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Discussion Starter · #1 ·
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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The internet is good at making it a much bigger deal than it is. If it's a side benefit than good, go for .040 or whatever......I wouldn't build around the arbitrary .040 idea.....
Stick to the 10:1 rule and it'll be fine.
 

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The internet is good at making it a much bigger deal than it is. If it's a side benefit than good, go for .040 or whatever......I wouldn't build around the arbitrary .040 idea.....
Stick to the 10:1 rule and it'll be fine.
Much controversary about the dynamic compression issue. As many people that believe it there are many who don't. Judge for yourself. Good luck.
 

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Only a controversy by those confused on it.
Those guys are talking about IVC, intake valve closing point, there isn't anything dynamic about it. Everything else is fluff trying to make it make sense.
 

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Only a controversy by those confused on it.
Those guys are talking about IVC, intake valve closing point, there isn't anything dynamic about it. Everything else is fluff trying to make it make sense. IVC IS PART of the dcr calculation. I've seen comments out here from some pretty savvy people that said they have built engines using it and were not happy with the outcome. I don't see where the confusion is. You input all of your engine info incl Ivc quench head gasket thickness etc and you get a Dynamic compression number. It either works or it doesn't.
 

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I have a 355 sbc with stock stroke 4340 LW crank, 6.125 LW rods, 383 pistons for 6” rods and 1.114 (?) compression height flat top pistons with 2 valve reliefs and 1.3mm Total Seal spacer rings(Weisco sold by Northern Auto Parts) Pistons and combustion chamber ceramic coated. .036 Cometic gasket .035” quench with aluminum heads with 1.6 full roller rockers. Solid flat tappet cam to 6900rpm. Performer rpm plenum divider notched 1” intake, w/750 vac secondary carb. Pan w/ windage tray and crank scraper. 1 3/4” headers. No problems on pump gas at 10.6 : 1 Runs amazing
 

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I've always thought that there's nothing at all "dynamic" about DCR calculations, because the simple formula does not take into consideration Volumetric Efficiency (cylinder fill or overfill) at high RPMs. Looks like you would need to know mass air flow, ambient air pressure & temperature, and cylinder pressure to determine VE.

I think the old rules about combustion chamber design, quench height, and cylinder head material will at least get you in the ballpark for determining static compression ratio.
 

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Somewhat realted, I thought I'd copy a post I once did here. I wanted to know if there was a correlation between compression test pressure and static compression ratio, and this took a while to research.

Basically, there is, but one needs to find the dynamic compression ratio first. Unable to sort out how to actually compute the DCR, I used an on-line calculator as per this link:

Wallace Racing: Dynamic Compression Ratio Calculator

I used what I knew about my engine and camshaft and calculated the DCR as 6.81:1


I thought the next step was using Boyle’s Law of gasses, but I was wrong. Apparently for an engine cylinder, one needs to use the adiabatic expansion formula (don‘t ask me to explain it, but apparently a pressure change is not proportional to volume change… likely to do with heat):

P2/P1 = (V1/V2)^1.41

Substituting V1/V2 with 6.81, I get 14.95 and;

(Comp Pressure + 14.7) / 14.7 = 14.95

The Comp Pressure works out to 205.11 psi which compares nicely to my measured pressure of 205 psi.

I’m not sure I’m satisfied with all this until I can understand it more, yet I’m pleased to find that (it seems) SCR can be mathematically massaged, along with engine and cam specs, to find the DCR and therefore predict a compression gauge maximum reading.
 

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Compression pressure changes with RPM so at 200 cranking rpm you might get your 205, it goes up through the rpm range, untill a restriction happens then it goes down again.
For example a big CI engine with a small 2v carb would still push 205 for example at 200 rpm cranking BUT what happens when the little carb becomes a restrictions and half the air needs at 5500 reduces the volume? pressure goes down again.
Thats why the math doesn't work.
 

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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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I agree that it's not the end of the world, and larger quench might mean you need to run 92 instead of 91, or bring the timing back to 35.5 degrees total instead of 36. It's not like it will really affect specific output in a way that makes it a lump or a winner.

I focus on quench mostly for efficiency. Not MPG efficiency, but thermal efficiency. That is to say, converting the most BTUs into torque, but it's a bit folly to make it the most important part. If you have an off-the shelf piston/gasket combo that gives you .050" quench, I wouldn't stress about finding custom pistons or a special gasket to get your target quench at the expense of missing your target compression.

My latest build is an LQ9 with LS6 heads. Everything specs out to 11.3:1 and 0.025" quench. Running flex fuel I don't anticipate any issues.

Regarding domes vs flat vs dish, flat tops tend to be the most thermally efficient. Dishes aren't bad, but domes (at least big ones) can suck for detonation tolerance. Forcing the flame front to wrap around and change directions often means that you need more ignition lead which can increase the need for octane. The size domes you would use are probably a non-issue, but when you get into something like a Hemi, those domes destroy efficiency... but they were also made in a time when 100+ octane leaded gas was 35 cents a gallon.
 

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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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Personally I have no interest in building a hot rod engine that's close to the limit compression wise. Sure an sbc will make more power NA pump gas @ 10.3:1 vs 9.3:1, but in the end the 9.3:1 has more potential when I turn on the spray. I'm a happy camper when I can pull down 20+mpg with 9.3:1 / .084" quench 8500+rpm stock block 355, then spray it well into the 5's at the track. I don't need no stinkin' trailer or $20/gal race gas to be happy!

Grant
 

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I've always thought that there's nothing at all "dynamic" about DCR calculations, because the simple formula does not take into consideration Volumetric Efficiency (cylinder fill or overfill) at high RPMs. Looks like you would need to know mass air flow, ambient air pressure & temperature, and cylinder pressure to determine VE.

I think the old rules about combustion chamber design, quench height, and cylinder head material will at least get you in the ballpark for determining static compression ratio.
That 's the point on Dcr calculation. If you build an engine using combustion chamber design quench height and cylinder head material that's the way it was done back when fuel was decent. My understanding is that with intake valve closing info (SUPPOSEDLY) you can use your static comp ratio higher than you normally could to run on todays fuels. Many live by it many shrug it off. ?????.
 

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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.
I doubt you could cut the heads enough to lower combustion chamber cc to where you want to get without intake fitment trouble
 

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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.
You probably have .025 on your undecked stock block just add .015 gasket for a .040 quench flattops and some 68cc chamber heads should be good to go. 9:8-1 range depending on size of valve reliefs.
 
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