Basically all these test have been done to some degree…. If the wastegate gas is brought back in properly there is absolutely no difference in power going to open air. We have tested this 4 times in the past…… They were all Greddy TD05 and TD06 set up with external wastegates. We tested them venting to open air and bringing them back into exhaust where they were directed back in with the flow of the exhaust and all the dyno tests were within a couple HP either direction going open or directed back in properly. You do want velocity, but you do not want ANY BACKPRESSURE with a turbo. Because the exhaust is still expanding coming out of the turbo, It needs more room as it comes out to keep from causing backpressure. Too large of a downpipe right away creates turbulance causing backpressure. Growing the pipe as the exhaust grows coming out of the turbo keeps from causing backpressure and because the hot exhaust is looking for cooler air, creating a lower pressure area drives the turbine even harder and ultimately increases velocity! The 2.5” downpipes have 6PSI backpressure in the downpipe at 15PSI boost with stock turbos. The 3” expansion downpipes have ZERO backpressure at 24 PSI even with sport 700 turbos. Think of a kids pinwheel.....If you blow on it it spins easy....then stick your hand behind the pinwhell and blow and it will spin much slower...... This is oversimplifed because tempurature and length of the exhaust come into play also , but it offers the general idea of the turbo spinning slower with any restriction. It is a balancing act to get the highest velocity without backpressure. The wastegate gas is only a issue with a open downpipe or if you bring the wastegate gas back into the side of the pipe rather than bringing it back in going the direction of the exhaust flow. The turbulence created in the open downpipe is crazy and is why turbo manufactures are making dividers built into turbos in newer designs…..It drastically improves the efficiency of the turbo with just a divider. Continuing with the divide and bringing back in after the turbulent exhaust coming from the turbine wheel has established good flow increases the efficiency even more. We had done tests on my car and Seb’s car with going back to 2.5” cat back and full 3” with the 3” downpipes and test pipes. Both of our cars had 700 turbos at the time and both showed gains on the top end, but nothing to write home about. The cars did feel better all the way through on the streets. The Huge gains were in the low end and mid range with going to the 3” expansion over the 2.5” super splits. We have done many stock turbo cars with all the crazy gains on every car in the low end mid range. How much full 3 inch helps on stock turbos we have not tested we have had some folks do it, but they were not dyno tested. What is interesting is the top end always feels better with full 3 inch. It could be the sound that offers that feel ? We have also tested exhaust cut outs vs. going through a good flowing cat back with ZERO change on two different cars. Hope that helps! Greg
