First, I know this isn't exactly advanced, but I want to try to get the best answer possible for this.

Setup stuff: 355, Pretty agressive Joe Overton custom cam, Comp R lifters.

On my new motor, I've been having issues with getting the rockers adjusted properly (and quiet). Basically, they're either loud and clacking with the motor running great (sounds like a sewing machine) or they're much more quiet with about 1/16-1/8 lash and the motor won't idle and surges. But that's not the question.

In either case, I have been watching my vaccuum. It's NEVER a nice steady pull, it's ALWAYS wavering around. With the rockers loose, it pulls about 14-17 In/Hg vaccuum, constantly bouncing in that area. With them tighter on the lash, it's more like 3-7, and throws a map code. I would think that's just too tight on the rockers, but it's barely any lash.

So, my question is this. Could the cam profile/ramp rates be so agressive that I'm just going to have a noisy valvetrain??? If the overlap is great enough (LSA <110) with a higher duration coupled with a very steep and agressive ramp, could this cause enough of an overlap issue to where some vaccuum would bleed off and cause a wavering indication???

I can't find any vaccuum leaks, and the only time I have thrown a low MAP code is when the rockers are too tight.

Also noteworthy may be the fact that with the rockers too tight and throwing a map code (34 I believe), if I unplug the map and then start the car in backup mode, it's fine.

Any thoughts to the genuises out there??? TIA

 

DTC 34 = 34 Manifold Absolute Pressure (MAP) sensor circuit (signal voltage low, high vacuum)

That's just the opposite of what you are seeing with the rockers too tight. You are probably getting DTC 33 = Manifold Absolute Pressure (MAP) sensor circuit (signal voltage high, low vacuum)

 

Is this a hydraulic cam?

YOu have the right idea about minimal pre-load. 0.020" is about the most you
want to set for high performance applications. With a roller valve train, that
0.020" will get you a good stretch of time between lash checks.

At zero lash, and 1/8th turn max pre-load (which should equate to about
0.020" on a standard rocker stud threading), the lifters will be happy.

The intake opening and exhaust closing points are going to affect the idle
vacuum immensly. If there is a great deal of overlap, the idle quality will
suffer (low vacuum/high pressure in the plenum).

The noise, if hydraulic, shouldn't be. With a cold/hot lash setting, the pre-load
will take up the shock and keep the valve train quiet.

How much are you adjusting the lash to drop ~10 inches of vacuum?

 

Injuneer, thanks for the correction. I was just wrecked (tired) last night and knew it was one or the other, and it looks like my 50% shot was wrong. Thanks.

Zero_to_69,

Yes, it is a hydraulic cam.

I'm adjusting it anywhere from flat zero lash, to about 1/4 turn for testing purposes. Anything over just the smallest amount past zero and the car doesn't like it.

Knowing that with a larger lift/duration cam that the idle vaccuum will suffer, you partially answered my question. I expect the idle vaccum to suffer some anyway just given the surface specs of the cam. But, should the actual vaccuum readings be wavering around like they are, or are they still expected to be a nice continuous pull like on my other car with the hotcam??

As for the valve lash, I can't for the lift of me figure out why the car doesn't like anything other than about zero lash. If the rockers aren't making all kinds of racket the car just isn't happy. I have tried adjusting them running (no dice), cold, and hot, and have the same result every time. Funny thing is that one time with it running I had them adjusted perfectly or so I thought. Car was idling fine, nice and smooth (for the cam), I thought "Hey, I got it!" But, after the car cooled and I buttoned everything back up, when I started it cold, it acted like they were WAY too tight for some reason. I can't seem to find any other problems, and the motor itself is brand new, still haven't even been able to break it in yet.

Could it be that they just need to be warmed up and they'll improve???

 

One thing that comes to mind that might be causing the poor idle with more
pre-load is that your cam lobes have very high duration specs at 0.050".

I'm thinking 245 degrees and more for the 0.050" duration is getting pretty
wild for a street application (tappet specs) . If the lobe separation is tight,
then the IVO and EVC numbers are going to overlap quite a bit which rides
the power production even higher up the RPM band.

By setting more pre-load, it may be possible that the lifters are pumping up
slightly causing the lifter seat to rise earlier and opening the valve a little
higher and sooner than it wants to be.

What sort of oil pressure do you have at idle? What rocker ratio are you using?

The bouncing of your vacuum gauge may be an indication of a leak, or the
bouncing could be a product of the cam's idle attitude. When you set the
pre-load to attain a vacuum of ~ 13-17 in./hg., does the needle bounce,
or remain fairly stable?

 

Well, neither the intake or the exhaust durations are greater than the 245* mentioned. I can't give out the exact specs, but I'll just say in duration terms there's many a person running something very similar and not having issues. I think my issues are stemming from the ramp rates on the lobe. I AM using Comp R lifters, which very well could come into play with what you said about the early pump-up.

When I have the rockers set to attain the 13-17, it's a wavering indication, to either side of 15 I'd say an average of +/- 1.5 to either side. I haven't managed to find any vaccuum leak, and checked with the visual and the carb cleaner.

My oil PSI at idle is showing at about 60 on the stock gauge.

My rocker ratio is 1.6.

Thanks for the help.