L-72 Vacuum/Idle Update

**Duke W.** · August 31, 2002, 01:03 PM

Re: L-72 Vacuum/Idle Update

Seems like strange behavior. The emission inspired ported vacuum advance of the seventies was set up so the vacuum advance signal line port was not uncovered until the throttle plate was above the idle position. The fact that your reading at the vacuum can signal line varies so much with idle speed seems odd to me. I wonder if there is something amiss with the carburetor, or maybe your car was originally sold in CA. In addition to AIR, the CA configuration may have had a different carb with a ported vacuum advance. I don't have enough '66 documentation, but if you research the '66 CSM and AIM you will probably find the answer.

Pre-emission vacuum can signals originated at a point that would be below the throttle plate for any idle setting, so the vacuum signal line always read full manifold vacuum.

I'd suggest temporarily teeing the vacuum can off the choke vacuum break line since it appears to be full manifold vacuum and see what happens. Also, since the OEM spec for your vacuum can lists max vac. advance at 12", 10" is not enough to pull it all the way, so I'd also recommend trying the 8" can off full manifold vacuum; close to 30 degrees of total idle timing should result in better idle stability, higher vacuum, and cooler operating temps.

Given your 5000 foot altitude, 12* of initial timing will give you a total of 42 at WOT high revs as your dist. is speced at 30. You might want to bump it to 14 and see if there is any difference.

Due to the lower air density at altitude, there is less tendency to detonate and the lower density also slows flame propagation speed a bit, so the engine wants a bit more WOT timing than at sea level.

Duke

**Everett Ogilvie** · August 31, 2002, 04:08 PM

Re: 1966 AIR system

Duke, I just checked Noland's book again to refresh my memory on Option K-19, AIR. The system was mandatory on all Corvette engines in CA except the L72. There is not even an ECL (Exception Control Letters) for L72. The Judging Guide also says the AIR system is not applicable to L72. This car was sold new in Los Angeles and I have a copy of the window sticker and K19 is not listed.

Last week I helped a friend do some tuning on another L72 car, and his idle and off-idle vacuum characteristics were similar; little to no vacuum at idle, with increasing vacuum as the throttle is opened. On both cars the behavior is almost like a "switch" - there is 2" when the idle is set to 750, and as the throttle opens it discretely jumps to 5", and appears to smoothly build from there with more throttle opening. If you adjust the idle below about 750, there is zero vacuum at this port. I just checked the Holley book again and they call it timed spark, but they don't specify which carbs are set up this way. I have a spare 3247 and I will look into the primary bores and the carb base again to try to determine visually what type port is used for the spark advance.

**John L.** · August 31, 2002, 09:14 PM

Re: L-72 Vacuum/Idle Update

Everett / Duke - Please keep this thread going. I also have an L72 and had vacuum characteristics almost identical to yours. I have a comp cam that has an alsmot identical grind as the original. I swapped out the vacuum can per Duke's suggestion with the one used on the FI car. It made a huge difference in the way the car idles and seesm to have really helped out my temp problem at idle. It seems to have also helped fuel economy. Real curious to see what ultimately comes out of this.

I am beginning to think that the L72 had ported vacuum to the can. I have looked at a spare 3247 that I have and I do not see any vacuum facility in the base or below the throttle plates which makes me think it is indeed ported. None of the ref books that I have list vacuum advance data for the L72. I have both 66 & 72 Motors Manuals as well as the 66 Chasis Service guide and Overhaul guide. The entries for the L72 are either blank or N/A.

john lolli

**Duke W.** · August 31, 2002, 10:43 PM

Re: L-72 Vacuum/Idle Update

I'd recommend you tee into the choke vacuum break line, and run a signal line to the vacuum can - at least as an experiment. This should deliver full manifold vacuum to the vacuum can under all operating conditions including idle. Try it out and see how it runs.

