I went from a custom 2.5" exhaust to a custom 3" exhaust. Peak backpressure measured at the top of the downpipe dropped from 10 psi to 5 psi. This is at 15 psi of boost.

I plotted 3 things: exhaust backpressure, TIP (turbine inlet pressure), and the difference (pressure across turbine).

Interesting that at 5500 RPM and below, reduction in TIP was more than the reduction in backpressure. Win-win for flow. I'm guessing that the resulting increased flow at 6000 RPM and up (peak power) is pushing the turbo a bit past its peak efficiency region ... it's bottlenecking so you don't see that same effect.

Also, at 4000 RPM, the turbo is almost operating in the mythical "crossover", where MAP > TIP. It would be up to 5000 RPM, if I had a zero backpressure exhaust.

 

Great information.

I have been thinking of performing a similar test to evaluate the effects of TIP + valve overlap on intake charge dilution. I am curious what kind of sensors you are using, and whether you have performed this test at various intake manifold kPa? (What is your TIP at part load and no load?)

 

JasonC SBB's contribution to the site: Technical data on backpressure reductions with larger exhaust

Hustler's contribution to the site: A technical assessment on how much booze can he drink before experiencing a reduction in bowel control faculties

 

Why type of manifold are you using?

 

I posted info on measuring TIP before. One can use a 3-bar MAP sensor and datalog that but I used a hardware store gauge and a video camera.

I've done this with my GT2554 at 10 psi and with my GT2560 at 10 psi.

More later.

 

I wonder if a 3" cutout on a high flowing downpipe would be just as effective.
I plan to do this on my next turbo build. Assuming I find one that actually seals that is