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Interesting (low) vacuum readings on 1982 W150 truck & 1
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Author:  spacecommander [ Sat Jan 24, 2015 8:18 pm ]
Post subject:  Interesting (low) vacuum readings on 1982 W150 truck & 1
Been cold here in New Mexico and haven't had a chance to put on any of the performance goodies that are sitting around (carb, manifold, ported/larger valve/milled head, torquer cam, etc). Just for grins hooked up a vacuum gauge to it to see how low it would go on WOT. It idles dead steady at about 18", but at full throttle all the way up to about 3700 RPM stays at ZERO. Highway at 65 reads about 5-6". I'm at 5500' altitude. Hooked the vacuum gauge up to the port tree in #6. Letting off the gas brings me up to about 23". Never expected to see zero at WOT nor 5-6" at cruise. It is a big heavy 1982 4WD truck.
Author:  SlantSixDan [ Sat Jan 24, 2015 10:46 pm ]
Post subject:  Re: Interesting (low) vacuum readings on 1982 W150 truck &am
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It idles dead steady at about 18"
…is excellent for your 5500-foot elevation…
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but at full throttle all the way up to about 3700 RPM stays at ZERO
…is normal for WFO…
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Highway at 65 reads about 5-6"
…is because you're pushing a big, heavy brick through the wind…
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Letting off the gas brings me up to about 23"
…is exactly what's supposed to happen…
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Never expected to see zero
Vacuum exists in the intake tract because the engine is throttled. Unthrottle it…and the vacuum goes away.
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It is a big heavy 1982 4WD truck
…is why it will always show you low vacuum and MPG numbers.
Author:  spacecommander [ Sun Jan 25, 2015 5:58 am ]
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I expected to see about 1.5" or so as the engine went about 3000 RPM. If I remember that's what happened on my 78 Volare (at sea level) before I put the 4BBL on - then it was zero. Thought there'd be some restriction from the 1bbl as the RPMs climbed. Was guessing it might be above 6.5 at 65MPH. Looks like I might need to rethink my power valve for the new carb.
Author:  spacecommander [ Sun Jan 25, 2015 6:02 am ]
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I'm getting 14.5MPG mixed. My 85 Dodge Ramcharger with the 360 engine got about 12 on the highway and literally about 6-7 around town - maybe not even that. Oh, that was back in CT and MA when the speed limit was 55 and highly enforced. it's 75 here and if you're going that slow you're a hazard. That's one of the reasons I had it painted bright yellow. :D
Author:  wjajr [ Sun Jan 25, 2015 7:23 am ]
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Oh, that was back in CT and MA when the speed limit was 55 and highly enforced.
You must not have been traveling in the brake-down lane on Mass Pike east of 128 during rush hour... LOL
Author:  nm9stheham [ Sun Jan 25, 2015 7:42 am ]
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Dan took all the words right out of my mouth... all sounds perfectly normal. Going down grade on a steep mountain at speed in my Dart Lite and the throttle closed in 3rd gear w/ manual trans, I could see 25-26" at times. (1000'-3000' elevations)
WOT will always be at or very near 0; it'll only come up a bit at higher RPM's when the carb throat is starting to choke down the engine.
Author:  wjajr [ Sun Jan 25, 2015 7:43 am ]
Post subject: 
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I expected to see about 1.5" or so as the engine went about 3000 RPM. If I remember that's what happened on my 78 Volare (at sea level) before I put the 4BBL on - then it was zero. Thought there'd be some restriction from the 1bbl as the RPMs climbed. Was guessing it might be above 6.5 at 65MPH. Looks like I might need to rethink my power valve for the new carb.
Yes, one will need to drop to lower vacuum rating on power valve with heavy vehicle and using a vacuum operated secondary four barrel select a heaver secondary spring to prevent secondaries from opening under normal cruise conditions for better fuel economy. Spring selection will be a trial and error process.

Also leaner than stock (out of the box calibrated at sea level) jetting is required for high altitude driving. The closer the carburetor tune gets to ideal A/F mixture the higher the vacuum readings will be.

You will find installing an Air Fuel mixture gage and o2 sensor along with a temporary vacuum gage with long length of tubing reaching dashboard will be helpful in dialing in a carburetor.
Author:  spacecommander [ Sun Jan 25, 2015 9:41 am ]
Post subject: 
Quote:
Dan took all the words right out of my mouth... all sounds perfectly normal. Going down grade on a steep mountain at speed in my Dart Lite and the throttle closed in 3rd gear w/ manual trans, I could see 25-26" at times. (1000'-3000' elevations)
WOT will always be at or very near 0; it'll only come up a bit at higher RPM's when the carb throat is starting to choke down the engine.
I figured the 1bbl would start to show some signs of vacuum somewhere above 3000 RPM like the old Volare but nope.
Have already leaned out the main jets for the 4BBl - will need to check what power valve I put in. Installed the quick change spring kit and have a selection of springs. can't find anybody here in Albuquerque who can weld cast iron - or is willing to do it anyway - wanted to make a dutra dual style front manifold so I could do dual exhaust to the muffler. Looks like I'll just be running the stock exhaust manifold for a while. The other car I was going to get fell through so I actually have a offy manifold with a 390 Holley (that's what I re-jetted) AND a offy 2x1 manifold with 2 carter weber 2bbls. When it gets warmer here I can play with both and see if one is better than the other. Cam and head won't go on till Spring.
Author:  spacecommander [ Sun Jan 25, 2015 8:12 pm ]
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Oh, that was back in CT and MA when the speed limit was 55 and highly enforced.
You must not have been traveling in the brake-down lane on Mass Pike east of 128 during rush hour... LOL
Western Mass and CT were insanely strict. To the point that back in the late 70's very early 80's if you got a ticket in CT for 10 MPH or more over the speed limit you HAD to make a court appearance - even if you were from out of state. You couldn't just pay the fine.

