Hooking up Dutra Duals

The use case of what you want out of engine is needed as well. Where you connect those pipes together makes a difference. Back when I was working with Doug Dutra on a cam design I talked to him a lot about the Dutras and what he had experimented with. I was running simulation models to zero in on a cam design and I started putting in things he had experimented with so we could get a warm fuzzy the model was in the ball park. The first thing is where do you want the torque peak. The further back you put that "Y" connection the further down in RPM that peak goes. I put mine right behind the crossmember as the model (and driving it agrees) predicted it would be at around 3200 RPM which is where I wanted it as that is in the area of if I was driving along (with the 3.23s) and wanted to pass someone I wanted that extra kick right there. Doug built one that put the "Y" all the way back by the rear axle both the model and his experiment said it should pull hard at 1900 RPM and he verified that it did pull very hard right off the line.

That brings up point 2. Everyone jumps on the bigger the better for pipe diameter which in reality is not always true. If too large the velocity drops to the point where the effect you are trying to achieve (the interaction between the two exhaust pipes at the "Y") is lost. That said I did run a lot of simulations and found you need to get up to 3" or so before that happens with the Duals. That also brings in trying to get two 2" pipes threaded down through the engine compartment and body. It can be tighter than you think. Large exhaust is really more for flow that that really depends on RPM. For an engine that you just want for cruising torque who cares if the exhaust choke it a bit at 5000 RPM and up? I didn't so I used the model to determine the smallest size that did not hurt performance for something that is running like an RV-10 or Oregon Cams 2106 (what I used). I didn't see any difference in the band I cared about until you hit 1.5" so I settled on 1-5/8" down pipes to the "Y". They were very easy to get routed and offered plenty of room around them to work on things in the future. My slant puts puts a smile on everyones face that drives it. I have never proven that it performs like the model but the seat of the pants dyno agrees with where the power should be and where it should start to fall off at high RPM.

My 2 cents

Jim