69 vacuum advance for L71

[Kenneth K.]

Re: 69 vacuum advance for L71

Duke - That’s a great idea to perform maintenance on the distributor while it is out. I have parts on the way. The only parts I can’t find are the 2 nylon buttons that go under the weights to reduce wear on the weight. I am following Dave Fiedler’s procedure that I found on the sticky pages.

I am glad I removed the distributor so I could remove the VAC. I never replace a VAC before so it was easier to see how to get it out with the distributor on the bench. It’s out and I am just waiting for the new VAC to arrive.

I have experience replacing the distributor so I don’t think I will have any problems. Years ago I clocked it one spark plug tower clockwise so the tach cable is not in such a bind. I will do the same when I reinstall or maybe I will clock it 2 towers clockwise so I can get my ignition shielding on if clocking it only one spark plug tower is not enough. I haven’t had the ignition shielding on so not sure about how many towers I need to clock it.

I checked the gas station where they sell the ethanol free premium. It is 91 octane by the (R+M)/2 method. Do you think I should reduce the initial timing to maybe 6 degrees?

[Duke W.]

Re: 69 vacuum advance for L71

I'm most familiar with C2 distributors and I know from decades of experience that when you start screwing around with wire indexing or don't get the dimple in the drive gear oriented properly you are going to have a boatload full of trouble.

I've been fixing screwed up C2 distributor assembly and installations for nearly 60 sixty years starting with my SWC back in 1965. Here's the latest one:

https://www.forums.ncrs.org/showthre...tion-corrected

C3 distributor orientation (and wire indexing?) is different than C2, and I have no experience with C3 distributors other than swapping centrifugal springs and VACs without removing the distributor, so I'll leave installation info to someone who does, but my basic recommendation is index the wires and install IAW the applicable service manual.

I can only give a rough estimate on any engine's behavior on whatever octane as long as I have a rough idea of the CR and exhaust valve closing behavior. One has to experiment on each individual engine. If you have 93 PON available even if it has ethanol, use it, since it will tolerate more advance.

If you use 91 start at 6... advance 2 degrees if it doesn't detonate and retard 2 until it doesn't. If 93 start at 10. Keep testing until you find the maximum initial timing that the engine will tolerate without detonation. The automatic is an advantage because you can't load up the engine at low revs like you can with a manual transmission because of the torque converter

Once you find the sweet spot you may never have to adjust timing again because the TI should maintain that setting until you have to remove the distributor again for whatever reason.

Duke

[Kenneth K.]

Re: 69 vacuum advance for L71

I receive the B28 VAC today and as you suspected it does not meet the specs. It opens around 4.5 “Hg (spec is 3-5) which is okay but it doesn’t fully open until 9 “Hg (spec is 5.75 - 8). I guess I will send it back and keep trying to find one that is fully opened at 8 “Hg or less…

Ken

[Duke W.]

Re: 69 vacuum advance for L71

Duke - The only parts I can’t find are the 2 nylon buttons that go under the weights to reduce wear on the weight.

I don't recall single point or TI distributors having those nylon "buttons" under the weights unless they were added late in the life of those systems. The HEI distributors DO have those parts. From my 1994 BWD paper Buyer's Guide, the part number is DG13, but thy are only called out for '75-'91 HEI distributors.

Duke

[Kenneth K.]

Re: 69 vacuum advance for L71

Duke-

My 69 TI has the nylon buttons. They are about 50% worn so I should be ok if I can’t find replacements. I also have a 64 single point tach drive that has the nylon buttons. They look the same size as my 69 but are more worn.

Do you think that the new B28 VAC would meets the opening specs (5.75 - 8) if it was at sea level? At 4500-ft in Colorado atmospheric pressure is roughly 85% of atmospheric pressure at sea level. So if the diaphragm in the VAC sees 15% less pressure at 4500-ft wouldn’t it take 15% more vacuum to fully open? 0.85 x 9 “Hg = 7.6 ‘Hg which is the nominal fully opening spec….

[Joe L.]

Re: 69 vacuum advance for L71

Duke-

My 69 TI has the nylon buttons. They are about 50% worn so I should be ok if I can’t find replacements. I also have a 64 single point tach drive that has the nylon buttons. They look the same size as my 69 but are more worn.

Do you think that the new B28 VAC would meets the opening specs (5.75 - 8) if it was at sea level? At 4500-ft in Colorado atmospheric pressure is roughly 85% of atmospheric pressure at sea level. So if the diaphragm in the VAC sees 15% less pressure at 4500-ft wouldn’t it take 15% more vacuum to fully open? 0.85 x 9 “Hg = 7.6 ‘Hg which is the nominal fully opening spec….

