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Is it worth smoothing the manifold?

Forums

Hi.

The carb manifold of my Dommie 99 is quite rough inside. I think it’s just the normal casting surface but it does feel rough. Is it worth smoothing the internal walls of the manifold? The bike is just going to be used on the road as a daily ride so I’m assuming there isn’t any point in giving myself more work (smoothing the casting). However, I just thought I’d ask in case the general opinion is that it would be beneficial.

Thanks

Regards

Tony

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Tony. You could go in to the deep darks world of advanced maths to calculate the fuel/air flow characteristic through the manifold or, make a Perspex model to view where all the flow disturbance of the fuel/air mix over the surface etc. Alternatively you can spend ages polishing the bores and smoothing/re-profiling just to get exactly what?

Personally I wouldn't bother as the benefits will be at best unnoticeable for normal daily use.

These performance enhancements are only any real benefit in the heady world of F1 when every bit of engine has been manufactured from high performance materials to micron level accuracy unlike the engine of your venerable Domie 99.

John

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As John said,very little, if any improvement in power will be achieved.However carefull balancing of tract and chamber volumes will allow a very even and slow tickover  to be realised (providing the same care is exercised on ignition timing,  carburation , valves etc.) I.did it for the fun of it.But then I have too much spare time on my hands.

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If it is raining or there is snow on the ground why not go for the smoothing.

Nothing to lose there.

Mike

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Years ago, when  former NOC president the late John Hudson was preparing an engine for a rebuild demonstration, someone asked him why he was polishing the crank flywheel all over.

"Well", said John..."Now it looks as if it belongs to someone".

Pride of ownership and doing the best job that we can must play a part. Isn't there a theory though that inlet ports should be smooth but not polished for best gas flow ?

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The inlet tracts are small diameter and so the boundary layer has a significant effect on flow. A bit of roughness will energise the boundary layer (put a bit of turbulence there) and detach it from the tract wall, improving flow. Polish the inlet tract and you could end up going slower!

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Thanks everyone. I have plenty of other things to waste my time on so the manifold will stay ‘original’. Although I am a bit disappointed as removing all that weight with my Dremel would definitely have made the bike go faster eh?  :-)

Regards

Tony

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I only own a single so I have no business to be in this thread but I can't help thinking that if I lost 10kg it would improve performance more than any polishing. Sadly, I can't be bothered to do either..

 

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