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Compression Vs Quench clearance

2229 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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· Registered
5,746 Posts
Not everybody is looking to build a modern race engine, let alone capable of it. I've used DCR in the past as a sanity check for what I'm doing. It takes 5 minutes and for the stuff I mess with at home, and people like me? Peace of mind for someone who doesnt build engines every day - is worth 5 minutes. Id wager that 90% of the people on here, who are assembling their own engine, are using parts that DCR calcs work as designed. So why not accept that for Stan the Streetrodder; its helpful. There are definitely people in the world who have hit the pause button because their list of parts in the shopping cart didn't add up as planned. Does that mean I'd throw my combo in the trash? No; but it would cause me to dig deeper and ask questions before I pulled the trigger.

Hell I preach a rule of thumb to use a combined gear reduction of 9.0-10.0:1 for a street car. Every day, at my job. However, this supposes that you have a blue collar, work-a-day engine (300-450hp or so and a fair amount of torque and midrange), tires in the 26-28" range and probably not wider than 9 or 10". It works well given its intended audience, the avg hotrodder will keep himself out of trouble, its easy to remember and you can impress the chicks at parties with your knowledge.
Is this what I use for my F.A.S.T. guys, gassers, the Land Speed guys, Pikes Peak guys or SVRA guys? Hell no, not even close. I tailor my approach to my customers needs.

· Race it, Don't rice it!
9,756 Posts
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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