custom exhaust tuning on a 91 CB250

[cbdork190]

I'm replacing the stock, double wall, chrome plated, heavy ass exhaust on my nighthawk with a custom (home made) ss 1.25" system and Vw 1.5" chrome flared tips.
The id of the the stock piping is .900" and the id of the ss piping is 1.100". That is actually 50% larger than stock. With all this extra flow I figure some sort of tuning is in order.
This is where I need help. Is baffling an exact science? Will an h-pipe help? What about an h-pipe with a butterfly valve?

 

[paddlesat16]

you posted this topic in the wrong section...

Regards

 

[mydlyfkryzis]

Are there any other modifications? Intake, Bore, Stroke, carburetion?

There is tuning, but to really take any advantage of it, you need to have the engine on a dyno. Small differences, if you are looking for the absolute max, can make significant differences.

For instance, is the inside wall have a higher or lower mach number then the surface of the stock?

Twice the are is half the velocity. Too low a velocity made reduce scavenging, and cause a loss in performance. Also, are you tuning to high RPM, Low RPM, mid range? The intake and exhaust are generally tuned to a small, specific range of RPM. Racers keep the engine in that range as much as possible.

For street riding, usually the low to midrange RPM tuning gives the best response.

So yes, tuning the exhaust is important. The stock exhaust were usually tuned for street, fairly wide range of torque, emphasis on the bottom end where most street riding is done.

You can tune for peak HP, but have a nearly un-ridable for the street tune.

Duplicating the stock exhaust in size and length will probably give you the best driveability.

Since you appear to be going larger, you need to follow some general rules of thumb...but you need to have a goal for it.

If you ask me, and you did ask for opinions, the greater performance would come in the reduction of weight a custom exhaust would have. If you duplicate the stock path, size and length but reduce the weight by 10-12 pounds, that would actually be a noticeable increase in motorcycle performance.

Shortening the exhaust, increasing pipe size significantly, may improve some places, lower others.

 

[cbdork190]

i just replaced the stock 26mm cv carb with a mikuni 28mm round slide. i plan on porting the head and replacing the stock exhaust with little larger ss pipes. the stock pipes are .9" id double wall. and what im replacing it with are1.1" id stainlless. im crossed between going 2into1 or seperate pipes with a crossover pipe. losing weight is a major goal so 2into1 sounds good, but im not sure if thats the best idea since thats not what the honda engineers used. there is no aftermarket performance engine support for this bike so porting is as far as i can go modifying inside the motor, untill i find high compressiion pistions from something else. the carb upgrade changed alot. so exhaust pipes and porting are next.