Vacuum gauge reading help

[old_man]

I am always fascinated at how much you can learn from a $20 Vacuum gauge, in minutes. Way underused old-school tool

Preach it brother. It, along with reading plugs and a compression tester will tell you more than the computer would.

 

[Ecomike]

Preach it brother. It, along with reading plugs and a compression tester will tell you more than the computer would.

Indeed, but I fear that reading chicken entrails is now a lost art, LMAO.

 

[Ecomike]

Everything that uses air is downrated for altitude. I'm at 6500 so I know it well. Altitude 'corrections' are made for compression, vacuum, ... even how much air your air compressor can make. In some cases you need to downrate the pressure capability of the air compressor too. Need a 2000 kW generator? Here you buy 2500 kW.

No doubt turbo charger numbers change as well. I bet they are a big help at high altitudes? Not as big a help (boost) as at sea level(?), but maybe more important? Would be interesting to read a summary on high altitude engine fine tuning some day.

 

[Ecomike]

I didn't even think of compression numbers being altered for altitude. I was concerned a rebuild was in store but can wait a while longer.

You should test it at or near sea level someday just for grins.

 

[Ecomike]

I will tell you ever time I've pulled a head and had a valve job, compression goes up, after normalizing for altitude I can go from 135-145 to 160-170. Never could tell a difference in drivability though. I was always pulling the head for another reason, the compressor numbers were never the driving force. I'd pull the valve cover and have a look. Time, heat, and miles and I've since the valve seals dislodged and riding on the valve stems... on 4.0's... If you're burning oil you can try a dry vs wet test which will tell you about the rings. If it changes, MMO or Auto-RX treatment can get carbon out of the rings, help them seal better, and your compression will go up too. While the valve cover is off, if you chose to pull the rockers, you can look at the tops of the valve stems. You want to see a bulls-eye pattern which will tell you if the valves are rotating. If its a line, then there is a TSB to manually rotate the valves to try and fix it. If it doesn't work, pull the head and go for the valve job. A well maintained 4.0 bottom end will go the distance but they still need valve jobs from time to time... Last time I pulled the head on my 01 at 315,000 I was still around 160 or so after I dropped a new head on (to fix the dreaded 0331 crack).

Interesting, Thanks!!! I find that very encouraging for my pending 300,000 mile head job on the 87 Wagoneer. The #1 and #2 valve guides are shot.

 

[dan1977p]

You should test it at or near sea level someday just for grins.

I'm actually moving back to Virginia at 0 elevation as soon as our house sells so I really want to check it. I'm looking forward to the added HP on the bike too.

 

[JeepNoob]

I was wondering about that, but since the gauge is measuring gauge pressure, ie. the differential pressure between inside and out side the cylinder, would the gauge reading drop that low, from 175 to 137 for a new engine spec? And the 120 psi manual spec is at sea level, but would still hold unless the CPS/Ignition timing is changed to compensate for the altitude???? If altitude conpensated, could it be OK at a lower reading?

I know Renix had a high altitude CPS to compensate, but never really gave the altitude issue much thought, till now.

Interesting.

For those new to this, at higher altitude the air is less dense, and there is less air mass sucked in to compress, and thus less pressure generated at TDC of the stroke. I think I just answered my own question.

Bingo! :cheers: This is something I've had to figure out the last few years living at 4,000+ feet or so in Colorado. Less air pressure up here, which is going to affect both your compression AND vacuum readings. Rule of thumb for vacuum readings is 17 to 21 in/Hg on a good engine at sea level. However, for each 1,000 feet above 2,000, you can subtract 1 in/Hg from that reading. So a good engine at 4,000 feet should have a vacuum reading of somewhere between 15 and 19 in/Hg.

Likewise, you can subtract 1 PSI of air pressure for each 2,000 feet above sea level. Doing a little bit of rudimentary, SWAG math here, 12.7 (pressure at 4,000)/14.7 (pressure at sea level)=.86395. 1-.86395=.13605 (13.605%, the amount you could expect a specified compression reading to drop at 4,000 feet.) Using 120 PSI at sea level as our bottom end compression figure, we can see that 120X0.13605=16.326. 120-16.326=103.674. Eureka! Our new, mathematical, scientifical, SWAGed, bottom-end compression figure for a Jeep 4.0 at 4,000 feet.