exhaust diameter 750cc

[Bjorn]

Hi,

Im doing some research for a new exhaust i want to build. I did a lot of searching on the forum for exhaust diameters, red a lot of topics. I particularly interested in the ideal exhaust diameter for a 750cc motor, of which I cannot find a clear answer.

I see that 1,5 inch OD (38mm) is mentioned alot in topics. Not sure if this applies to just the 650cc motor or to the 750cc too?

My application: 750cc road bike, not racing. Not ported, 34VM carbs, 277° with shell cam. Im looking for normal street-power, so the big diameter pipes are not ideal.

I got a second hand stock Triumph street scrambler exhaust I would like to use, I like the looks and the sound it makes on the triumph. And it should hopefully be a bit more quiet compared to my current exhaust. The triumph exhaust works with 38mm OD (1,5'') pipes.

I also noticed that modern bike have quite small diameter headers. For example my Yamaha MT07 makes 75 HP , 270 crank, 700cc and works with headers < 38mm OD.

 

[Signal]

There is no definitive answer to the right sized exhaust . I would use 38mm OD on a road 750cc XS in preference to larger diameter.
I also suggest you make a set of exhaust port optimisers. They are a bandaid for the XS but they do seem to work. I fitted a set to a bike with the same tune as yours and it did improve things.

 

[Bjorn]

Thanks for the reply, i understand without data its not an exact science.

I have been running 40mm pipes for the last years, that is with a stock 650 motor. Might have been to big.

I find the whole topic of exhaust technology very interesting but also kinda hard to graps. Same with sound/noise. You never really know how a pipe is going to sound

 

[Jan_P]

I remember reading About Harley Owners using straight pipes

straight pipes so the torque dies

Adding a rotating washer / disc at the end of the pipes .. Getting the option of adjusting the pressure and velocity also I suppose
getting better results.

Would you be interested taking on small fabrication jobs TIG welding and Machining.or so Should something come up
I can pay UP front if that is a preferred option .

 

[Grewth]

You could get in way deep in the theory and mathematics of this.
But unless you're building a competition bike and you need to wring every last ounce of power out of it, I think just trial and error is as good a method as any.
I remember that back in the sixties, Triumph development engineer Doug Hele coaxed the last couple of horsepower out of the Bonneville engine by reducing the diameter of the header pipes to speed up the gas flow.
Made it possible to tune the exhaust correctly.

 

[Charles13]

If you ride at a steady state throttle (cruise) a lot; smaller is better, or you will need optimizers to stop it from bucking around. The larger pipes are fine if you are on and off the gas (making noise)! around town.

 

[40north]

lollypops, snuffers, can be useful. "optomisers", little stainless nozzles that fit at the headpipe inlet, are highly good. I fitted both to my XS, with shorties the guts of which I fabricated, after hurting my head studying the matter. Snuffers can be rotated to reflect the supersonic part of the exhaust pulse, though I seldom bother to do that. No ticket no problem. Best of luck brother. I take it that your "1/5 inch diameter" refers to the OD. Some pipes are double wall... Obviously everything changes with the double wall, the heat loss, ie temperature of the gases, and the velocity characteristics, the Reynolds number, everything.. https://en.wikipedia.org/wiki/Reynolds_number It's not rocket science, it's far more complicated! But generally it doesn't matter, so long as it goes nice and you are happy.

 

[40north]

https://strikeengine.com/exhaust-pipe-size-calculator-horsepower-vs-diameter/

Interesting. Maybe of value, can not say.

 

[Mannyroad]

Hi,

Im doing some research for a new exhaust i want to build. I did a lot of searching on the forum for exhaust diameters, red a lot of topics. I particularly interested in the ideal exhaust diameter for a 750cc motor, of which I cannot find a clear answer.

I see that 1,5 inch OD (38mm) is mentioned alot in topics. Not sure if this applies to just the 650cc motor or to the 750cc too?

My application: 750cc road bike, not racing. Not ported, 34VM carbs, 277° with shell cam. Im looking for normal street-power, so the big diameter pipes are not ideal.

