I'm changing my mufflers to the Magnaflows and need to know what size piping to run from the Y-collector to the mufflers with no resonators. I plan on going turbo in about a year so I don't wanna have to change the rear piping again. Should I use 2.25 or 2.5 for an SC300.

 

what i'm going to do on my first gen gs300(same engine) is run 3 inch on the Y pipe and 2.5 inch with no resonators down to my magnaflow's

 

3 inch if you're going turbo

 

I would think that 3" duals from a 3" mid pipe would be a little much but I could be wrong. How would low torque be affected with dual 3"?

 

If not, stay at 2.5

 

Ditto. I have 2.5 and they are much better than the stock 2.25.

 

Do not be fooled with the myth that surounds larger piping diameter. If you are adding a turbo, use the largest you can use. There will be no loss, only gains. Turbos do not need any back pressure.

 

Correct. Backpressure is not needed. The reason is that the increased flow helps the turbo spool quicker and at a lower RPM--increasing torque. Low end torque on my vehicle increased dramatically just from installing a larger 3" mid pipe, and the spool point went from around 2200rpm down to 1800rpm. It now puts out measurable boost at 1800rpm. 3" on the mid pipe is as small as you'd want to go if you're planning to turbocharge. If I did it again, I'd do 4" to dual 3"

 

Thanks guys I guess I will go for 3" duals.

 

Just installed hybrid steel turbo's to my 2.5 TT, we had uncontrollable runaway boost until we fitted a 2" restricter in the 2.5" down pipe to add some back pressure, go figure?

 

what would be the best diameter size for an sc400? thanks

 

London Bill...
Why did you do that?
I've always heard that boost creep was a result of inadequate wastegate, improper a/r, etc. Sounds like you're leaving hp on the table by causing an artificial restriction in the dp.

 

Like I said, I like the 2.5-inch the best, but I want to give 3-inchers a try. The car looks veeery aggressive with fat pipes running down the back. Backpressure loss effecting performance? I haven't felt it.

 

Quote from SC300T;

London Bill...
Why did you do that?
I've always heard that boost creep was a result of inadequate wastegate, improper a/r, etc. Sounds like you're leaving hp on the table by causing an artificial restriction in the dp. End quote.

This was not boost creep as in too small on the wastegates, it was total uncontrollable full boost (the hybrids should run 28/30psi and headed that way until the FCD cut in at 22psi), although the wastgates and actuators where operating correctly , we ran it up on a dyno to check but the boost just shot off the gauge with the wastegates fully open.

I had several tuning guys and the turbo designers investigating the problem, another pair of these CT12A hybrids had been fitted to a 1JZ Supra with no boost problems and they ran 12.5psi standard the major difference being his stock exhaust v my straight through low back pressure system. We decided to introduce some back pressure as a test hence the 2" reducer in the downpipe, exhaust back pressure is a wierd thing as there is an ideal level for any engine set-up.

My concern is that the restriction will rob power in other areas, our resident professor on the LSOC said if the restriction is short and within 20" of the turbos (it is), then the mid/high range torque will be improved, I am concerned about spool rates/turbo lag with the restriction in place, it is going back on the dyno next week so we can check on fuelling then we can see what the boost curve is like, I would hate to have to remove them as they are a pig but if bigger wastegates are needed they will be going back to the builders.