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Registered User
Join Date: Jun 2009
Posts: 75
intake air velocity

I will start by making the assumption that the higher the velocity in the intake, the better. Please correct me if I am wrong. My question is, does increasing the compression ratio also increase the charge velocity in the intake. I am thinking that the higher compression engine will accelerate faster and the increased inertia will foster faster air. This is a laymans perception. Would appreciate any feedback. Thanks, Bob
Old 02-09-2019, 12:19 PM
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Walt Fricke's Avatar
Join Date: May 2004
Location: Boulder, Colorado
Posts: 5,122
Maybe an engineer will give a really sound answer.

But I think your premise is wrong. Velocity is going to be a function of displacement, cam, intake sizing and characteristics, and RPM, seems to me. Increased intake air inertia may affect how the column of air packs into the cylinder (though exhaust scavenge and so on has a role in this packing also). All of which affect volumetric efficiency (VE).

Increased compression increases torque (which, in turn, increases horsepower). But that all happens after ignition, and the intake air flow has long ago stopped.

Increased compression also increases the negative work of the compression stroke. But by then the intake valve is closed, or nearly so.

I also think that velocity, while important, is not the end of the line. Imagine a super small carb venturi - velocity there would be super high, but flow would be very poor.

Easy to make the intake stuff too large or too small. Hard to get it just right.
Old 02-10-2019, 07:26 PM
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Cajundaddy's Avatar
Join Date: Sep 2010
Location: Cucamonga CA
Posts: 3,001
It's complicated. At least a dozen books have been written on this subject but intake velocity and compression ratio are unrelated. To generate more power you want to pack the most F/A mixture into the cylinder and after the valves close, the compression ratio engages until ignition.

To increase the #of O2 molecules into the cylinder for combustion, volume, density, and velocity all play a role. Increasing the diameter of the intake will increase volume and reduce velocity which tends to improve high rpm performance. Decreasing the diameter of the intake will increase velocity and decrease volume which tends to improve the torque curve across the rpm range. Turbo, intercooling, and acoustic resonance are proven methods to increase density.

The trick is deciding where in the RPM range you want to maximize engine performance and then engineer an intake that will deliver the goods.

It's complicated.
2009 Cayman PDK
With a few tweaks
Old 02-10-2019, 08:12 PM
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