» 2.5" vs. 3.0" diameter piping on a 3SGTE (MR2)
2.5" VS. 3.0" Piping
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In Corky Bell's book, "Maximum Boost" it is stated that 2.5" exhaust doesn't become a restriction until somewhere between 450 and 500 horsepower (Chart above taken from "Maximum Boost" by Corky Bell). Unwilling to simply accept that at face value, we have been conducting our own testing, to determine where 3 inch diameter pipe becomes an advantage on the 3S-GTE, found in the MR2 Turbo (SW20), and Celica All Trac (ST165, ST185 & ST205). We have found that point to be approximately 300 rear wheel horsepower, which is substantially lower than Corky would have us believe. At 300 rear wheel horsepower, we were able to make the same RWHP at one less PSI of boost, and reach 200 foot-pounds of rear wheel torque 300 RPM sooner by upgrading from 2.5" to 3" down pipe and exhaust. There does not appear to be a significant disadvantage to running a 3" system on a lower horsepower application, but there also has not been any real advantage shown with properly conducted back to back dyno testing either.
2.5" VS. 3.0" Dyno Graph
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Neither the Toyota CT-26 nor the CT-20b turbochargers can flow enough to achieve those power levels. The exit diameter of the ct26 and ct20b are about 2.4" -- this means that running 3" piping can cause more flow loss right at the exit of the turbine due to turbulent 'eddy' effects.
3" pipe would require larger bends along with the larger pipe, and clearance is an issue without resorting to even more harsh angle joints in the downpipe or 'cheated' cuts, or tight radius bends that are not as ideal for flow. Fastener access where the down pipe secures to the turbine is very difficult with a 3 inch down pipe and significantly less so with our 2.5" design. Many popular aftermarket exhausts are 2.5 inches where they connect to the downpipe making a better transition from our downpipe to the exhaust. A disadvantage of having a 3 inch downpipe going into a 2.5 inch exhaust inlet is the 'step' transition to the smaller pipe would created by the dissimilar diameters. This causes a significant restriction to the flow of exhaust gases. These are just SOME of the reasons why KORacing's CT26/CT20b downpipe utilizes 2.5" mandrel bends.
Based on our testing results, we now exclusively offer a 3 inch down pipe on our Street Brawler turbo kits, and we have released a new version of the TKO exhaust, which is a full 3 inches from where the b-pipe mates to the down pipe to where the exhaust splits into dual 2.5 inch pipes. We will continue to offer the 2.5 inch CT26/20b down pipe and 2.5 inch exhaust for those customers who don't plan to make more than 300RWHP.
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