Quench vs. CR

I need to order a headgasket for the new motor. My choices are .052 (OEM) or the Mopar Performance .045. The choice does this to my quench and CR:

.052
9.17:1
0.0525

.045
9.29:1
0.0455

I want to run 87 octane. So would you go with a tighter quench to fight detonation at the expense of a higher CR, or stay with the lower CR and hope for the best on the quench?

CRASH

CRASH said:

I need to order a headgasket for the new motor. My choices are .052 (OEM) or the Mopar Performance .045. The choice does this to my quench and CR:

.052
9.17:1
0.0525

.045
9.29:1
0.0455

I want to run 87 octane. So would you go with a tighter quench to fight detonation at the expense of a higher CR, or stay with the lower CR and hope for the best on the quench?

CRASH

Click to expand...

There will no difference with these numbers.

Are they just so close that it doesn't matter?

CRASH

That's pretty much it. If one of the gaskets was to take you to over .060QH, then it would be time to worry.

There is a good discussion on qhench by a motorcycle mechanic I'd found somewhere that really makes sense of the issue - but I don't have the url handy. I found it with a Yahoo! search, so it should still be out there.

Something to bear in mind - with our OEM compression ration being below 9:1, quench just isn't that much of an issue for us - worry about it when you build a stroker.

5-90

I AM building a stroker......see this thread:

http://www.naxja.org/forum/showthread.php?t=19226

CRASH

oh...

Either gasket will work for you, but I'd probably tend toward the higher clearance.

The two CRs you give aren't far enough apart to matter, and both are low enough to run pump gas with proper quench (and you are there.) If you are worried, I'd round off the edges of the combustion bowl slightly, both to prevent heat risers and to encourage "quench" airflow.

5-90

Crash,

Victor also makes a .045 gasket without the "take it without KY" attitude of Mopar performance.

I am with the others. I do not think it will matter which of these gasket you go with.

As long as your quench is close, I think predetonation is more dependant on cam choice.
I had some predetonation without premium with my first cam (250*), but I can now run 87 to 88 oct on the new cam (260*) with only occasional light ping. I think the main difference is in the longer overlap and the resultant change in the dynamic compression. I like the new cam because of the more versatile fuel choices, but the original cam had more seat of the pants low end pull both at 600' above sea level and in the mountains. Sometimes I wish I had the old cam back.

As a point of reference: I run a 4.6L Accurate Power based stroker with 28 cc dished forged pistons ringed with Total seal rings and stainless steel stock sized valves, all fed with FoMoCo 30 lb injectors and vented via a wrapped Borla header. Fire comes from a MSD blaster coil and Taylor Spiropro wires on a stock cap and rotor set up. I went 28cc dish pistons in hope of a future forced induction set up. I also run a 0.000 to -0.005" deck height to be able to step up to a thicker head gasket if I do go a small amount of boost in the future (to decrease CR and still keep in quench).

Bones :skull1:

There's no reason a stroker can't benefit from a tight quench distance. For a stoker or an engine without a tight piston, 0.045" quench is ideal distance. Anything beyond 0.060" is ineffective as it doesn't create the shock wave in the compression chamber. From what I understand this "close collision" not only equalizes temperatures to reduce detonation but stirs the gasses to create an more efficient burn and increases power. So far I can't find a down side to running a close quench distance, a rarity in the performance world! :worship:

An excellent article on this topic:
http://www.beckracing.com/page05.htm

...and these technical articles:
http://kb-silvolite.com/article.php


All written by
John Erb
Chief Engineer,
KB Performance Pistons

Bones said:

Crash,
As long as your quench is close, I think predetonation is more dependant on cam choice.
I had some predetonation without premium with my first cam (250*), but I can now run 87 to 88 oct on the new cam (260*) with only occasional light ping. I think the main difference is in the longer overlap and the resultant change in the dynamic compression. I like the new cam because of the more versatile fuel choices, but the original cam had more seat of the pants low end pull both at 600' above sea level and in the mountains. Sometimes I wish I had the old cam back.

Click to expand...

The cylinder will start to build pressure when the intake valve closes. If you are building too much cylinder pressure you can retard the cam to prevent detonation but this is not ideal.

BTW: the stock 4.0 deck height is 0.022" does anyone out there in XJ land know of a useful head gasket with a compressed distance around 0.023"? I know of SCE copper gaskets but they require additional machine work to the block. I'm wondering about getting appropriate quench without machining the block.
...hell a custom gasket would be worth it.

kreature said:

The cylinder will start to build pressure when the intake valve closes. If you are building too much cylinder pressure you can retard the cam to prevent detonation but this is not ideal.

BTW: the stock 4.0 deck height is 0.022" does anyone out there in XJ land know of a useful head gasket with a compressed distance around 0.023"? I know of SCE copper gaskets but they require additional machine work to the block. I'm wondering about getting appropriate quench without machining the block.
...hell a custom gasket would be worth it.

Click to expand...

Getting close to a 0.040" quench without decking the block is only possible if you use custom pistons with 4.0 rods. Otherwise, you'll need to mill the block deck, go for zero deck clearance, and use the Mopar Performance 0.043" head gasket. The thinnest head gasket available is the SCE Copper 0.032" jobbie but that ain't cheap.

Dr. Dyno said:

Getting close to a 0.040" quench without decking the block is only possible if you use custom pistons with 4.0 rods. Otherwise, you'll need to mill the block deck, go for zero deck clearance, and use the Mopar Performance 0.043" head gasket. The thinnest head gasket available is the SCE Copper 0.032" jobbie but that ain't cheap.

Click to expand...

Hey Dino,

Yeah I'm still looking, not that I think your info is incorrect just that there might be other options if I look hard enough. I've heard mentions in the V-8 Mopar world of a head gasket around 0.020".

I'm not going to go the stroker route and the bottom end of the 4.0 is plenty strong so I wanted to see just how much bang for the buck was achievable without screwing with the block. If I got new pistons then I'd bore it .030" over, lots of other machine work since it's already at the shop and all new everything. So what started out as a simple port & polish, valvetrain upgrade and compression increase ends up being thousands of dollars just to get the quench right.

Oh BTW according to Clifford Performance, it's necessary to "O-ring" the block to use the SCE Pro copper gasket. Not sure exactly what that is but my guess is it's the fire ring. So I'm still looking and if I cant find anything I'll probably ask if SCE can make me a custom Pro Seal gasket (includes fire ring) @ 0.023" to 0.028" to achieve 0.045" to 0.050" quench.

In my opinion even if it's $150 it's worth it just to get a better burn even without detonation prevention. :cheers:

CRASH said:

I need to order a headgasket for the new motor. My choices are .052 (OEM) or the Mopar Performance .045. The choice does this to my quench and CR:

Click to expand...

Crash you will get higher compression with the Mopar performance gasket.

Based on a stock engine:
Static compression with 0.052 head gasket = 8.883:1
Static compression with 0.045 head gasket = 9.013:1
But neither will offer adequate quench of 0.060" max.

Also I'm waiting to hear back from SCE gaskets, they make a 0.021" Pro Seal head gasket, I'll see if they make one for the 4.0.

Static compression with 0.021 head gasket = 9.492:1 without any milling of head or block and offers a quench height of 0.043" but I have to see if they'll even make it. This might be too high to run on 87 octane but it really depends upon your dynamic compression.