High Vacuum Reading

[Violator]

This kind of goes along with my bouncing Alt gauge issue but I think it'w worth a new thread. After replacing the voltage regulator and setting the timing with a timing light I decided to check and see what the vacuum gauge would have to say. I connected at the lowest port on the carb and at idle it was pulling a steady 25 inches of vacuum. From what I understand that seems pretty high. Is that a sign of other issues and what would you suggest checking or tuning? When I opened the throttle wide open it slammed right down to 0 inches and then came right back up to 25 again when I let off.

 

[65 mustang]

Sounds normal to me. Somebody will tell me if i'm wrong, I don't remember any " normal" engine having vac readings above 30. Big cams can make that number in the teens. Scott

 

[Viperwolf1]

You might have one of those miniature black holes growing inside your engine.

It just means you have good cylinder sealing.

 

[Lmfp]

That's one of those good problems to have

 

[Rustytruck]

That is high but good. it really doesnt matter too much what the reading is as long as at idle it is steady as a rock. bouncing needle is bad the worse it bounces the worse it is.

 

[Broncobowsher]

That high at idle is really good. To the point I don't believe the gauge. Lucky to see that much vacuum during engine braking.

 

[Violator]

Well glad to hear all those comments. Just one less thing to worry about I guess. I just thought it seemed high and even removed the hose to the vacuum gauge and put it back on a few times just to make sure it was reading correctly. Still trying to track down the bouncing gauge and now I noticed the lights are pulsing now. Never did that before.

 

[00gyrhed]

vacuum gauge might be off. Or maybe its not in inches of mercury??

Its been a while since I was in school but if I am remebering this right atmospheric pressure is 29.9" of mercury. What you read on the gauge is actually how far below atmosheric pressure you are reading. you can't go any lower than 0 inches of mercury, which will read 29.9 on your gauge. PERIOD thats physics. A reading of 30" or more is impossible. So for you to read 25 is possible. There is more than cylinder sealing involved in manifold vacuum readings. Ignition timing will alter vacuum readings, as well as the cam lift, duration, and cam timing. technically nearly anything in the system from the air intake/filter to the muffler and out the exhaust tip can alter manifold vacuum readings.

I was always told normal heathy stock engine would idle at 18-20 inches of mercury, and that has been my experience. I have seen some short duration high lift cams that didnt make 25 unless you reved it up and closed the throttle, and they generally make vacuum better than stock profile cams. An engine 3" of cranking vacuum will ususlly start.

 

[rguest3]

I have a 351w with about 18 inches of vacuum with a nice cam installed with a little added duration.

25 inches sounds high but may just be a nice sealed engine.

 

[73azbronco]

Is it a stock cam, curious because the only way to get that hi is a bad gage or a camshaft which drives the vacuum to this level. Bottom line, I would not worry, you have good vacuum.

 

[pcf_mark]

My '68 289 with headers, Edelbrock intake and Holley 600 makes 22". Your carb tuning and ignition timing at idle must be very good. I worked mine up to 22 from 18.5 with tuning.

 

[WheelHorse]

What is your idle speed at?

Are you port timed or manifold vac for vac advance?

When I ran full manifold vac to my advance canister, my idle vac was 23-24...which indicated too much timing at idle. When I switched over to port vac, she settled into a steady 20 on the money.