Hi guys, looking for input on hooking up a vacuum advance. I'm running a GM HEI on a freshly built 462.5 approximately 500 HP, with a TH400 2000 stall. Currently I run it at 36 degrees all in, 16 base, without a vacuum advance hooked up. I drive primarily on the street, no racing or drag time. I'm in the process, waiting on parts, to hydra boost the brakes due to 12Hg vacuum at idle, 800 RPM in gear. 3.31 out back with 28-inch tires. I know a vacuum advance might increase the MPG used, which would be a benefit, but not sure my setup would benefit from it. My engine builder said no, he races BB's. I have a Vacuum advance limiter installed that should only give me an additional 10 degrees advance, but never hooked it up to the vacuum, following his advice. I know about the try it, if it pings, retard the timing aspect, but I have to admit, my confidence isn't high enough to try it. Zero decked 22cc domed pistons with 781 heads shaved to approximatly16cc chambers, about 10:1 compression, 104 octane. What say you? I'm not a wealthy man to rebuild an engine again but for the same reason some MPG would be a plus.
Vacuum advance
unclern
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Hi guys, looking for input on hooking up a vacuum advance. I'm running a GM HEI on a freshly built 462.5 approximately 500 HP, with a TH400 2000 stall. Currently I run it at 36 degrees all in, 16 base, without a vacuum advance hooked up. I drive primarily on the street, no racing or drag time. I'm in the process, waiting on parts, to hydra boost the brakes due to 12Hg vacuum at idle, 800 RPM in gear. 3.31 out back with 28-inch tires. I know a vacuum advance might increase the MPG used, which would be a benefit, but not sure my setup would benefit from it. My engine builder said no, he races BB's. I have a Vacuum advance limiter installed that should only give me an additional 10 degrees advance, but never hooked it up to the vacuum, following his advice. I know about the try it, if it pings, retard the timing aspect, but I have to admit, my confidence isn't high enough to try it. Zero decked 22cc domed pistons with 781 heads shaved to approximatly16cc chambers, about 10:1 compression, 104 octane. What say you? I'm not a wealthy man to rebuild an engine again but for the same reason some MPG would be a plus.
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I would certainly hook it up.
10* vacuum advance should be perfect.
FWIW, I have about 12" of vacuum and no hydra boost. Brakes work fine for me.
10* vacuum advance should be perfect.
FWIW, I have about 12" of vacuum and no hydra boost. Brakes work fine for me.
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^^^^^^^^^^^^^^^^
if mine I would hook it up.
if mine I would hook it up.
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Unc, Man I feel ya. My builder ALSO told me NOT to hook it and keep it at 36 degrees TOTAL. I have my scroll plate set to give , literally 3.5 degrees,( thank you Dave Ray! ) and I am scared to disobey orders. He simply DOES NOT want me to ruin a very well built machine.
The +10 is old time advice however.
Crappy mpg = 16.3 en route to the MCC show last year. About a 400 mile haul. No vac advance yet! 9.9:1 , Big 850 race holley, very similar to yours, so...... why again?
PS I did before, with a different engine, 427, and on hot days after a few hours runtime, I would often have to pull over and plug the vac can.
Next time lets build 9:1 BBCs, give up 50 ft lbs ( unless we stroke them) , and use near 50 degrees of advance total!
The +10 is old time advice however.
Crappy mpg = 16.3 en route to the MCC show last year. About a 400 mile haul. No vac advance yet! 9.9:1 , Big 850 race holley, very similar to yours, so...... why again?
PS I did before, with a different engine, 427, and on hot days after a few hours runtime, I would often have to pull over and plug the vac can.
Next time lets build 9:1 BBCs, give up 50 ft lbs ( unless we stroke them) , and use near 50 degrees of advance total!
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well my brakes work, it just feels like manual brakes instead of power brakes that I got accustomed to before this engine was installed.
You mentioned 104 octane....you don't need that with 10:1 compression.
My engine in the 70 has 10.7:1 and runs just fine on 93 octane with 35* total timing.
I also have a few other big block Chevy builds in some of our other rides that are 10:1 and 93 is all they need.
My engine in the 70 has 10.7:1 and runs just fine on 93 octane with 35* total timing.
I also have a few other big block Chevy builds in some of our other rides that are 10:1 and 93 is all they need.
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Lost me on the scrool plate set at 3.5. Wish I could get 16 MPG, I'm closer to 6 (just a guesstimate) around town but haven't actually had a chance to highway drive it yet. I have less than 100 miles on the engine since my stock used TH400 only lasted till I stood on it the first time. I just got the new trans installed, ready for round 2.Unc, Man I feel ya. My builder ALSO told me NOT to hook it and keep it at 36 degrees TOTAL. I have my scroll plate set to give , literally 3.5 degrees,( thank you Dave Ray! ) and I am scared to disobey orders. He simply DOES NOT want me to ruin a very well built machine.
The +10 is old time advice however.
Crappy mpg = 16.3 en route to the MCC show last year. About a 400 mile haul. No vac advance yet! 9.9:1 , Big 850 race holley, very similar to yours, so...... why again?
PS I did before, with a different engine, 427, and on hot days after a few hours runtime, I would often have to pull over and plug the vac can.
Next time lets build 9:1 BBCs, give up 50 ft lbs ( unless we stroke them) , and use near 50 degrees of advance total!
What he means is, his vacuum advance it set to only add 3.5 degrees
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I expect to be able to run 93, I just had good gas on hand and decided to use it up during break in. Are you using any Vacuum Advance on those builds?
