I have been trying to tune out some high RPM boging and cant seem to get anywhere. I think I have had this problem since I changed the cam, the springs are Crane # 99848 which the web site says they are rated to .585 gross valve lift... the new cam with the 1.6 rockers is pushing .507 lift so I did not change them... however I did have comp send the springs that were matched to the cam but I faild to tell them that I had 1.6 rockers... I called comp and they told me max lift for these spring they sent me is .500, the # on the comp springs are 981 but the crane (99848) specs are pretty close to the comp springs so I dont understand. Basicly the problem is when you go into WOT you accelerate to certian speed/rpm and the you seem to not be accelerateing anymore, if you slighlty back your foot off the gas you accelerate again... any ideas?

How do they determine max lift of the spring?
What are characteristics of valve float?

 

max lift: basically, installed height - coil bind - small margin for safety = max valve lift

valve float: power takes a dive off the deep end.

For those springs, i would go for a .500 or so max lift. .510 TOPS.

What cam are you using?

 

So these 2 springs are equivelant... installed height on both are 1.70 and the comp coil bind is 1.15, crane is 1.153... how does crane rate theirs at .585? Crane springs

Can you elaborate on what really happens durring valve float, in ways of how the engine runs... im still trying to determine if that is what is going on or not.

my cam is 08-413-8

 

Directly off their site:
You have 99848, not 99846.

Valve float is when the valve literally bounces bac off the valve seat after it shuts because there is not enough seat pressure to hold the valve closed.

The springs you have are on par with the Comp 981's. However, bear in mind that thats a 4x4 cam. 4x4s never really rev high enough for valve float. I would go for something more like the 986 or 987 spring with about 130 lbs of seat pressure.

 

What you're experiencing sounds more like a fuel delivery problem or the ECM not optimizing the fuel map at WOT.

It doesn't sound like valve float.

 

my bad, thought the site was talkin about the 99848's...

What is reving high enough for valve float... the cam is rated to 5500 rpm, it hits that easy, the engine starts doing what I was talking about around 4000... how can a cam not be able to rev high enough for valve float?... if my spring are insufficient the valves will float right, has nothing to do with the cam, just the amt of lift vs. the seat pressure to hold down the valve.

So do you think this is valve float or not, how do I verify if it is and what should I check if it aint?... you said cam dont rev high enough, so it should happen at high rpm then which it is.

Good call on the 986 and 987, only promblem is they wont fit in the stock hole and I dont wanna take the heads off. Comp recomended the 941 cause they will sit in the stock hole and they have 130lbs seat pressure.

 

Yep. Doesn't sound like valve float to me.

 

Valve float is what Stekman said...

This is usually caused by excessive RPM, or more RPM than the spring can control.

It can be caused by oscillation of the spring from not having a damper, but that rarely happens. Kinda like coil springs on a car with bad shocks. The car keeps bouncing long after hitting a bump in the road. The damper is kind of like a shock absorber.

From what you described, valve float (or oscillation) is most likely not the cause.

 

I'll get a fuel pressure guage inside the drivers compartment and see whats going on then, thanks for the help. I hope it is fuel, if it drops really low at WOT I'll change the fuel pump and sell these springs on ebay or something.

Thanks
Shaun

 

I'd start with the fuel filter first. Start small and cheap.

I'd also get some time of diagnostic tool. I'm sure you'll find a "cheap" one in the DIY PROM board.

 

Is your engine carb'd, or fuel injected? The stocker FI is built to go around 4,000 RPM before it cant really handle much more (from what I hear... I have a carb'd setup, so please correct me if I'm wrong)

(EDIT) So it would make sense that with a lumpier cam than the stock one, you would go to a stand-still on power gains.

Also, something to think about is that your distributor is about 20 years old. It is probably not exactly in the same shape as when it came out of the factory.... it could not be able to advance the timing enough, or figure out how much timing advance it needs, in order to fire the cylinders at the correct time, which too would give you a stand-still in power gains at a certain RPM. Though this is less likely. Than the FI problem.

 

Not the filter, I just cut the old one out last week, it was about 6 years old, it was so rusted I could not change it before the engine swap... 2 years ago. I do have ALDL hooked up and tune my own chips, neither of which seems to want to tell me anything no matter how far to either extream and every thing in between I go... I just got back from autozone, got some stuff to hook up my FP guage and I might try it out tomorrow... got a feeling that it is gonna drop to about 5 psi from 12 at WOT...

 

Alright I just checked my WOT fuel pressure... its a TBI setup and they only run aout 12psi... the guage I have only goes up to 11, it never droped below 11 at all so I am thinking its not a fuel delivery problem. Any other thoughts?

 

So did you ever figure this problem out? I know this post is almost 10 years old but I am having the same exact problem!

 

having similer issues on my carbed l31 motor. help?

 

Went through something similar. Ended up the timing chain sprockets were off from each other a tooth