I got a second hand stock Triumph street scrambler exhaust I would like to use, I like the looks and the sound it makes on the triumph. And it should hopefully be a bit more quiet compared to my current exhaust. The triumph exhaust works with 38mm OD (1,5'') pipes.

I also noticed that modern bike have quite small diameter headers. For example my Yamaha MT07 makes 75 HP , 270 crank, 700cc and works with headers < 38mm OD.

View attachment 330310

I've been having the same discussions with a good friend of mine who has many years experience building and tuning Triumph 650/750 twins from road going machines right through to record holding lands peed record bikes. His bedtime reading is literature on tuning, exhaust design, oil specs etc (yes, seriously), and my plan was to pick his brains or get him to do the maths to determine the correct exhaust parameters for my new 750 big bore 277 rephased motor, running vm34s. First thing he pointed out to me was that I probably wouldn't like the answers as they would strongly support the use of 35mm headers, as opposed to 38mm ones that I'd prefer to use, for aesthetic reasons. Before I can move forward with looking into this I need to get accurate profiles for the cam, so not there yet. Be interesting to here what others suggest and what their suggestions are based on.

 

[gggGary]

Seems you know the answer 1 1/2 for the street, big pipes work at high RPM but low, mid range often suffer.
That said I have 1 3/4 pipes and port restrictors with "fairly restrictive" mufflers on madness, after a lot of tuning work it's fine for me.
Yeah it's about the look.

Norton 750/850 ran 1 3/8" head pipes.

Brit guys expound on headpipes; https://www.accessnorton.com/NortonCommando/exhaust-pipe-diameters.21267/

 

[JesseeS]

One of my projects for my bike was going to be attempting to make my own stepped headers. Looking at different sweeps and lengths currently

 

[zrx1100]

I have 1.75" pipes on my 700cc build and ended up using 1 3/8" TPOs (inserts) to get rid of a reversion issue while using commandoes. With open core mufflers my torque is excellent and peaks at 4K. I am thinking the 1.5" exhaust would be a great balance between great torque and horsepower. I do have a dip in torque around 2.5K rpm and I am convinced it is a reflection from the end of my headers where the 1 3/8" TPOs open up into 1 3/4" mufflers.

https://www.xs650.com/threads/700cc-kit-with-a-shell-1-cam-in-a-78xs650e.65405/

 

[Ratranger]

I'm running 1.5" at the head, stepping up to 1.625" after about 12", then it goes into the mufflers. Mufflers have a long cone to a straight 3.5" section with reverse cone. Feels a little sluggish under 3500, but pulls all the way to 8000. These pipes are in no way optimized. I looked at pictures of jemco pipes to guess measurements.

 

[dave_a]

I've been trying to convince my wife "smaller is better" for years. I'll have her read this thread.
Maybe those engineers who put those 1.25" headpipes on finally figured it out.
EXUP

 

[Bjorn]

I've been having the same discussions with a good friend of mine who has many years experience building and tuning Triumph 650/750 twins from road going machines right through to record holding lands peed record bikes. His bedtime reading is literature on tuning, exhaust design, oil specs etc (yes, seriously), and my plan was to pick his brains or get him to do the maths to determine the correct exhaust parameters for my new 750 big bore 277 rephased motor, running vm34s. First thing he pointed out to me was that I probably wouldn't like the answers as they would strongly support the use of 35mm headers, as opposed to 38mm ones that I'd prefer to use, for aesthetic reasons. Before I can move forward with looking into this I need to get accurate profiles for the cam, so not there yet. Be interesting to here what others suggest and what their suggestions are based on.

SOoo, any more explanation on his advice?

This project of mine is still in the works but on the "slow burner", just waiting till winter hits

 

[zrx1100]

A bit of an aside, whether it is exhaust or radio frequency circuit design, waves are waves: https://www.linkedin.com/posts/juli...content&utm_medium=g_dt_web&utm_campaign=copy

 

[Mannyroad]

SOoo, any more explanation on his advice?

This project of mine is still in the works but on the "slow burner", just waiting till winter hits

Sorry, not yet. Haven't sorted the cam for the motor yet, so can't really move forward, though may just sack off going down the calcs route and shove a set of 38mm slash cut headers on and sort out the tuning later. Need to get over to my mate's to discuss it further.