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Compression Vs Quench clearance

2234 Views 41 Replies 15 Participants Last post by  johnsongrass1
Understanding that tight deck clearances (piston to head) in the 0.040" range are beneficial to controlling flame-front propagation, how much more conservative must one be when tight deck clearances are not attained?


As an example, a Gen 1 chevy small block:

Using a modern aluminum head with a fast burn chamber, tight quench clearance and flat top pistons, running north of 10:1 static is not that risky on 91 octane.

how much more conservative must one be when you can't get that tight quench clearance, and/or resort to domed pistons to get the compression up? understanding that all of these things negatively influence flame-front control.

I'm aware of how cam timing influences static timing, so if one wants to respond in terms of dynamic compression, I'm good with that too as i can easily back my way into a static.

In my mind, with the proper cam timing one should be able to run 10.2:1 on 91 octane if the deck clearances are optimized. but with a larger quench clearance around 0.060", that number should probably be quite a bit lower. my gut says 9.75:1, but it is just a gut feeling and isnt really based on anything other than a basic understanding of the priciples in action.

in the end here, i'm trying to pick a combination of pistons and head chamber volumes to build something that will run decent on 91 octane.
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Discussion Starter · #11 · (Edited)
let me try this from a different angle:


I'm not decking the block, so i'll have the 0.020 built in clearance. with the heads i've picked (68cc) my choices are flat top pistons with 6cc valve reliefs which with a 0.039 gasket gets me a SCR of 9.31 and a DCR of 7.59. lower than i would like.

alternatively, i could pick small domes which bumps the SCR to 10.34 and DCR to 8.41, which i'm not sure if that's too high.


In my mind, if the quench distance was good with flat top pistons, the 10.3:1 might be fine, but with the wider clearances I feel like it is probably a little too high.



That leaves me with the flat top pistons and milling the heads to reduce the chamber volume (or just accept it as an 87 octane build). And if I’m going to mill the heads to get the compression up, how far can I go?

or, i could possibly get the domed pistons and have some of the dome milled off in a lathe to get the compression down, but if i decide to do that - how much do i need to take off.


What I don’t think is a good idea, is to use the domes and a 0.060 thick gasket on top of the 0.020 built in clearance to get the compression under 10:1. To me that sounds like all the things that we learned not to do in the late 1970’s.

i really just need some kind of reasonable target, since i lack the real world experince of doing this a dozen times to know what doesn't work.
 

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Discussion Starter · #14 ·
I found some 65cc heads. That gets me 9.6:1 static, which is better. Still a couple tenths lower than I wanted but better.
I’d build it at 10.3, but I’m afraid it would be just on the wrong side of the line.
 

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Discussion Starter · #25 ·
Running the math with those shim gaskets and assuming a 0.025 deck clearance, the compression ratio with flat top pistons (4-valve relief) gets me 10:1. Assuming a 0.020 deck clearance bumps me up a tenth.
With the tighter quench clearance, I’m good with that.

Are the 1094’s going to reliable for a street motor? I only ask because the description references racing applications.
 

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Discussion Starter · #29 ·
I’m really liking the coated shim gasket. That gets my quench and compression ratio exactly where i was hoping to get it. I’m thinking that is going to be the plan.

so right now the build is looking like flat top pistons with a ball hone. 65cc aluminum heads with 2.02/1.60 valves and the z28 springs. Taking an earlier suggestion on the cam and found the comp cams high energy 268h cam. 268/268 - 218/[email protected] 110 LSA/106 ICL.
I’m thinking that with the higher compression and the tighter LSA this should be a fun little build.
Have to tear the block down now to see if a simple hone and re-ring with flat top pistons is going to work clearance-wise.
 
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