I would like to provide an update here concerning my L72. As has been suggested by Duke and discussed in some detail on the DB, I reconfigered my vacuum advance line and re-connected the hose to the port opening located on the rear of the intake manifold. At this opening I am getting full manifold vacuum with a reading of 14 to 15 inches of vacuum at idle. The vacuum can is the orginal can, which I tested, and starts to provide advance at 7 inches of vaccum and provides its full 12 degrees of advance at 15 inches of vaccum. With this new set-up, I drove the car for the first time yesterday and noticed that its seems to idle better and has better response off the line. The real test will come this summer when the weather gets hot again to see if this new ste-up will help with my over-heating condition at extended idle. One question for Duke, if running the vacuum advance line off full manifold vacuum provides superior results to the oinal factory set-up off the carb port opening, why didn't GM engineer this set-up in the first place?
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Re: C2/L72 Vacuum Advance Update
They did, all the early vacuum systems (at least thru '65) were setup that way.Bill Clupper #618 -
Re: C2/L72 Vacuum Advance Update
John - I have followed this thread fully as well fo my L72 when I finsh the engine rebuild. They did run full ported vacuum on many, I remember Duke was surprised to find the 66 L72 ported. Apparently, this was some of the early attempts at SMOG control, remember the added the A.I.R. in CA for the first time in 66...CraigComment
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Re: C2/L72 Vacuum Advance Update
Craig, How is your L-72 currently set-up? Is your vacuum line running off a ported or full manifold source? Also, are you using the factory original vacuum advance can or did you switch over the vacuum can recommended by Duke which I believe provides 16 degrees of advance at 8 inches of vacuum?Comment
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Re: C2/L72 Vacuum Advance Update
During this discussion it was briefly noted that a 1967 L-71 might also receive its vacuum signal from a ported source. I followed this thread with great interest because I was having a difficult time adjusting the carberation and timing to get a steady idle.With the vacuum gauge on the throttle body plate,the gauge read 2" of vacuum at an idle speed of 950 rpm.With minor movement of the throttle,the gauge read 10-15" at 1200-1500 rpm. Closer examination of a spare Holley center carberator revealed that the vacuum port for the distributor line is partially covered by the throttle blade; only slight movement of the throttle is necessary to achieve full vacuum. Regards,Kieran #11373Comment
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Re: C2/L72 Vacuum Advance Update
John - my 66 L72 had an NOM 68 4 bolt rect port engine, and I now have and am assembling all of the correct block and heads....my Holley and the intake were original however, but all the vacuum plumbing was wrong. I have the corrrect 360 (In think that was the last 3 digits) can on the TI dist, but I bought a VC 1810 per Duke's posts, and was just going to hook it up that way after I get the Holley restored. Sorry I don't have any current info to help you out...CraigComment
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Re: C2/L72 Vacuum Advance Update
In one of the earlier threads on this subject, it was pointed out that CA L-72s DID NOT have AIR. This was new to me.
All I can figure is that GM decided to do one L-72 configuration to meet CA specs, and they also sold the same configuration to all other markets. It may have been motivatied by cost and volume considerations for the L-72. Ported vacuum advance was purely an emission control strategy to keep EGT higher, which also contributes to higher engine temps in marginal cooling situations like idling on a hot day.
John - would to check idle vacuum again now that you have full vacuum advance. It should pull a bit more vacuum than before at the same idle speed.
My Corvette News spec says the '72 L-72 max vacuum advacne is 15*@12". Did you tranpose the numbers in your post?
DukeComment
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Re: C2/L72 Vacuum Advance Update
Duke, the original 360 vacuum can on my car has a 12 on it which I undertand indicates that the can produces maximum advance of 12 degrees. When I tested it, maximum advance was achieved at 15 inches of vacuum.Comment
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Re: C2/L72 Vacuum Advance Update
Duke, the spec on the 360 can is 12. I have 3 of them and they all provide 11 or 12 degrees of advance.
AIR was indeed not available on the L72.
While testing three different 093 distributors (this is the L78 and L72 TI distributor) I have learned that there were (at least) two different sets of centrifugal advance weights in the 093 distributor. One set is "light weight" with holes drilled in the ends, and the other set is "heavy" without holes. I have weighed them and there is about 7 or 8% difference in the weights (I have plotted the two advance curves and the heavy curve is quite aggressive). Based on limited data, it COULD be that late 093 distributors came with the heavy weights, while earlier distributors came with the lighter weights. I have one more early distributor that I need to check out to see if this theory holds. At any rate, it appears that Chevrolet was testing/changing combinations of vacuum and mechanical advance during 1966 production.Comment
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Re: C2/L72 Vacuum Advance Update
Okay, it sounds like Corvette News transposed the spec numbers, 12*@15" it is.
If the L-72 will pull only 14" at idle then a can with max advance at not more than 12" would provide the best idle stability and quality. Of course the NAPA/Echlin VC 1810 with 16*@8" would meet this criteria.
I think both of you have the vacuum advance document with all the specs. You might want to take a look at that and see if there is a can that provides about 16*@12".
Regarding the two sets of weights in 093 distributors, I would postulate that someone changed the weights somewhere along the line. If Chevrolet changed the centrifugal spec during the year, they would have assigned a new part number to the distributor.
DukeComment
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Re: C2/L72 Vacuum Advance Update
When I first encountered the different weights I thought someone had been playing around with one or all of these distributors, and I was not sure which weights would be the originals. But after talking to Dave Fiedler of TI Specialty, he informed me that the 093 came "both ways", although he did not go into detail about when changes were made. I won't claim to be sure about this - only passing along what I was told.
Some of the (L72) changes that appear to have been taking place during '66 add up to what could have been attempts at improving idle stability and/or early emissions calibrations, or both. Recall that during '66 production, Chevrolet opened up the solid lifter valve lash - from 020/024 to 024/028. First edition Owner's manuals list the tight spec - the 2nd edition lists the loose spec. Maybe they tried to improve idle quality due to ported vacuum and lazy vacuum can characteristics, by loosening up the valves?Comment
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