I have a mildly modified 350ci..rebuild eng and heads with larger valves, cam, compression, and aftermarket intake and carb. Now the size on the hooker headers (4 x 1 7/8) and sidepipe is larger than what is recomended for that set up. There was even artical in "chevy hp" and it looked like i might loose some low end tork and hp. Will this even be noticible?? If that is the case then would the reverse flow muffler be better so to add a little back presure?? Ive never done the before and have flying be the seat of my pants, if is all comes together it will be a small miracle!!


Thanks, Ron

Ruready2go said: . . . Now the size on the hooker headers (4 x 1 7/8) and sidepipe is larger than what is recomended for that set up. There was even artical in "chevy hp" and it looked like i might loose some low end tork and hp. Will this even be noticible?? . . . Thanks, Ron

What is the recommended size for your set up? If the oversize difference is more than 1/2" I would expect the added back pressure of reverse flow to make a noticeable difference (in a good way).

My two-cents worth.

John

I have seen quite a few conflicting articles on whether header pipe size really makes a dif... on hp/torque

the ones that seemed credible made the case that they didnt hold the heat as well and therefore didnt scavenge exhaust as well..

of course they recommended header wrap to fix this..

you have jet-hot and therefore this should be a nonissue for you.

getting backpressure to smoooth out your engine ??

i would think the glasspacks are enough

what mufflers did you get from jet-hot.. are they the glass pack??

I just ordered the header and sidepipe from jet-hot, im still trying to decide on muffler..glasspack, reverse flow or something else...do you know if glasspack are to loud? I want the nice sould but dont want to break window in neighborhood when leaving :-)

i dont know first hand but from an article in corvette fever.. the reverse flow are wimpy and have no sound..

the guy with blower vette here has that problem..

the glass packs get annoying on a long drive..

other than straight pipes.... which sounds like an exceptionally loud harley

these are the only options I know of.

Using back pressure to offset large header tubes may help idle a bit, but it won't help power, and may hurt. There are many factors involved.

When the exhaust valve closes, the exhaust traveling in the pipe is an object in motion. It wants to stay in motion. This motion actually creates a vacuum in the exhaust pipe when the exhaust valve closes. This is why headers work. They balance the exhaust pulses evenly, and time it so that each tube creates a vacuum pulse at the collector, thus applying vacuum to the other tubes. When the next exhaust valve opens, it has vacuum applied which pulls the exhaust out of the cylinder faster than it would before. This allows more fresh air/fuel in, so there is more power produced.

If the header tubes are too large, the exhaust flows slower. Much like a wide lazy river as opposed to a narrow fast moving spot. The slow flow does not create the vacuum. The cylinder is not scavanged as well, and power is lost. At higher RPM the flow is increased and the problem corrects itself, but low end is lost. It can even be worse than stock manifolds at low end.

If the tube is too small, it gets restrictive at high RPM and top end is lost. Low end works well.

If you have large tubes and the flow is too slow, creating a more restrictive flow and increasing backpressure just slows down the flow more, making the problem worse. It may idle better, but performance typically suffers.

This is all a balancing act based not only on engine size, but cam selection, compression ratio, valve size, and everything else in the engine. It all affects air flow. Even the intake manifold makes a difference.

The reason the header wrap or coatings work is they keep the heat in the exhaust. If the exhaust cools, The exhaust gas contracts and reduces volume. The lesser volume means slower flow, and less scavanging.

There is no one answer. It depends on how you set up the engine and the RPM range most used. Final drive and tire size also have an affect.

The most common mistake is choosing too large of header tube. And many people use a system with too little back pressure for a mostly stock engine. It can actually slow you down.

Kstyer is right on, I went to the Hooker page and the OD on the side pipes are 1-7/8" so the ID is 1-3/4", Hooker has a phone
number to a tech line, you might want to call and tell them what you got and see what they say. Holley owns Hooker so go to the Holly page first,and then to Hooker. I had header- sidepipes on my 69 big block, they were fun at first but they get old after a while, you might want to consider an under car 1-5/8 header with a 2-1/2 exhaust, if you do, make sure your muffler guy double flanges the primary exh. pipe on the engine side of the trans crossmember at the collector so you can pull it without pulling the headers,incase you have to pull the trans or clutch.

Ken is right, not that I can add to his knowledge...but I have a bone stock L-48 and I didn't ask the right questions...so I have the biggest available Super-Competition Hooker Headers on there. Taking it home from getting those put on and it was just sad...the sound is much better, but off the line torque? HAH! Used to be able to chirp tires off paint at stoplights...not anymore(and that is truly a low day for a Corvette)...however, at full RPMs when it shifts, it'll chirp from 1-2 and 2-3...

Scot

My '73 sounds similar to yours. Rebuilt engine, new heads, cam, etc. Contacted Hooker before changing from the stock exhaust. They said to go with 1 5/8" Competition or Super Competition. Added a 2 1/4" custom mandrel bent exhaust with a good flow muffler. Good sound, not too loud. (With a pearl orange '73 Vette the cops can see me coming from a mile away. Figured they didn't need to hear me as well.) Hooker's recommendation seems to be right on. With my mild cam (Comp Xtreme Energy 212/218) I've dyno'ed at 367 hp and 408 lb-ft. My $0.02 worth. Hope it helps.

I think I have 2 1/2 or 2 3/4 Hooker Super Comps....Could be running a LOT of boost and still not need a bigger exhaust...

Scot
Got Snookered.