Low vacuum with Comp Cams 252H

I am working on a 67 289 with a Comp Cams 252H cam in it. Best manifold vacuum is 12” hg. That’s with ignition timing at 15 degrees and about 750 rpm. I am thinking it was installed retarded. I’ve never dealt with the effects of retarded cam timing. Does this make sense

Can you provide the cam duration @.050 and tot lift for both intake and exhaust?

Most Comp Cams come advanced from the factory.
Perhaps the timing gear marks were incorrect.
It would be a good idea to check the damper to see if it has slipped - I had one that slipped over 30 deg.

Duration at .050 is 206 degrees (intake and exhaust). Lobe separation is 110 degrees. Intake center line is 106 degrees. Total lift with factory ratio arms is .433

That’s a pretty small cam, so vacuum probably shouldn’t see be that low. I’d suggest you make a piston stop and put it in the #1 spark plug hole and determine tdc and make sure tdc average is 0 on the pointer. If that checks out, you should probably remove the front cover and degree the cam to determine what centerline the cam is installed at. I’ve seen timing sets several degrees off. And your correct, retarded cam timing will reduce vacuum.


You could also check vacuum at 2000 rpm and see if it’s much higher, that would also indicate retarded cam timing.


I will check that specifically. My recollection was that it was much higher. Who ever did this also seems to have limited ignition advance. Even though it starts with almost 15 degrees advance all in is about 40.

Compresdion check will be the most accurate indicator of cam timming. Get a baseline on that. Then perform a Cylinder leak down check before tearing anything apart. Those two values will draw the map of where your engine is going.

Can you help me understand what I am looking for? Is this for a comparison before and after changing cam timing?

Exactly ! Cam timing directly effects cranking compression
And the compression values when comparing all cylinders to each other tells you if you have an issue with 1 cylinder or another

Vacuum is rock steady but low. Typically a weak cylinder shows as a tick on the vacuum gauge. I don’t think that is my issue. I think where I am at now is that if I don’t find the cam is installed out of spec timing I will just replace it and a set of lifters.

A misfire or stuck valve will show as a fluctuation but if you have a smooth running engine with high blow by on a weak cylinder your vacuumm will just be low

Instead of replacing the cam…

  1. use a compression gauge to check average cranking compression now.
  2. pull the timing cover, check position and advance the cam if needed
  3. before putting things back together, recheck cranking compression to see if there is an increase…if so you may want to put it back together and see what you have for vacuum.
    It seems like a 289 with that cam should generate 16-18 in of vacuum at idle with proper initial timing. If that cam were in a 351, I’d suspect slightly more vacuum. Comp has intense lobes, so the advertised duration is short enough that the 110 lobe seperation won’t knock down too much vacuum. Or, is this bottom end worn out where it’s puffing oil mist out the breather?

If all of your cylinders are equally worn with low compression this could make sense. What is the concern your chasing? Low power? runs rough? etc.

The engine actually runs pretty good. No blow by or other issues. The back story is that the engine was rebuilt in late 1998. The car was driven very little and then parked. The previous owner got cancer and passed away. The widow kept the car for several years until she remarried. Her new father in law went through the car and did a lot of work on brakes and everything else that dies from sitting too long, and got it running. As it turned out she never was comfortable in the car and decided to sell it. When I bought it the brakes would lock up if you touched the pedal. The booster was bad. There were also a ton of electrical “fixes” that I had to unfix and get it all working. Once I got it running I discovered that it made very little vacuum, not enough to make the booster work right and it was not wanting to shift out of first.

I found the paper work from the rebuild and it all looks reasonably good. TRW pistons that are .030 over L2218E. Flat top 4 relief design. Should make 9.6:1 compression with a 54cc head.

Summit used to offer kits. This was the FEM-CSMHP728-311: pistons rings main bearings rod bearings cam bearings oil pump and of course this cam.

At some point it also got an Edlebrock carb and single plane intake manifold.

It had some little glass pack mufflers that were very loud. I suspect he might have been looking for the thumper sound as one possibility, or just didn’t realize that the cam was ground with 4 degrees of advance built in and maybe tried to take that advance back out. Or… he bought a different cam all together.

It’ll be a few days before I can dig into it and see.

Curious. If cam has built in advance, does retarding initial timing cancel that out. Or is it more complicated than that.

I had a Comp Cams 260H back in the day in my first Cougar with a 289. It ran amazing with a F4B intake, Holley 650 and headers, although it maxed out RPM at about 5000, past that was just valve float. But vacuum was like 15-18 if I remember correctly…

Ignition timing is a separate issue. It’s related but you can’t fix cam timing by changing ignition timing

single plane intake manifold.

Am I wrong that these don’t make as much vacuum as a dual plane at lower RPMs?

Maybe it has some of them there bleed down type lifters like Rhodes makes.
Could be the rocker ratio got changed.