There have been a lot of threads recently about turbo exhaust and downpipe sizing and stuff. I was under the impression that even with a turbo there is a point that an exhaust can be too big. There were some people who indirectly backed me up in different threads, but a lot people said I was stupid and needed to learn some more. Since I know that there are people on here that have waaaaay more experience than me, that's exctly what I did. I went a re-read some of the parts in "Maximum Boost" and here is what I found.

Direct quote from "Maximum Boost" by Corky Bell. Chapter 5 Intercooling. Pages 61 & 62.
"TUBE SIZES AND SHAPES. There is probably a magic number that airflow velocity in a tube should not exceed, for reasons of rapidly increasing drag and consequent flow losses. I suspect this number is around Mach .4, or about 450 feet per second, since drag, and therefore flow loss, increases significantly after this. Tube size can easily be checked by calculating the maximum airflow atainable, dividing by the area of the tube in square feet, and dividing again by 60 to convert to feet per second. An approximate value for maximum airflow can be obtained by multiplying the desired bhp by 1.5. (Then he does some calculations that I don't know how to recreate on my computer) Thus, the 2.5-inch-diameter tube will to flow 600cfm without unreasonable drag."

Chapter 11: Exhaust Systems Pages 132 & 133
BASIC TUBE SIZE. It is easy to get overeager on fitting large-diameter pipes into an exhaust system. "The larger the better" is not the case. As indicated in Chapter 5, there is an exhaust gas velocity that ought not to be exceeded. I am going to suggest that for exhaust calculations, this velocity is approximately 250/ftsec. The considerable expansion of exhaust gas due to the temperature increase alse requires a significant increase in the desired volume of the tailpipe. The tubes for the hot gas on the exhaust side should therefore be larger than the tubes for the cooler intake side. Base the caculations on the same conditions as for intake tube, but use a maximum velocity of 250 ft/sec rather than 450 ft/sec. To size a tailpipe, you can adhere to this exhaust gas velocity or to the simple guidline of selecting a tube diameter approximately 10% larger than the turbine outlet diameter. "

Chapter 15: Trouble Page 176

"Inspecting the turbo system for malfunctioning: TAILPIPE. The tailpipe can frequently cause an overboost problem. Often, the wastegate depends on an increment of back pressure in the tailpipe to function properly. This is particularly true with integral wastegates. The problem can be further aggravated by the OEM's tendency to use smaller-than-reasonable turbos. These factors can commbine to cause overboost when something in the pipe fails and reduces back pressure. Wouldn't it be fun to have a rust hole in the muffler of your expensive turbo car cause an over boost problem that leads to engine failure? No wonder Yankees park thier performance cars in the winter. It could be argued that some ought to park them regardless."

What is your guys' take on this info? There is some nice solid evidence that the saying "bigger is better (even on a turbo car)" is only true to a point. This is what I was trying to say in my other post if anyone remembers (or cares). Anyway, sorry if there are any typos, I tried to proof read as well as I could but it is almost 5:00am. Let me know what you think.



[Modified by Stu, 11:47 AM 2/14/2003]