When I was building my motor I used "Desktop Dyno" to get a rough idea of what changes would affect. Sure, its not perfect but it will show gains or losses and where they'll be and that's good enough for me and my street motor. Since I was building a torque motor with the intention of running a 3.54 gear, the resulting decrease in torque from going to a 2" header versus 1-7/8" was what I considered "affordable". Keep in mind that a gain in one place is a sacrifice elsewhere. You'll see a lot of cars out there with Doug's 452's !!

For a street car I would use the 1-7/8. Myself I had 1-3/4 headers on my old 440 so when I built my 493 I bought TTI 2" that I use now. But if I would have had a nice set of 1-7/8 headers I would have used them on my car. Since I wanted bigger then the 1-3/4 I figured go 2" in case I ever step up more. Ron

buy this software http://maxracesoftware.com/pipemax36xp2.htm

I have used it for years when building headers, it will calculate best tube diameter and length for any combination, also will calculate two or three step tube combinations. If you put good numbers into it the results are as good as you can get. It also has an engine dyno program and much other info as well.

If your 600+ HP, then 2" would be better for performance.

The mild 505" stroker (Maybe 550 HP?) in the convertible uses the TTI 1-7/8" headers just because 2" headers can be a pain for fitment and burning up plug wires.
Also, because it is a street car, I am just running dual 2-1/2" exhaust pipes.

The 500" stroker (700+ HP) in the Charger has the 2" Hooker Super Comp headers, and I don't really like how they fit. Originally I ran dual 3-1/2" exhaust, with race mufflers, but it was way too loud on the street. Went to the TTI 3" exhaust and I have been happy with it.

I haven't used the TTI 2" step headers, but I have been thinking of getting some to replace the Hooker headers.

FWIW I went from cheap 1 3/4" headers to hooker super comps 1 7/8" and picked up nothing in et and only 11.2 MPH. 440-6 that ran 11.30's

11.2 MPH increase on the header change only

On my 383, 1-7/8" headers made 13hp more than 1-3/4" headers, peak to peak........ But in the upper rpm ranges there was about 20hp difference.

TQ was almost exactly the same between the two, the bigger tubes just allowed the numbers to hang on better up top.

Do you mean that the the hp difference became larger after peak hp?

Yes....... It's all right here:

http://www.moparts.org/Tech/Archive/bb/26.html

And

http://www.moparts.org/Tech/Archive/bb/25.html

1-3/4":
472.5 @ 4200 TQ
473.4 @ 6000 HP
450.8 @ 6800 HP

1-7/8":
472.0 @ 4300 TQ
486.7 @ 6200 HP
471.7 @ 6800 HP

We seem to race dynos a lot on this forum, but not much discussion on the other dyno details that matter more than peak power. Carrying the power like that matters a bunch in a car if you are shifting up there. Higher rpm shift recovery and a lot more area under the curve.

Wish i would have swapped from 1-7/8" to 2" and done nothing else.
I gained with the header upgrade, but i also swapped to a larger carb at the same time
Shift 6,200/6,000....trap 6,000

I would go with 2" myself. There may be a slight trade off with the low end grunt, but the 2" header up top should make more power. If nothing else, if you decide to make changes down the road, the 2" will give you room to grow. That being said, if I already had the 1 7/8, I would be completely fine with running them with that combo.

I think part of the "problem" some people have, when they are racing dynos is....... They often don't have the correct equipment/set up in the car to exploit the combo they ended up with while they were chasing numbers on the dyno.

I've had pretty good success in coming up with a set up for the car that will compliment the power output and horsepower curve I got from the motor on the dyno.

You just have to get everything "on the same page".

The OP's motor will easily be solidly over 600hp..... Probably more like 650hp.
With 505 cubes and an induction system that will support a hp peak at over 6000rpm, I think the 2" header is the right choice......... Unless the plans are for a stock 12" converter and 2.94 gears to go behind the motor....... In which case several other aspects of the build wouldn't be right either.

The numbers from the 542 I referenced earlier;

1-7/8 x 3.5 hooker fenderwells-
673.3 @ 4500 TQ
678.0 @ 5900 HP
648.0 @ 6500 HP

2 - 2-1/8 x 4 dyno headers-
677.2 @ 4500 TQ
717.7 @ 6000 HP
702.8 @ 6500 HP

I am thinking he means 1.12 mph since he said no change in et ?? Ron