Topic: Custom Exhaust Question CB750 SOHC

[86FJNJ]

It looks like the stock exhaust is 1.25" diameter (about 31mm). I want to build my own exhaust but can only seem to find 1.5" diameter (38mm) exhaust pipe that has some premade mandrel bends. I'm fine to piece each pipe together but don't want to do all pie cuts, so looking at pre bent pipe.

Does anyone have any experience if going with 1.5" diameter pipe for the exhaust headers would be a big negative impact on performance on a stock 736cc? I would likely do a 4-1 or 4-2-1 with the single collector being smaller, maybe around 1.75" (45mm) or 2" (50mm) so the smaller collector may help balance things out.

I'm sure I would need to increase the jet sizes and move the needle up to add more fuel for the more free flowing exhaust but interested what people think about larger headers on an otherwise stock CB750 engine. 1.25 to 1.5 is about a 20% increase which seems significant enough to ask the question.

Pic for attention.

[Don R]

 I recently measured a MAC 4-1 header, the primaries are 1.5' OD. it was on an 1124cc engine though. Others said they have used them on stock motors.
 Ebay seems to be loaded with 1.25" pipe and bends, both stainless and mild steel.

[Don R]

 Cone Engineering has 1 1/4, 1 3/8" and 1 1/2" mandrel fittings, I'd consider a step header increasing one size a few inches after the top bend. 
 I'm doing a 2 1/2" 304 stainless car exhaust system with Cast Iron headers and an X pipe.

 I have a CB750 spider turbo header that was re-welded off of the engine, it pulled like crazy. I had to make small pipe jacks to spread it apart to install it at the head. Hopefully heat will let it relax. So be sure to keep yours bolted to the head as long as possible.
 If you need pictures or measurements of any particular header, I have V&H, Mac,  Honda F1 and F2, RC Eng, Hooker 4-2 and a couple unknown brands. I always wondered why no one ever copied the CB400F header for a 750. It's either sexy or ugly depending who you ask.

[86FJNJ]

Thanks, I’ll poke around eBay more. I agree a stepped design would be nice hopefully keeping 1.25” for at least the first 6” of the header.