40 MM vs 38 MM Exhaust Headers

[jsey]

Hi All,
My 1978 R100RS came from the factory with with a 40 MM exhaust system. After all of these years it is time to change the exhaust headers. I am considering the the Keihan 40MM stainless offerings. Come to find out that these pipes have either end expanded to 40MM but the rest of the pipe is actually 38. My old headers are 40 MM all of the way for the entire length. The question I have will this necked down approach have any kind of noticeable effect on how the engine performs? Thanks

 

[20774]

I wouldn't know how to quantify it, but a reduced cross section most certainly changes how the engine performs. If the valves are putting through their normal amount of output and they meet a smaller header diamter, the air won't go out as easily and thus can "back up" or stagnate.

 

[PoorUB]

I am going the other way.

I don't think 2 MM will make a hill of beans difference on a stock engine. If you put the bike on dyno before and after you might record a difference.

Sometimes smaller tubes will produce more torque in the lower RPM range, at the loss of a couple HP at wide open throttle.

Where do you do most of your riding? Wide open throttle or 25% throttle.

Either way I wouldn't lose much sleep over it.

 

[stevenrankin]

Notice

Honestly unless you are flat out on a race track, you won't notice the difference. There has always been a big debate as to any advantage of 40mm exhaust versus 38. Some say you get a bit more power at the top end of the RPM range with the 40mm system. That would be great if most of the time you spend your riding there. I have ridden both 38mm system RS and 40mm RS bikes, and could never feel any difference.

Why are you changing the headers? Are they so badly rotted? You can have them rechromed for not a whole lot of money if it is only a finish issue with them. St.

 

[barryg]

Is this bike rated at 60 or 70 horse power?