock.

When trying to do this, remember that the shape and depth of the piston dish can be a complimentary tool to the chamber.
In short, when the piston dish diameter and depth is well matched with a modified chamber, squish can be optimized to a relatively high level and aggressive timing without knock is the benefit.
Although, if you utilize a standard piston, the benefits are still there !!
A good point of reference - The spark advance that required a racing fuel mix should now be available with pump gas.
That can be approx. 5-7 degrees of spark advance improvement and even higher if its an optimal arrangement.
Add the race gas back into the equation with additional spark and the true benefits can now be realized.

Note: depending on how much needs to be removed from the deck on your engine, you may require a shorter cam belt.

I hope this helped.

[4 l-bodies]

Or, if you are ordering custom forged pistons you can alter the CH height bringing pistons past TDC and out of the hole. No issues with timing belt being too long this way, or cam timing being substancially retarded. However this can give you other issues like valve relief depth if running bigger camshaft lift, and increased compression ratio. I typically run all my 8v pistons about .015 out of the hole and it likes it. That puts me about .050 squish running Cometic gasket. That is about .025-.027 higher than factory.
I personally think the factory turbo piston design and being at least .010-.012 down the hole to begin with, makes for far less than optimal squish/quench. Seems like lots of area for impovement.
Todd

[4 l-bodies]

I think that is something well worth pursuing!

Some of us have been for many years (lol).

[88C/S]

Or, if you are ordering custom forged pistons you can alter the CH height bringing pistons past TDC and out of the hole. No issues with timing belt being too long this way, or cam timing being substancially retarded. However this can give you other issues like valve relief depth if running bigger camshaft lift, and increased compression ratio. I typically run all my 8v pistons about .015 out of the hole and it likes it. That puts me about .050 squish running Cometic gasket. That is about .025-.027 higher than factory.
I personally think the factory turbo piston design and being at least .010-.012 down the hole to begin with, makes for far less than optimal squish/quench. Seems like lots of area for impovement.
Todd

So that would require increasing the valve relief depth(.025 -.027) by the same amount?

[glhs613]

Todd/5DIGITS,

Thanks for the response. Per Todd's comment "factory turbo piston design and being at least .010-.012 down the hole to begin with." That is pretty much exactly what I measured on my 2.2L 8V buildup. With that said, Cometic makes custom head gaskets of various thicknesses and they show one in their catalog at 0.035." I CC'd my cyl head and piston bowl so I know where I am from that standpoint. I think I can achieve the "Tight Squish" I'm looking for with the thinner gasket but I will have to machine material out of the piston bowl or combustion chamber to get the proper C.R. I'm looking into which of those options right now but I was hoping to get some feedback on where to set my C.R. Stock is 8.2:1. With the .035" squish I'm comfortable bumping up a few tenths while still running 92 octane pump gas. I just don't know if I should venture above 8.5:1. Your thoughts?

My hard-parts list:
-Forward Motion S3 Cyl head - Early Non-CNC version with 53cc chambers
-FWD-Perf S3 Cam
-Wiseco Forged Pistons
-Garrett GT3076 DBB Turbo
-BIG Air-to-air IC
-Tunable Fuel/Ignition

Thanks,

Steve Coghill

[4 l-bodies]

Todd/5DIGITS,

Thanks for the response. Per Todd's comment "factory turbo piston design and being at least .010-.012 down the hole to begin with." That is pretty much exactly what I measured on my 2.2L 8V buildup. With that said, Cometic makes custom head gaskets of various thicknesses and they show one in their catalog at 0.035." I CC'd my cyl head and piston bowl so I know where I am from that standpoint. I think I can achieve the "Tight Squish" I'm looking for with the thinner gasket but I will have to machine material out of the piston bowl or combustion chamber to get the proper C.R. I'm looking into which of those options right now but I was hoping to get some feedback on where to set my C.R. Stock is 8.2:1. With the .035" squish I'm comfortable bumping up a few tenths while still running 92 octane pump gas. I just don't know if I should venture above 8.5:1. Your thoughts?

My hard-parts list:
-Forward Motion S3 Cyl head - Early Non-CNC version with 53cc chambers
-FWD-Perf S3 Cam
-Wiseco Forged Pistons
-Garrett GT3076 DBB Turbo
-BIG Air-to-air IC
-Tunable Fuel/Ignition

Thanks,
Steve Coghill

Steve,
If running Cometic gasket make sure RA finish is fine on deck and head. Obviously you should not assume deck is flat to begin with. It needs to be if using Cometic gasket as they do not compress much (if any). If your using swirl head, you might gain a couple more cc's by unshrouding valves further. With your ability to tune ignition and fuel, you should be able to run a bit more compression than the factory. That is of course dependent on available fuel, amount of boost, and environment your in. I've personally heard of many guys running standalone EFI running 8.5 or even a bit more on pump fuel.
Todd

[GLHS60]

This is a great topic. I have always considered .035 to be ideal squish but have been reading that in some cases highly boosted engines can actually benefit from reduced squish. As in approx. .110 clearance. Any one have any input??

Thanks
Randy

[4 l-bodies]

This is a great topic. I have always considered .035 to be ideal squish but have been reading that in some cases highly boosted engines can actually benefit from reduced squish. As in approx. .110 clearance. Any one have any input??

