I need to learn or find out what this compression ratio and camshaft pressure bleed off is all about. I am going to running aluminum heads and need the highest comp ratio I can get and still be able to run 93 octane, maybe I'l have to mix some 93 with 110 to get more octane to do what I want.. my motor I just took out had a few detonation problems cause of high compression ratio and the cam didn't bleed of much pressure. It had 9.9:1 CP with 180-185 crankin PSI... the cam had 2 degress of overlap.

The new cam has 14 degrees overlap.

I know the more valve overlap you have the more cylinder pressure the cam bleeds off. I know the more pressure bleed off the lower the compression ratio gets..


So is there a formula for how much pressure that is bleed off and how many points it drops the comp ratio?


What ranges in pressures should I be around in a sbc.?


Is there a theory on how much pressure is bleed off per degree camshaft movement?



help me guys...

make me understand

Travis

No, doesnt bleed off "compression ratio", just bleeds off cylinder pressure. CR is entirely dependent on bore, stroke, and total charge volume at TDC.

Cranking pressure is your best indicator of bleed-down. Hoe the cam is degreed (advanced, retarded) also makes a difference in cranking pressure.

High CR and big cam make for decent liveability with a given octane fuel. Keep in mind, that at higher revs, the big cam doesn't give up cylinder pressure over a small cam. In fact a big cam at a specific rpm will increase volumetric efficiency in some cases to more than 100% with complimentary pieces on the intake and exhaust side.

Need to watch the timing and look for speckles on the plug insulator a lot while tweaking it in...

Traviss-
See if you can find a copy of this book-

"How To Build & Modify Chevrolet Small-Block V-8 Camshafts & Valvetrains"
by David Vizard, Motorbooks International,1992 (Amazon.com might be a source?)

This book gets very specific into cam and related parts theory, and I found it very helpful in building SBC's.

Rocker ratio is another thing that affect cylinder pressure bleed-off, i.e. 1.6's on intake only (something I found to be effective, and an accepted practice for building a little more power)
Jim

yea, what mc said. I dont know squat about small blocks but I know my BB cranking pressures (using starter) are quite low because of the cam overlap and cam timing I use. I doesnt mean my actual compression ratio has changed.

Thanks guys.

Travis,
I would run 10:1 static compression and 93 octane fuel. I would call Steve Tanzi at Erson Cams (I have an 800 number at home I will post it later). He is very good especially if you have some baselines to give him, which you do. Your ideal cranking pressures with that compression is around 160 to 165 I would guess. That will keep you into pump gas and still give you plenty of low and mid range torque. Dynamic compression is what we are talking about here and it is just as important if not more impoprtant than static compression. With that said give Steve a call and be ready for more technical BS than you can handle, and get the best cam for the combo. If I were you I would run solid rollers and turn 6000 RPM.
Craig

traviss- give me a fax#, and I will send you a couple of pages that thoroughly explains what you are looking for.
Jim

Wette vette.. Thanks for more info I needed I really need to understand this whole subject. Cause I do not want to make the mistake I made before. What do I gain by switching to solid roller lifters instead of hydraulic roller lifters?



Jafo.. sorry bro no fax I'll pick the book up at one of the local stores, they had them last time I was looking...


Travis

trav-
When you get the book (you'll probably wear it out like I did), look on pages 25-26; it will explain everything you need to know about comp ratio selections vs. camshaft selection. Also check out pages 47-48.... there is a real good explanation and graphs on the rocker ratio issues. Hope this helps, it did for me! (I'm around turbines way too much to remember the recip stuff)
Jim

I just got back form getting the book. I'll break into it this evening... I'll pay close attention to those pages you said



Its funny you go to a book store to get a book and you walk out with the book and about 5 different car and boat mags LOL!




Thanks again you guys


Travis