Blown88

My procharged vette has the stock mufflers on it, which makes it nice for road trips as it is fairly quiet...but...it's a vette...and it's quiet! My NA vette has the front Y pipe without the precats and the CC muffler eliminator pipes on it and I get compliments on it's sound all the time.

The question is...what will a free flowing exhaust do for boost? I spoke with a tech at procharger and he seemed to think that I need the backpressure of the mufflers/cat/etc to keep the boost level up. Doesn't make sense to me. By the time the pressurized air has made it's way into the cylinder and the intake valve has closed, does the air going out the exhaust valve really care what pressure lies on the intake side of the closed intake valve? The car has headers on it that were there when I purchased it - another thing that didn't make sense to me with stock mufflers.

mn_vette

Well, the procharger tech is correct you will make less boost, HOWEVER, with a free flowing exhaust you will make more horsepower.

That being a blanket statement, it really depends on how much change you do to the exhaust and where the efficiency range that the blower is spinning at.

On alot of cams there is some overlap of the intake and exhaust valves being open at the same time. The amount of valve overlap that you should have really depends on your setup. Its good for high rpm horsepower, bad for fuel economy and low end torque.

Lets just say that there is zero valve overlap in a setup. As the exhaust stroke goes out and the valve closes you have exhaust gasses trapped inside the chamber. This gas can be pressurized fairly well, sometimes even more than the pressurized intake charge. These trapped gasses will cause less intake charge to be put into the intake stroke.

By reducing the exhaust back pressure you will reduce the amount of gasses trapped inside the combustion chamber after the exhaust stroke. This will allow more intake charge to be put into the intake stroke and more power produced.

If the exhaust back pressure is reduced the blower will spin at the same speed. Unless your blower is running on the left side of the compressor map this increased flow of air will result in a decrease of boost pressure. You must also remember that horsepower is going to be determined by how much air you flow though your engine and not how much boost you produce. With the increase in airflow you will see an increase in horsepower. And it will also be safer horsepower at the lower boost level.

But even with a stock cam there is some kind of valve overlap. This tends to help move out all of the extra exhaust gasses by keeping the momentum of the exhaust gasses going out and letting the intake charge fill the combustion chamber before the exhaust valve is closed. Stock cams tend to be on the conservative side of valve overlap. As the backpressure of the exhaust system decreases this will increase the effeciency of the valve overlap.

As far as the headers go they do have an advantage. With the long tube headers you get what is called a scavaging effect. Since the exhaust valve is only open for about a 1/4 of the time you get pulses of exhaust gasses going though the pipe. These pulses tend to create a small vaccume behind them as they flow down the tube because of the velocity of the air particles and the lack of something to fill the space when the exhaust valve is closed. Now when you open the exhaust valve you are letting the gasses out into a less pressurized environment and they will leave the engine quicker.

With stock manifolds you do not get this effect because the length of the runners is not long enough to maintain the individual pulses in the header. Once the runners combine it decreases the scavanging effect dramatically.


I hope some of this made sense and wasn't just me babaling on.

69427

You made perfect sense. Keep up the good work!

Blown88

So it sounds like the shop that did the install was looking for low end torque in preference to loads of hp (horsepoo?). That would be consistent with the other mod - higher ratio rear end.

Guess if I ever make it to the dyno I'll unclamp the mufflers for a run and see what changes!

Thanks for the explanation.

-Eric