If it is quarter mile performance there is more to going fast than headers or manifolds. As for Exhaust Manifolds and they are a nice factory piece I would leave them alone and find ones that are already altered or buy a set of repros and port them with no hear riser. At one time in I isle at Carlisle Mario sold Auminum Hemi exhaust manfolds. You save a lot of weight running those and I believe a few FAST hemi cars have them on. Every area of the motor can be altered for more HP like the stock intake manifold with a Vanke Mod,, changing rocker ratios. Leaving a little on the table is not a bad thing.

Bringing this back from the dead.

The 612 vs 588 horsepower, was that on as-cast or ported manifolds? Trying to decide between headers and porting my manifolds.

Thanks!

I'll provide some at the track results from two of my cars, both wedge powered, not hemiroid powered, with the muffs and pipes on versus no muffs and pipes removed.
Example #1 was my 1969 Dart GTS 440 powered street and strip bracket car at LACR 1/4 mile back in the late 1990s or maybe early 2000, the car ran 12.29 ET at 109.4 MPH with a new set of M/T 105x28.5x15 with 11.0 lbs. in them on the first run at around 6:000 PM on a Friday night T&T with the 2.5 inch exhaust system that consisted of a set of Hooker 1 7/8 A body fenderwell headers hooked up with about 16.0 inches of 2.5 head pipes to a set of Thrust 2 1/2 inch in and out Corvair Turbo muffs with a set of 2.5 inch side pipes going over to the rocker panel directly in front of the rear tires. 2nd run was with 10.5 Lbs. in the slicks 45 minutes later with no other changes, the car ran 12.27 at 109.5 MPH, the last run that night was around 9:00 PM with 10.0 Lbs. in the slicks which resulted in a run of 12.23 at 109.7 MPH.
This was a 3 day event with T&T on Friday night with the gates opening at 4:30 PM , first TT at 6.:00 PM stopping at 10::00 PM. Saturday the gates opened at 8:00 A.M with the first TT at around 9:00 :AM, closing at 10:PM Sunday had the same schedule with closing at the end of racing.
I got to the track around 9:00 AM and removed the exhaust system from the headers back and let the tires down to 9.5 Lbs. with no other changes and made the first TT run around 10:30 AM, it ran 11.39 at 117.6 MPH I didn't believe it so I made another run at around 11:10 AM on the other side (L/S) of the track, which with that car was normally around .02 to .03 ET slower. It ran 11.42 at 117.7 MPH
I did some similar testing with my last street and strip car at Madras, OR a 1/8 mile track it had a pump gas 512 C.I. low deck stroker motor with a six pack and a complete 3.0 exhaust system with the same Hooker headers off of the old Dart fed into 3.0 inch pipes ran to the rear of the car with a set of Magna Flow muffs that where 3.0 inch side inlet by 7.0 inch thick by 14.0 oval with a 3.0 inch center outlet with a piece of perforated straight through pipe in it, the muffs where mounted at the rear bumper with 45 degree turn downs straight out of them. That car ran a best of 7.01 ET at 95.3 MPH with the system on and it picked up to 6.98 ET at 95.5 MPH with the complete system removed I'm thinking the weight was where the small perforamnce gains where made, not the exhaust system restrictions
Happy Thanksgiving to all