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hi
All of a sudden a machinist becomes an engine tuner !! wow
vac advance operates under light throttle cruise
----why----
to give ignition enough/more time to fire a less dense mixture
-------------Point in time at which events happening ---------------
vehicle speed 45--55 mph .. +/- 5mph
engine vacuum at those vehicle speeds
--------
how much ???
trial and error driving ,listen for ping or use knock ears
measure vacuum at 40 45 50 55 mph
---setup ---
Have the distributor tuned on a tuning machine and setup vacuum advance on that machine to verify when vac advance starts and to measure total vac advance .
*****For a street car and street engine vac advance done properly ----works .
To say other wise [angry] the person should be paying for fuel cost .
Then again we all should drive horse/buggy because of reliability issues with a gas engine .
All of a sudden a machinist becomes an engine tuner !! wow
vac advance operates under light throttle cruise
----why----
to give ignition enough/more time to fire a less dense mixture
-------------Point in time at which events happening ---------------
vehicle speed 45--55 mph .. +/- 5mph
engine vacuum at those vehicle speeds
--------
how much ???
trial and error driving ,listen for ping or use knock ears
measure vacuum at 40 45 50 55 mph
---setup ---
Have the distributor tuned on a tuning machine and setup vacuum advance on that machine to verify when vac advance starts and to measure total vac advance .
*****For a street car and street engine vac advance done properly ----works .
To say other wise [angry] the person should be paying for fuel cost .
Then again we all should drive horse/buggy because of reliability issues with a gas engine .
I actually do not. We aslo have a 69 SS396 that runs 12 teens @ 111 MPH at the drag strip with 8.9:1 compression and burns 87 octane. This build would be the best to use v/a but it runs just fine without it.And has been running great for 15+ years
All due respect, whom are you referring to specifically?
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Ah, was wondering what he meant. I have the Moroso limiter installed on the D setting. On a previous build it was right at 10 degrees but that car had lots of vacuum at idle. So not sure how a lower idle vacuum will come into play with this build. I don't think the stock can has an adjustable spring if I need it.
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Yeah I tried to find someone with a Dizzy machine awhile back. No luck. I did use this distributer in another small block build and had time get it tuned to my liking but that build dropped a valve and did a lot of damage. Decided that a Chevelle should have a Big Block in it, well here we are. Working out the bugs.
Nope, not getting into it here, as I have already outlined how to fix it, over 200 times, on all sorts of boards.
But, idle too high, engine builder is not a tuner by any means, hasn't got a clue as to ignition and carb for STREET engines, and, this ain't that hard to fix.
Remember, vacuum advance to FULL MANIFOLD VACUUM, NOT PORTED.
The info is out there, here, there, other places, look for the common sense stuff, not the Top Fuel/Funny Car for the street idiocy.
But, idle too high, engine builder is not a tuner by any means, hasn't got a clue as to ignition and carb for STREET engines, and, this ain't that hard to fix.
Remember, vacuum advance to FULL MANIFOLD VACUUM, NOT PORTED.
The info is out there, here, there, other places, look for the common sense stuff, not the Top Fuel/Funny Car for the street idiocy.
Your engine and car sound like a perfect candidate for vacuum advance plugged into full manifold vacuum and pump gas.
1970 El Camino SS396, VortecPro 467, ATI TH400, Moser 12 bolt /TruTrac 3:42, Hedman headers
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1,682 Posts
NOTED that some engine builders do not recommend using vacuum advance ( one mumbled something about top ring issue, but nothing further), in a street driven motor I believe that VA is a plus, better economy & poss. longer plug life with all things being equal. 10 degrees in the can appears to be the recommended standard (OK Dave ?), mfg's used a lot more VA in choked down emissions motors to attempt to offset slow or retarded advance curves. NOW...manufacturers used VA in the majority of their street driven "Super Stockers", with a few exceptions, but today your electronics advance the timing when the injection goes into the "cruise mode" to gain economy and reduce emissions. The manufacturers are not stupid when it comes to power, emissions and EPA compliance...ie factory Hi/Perf. motors like the Hellcat hemi, the blower Camaros and Mustangs use an electronic "vacuum advance" @ cruise.
One way to check out in our older cars if you have too much dist. timing is to drive into a long slow rising hill in high gear and SLOWLY ease into the throttle, listening for a rattle or pinging (lean tip-in). If it pings/rattles try the hill again with the VA disconnected and listen, if it still pings back the initial off a couple of degrees and try again. A quick way to sort out if you have too much timing and if it is the VA or the initial timing or too rapid a centrifugal advance curve
One way to check out in our older cars if you have too much dist. timing is to drive into a long slow rising hill in high gear and SLOWLY ease into the throttle, listening for a rattle or pinging (lean tip-in). If it pings/rattles try the hill again with the VA disconnected and listen, if it still pings back the initial off a couple of degrees and try again. A quick way to sort out if you have too much timing and if it is the VA or the initial timing or too rapid a centrifugal advance curve
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7,406 Posts
Only time I have ever used a vacuum advance was with a puppy cam or stock one. I tried to add 10° to my Vortec 350 and going down the road the engine ran like crap so I took it off, Vortecs don't like much more than 32-34 total. I have never noticed much fuel mileage difference with the vacuum advance on my cars. Helped out an old codger at the car show last month as his sweet truck ran like crap he said, so he had a timing light and I took a look. With the total timing and vacuum advance in his engine at idle was like 65° YIKES! so I was able to reset everything and told him to run the vacuum with this. Got a call as he was going home and he said it was like driving a new truck with the 40° I put in it for him. I guess some thing if a little is good, a bunch should be better.
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What I've found in hi comp engines, is you get detonation in the upper rpms under light acceleration with the vacuum advance connected even with 10* added. The builder wants to be safe not sorry.
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