Thanks
Randy

Not saying I'm in this camp as I try to tighten up squish in my 8V applications, but there are people out there that think squish/quench is not particularly important in forced induction applications. I've been debating this with my Cosworth motor for a while now. The factory Mahle forged pistons have a ridicuously large 49cc dish piston. That makes for a far heavier piston than it needs to be. Also the flame propagation or travel can't be ideal with a piston design like that. Anyway, a couple guys had some Cosworth pistons made by Ross a few short years ago. They sent sample Mahle Cosworth pistons to them, and Ross determined their design would use a flat top piston. They were very light and they just ran the piston WAY down the hole. Another forum member bought a disassembled 2.0 Cosworth motor from a Bonneville salt flat racer. He used Aries pistons. They also came back designed as flat tops as well, They were designed for a forced induction application. Like the Ross, just ran it down the hole like .250! Obviously with no regard to any squish whatsoever. The Cosworth is an extreme example as their chambers are only around 29cc (compared to 50-56cc for the 8V heads). To run pistons up near TDC required a HUGE dish piston if using in a forced induction appication. All the pics I've seen of the the Hans Herman head pistons that went into #001 appear to be flattops as well. I don't think the chamber had nearly big enough cc to do this without running the piston quite far down the hole like the Cosworth. Obviously using the same "screw the squish" approach.
Todd

[5DIGITS]

... but there are people out there that think squish/quench is not particularly important in forced induction applications.

A good example of benefits and need for squish is the effort put into the SRT Turbo 2.4.
The engine was originally developed by Rousch for the Mexican Stratus turbo but it was lethargic in many respects.
A key piece to improving the output of the engine while reducing knock sensitivity was the 'ski ramp' piston tops.
16V chambers do not offer much shrouding when compared to the 2V chamber, therefore the piston top is modified to bring it out of the hole and closely mate with the chamber.
This supported a 7 degree spark advance increase across the board and 10-11 degrees in specific areas and therefore a significant gain in torque and HP, over the original piston.
Although, there is an obvious down side to adding height to the piston - weight! This added piston weight, above the wrist pin, can induce added wear on piston skirts due to the pistons tendancy to 'bobble' in the bore, between combustion events. Therefore, the added weight obviously needs to be concentric and/or well balanced with respect to the wrist pin axis. In short, in cases where decking the block will not provide all of whats needed, it's a justifiable trade-off when done properly.

[glhs875]

I have been involved with the tuning of the 2.4 engine using flat top pistons which decreased the quench. The combo was VERY sensitive to ignition timing and was unable to run much timing advance without running into detonation. Although it still made alot of power.

[Vigo]

I was moved to give this thread 5 stars.. and i dont think ive ever rated a thread before! Good info in here!

[GLHS60]

To expand the discussion even further some feel squish and quench are not exactly the same thing.
Please discuss.

Thanks
Randy

[GLHS60]

Making higher power with less advance is not a bad thing.

Thanks
Randy

I have been involved with the tuning of the 2.4 engine using flat top pistons which decreased the quench. The combo was VERY sensitive to ignition timing and was unable to run much timing advance without running into detonation. Although it still made alot of power.

[glhs875]

I don't disagree with your statement. It would of been interesting to see how the combo would have performed with more quench being the only change!

[zin]

To expand the discussion even further some feel squish and quench are not exactly the same thing.
Please discuss.

Thanks
Randy

Technically, they aren't, kind of like pre-ignition and detonation, similar, but not quite the same...

Quench is what happens to the flame front when it gets to withing a few thousands of something, like the cylinder wall, chamber, etc. In effect, the cooler (compared to the flame) piece quenches/extinguishes the flame front. This can be beneficial if the part you are quenching is the volatile "end gasses" that are prone to detonation. Not so good for emissions as those same surface areas will have a bit of unburnt hydrocarbons that will exit with the exhaust, driving up emissions, or at least adding to the burden of the catalytic converter.

Squish is what happens when you bring two movable surfaces together, in this case the piston and the chamber. You can imagine what is happening to the mixture when this occurs, they are "squished" out into the open part of the chamber, the resulting turbulence is great for combustion efficiency.

It is even better if the part you're "squishing out" is the end-gasses (the last part of the mixture to burn), as they are what is going to "detonate" due to their exposure to the highest heat for the longest time.

One of the reasons these two things get confused, is that you rarely have one without the other, though I suppose you could have squish without quench if the two surfaces don't get that close to each other.

In order to use quench as an advantage, you'd want it to be where the end-gasses will be. The idea is to use the two surfaces to "blow" the remaining mix into the oncoming flame front, while snuffing out/quenching the part of the chamber/mixture that is most prone to detonation.

Sounds complicated but it really isn't. Basically you'd like to have a chamber and piston that come close to each other, .035 is a good place to start, much wider and you loose much of the effect, tighter and you're likely to have things "talking to each other".

Aside from welding up the chamber, I don't see much that can be done to make things much better besides running the piston higher/decking the block to get the .035" clearance, but perhaps 5DIDGITS has some insight he'd care to share?...

Mike

PS I agree with you in regards to making max HP with min timing, that's what we should be shooting for, and these mods will help in that area, as well as making the engine more tolerant to fuel quality, ie more boost on lower octane.

[GLHS60]

I'm a one finger typer so I have to simplify things.

Squish - squeeze

Quench - cool

Detonation - after plug fires

Pre-ignition - independant of plug firing

Thanks
Randy

[GLHS60]

Yes, I'm with you there. While there is no doubt squish is important N/A, the question is, at what boost level and circumstances might it become a negative. It seems there might be a point when there can be too much turbulence.

Thanks
Randy

I don't disagree with your statement. It would of been interesting to see how the combo would have performed with more quench being the only change!