Exhaust back pressure By royldoc (Read 2,899 times)

[royldoc]

I have been building small engines of various types and horse power for many years now. This whole time I was taught an engine needs to breath to create more horse power. This is why on race motors they use bigger valves larger headers and no mufflers. So why is it many people say a 4 stroke scooter engine needs back pressure to run properly? I'm just trying to figure some reasoning behind this theory.

Roy

[jct842]

there is at least some truth to needing some back pressure. no header and valves can be burned. the tuned headers on a dragster are critical to extracting max hp. I would like to see where some one with a repeatable dyno or engine brake do tests with variable length and diameter head pipes on a gy6. I expect a 4 stroke to exhibit some of the same things a 2 stroke wants as far as a tuned exhaust. when the pipe is the right length there is negative pressure that actually sucks exhaust out of the engine. an engineer could explain the exact reasons for this. what i do understand is it has to do with a reflection of the burst of exhaust gases. john

[Bashan]

I know a guy pretty well that used to work at the Harley R & D facility in Talladega, AL while they were still open. He had been everything from a test rider to working in engine development. I asked him about taking the cat converter off of my Vulcan and he told me to leave it. He said that all engines big and small are designed for a certain amount of back pressure, and by certain he meant a specific value. I can see why you wouldn't want the air/fuel mixture storming through just as fast as possible all the time, a lot of it wouldn't get burnt. After struggling to get a performance exhaust dialed in once I decided to go back to the stock. I gained a little top end but it ran like crap in the mid-range no matter what I did. I figured that 150 wanted that exhaust and I was done arguing with it. Rich