Does High Lift Cam Affect Vacuum/Lights?

[Vettehead Mikey]

Hi All,

I've been having problems getting my headlights to function. The lights turn on, and you can hear them trying to actuate... but they do not lift up.

I had installed the Edelbrock Perrformer setup, including a .510 lift cam, so the engine has quite a lope to it.

My question is --- with this high lift, is the vacuum therefore adversely affected, i.e., weaker? If so, does this sound like a likely reason the lights wont actuate? They worked fine before my rebuild. :confused

Thanks.

If you've got enough vacuum to operate the engine, you've got enough to raise and lower the headlights.

There's most likely a major leak somewhere in the system and it comes down to long hard hours of troubleshooting to find it. Good luck.

 

[GerryLP]

If you've got enough vacuum to operate the engine, you've got enough to raise and lower the headlights.

There's most likely a major leak somewhere in the system and it comes down to long hard hours of troubleshooting to find it. Good luck.

Mike,

I don't doubt that a leak may be present, but the problem is not that an engine does or does not produce a strong vacuum signal with a loppy cam. The problem is that with a relatively loppier cam that vacuum signal can only be produced at a higher RPM.

Getting back to the issue at hand, I would recommend anyone who wishes to stay with their selected cam profile, and if they are experiencing slow head lights opening or stowing, then one option is to add a vacuum canister. The cannister helps accumulate vacuum signal for those car accessories that operate from engine vacuum. For example, while the car is at freeways speed or otherwise higher RPM, the cannister operates as an accumulator of vacuum signal. Then when one is at the stop light and the car is shaking from the vacuum signal loss created by the overlap on intake and exhaust valves duration, then if you need to turn-on the wipers (older Vettes) or if you need to turn-on the headlights (C3's and earlier), then you'll be able to do it without having to bring the engine to 1.5k RPM's or higher.

And if one does install one of those babies, it won't work unless one does check the system for leaks like Mike said.

GerryLP:cool

 

[fine69]

I am running a cam (CC XE274) with similar specs, but on a 383. Manifold vacuum @ idle is 13". For my 350 street engine w/HE268 cam and stock heads, I had 15-16" at idle.

That Edel cam seems a little much, on duration, for a stock or near stock build.

I have seen others complain that the Edel. Performer RPM cam was too much for a near stock street engine.

Thanks for all the replies!

I completely overhauled my engine last summer and it runs great with the Performer RPM setup - very strong.. the cam is not at all too much.

I did a rudimentary check (with my finger!) of the vacuum off the manifold - it appears to be strong.... I'll have it checked with a gauge. The vacuum hoses on the other hand are the original ones and look pretty shabby... so hopefully the replacement kit will solve the problem.

 

[fine69]

Mike,

I don't doubt that a leak may be present, but the problem is not that an engine does or does not produce a strong vacuum signal with a loppy cam. The problem is that with a relatively loppier cam that vacuum signal can only be produced at a higher RPM.

Getting back to the issue at hand, I would recommend anyone who wishes to stay with their selected cam profile, and if they are experiencing slow head lights opening or stowing, then one option is to add a vacuum canister. The cannister helps accumulate vacuum signal for those car accessories that operate from engine vacuum. For example, while the car is at freeways speed or otherwise higher RPM, the cannister operates as an accumulator of vacuum signal. Then when one is at the stop light and the car is shaking from the vacuum signal loss created by the overlap on intake and exhaust valves duration, then if you need to turn-on the wipers (older Vettes) or if you need to turn-on the headlights (C3's and earlier), then you'll be able to do it without having to bring the engine to 1.5k RPM's or higher.

And if one does install one of those babies, it won't work unless one does check the system for leaks like Mike said.

GerryLP:cool

Gerry,

XLNT suggestion! Is this something that is available at automotive shops, etc - or is this usually a custom-made item?

Thank you.

 

[Vettehead Mikey]

Gerry,

XLNT suggestion! Is this something that is available at automotive shops, etc - or is this usually a custom-made item?

