I got headers for my F600, but we did't realize my 391 has a 5th exhaust port. I've been advised not to weld them. Anyone have experience w this issue?

 

I think the 5th exhaust port is the crossover under the intake. If you weld it up the truck will be exceedingly cold natured.

 

Yeah, you really need that crossover. Not to mention, the exhaust ports in the 391 are much smaller than the header, aren't they? It might hurt more than help.

 

Well, after much thought and dinkling around, I removed the one I had installed and sent the headers back. I am installing dual exhaust instead, since I want more flow/less restriction. This seems to be a more the viable option. Thanks for your help everyone!

 

From what I remember the exhaust manifolds on an FT engine are pretty big and free flowing. Duals should help quite a bit.

 

The crossover is simply a mechanism to warm up the intake manifold to allow fuel to vaporize when it is cold. The down side is it is a power robber when it is warm. It warms up the entire intake charge of air and fuel making it less dense which translates to less power. Many engines have this and many aftermarket intakes and heads eliminate the crossover because it is a power robber.

 

imabass, that's a good idea for a race engine, but not good on the street. The best heat for a crossover is water, but it takes a while to get warm enough, however it is a very predictable heat. 401,477 and 534 engines use that as did the MEL 383, 410, 430 and 462 engines.

 

The massive cast intake is different than aftermarket alum which is probably not available for the 391 unless you do a full swap to 390 heads and intake. If you want a big increase in power the 390 heads and intake will breath way better and give you sizable increase in power.

Not sure on your 391, but the one I have on my engine stand has massive dished pistons with super low compression. I am in process of building with 360 pistons and 390 heads/intake/cam. It is for a 40" bluebird bus.

 

If you are increasing the compression ratio from the stock 7.2 to 1, that is a bad idea in a heavily loaded engine as it will not last that long due to excessive combustion chamber temps.

Ford (and GM, and Chrysler, International, Reo, and all of the others) made all of there medium and heavy truck GAS engines with lower compression ratios for a reason.

Slogging uphill, throttle down is where it is at with a relatively small displacement for the weight moved engine and that lower compression engine DESIGNED for this type of service is what will hold up, and there is no way that any of us at FTE can spend the amount of engineering $$$ getting to where EVERY medium and heavy duty gas engine maker got to with there engineering $$$.

David

 

You are correct. Plenty for the designed maximum operating rpm of this engine - less than 3800 rpm.

David