69 vacuum advance for L71

[Kenneth K.]

Re: 69 vacuum advance for L71

Duke-

I have the 3:08 rear end with posi. I think the 3:08 and the automatic tames the L-71. It’s not a beast at all to drive, in fact, it’s not any more wild than my 1995 LT-1. Really a lot of fun with the tri-power and not having to shift around town. Very thumpy cam in drive when you’re just idling around like in a parade. Of course on the road is a different experience. Hot, rough ride and noisy! AC was not an option with the L-71 automatic.

Not sure what the stall speed is maybe around 2500 RPM?.. I don’t know a lot about the TH400 other than it is HD compared to the others offered on the milder engines. I also remember reading that the torque converter has 6 bolts so it’s HD. If anyone has information please post.

I set the initial timing to 8 degs with the VAC plugged off and around 750 RPM so that is below the speed the centrifugal starts.

I was able to do a little testing today. Before I converted to full time vacuum and initial timing at 8 degrees I was running initial timing at 12 degrees and no VAC. So I am not sure if some of the difference I noted was do to the change in initial timing or the VAC.

I only had a little av gas and 91 octane mixture left so I could not do a lot of street testing. It did seem to accelerate better with more power from WOT start and low speed response but I didn’t see any obvious difference at higher speeds. I don’t like revving it more than 3500 RPM. It’s a numbers matching engine and I am a little nervous that I might damage it. Maybe I am more cautious than I need to be.

I simulated the parade route to see how hot it got. I was running 210 degrees at the end of the route which should be safe but it was only 83 degrees today. I am going to get more fuel tomorrow and test it again. It is supposed to be in the 90s so that will be a better test. I am wondering why my total advance is only 19 degrees (8 degrees initial timing plus 11 degrees VAC). If I could get the full 16 degrees VAC maybe it would run cooler.

I did experience a nasty backfire when I when to WOT from 35 MPH. When I was running without the VAC and initial timing set at 12 degrees I had to reduce my accelerator nozzle a couple sizes because I had a hesitation at WOT. Reducing the shooter size was the only way I could eliminate the hesitation. So I am thinking maybe it was a lean backfire caused by the small shooter. I replaced the smaller shooter with one size larger and didn’t get any backfires but I didn’t do a lot of tests. I’ll do more WOT testing tomorrow.

[Duke W.]

Re: 69 vacuum advance for L71

The VAC should add about 16 to initial, assuming the engine is below the start point for centrifugal. So if you're sure initial is 8 (measured below 900) but total idle advance is only 19 at 10" then it sounds like the 4V1053 is out of spec in terms of maximum advance. Maybe that's why the pin is at the limit at 8".

Duke

[Terry M.]

Re: 69 vacuum advance for L71

The Restorer

Ken
There was a story about yours and mine TH400 Transmission in The Restorer some years ago. The title involved the code for the TH400 you are referring to -- CY. I can't even remember whose tenure as Editor it was published under.

[Kenneth K.]

Re: 69 vacuum advance for L71

Duke-

The specs for the B28 VAC in the article you posted on the sticky pages says it starts at 3-5 “Hg and pulled fully back at 5.75-8 “Hg. I tested the B28 VAC from RockAuto out of the box before I installed it in the distributor and it starts at 5 ‘Hg and fully pulled back at 8 “Hg so it meets the upper end of the spec.

I retested the VAC in the car and it is almost fully pulled back at 8 “Hg but needs 9 “Hg to ‘snap’ it tightly agains the stop. Apparently the resistance in the distributor requires a little more vacuum. However, the timing line is nice and steady and doesn’t bounce around so it appears to be locking in the total advance. With 8 degrees initial timing I see about 19 degrees total with the VAC connected. So the next test I did was connect my MytiVac to the VAC (with the manifold vacuum line plugged) and pulled about 17 “Hg. The total timing was about the same; around 19 “Hg. It won’t advance anymore than that.

I can’t pull any more timing because the rod is fully pulled back to the stop at the end of the slot. To increase the timing I would have to file the stop to increase the distance the rod can pull back. But if I do that then it will require more vacuum and I would no longer meet the two inch rule. Correct? I also compared the length of the slot in the B28 with my OE VAC and it is the same length so it appears the the length of the slot in the B28 is also to spec.

Do you still think the B28 is out of spec or maybe the advance in the distributor needs adjusted? Is the vacuum advance in the distributor even adjustable?

I found another VAC (VC1825/AR1) that starts at 3-5 “Hg and fully pulled back at 6-8 “Hg. This is identical to the B28 but the VC1825 provides maximum advance of 18 degrees rather than 16 degrees. Do you think this might be a partial solution? It would potentially give me 2 more degrees of advance. This VAC is manufactured by Standard Motor Products under the part number VC246. It is available from RockAuto and I saw it listed on the Autozone site as part number DV1825.

I fueled up the car to do another street test and realized I didn’t adjust my idle mixture screws after I installed the new VAC as you mentioned I should do…. I found that the mixture was very lean. After properly adjusting the mixture the idle is very nice and the car runs really good. In fact, with the full time vacuum advance and the valves adjusted according the article by Ken Anderson this is the best the car has run since I owned the car.

I retraced the parade route when the ambient temperature was 91F (about 8 degrees hotter) but it still ran up to 210 degrees.

I am still waiting for my dial back timing light. The timing light has a tachometer so I can see the timing and the RPMs at the same time. I had a 25% off coupon from Harbor Freight so it seemed like a good deal. It hasn’t shipped yet. Go figure.

Thanks,
Ken

[Duke W.]