Thank you.

All C3 Corvettes came from the factory with a built in canister, more than adequate in volume to open/close the lights while at minimum vacuum situations.

When new (or rebuilt properly), you could shut off the engine, walk away for a few hours and still have enough vacuum to open and close the lights once or twice. Very few C3s can do this today due to system deterioration.

 

[GerryLP]

Gerry,

XLNT suggestion! Is this something that is available at automotive shops, etc - or is this usually a custom-made item?

Thank you.

Fine69,

Here is an example of the CraneCams model: http://www.jegs.com/webapp/wcs/stores/servlet/CategoryDisplay?lang=-1&catalogId=10002&storeId=10001&categoryId=13767

CompCams Model: http://www.jegs.com/webapp/wcs/stores/servlet/CategoryDisplay?lang=-1&catalogId=10002&storeId=10001&categoryId=23405

Changing a cam is an expensive (relatively) option, so the cannister could work, but I can't over-emphasize how important is to correctly tune the idle mixture ratio, ignition timing, and idle speed to obtain the highest vacuum signal. Perhaps you'll gain some vacuum signal and the cannister won't be indispensable.

GerryLP:cool

 

[GerryLP]

All C3 Corvettes came from the factory with a built in canister, more than adequate in volume to open/close the lights while at minimum vacuum situations.

When new (or rebuilt properly), you could shut off the engine, walk away for a few hours and still have enough vacuum to open and close the lights once or twice. Very few C3's can do this today due to system deterioration.

Good point, Mike!

If one thinks about it, the 1970 LT-1 had a radical cam. So did other C2 and C3 Vettes that also operated vacuum-assisted accessories.

GerryLP:cool

 

[GaryS]

Gary,

What should the manifold vacuum be. I took mine the other night and it is around 22

You might want to borrow another vacuum gauge. I seriously doubt that you are getting 22.

Gary

 

[JOE BYRNE]

Gary,

I have two gauges....I did run the idle down to 700 and checked it and it is at 18 off the manifold

 

[GerryLP]

Gary,

I have two gauges....I did run the idle down to 700 and checked it and it is at 18 off the manifold

Joe,

Is there a hose comming-off the valve covers, or perhaps is there a vented cap that serves as an oil filler cap?

What I am suggesting is to check the PCV valve for clogging or perhaps instead of a breather cap, there may be a cap clogged that is not allowing the engine crancase ventilation to take place.

With that much vacuum signal, you should not be having problems with vacuum-operated accessories. Refer to earlier post by Mike on a hose leak.

GerryLP:cool

 

[JOE BYRNE]

The breather cap is new and the PCV valve sounds like a baby's rattle. I was really just wondering if it is ok being that high

 

[GerryLP]

The breather cap is new and the PCV valve sounds like a baby's rattle. I was really just wondering if it is ok being that high

Joe,

It does sound quite high (22 InHg of vacuum), and perhaps is just my limited knowledge, so I cannot explain it away, but what size of carburetor is installed on the engine and what is the CR for your engine?

The highest I have ever experienced is about 18 InHg on the stock L81 at sea level. Up at the 5k feet of altitude I am at, that normal vacuum signal would be between 15.5 and 16 InHG of vacuum signal. In my 9.5 CR play engine, the XE274 cam allowed at best 10.5 - 11 InHg at idle. However, I have obtained as much as 12.5 - 13 InHg by installing a smaller carburetor.

One thing is certain! If your engine really produces 22 InHg of vacuum signal, then you should not be having issues with your vacuum-operated accessories if they are in good order.

GerryLP:cool

 

[JOE BYRNE]

It's a stock rebuilt Q-jet for a 73 350 that is running rich like the old one (runs good but you can see black out the back on heavy acceleration in 2nd and third)...but thats a different thread. I dropped the engine idle down to 700 and it's about 18 hg. Idle by the book is 900 on a 73 with a manual trans. I don't think im going to loose ant sleep over it. Thanks for the help Gerry!!!!!!!