I hope this post isn't a bit too late. After a recent trip to the track and discovering a upper rpm horsepower loss, (26 rwhp peak after dynotesting) I thought I may have traced the problem to weak valvesprings. This is something I have never attempted before but thought I would give it a try. The tool was pretty simple and only presented a problem when trying to work on the springs located under the cowl. The springs that were recommended were K-Motion 750s which are supposed to be good up to .610 lift. My cam is relatively small (.533/.544) so I figured it should pose no problem for those springs. The install took most of the day and I really wanted to get it completed since sunday's forcast was just perfect for a day at the track. The new springs looked almost identical to the old springs so I didn't replace anything besides the springs themselves. I adjusted the rockers by making sure that the valve I was working on was fully closed, usually by making sure that the other valve in that cylinder was fully open. I would then tighten the nut until I could no longer turn the pushrod by hand then turn the nut and additional 1/4 turn before locking it in there. I made it to the track today only to experience a problem with the car breaking up pretty badly at around 5200 rpms. I did not have this problem before the valvespring replacement. Is it possible that I may have misadjusted the rockers? One interesting note is that the scanmaster was showing 785 mv which is very lean. Could it be coincidence that maybe the factory 101k mile fuel pump may have picked this same time to quit? Any help/advice you guys can give will be greatly appreciated...Thanks

 

Straight from CraneCams :

For hydraulic lifter camshaft valve adjustment, turn the
engine in the normal direction of rotation until the exhaust
lifter on the cylinder that you're adjusting starts to move up,
then adjust the intake valve on that cylinder to zero lash
with no preload, then 1/2 to 1 full turn more. Turn the
engine over again until the intake lifter has come to full lift
and then is almost all the way back down.

And if you think it's a fuel pressure problem, I'd recommend taping a fuel pressure gauge to your windshield and having a buddy watch it while you got WOT. This can also be done on a dyno.

-Alex

 

Over a year ago I had someone videotape my fuel rail mounted fuel pressure guage while I made a pass on the dyno. As the rpms climbed close to 6k rpms, the fuel pressure started to fall off by as much as 15 lbs. I thought this was normal considering that I had seen another lt1 do the same exact thing while on the dyno. I didn't suspect a problem since my performance hadn't suffered...at least I didn't think it did. The most recent trip to the dyno that showed the power loss also showed that the a/f ratio stayed at 13.1-1 which I thought was normal. Why now, after the spring swap, would I have a lean condition?

 

Your fuel pressure should not drop at all...well, maybe 1 or 2 lbs, but 15 lbs? No. You are either pushing your stock pump too hard (are you over 420rwhp?) or it's going bad. It may also be worth your while to try a new fuel filter and/or FPR.

-Alex

 

Dave, lets have a LT1 tuneup day at the shop! Im in search of my missing 2 mph..

 

I'm not making that much power at all (345 rw) but how much can the factory fuel pump take? Exactly where is zero lash? I was under the assumption that zero lash was when you could no longer turn the pushrod by hand. Is this wrong? Again, would misadjusted rockers cause the car to 'break up' and is there a theory behind why? Could the new springs be the problem or possibly just amplifying the problem, considering this problem wasn't there before the spring change?

 

Zero lash to me is when the pushrod JUST starts to tension under the rocker arm. I'd try re-adjusting using the method I copied from Crane's site.

The fuel pump should easily be able to handle 345rwhp...but it could be worn or pushing through a bad FPR or fuel filter. Fuel filter is cheap, try that first if fuel pressure is dropping.

-Alex

 

Thanks alot for your help/responses Alex.

 

If you go until you can no longer turn the pushrod by hand, then you have already gone too tight.... Let alone the additional 1/4 - 1 turn.


That could be your problem ...that they are set too tight. Which can cause problems such as , bad idle , and power loss.


Try your zero lash point as being when there is no more vertical play, instead of when you can't twist the pushrod by hand, which again can already make the valves set too "tight".


Going until they you cant twist anymore is kinda like going 2 or 3 plus turns.

 

ive heard until you just feel drag on the pushrod

but Ive never done it soo

 

This is the method I've used numerous times with no problems,but that doesn't necessarily mean it isn't wrong. Today I readjusted one side by using the method you guys have suggested and will try the other side, the tough side, tomorrow.

 

Could it be your 3.90's! . That rear sucks up more power Jeff.

Dave, It sounds like a fuel problem to me. If you're losing 15psi on the dyno, and you're breaking up that badly up top, I'd try replacing the pump (Assuming you've replaced the filter recently, and have checked the regulator).
If your rockers were too tight, they'd most likely make some noise, and if they've been the same way for awhile, would probably show some wear.

Your car was blown before, right? (if I remember correctly) If you had the stock FP in then, that could have worn it right out.

 

I did have a blower before but I was using the inline pump that came with the ATI kit. In fact, I might try to reinstall that pump instead of going through the trouble of trying to drop the tank to get to the other pump. What is the best way to find if the regulator is working correctly?

 

That could be your problem ...that they are set too tight. Which can cause problems such as , bad idle , and power loss.


Try your zero lash point as being when there is no more vertical play, instead of when you can't twist the pushrod by hand, which again can already make the valves set too "tight".


Going until they you cant twist anymore is kinda like going 2 or 3 plus turns. [/B][/QUOTE]


i agree with this, if you can't turn it, your way to tight

 

This loss is before the 3.90s went in, I havent raced the car since