I liked the Wichita, KS to OKC commute - only did that a few times but it was bumper to bumper! Oh, bumper to bumper at like 90!
Author:  wjajr [ Mon Jan 26, 2015 4:50 am ]
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Western Mass and CT were insanely strict.
Copy that. We lived in Framingham area, and I worked easterly and north of Boston from 128. Speeding tickets were a second breath of life for Taxsachusetts in the day.

I even got culled from the herd several times whistling down the Pike and up 128 in my then spanking new 82 LeBaron; a car that takes two exits and a good tailwind blowing to get up to 60...

Bastards in their old crappy beat Mass Highway work trucks equipped with radar parked in median and cop radio equiped to tell the wolf-pack over the brow of next hill who needs to join the ticket writing gangbang. Oh yeah those were the days that took off a hundred miles or more of travel away during a ten hour drive.

Yeah for Jimmie Carter opening up a second revenue stream off the backs of the working man trying to get to work; remember setting your thermostat to 62?

Nice Conn you had going down there 10 over, and a trip to court...
Author:  spacecommander [ Mon Jan 26, 2015 7:51 pm ]
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Yeah for Jimmie Carter opening up a second revenue stream off the backs of the working man trying to get to work; remember setting your thermostat to 62?
I remember his "Sweater talks." Put on a sweater. Even and odd plate numbers for gas rationing. No thanks.

Still a little confused regarding the zero vacuum at 3500 RPM with a dinky 1bbl. I do remember something between an inch or two on the Volare and it then going to zero with the 390CFM 4bbl. Long time ago - maybe that was the reading at 4500 rpm - would make a big difference. Thought the whole idea of the bigger carb was to ditch any restriction which wold then drop the manifold vacuum to zero throughout the rpm range at WOT.
Author:  WagonsRcool [ Wed Jan 28, 2015 3:10 pm ]
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Still a little confused regarding the zero vacuum at 3500 RPM with a dinky 1bbl.
All it means is that the 1bbl carb isn't the biggest restriction to airflow through the engine below 3500rpm- as it is presently configured. There are ALOT of differences between your pickup & a Volare. Vehicle weight, gearing, (possibly camshaft timing), ignition timing, altitude, actual/ effective compression, exhaust restrictions, etc. You can't really have a true "apples to apples" comparison with so many variables.
Author:  spacecommander [ Wed Jan 28, 2015 7:30 pm ]
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Still a little confused regarding the zero vacuum at 3500 RPM with a dinky 1bbl.
All it means is that the 1bbl carb isn't the biggest restriction to airflow through the engine below 3500rpm- as it is presently configured. There are ALOT of differences between your pickup & a Volare. Vehicle weight, gearing, (possibly camshaft timing), ignition timing, altitude, actual/ effective compression, exhaust restrictions, etc. You can't really have a true "apples to apples" comparison with so many variables.
I'm aware of that but expected a greater vacuum not less. Huge heavy vehicle. Only big difference in the opposite direction is altitude. It does have hydraulic lifters. Hopefully the new head with the OS valves & porting and the torquer mechanical cam will make a decent difference when installed. If the timing marks are right it idles at 11BTDC. Was at about 6 BDT, I advanced it when I replaced the distributor as the old one had a blown out vacuum advance can. I run hi-test and detect no pinging. It does run very rich (black soot) due to stock jetting at 5500'+

The restriction to power appears to be stock cam/head related as the power curve just flattens out at about 3000RPM. Oh, it has an upgraded exhaust - stock exhaust manifold for now (will make dutra duals eventually) but a 2 1/2 inch exhaust to a huge walker quietflow and out the driver side in front of the rear wheel. No dozen bends going up and over the transfer case and such . . . .
Author:  spacecommander [ Wed Jan 28, 2015 8:11 pm ]
Post subject: 
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. . . . Was guessing it might be above 6.5 at 65MPH. Looks like I might need to rethink my power valve for the new carb.
Yes, one will need to drop to lower vacuum rating on power valve with heavy vehicle and using a vacuum operated secondary four barrel select a heaver secondary spring to prevent secondaries from opening under normal cruise conditions for better fuel economy. Spring selection will be a trial and error process.

Also leaner than stock (out of the box calibrated at sea level) jetting is required for high altitude driving. The closer the carburetor tune gets to ideal A/F mixture the higher the vacuum readings will be.

You will find installing an Air Fuel mixture gage and o2 sensor along with a temporary vacuum gage with long length of tubing reaching dashboard will be helpful in dialing in a carburetor.
For the 390cfm 4bbl I'm getting ready to drop in - dropped from 51 main jets to 49, went from a 9.5" Power Valve to a 6.5" - but upon seeing vacuum readings think a 4.5" or even 3.5" PV will be needed.
Author:  SlantSixDan [ Wed Jan 28, 2015 8:56 pm ]
Post subject: 
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Put on a sweater. Even and odd plate numbers for gas rationing. No thanks.
Welll, god promised America endless cheap gas forever; it's right there in the bible, so don't you fret about it none, y'hear?
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