Trying to turn exhaust flow numbers into something meaningful

[travis]

The more I learn, the more questions I come up with

Gary and I flow tested my new Promaxx 9175 sbf heads last night. The intake ports flowed IMO amazingly well for a cheap 180cc head. The exhaust side...not quite as impressive and well under the advertised numbers. However...we tested without a pipe on the exhaust. I was kind of bummed at the numbers because the intake side flowed so close to advertised. We tried to find a 1 5/8” tube of some sort to stick on the exhaust port to see what kind of difference it would make, but the closest thing he had was, iirc, a short 1.89” ID piece of round pipe. Sticking against the head and playing with its location relative to the port showed an immediate increase of 20 cfm. What I gathered from doing some reading is that just flowing the exhaust without a pipe throws the readings off because the short length port hasn’t “straightened out” the airflow yet...essentially causing turbulence right at the exit flange. So...does this make the exhaust flow numbers without a pipe meaningless??

On top of that, since testing on bench doesn’t take into account exhaust temps or realistic running pressures, does that make the numbers even less meaningful?

How can you come up with a “proper” exhaust lobe with these flow numbers (or can you)? Or is it more of a matter of X cubic inches at Y rpms needs Z duration to support whatever RPM’s, and the actual flow bench numbers don’t really mean that much? In other words, would the exhaust lobe be selected more based on valve size, CSA, displacement, compression, etc than actual flow numbers?

[PRH]

To confuse the issue even more........

Most ex ports pick up flow with a flow tube attached.
Most intake ports lose flow with the manifold attached........ and sometimes they lose a bunch.

Since the motor has to have an intake and exhaust system........ just how useful are the flow numbers you get from a bench when there’s no intake or exhaust system in place during the test??

[ClassAct]

The more I learn, the more questions I come up with

Gary and I flow tested my new Promaxx 9175 sbf heads last night. The intake ports flowed IMO amazingly well for a cheap 180cc head. The exhaust side...not quite as impressive and well under the advertised numbers. However...we tested without a pipe on the exhaust. I was kind of bummed at the numbers because the intake side flowed so close to advertised. We tried to find a 1 5/8” tube of some sort to stick on the exhaust port to see what kind of difference it would make, but the closest thing he had was, iirc, a short 1.89” ID piece of round pipe. Sticking against the head and playing with its location relative to the port showed an immediate increase of 20 cfm. What I gathered from doing some reading is that just flowing the exhaust without a pipe throws the readings off because the short length port hasn’t “straightened out” the airflow yet...essentially causing turbulence right at the exit flange. So...does this make the exhaust flow numbers without a pipe meaningless??

On top of that, since testing on bench doesn’t take into account exhaust temps or realistic running pressures, does that make the numbers even less meaningful?


If you port an exhaust by flow you'll make the port too big.

How can you come up with a “proper” exhaust lobe with these flow numbers (or can you)? Or is it more of a matter of X cubic inches at Y rpms needs Z duration to support whatever RPM’s, and the actual flow bench numbers don’t really mean that much? In other words, would the exhaust lobe be selected more based on valve size, CSA, displacement, compression, etc than actual flow numbers?

[GARY C]

To confuse the issue even more........

Most ex ports pick up flow with a flow tube attached.
Most intake ports lose flow with the manifold attached........ and sometimes they lose a bunch.

Since the motor has to have an intake and exhaust system........ just how useful are the flow numbers you get from a bench when there’s no intake or exhaust system in place during the test??

To add to that you make more power porting the head then you do the intake and although the ex would probably flow better with a bigger header the bigger header may actually hurt power.

[Erland Cox]

The pipe on the exhaust port when bigger than the port slows the velocity of the flow into the atmosphere.
Pressure recovery.
If fast air flows into still air it is pinched by the surrounding air because its static pressure is lower than the atmospheres.
Flowing without a pipe can fool you into making the exhaust entry a lot bigger than what is good for the engine,

Erland

[Stan Weiss]

With the way most people flow an intake port are we not really just seeing the difference in efficiency between their radius entry and the intake manifold?

Stan

[cjperformance]

With the way most people flow an intake port are we not really just seeing the difference in efficiency between their radius entry and the intake manifold?

Stan

Yes, that and restriction from size and shape of intake runners.