12.5-1 compression Ratio

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Mar 31, 2014 | 10:03 PM

jtwmd83

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12.5-1 compression Ratio

Whats going on. I have a 1995 impala SS My build is:

Lloyd Elliot Trickflow 21 degrees..58 mm TB/intake..
Cam: 239/247 .587/.600 110 LSA..
Bottom end
Karl Ellwein forged 383 (12.5 over 1 CR)...
Quad one headers..3 in clearimage exhaust w/ x-pipes/spin techs
Level 3 trans (finishline)..3600 yank T/Q
3.73 gears w/ abs reclutor..Eaton posi ..Moser axles

I was wondering on the rpm range for idle. I have a pcm4less mail in tune, but I understand that I need to still get a dyno and tune (possibly). Seeing that my CR is 12.5-1, I was told that my idle rpm range should be around 900-1000 rpms. Is this true? Also Why? I'm currently doing my own research as well (via internet). Also anyone else running this kind of set up, I would like to hear your story on the streetability of your car.

Thanks in advance

Mar 31, 2014 | 11:55 PM

DUDCOUPE

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Depends on what the motor wants to stay alive.

Apr 1, 2014 | 06:20 AM

smitty2919

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Quote:

Originally Posted by DUDCOUPE

Depends on what the motor wants to stay alive.

That would be assuming no issues I would think. A small vacuum leak or something isn't set up right would cause idle issues. TB/TPS voltage range should be correct.

BUT, I could see how "depends on what rpm the motor wants to stay alive" could apply.

Apr 1, 2014 | 09:52 AM

moehorsepower

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From: Texas

Quote:

Originally Posted by jtwmd83

I was wondering on the rpm range for idle. I have a pcm4less mail in tune, but I understand that I need to still get a dyno and tune (possibly). Seeing that my CR is 12.5-1, I was told that my idle rpm range should be around 900-1000 rpms. Is this true? Also Why? I'm currently doing my own research as well (via internet). Also anyone else running this kind of set up, I would like to hear your story on the streetability of your car.

Thanks in advance

Its not the compression that dictates the idle, its the duration and LSA of the cam, The more the duration and the tighter the LSA, yes you have to raise the idle, The engine will not sustain a stock idle with said cam, Reason? if you try to lower the idle it will load up and die, and as said, you set the idle where the engine is happy. with your setup, about an 800-850 rpm should do, but also, all builds that are the same are different, clearances, rotating weight ect. One engine with the same setup might like a 1000 RPM idle..

Apr 4, 2014 | 03:12 PM

KW Baraka

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From: S.A., TX

Quote:

Originally Posted by jtwmd83

.....I was told that my idle rpm range should be around 900-1000 rpms. Is this true? Also Why?......

A minimum idle of 900 RPMs would be in order. That's about what I run on my 398ci; I'm idling a shade over 900 RPMs.

Why, you ask? Because given the amount of cam overlap in your system, anything less than 900 RPMs would stall the engine.

Best of luck.....hope you have access to 93 octane fuel