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Quench and Piston Rock

Old 02-01-2021, 12:21 PM
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Default Quench and Piston Rock

Guys... I had this posted in my engine build post in the Formula subsection. I'm hoping the amazing collective here can help me get pointed in the right direction. This is for choosing the right head gasket to get quench correct on one of my engine builds. I'm close to being done with this one... just need the head gaskets.

The piston rock is making things difficult to get nailed down. We followed JE’s recommendations for piston and ring clearances, but this introduces a good amount of rock at TDC. Tightest is right around .004 with the deepest about .018. Averaged out, I come up with .0105 (let’s call it .011). Those measurements are combined averages including rock at the intake sides and exhaust sides. If I measure at the wrist pin flats, I come in at .012 as an average. I know the piston will expand thus eliminating some rock and that rock really is greatest on the power stroke anyway. The head-scratcher for me... should I focus on the average numbers or should I focus on the absolute minimum measurement? I’ve probably answered my own question based on how I explained (expansion, rock on power stroke, measurement at the writ pin), but I’d like to know how others are going about it. I want quench to be good, but I also don’t want pistons crashing into the head at 5600.

I’ve held off on ordering head gaskets because of this. The .027 Cometic with the wrist pin measurement goes right at .039 and .038 at the averages. Taking minimum clearances, it ends up at .031-.032. That’s too close and would make the .039 Fel-Pro a safer choice. But the averages put the clearance over .051 (minimizing quench benefits).

Hoping you guys can give me some direction here. TIA!

Last edited by TomZ; 02-01-2021 at 03:43 PM.
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Old 02-02-2021, 06:24 AM
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What are the rest of the specs of your builds? Cam, target compression, heads, what fuel are you planning to use?
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Old 02-02-2021, 07:04 AM
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I have a big block with a 4.500" bore with a deck measuring about .005 . deck ht. I did use the .027 thick gasket. Then I got scared after it was together so I got some .062 solder and checked my clearance. It wound up to be about .040. All good so far..
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Old 02-02-2021, 08:23 AM
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Originally Posted by fbc25el View Post
I have a big block with a 4.500" bore with a deck measuring about .005 . deck ht. I did use the .027 thick gasket. Then I got scared after it was together so I got some .062 solder and checked my clearance. It wound up to be about .040. All good so far..
I'm probably making a mole hill out of an ant hill. I have the solder here. It wouldn't be anything to bolt the heads on to see what it looks like. I have a set of .039 Fel-Pro gaskets hanging on the wall and can use those as a control and go from there. Thanks for the idea!
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Old 02-02-2021, 08:32 AM
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Originally Posted by Unlimited jd View Post
What are the rest of the specs of your builds? Cam, target compression, heads, what fuel are you planning to use?
509
Eagle 6.135 rods
SRP mini dome - 1cc dome
AFR 325 cut down to 112cc
Cam is a custom roller, 235/241 .646/.646 113
Compression target is 9.5 (if using a .027 gasket, just under 9.6)

Fuel will be whatever it needs, but would prefer using the lowest octane that I can get away with. It will be run on the dyno in marine trim.
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Old 02-02-2021, 10:10 AM
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You looking to peak hp around 5,500. Looks like a nice build.
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Old 02-02-2021, 10:59 AM
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Originally Posted by getrdunn View Post
You looking to peak hp around 5,500. Looks like a nice build.
Thanks! Im hoping theyll run nice. The cams are the ones I bought from Rookie. 8.1/LS firing order so should be fairly decent with good dock manners but still rip. Itll be closed cooled too.
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Old 02-02-2021, 12:01 PM
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I knew I saw those specs as of recent. Should run strong and reliable.
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Old 02-03-2021, 02:42 PM
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2 things,
1 a buddy of mine recently was checking quench height and found a significant difference between one side of the piston and the other. Ended up finding the brand new scat rods had a twist in them.
2) Im not seeing a huge benefit in your build to aim for such tight quench. Id run the .039 and call it safe.
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Old 02-03-2021, 03:27 PM
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So clearance of .051 or so is still workable? No concerns about increased detonation risk to worry about? That's been my greatest concern vs squeezing out any extra power.
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