VAC for L79 (1966 4 speed)
Having read so much pertaining to the ignition timing being critical to idle tempreture, I advanced the timing by ear. The water temp dropped and stayed stable, which has been a problem since purchase. The current VAC is a a B26, idle speed 750 - 800 RPM, and the initial timing is 12 - 14 degrees at idle with vacuum hose disconnected. Can someone suggest a VAC that will be all in at idle and is available? The numbers I have been chasing seem to all be obselete. My experience with reproduction mechanical parts has been less than something a person would want. What brand and Part # are you using to replace the B26. Or, is there a better choice on the market. At this point function is the first issue! Been using the search feature with no good results. Recently thought there was a list of currently available replacements listed in a post but simply cannot locate the post. Any help or shared experience is appreciated.
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Re: VAC for L79 (1966 4 speed)
Are you absolutely, positively sure that it has an OE L-79 equivalent cam. Unless you've owned it since new and have detailed records there's no way to be sure. So measure idle vacuum at 750 and report.
If you have a real L-79 cam idle behavior should be 14-15" @ 750, so a 12" B26 passes the Two-Inch Rule, but test it... start at 6" fully deployed at 12"
OE L-79 max centrifugal is 30 @ 5100, so you should run no more than 10 initial. A better option is to buy a Mr. Gasket 928 spring kit install the black springs, and set total WOT advance as high in the 36-40 range as the engine will tolerate without detonation at a few hundred revs above where centrifugal is all in which shouldn't be more than about 3500..
Search this site for threads started by me: L-79 blueprint overhaul and the Web: Duke Williams San Diego Corvette for the tuning seminar.
Duke -
Re: VAC for L79 (1966 4 speed)
John------
I believe that all of the vacuum controls currently available, regardless of brand, are manufactured by Standard Motor Products. So, as far as brand goes, it really does not matter. Keep in mind that it's been a very long while since these controls were used on production vehicles. The number of manufacturers has dwindled to one. At some point it will dwindle to none.In Appreciation of John HinckleyComment
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Re: VAC for L79 (1966 4 speed)
I've purchased a few Standard Motor Products #VC181 from www.rockauto.com and/or Amazon. Stamped as B26, tested accurately with vacuum pump/gauge, and about $11 each (watch shipping costs with RockAuto). I'm sure other places have them as well.Comment
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Re: VAC for L79 (1966 4 speed)
...absolutely agree. These "B-number" VACs are not "reproductions", but OE replacement parts, and SMP is a Tier 1 OEM supplier to the automotive industry.
There are some "reproduction" VACs sold by the various Corvette parts vendors - source unknown - that have the correct OE ID number, but some aren't even close to the original OE spec for that number.
A year ago I bought a B26 as a birthday present for a buddy to convert his '67 L-71 to full time vacuum advance. I found one, Standard Brand VC-181, on Amazon for a little over ten bucks. I could clearly see the "B26" in the photo.
A neighbor ordered it for me with her Prime account, so no shipping charge and two-day delivery. I tested the start/stop points and it was well within spec.
DukeComment
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Re: VAC for L79 (1966 4 speed)
Duke------
I would totally steer clear of the "reproduction" vacuum controls with original embossments and configuration or, at the very least, check them before installation. As with may reproduction parts, I think they're more about re-creating original configuration than they are about re-creating original functionality. Even if they pass a performance check, I'd still be concerned about durability.In Appreciation of John HinckleyComment
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Re: VAC for L79 (1966 4 speed)
Thanks to all! Ordered the VC181 from amazon, will arrive Friday. Have the Mr. Gasket spring assortment and will install the black springs today. An interesting side note is that with the advance weights held in the "unadvanced" position with rubber bands, and the initial ignition timing set at factory settings there is an additional timing mark on the harmonic balancer that lines up with the (0) mark on the indicator. So in a nutshell there are two grooves in the balancer, one indicating TDC, the other indicating Total VAC advance with centrifugal weights disabled. Has anyone else a balancer such as this. The line looks far too "factory" to be something that a previous owner had added. Could find no information on this in any reference material I have. Any comments on this?Comment
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Re: VAC for L79 (1966 4 speed)
The material giving the cross reference part identification and specs was published as a joint paper written by lars and Duke. If you are having problems locating a VAC you would do well to read this - lots of information and background and help in finding a part which seems to be on the endangered species list of replacement parts that are built to specifications that will do as they should. Thanks Duke and Lars for all you do for the Corvette restoration community!Comment
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Re: VAC for L79 (1966 4 speed)
Rubber bands! NIST calibrated type?...lol
Another (general) suggestion IF numbers matching isn't as important as performance is to get an adjustable "travel" (NOT adjustable rate) canister. This will allow you to set the travel limit and fine tune till your hearts content.
