how to raise max RPM's

by the way that 650 drag bike is a real cool little bike that would be a fun ride.

timebomb33 wrote:

a good freind of mine who by the way is an instructor at the same college as me has a gs 750 two valver 1978 i think that runs on a alcohol and has an out of the box mr.turbo system on it and i personally have seen 240 horse on a chassis dyno dam impressive little bike with a 650 set up on gas i wouldn't be suprised if you could between 180 and 200 horse with a turbo and still have a fun civilized ride most of the other time and with the boost contrls that are around now it could be totally streetable.

Had a friend in the late 70's that put an ATP turbo pack on a Stock 650,and had the fastest thing in town, if you do the turbo w/ an 810 you should be more than satisfied, if not, a lil shot of nitrous might help...

The 8 second(s) 650:

I'm still stuck on the 810 turbo that goes 211 mph....

That sounds a little fast for 810cc....

^ Look up Lorcan.

KruZer wrote:

I'm still stuck on the 810 turbo that goes 211 mph....

That sounds a little fast for 810cc....

heres a shot of the madman and his bike,hes got more balls than any 3 guys i know! :laugh:

turn up the speakers boys, sweet music comming.

The reason these older bikes don't make horsepower above 10k or so is because of the bore and stroke. The bore of older bikes was much smaller and the stroke much longer for the same displacement.

Additionally the two valve heads do not provide as much air flow capability as the 4 valve heads.

But, the main issue is bore and stroke. As someone pointed out, the rpm limit of most engines is a function of piston speed. Kevin Cameron stated some years ago that 4500 feet per minute of piston speed is a reasonable upper limit for most engines. Let us take a look at how this applies to our motorcycle engines with a specific example in which we compare a Kawasaki 1000 J motor with a late 80's / early 90's ZX10 motor:


Old tech:
J motor Kawasaki with 2 valves per cylinder
Displacement = 998 cc
bore = 69.4mm , stroke = 66 mm
piston feet per minute at redline (8500 rpm) = 3683 fpm

Newer tech:
1989 ZX10
Displacement = 997 cc
Bore = 74 mm, stroke = 58 mm
(I don't know the redline for this engine. Instead, let us solve the fpm equation for rpm rather than fpm. First, assume the same fpm as the KZ1000 J (3683 fpm) Now, solving for 3683 fpm with 58 mm stroke, the equation will yield 9692 rpm. What I am saying is that the ZX10 with it's 58 mm stroke can turn 9692 rpm before its pistons reach 3683 fpm.

This means that although the two engines have the same displacement, the ZX10 can spin to 9692 rpm and be under no more stress than the J motor at 8500 rpm.

Summary - An engine with larger bore and smaller stroke will have more rpm capability than one of the same displacement that has smaller bore and longer stroke.

Now, what else can be said about the larger bore motor? We already see that it has more rpm capability. But, since the bore is larger, there will be more room for larger valves in the head, since the head will have a larger combustion chamber diameter to go along with the larger bore. This is true whether the head is two valve or four valve.

This means that the larger bore motor can be fitted with larger valves thereby providing more valve area to provide air flow to feed the motor at the higher rpms that it is capable of turning. With more valve area to work with, the lift doesn't have to be extremely high to gain adequate flow to feed the motor. This means that the valves are not as stressed at higher rpm.

To help fill the larger bore cylinders the manufacturers switched to four valves per cylinder (Yamaha did 5 valves for a while). Four smaller valves will provide more air flow than two larger valves and will be more reliable at high rpm because they will weigh much less. You won't be dropping valves or breaking valve stems off the valve heads with the smaller, lighter valves - less stress.

More RPM and more air flow equals more horsepower potential.

That is why you can't build the old motor to make the same horsepower and turn the same rpm reliably as the new ones do.

There are probably many other factors including valve angles, intake tract shape and size etc, but the key to it all begins with the larger bore sizes.

These are not my opinions, but those of people more learned than I in the ways of internal combustion. I am just spouting back what I have read over the years - but it does make sense to me.

Hope it helps.

P.S. The formula for piston feet per minute follows. You have to supply the target rpm value such as 8500 rpm:

fpm = (stroke in inches X 2 X RPM / 12
or
fpm = (stroke in inches X RPM) / 6

to convert your bore from mm to inches:
stroke in inches = (stroke in mm) / 25.4


Kirk

bigger valves will only get you so far the way the kz style head is laid out you run out of room too easy.but thats also a good thing since big valve big cammed heads arent very street friendly in my book

kawsakiman wrote:

turn up the speakers boys, sweet music comming.

Take that potato heads!!!

PLUMMEN wrote:

if you really want to build a reliable 650 based motor thats capable of making 90/100hp when needed its very easy.start with the best possible bottem end you can build including those carillo rods,id still have those rods checked including magnafluxing and new bolts at minimum.if anything looks questionable in there change it! id go with a reasonable street compression set of 810 pistons and a gpz style head as Larry said,with a STREETABLE set of cams and some carbs in the area of 29mm or fuel injection if you can swing it.im guessing this should put you in the 75-80 hp area with a motor that can be ridden without having to constantly tweak things on it,once you get it running the way you want then you can put a small nitrous bottle on it witha 20hp shot for those moments you really feel the need for it i think youd be much happier with that combination than a peaky buzz bomber that makes its power from 8000-12000rpm.oh wait kawasaki made one of those,it was called the H2 750 triple 2 smoker! :woohoo:

If you spin a stock H2 to 12,000rpm it will likely come apart like a cheap suit. Those engines made great power from 2000 all the way up to about 8000rpm.

the warmed over episode i had seemed like it reved higher than that,i remember rings were seasonal at best! :laugh: