Idle to 4k rpm engine for light towing, what intake + exhaust?? - Hot Rod Forum : Hotrodders Bulletin Board
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Old 04-30-2011, 05:06 PM
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Idle to 4k rpm engine for light towing, what intake + exhaust??

350 cubic inches, 10.25:1 compression, power band that goes from idle to 4000rpm.

The engine is done and is basically set in stone, not looking to change anything there. Same for the trans (3 speed auto) and the rear end (3.55:1 w/ detroit locker).

What I am not sure about is what to do with the carb and exhaust and how to tune one to the other. Also, not sure what to do for an ignition system.

I am looking for fuel economy, and low rpm performance.

Any suggestions welcome.

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Old 04-30-2011, 06:04 PM
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HEI ignition with a factory intake and rams horns is about as good as it gets for that app..
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Old 04-30-2011, 06:14 PM
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Quote:
Originally Posted by OneMoreTime
HEI ignition with a factory intake and rams horns is about as good as it gets for that app..
Pardon my ignorance, but what are rams horns?

I don't have any original parts nor do I have ready access to factory parts in good shape that will fit my application, but I do have new aftermarket parts. I have a weiand intake with the idle to 4k band.

How would I take an Edelbrock carb, say a 1404 500cfm, and tune it to match a factory style exhaust system. In this case, shorty headers with factory sized and placed collectors that go to a dual 2 1/4 - single 2 1/2 y pipe?

What I am trying to do is match the intake, exhuast and ignition to an idle-4k engine.
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Old 04-30-2011, 06:37 PM
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You have NOT built an idle-4K motor, not with 10.25:1 static compression ratio you haven't. If you use a cam that matches up with the SCR (222-232 degrees intake duration @0.050"), your operating range could be anywhere between 2000-5800 and 3000-6400. If you use a short cam with that 10.25:1 SCR, the cylinder pressure will be too high to operate on pump gas and the motor will detonate itself into little pieces in short order.

An idle to 4K operating range would call for a static compression ratio of 8.00:1 and a cam with 184 degrees intake duration @0.050".

A flat intake manifold and shorty headers or stock cast-iron ram's horn log exhaust manifolds will leave about 80-90 horsepower on the table, over a dual-plane, high-rise intake and 1 5/8" long-tube headers. A 500/600 CFM carb will work fine. Matter of fact, a Holley 390 would work fine, but it is more money than a 500/600.

Last edited by techinspector1; 04-30-2011 at 06:49 PM.
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Old 04-30-2011, 08:19 PM
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Quote:
Originally Posted by techinspector1
You have NOT built an idle-4K motor, not with 10.25:1 static compression ratio you haven't. If you use a cam that matches up with the SCR (222-232 degrees intake duration @0.050"), your operating range could be anywhere between 2000-5800 and 3000-6400. If you use a short cam with that 10.25:1 SCR, the cylinder pressure will be too high to operate on pump gas and the motor will detonate itself into little pieces in short order.

An idle to 4K operating range would call for a static compression ratio of 8.00:1 and a cam with 184 degrees intake duration @0.050".

A flat intake manifold and shorty headers or stock cast-iron ram's horn log exhaust manifolds will leave about 80-90 horsepower on the table, over a dual-plane, high-rise intake and 1 5/8" long-tube headers. A 500/600 CFM carb will work fine. Matter of fact, a Holley 390 would work fine, but it is more money than a 500/600.
Well, I was slightly off. The cam specs say exactly that the basic RPM range is 1250 to 4500 rpm.

Intake duration @ .050" is 206.

Cranking compression is 200psi.

I am also using aluminum heads, so I would tend to think that I can get away with some extra cylinder pressure relative to what iron heads would allow.
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Old 04-30-2011, 09:25 PM
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Quote:
Originally Posted by z-adamson
Cranking compression is 200psi.
I don't think any further comments from me are necessary. Good luck.
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Old 05-01-2011, 09:00 AM
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Quote:
Originally Posted by z-adamson
Well, I was slightly off. The cam specs say exactly that the basic RPM range is 1250 to 4500 rpm.

Intake duration @ .050" is 206.

Cranking compression is 200psi.

I am also using aluminum heads, so I would tend to think that I can get away with some extra cylinder pressure relative to what iron heads would allow.

SOME, yes... maybe 9:1 with a conservative timing curve. Hope you have a steady supply of 110 octane.

Do you have E85 where you live? That's about the only way you'll get this combo to work.
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