I use straight manifold vac not ported. You can bring the vac adv all-in at idle. 10-12* base 8-10* on the can for 18-20-22* at idle or more if desired. I usually suggest giving the engine as much timing on all three event as it will tolerate before detonation. But caution as it can be undetected as the ear wont always here it. Better to lean on the safe side and go conservative.
Of course all other systems must be up to snuff no vac leaks etc...
The A/F ratio gets adjusted as the timing gets adjusted. Go back n forth between the two until you find the engines sweet spot. which is the highest vac signal at curb idle.
Many variables come into play with fine or super tuning...gas and octane, engine characteristics(cam compression timing gear slop) elevation, driving habits...et al.
FYI you can mix match advance springs to get the total to come in at your desired RPMsComment
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Re: VAC for L79 (1966 4 speed)
All of the mods to the distributor are finished. Mr. Gasket "black " springs and B26 VAC. Here is the ignition timing map that was created: 700 RPM - o* advance. 800 RPM = 2* centrifugal advance, 1000 RPM + 4* advance, 1500 RPM + 7*, 2000 RPM + 13*, 2500 RPM + 13* also. The total centrifugal and initial advance now total 36*crankshaft. The new VAC starts to move at 5" HG, and is all in at 12.5 " HG. So if the engine is at idle the VAC is fully advanced. The initial static advance was set at 10*, giving a total at idle spark advance of 36* plus 2* centrifugal (idle speed 825 RPM) So the running engine sees 38* at idle. The engine runs with what I will call "crisp" feeling, and lugging in top gear produced no rattle or ping that I could hear. The total centrifugal advance would be 36* (26 cent. and 10 initial) which should be no problem although I doubt the car will ever see much or any full throttle driving. Now for the best part! For the last two years I have been fighting an overheating problem at idle.
For a few minutes no problems but after ten or so minutes the temp gauge would begin to creep and I suppose would go to 250 if one let it run. All of the usual suspects were inspected and many improved such as radiator seals from AC car, 7 blade fan, rebuilt stock fan clutch as well as a new Hayden replacement, DeWitt alloy radiator, 16 lb. pressure car, restricting the by pass hose on the water pump to intake 90* rubber hose to 1/4 inch, restricting completely the little hole in the water pump on the passenger side that should be blocked with the 350HP engine, 180* stat etc. Still overheated!! With this new timing map and VAC the engine will now run I guess until it runs out of gas. The temp here has been in the low to mid nineties so this was a good opportunity to test. After a one hour interstate drive at 70 - 75 MPH, in town for 20 minutes missing most stop lights, the car was let to idle 45 minutes and the temp stabilized at 193 and never, ever moved higher. This was with an ambient temp of 93*F. As you would expect with this temp there was no "puke" after shutdown. Engine starts immediately when cold, and as mentioned runs like it is on ball bearings. Just so wonderful to have the pesky overheating problem in the rear view mirror! I want to thank all of you for the tips, messages and well wishes during this project. Duke and Lars helped so much with their well written tips and information so a special thanks to both for sharing so willingly their knowledge. The bottom line is that this particular car likes lots of idle spark advance to keep cooler engine temps. Hope that this may help someone in the future with an overheating problem at idle.Comment
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Re: VAC for L79 (1966 4 speed)
... glad your problems appear to be solved. One comment is that the OE max centrifugal spec is 30 degrees, but you only measured 26 if I'm interpreting your data correctly. (It would help if you use liberal paragraph spacing... half a screen of single spaced text is hard to read.)
One test that should be done is to bring revs up to a few hundred above what you measured for max centrifugal, then give the throttle a hard blip to ensure that there is no more advance.
If you set the initial advance at 10 (at below 800 where centrifugal starts) then total idle advance should be 10 plus 16 vacuum plus 2 centrifugal at 825 idle speed for a total of 28.
Also, setting total WOT advance as high in the 36-40 range as the engine will tolerate without detonation is usually the optimum. I shoot for 38-39, and if it doesn't detonate, leave it be.
DukeComment
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