Intake & exhaust modification 1

[snobal]

Engine:
Stock Honda 900cc motorcycle
4 overhead camshafts
4 valves per cylinder
4 carbs
4 into 4 exhaust
I am told any mods to the intake or exhaust on this engine will cause major problems. I would like to put 4 into 2 or 4 into 1 exhaust on this engine but do not want to re-jet the carbs. Does anyone have any thoughts or experience on this. Thanks

 

[mcgyvr]

Why don't you want to re-jet the carbs? Increasing airflow through the engine will typically require rejetting to avoid a lean condition.

 

[patprimmer]

Any engine when modified in any way that has an effect on pulsing in the manifolds or a change in piston crown shape or compression ratio can impact on air:fuel ratio at certain speeds and on where the power band is. If one component moves the power band, you may need to optimise other components to the new power band to optimise performance.

This is not restricted at all to your engine, but the more finely an engine hs been optimised to an application and the vehicle optimised to that engine (gear ratios etc) more lkely you are to get a decrease in performance rather than an increase

Regards
Pat
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[snobal]

Thanks for the replies. I thought that was the case. As far as re-jetting the carbs, I wouldn't know what size to buy if I could even find them. Also, removing the carbs looks to be a complicated procedure with all the cables, etc. I guess I will just leave them as they are. The engineers at Honda know more about this stuff than I do. I don't want to burn up the valves or something like that. The mufflers on this thing are big and heavy and I thought it would be nice to have something smaller and lighter. Well, at least they lower the center of gravity a little. Mike

 

[patprimmer]

If you change the exhaust you may go either rich or lean on the carby or even stay the same. I would go up by about 10% on the cross sectional area of the jet to be safe, then test then adjust as necessary to avoid risk of burning valves.

On most carbies you can access the jets by removing the top of the float bowel with the carby in place.

Regards
Pat
See FAQ731-376 for tips on use of eng-tips by professional engineers &

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for site rules

 

[snobal]

Thanks for the help. This bike has 4 carbs with the float bowls on the bottom so they have to be removed for access. I have a problem with loosing tiny parts so I think I will leave well enough alone. Mike

 

[bentwings2]

We have a 954 on a sand buggy. It is the fuel injection motor. I had to redo the exhaust to fit the buggy chassis. I really wanted to get rid of whatever the rotary valve is called but I needed it for a spacer so we just left it there and wired it open. I split the pipes to get a dual exhaust but maintained the 2 into 1 relationship. They are a bit longer especially in the 1 length. Then I made a pair of aluminum straight thru mufflers with a glass packing. these are bolt together so the packing and cores can be replaced.

The motor is pretty quiet at idle and cruise but barks pretty loud over 6000. The power band is good up to 6000 but just explodes after that all the way up to 11,500 which is as high as we've taken it. We did have the entire exhaust system except the mufflers ceramic coated which reduced the temp around the headers significantly.

We are happy with it and the system stays together.