Should I connect my vacuum advance?
07-12-2011, 02:35 PM
#21
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Some combinations were used for OEM tuning/driveability reasons and others were used for competition reasons. This didn't just apply to Chevrolet either.
The "270" distributors were used in the solid lifter engines to provide increased dwell time for the coil to build via the dual points. The vacuum advance was eliminated to provide the added dwell stability due to a fixed breaker plate vs a moving breaker plate for the vacuum distributor. As noted above, the centrifugal only distributors had a more aggresive advance curve which gave better performance under competition conditions. An unmolested vacuum advance distributor will not have this feature.
As most here know, the vacuum advance is not functional under wide open throttle which was the anticipated usage of the solid lifter engines.
Properly calibrated for a street engine, the vacuum advance wins hands down.
07-12-2011, 05:54 PM
#22
Melting Slicks
John, your article ends with:
Tune in next month, when we’ll show you
how to check out those systems, how to “map” your advance curves against their design
specifications and verify proper operation, and pass along some simple tips and techniques for
improving your Corvette’s performance by “tweaking” its advance systems for peak efficiency.
Can you post that paper too?
Tune in next month, when we’ll show you
how to check out those systems, how to “map” your advance curves against their design
specifications and verify proper operation, and pass along some simple tips and techniques for
improving your Corvette’s performance by “tweaking” its advance systems for peak efficiency.
Can you post that paper too?
07-12-2011, 09:27 PM
#23
Racer
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Guys, just for kicks (being the moron that I am about tech stuff), what would happen if I DID connect my vac advance line (assuming that my vac can is working) but DIDN'T change the timing? Consequences? Symptoms?
Dave
Dave
07-13-2011, 12:17 AM
#24
Le Mans Master
In the 1950's, lot's of different distributor combinations were commonly used. Mechanical advance only, vacuum advance hooked to part time vacuum and vacuum advance with full time.
Some combinations were used for OEM tuning/driveability reasons and others were used for competition reasons. This didn't just apply to Chevrolet either.
The "270" distributors were used in the solid lifter engines to provide increased dwell time for the coil to build via the dual points. The vacuum advance was eliminated to provide the added dwell stability due to a fixed breaker plate vs a moving breaker plate for the vacuum distributor. As noted above, the centrifugal only distributors had a more aggresive advance curve which gave better performance under competition conditions. An unmolested vacuum advance distributor will not have this feature.
As most here know, the vacuum advance is not functional under wide open throttle which was the anticipated usage of the solid lifter engines.
Properly calibrated for a street engine, the vacuum advance wins hands down.
Some combinations were used for OEM tuning/driveability reasons and others were used for competition reasons. This didn't just apply to Chevrolet either.
The "270" distributors were used in the solid lifter engines to provide increased dwell time for the coil to build via the dual points. The vacuum advance was eliminated to provide the added dwell stability due to a fixed breaker plate vs a moving breaker plate for the vacuum distributor. As noted above, the centrifugal only distributors had a more aggresive advance curve which gave better performance under competition conditions. An unmolested vacuum advance distributor will not have this feature.
As most here know, the vacuum advance is not functional under wide open throttle which was the anticipated usage of the solid lifter engines.
Properly calibrated for a street engine, the vacuum advance wins hands down.
07-13-2011, 12:34 AM
#25
Le Mans Master
Dan, here is a pic of my distributor taken from page 198 (6Y-2) of ST-12. It's the one on the right. Is that a vac can on it or is that the FI pump drive you mention?
URL: http://i1212.photobucket.com/albums/...s/gijfbhjb.png
URL: http://i1212.photobucket.com/albums/...s/gijfbhjb.png
07-13-2011, 02:16 AM
#26
Le Mans Master
For wide open throttle operation Mechanical advance is all that is needed. For all other engine operation you will find a more efficient running engine with the Vacuum hooked up.
You may need to tune your vac advance to maximize the results. Modern technology allows the timing to vary from 0 to 50+ degrees of advance depending on your driving situation. Running a properly tuned distributor with vac advance gets you as close to modern as possible. Better torque curve, better MPG same total Horsepower. How does this sound BAD?
You may need to tune your vac advance to maximize the results. Modern technology allows the timing to vary from 0 to 50+ degrees of advance depending on your driving situation. Running a properly tuned distributor with vac advance gets you as close to modern as possible. Better torque curve, better MPG same total Horsepower. How does this sound BAD?
07-13-2011, 02:21 AM
#27
Le Mans Master
In the 1950's, lot's of different distributor combinations were commonly used. Mechanical advance only, vacuum advance hooked to part time vacuum and vacuum advance with full time.
Some combinations were used for OEM tuning/driveability reasons and others were used for competition reasons. This didn't just apply to Chevrolet either.
The "270" distributors were used in the solid lifter engines to provide increased dwell time for the coil to build via the dual points. The vacuum advance was eliminated to provide the added dwell stability due to a fixed breaker plate vs a moving breaker plate for the vacuum distributor. As noted above, the centrifugal only distributors had a more aggresive advance curve which gave better performance under competition conditions. An unmolested vacuum advance distributor will not have this feature.
As most here know, the vacuum advance is not functional under wide open throttle which was the anticipated usage of the solid lifter engines.
Properly calibrated for a street engine, the vacuum advance wins hands down.
Some combinations were used for OEM tuning/driveability reasons and others were used for competition reasons. This didn't just apply to Chevrolet either.
The "270" distributors were used in the solid lifter engines to provide increased dwell time for the coil to build via the dual points. The vacuum advance was eliminated to provide the added dwell stability due to a fixed breaker plate vs a moving breaker plate for the vacuum distributor. As noted above, the centrifugal only distributors had a more aggresive advance curve which gave better performance under competition conditions. An unmolested vacuum advance distributor will not have this feature.
