Exactly. That's my limit, as i am using the Cossie cams with stock valvetrain as i was suggested...

Won't raising it until i build head and probably bottom...so it will take a while to see higher than 7.8K...

 

The biggest enemy to your rods/rod bolts is engines revs. The higher you rev the more stresses multiply.

For instance, this is a Honda K20A(for example only):

Piston acceleration at TDC for K20A @ 8400 rpm = 4300 G's
Piston acceleration at TDC for K20A @ 9500 rpm = 5600 G's

As you can see, the acceleration of the piston multiplies a lot as revs increase. Of course, this particular engine has a 86mm bore and a 86mm stroke, it's a square engine. It also has a rod ratio that supports high revs from the factory. This is not a 4G63, as the 4G63 is a much different engine. But it illustrates my point very well.

This is not even taking into account the valvetrain and what it's physical limits are in regards to valve float. I'm not too sure what the max RPM is of the stock valvetrain on the 4G63 is. But, if you plan on running high RPM's, upgraded valve springs/retainers are a must, no matter what you do with the bottom end.

I would recommend staying within 500rpm of stock fuel cutoff. This is a good area to stay in with the stock valvetrain and stock bottom end.

Another thing to think about is your fuel needs. For instance, if you are running stock injectors@a given amount of boost, and your injectors are at or above 85% duty cycle at 7500rpms, as rpms increase, so will your injector duty cycle(even if boost tapers off at high RPM's. At 8500rpms you could very well run out of injector head room and start to run lean. Thats not even taking into acount atmospheric condition such as humidity and most importantly, ambient temperature. As ambient temps fall, fueling needs increase. Just something else you need to think about before upping your fuel cutoff.



CJ

 

Staying within 500 rpm would mean a max of 8100, so our suggestions have been good.

 

Correct. 7800-8100 rpm should be fine. You would rather have some valve float and a piston/valve contact event then have your rod go putting a nice large window in the block, rendering the whole bottom end useless. Even without knowing the stock spring/retainers limits, i would still say, based on my knowledge of how piston acceleration multiplies, i would say 8500rpms is really in the RED area. Again, i would be mostly concerned with rod failure at that point.

Valvetrain limits using the stock cams is one thing, but some aftermarket cams that don't require aftermarket valve spring/retainers still have aggresive ramp rates. This can only multiply valve float issues at these high revs. If anything, i would say deffinatly not to go over 7800-8000 rpm's in this case. Of course, if you are running cams that are using upgraded valve springs/retainers, then this would be a different story. But again, this does nothing for the piston acceleration problem and again my real concern would lie in the bottom end(i.e. rod/rod bolt failure).

My advice for people looking to run ultra high revs, is to properly build the bottom end and valvetrain for the duty at hand. Of course, this would require you to have a setup(proper turbo/manifold/support mods/injectors/fuel pump...ect) that will take advantage of high revs. Don't expect too much from the stock engine when it comes to high revs, keep it safe and you will be much happier with the end results.

CJ

 

I set mine at 7200rpm. I feel it is safer for the valve train until I get new springs and retainers. Then 7800 will be the limit.

 

You can run stock fuel cutoff at 7600rpm. There is no need to not at least run stock cut off unless the cams you are running suggest to run less than stock due to you not having upgraded valve springs/retainers.

CJ

 

warr,

i have actually set a few customer cars at 7.5, most of them to 7.65K

i have seen customers with less than efficient shifting skills pin the car in a high rev limiter when they shift hee he... so i bring it down, to bump it a little sooner....

cb