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Pressure recovery...am I understanding it correctly?

General engine tech -- Drag Racing to Circle Track

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travis
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Pressure recovery...am I understanding it correctly?

Post by travis » Mon Dec 02, 2019 4:30 pm

Comparing 2 heads exhaust port flow numbers...my sbf Promaxx 9175’s vs my barely ported 601 305 heads. The Promaxx heads have a 1.60 undercut stem valve, a short perfectly straight port with a “textbook” short turn, a short length, and a square exit that is obviously bigger than the 601’s. The 601’s have an undercut 1.50 valve, more length, a slight bend in the port (exit isn’t right inline with the valve), and a smaller more rounded exit.

Flow bench results between the 2 heads show that with or without a pipe, the smaller more round exhaust port of the iron heads flows better at all lift points EXCEPT at .300”, where the Promaxx is 11 cfm better. The Promaxx also flattens our above .500” lift, only gaining 2-3 cfm at each point above that, where the 601’s are still gaining 9-12 cfm at each point above .500”.

I’m trying to wrap my brain around what is going on here. Pressure recovery has been mentioned before on similar questions, so I am trying to make sense of that. Here is what I am thinking...the smaller CSA of the exhaust is creating a higher velocity “slug” of air that can more easily penetrate the atmosphere outside of the port, where as the larger CSA, slower moving air from the aftermarket heads sees atmosphere as more of a wall, so to speak. Similar to a pointed nose bullet vs a flat nosed bullet hitting and penetrating the same material.

Am I visualizing this correctly?

In general, if you had the same intake port, and could use either of these 2 exhaust ports...would the smaller faster exhaust make more average power?

Walter R. Malik
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Re: Pressure recovery...am I understanding it correctly?

Post by Walter R. Malik » Mon Dec 02, 2019 5:55 pm

travis wrote:
Mon Dec 02, 2019 4:30 pm
Comparing 2 heads exhaust port flow numbers...my sbf Promaxx 9175’s vs my barely ported 601 305 heads. The Promaxx heads have a 1.60 undercut stem valve, a short perfectly straight port with a “textbook” short turn, a short length, and a square exit that is obviously bigger than the 601’s. The 601’s have an undercut 1.50 valve, more length, a slight bend in the port (exit isn’t right inline with the valve), and a smaller more rounded exit.

Flow bench results between the 2 heads show that with or without a pipe, the smaller more round exhaust port of the iron heads flows better at all lift points EXCEPT at .300”, where the Promaxx is 11 cfm better. The Promaxx also flattens our above .500” lift, only gaining 2-3 cfm at each point above that, where the 601’s are still gaining 9-12 cfm at each point above .500”.

I’m trying to wrap my brain around what is going on here. Pressure recovery has been mentioned before on similar questions, so I am trying to make sense of that. Here is what I am thinking...the smaller CSA of the exhaust is creating a higher velocity “slug” of air that can more easily penetrate the atmosphere outside of the port, where as the larger CSA, slower moving air from the aftermarket heads sees atmosphere as more of a wall, so to speak. Similar to a pointed nose bullet vs a flat nosed bullet hitting and penetrating the same material.

Am I visualizing this correctly?

In general, if you had the same intake port, and could use either of these 2 exhaust ports...would the smaller faster exhaust make more average power?
Personally, I have never thought about Pressure Recovery from an exhaust port; only intake air flow entering the combustion chamber.

On the exhaust side it seems that whatever is placed immediately after the port would have the most affect upon the airflow leaving it.
There would be no mixture separation or wall wetting difference to have any effect upon combustion efficiency so, I am not sure that it would make any difference at all; other than mass air flow out.

Good brain teaser...
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PRH
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Re: Pressure recovery...am I understanding it correctly?

Post by PRH » Tue Dec 03, 2019 10:58 am

Stating the obvious........ if you have a “barely ported” 601 ex port out-flowing an aftermarket performance head that has a bigger valve....... the aftermarket head doesn’t have a very good port.

Have you tried turning both heads around(ex facing out)?

Tried a proper flow tube on both?

I’d be interested in seeing what the valve job/bowl/chamber looks like on the Promaxx head.

Mostly what I work on are Mopar heads.
On the ex side of the import heads, it’s not uncommon to see a 25-30cfm gain by recutting the seat and doing some very mild blending.

My experience is that SBC heads of that era don’t have ex ports that are anything special.

Lastly, if you’re using a SF-110 bench....... I had one of those for 14 years and noticed often that you’d get different flow out of left and right ex ports.
If you want to test for that, do the same work you've done to the port you’ve tested, but on the opposite side port(as in #1 vs #7)....... see if the numbers repeat.
This if for heads with left/right ports.

I recently had an Ede RPM SBF Head here.....ex flow on 4” bore, no blending:
Lift——ex
.300—131
.400—153
.500—166
.600—171
.700—173
Somewhat handy with a die grinder.

mag2555
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Re: Pressure recovery...am I understanding it correctly?

Post by mag2555 » Tue Dec 03, 2019 1:41 pm

The center Exh ports on the common factory SBC iron heads up until the LS motors have a pretty good bias in them in regards to pointing towards each other, just like Exh ports in Ford 2bbl Cleveland heads, and if you do not port them with that in mind then the outer and inner Exh ports will never flow close to each other.

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