Re: 69 vacuum advance for L71

About a year ago I worked on a friend's '66 427/390 four speed. The idle speed was constantly varying and the problem was a sticky vacuum advance. So we replaced it with a new 12" B26. Now it idles consistently at 600 pulling about 15". We also installed some lighter centrifugal springs because the OE centrifugal isn't all in until about 5000, but then we noticed a problem. It was "trailer hitching" slightly below about 1500 in any gear, so I figured it had too much low end advance.

We swapped out the 530 cam assembly with a 732 that gave us a couple of more degrees centrifugal. In fact the '67 L-36 centrifugal was increased to 32. Then we backed initial down a few degrees... I forgot how much, but the total WOT advance was still at least 36 and is doesn't ping on 91 PON California cat piss.

The bottom line is that I don't think big blocks like as much low rev/low load spark advance as small blocks. It might be that compact chamber design with the two quench areas.

So I recommend the first thing you do, which is easy is bump initial up to 10, which should give you about 21 degrees of total idle advance, and 40 degrees total WOT advance. If it doesn't ping, I think it's pretty close to optimum.

Maybe the fact that the VAC only provides 11 max is a good thing.

If the engine temp only gets to 210 idling along in Drive in 90 degree heat, I think that's pretty good. A 50-50 ethylene glycol-water mix with a 15psi pressure cap doesn't boil until 265, and some modern Corvettes don't even turn on the fans until 230.

I expect your fan clutch is operating properly, but check it. Turn in cold, then shut the engine down at 210 and it should require noticeably more force to turn it. Also, in a parade you can always pop it into neutral and rev the engine to 1500-2000 to increase radiator air flow.

Duke

[Duke W.]

Re: 69 vacuum advance for L71

Duke-

the B28 is out of spec or maybe the advance in the distributor needs adjusted? Is the vacuum advance in the distributor even adjustable?

I found another VAC (VC1825/AR1) that starts at 3-5 “Hg and fully pulled back at 6-8 “Hg. This is identical to the B28 but the VC1825 provides maximum advance of 18 degrees rather than 16 degrees. Do you think this might be a partial solution? It would potentially give me 2 more degrees of advance. This VAC is manufactured by Standard Motor Products under the part number VC246. It is available from RockAuto and I saw it listed on the Autozone site as part number DV1825.


Thanks,
Ken

I'm pretty sure that VAC (stamped AR1 I assume) is for HEI distributors, despite the NAPA Web site photo being what appears to be a single point/HEI VAC.

https://www.napaonline.com/en/p/ECHVC1825

The applications are all 75-79 and '75 was the first year for HEI across the board.

In any event 18 degrees vacuum advance may be too much with initial timing in the 6-10 range.

The only thing you can do to modify an OE type VAC is to install a bushing over the pin or procure a limiting device from Lars. Some OE Delco VACs have a bushing on the pin to limit advance. The bushing will reduce maximum vacuum advance at a lower vacuum level.

Duke

[Kenneth K.]

Re: 69 vacuum advance for L71

Duke-

Finally got my dial back timing light. I should have bought one of these earlier. I’ll report some of my findings below then give you a call.

1. RPM- The RPM readings with the dial back timing light indicated my mechanical tach in the dash is off by 100 RPM. I verified this by comparing the dial back with my street rod that I connected to the OBD and the 2 tach readings agreed. So I was setting my idle at 750 but it was really idling at 650. I always thought my vacuum at idle was a little low so now I know why. After setting the idle correctly at 750 RPM using the dial back and with the full time vacuum I am getting about 10 1/2 “Hg in drive. Corrected for my altitude at 4500-ft this is about 15 “Hg at sea level which seems to be right on.

2. B-28 VAC- I guesstimated the total advance with the B-28 was only 11 degrees because the degree line on the dampener was above the degree indicator bracket and could not be read accurately. Using the dial back I found the maximum advance for the B-28 is actually 14 degrees. With 14 degrees from the VAC and 10 degrees initial gives me a total of 24 degrees advance timing.

3. Centrifugal Advance. I measured the degrees advance for the OE springs in the distributor and for some lighter springs. I tested silver springs, 1 silver and 1 black spring, and black springs at several different RPMs. Results are summarized in the posted table. The OE springs advanced 2 degrees at 1400 RPM. The CSM spec is 2 degrees advance at 1100 RPM. The black springs advanced 2 degrees at 1100 which meets the CSM spec so I road tested the black springs and found much better throttle response. In fact, this is the best throttle response ever. It also seemed to develop more power at WOT from a standing or slow rolling start. No pinging, no backfire, no hesitation. Still using 50% AV gas and 50% 91 octane ethanol free premium. So I am I ready to start testing 100% 91 octane or should I road test one of the other springs?

Adv deg data.jpg

[Duke W.]

Re: 69 vacuum advance for L71

The black springs should be the lightest, so they bring in full advance earlier than OE and the other springs in the kit. That's why the engine feels stronger. They may be a bit too aggressive for your highest available pump octane, but the only way to know for sure it testing.

The L-71 centrifugal curve ('67 to '69) is actually pretty aggressive compared to L-72, which is 30 @ 5000. Your OE spec is 30 @ 3800.

You can approach octane testing two ways - start with the OE springs and go lighter from there assuming no detonation, or start with two black springs. If there's detonation install one stiffer spring, and go from there changing one at a time. The next stiffest after black is silver, then gold so least stiff to stiffest combinations should be as follows.

black/black
black/silver
silver/silver
silver/gold
gold/gold

Duke