Vacuum source question.
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
Vacuum source question.
Trying to increase vacuum to my advance just a little. Would a intake manifold vacuum port pull slightly better than a manifold port on carb or would both be equal? I just switch back to a carb from FiTech and trying get everything dialed in. I've already tuned for max vacuum at carb.
Problem is.....Car idles fine at 850 in park but drops to 700ish in drive with a surging idle. I believe its because my vacuum drops too low in drive and vac can is pulsating pulling in erratic timing. The surging is not that bad, and car does not stall but I would like it to be correct.
Problem is.....Car idles fine at 850 in park but drops to 700ish in drive with a surging idle. I believe its because my vacuum drops too low in drive and vac can is pulsating pulling in erratic timing. The surging is not that bad, and car does not stall but I would like it to be correct.
Burning Brakes
Joined: Jan 2012
Posts: 943
Likes: 73
Trying to increase vacuum to my advance just a little. Would a intake manifold vacuum port pull slightly better than a manifold port on carb or would both be equal? I just switch back to a carb from FiTech and trying get everything dialed in. I've already tuned for max vacuum at carb.
Problem is.....Car idles fine at 850 in park but drops to 700ish in drive with a surging idle. I believe its because my vacuum drops too low in drive and vac can is pulsating pulling in erratic timing. The surging is not that bad, and car does not stall but I would like it to be correct.
Problem is.....Car idles fine at 850 in park but drops to 700ish in drive with a surging idle. I believe its because my vacuum drops too low in drive and vac can is pulsating pulling in erratic timing. The surging is not that bad, and car does not stall but I would like it to be correct.
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
Do you know what vac can you have in terms of inches Hg? I changed mine out for a lower spec than what was on the distributor.
Last edited by Bloodzone; Aug 1, 2018 at 01:39 PM.
Burning Brakes
Joined: Mar 2016
Posts: 1,008
Likes: 163
So I think you may be chasing this the wrong way. You should size your vacuum can to your motor. So if you only have 8 inches of vacuum at idle, you need to find a vacuum canister that gets full timing at 8 inches.
Since you may have a hard time finding that (what cam do you have in your motor that is only giving you 8 inches at idle??) your best bet would be to get an adjustable vacuum cannister with a timing limiter plate. I did this for my distributor a couple months ago. The little allen key in the vacuum cannister adjusts how quickly the timing comes in (not the amount of timing allowed) so you can set it to be all in at whatever vacuum you need. Then the limiter plate goes in your HEI to set the maximum vacuum advance (mine is set around 12 degrees). This way you can have your cake and eat it too: the timing you want at idle without sacrificing your cruise timing and MPGs.
I believe I used the Crane vacuum canister and timing limiter.
Since you may have a hard time finding that (what cam do you have in your motor that is only giving you 8 inches at idle??) your best bet would be to get an adjustable vacuum cannister with a timing limiter plate. I did this for my distributor a couple months ago. The little allen key in the vacuum cannister adjusts how quickly the timing comes in (not the amount of timing allowed) so you can set it to be all in at whatever vacuum you need. Then the limiter plate goes in your HEI to set the maximum vacuum advance (mine is set around 12 degrees). This way you can have your cake and eat it too: the timing you want at idle without sacrificing your cruise timing and MPGs.
I believe I used the Crane vacuum canister and timing limiter.
Race Director
Joined: May 2006
Posts: 18,692
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read the comments on the AR31 in the second post in this thread
https://www.corvetteforum.com/forums...e-shipped.html
https://www.corvetteforum.com/forums...e-shipped.html
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
I'm getting 9Hg at idle in drive, 12Hg in park. My Can is rated 8 at full advance. But that may not be what its really pulling. The AR31 should be correct for my needs but mine could be out of spec and not fully advancing till10
I'm going to try another port, maybe T off the brake booster and see if I can get a little more to the distributor. If that doesn't work I'll either live with it, try another advance canister or switch to ported vacuum for my advance.
I'm going to try another port, maybe T off the brake booster and see if I can get a little more to the distributor. If that doesn't work I'll either live with it, try another advance canister or switch to ported vacuum for my advance.
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Burning Brakes
Joined: Jan 2012
Posts: 943
Likes: 73
Is this a stock engine?
Last edited by BlackC3vette; Aug 2, 2018 at 08:43 AM.
