Where to set the rev limit?

Talked to APE about my crank last night, it should be ready in 3-4 weeks. I asked how many RPM it would be safe to turn after it was lightened, ballenced and nitrided and I was told it should be good up to 14,000 rpm.

That number sounds high to me considering what was discussed in other threads about max RPM's. I wonder if the guy I was talking to factored in oil flow and bearing limits before giving me sugh a high number...

I am afraid with the lighter crank, clutch, and rods the motor will rev up quickly, so a rev limiter seems like a good peice of kit for insurance. The top end will be mostly stock kz750 parts.

So does anyone have a good idea as to what would be a safe limit?

Am I correct to think that it SHOULD be valve float that will cause engine failure first at high RPM?

If valve float is the key factor is there a formula to calculate when the valves will float (figuring in spring tension, valve weight and rpm)?

Thanks for any input.

Set at 13,750 and let it eat some rubber!!!

Yeah, what he said..Let 'er spin...

if your riding this bike on the street id set it about 11,000. you should have plenty of oil pressure to be able to run it there safely if your going to the track bump it up to 12,000 you may want to do some dyno runs when you get the motor back together to see where your making peak horse power and set it about 500 to 1000 rpm higher then that rpm no sense spinning it if your not making anymore power. on my drag motor i have my high end set at 11,000 and i shift it at 10,250.

I have a formula for it but its in my four-stroke tuning book by john robinson. I'm pretty sure its back at college but I'll see if I can't find it in my vehicle.

There are a few factors to consider when picking an upper limit. Valve float is likely the first mechanical limit.
If u put in stronger springs u can raise that.
And as timebomb said , when it stops making power stop reving.
So then u look at what stops it from making power, fix that and u can rev higher. always keeping in mind the mechanical limits of course.
1 thing that commonly stops it from making power, is its ability to breath. so then u need to analyze that. the usual things are small parts. ports, valves, cams, carbs.
Keep in mind that as all of these things grow, your torque curve goes up with them. so now u have a motor that will scream at the top rpms :woohoo: , but lacks that beautiful torque on the bottom.

I don't mind losing low rev torque... that is one of the reasons I went to lightened bottom end parts. I had alot of practice on my hyosung 125 and 250 in Korea keeping them in the proper rev range to get max power; and I find the noise a motor makes at 10,000 to be better music than anything ever put onto a CD.



I will need to buy some dyno time, it will be good to make sure my air/fuel gauge matches the dyno's sensor and to tune the bs32's I will be going with. It is unfortunate that I will need to re-do everything on a dyno in another year when I have the funds/time to tackle the top end. But at least my project is more than a worn out picture of an empty frame.


So until I get to a dyno 11K RPM would be a good start?

But at least my project is more than a worn out picture of an empty frame.





and it goes on........:S

if you have decent valve springs 11,000 shouldn't be a problem.

decent as in fairly cheap aftermarket springs or decent meaning stock springs that are not whipped?

I had not planned to touch the head this winter (cause if I start replacing valve guides, well then I will need to have the seats cut, and hell oversized valves are only a few hundred bucks so I should go ahead and add those while I need to get the seats cut anyway, then you of course a cam to make them work "right" oh hell; and some light porting to blend the new seats in.... )

I know when it is time to do the head I will spend at least 1K on it.... but until then I am just going to use a stock gpz750 head with stock springs (I will test the spring tension as per the FSM's instructions).

with stock springs i wouldn't go much past stock redline the stock springs don't have alot of pressure and it can cause valve float springs are reasonably cheap do your self a favor and by a decent set and save your motor if you going to be tearing the head apart anyway.

keith1 wrote:

But at least my project is more than a worn out picture of an empty frame.





and it goes on........:S

Insert picture of dead horse.......Here :laugh: