Mr_Q

iTrader: (1)

Anyone know of the vendor? I saw on the SpikeTV shows "trucks" a powered exhaust redirect. After the header it directs flow using a ball valve to side pipes with the flick of a switch. Bypassing the cat and muffler.

Now that would be bad ***. But I didn't hear who made this kit.

drdisco

I remember seeing a post about this a while ago. It was installed on a camaro though. One side directed the exhaust into the factory pipes, and the other side dumped out into a straight pipe. Not sure how legal it is and nobody could really confirm that it would work well on a Z. The general concensus, if I remember right, was that the Z needs a bit of backpressure from the exhaust.

drdisco

Did some research. Summit has a cable operated one. Part number mrg-5424.

Here is an electric one.

QTP

Mr_Q

iTrader: (1)

Dude, rock on and thanks!

HyperSprite

iTrader: (6)

Eeeeeeek, it's the back pressure myth again!


but I digress


Mr_Q cutouts are in fact illegal for use in California (just so you know). I think they are cool!

Chris

Mr_Q

iTrader: (1)

Yah, but if you only use them at the strip or the ocassional stop light...heh.

As far as passing visual...I have a couple of nice places to go fo that.

drdisco

Yeah, the backpressure thing doesn't make much sense to me either but I was just regurdgitating what I had heard previously.

I am pretty sure they are illegal around me too, but the half-wits at the state emissions inspection stations around here would probably never even notice. (They once tried to put front wheels of my rwd t-bird on the barrels to do the "rolling" test)

wren57

iTrader: (34)

Backpressure is needed on an NA engine. I ran without a y-pipe for about 10 minutes and felt a CONSIDERABLE loss in power below 4k. Above 4k it felt about the same.

shopdog

Backpressure due to exhaust restriction is *never* a good thing.

*But* you do want the exhaust system to be *tuned* such that it helps extract (scavenge) exhaust gases and prevents reversion. The wrong length can pull raw fuel through the engine during the time both the intake and exhaust valves are open (overlap period), or cause fuel to backflow into the intake (reversion). Both are bad, affecting both power and economy.

The length and diameter of the exhaust system tunes it by supporting standing waves over a particular frequency range (RPM range). There are two parts to this tuning. The header tube length to the collector is one. This is most important at high RPMs. The total length and diameter to the exhaust tip is the other. This basically affects the amount of low end torque the engine will produce.

There are equations engine designers use to determine these lengths and diameters for a particular engine. The engine displacement, cam profile, intake configuration, and *intended operating profile* all interact to determine the ideal exhaust system configuration. Deviating from that configuration can cause substantial loss of performance or economy over various parts of the RPM range.

Tinkering with the exhaust system can also cause our engines to run lean, which is very bad since our ECUs only have a limited range over which they can adjust mixture to prevent detonation. (In the old days, hotrodders would simply rejet the carburetor to compensate for exhaust system changes. We can't do that. We have to either get our ECUs reflashed, or add on a piggyback controller to correct the A/F ratio.)

A very good book which covers all this in detail is "Design and Tuning of Competition Engines" by Philip Smith. He devotes a couple of chapters to exhaust system design. It is critical to good engine performance. Nissan engineers did a good job of employing these design equations to give us an engine with good performance characteristics. Mucking about with the exhaust system without a good understanding of these design factors can yield a car with inferior performance. It is even possible to cause engine damage.