327/300 Powerglide idle - vacuum advance?

[Joe R.]

I have been chasing a rough idle problem in my 67 327/300 ever since I bought the car last year. I have methodically worked through the fuel and ignition systems (which needed attention anyway for Flight judging) but this has had no effect on the idle problem. Currently I am looking more carefully at ignition timing.

The engine idles reasonably well in neutral at 800 RPM, although I think it is a bit more lumpy than some other 327/300 engines I have listened to. However, I don't think I would consider the idle quality a problem if I had a manual transmission.

My problem manifests itself mostly because I have a Powerglide transmission. If I set the idle speed to 700-800 RPM in neutral, the engine will stumble and stall when I put it into drive. To keep the engine from stalling when I put it in drive, I have to set the idle speed to 900 RPM in neutral. In drive, the idle speed drops to about 600 RPM and the idle quality is poor.

Vacuum tests show that the idle vacuum is about 17 inches at 900 RPM in neutral, but vacuum drops to 9 inches at 600 RPM in drive. Timing tests indicate that the total advance at 900 RPM in neutral is 20+ degrees, but it drops to 6 degrees (the setting for initial timing) in drive. In other words, the vacuum advance drops out completely when I put the car in drive at idle.

I think I may have a chicken-or-the-egg problem here, since low vacuum causes the vacuum advance to drop out, and the loss of the vacuum advance causes low vacuum.

The factory specs for my vacuum can specify 0 degrees at 6 inches and 15 degrees at 12 inches. Bench tests on the actual can that I have show that it begins to move at 7 inches and is all in at 16 inches. So, it may be slightly out of spec but not a lot.

Anyway, some simple experiments with higher initial advance show a noticeable improvement in the problem. This suggests to me that I might want to switch to a vacuum can that provides full vacuum advance at vacuum of 9 inches, so that I get full vacuum advance even at 600 RPM in drive.

My apologies for the long description of the problem, but this leads me to a couple questions that I hope other board members can help me with:

1) In a stock 327/300 with Powerglide, what should the vacuum level be at idle with the transmission in drive? If someone out there has a similar car that runs well, I would appreciate hearing the results of an actual test.

2) If normal idle vacuum in drive really is in the 9-10 inches range, would it be okay to switch to a vacuum can like the L79 version, which is all in at 7 inches? Is there any risk of damage to my engine if I make this change?

Thanks in advance for any insight that you folks can provide.

[Wayne W.]

Re: 327/300 Powerglide idle - vacuum advance?

A 300 HP engine is very smooth and should idle down very well. Assuming the engine is OK, it sounds like someone has put a camshaft in it that is of rather high duration and overlap, like the 350 HP cam. Lots of us like the way that one sounds and performs, including me. I put one in my 67 coupe which is also an automatic. It was necessary to use the 350HP advance, because as your symptoms suggest, it teeters on the border of the advance curve. Works fine with the HPerf advance.

[Duke W.]

Re: 327/300 Powerglide idle - vacuum advance?

Your vacuum can is far enough out of spec that it should probably be replaced. The NAPA Echlin VC 1765 is closest to OE spec, 16@12", but I don't think this is the source of your problem.

Idling in neutral at 500 RPM, a 300 HP engine should pull at least 16", preferably 18", and it should show about 23 degrees total idle timing with 8 initial. P/G should idle at 450-475 in Drive. I don't have any manifold vacuum data for this condition. but it will undoubtedly drop a couple of inches from idling in neutral due to both the reduced speed and converter load, however, with a 12" can the total idle timing should be unchanged from idling in neutral.

I would be willing to place a substantial wager that your engine does not have an OE or OE equivalent 300 HP camshaft, which is likely the root cause of the problem. It's idiotic to put a "hot cam" in a P/G car, but bubba does it anyway.

You should run a lobe lift check and compare to OE specs. You didn't state the MY. The '66 and '67 300 HP cams are slightly different in spec, but essentially identical in operating characteristics. The "early" 300 HP cam is obsolete and no longer available. The "late" 300 HP cam,, 3896929, that went into production in '67 is the correct replacement for all earlier base cam engines. Specs are in your CSM.

A lobe lift check will likely tell you if you have an OE or some other cam. If it's not OE, well... you know what the "fix" is.

Duke

[Joe R.]

Re: 327/300 Powerglide idle - vacuum advance?

Hi Wayne:

I have wondered about the possibility that that the previous owner (now deceased) had the 350 HP cam put in during a rebuild that occured in 1988.

