I looked up the specs for the 300hp crate motor cam.
250/264 advertised duration, .385/.401 lift, 110lsa.
I really don't think that's very representative of what most people are running in their hot street SB combos.
I think a more realistic test would have been by using something like a Comp XE268 or a Voodoo 268 in an E headed 9.5:1 360.
I think in a more typical build like that there would have been noticeably more difference between the 318 manifolds and a set of headers than 8hp.

I would agree but it is the only back to back dyno test of three sets of manifolds and three sets of headers I'm aware of for a small block mopar.

The engine in the Duster in reality isn't that "hot"

It has 1.92 valves, a cam that tops out around 5500, dual plan intake and 9.5-1 compression.

In comparison my old 340 had 10.5-1, aluminum flywheel, 528 DC mechanical cam, ported 2.02 X heads and a TM-5 intake. It would pound through the traps on street tires at 112 mph with a 3400lb stick Dart.

On the street the Dart would have the Duster's lunch as it was set up for street racing and was never run with slicks. With 4.10 gears and a 3500 rpm launch the car was hard to beat out of the hole.

"The engine in the Duster in reality isn't that "hot" "

And yet it still picked over a second in ET and 10mph by ditching the manifolds.
Obviously, way more than an 8hp gain.

This much is true!

Well the reality is it picked up 5 mph and 5 tenths, the rest was in tuning and maybe shift points which allowed the car to take advantage of the better breathing.

I guess I was under the impression that the car was essentially "stuck" at 14.2 @ 95mph with the manifolds on it.

It was.

The best time I was able to get out of it was 14.1 at 95 with a 2.0 short time. Once the manifolds came off with no other changes it ran a 13.49 at 100 mph.

And then I started tuning again and pulled another 4 tenths out of it.

I think I'm just looking at it from this side of the testing.
Now that you've had a chance to play with the tune up and shift points, do you think it would only slow down 5 tenths and 5 mph if you reinstalled the manifolds?

Before the testing the question was how much would the headers gain you.
After the testing and tuning the question becomes what would the manifolds cost you.

On a semi-related note, I was talking with a customer last night who was running a low deck 499 with stealth heads and manifolds. He had contacted me over the summer to see about stepping up the performance of his combo. I outlined a few changes I thought would really wake his motor up. Different intake, carb, cam, and add headers.
The changes were worth 90tq and 85hp. Before he took the motor off the dyno he tried the manifolds again. The motor lost 65tq and 43hp, and used 90cfm less air at 6000rpm.

I think it would drop right back to 14.1 at 95 mph again.

"On a semi-related note, I was talking with a customer last night who was running a low deck 499 with stealth heads and manifolds."

Dwayne - have you spoken to Todd? He had his dyno'd with the headers, pipes, and carb.

That sure sounds like 1 second and 10mph to me

Dave, no I haven't spoken to him yet. I'm sure I'll get a call at some point.
The motor I referred to was a pretty similar combo to Todd's though.

Now that is informative.

Can you share the absolute numbers? And the difference in the cam specs.

Do you think that comparing peak power is the best measure? Tighter LSA seems to favor higher peak power and headers, but hurts manifolds.

It does not seem quite right to solely compare peak hp numbers between manifolds and headers without some discussion about cam design, and average power.

Thank said, 10 mph (70 hp-ish on a 250 hp motor) is quit incredible.

Thanks

From what I read, the car picked up 5MPH with the replacement of manifolds with headers. The car picked up 4MPH+ by opening the headers and bypassing the mufflers. That increase seemed to show that there was still a big restriction behind the manifolds/headers or the AFR is way off. I have really enjoyed this thread!

Yes.



Almost- The car gained another 1 MPH by adding more fuel (I assumed it would need more as it was breathing better.

When the headers were uncorked it went from 101 mph to 104 mph and then I lost 1 mph when I lowered the slick pressures which was expected.

The initial bottleneck was the manifolds.

After they were replaced and the output increased other restrictions started to come into play like the mufflers only being so-so but that was expected by me.