Idle Vacuum Question

[Dusty_Duster]

OK, this came up in another thread, but I'm going to post the question here. I have an '88 Turbo I motor with the automatic transmission, intercooler, fully-built bottom and top end, etc. At idle, my car's vacuum gauge reads 10 inHg. During deceleration, it is around 20-22 inHg. For as long as I've had this motor, this is what the vacuum was at.

I've been told by some people that this is too low and that the cam timing is off. I've been told by other people that this is normal; the high-profile cam that I have is what's causing the low vacuum.

I do know that my car does not have much bottom-end power at all. It doesn't really "move" until it hits 8 psi on the boost gauge. (G-valve is set at 12 psi.) Then it throws you back in your seat. The high powerband is probably caused by the cam and turbo lag caused by my rather crappy intercooler.

What's the deal? Is my cam timing off, or is this normal?

[Speedeuphoria]

I do know that my car does not have much bottom-end power at all. It doesn't really "move" until it hits 8 psi on the boost gauge. (G-valve is set at 12 psi.) Then it throws you back in your seat. The high powerband is probably caused by the cam and turbo lag caused by my rather crappy intercooler

I think the cam may be hurting spool and bottom end

http://www.turbo-mopar.com/forums/sh...ad.php?t=14393

not sure how much it helps, but I think you may be off some

[Dusty_Duster]

Crap. That's what I was afraid of. I really don't have the tools necessary to fix the cam timing.

Is there anybody in central Ohio (near Mansfield) that could help me adjust the cam timing?

[Dusty_Duster]

Me and a friend are going to check the cam timing tonight. I'm hoping that that's the issue and it can be fixed tonight. *crosses fingers*

[ssheen]

Do not forget to take into account your elevation. Loose something like 1" for every 1000ft or so.

[Dusty_Duster]

A coworker let me come over and use his garage. We checked the cam timing; it's spot-on perfect. We also adjusted the ignition timing. It was a little off. I'm idling at about 11 inches of vacuum now. It's a little better, but not much. But the car is idling smoother and seems easier to start. We also called up a friend who's a master Chrysler mechanic. He asked if there was a performance cam in the motor. I told him yes, and he told me that's probably why the vacuum is so low.

It's running a little smoother now and seems easier to start.

Thanks for the help, guyz!

[sdac guy]

If it is an aftermarket cam, then checking to make sure it is installed straight up (which is probably what you did) is of little use. If the person that installed the cam originally did not degree it in to it's centerline, then the sprockets can all be aligned properly but the cam could still be off enough to cause the problems you are having.

To properly check it, you need a degree wheel you attach to the crank pulley, a pointer of some type to reference the degree wheel as the engine is rotated, and a dial indicator to measure valve lift. But most importantly, you would need the specs for the cam indicating the proper installed centerline. I suspect you had none of these when you checked the cam timing.

Depending on the cam itself, that could be the source of the problem though. The Mopar Performance turbo cam with the part number ending in "314" has enough overlap in it's design that it has a low vacuum no matter what installed centerline is used. And low vacuum will lead to increased fuel use as the engine controller looks at vacuum to tell if the engine is under load. The lower the vacuum reading, the more it richens up the mixture.

That particular MP cam also has a narrow powerband which starts at about 3300-3500 rpm. So the cam design can cause the problems you are having such as low vacuum, poor mileage, and no low end torque. But so can having a cam installed at the wrong centerline.

Since you don't really know which cam is in your car (FM sells a few different ones), your best bet would be to get an adjustable cam sprocket and try different settings, advancing and retarding the cam slightly to see where your best idle vacuum is, and where the car runs the best overall.

Barry