You guys think 11.2:1 static, 9.4:1 dynamic is too high for 93 octane on aluminum heads?

-- Joe

 

8.5:1 dynamic is borderline.

how do you manage 9.4 dynamic with 11.2 static??

 

Cam.. intake closes at 59 degrees. (44 at .050).


A lot of guys shoot for low 9s dynamic, but I'm concerned that the cranking pressure will be too high at 11.2 static.

Calculated out about 210-215psi cranking pressure.

Depending on the calculator you use, and if you choose to use the IVC at .050, advertised, or .050 + 15 degrees you get anywhere from 9.4 dynamic to 10.1 dynamic..


-- Joe

 

Static is fine, but the dynamic should be around 8-8.5 like 5678TA mentioned. You can change that by using a cam with a later closing intake (bigger cam).

 

Interesting. If I use the pat kelley calculator, which uses advertised numbers it calculates out at 8.6.. big difference.

So who do you trust ??

timing card says ADBC 44 at .050"..

When using .006 duration specs, the program calculates the ABDC to 69.5.

If I use the kb method of .050 + 15, it comes out to 59.

Obviously the difference is huge, so it's kind of hard to make a decision.

In theory, if we go by the "8.5 max", Pat Kelley says I can run a 58cc chamber, and be at 8.4 DCR, and 10.9 SCR. About 180psi cranking.

If I go by the kb-silvolite calculator, and use the .050 + 15 degrees for a total of 59 degrees, I get 9.1 DCR with the same 10.9 SCR (58cc heads).



So who do you believe..

-- Joe




-- Joe

 

Use the .050 numbers if possible... they will be more 'accurate' (for lack of a better word).

 

Personally Kelleys was closer to my real world experience. By the Silvolite calc my DCR was so high the engine should not have run on the gas I was using.
I don't believe that at 0.050+15 the valve is on the seat enough for compression to begin?

FWIW
my new 383 combo is 12:1 CR ; 8.7 DCR and cranks fine with an orginal Denso starter from a '89 Vette

 

Great article on dynamic compression ratio at this link. There's a calculator to download as well.

http://www.empirenet.com/pkelley2/DynamicCR.html

I would think that the advertised duration would give more consistant results when comparing cam to cam (no disrespect AirAdam). With lobe profiles changing so much with respect to opening rates, there would be less discrepancy using an advertised or .006" value. If you think about it, if the valve is opened .006" it's not sealing the cylinder so there's no pressure to build. No pressure, no compression.

 

That is the calculator Anesthes is referencing in his comments above ."Pat Kelley"

 

Thank you guys for the responses. I appreciate it.

At this point, I have my shortblock and cam. I have not ordered heads, because I'm slightly torn between what size I want the chamber to be (including if I have to mill them) to get my desired effective compression ratio.

Pretty much all the cams I was looking at had about the same ABDC, so the variable that is easiest to change is obviously the chamber size to dial in the compression dead nuts where I'd like it to be. But with the discrepancy between the calculators, I was a little uneasy shooting for a specific compression ratio.

Of course, if I decide to go with a blower, it kind of makes this whole exercise useless because we can tune to pulley size to get optimum power, as the starting compression would be in the 8s/low 9s and dcr burried in the 7s..

The cam I have is actually somewhat between a blower cam and a n/a cam.. Oddly enough, after buying the cam I found an article where chevy high-tech mag did a 383 with a vortech, same cam specs, and put down 700hp (flywheel). I was impressed.

-- Joe

 

It's been along time since I've read the article in it's entirety. That's what I get for not being thorough.
I hope you found it good read none the less.
Good luck with the build Anesthes.

 

Thank you!

-- Joe