I'm looking at either the edl. performers or the performer RPMs to bolt on my street driven 350. It seems the only difference is the heat crossover. Is it something that's needed? I would also like to run a performer air gap to top things off, but I don't think it has provisions for the cross over. Just wondering what difference the cross over makes if any. Any advice is greatly appreciated.

Your correct on the heads, the crossover is there to preheat the air/fuel for cold weather driveability. Depending on how much you drive the Airgap might give you fits during cold weather.

Air gap makes more power, heat in the manifold makes them so much nicer to drive. One thing I will never do again is try to run an auto trans without a heat riser, it is very anoying! Next issue is sound! If you run without a heat riser I would want an "X" pipe or some other cross over balance conection in the dual exhaust. Without a balance pipe in the exhaust or a heat riser cross over the exhaust sound is ugly! I do not mean ugly loud I mean braaaaaaat ugly! I recently did a 454 conversion from 350 on my 72 and bought a no heat riser manifold used. My neighboors would plug there ears when I started it and it was not mocho, it was ugly! Put a heat riser manifold on it and the car is a joy to own.

Thanks for the info, looks like I'll be sticking with the regular performer manifold and perf. heads. It's nice to find this out sooner than later.

I run no crossover on my old 307 and my new 454. I have never really had a problem with cold weather, I even daily drove it with both engines. But, I do have electric fans so I just leave them off till the engine warms up.

Also, if you do use the heat crossover, make sure you run a carb heat shield, the hot exhaust in the manifold may boil the fuel bowls.

Thanks, I'll definitely run the heat shield.