Are you saying that you put the 350 HP cam in your PG car and that using the 350 HP vacuum cam was all you had to do to make it run right with the PG? Do you have any data on what the vacuum levels are in neutral and in drive?

[Joe R.]

Re: 327/300 Powerglide idle - vacuum advance?

Hi Duke:

I agree that the engine may not have the OE 300 HP cam, and that a lobe lift check would be the best way to check. That will likely go on my list of tasks for this winter.

In the meantime, though, do you see any risk of damage to the engine if I experiment with substituting the NAPA VC1810 can that you have mentioned in previous posts? I believe that can is specified to be all-in at 8 inches of vacuum.

It sounds like you believe that full vacuum advance is still desireable when the transmission is in drive at idle, so the VC1810 can would ensure that I still have full vacuum advance in that condition.

It's not clear to me how the vacuum advance affects overall timing at conditions other than idle. My guess is that the vacuum advance will pull in under most off-throttle driving conditions, so the VC1810 can would tend to kick in a bit more than the standard 300 HP can whenever vacuum goes up. I'm just concerned about creating pre-ignition conditions or some other undesirable condition with the VC1810.

Perhaps another way to state the question is why GM tended to use a higher vacuum threshold for the vacuum advance to kick in on their low performance engines, and only resorted to low thresholds like the VC1810 has when they used high overlap cams that had poor idle vacuum. I don't want to mix and match without considering possible side effects.

By the way, on the theory that perhaps a previous owner substituted the 350 HP cam, do you know what idle vacuum (in neutral) that cam typically produces? I thought it would be less than the 16 inches that I measure at 700 RPM.

[Duke W.]

Re: 327/300 Powerglide idle - vacuum advance?

Go ahead and try a VC1810. It might improve the situation, but watch for detonation.

The L-79 cam pulls about 14-15" at 750-800 idle speed with about 25 deg. of total idle timing.

Duke

[Joe R.]

Re: 327/300 Powerglide idle - vacuum advance?

Thanks Duke. I think I will give the VC1810 a try.

To simulate the effect of maintaining full vacuum advance at idle in drive, I had experimented with disabling the vacuum advance and setting the initial advance to about 24 degrees (all tests performed at idle speed only). The engine seemed to idle better in drive, but it still had only 11 inches of vacuum at 600 RPM in drive. The VC1810 should guarantee full vacuum advance at this condition.

I think it is quite possible that my engine has a non-stock cam in it, possibly the 350 HP cam. Once the car is in for the winter I'll pull a valve cover and measure the lobe lift.

[Bob R.]

Re: 327/300 Powerglide idle - vacuum advance?

If the cam was changed I would think you would need to idle at 750 rpm in drive to get a smooth idle.
My 63 with a 300 hp 327 won't idle at the manufactures recommended 450 rpm. But it does run nicely at 750 rpm.
The cam was changed when the engine was rebuilt a couple of years ago.

[Joe R.]

Re: 327/300 Powerglide idle - vacuum advance?

Hi Duke:

I swapped in the VC1810 vacuum advance unit, and it made a dramatic difference in how the car idles in drive. The engine now retains full vacuum advance when I put it in drive, and it is a lot happier with that.

I was able to reduce the idle speed in neutral to about 750 RPM, which drops to about 550 RPM when I put it in drive. Now, though, there is no tendency to stall when I put it in drive. The idle vacuum is about 17 inches in neutral, and drops to about 11 inches in drive.

I think you are right that a previous owner probably put in a hotter cam, most likely the 350 HP cam. What I think was happening is one of those positive feedback latching mechanisms, where putting the car in drive caused the vacuum to drop, which cause the advance to drop, which caused the vacuum to drop further. At idle in drive, I had only 9 inches of vacuum and no vacuum advance at all.

Anyway, the car is much more drivable now. It's idle performance is not as smooth as a stock 300 HP engine, but it is entirely acceptable. The engine seems to run quite strong otherwise, so I suspect (and hope) that the problem was simply due to a mismatched cam and vacuum unit.

This winter I may do a lobe lift check to try and confirm that a hotter cam was the cause of the problem. I hope that it turns out to be a hotter cam, because then I can just relax and enjoy the car. If it turns out to have a stock cam, I'll have to keep chasing this problem some more.

Thanks for your help getting this sorted out.

[Duke W.]

Re: 327/300 Powerglide idle - vacuum advance?

I'm sure it's not an OE 300 HP cam with those vacuum readings, but if you're happy with the way it runs then it's probably best to leave it alone other than doing a lobe lift check if you wish to try and verify the installed cam.

Duke