New engine - which intake??

[fried_guy]

I'm building a new engine for my 77 blazer.

I've got 2 choices for intake manifolds. I have a used (but perfect) performer RPM airgap and a brand new, still in box performer EPS... here's my engine specs.

It's a small block 400 + .030"

Carb is a 670 holley truck avenger.

Camshaft is the comp cams extreme energy 4x4 hydraulic flat tappet cam at 270/278 duration (P/N: 12-243-3).

I'll be using rhoads lifters to increase low end and still have a strong midrange and low-topend.

Heads are dart iron eagle 200cc with 2.05/1.60 undercut valves.

Around 9.1 compression ratio.

The only used part in the engine is the block and it's been modified a bit.

I'm kind of torn because the block is a 400 and can flow more at lower rpms. The RPM airgap flows 1500 - 6500 on a 350. IIRC, a 400 sbc lowers the rpm range of products about 1000 rpm. So it would flow approx idle - 5500?

Anyway, considering the rest of the combo, which manifold would be best?

TIA, Case.

 

[blazinzuk]

the heads you are using are what dictates flow, not the block. That being said whats your projected max rpm with this motor.

If it were me I would use the air gap manifold. I have never used one but reading about them they seem to be one of the better all around manifolds. I don't think you will change your power band much at all with either one.

 

[fried_guy]

I agree, the heads dictate the flow, you are correct, that was mis-stated.

But a larger displacement engine will need more flow than a smaller displacement engine because each stroke pulls in more air throughout the entire rpm band. And, because a larger displacement engine uses more flow than a smaller displacement engine at the same rpm, velocity increases accordingly. That's why if you wanted to keep the same power band in an engine with 10% more displacement, for example, you should "increase" everything else by 10% as well... the heads, the intake manifold, the exhaust, etc... to keep everything in proportion.

I'm not designing this engine with a "max" in mind. As long as it has a high and flat torque band to about 4500 RPM or so.

And... with the epilog above, I think I answered my own question (and you did as well). I think the RPM air gap is the way to go.

Anyone else?