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Post by andy on Oct 5, 2007 13:27:33 GMT -5
hi to all,possibly thinking of a top end re-build this winter and i'm considering a cam swap,an area where i could do with some advice.i'm currently running a straight thro' exhaust and a big k&n filter,whilst i'm happy enough with the performance and top end speed i would like things to happen a bit lower in the rev range,it seems the bulk of the power comes in at 4,000 ish.not looking for extra work like stronger springs,head re-shaping etc.please forward plusses&minuses on this subject based on your own experiences.i suppose i'm looking for more grunt lowdown rather than topend speed cos i think much above 80 m.p.h becomes uncomfortable after a while and these bikes aint high speed tourers!
regards to all.
andy
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Post by milkman on Oct 5, 2007 14:23:15 GMT -5
maybe restricting your pipe a little will help make some of that grunt come in earlier. If its straight through as you say, it might flow too freely for what you're after
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Post by fenz on Oct 5, 2007 16:49:36 GMT -5
Yep a bit of back pressure should move the power more down low.
There is a lot of factors that come into play on this one e.g pipe carb etc.
If the bike has seen a few miles chances are the cam is worn any way and this will rob power from the entire rev range.
The last couple of cams i have removed from engines i have been setting up in the lathe and using a dial indicator measuring the lift.All cams showed a considerable amount of wear on the exhaust lobe (more evidence to do the oil line mod)
You think about a replacement cam being more lift and duration to give you more power a cam that is worn may be missing a couple of millimeters of lift witch will then screw with the valve duration.
All this will result in less power.
Most cam grinders will give you options for a cam that gives better low end tourque with out the use of springs.But bear in mind that the valve springs are most probably near 30 years old and dont have any wear near the seating pressure that they should have.
Hope i havent confussed you on the subject.Have a play with the exhaust/carb and see what difference you can make before opening the motor up as this ussually co-insides with opening your wallet up....
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Post by wizz954 on Oct 6, 2007 4:29:02 GMT -5
I agree with Fenz, all sorts of things you can do to increase grunt, depends on whether you want to do it with bolt on parts or engine work.
Without engine work, you can trade top end power for bottom end grunt by using a different carbie and pipe, usually a smaller bore carbie and smaller header pipe diameter. You can also retard ignition timing for more bottom end and less top end.
Engine work to increase power will usually increase grunt at the same time. Higher compression piston will improve grunt, but I wouldn't go above 10:1 for the road. If you decide to change the piston, you will need a re-bore as well, so you may as well go up a few oversizes to get a bigger capacity hence grunt.
Flowing the head is not necessary and this is expensive to get done. Aftermarket cams are usually designed to give more top end power, so I wouldn't consider that as part of your plans for bottom end power.
I'm sure you're really confused by now, but you'll learn a lot by the time you do the rebuild!
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