I almost forgot about what paul mentioned earlier as well. I think the headers may be too small and acting like a small exhaust valve. I think you would be better off using 1 3/4 or even better 1 3/4 stepped to 1 7/8 into a 3 inch collector. That may be the 500 or so lower RPM peak right there. BUT as every "hobbyist" racer has too look at money per 10th................... Is it worth the money?
I'm running an Edelbrock Air Gap, 91 octane pump gas. I was in the room for all the runs and he would just hit the print button. The Demon did have the same lean spot in AFR at the same RPM range even though it uses bigger jets than the Holley. I was told the 1 5/8 headers would give me good bottom end torque and should be good all the way to 500 HP. I'm OK with the numbers for now but I have that "anal" desire to push things as far as reasonable. I didn't feel that it was reasonable that a cam would fall 1000 RPM short which is why i'm grateful for all the info and ideas i've been getting. Unfortunately, I have to wait for my funds to build up again before I can consider even moderately affordable improvements like valve springs but dyno time, i can't even consider. There will only be a couple of months of racing left out here by the time i get it together, so I'd be 'get a move on'
Your problem is that you have headers that are larger than you need. The interference of the exhaust pulses might be causing that dip in the torque. A longer tube would probably take care of some of it but a smaller tube would invrease the torque and give higher average torque and power. You might lose a little peak hp at the top but you might not - either way the averages would likely be higher.
A 1 5/8" primary is just about right in my opinion. Although if you listen to Headers by Ed, he would probably tell you that 1 5/8" is overkill. There is a lot to be said for scavenging the exhaust with a properly built set of headers with equal lengths. I got a lot of data from Ed back when I was going to build some headers for my Mustang 10 years ago, and I can't argue with his results. Paul may be on to something.
I'm a curious individual, so I'm willing to try anything with this combo. As it turns out, I can't even bolt in the Hooker 6901's. I have to get them modified so that tube #7 doesn't hit the floor by the speedometer cable. I'll bolt on my 1 1/2" headers for now. Since the dip in the graph is right where I launch, a difference in the torque curve should materialize as difference in my 60' and 330' times between the 2 headers. Does anyone know where I can get headers that expand as the rpm's go up?
