I heard this week that W30 exhaust manifold flow better than any headers you can buy. Is this true? And how do the after market manifolds compare to OEM?

 

No, that is not true.

 

I agree with Ken. If they were as good as headers, why would you put them on, big cost, and on a Olds a big PITB, to install. If you are just driving the car, I see no reason to to put them on. If you go to the track, you would probably see the difference in your time slip.

 

The W/Z manifolds have internally divided runners and DO flow better than other open log style manifolds, but they are NOT better than equal length, long tube headers. Now, compared to shorty headers, they are probably a lot closer. Of course, this also depends entirely on the engine. If the engine is relatively stock, you won't see much difference. The more modified the engine is, the more difference real headers will make.

 

I just got talked out of exhaust manifolds on a lightly modded 500cid Chrysler, my engine builder dyno'd back to back headers vs manifolds. The manifolds were 100hp less than headers. Maybe Olds are a little better, but unless it's a concourse restoration I'm not giving up that much hp. Best of luck with your project.

 

You will lose 30 hp vs properly sized full length headers. I would bet even Thornton new shorties will beat them by at least 10 hp especially with 2.5" pipes and bolt in place of manifolds. The rest of the exhaust makes a difference as well.

 

Anyone ever noticed the horrible transition ( by pictures) from square port to round? All pics ive seen looks that round pipe is just pushed at the flange and welded, and then promoting that abrupt change of shape in less than 1/2 inches after it leaves exhaust port in head. When you look some cheapo 150$ long-tubes, even they have it better looking.
Just an observation.