Jay reading TGFEIC again and it got me wondering about the Dyno-Max Truck

[Qikbbstang]

Headers doing so well in your testing and your explanation of the straight shots out of the ports as to why.... I've read of the importance of the straight shot since way back in Smokey's Power Secrets and there is no question of the importance in making power.  One thing I noticed that makes the typical FoMoCo cars "shock-tower impaired" headers even worse is the header Mfgs take "short cuts" when fabricating the headers. The worst is, I have a set of Hooker super-competition 429CJ/SCJ headers for all the 71-73 shock tower cars that have their primary tubes almost laughable severely diagonally sliced (we all know 429CJ/SCJs have tall exhaust ports in excess of 2-1/2"). Ideally you would want a very tight uniform mandrel bent ID coming off the flange face, but of course these are difficult to manufacture and expensive.  If you look closely you imagine the worst result - pulses literally fly out the port and ricochet off the side of the primary tube upon exiting the head and then proceed down and out of the primary tubes.
            I see this as double trouble: first thanks to the diagonal slicing the pulses instead of going through an even diameter tube, the pulses must go through a smaller cross section of tube and pulses must ricochet off the wall to turn making a mess of the pulses. Second I'd bet the pulses hitting that primary tube may even reflect pressure waves back into the ex-port, by any book that is a disaster.
    So here's the deal on FEs --everyone always opens up Ex-Ports "except" on the floor because flow is said to not follow the bottom of a port..........However with "shock-tower impaired" header primaries THAT low path is exactly where the flow is forced to go.  So finally would it not make sense to clean up the bottom of the exhaust port particularly by the flange to make it easier for those pulses to leave town?

[jayb]

I think that does make sense, BB, but only if you have headers that will match up to that downturned floor.  Of course, the best way to really quantify this would be to modify an exhaust port as you suggest and flow test it with a matching primary tube.

[Joe-JDC]

Almost all the flow in a FE exhaust port is in the upper, outside 1/3 part of the port.  If you start the contour of the exhaust back in the port and turn it down like the factory tried to do, you can help the flow increase.  Check out a GT-40 iron head on the exhaust side.  The new CHI heads turn the flow in almost a 180* turn down out of the port from the valve.  If you could keep the lower part of the FE exhaust floor in a curve parallel with the roof, it would increase flow.  What works on some heads is to bias the flow out the side of the port instead of the roof.  It takes work, but can be done with patience and bench testing.  A Pond head is a prime example of no short turn on the exhaust, but the port can be made to flow 250cfm by tricking it out sideways.  Joe-JDC.

[Qikbbstang]

Maybe I'm missing something but I'm thinking this is not so much about achieving high flow rates but rather optimized scavenging via maximized reflected negative exhaust pulses just downstream of the exhaust valve. I think of exhaust pulses as puffs of gas that carry impressive power that can travel a surprisingly long way even without the confines of a tube. Anyone who has ever witnessed a single cylinder motors exhaust pulses randomly moving targets of grass, leaves etc 3-10ft away from the pipes end has seen it in action.
 I learned the hard way that even moderate bracket race cams can suck discharged exhaust jacketed cooling water at low rpm's ass-backwards 3-6 feet up the pipes and log manifolds all the way back into the exhaust ports. I have even seen evidence of salt water getting into the forward most chamber/closest to pipes end through the exhaust port on a hot-powerboat.  The only way I figure this happens is at low rpm overlap sucking follows the inside radius of pipes IDs and the hard stronger ex-pulses by their mass cling to the outer radius of the pipes IDs. Obviously there can be both gasses flowing both in and out of an exhaust at the same time, the gulps of gasses can walk the water right back along all the inside radiuses.
    I always thought it was pretty straight forward that pulses in a primary tube act almost like a piston creating a strong low pressure area in their wake.....Can a flow bench aid in the fine-tuning of exhaust scavenging?

this linked article on race headers

http://www.epi-eng.com/piston_engine_technology/exhaust_system_technology.htm
 

[Qikbbstang]

Joe-JDC here's cut-away of iron GT40 head Ex.
Re:" If you start the contour of the exhaust back in the port and turn it down like the factory tried to do, you can help the flow increase.  Check out a GT-40 iron head on the exhaust side. "

I can't figure what your saying, looks to me like the ex-ports are aiming high:

[Joe-JDC]

That is not a GT-40 head.  Sorry, the picture is wrong.  On the exhaust it turns down nearly like looking at your finger on a trigger.  If I were more computer savvy, I would send a picture.  Joe-JDC

[Qikbbstang]

Hopefully THIS is GT w/ it cast on it. Joe I think I see what you mean about the floor going for the floor but sure wish this picture showed the top of the ex port. Exactly what I was proposing but for FoMoCo to use those turned down ex-ports when not forced to by shock towers or -?- screams at the classic high-port theory.

[Joe-JDC]

The roof of the port is parallel with the floor and almost round/oval shape for good flow as cast.  With a little tweaking around the back of the valve guide, the port will flow near 200 cfm.  Tweak it wrong, and you can kill the flow back to 160cfm.  Joe-JDC