Proper Engine Vacuum?

[capriceforever]

Does anybody know what the correct engine vacuum is for a 1979 Caprice, 350 V8, at idle and warmed up?

I'm reading 16, but I'm pretty sure my 88 and 90 Caprices with the 305/307 is supposed to be 18-21.

Just curious, thanks.

[silicon212]

Originally Posted by capriceforever

Does anybody know what the correct engine vacuum is for a 1979 Caprice, 350 V8, at idle and warmed up?

I'm reading 16, but I'm pretty sure my 88 and 90 Caprices with the 305/307 is supposed to be 18-21.

Just curious, thanks.

With stock components, vacuum should be in the ballpark of 18 in/hg. 16 is acceptable, if the engine is running fine then there is no need to worry. High performance parts added can affect engine vacuum (intakes, cams, lifters, rockers etc) and if your engine has any of these parts, then that would be the reason for the slightly lower vacuum you have (which is still in acceptable range).

[capriceforever]

Thanks. Car runs like a million bucks, so I'll keep my hands off!

[hi-tech]

You didn't mention if you were measureing venturi or manifold vac, but I assume it is manifold. The biggest suspect in measuring vacuum is the gauge. Over the counter guages are notoriously inaccurate. Kinda like a cheap bathroom scale. Never reads the same thing twice.

[capriceforever]

Good point, I didn't realize there was a difference, but it would have been venturi vacuum, since I measured it off the port on the front RHS of the carb. This is the port that runs the vacuum advance. I suppose I should have tapped off the port on the rear/base of the carb instead.

The reason I asked was that I am getting engine ping at low altitude when the engine is well heated up. All my temp sensors are working right. Timing is right, air cleaner thermostat, EFE valve, vacuum advance etc. Just cleaned my EGR valve/passageways. But I was wondering if maybe my engine vacuum was a bit too low, possibly I wouldn't be getting enough vacuum to fully open the EGR at wide open throttle to cool down the combustion temperature.

Thanks for the advice.

[silicon212]

Originally Posted by capriceforever

Good point, I didn't realize there was a difference, but it would have been venturi vacuum, since I measured it off the port on the front RHS of the carb. This is the port that runs the vacuum advance. I suppose I should have tapped off the port on the rear/base of the carb instead.

The reason I asked was that I am getting engine ping at low altitude when the engine is well heated up. All my temp sensors are working right. Timing is right, air cleaner thermostat, EFE valve, vacuum advance etc. Just cleaned my EGR valve/passageways. But I was wondering if maybe my engine vacuum was a bit too low, possibly I wouldn't be getting enough vacuum to fully open the EGR at wide open throttle to cool down the combustion temperature.

Thanks for the advice.

In your case, it isn't "venturi vacuum" it's "ported vacuum". Ported vacuum is basically shut off when the throttle is closed, increasing as it's opened, to a point. Manifold vacuum is the amount of vacuum present in the plenum of the intake manifold, or near it.

If you used a vacuum gauge on a ported vacuum port, then you wouldn't have gotten any sort of reliable reading. The fact the distributor vacuum advance was disconnected will further taint the readings, as idle advance affects idle vacuum readings. Connect your gauge to the port the air cleaner vacuum motor runs off of - this is manifold vacuum and will give you a proper reading.

[hi-tech]

Actually, ported vacuum referes to the vacuum reading above the throttle plates in a TBI unit. Since this is a '79 Caprice, it is carb'ed. Hence, venturi vacuum. I should say, this is how we always designated it when I was in the shop. It really is the same thing though.

[silicon212]

Ported vacuum refers to vacuum pulled above the throttle plates on a carb, too. I've been doing this for years, before TBI was standard equipment on any car!

[hi-tech]

Not sure why you decided to pick this unimportant issue apart, but I'll be the bigger man and appologize for stepping on your ego. As I stated at the end, venturi vacuum and ported vac are really the same, we just used the terms to differentiate the difference between fuel delivery. From now on, I will defer to your superior wisdom. I guess my years of being a Chevy Master Tech and being rated in the top 100 nationally have very little meaning. And that's fine with me. I don't let what I do determine who I am.

have a nice day

[silicon212]

Did you used to beat up little kids at school for lunch money? Re-read what you wrote, how you responded to me, and then you will see plainly who has the ego problem. Hint: It isn't me.

I only do this as a hobby, but while we're engaged in a pissing contest, I will let you know that I built my first SBC in 4th grade at age 9. Of course, that doesn't mean anything and niether do your credentials which you feel the need to plaster on the internet. I let my work speak for itself, I never feel the need to espouse credentials. Go away.

[hi-tech]

Like I said, not sure why you chose to pick such a minor issue apart. Again, my applogies for threatening your ego. And again, have a nice day.

Looking it over, I was simply referring to your incorrect statement that is wasn't "venturi vacuum" it was "ported vacuum". I responded by trying to explain how WE differentiated between the two, and then ended by saying there are really the same thing. It was you who implied they were different. It was you that missed the fact the original poster didn't specify, and it was me that caught it. Again, so sorry for stepping on you little toes. And again, sorry my credential somehow threaten you. Perhaps some time in a therapist office may help you work that out. Never intended to get sucked into a pissing contest with someone like you. Won't happen again, I can assure you. And if me going away will make you feel better, maybe you should ask yourself why you let other people exert so much control over your life. I could go on and on, but I won't.