As most here know, the vacuum advance is not functional under wide open throttle which was the anticipated usage of the solid lifter engines.
Properly calibrated for a street engine, the vacuum advance wins hands down.
07-13-2011, 06:24 AM
#28
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If you hook up the vacuum, nothing may happen if the can is no good or non-functional. The worst that could happen is you'd wind up with too much timing and you'l likely hear the engine spark knocking or bucking/surging at lower speed.
If your initial timing isn't set any faster than 6-10 degrees and you hook the vacuum up to manifold vacuum, chances are the only thing you'll notice is the engine seems more responsive and the idle speed increased.
No guarantee though until you know for sure what specs the distributor is running.
What's holding you back from trying it? Fear and common sense?
07-13-2011, 06:56 AM
#29
Melting Slicks
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Just go purchase a timing light and check where it is at now. You don't have to change anything to do this. Very, very simple. Once you know where you are at now, others can help you about what to do. If just checking the timing is more than you want to tackle that is fine, then you should just take your car to a mechanic and not even think about these things. But that way you miss all the fun.......Just do it! And good luck.
07-13-2011, 08:16 AM
#31
Racer
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Okay, guys. I'm gonna get a timing light and see where my timing is set now. I'm also gonna get some vac hose and hook up my vac can to the manifold port (without changing timing). I'll report the results and y'all can coach me from there.
What a super resource your collective knowledge is! Thanks again, to all of you. Stay tuned.
Dave
What a super resource your collective knowledge is! Thanks again, to all of you. Stay tuned.
Dave
07-13-2011, 08:27 AM
#32
Le Mans Master
John, your article ends with:
Tune in next month, when we’ll show you
how to check out those systems, how to “map” your advance curves against their design
specifications and verify proper operation, and pass along some simple tips and techniques for
improving your Corvette’s performance by “tweaking” its advance systems for peak efficiency.
Can you post that paper too?
Tune in next month, when we’ll show you
how to check out those systems, how to “map” your advance curves against their design
specifications and verify proper operation, and pass along some simple tips and techniques for
improving your Corvette’s performance by “tweaking” its advance systems for peak efficiency.
Can you post that paper too?
I have John's various tech papers (and many others) hosted up on my website's tech articles page here:
http://lbfun.com/Corvette/Tech/vettetech.html
you are welcome to go thru all the articles I have hosted up.
To find the specific article from John about mapping your advance curve click on the title header called "Timing and Vacuum Advance" than click on the article called:
Mapping Advance
by: John Hinckley
07-13-2011, 12:23 PM
#33
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Here's a link to the full/expanded version of the "Timing 101" article, as published originally with photos in "Corvette Enthusiast" magazine; Barry, you can put this one up on your site as well if you like. 
http://www.camaros.org/pdf/timing101.pdf
http://www.camaros.org/pdf/timing101.pdf
07-13-2011, 12:50 PM
#34
Le Mans Master
John, that version of your Timing101 article IS the version I have hosted up on my site. You sent it to me a while back to replace the older version without pics I originally had. 
you've always been pretty good about keeping me and my website with the most up-do-date versions of your articles.
you've always been pretty good about keeping me and my website with the most up-do-date versions of your articles.
07-13-2011, 12:59 PM
#35
Safety Car
Dave,
One other thing to keep in mind if you find that the initial timing can't get to spec with the distributor you have. Because the can hits on the manifold in one direction and the coil in the other direction, it is not all all uncommon to find that the distributor can not be moved enough to get the initial timing correct. Unfortunately, moving the distributor gear one notch will move it too far in the other direction. The only solution is to remove the distributor, punch out the pin that holds the gear to the shaft, and rotate the gear 180 degrees on the shaft and reinstall the pin. This will effectively give the equivalent of moving the distributor 1/2 a tooth. I have put the distributor on a couple of 270 hp engines, and I always had to rotate the gear to get it timed. There just ain't much adjustment when trying to use a vac advance distributor on a 270 horse engine!
I would guess that this is the very reason that GM did not mount a vac can distributor on these engines.
Regards, John McGraw
One other thing to keep in mind if you find that the initial timing can't get to spec with the distributor you have. Because the can hits on the manifold in one direction and the coil in the other direction, it is not all all uncommon to find that the distributor can not be moved enough to get the initial timing correct. Unfortunately, moving the distributor gear one notch will move it too far in the other direction. The only solution is to remove the distributor, punch out the pin that holds the gear to the shaft, and rotate the gear 180 degrees on the shaft and reinstall the pin. This will effectively give the equivalent of moving the distributor 1/2 a tooth. I have put the distributor on a couple of 270 hp engines, and I always had to rotate the gear to get it timed. There just ain't much adjustment when trying to use a vac advance distributor on a 270 horse engine!
I would guess that this is the very reason that GM did not mount a vac can distributor on these engines.
Regards, John McGraw
07-18-2011, 08:28 AM
#36
Racer
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Thank you for your suggestion, John. The distributor pictured in John Hinckley's superb article is the exact one I have. I can see your point about limited rotation due to the close quarters for the vac can. Maybe that's why whoever restored this engine capped the vac line. I'll try to get up enough nerve to rotate that gear as you suggest. :-) Thanks again, John.
Dave
Dave
07-18-2011, 10:28 AM
#37
Race Director
Fi distributors are actually two piece units...the top rotates to get the advance and the bottom always sets in the same place because of the fuel pump and tach drives.....they {the bottom} can never be in a different location or rotated.
