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Subversive
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Discussion Starter #1 (Edited)
Hey folks!

My bike (1981 CB900C) is soon to go under the wrench for some motor work...

To make a long question shorter, I was looking around at some performance parts and I found some valves that come with the stems machined to allow better flow..

These aren't cheap but would still be an easy swap for the stock valves.. My question is how much, if any, would this kind of change make in the overall power output of the engine, assuming all else was stock?

Also are there any other changes that would make the above change better without costing a ton of money or any other mods/parts that might be lowish cost? I would like to see perhaps a 15-20 HP gain or so..

Thanks for any thoughts...
 

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Gone
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23,907 Posts
My guess is that the performance change would be negligible by changing the shape of the valve stem. The valve is still opening the same amount, and the piston is still applying the same amount of vacuum to pull in the fuel mixture.

You could likely get a small increase in horsepower on the top end of the RPM range by changing the exhaust system and the carb jetting to allow more fuel flow at higher RPM. Stronger valve springs could allow a higher red line.

I'm not sure if all of this would get near 15-20 horsepower, however. It may shave a few hundredths of a second off the 1/4 mile time in a drag race.
 

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Save them all!
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My guess is without making other changes to the head or cam you're only going to see a slight improvement. Usually having your valves machined is done along with porting and cam work, and that total package might give you the 15-20 gain you're looking for.
 

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Retired twice: Navy and as a govt contractor
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^^Yea but you are talking percentage not HP. 20% of 55 HP is 11 HP and that will not be reached with just a ported and polished head.

HP cost money
 

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Subversive
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416 Posts
Discussion Starter #5
Yeah I think you guys just saved me some bucks...

It would be cheaper to buy a faster bike...


The cams they sell are like a grand I think and to do more work, well would cost even more...

Critter where did you get 55HP? Just wondering... :/

Thanks guys....
 

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Retired twice: Navy and as a govt contractor
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Pulled it out of my @$$ as a point of reference for the example
 

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ZAMM Fanatic
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The most you could hope to do by machining the valve stems to a smaller diameter would be to increase oil consumption and decrease internal resistance by perhaps 2%. Don't waste your time. If you already have your valves out polish the stems with emery cloth and call it good.
 

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Save them all!
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Yeah I think you guys just saved me some bucks...

It would be cheaper to buy a faster bike...
There is so much truth in that statement...

"Speed costs money. How fast can you afford to go?"
 

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Subversive
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416 Posts
Discussion Starter #11 (Edited)
The most you could hope to do by machining the valve stems to a smaller diameter would be to increase oil consumption and decrease internal resistance by perhaps 2%. Don't waste your time. If you already have your valves out polish the stems with emery cloth and call it good.
Well as I understand it.. (these are AM valves) The top of the stem is made slightly more narrow (smaller dia) to allow better flow, same as in pneumatics with stem mods... The part of the stem where the seal rides is the stock 5.5mm dia AFAIK...
 

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Subversive
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416 Posts
Discussion Starter #12
My guess is without making other changes to the head or cam you're only going to see a slight improvement. Usually having your valves machined is done along with porting and cam work, and that total package might give you the 15-20 gain you're looking for.
Yeah I guess it would require the whole cam thing and some head work... Might as well just buy a new bike. It would be fun to see how the performance changed with a low torque cam but alas they want big bucks for these parts...

In the old days when I got a cam, intake and carb for my chevy the parts cost next to nothing compared to the parts they sell for my (old) bike now...sheesh...

Stock it is!
 

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Subversive
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416 Posts
Discussion Starter #13
You could likely get a small increase in horsepower on the top end of the RPM range by changing the exhaust system and the carb jetting to allow more fuel flow at higher RPM.
Which really isn't what I want anyway...

This engine stock starts making power over 5K... And the gearing isn't right for the engine IMO...

They should have made this bike with a 6 speed tranny and a lower final drive ratio... It's a little too high and takes forever to get through first gear..

This company sells a cam with much lower rpm peak power, which I think would be a good fit for this bike... but they want some insane amount of money for it ....

If I could get an alternate (lower) Lo gear for the secondary transmission that might work as well..
 

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ZAMM Fanatic
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The state of the art in valve grinding today is what's called a 3-angle grind. You can find pictures online. Back in the day one "lapped" valves into their seats with lapping compound (abrasive) and a suction cup tool.

Today valves and valve seats are all cut by specialized machine tools.

There's plenty out there on the Internet about it.
 

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Subversive
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416 Posts
Discussion Starter #15 (Edited)
The state of the art in valve grinding today is what's called a 3-angle grind. You can find pictures online. Back in the day one "lapped" valves into their seats with lapping compound (abrasive) and a suction cup tool.

Today valves and valve seats are all cut by specialized machine tools.

There's plenty out there on the Internet about it.
AFAIK lapping is exactly what this gurl is about to get... LOL sounds kinky...

I'm probably going with the stock valves because of costs. If the machined valves would have made a real difference I might have went for them but no bang for the buck there at $23 per valve x 16 valves... And the cams are out of this world in price.. I'll just enjoy the freakish power combo they came up with for this bike....(she loves the highway) minus the oil leaks and failing valve seals...
 
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