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911st 911st is offline
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Join Date: Jan 2004
Location: Sacramento
Posts: 7,269
If the exhaust is 34mm it would seem something in the 34-36mm would be a fit looking at what Porsche did on there NA motors.

How do we come up with an ideal intake or exhaust port size?

I am guessing we do not want to go any larger than needed to try to keep intake port velocity up which seems to be relevant with CIS looking at what Porsche did with the SC reducing its intake port size half way through production.

As a point of reference I have one friend that made 425+ rwhp through stock intake ports and SC cams. He did however have an extrude honed intake manifold with tapered injector blocks down to the stock ports.

I wonder if we can use the relative difference between intake and exhaust valve sizes and a valid point of reference.

The exhaust valves are 41.5mm diameter and or intake valves are 49mm. Thus, the intake valve has an 18% larger curtain area under its valve seat.

It would seem we would want an 18% larger intake port cross section to support the 18% larger area under the intake valve seat.

A 34mm exhaust port has 1.42 sq inch volume. If we want the intake to have 18% larger cross section it would need to be 1.68 sq inch. A 37mm intake port has an 18% larger cross section than the 34mm exhaust port.

However, the intake valve is also open longer than the exhaust with most cams so 37mm might be larger than needed.

Thus, it might be that unless the exhaust ports are opened further anything larger than 37mm for the intake is probably not going to be much of a benefit.

Of course there are more variables than this.

I think if I was doing a motor today I would look at something in the 34-36mm range with the right cam.
Old 02-25-2010, 08:42 AM
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