The vacuum advance spec for '66 L-72 is 0*@6" and 15*@15.5". This is from the '66 Corvette News that has all the '66 tuneup specs. Back then the second or third issue of each volume had a table of all the tuneup info for the current model year including the centrifual and vacuum advance specs.

I suggested to Everett that he research the CSM and AIM for any information on the carburetor including if the CA version has a different calibration. You might want to do the same and see what pops out. I'd help, but I don't have a '66 CSM or AIM, but I'll take a look at what Noland has to say this evening.

It's beginning to look like the L-72 (and possibly the L-71) did not have a conventional vacuum advance system, but a "semi-ported" setup. If that's the case, it's a new one on me, but I learn something new everyday on the board. Such a system would have helped meet the '66 CA tailpipe emission standards, but there were no tailpipe standards for the other 49 states until the first federal standards were promulgated for the '68 model year.

Duke

**John L.** · September 1, 2002, 07:24 AM

Re: L-72 Vacuum/Idle Update

Duke

I am getting full advance at idle with the FI can hooked up to the regular port. It is currently running very well.

john lolli (a.k.a jl66redcpe)

**Duke W.** · September 1, 2002, 09:59 AM

Re: L-72 Vacuum/Idle Update

So if you tee into the vacuum can signal line with a vacuum gage, how much vacuum do you read at what idle speed?

Duke

**Everett Ogilvie** · September 1, 2002, 10:30 AM

Ported !

Just finished examining my spare 3247 - the vacuum advance is definitely ported. In the passenger side primary bore there is a round hole that is perfectly coincident with the throttle blade. I traced the path of the vacuum pipe connection from the outside of the carb to this port with a short piece of thread that I pushed through. Then I was able to cover the various openings and blow through the path to confirm - at the current throttle blade idle position (which was about 800 RPM on that carb when it was installed) the port is about half way covered by the throttle blade, and when you open the throttle ever so slightly it becomes fully uncovered and you can blow through it freely. This explains the vacuum performance variations, which are completely dependent on throttle blade position (idle speed setting). Recall that when I connected the vacuum gauge to the choke pull-off line, there was 10" of vacuum even down to idle speed of 750, but at the advance connection there is zero vacuum with idle speed set to 750 or lower.

**Everett Ogilvie** · September 1, 2002, 10:35 AM

Service Manuals and the AIM

Duke - I checked the CSM (Chassis Service Manual), the COM (Chassis Overhaul Manual), and the AIM - none of them refer to different versions of the 4150 Holley carb, and they only give high level setup and tuneup procedures. The service manuals do not specify what vacuum should or should not be present at the vacuum advance port either. However, see my other post about verifying that the vacuum advance source is definitely a ported source.

**John L.** · September 1, 2002, 04:32 PM

Re: Ported !

This is great information !! However now what does all this mean re advance at idle. Did Chevrolet goof in the context of retarded timing at idle ?? Is that why so many big blocks overheat at idle ?? Do you folks have a stock cam in your L 72 ???

**Everett Ogilvie** · September 1, 2002, 05:35 PM

Re: Ported !

John, I think Duke's advice about setup is excellent (using the earlier advance can and maybe using a tee to tie it into the full vacuum line that the choke pull-off uses). This should improve idle stability, and probably reduce the idle temperature. I have two L72 cars and one is a 5 Star Bowtie car, so I will not alter that car from factory configuration. FYI - I have leaned out the carb with smaller jets and the timing is well advanced b/c at high altitude you can get away with more. The Bowtie car actually idles very well at about 950. On the hottest days if it idles for a while it gets pretty warm but never hot enough to spill out the radiator cap, so I think it is operating acceptably. It even starts fairly well on hot starts. If your car is not a Bowtie car or candidate, you could consider one setup for driving (Duke's recommendations) and the original configuration for judging, if you desire. I believe that GM and Holley intended the carb to not have vacuum advance at idle to reduce some aspect of emissions - Duke would know more about this.

**Duke W.** · September 2, 2002, 01:06 PM

Re: Ported !

