90deg military exhaust manifold

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90deg military exhaust manifold

Postby needlenose » Sat Feb 25, 2017 7:13 pm

I'm running the 90deg exhaust manifold out of some kind of military truck(seems like an 8.xL cummins). While the ports are the same configuration, the runners are larger in this manifold than the stock manifold. Will this hurt my boost? I know it will flow more, but I was told a long time ago that turbo response has something to do with keeping the exhaust hot and compressed so it expands in the turbine? Or something like that....

It's on a 92 12 valve. I used the 90deg because it's swapped into an OBS F350 and it's perfect for clearing the evap box. I went with the 16cm housing but it doesn't seem to have the same power as it did when it was in the Dodge.
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Re: 90deg military exhaust manifold

Postby DMan1198 » Thu Mar 02, 2017 6:47 am

Only experience there's no appreciable gain, or loss in power/boost going from stock to high flow as I put what is the highest flowing aftermarket manifold on mine without changing anything else, and nothing changed.

You feeling of loss might be from the additional weight of the new vehicle.
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Re: 90deg military exhaust manifold

Postby Begle1 » Mon Mar 20, 2017 4:55 pm

I'm sure there's science and a lot of math when it comes to exhaust runner geometry on our antique high-boost engines.

Playing the logic game, having a 100 gallon air compressor tank for an exhaust manifold would hurt performance. You'd have all that dead air in the tank to compress and push out before getting to the turbo, and it'd take longer for the air at the turbo to get hot as well. I can't see how that wouldn't hurt spool-up.

I don't think it's a very sensitive thing though. In most applications it just comes down to "get more turbo and fuel to go faster".

It is definitely possible for vehicle weight and gearing to do subtle things to turbo power-bands too. A lighter vehicle will usually "get out from under" itself faster, maybe let the engine spend less time in an inefficient RPM band before getting into a higher-RPM, higher-boost sweet spot. Same engine with more weight might choke on over-fuel for too long and never light.
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