Hi all! Just curious for those of you who run vacuum gauges on your trucks, what readings do you typically get? I rebuilt my motor (390 using original pistons w/ new rings, bearings, etc.) a few years back and put an "RV" cam in, so nothing radical. Just a good torque cam. The engine seems really cold blooded, and takes its sweet time to warm up.

My vacuum gauge reading when cold is very low....ie...~10, give or take. But, after it warms up and I take it out and drive it for a few minutes, the vacuum then gets up to around 15 at a slower idle, and 17 at a faster idle, depending on what mood it is in on any given day. It was a good day today. Warmed up nice, ran good, and was idling a little faster with a reading of 17. I would like to hear what kind of reading you guys are getting!?

 

Can't say I've noticed too much difference in engine vacuum warmed up or cold. 17" isn't too bad, but there are some things to check.

Important thing is that it's steady. Any stock engine at factory idle (550 to 650 say) should pull between 18" and 20" Hg at sea level. A different camshaft can alter this dramatically. Altitude plays a big role too. Retarded or late ignition timing will reduce manifold vacuum, also mis-adjusted carburetor, and tight valve lash on solid lifter engines.

There are a lot of cues and clues to be gained from a mechanic's vacuum gauge if you know how to read them. There are at least a half dozen charts or so you can download or print with each having a different test or two, everything from weak valve springs, poor compression, exhaust restrictions etc. AKA "Poor Man's Sun Machine" or "SteamPunk ScanGauge."

 

Thanks Tedster! I keep thinking that the engine is too "doggy" when I first start it. It takes a 10 minutes to get the vacuum up to at least 15. The first chance I get, maybe tomorrow, I am going to play around with the carb idle screw settings, and timing. It is set at 10 deg BTDC right now. Gonna see if I can't get the vacuum up some. But, I also wonder if a larger cam is also playing a role in the lower vacuum readings. A stock cam has will probably give higher readings. Like in a race car with a high lift, high duration cam, vacuum does suffer. While my cam is no race cam, I do wonder whether the slightly higher lift/duration in an "RV" cam can lower the vacuum reading a bit. Anyway, I will play around with it and report back if I can get it closer to 18-20.

 

Yep, it has to do with valve overlap.

Performance applications and engines that don't start smoothing out till 4500 RPM or whatever, don't idle well. And consequently have poor manifold vacuum. 10" might be doing well. A mild cam like yours might have a slightly lower vacuum. I think you're probably OK. Check for the usual -

https://www.classiccarrestorationclu...oubleshooting/

 

Thanks for that great information Tedster! Very informative. Well, I have been fiddling with the truck today, and I can't coax a reading of more than 16/17 out of it at idle in park (750 rpm). When I drop it in to drive with the engine running at idle, she goes to 13/14 on the vacuum.

I am not really happy with those readings, and wished it was closer to 17-20 in drive at idle, but, it won't do it. I don't know if I have a problem or not, but, the truck runs like a scared pup on the road, so I am going to call it a day. Not sure if I may have damaged my cam during break in, or just what, but, this is just the way it has run since the rebuild.

I did take the timing from 10 up to 11/12. I adjusted the idle screws out around 2 turns, and fine tuned them from there.

 

If your vacuum readings aren't steady when cold or hot I would suggest checking for vacuum leaks....starting with the carb's base, throttle plates, look for any brittle or missing rubber plugs on the carb, vacuum tree and brake booster line and swivel.

If you have a vacuum gauge can you throw up a video of what it's doing when the engine is running?

 

This vintage of truck is notorious for that.

 

I can try to post a video of the vacuum gauge while the truck is running as I get time. I think I tried to post a video before, but, was rejected due to the file being too large.

The vacuum is very steady at 16/17 with very little fluctuation. When I give the throttle a quick snap, the vacuum seems to act as it should and drops to ~5 goes up to ~25, and drops right back to 16/17. That is what I have read it should do. I next need to look for any vacuum leaks. Thanks for the replies. All very helpful.

 

Steady vacuum is good. A little low on the scale but good..(a steady reading).

When goosing the throttle the vacuum is great, acts like a healthy engine...again a bit low.

My only concern is the different readings you get when cold/warming up/operating temperature.

16/17 inches Hg would be reserved for an engine that has gross lift in the low 0.500," duration somewhat around 0.235s and and LSA of 110 or so. None of which I'm sure your cam has......if it did, you'd feel it as well as hear it.

 

It seems when I had my old cam in the truck, I remember getting vacuum of closer to 18/19. I believe the original owner of the truck had the engine rebuilt at one point, but, I don't know if he switched the original cam out or not. I just went and dug out my paperwork for the rebuild. My machine shop guy really likes Howard's Cams, and that is what he wanted to put in. Okay, I guess! I was looking at a Comp Cams, but I let him run with it. It is a Howard's 250021-11 cam that says Ford FE on the card profile. Here are pics. How about that bitchin' Howard's Cam sticker? That is going in the rear window tomorrow!

Please let me know if anything looks out of whack on the cam specs which may cause a low vacuum reading.

 

I have been tracking down another couple potential reasons for my relatively low vacuum. They are:

Howards's Cams: Talked to a Tech Rep this AM. He says that 15-17 is normal, and that is okay for a "performance" cam. Well, not sure I would categorize my cam as a performance cam, but, hey, I will take a compliment where I can get it!

Pertronix: Talked to a Tech Rep there too. Got to thinking maybe my EI is the reason for the sluggishness when cold, and lower vacuum. I have my pink "ballast" wire from the ignition feeding voltage directly to the + on my Flame Thrower coil. The Tech guy said I should feed a full 12 volt power feed to the + post on the coil. One thing I can do to see if this is the issue is run a jumper wire from the + on the battery to the + on the coil to see if that helps things. Plus, I think I want to run a relay on the ignition and have the pink wire be the trigger for the relay, and run the power from the fusebox into the relay, and out to the + post on the coil. So, we shall see if feeding a full, and constant 12 volt feed to the coil will help. As I understand it, the pink wire, which used to feed the points set up, does not have a full 12 volts. Let me know if any of this makes sense to the experts.

 

Correct. The ballast wire drops the voltage down, but the reason for that is to reduce the current that flows through the primary ignition circuit.

Pertronix Ignitor 1 module wants to see no more than 8 amps. If the ignition coil primary has at least 1.5 ohms resistance you are good to go. Remove, or bypass, the pink wire. A good hot ignition and/or tuneup should noticeably improve warmup time. But I suspect you could probably use a new thermostat. Don't cheap out on these, they aren't that expensive anyhow. Motorcraft 195° F and see how that works.

 

Thanks Tedster! You are a wealth of knowledge. I will try those things. I asked the Pertronix rep about the ohms question. He said the Flame Thrower coil has 1.5 ohms built in to it, so I think I am good there.

Next step, bypass the pink wire, and put in a new t-stat.

 

Try timing it higher. Time it with the Vacuum Gauge. Look for the highest reading without going past that.

My 351 will idle at 19 and cruise at 12-15 depending on conditions.

 

I gave this a shot today. I had the timing way off the charts and it only got to ~18 Hg, but, the idle RPM's were higher too. In fact, I think the higher RPM's actually caused the vacuum to read higher. So, I took it back down to 12-13 BTDC and called it good. I normally run it at 10 BTDC. Vacuum is about where its always been at 15/16 Hg. I do appreciate your idea.