Manifold Vacuum Experiment
- Thread starter RustyRatRod
- Start date
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The high horsepower Chevelles of the day, the LS5 and LS6 cars all came from the factory using manifold vacuum. The LS6 was 11.25:1.
What people seem to miss is, changing to MVA really doesn't change much. I have the SAME initial timing with MVA with the can unhooked than I did using ported vacuum. The only change is WHEN the can is activated.
When I connected to MV, I had to reduce the static timing to 5* since my happy place is 25* and the can ads an extra 20*. Now I have to rethink my curve because..... When MV drops off when accelerating from idle, the centrifugal is starting from 5* as opposed to 25*.
Might need to reduce the total that the vacuum can can supply.
That's sometimes necessary, depending on what the vacuum can pulls in. But you already knew that. lol
That's why I am starting to re-think MV for my application. Since I only need 25*, static may be Ok if the starter can handle it.
Just keep in mind, the vacuum can has zero effect on startup. It's only activated when the engine is running.
So we took Vixen to town today. No spark knock that we noticed. She was noticeably less cold natured. Not that she was bad to begin with. But from what I can tell, she's running cooler. It was a cool day, but, even so on cool days like this before, she always ran a touch hotter when she got up to temp. Today she ran a tic cooler. Even though the idle is noticeably a tad more rough, it's cleaner too.....if that somehow makes sense. Evidently, when I tried this before, I did something wrong, because I believe she runs a little better with MVA. Definitely not the night and day I got from the truck, but there is improvement.
mopar head
Well-Known Member
Good thread Y`all.
Definitely, each engine has its own uniqueness/variables.
I know, I promised vacuum measurements. I JUST NOW found my vacuum gauge. Some idiot didn't put it back where it belongs when he used it last. I'll have some vacuum readings in a day or two.
Oh yeah. I'm dumb, I aint styoopid. lol
So I put the vacuum gauge on Gladys yesterday and was somewhat surprised. Before this experiment, she idled along bouncing between about 5.5 and 6.5hg. Now she has between 8 and 9hg, so there was improvement there. We've been driving her pretty much everywhere we need to go these past few days so I can get a handle on how she runs and I must say with Gladys, this has been a day and night experiment. She's running better now than she ever has. She's idling a tic high now, but I'll go out today and get her down around 700-800 and call her done.
Vixen on the other hand, while showing improvement, isn't the night and day story Gladys is. She never was what I'd call cold natured, but with manifold vacuum on the can, she is even less so. Also, it does seem like she runs a tad cooler than she was. Lastly, I can feel that same "more" with part throttle that I feel with Gladys. In other words, it takes less of the gas pedal to keep her pushing down the road. There's no doubt about that in either case. So I'd say all in all, this has been a success on both counts.
If there are any other vacuum readings yall want, just say so and I'll do it. Since I found my vacuum gauge now, it'll be easy. lol
Vixen on the other hand, while showing improvement, isn't the night and day story Gladys is. She never was what I'd call cold natured, but with manifold vacuum on the can, she is even less so. Also, it does seem like she runs a tad cooler than she was. Lastly, I can feel that same "more" with part throttle that I feel with Gladys. In other words, it takes less of the gas pedal to keep her pushing down the road. There's no doubt about that in either case. So I'd say all in all, this has been a success on both counts.
If there are any other vacuum readings yall want, just say so and I'll do it. Since I found my vacuum gauge now, it'll be easy. lol
Truer words were never spoken.
So we took Gladys into town today and I took some measurements on both manifold and ported vacuum. On manifold vacuum, at hot idle Kitty recorded 9hg. That's a big improvement over the 6-7 we had on ported vacuum. At 2200 at a steady cruise, we had 15hg. That's an improvement of 1hg before on ported vacuum. Small, but an improvement nonetheless.
On ported vacuum idling hot was zero, of course. At 2200 at a steady cruise, we had only 11hg. So that's 4hg less than manifold vacuum at a steady cruise. So clearly manifold vacuum is pulling in more timing at cruise than on ported vacuum. I must admit, I am surprised, as I figured they would be the same at cruise RPM, but they're not. This proves the theory that manifold vacuum pulls in more timing at cruise, at least in my instance.
On ported vacuum idling hot was zero, of course. At 2200 at a steady cruise, we had only 11hg. So that's 4hg less than manifold vacuum at a steady cruise. So clearly manifold vacuum is pulling in more timing at cruise than on ported vacuum. I must admit, I am surprised, as I figured they would be the same at cruise RPM, but they're not. This proves the theory that manifold vacuum pulls in more timing at cruise, at least in my instance.
mopar head
Well-Known Member
About a month ago I decided to hook mine up on the big block.
I haven`t noticed any economy gains, but it did make it run about 5 deg. cooler, which will come in handy mid summer.
Last summer I experimented with a cool air/ram air system that some may look at it and say
. but lemme tell ya, that dropped my temps down on sustained interstate highway speeds by 5 deg. also. But, I now have been jetting up cause my plugs look like it`s running lean.
I haven`t noticed any economy gains, but it did make it run about 5 deg. cooler, which will come in handy mid summer.
Last summer I experimented with a cool air/ram air system that some may look at it and say
. but lemme tell ya, that dropped my temps down on sustained interstate highway speeds by 5 deg. also. But, I now have been jetting up cause my plugs look like it`s running lean.
I found the source of my off the line stumble this afternoon. The primary accelerator pump is weak. I've gotta hunt down what's causing that now.
Aight den. I need some ideas. This carburetor has what I call "pink" pump cams on it. I have adjusted the pump correctly, tried both holes in the cam. Hole #1 works better, but I still have a stumble right off idle and I KNOW it's the primary accelerator pump, because I can see that it's a weak shot. The secondary side is like Niagara Falls, but the primary side is weak in comparison. I've disassembled it and run small drill bits into the carburetor body through the pump passage and down the nozzle hole on top to make sure there are no obstructions. Blew it out real good with the air hose, but the pump shot is still weak. Any ideas?
Newbomb Turk
Well-Known Member
The blue cams have the most lift and duration. I think the pink cams are right in the middle.
One thing I’ve been finding lately is the part of the lever that rides on the pump cam has to be “tuned up” so that it lays flat all the way across the pump cam.
If that lever is riding on just the edge of the pump cam, it changes the geometry and that affects the pump shot.
I've Leaned it out on the idle circuit with leaner idle air bleeds. 4 steps first, then 4 more. Made it worse each time. Trust me, it's the pump. I can SEE the difference front to back. It's huge. I think I'm going to check the angularity of the pump arm and then I may swap pump cams. Something simple and costs nothing.
I thought about that and I thought about just removing it, but I left it there.
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