The L-72 turns out to be a strange one, indeed - certainly interesting, but I'm still scatching my head about how the engine is set up.

The fact that K-19 was NOT required on CA L-72 versions is enigmatic, and a revelation to me as the L-72 was probably the dirtiest engine in terms of emissions.

Since there appears to be only one carb. - the Holley 3247 - it appears that it was calibrated to meet CA emissions, so all L-72s met CA standards because they were configured the same in all 50 states. Unless someone can come up with data from the CSM, AIM or other reliable source that shows something different between the CA and 49 state configurations, this has to be the case because we know that L-72 was available in CA.

The first CA tailpipe emission standards in '66 were not that tough to meet, and I believe they only addressed CO and HC, not NOx. The early strategy to control CO and HC was to retard timing and inject air. By retarding the timing, EGT went up and the combination of higher EGT and injected air would promote enough oxidation in the exhaust system to meet the standards. As far as I know, these early air injection engines still had full manifold vacuum advance, but the initial timing was retarded from 49 state versions, so the total idle timing was less.

The vacuum can signal for the L-72 does not appear to be quite like the later"ported" systems as they would not provide any vacuum advance until well above idle, whereas the L-72 will provide some vacuum advance if the idle speed is set above the 750 spec, however, it's not clear to me how eliminating or reducing vacuum advance at idle without air injection would lower emissions. Unless there is sufficient free oxygen in the exhaust, there can't be any oxidation reaction in the exhaust system. SHP engines typically need a very rich idle mixture because of all the exhaust gas dilution of the fresh charge that is a result of the valve overlap, so I would think virutally all the O2 would be consumed during the combustion process.

Is there a special procedure in the CSM to adjust the idle mixture on CA L-72s - like the "RPM drop method" that was common in the seventies? With this procedure, you essentially set the mixture by the traditional method, then lean it out to provide about a 50 RPM drop in idle speed. This yields some free oxygen in the exhaust, but increases idle roughness because of the lean mixture.

The L-72 should have better idle quality and run cooler if full vacuum advance is applied at idle, and this can be done by teeing off the choke vacuum break, It's not clear to me if the OEM 12" can would provide full advance, but it might. In any event, the 8" can will.

If I owned a L-72 Corvette, I would probably figure out how to modify the carb internally to provide full manifold vacuum to the vacuum can signal line at idle. This would be undetectable during judging, and I doubt if it could be caught during a PV either.

I can give you guys some advice on how to improve the operating characteristics of these engines, but you'll have to run the experiments to see how a different configuration works out. Based on theoretical considerations and my experience with SHP small blocks, the '65 SB SHP/FI spark advance map is the way to go, and the same should apply to SHP BBs since they have the same basic manifold vacuum characteristics.

Duke

**John H.** · September 2, 2002, 01:53 PM

Re: Ported !

Duke -

Just as a reference point, my (stock) '69 Z/28 302 (with A.I.R.) and the huge 800CFM Holley also has ported distributor vacuum - zero vacuum at 900-950 rpm idle, starts showing vacuum at about 1100; at the manifold fitting, about 10" Hg. at 950 rpm. I run the distributor vacuum line teed into the choke pulloff hose (manifold vacuum), and use the VC-1810 vacuum can so it's fully deployed at idle.

**George J.** · September 3, 2002, 08:34 AM

Re: Ported !

John,

did they deduct for that at the show this weekend? You may have to return any award

George

**John H.** · September 3, 2002, 05:53 PM

Re: Ported !

George -

NO, they didn't spot it, YES, I got the trophy, and NO, I'm not gonna return it! (not many people know what a correctly-configured '69 Z/28 A.I.R. engine is supposed to look like anyway - there's no NCRS for 1st-Generation Camaros ). There were four other '69 Z/28's in my class, and mine was the only REAL one (and only two of the four owners knew theirs weren't real - sound familiar?); no, I didn't burst any bubbles - not my place to do that - I just let them talk.

L-72 Vacuum/Idle Update

L-72 Vacuum/Idle Update

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