Is it possible to bleed off enough cylinder pressure to run 11:1 or 11.5:1 compression on a pump gas small block (383-427ci engines) without killing a bunch of timing? Obviously I'm talking about an aluminum head motor with a proper cooling system. Would an aluminum block make a difference at all?

 

big cams will bleed off low rpm cylinder pressure allow you to run that kind of compression with no problems.by big i mean a cam with a large amount of overlap

 

What counts is dynamic CR, not static. The key cam spec is the intake closing point. The later the intake closes, the lower the dynamic CR. You can achieve later intake closing by increasing duration of the intake lobe, widening the LSA, or retarding the intake center line. To a small extent, a steeper closing ramp on the intake lobe will also lower DCR, but this is pretty much negligible. For an NA motor aim for ~9:1 DCR. The calculator at http://cochise.uia.net/pkelley2/DynamicCR.html is a neat little tool for estimating DCR.

An aluminum block won't make any appreciable difference. Coatings (piston crown and exhaust valve especially) may also decrease the tendency toward detonation and thus allow slightly higher CR.

Rich Krause

 

Lots of folks run 383+ cube motors with around 12:1 compression. On the same 91 octane owl **** you're stuck with, I ran 32 degrees of total advance and 12:1 compression in a 230/.603/110 cam'd 383. How much of that has to do with reverse cooling is questionable.

IMHO, total advance isn't really the issue, but rather pulling out enough timing down low to prevent detonation under high load/low-RPM operation street driving.

 

These two guys have it rigth.

Intake Closing point it very important so I would rather put that where you want it and use a compression that suits it.

The overlap area is going to help more than anything.

Also the timing down low will determine if there is detonation or not.

That being said. 214/224 engine with zero overlap @ .050 on 11.5:1+ and 35deg or total timing a 9:1 DCR and a steed distributor curve down low on 92 octane will have very little detonation, change the distributor curve a little and it's fine.

Revearse cooled heads will help, the constuction of the chamber will help also. Coatings will protect the piston from damage but they will most likely add to detonation since they make the total chamber hotter not colder.

Bret

 

Bret: I bow to your superior knowledge, but I thought that coatings will, in general, decrease the tendency toward preignition (I know that ins't the same thing at "detonation" in the strict sense of the word) by preventing hot spots.

Any explanation would be welcome.

Rich Krause

 

Rich, you know alot too.

Well when I was working with the first Engine Masters engine, Old SStroker and I did some digging on this and coatings. I wanted coatings from the start, but we found a atricle by Warren Johnson about short duration engines not needing them. Anyways, I think they are an even better idea now. Since I was looking into super high static and dynamic compressions on 92 octane. It seems that it actually promotes detonation or preignition with high comp ratios. I still think the benefits of the coatings are more than the extra .2-.3 of a point of compression that they might limit.

They will protect parts from detonation, but it will not prevent it.

Preventing hot spots is something we all do by eliminating sharp edges in the chamber and on the piston tops. That's one reason why pock marks in pistons from detontaion just cause more problems. It does even out the heat in the chamber (supposedly) and that will also elminate hot spots.

I've come to the conclusion that coatings are really beneficial but you need to have the rest of the parts there first before you use them.

BTW. the 1000hp tread. Hot Rod and PHR have some 632 cube big blocks this month. One is 750hp! and one is 1,100 hp! The reason is the heads, the 750hp one is using junk Edlebrock (360cfm) ones and the other is using 500cfm ones. You have to have the parts first, then the coatings are worth something.
They also have a use in classes that don't specificly outlaw them but limit the parts to stock parts. Say something like A sedan in SCCA. (I still want to make one of those cars/engines)

Bret

 

Again, we're talking dynamic compression. If you do a search you will find another thread on this same issue from maybe two months ago. May help, may not.

Coatings: This extends to a little more than simple autoignition issues. The real benefits are found through the level of tuning coatings allow. I think they are given a little too much credit on the "power increase" side of the coin. It's not just about the coating.

Take care

 

Brett, what was the inake valve closing point of this motor? The LSA? Im guessing it was retarded a few degrees.

 

My 11.3:1 compression NA SBC 427 with 18* heads made 650hp/580tq on 93 octane with full timing. I had it tuned to run as much as 34-36 degrees of advance with full advance in by 2800rpm. I was running a custom mech roller with 256/264, 112LSA and it never detonated even on hot, humid days.