95Q Exhaust

[jamesmost]

My 95Q has a modified cat back exhaust from STILLEN it is 2.5" stainless steel (so claimed) mandral bent. I was having a small section of pipe replaced, when discovered another leak, the seam on the rear can.---Does anyone know what brand stillen used- i think it has a 3" input and output it may be 4"??thanks j

[autotech43]

Exhaust system backpressure is very critical on an engine, although in my opinion the Q engine does have certain speedster/racing characteristics! You open that engine up too much you will lose all kinds of low end torque. I have some info regarding header tubes, lenghts and exhaust backpressure that might give you an idea on what I'm talking about.

Unlike exhaust manifolds, headers have individual tubes for each cylinder that join together into a collector. The tubes are of equal length and tuned. "Tuned" means the tubes are of the proper diameter and length to perform optimally for a given engine displacement and RPM range.

Exhaust gases don't flow continuously, they flow in pulses from a given cylinder with a new pulse coming with each exhaust valve opening. Each pulse creates a point of high pressure traveling through a header tube. Between each pulse the pressure is relatively low.

Back Pressure - Header tubes are equal length. Each exhaust valve opens at a different time - 120 degrees of crankshaft rotation apart for a 6 cylinder engine. Because of equal tube length and non-concurrent exhaust valve opening, exhaust pulses from different cylinders don't collide in the collector. The high pressure pulse from one cylinder arrives at the collector in the low pressure zone between pulses from other cylinders. This reduces overall back pressure of the system. If the tubes where not equal length the pulses would collide in the collector and increase back pressure.

That explains the reduction in back pressure. But why not eliminate the exhaust system entirely and let the valve vent directly to the atmosphere? That would surely deliver even less back pressure. Yet ALL race cars use headers. It turns out that headers do more than just reduce back pressure. Read on.

Scavenging - Each pulse of gas has mass, and as it moves down the tube it develops momentum. If you suddenly try to stop the flow of that pulse by closing the exhaust valve, it will attempt to keep moving (a body set in motion will remain in motion). The result is that something of a vacuum is created behind the pulse. If the exhaust valve is still partially open, that vacuum draws the residual exhaust gases out of the chamber, improving evacuation. This is called "scavenging" and is one of the key benefits of a header system. If the cam has a bit of overlap (intake valve and exhaust valve open at the same time) the intake charge is sucked into the cylinder, delivering a dense uncontaminated intake charge.

Sizing - The diameter and length of the header tube are critical. For a given engine displacement, a smaller tube will cause the exhaust pulses to flow faster down the tube, thus increasing the momentum and the scavenging effect. Too small a tube and back pressure increases. Long header tubes provide superior low RPM performance while shorter tubes work best at high RPM. Optimal tube length and diameter depends on displacement and the desired RPM for the power band. Hence big race engines - big tubes, small street engines - small tubes.

Bonus information for reading to the end - intake runners have a similar tuning effect! Engine components need to be balanced - exhaust, cam, intake, RPM range and displacement all have to work together.

[Q45tech]

On a Q the headers, precats, and cats provide enough backpressure [and isolation] so that what you do after the last cat is immaterial to the engine as to scavanging and reversion during overlap.............you must then just decide how much sound you want as long as you keep the exhaust cross over [H] pipe in the same location.

Sure a properly designed loud exhaust will allow lower back pressure for the system but not as much as no exhaust at all.......turn out pipes under the doors.....we drive Q on the lot with the exhaust system off.......just the cats exposed to the air........NASCAR SOUND..........it still won't spin the tires anymore.

[jamesmost]

OK awsome info, my question is thisi had a cat back system from stillen, removed the lower cats, removed the resinators, my rear can is shot on the seam, thinking about patching it, from what u said my rear can is insignificant, thus i can put any muffler with the proper diameter openings??

[AZhitman]

Probably a Ultraflow SS unit. They make a dual-in / dual-out unit that will fir in the stock location, good flow and last forever (stainless steel). I think the part # is 17515 (check website).

James, tell me more about your system - It used no "resonators" (mufflers under the car) and was 2.5" from the headers back? Was it loud? Did you notice a difference in driveability / performance? No lower cats? You must live somewhere where that's legal...

[jamesmost]

my mistake i removed the mufflers under the car as well as the 2 lower cats rippin loud sounds like a worked chevy 350 , my friend was upset that my car sounded better than his '96 trans am pace carshe really opens up around 40-50 mph and higher, i also have a k&n and e-prom re-program , my "stainless"- ultra flow rotted at a seam