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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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· Race it, Don't rice it!
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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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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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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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Love ya to death, but you missing something more important than justification of an arbitrary number.
When the fine begins over scavaging the cylinder....your DCR is thrown out the window. In different words, your DCR number is derived from IVC, and your DCR number is is calculated in the cam power range.
oh, but the thing is cams often operate way outside the range. Throwing the DCR out the window. Try swapping heads to a very small very good flowing port, then to a very large poor flowing port and think about what's happening. IVC didn't change, DCR didn't change. The engine is radically different in both characteristics and power curves.
Now you COULD estimate a typical street engine and make the assumption of DCR and be within the tolerances but to use the BS math for all engines, you would wrong more than right.
Remember this....not one single professional head porter or cam designer uses DCR calculations. Why not?
The real recipie is the smallest port that flows enough, then opens and closes the valve the fastest without breaking something, and the highest compression ratio the fuel will tolerate in the rpm range it's intended to use.
DCR as it's commonly calculated simply assumes way too many things to be of use.
 

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I don't care push a theory that saves guys from themselves. That's their responsibility.
The problem as I see it, is pushing a narrative to save guys from themselves quickly becomes the gold standard and when the time comes, a professional builder laughs and smirks in their faces because the hotrodder guys tells the professional engine guys they read it on the internet, and its a fact.
It's junk science, it's junk math. I see this crap walk in the shop all day everyday.

Guys looking for a high rpm screamer 327, 3/4 race cam, high rise intake elderbrok, double bump heads, and a 850 double humper carb. (Misspelling on purpose)
I always ask Why? Are you trying to go slower?

Lets educate, and help guys put in the correct parts, instead of just preventing them from chasing the really wrong ones?
Maybe it's just me.
 
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