| Message |
Boy this is one subject that comes up a lot,, lol. Put in exhaust piping or so in search and you could read for days. Now saying this, you will see some have 3" exhaust on stock turbo's ( like myself ). I have posted my Dyno sheets in data form to show they didn't hurt my cars performance at all. But given that I've read a lot on here and through other racing sources. The exhaust system on a turbo car is based on the Max CFM ( flow ) of the turbo ( turbo's x 2 pre ea. side ). Given that a stock turbo CFM is 304 ( x 2 = 608 ) and the displacement of the motor. I've always understood that you want to keep the exhaust gases moving, even once you get them out of the cylinder. One thing that will cause the gas to slow down is a sudden decrease in temperature, which is one reason why auto makers don't try to cool the exhaust manifold. The other thing that can cause a decrease in flow velocity is rapid expansion and turbulence, like going from a small passage (like the exhaust port in the head) to a 6-inch pipe. A smaller-diameter pipe geometry will tend to keep the flow rate up, but it will also lose heat more quickly (less exhaust gas per linear inch of pipe). However, a large pipe will slow the velocity due to expansion. Worse still, the exhaust is constantly cooling from the moment it leaves the cylinder, meaning it's getting denser and slower. See the problem here? It's all about compromises. The proper pipe size is going to be influenced by the flow rate (volume rate, which is related to RPM and engine displacement), exhaust velocity (again related to RPM), exhaust temperature and undoubtedly an array of other factors. All of this is dependent on the application: is this a street car, a race car, or something in between? Where will the engine spend most of its time? Idle, full throttle, part throttle? I've also heard the thing about back pressure, but I believe that they're really talking about flow velocities. That is, if you can keep the exhaust gases moving in the exhaust pipe, they will cause a small reduced-pressure area behind the closed exhaust valve, in the exhaust. This happens because the gases have momentum. They move away from the valve, creating a localized reduction in pressure. When the exhaust valve opens, this reduced-pressure zone will help evacuate the burnt gases from the cylinder. Or so the theory goes, as I understand it (two disclaimers there). The bottom line is that Turbo manufactures have probably already considered all this and other factors when sizing the exhaust to the turbo's CFM rate. Unless you're significantly increasing the flow rate, you're likely to cause more harm than good by throwing on a pipe to big for the turbo. I like many was planning on the 3" for down the road Turbo's and that's why I have them on the car. Yet seen and proven with my own dyno's. The cars don't preform any less with 2.5 or 3" exhaust systems. My car as it sits, pulled a corrected 400.1RWTQ at 3600rpm's on all stock crap ( turbo's, injectors, IC's, timing, cooling system etc. ) That's a 4th gear pull at 2000rpm. So from 2000 to 3600rpms ( 1600rpms ) it pull that 400ft lbs all in 3 seconds. See pic's and little story. [ http://www.twinturbo.net/net/viewmsg.aspx?forum=technical&msg_id=830908 ]
 STAGE V++++
Going to XV+ and beyond :) Yellow Z STAGE II
Going to XI+ |
 |
