open pipe vs glasspack = mpg
 

ive got a stock 350 with center dump manifolds with 2 ft of 2in pipe leading into a 26in glass pack and then dumps out the end of the glass pack, its true dual without a crossover and i was wondering what if any affect cutting the glass packs off and leaving just 2ft of pipe off the manifolds with do to the mpg???

Re: open pipe vs glasspack = mpg
 

I am not the best on this, but I spent some time with the Flowmaster guys (tech line) before I built my system on my 84 truck. Are you really after gas mileage or loud noise. The engine needs a little backpressure to make the gas exchange in the cylinders work effectively. You DO need an exhaust that will create a slight vacuum effect to draw the gases out and thus pull more fuel enriched air into the cylinder. The short pipes that you are suggesting could cause a severe cooling effect on the exhaust valves after they close on each cylinder and may shorten their lives. The crossover behind the headers or the exhaust manifolds helps to build low end torque and running TRUE seperate duals negates that effect. An "X-Pipe" has been found to make the systems perform the best and there are many on the market. Hiowever, even an "H Pipe" crossover is good for torque numbers. That should be followed up by a muffler design that is effective at doing a sound drop, but keeps the flow characteristics at the max. That is what companies like Flowmaster, Edelbrock, and many others are working at. Even a glasspack is poor in flow characteristics, because it is made to have many little compression points to cause the sound wave to revert on itself in the tube. This causes turbulence inside and that causes an impingement on the flow. The exhaust pipes need to be of a size that as the gases cool, they create that vacuum I mentioned and it sweeps the system clear to help remove the gases. In other words BIGGER is not always better---so don't go to 3" unless you really have a monster Big Block motor. Don't go to small, either. Look at what kind of driving that you do and then decide on what is needed by talking to the techs at one of the manufacturers. I am running full length dual Flowmaster mufflers and a 2 1/2" system on my 84 SWB with a383 stroker motor and this gives me a good combination and some real sound also. I get a good 15 MPG with a 3.73 rear end gear and a lower profile tire.

Re: open pipe vs glasspack = mpg
 

I don't agree that they need backpressure but I do agree you dont want to go to big. If you go to big the exhaust will expand causing it to cool down which then makes the gases tumble and slow down. So 2 1/2" is as big as I would go on a small block, 2" is to small. X pipes do work well but in my experience all of its gains are in the high rpm range (above 6000) thats why nascar uses them, so on a street driven vehicle you dont get the advantage over a H pipe. Mufflers are very important because they cause the exhaust to expand creating a scavaging effect (vacuum) which is good. I personaly like magnaflows over the flowmasters because they flow better but probably not enough to feel the difference, mostly sound difference. If you use a glass pack make sure it is a good quality glass pack not a cheap thrush cherry bomb that you buy out of a trash can at the local auto parts store, it should have a 4" outer core so there is more room for expansion. But even then they are not as good as a good turbo muffler.