What are you guys setting as your Rev Limit on the Big Bores? I am looking at setting mine at 10K, is that conservative?

What are you guys setting as your Rev Limit on the Big Bores? I am looking at setting mine at 10K, is that conservative?
Thats No Problem on a Balanced Assembly :thumb: !The Race Bikes Go Just a Little Higher then that! ;)

Doesnt it make a difference based on how well balanced your Crank is and whether it is a full circle crank or not?

My son works at the test facility and he told me this weekend that he was talking to an oldtimer that was there when the Rev was under test. He told him they ran at or above 15K rpm during testing. ;)

What are you guys setting as your Rev Limit on the Big Bores? I am looking at setting mine at 10K, is that conservative?
I thought the motor does not make more power on a big bore after about 9k???

I thought the motor does not make more power on a big bore after about 9k???
I would imagine that if the cams are changed it will..

If your new bigger pistons have the same weight as the stock pistons (i.e. design with shorter piston shirt) and you still have your stock piston rods installed then 10K is no problem.

If they are heavier and you still have the stock rods then I'd guess that balancing the crank is not an easy task.

Ideal solution would be to have a lighter rod matching the bigger piston, thus compensating the oscillating mass difference to the stock engine. What does the supplier recommend?

I suspect you will run out of air before you get to 10K rpm.

I have no problems reaching 10K at all (in 1st to 3rd gear, ok?) since I tuned the AFR right and gave plenty of spark advance.
That with stock bore size of course.
As long you don't go topless it will do 10K.

Hi Guys,
Balance is not really the high rpm problem it is piston speed when you increase the stroke the piston speed increases at the same RPM ,as speed increases weight must decrease to keep the same stress on moving parts, A 1/2" stroker motor turning 9500 rpm has more piston speed than a stock stroke at 11,000 rpm

Paul,

that's right of course. But Scott mentioned Big Bore, not stroker.
So piston speed should be the same.
What's your opinion on Big Bore balancing?

Mine is set at 9,400. It is my understanding that the higher you rev the engine - the greater the stress on the motor. So unless you are 1. a racer and 2 a racer with a sponsor or a racer with money to burn then why add extra stress to the motor. Actually - my view is that unless you are on a track you should never see North of 9,000 RPM as life is getting way to fast for an uncontroled environment like the street..... (I have run stok at 9,800 with shift light at 8875.... but currently running shift at 8,375 with rev limiter at 8,375) higher rev limit is for head room only. Of course I run a conservative set up - and have a perfect attendance record at the track.... nothing ever breaks. Kevin
Kevin

Paul,

that's right of course. But Scott mentioned Big Bore, not stroker.
So piston speed should be the same.
What's your opinion on Big Bore balancing?
Hey your right, I was thinking faster than I was reading, We have ran stock stroke with stock valve train to 10,200 over that we change valve train. our big bore pistons weight less than stock

I`m set at 9500, bike goes like hell. Paul did the big bore 2 years ago and it`s still together.

I`m set at 9500, bike goes like hell. Paul did the big bore 2 years ago and it`s still together.

Peter,

How many miles? What size BB?

Frank

Peter,

How many miles? What size BB?

Frank
10,000 miles and 4.250 bore

10,000 miles and 4.250 bore


Is it stroked?

Frank

Is it stroked?

Frank
No, just a big bore kit, dyer rods, webber cams(timed properly). I did do some extensive head work and 58mm intake w/tuned stacks. It`s putting 140HP to the rear wheel and 98 foot pounds of torque. I`m a happy guy, plenty of power, good fuel mileage and it looks and sounds stock till I crank it on.

Frank, I test rode Peter's Bike and it really is a sleeper, you would never knew it had a big-bore if you were next to it until he hit the throttle. Its very quiet at idle.

Brent,
Thanks for the positive feedback.

Unless you change your valve springs, i wouldn't go much over 10K rpms. I know a guy who set his @ 10,500 and floated the valves just over 10250rpms. Needless to say he needed new heads, pistons, and a crankcase. Can you say grenade?

Dear VRTURK,

please define "floated" (was it possible to judge what actually broke first from what was left ?), thanks.

Unless you change your valve springs, i wouldn't go much over 10K rpms. I know a guy who set his @ 10,500 and floated the valves just over 10250rpms. Needless to say he needed new heads, pistons, and a crankcase. Can you say grenade?

I assume that was with stock heads, not Screamin' Eagle heads. Correct?

Dear VRTURK,

please define "floated" (was it possible to judge what actually broke first from what was left ?), thanks.
:stupid: it's usually a Bearing in the Bottom end taking out the Crank and Rods! :hitfan:

Does floating mean that the valves don't close any more because they are "jumping" on the cams (mass inertia) - thus finally hitting the piston crown?
Aren't 10500 1/min a bit early for that to happen, considering the stock revlimit setting being just 15% below that?

Maybe a faulty valve spring broke first?

Jan-Dirk,
Floated=Cyl head valve spring tension not great enough to keep up with cam speed. Above 10200rpms the valve train has a very hard time keeping up without stiffer more robust springs in the valvetrain. The point I tried to get across was at high revs the stock springs cannot exercise as quickly as needed.

It could have been a broken spring, I don't know, I didn't tear it down myself. When they "float" they kiss the pistons for sure.

Instead of stronger springs could lighter valves be used? Does anyone make lighter valves?

I would think that stronger springs would increase wear quite significantly although if you are wanting to increase the rev limit I doubt this is a concern.. :)

:2cents: Two things to also consider, not just valve float, ie no contact with cam, but also bounce ie slams into the seat so hard and fast it rebounds off again. Piston speed is a limiting factor for rpm, but more importantly is acceleration - the more you rev, the less time the piston has to reciprocate, so it has not only to speed up, but also speed up faster thus accelerating. The more you stroke, the bigger and more significant the effect on this. :blahblah:

Instead of stronger springs could lighter valves be used? Does anyone make lighter valves?

I would think that stronger springs would increase wear quite significantly although if you are wanting to increase the rev limit I doubt this is a concern.. :)
THE 6MM STEMS LIMIT HOW LIGHT YOU CAN GO THEY ARE ALMOST 7 IN
LONG INCREASE THE HEAD SIZE (OVER SIZE) YOU HAVE A PROBLEM.
I WOULD GUESS THE ABOVE PROBLEM DEPENDING ON THE AGE OF THE
INSTALL THAT A SPRING BROKE.HAPPENED TO US UNTILL FERREA CHANGED
MATERIAL.

Instead of stronger springs could lighter valves be used? Does anyone make lighter valves?

I would think that stronger springs would increase wear quite significantly although if you are wanting to increase the rev limit I doubt this is a concern.. :)

Yes! I run a lighter valve with stock spring & retainers. (11250rpm - Not B/Bored)

Again, I pose the original question rephrased:
With a Scream'n Eagle 1250 Big Bore Kit, would 10,000rpm Rev Limit be safe to run?