Burning Brakes
Joined: Jan 2012
Posts: 943
Likes: 73
I'm getting 9Hg at idle in drive, 12Hg in park. My Can is rated 8 at full advance. But that may not be what its really pulling. The AR31 should be correct for my needs but mine could be out of spec and not fully advancing till10
I'm going to try another port, maybe T off the brake booster and see if I can get a little more to the distributor. If that doesn't work I'll either live with it, try another advance canister or switch to ported vacuum for my advance.
I'm going to try another port, maybe T off the brake booster and see if I can get a little more to the distributor. If that doesn't work I'll either live with it, try another advance canister or switch to ported vacuum for my advance.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
Your first line of action would be to identify any leakage within the vacuum system. That system connects with the headlamps, HVAC system, vacuum storage device, vacuum advance system, and emission control devices/sensors. To determine the MAXIMUM (baseline) vacuum your engine is capable of producing, you need to disconnect ALL vacuum devices from the engine (and cap any open lines), then check vacuum level available. Optimizing timing at idle can also result in manifold vacuum level gains. If the baseline vacuum level is no more than 1-2" Hg higher than with all vacuum system connected, you have very little leakage. If it is more than that, you need to repair the leaks.
Once you have gotten engine vacuum level where it should be, and you still find the level to be below what is needed for a stock vacuum advance can, you can always swap out that can for one which will provide the advance you need with the vacuum level you actually have.
Once you have gotten engine vacuum level where it should be, and you still find the level to be below what is needed for a stock vacuum advance can, you can always swap out that can for one which will provide the advance you need with the vacuum level you actually have.
Le Mans Master
Joined: Aug 2017
Posts: 8,336
Likes: 2,809
From: Cool Northern Michigan
Technically, the booster should have a dedicated Vac line with no "Ts". Not always easy to do on a Vette, that has so many vacuum needs anyway.
Last edited by HeadsU.P.; Aug 1, 2018 at 08:03 PM.
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
I use 3 vacuum lines. Brakes, Advance and PCV. Happy to be able to say I have no emission control junk and I run electric headlights. So leaks are no problem.
I agree about the booster line. But I'm gonna try a few different configurations just to see what I get. It may be as simple to running a higher idle in park like 1000.
I agree about the booster line. But I'm gonna try a few different configurations just to see what I get. It may be as simple to running a higher idle in park like 1000.
Team Owner
Joined: Jan 2006
Posts: 37,637
Likes: 3,116
From: Crossville TN
If leaks were not a problem, you should have plenty of vacuum for the advance can. Either you have leaks (maybe IN the advance can) or you have a radical cam in the engine which will not support adequate vacuum level. As I mentioned previously, if you are satisfied with the vacuum level being correct, just install a vacuum can that provides the advance you need with the vacuum you have.
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
Maybe you need more initial advance?
14 initial +16 from can at manifold vac gives me 30ish at idle, I get 20-21 mech advance for total timing of 35 and a total + advance of around 51
I just need one more Hg for my current AR31 can to work. Hopefully I can find it.
Racer
Joined: May 2018
Posts: 297
Likes: 43
I'm getting 9Hg at idle in drive, 12Hg in park. My Can is rated 8 at full advance. But that may not be what its really pulling. The AR31 should be correct for my needs but mine could be out of spec and not fully advancing till10
I'm going to try another port, maybe T off the brake booster and see if I can get a little more to the distributor. If that doesn't work I'll either live with it, try another advance canister or switch to ported vacuum for my advance.
I'm going to try another port, maybe T off the brake booster and see if I can get a little more to the distributor. If that doesn't work I'll either live with it, try another advance canister or switch to ported vacuum for my advance.
Had a 1976 L-82, 4-sp
Joined: Mar 2011
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From: Some days your the dog and some days your the hydrant.
Royal Canadian Navy
I don't think you are 'giving the engine what it wants'. I would focus first on increasing initial and reducing mech'l with vac can disconnected. If your vacuum increases at the same idle speed, your moving in the right direction. If your tuning for performance, you may need to eliminate the vac can and take a hit on fuel economy.
Thread Starter
Burning Brakes
Joined: May 2014
Posts: 884
Likes: 128
From: Rhine Georgia
I don't think you are 'giving the engine what it wants'. I would focus first on increasing initial and reducing mech'l with vac can disconnected.
If I limit my Mech to 16, raise intial to 18 I should gain some vacuum that wont drop off from park to drive.
Last edited by Bloodzone; Aug 2, 2018 at 10:04 AM.