Superformance Exhaust

[wanab5150]

I changed the plugs this morning and ran it about 50 miles, mostly at 3000 RPM cruising. I jumped on it in 3rd, 4th, and 5th and it ran great. No issues at all. It has perfect throttle response with no bogs, hick ups, stutters.....nothing. I drove west toward Sacramento to maintain seal level as much as possible. The temp was 80 degrees.
I drove home and checked the plugs, and they looked like they have never been run. I wasn't expecting brown or even a medium tan, but I mean clean! There weren’t any signs of lean either. They were clean to the point it scared me. Fifty miles isn’t much, but I expected some coloring.

I called Holly and discussed the 770 Street Avenger which is the carb I have. They said that no re-jetting would be required as Holly's are flow tested and a pipe change would make no difference. The more you put your boot into it the more air enters and takes the metered fuel with it....regardless of RPM or exhaust.

Well, jetting Harley’s and race bikes always required going up a few jets, but it also entailed changing either the air cleaner or air box. In this case only the pipe diameter was changed. Since I was not reassured by Holly’s response (too simple) I called Roush and told them my tale. John, the tech guy, said that the Holly 770 CFM was a really good match for the 427R and they switched to that carb and dropped the 750CFM a while back. He said they test a variety of Holly’s and this one was the best all around carb for torque and HP. I asked him what a good replacement would be if I wanted one and the phone went silent for a few uncomfortable seconds, and then he said “I would keep the 770”! OK, I said. It was clear that I Sh%* in his Cheerios at that point.

I stumbled into my exhaust story before he hung up on me, and asked about the jetting issue. John said the same as the Holly guy. He said these Roush engines are set up with a straight header when their dyno’ed and are sold to thousands of customers with a variety of systems, and the exhaust size has no bearing on the jetting unless you change to an odd-ball carb , manifold , or head. They said it was set up for that motor and if it’s been running good for the last 6000 miles, it should remain in the correct jet range.

Now that I have listened to two complete professionals in their field, and been told to leave it the hell alone, it was time to completely disregard what they said and venture into uncharted territory. I pulled the 70 primary jets and replaced them with 71’s and changed the secondaries from 73’s to 74’s…….what’s the worst thing that could happen.
I took the car out and ran it through the gears like this morning. Performance was not much different except for a slight surge when I hammered the throttle through 3rd gear and 4th gear. It didn’t do that earlier with the stock jets. There was also a touch of a stumble at 2500RPM hammering the throttle, also NOT THERE EARLIER! It was 90 degrees at that time but I can’t believe 10 deg. would make a difference…….But!

I should mention that the car is downright scary in 3rd. and 4th. Red line is there instantly. 1st and 2nd are out of control anyway as they were before. But after it straightens out it hits hard. I can’t say that I gained a hundred HP, but it is substantially faster through the gears. It seems that up to 3000RPM is the same (no loss at all) but above 3000RPM it’s nuts. I’ve run it up to 120 before and I would say it does feel 15-20% quicker. That’s the only way I can describe it. I did this for the exhaust sound in the beginning so I’m completely satisfied. The HP gain is a side benefit, and I need a hundred HP like I need an extra ass! But, I’ll take it.

I changed back to the stock jetting this evening and will test tomorrow when the sun is high and the CHP are having coffee! I think I’m done. Tom