Hi, I have a xv1000 self made bobber. I want to raise compression and add other cams (megacycle 258x5) and other ignition. I already have dynojet hitachi carbs, kn filter and self made exhaust with reverse cone mufflers.
Most use xv750 heads but these have smaller valves.
Do these smaller valves not limit power in the 6000 - 7000 rpm range a lot? Or is a 3 or 5 angle job on smaller valves (43 mm inlet) just as good as the normal valves (47 mm)? Or is it possible to put the larger valves in the 750 head without too much trouble?

I want to raise hp by raising the torque without shifting the powerband too much (so not lower the max hp rpm).
Also what about xv700 heads, are they smaller, to raise even more compression?

Thanks

Hi Richard,

judging by your claim, that you're mostly after torque, I'd dare to say that the smaller valves will help you to get some additional grunt down low. Judging by the valve sizes, my XT500 and SR500 are in between the XV750 and XV1000, when it comes to valve size and there's not the slightest hint of breathing problems.

I didn't find a curve for the megacycle cam compared to stock, but make sure, you don't go for a too hot cam, otherwise you will annihilate all the gains you may get from the higher compression and smaller valves. A bit more lift and a tad more duration won't do harm though.

Cheers,
Greg

Thank you,

I also thought that smaller valves give more low grunt. However I am not only after torque but after torque and hp. As hp is torque with rpm, if I raise my torque (also in thr 6500 rpm) I will also raise my hp.

My problem is, larger displacements need larger valves. If I get smaller valves will I also lower my max hp rpm. The xv1100 has a larger displacement, but the same valves as the xv1000, wich gives it smaller valves per displacement. And the xv1100 hp is at 6000 instead of 6500.

I would like to keep my hp in the 6500 - 7000 rpm range. Is that possible with the xv750 head? And how?

What do you want with a bobber over 6000 rpm, better to take the next gear.......you have five.
S.

Well with my project I did not wanted a typical bobber that is more a cruiser with apehangers, straight pipe, hardtail, no handling etc. In my vision, I want a motorcycle more based on the race / dirt bikes of america in the 50's / 60's with also light weight, performance and handling. Simular to cafe racers but than different style, maybe call it a "bob racer". So just like somebody want to improve the performance on a cafe racer, I am seeking after that to, prefferable in all the rpm range. Offcourse this is only my oppinion, but this is how I feel about it. That is also why I don't like a harley for my project, because I wanted something that revs higher, but is still torquey.

I am curious about what other people thought or did about the smaller valves when they did a 750 (or 700) head swap. Do you replace the valves? Do you just do a valve job and maybe a port job? Or are the smaller valves big enough to support power also in the higher rpm?
Bassically I'm asking what are other people oppinions and experiences about this? This would realy help me figuring out what I want to do with my project during the holidays of my study

R.