Kenneth-------


The nylon buttons on the distributor cam's weight base were GM #1959329 but were discontinued without supercession in June, 1993. The same buttons were also used on the rear of the cross gear housing in later Corvette tach drive distributors.

According to GM some 1968-69 427 distributors and 1970 LT-1 distributors had the nylon buttons on the cam's weight base but I've never verified that. All 1971-74 Corvette distributors had the buttons on both the weight base as well as the cross gear housing. In addition, all Corvette SERVICE distributors for pre-1971 applications manufactured after about 1971 had the buttons at both locations.

[Duke W.]

Re: 69 vacuum advance for L71

Duke-

My 69 TI has the nylon buttons. They are about 50% worn so I should be ok if I can’t find replacements. I also have a 64 single point tach drive that has the nylon buttons. They look the same size as my 69 but are more worn.

Do you think that the new B28 VAC would meets the opening specs (5.75 - 8) if it was at sea level? At 4500-ft in Colorado atmospheric pressure is roughly 85% of atmospheric pressure at sea level. So if the diaphragm in the VAC sees 15% less pressure at 4500-ft wouldn’t it take 15% more vacuum to fully open? 0.85 x 9 “Hg = 7.6 ‘Hg which is the nominal fully opening spec….

The VAC works on differential pressure, so I believe that it should test the same at altitude as at sea level, however, I believe that any engine configuration will show less idle speed/vacuum at altitude, so idle speed must usually be adjusted up to equal sea level idle speed or a bit more, but manifold vacuum will be less than at sea level at the same idle speed.

Another thing to consider is that increasing total idle advance by converting ported to full time manifold vacuum usually increases manifold vacuum by a measurable amount at the same idle speed.

Duke

[Duke W.]

Re: 69 vacuum advance for L71

Kenneth-------


The nylon buttons on the distributor cam's weight base were GM #1959329 but were discontinued without supercession in June, 1993. The same buttons were also used on the rear of the cross gear housing in later Corvette tach drive distributors.

According to GM some 1968-69 427 distributors and 1970 LT-1 distributors had the nylon buttons on the cam's weight base but I've never verified that. All 1971-74 Corvette distributors had the buttons on both the weight base as well as the cross gear housing. In addition, all Corvette SERVICE distributors for pre-1971 applications manufactured after about 1971 had the buttons at both locations.

Looking in my 2/77 P & A catalog I see a "button" called out in Group 2.381, p/n 1957329 for all 71-74 Y (Corvette) It expect this is the same as the B-W DG13 I referenced earlier, and I'm sure it's sold in various "brands" if it's still manufactured, so if your convenient parts store doesn't sell the B-W brand, they can probably cross reference either the B-W or GM number to whatever brand they sell if it's still manufactured.

Duke

[Joe L.]

Re: 69 vacuum advance for L71

Looking in my 2/77 P & A catalog I see a "button" called out in Group 2.381, p/n 1957329 for all 71-74 Y (Corvette) It expect this is the same as the B-W DG13 I referenced earlier, and I'm sure it's sold in various "brands" if it's still manufactured, so if your convenient parts store doesn't sell the B-W brand, they can probably cross reference either the B-W or GM number to whatever brand they sell if it's still manufactured.

Duke

Duke------


I cannot find that the button remains available from Borg Warner. Borg Warner, like many other automotive suppliers, has had a "metamorphosis" and now focuses on turbocharging systems.

[Kenneth K.]

Re: 69 vacuum advance for L71

I did a search on “1957329 nylon buttons” and got a couple of hits. Corvettepacifica sells them of $3 ea. so I am going to order a couple. Thanks for the help. Interesting none of the large corvette suppliers sell this part….

I sent the new 4V1053 VAC back to Rockauto.com where I purchased it since it does not meet specs and asked them to send me a replacement. I don’t have much hope that the replacement will be any better and my luck it will be even more out of spec. I could not find an auto part store in town that could even order one. I tried them all, Autozone, Oreilly’s, Advanced Auto, NAPA…. Rockauto was the only source i could find. Although I did see a couple on eBay and I have asked them if they could check to see if their VAC meets specs. We’ll see….

If I can’t find a 4V1053 that meets specs I guess my options are to stay with ported vacuum or try and find an adjustable VAC that I can set the start/stop points and total advance. Do they make an adjustable VAC that you can do that? Sounds like the Accel variable VAC that Mark posted varies the vacuum point for zero advance, maximum advance, and the amount of maximum advance. Does that mean you can’t set the start/stop points and total advance on this adjustable VAC?

