soxx35, your numbers seem to be right on.

It's my understanding that torque begins to suffer when you increase your header or exhaust size beyond the ideal.

Bigger is not necessarily better.

Scott80L48, My 79 crossmember IS welded but my 78's are both bolted in. Maybe earlier C3's are also bolted. I also see crossmembers listed in Paragon Reproductions catalog. Around $300. Mike

mkapp7879,

Thank you for the information. Is your 78 "double-humped"? Is there a web site for Paragon Productions? I figured I would be spending around $300 so if there's is a bolt in it should work. Thanks again for the information.

Scott

Scott80L48, www.corvette-paragon.com For about the price of just the crossmember, Mid America has a dual system for your year that utilizes the single "hump". Check it out, probably lot simpler. Mike

Ahh, the auto instructor in me is comming out. The pipe size depends on two things, One the RPM range you wish to operate at, and two the volume of air the engine moves. A big block will move a lot more air than a small block.

Here is what happens. An object in motion tends to stay in motion. This includes exhaust gas. As the gas rushes down the pipe, at a point in time, the exhaust valve closes. The still flowing exhaust now has no supply and actually creates a vacuum at the manifold. This vacuum is relieved when the next valve opens, but the vacuum applied actually helps to suck the exhaust out of the cylinder that just opened. This gets more burned gas out of the cylinder, and more fresh fuel mix in. With more fresh air fuel mix in the cylinder, you create more power. Back pressure really is not the issue, flow is.

Now we get stickey. If there is too much backpressure, (Small pipes)there is a reduction in flow, and less purging of exhaust from the cylinder. Less power. BUT if the pipes are too large there is too much volume in the pipe and the gasses flow slower. Much like pouring a gallon of water though a hose, or a 10 inch pipe. With both fairly level, and both full, the water will flow much faster in the smaller pipe than in the large one. This slower moving flow creates much less vacuum due to lack of velocity. Once again, less vacuum, less purging, less power.

The secret is to match the flow to the engine and desired RPM. If you are after low end torque, smaller pipes will do it, if you don't go too small and choke the exhaust. If you want high end torque, use a larger pipe, but you will loose some low end. Too high you loose again overall.

Don't forget the engine displacement, and the cam timing, valve opening, etc. All make a difference in the total flow requirements. How radical is the engine built? Is it stock. Where do you want most power? Stop light to light, or top end?

Tuned headers work due to the fact that the vacuum effect is distributed evenly at every exhaust valve. Tuned headers all have the exact same length/volume pipe. Untuned headers are a step down, and stock manifolds another step down in balancing the flow. This means different cylinders make different power. You can't feel that per cylinder, but the overall power you can feel.

The chart posted above is a good guidline due to the fact if an engine is built to produce more power, it will have more flow, and need larger pipes.

I hope I didn't bore anyone, and this helps clear up why. More is not always better, it can hurt you.

By the way, when I get my project 75 L48 ready to go, I am going to go with the factory 74 dual exhaust. No cat converter. Even if I go one step up on the cam shaft, which I may do, this will be about the best for my mostly stock fairly whimpy engine. Unless I go with headers, but I don't want to do that. I would rather have the stock look, and with my engine it won't help all that much.

My 73 L82 has 2.5" from the manifold to the mufflers. Was necked down at the manifold to 2.25". My transmission cross member has holes thru it for the exhaust pipes and is welded in. I believe 4 spd are welded and autos are bolted.

Marc

Mid America sells a true dual exhaust for an 82 and it works great with the 82 crosmember. Double hole crossmember does not work very well on an 82 because an 82 has a 700R4 and there is very little room on the left for the pipe.

Crossmembers: welded on in earlier C3's are for a 4 speed. Bolted on = automatic.

My question is whether to go with madvet's dual exhuast kit for my '79 for around $230,or have an exhaust shop put it together for me? I have to modify Mid-Americas anyway when I bolt on headers.

C3rVette,

The determining factor in that would be price. I doubt if you would get a large enough difference in quality from either on to make a difference on the decision. Your local muffler shop will make one specifically for your car, but MAD has already done all the work for you.
Just from experience the table listed above for horsepower is pretty well accurate. Smaller exhaust can sometimes be a benefit! Believe it or not so can smaller ports in your heads. Like KStyer was saying earlier, flow is what is important, not size. If the pipes are too big you won't create any vacuum to help suction the exhaust out, or pull the fuel/air mixture in. This is also known as "scavenging"

Straight from Dynomax.....

Pipe Diameter

A common misconception is that the larger the diameter, the better the system. But bigger isn’t always better. Systems that are too large in diameter can actually hurt performance.

As a general rule, switching to a performance system that is 1/4” to 1/2” inch larger than stock will provide you with the best horsepower increases. To determine which pipe diameters will be best for your system, decide what RPM range your engine will operate at, most of the time. Smaller diameter pipes will produce low- to mid-RPM torque. Larger diameters produce mid- to high-RPM torque.

Balance Tubes

Balance Tubes (or ‘H’ pipes) help to even out the pulses in the exhaust gases and balance the backpressure between both banks of your engine. They not only help improve horsepower, but sound as well.

To find the right location to install your balance tube, apply a strip of paint down the front portion of the exhaust pipes. The balance tube should be placed where the paint burns off (bubbles) on the exhaust pipes. Generally, put the balance tubes as close to the headers as possible. The balance tube diameter should be at least 75% of the diameter of the pipes with which it connects.

Hope that helps!!!!!!!!!!!!

Semper Fidelis!