[Mark E.]

Re: 69 vacuum advance for L71

I did a search on “1957329 nylon buttons” and got a couple of hits. Corvettepacifica sells them of $3 ea. so I am going to order a couple. Thanks for the help. Interesting none of the large corvette suppliers sell this part….

I sent the new 4V1053 VAC back to Rockauto.com where I purchased it since it does not meet specs and asked them to send me a replacement. I don’t have much hope that the replacement will be any better and my luck it will be even more out of spec. I could not find an auto part store in town that could even order one. I tried them all, Autozone, Oreilly’s, Advanced Auto, NAPA…. Rockauto was the only source i could find. Although I did see a couple on eBay and I have asked them if they could check to see if their VAC meets specs. We’ll see….

If I can’t find a 4V1053 that meets specs I guess my options are to stay with ported vacuum or try and find an adjustable VAC that I can set the start/stop points and total advance. Do they make an adjustable VAC that you can do that? Sounds like the Accel variable VAC that Mark posted varies the vacuum point for zero advance, maximum advance, and the amount of maximum advance. Does that mean you can’t set the start/stop points and total advance on this adjustable VAC?

If you look at the advance graph posted for the Acell variable VAC, turning the adjustment screw varies the vacuum points when the curve starts, ends, and the total advance amount. In practice, advance the adjustment until detonation occurs during part throttle or high vacuum conditions, then back off a nudge.

I've had good results with this product but I've never used one on a solid lifter engine.

[Kenneth K.]

So I meet the 2 inch Rule!

[Duke W.]

Re: 69 vacuum advance for L71

Congratulations... you're making great progress. As I started reading your post I recalled that I neglected to tell you that since the full time vacuum advance increases total idle advance, idle vacuum at 750 in Drive would likely be greater, so the first 4V1053 might have been okay, but I'd glad you requested a second one and it meets spec.

You didn't say what RPM you set initial timing at, but it has to be below the speed that centrifugal starts, which I recall is 900 for your engine, but verify with the CSM. So you need to set it at below centrifugal start speed, in neutral, or you can set it at 38 with the VAC disconnected and plugged at above the max centrifugal speed that I think is 3800 on your engine, but again, verify with the CSM.

I think you'll find the new dial back timing light a great tool to have.

Once you get the timing dialed in you can do some road testing. I'm interested if you notice any difference in driving characteristics, particular low speed throttle response. Also, note any changes in typical operating temperature, particularly around town.

Then it's on to octane appetite testing.

I believe '69 was the first year that a mechanical lifter engine was offered with an automatic. The standard axle was 3.08 with 2.73 and 3.36 optional. Do you know which one you have?

As a general rule automatics don't work well with high overlap cam engines, but that big block mechanical lifter cam is not all that wild, and IMO it's more docile than a 283 with a Duntov cam. Longer stroke engines can usually tolerate more duration and overlap than short stroke engines. So I'm interested in your driving impressions, especially around town. Is is a cantankerous beast or acceptably docile? Also what's converter stall speed. To test firmly apply the brakes at a standstill, then floor the throttle and lift off once the engine speed no longer increases. It only takes a second or two.

Keep up the good work!

Duke

[Joe L.]

Re: 69 vacuum advance for L71

I believe '69 was the first year that a mechanical lifter engine was offered with an automatic. The standard axle was 3.08 with 2.73 and 3.36 optional. Do you know which one you have?

As a general rule automatics don't work well with high overlap cam engines, but that big block mechanical lifter cam is not all that wild, and IMO it's more docile than a 283 with a Duntov cam. Longer stroke engines can usually tolerate more duration and overlap than short stroke engines. So I'm interested in your driving impressions, especially around town. Is is a cantankerous beast or acceptably docile? Also what's converter stall speed. To test firmly apply the brakes at a standstill, then floor the throttle and lift off once the engine speed no longer increases. It only takes a second or two.



Duke

Duke------


Yes, 1969 was the first year for use of an automatic transmission with a mechanical lifter engine in a Corvette. It was a special HD version of the THM-400.

It was offered with the L-71, L-89, L-88, and ZL-1.

While the L-71/L-89 cam was not very wild, the L-88/ZL-1 cam could be considered pretty wild.

[Duke W.]

Re: 69 vacuum advance for L71

About 20 years ago I got a brief ride in a '69 L-88 with a TH-400 in Scottsdale. It was a beast!

Do you know what the converter stall speed is and what parts were different from a the TH400 offered on the milder engines?

Duke