C3: Vacuum advance curve

[Robert E.]

Hi all,

Last spring, I had the engine in my '71 LT-1 rebuilt. The shop that did it for me, decided not to attach the vacuum advance back up the distributor. After reading the benefits of having it hooked back, I decided to verify the advance curve first.

Since I've never done this before, my I explain what steps I took and the results I obtained? I'd appreciate any guidance and criticism of the results.

1. I removed the cap and rotor and held the advance weights with a rubber band to set initial timing. (I noticed the springs were of different strength)

2. idle timing was set at 8* BTC.

3. Removed the rubber band, then checked the centrifugal advance. This was my findings:

rpm// timing mark
idle // 8*
820 // 8*
1000 // 8*
1600 // 16*
2000 // 25*
2500 // 28*
3000 // 28*

4. Then I hooked up the vacuum advance (hooked up to full manifold source), readjusted the idle. This was my findings:

rpm// timing mark

700 // 25*
800 // 26*
1200 // 29*
1500 // 37*
2000 // 43*
2500 // 45*
3000 // 45*

Do these reading look OK? What about the different springs?

Thanks in advance

Robert
41801

[Duke W.]

Re: C3: Vacuum advance curve

You should check your measurments against the AMA specs or CSM specs, however, if two different springs are installed, someone has probably changed them.

You should check the vacuum advance with the centrifugal tied up by using a vacuum pump and gage. Your data indicates that your engine has full time vacuum advance rather than the OE ported vacuum advance, which is good.

The sum of initial plus full centrifugal should be in the range of 36-38 degrees, and you have to rev the engine until it stops advancing to know you are at max centrifugal. On some engines this could be about 5000 RPM, but yours appears to be less than 3000 because of the lighter than OE springs, which is good, but the total is 10 degrees short.

With full time vacuum advance the vacuum can should provide full vacuum advance at no less than 2" below typical idle vacuum. For the LT-1 a 8" can is probably the way to go, but a 12" can might work. An engine idle vacuum reading is required to make this determination.

Emission controlled engines typically have less initial advance and less total centrifugal than pre-emission engines, so they usually won't achieve the desired 36-38 degrees total WOT timing - the sum of initial plus full centrifugal. This is the case with the data you presented.

Emission controlled engine vacuum cans typically have more advance than pre-emission engines, but start and end at much higher vacuum, which makes them unsuitable to full time vacuum advance.

From your data it appears that the centrifugal adds only 20 degrees (check specs). If this is the case I would recommend filing out the slot to increase it to 24-26 degrees and set the initial timing at 12-14. The centrifugal shoulds come in more quickly - as quickly as possible without detonation. Yours appears to be fairly quick, probably due to non-OE springs, but the total centrifugal is insufficient. Since your engine was originally set up to run on regular unleaded, you can have an aggressive centrifugal curve if you are willing to upgrade to midgrade octane of 89 or so.

You need to characterize the vacuum can by determining the vacuum required to start pulling the plunger and the vacuum required to pull it to the limit. The total of 17 is in the ballpark.

What is the ID data stamped on the vacuum can? And what are the OE centrifugal and vacuum specs?

Duke

[Robert E.]

Re: C3: Vacuum advance curve

Duke,

Thanks for your detailed response. I did forget to mention that I am using a Napa VC1810 vacuum advance canister. You mentioned a couple of things I'd like to get more information about. How do I know what springs are factory correct for my T.I. system, or better yet, ones that will do a better job for my application. As is is set now (or without the vacuum attached) under steady acceleration, there is little to no detonation up through 4500 rpm and probably beyond, but if I suddenly step on it, it will detonate.

What can I do to set this up properly vacuum advance to get the most out of her?

Thanks

Robert

[Robert E.]

Re: C3: Vacuum advance curve

Duke

Sorry, but you also mentioned that my OE application should have ported vacuum advance. Please correct me if I'm wrong, but there are no ported vacuum ports on my carb. On my List-4801 (and all others), all vacuum access is below the throttle plate.

Robert

[Duke W.]

Re: C3: Vacuum advance curve

The VC1810, which is an 8" can may be "too aggressive" and cause transient detontion. If idle vacuum is high enough, a 12" can (VC1765) would be okay at idle and should reduce transient detonation.

There's no real way to determine the OE centrifugal springs, but I would not recommend them anyway because they cause a very slow curve. The current centrifugal curve appears to be amaxed out at about 2800, which is okay.

Increase the initial timing to 14 degrees which will get the total WOT timing to about 34 and the total idle timing to about 31. Then check idle vacuum and see if you can achieve 14" at 900. Take readings at idle speed ranging from about 700 to 1000, but I think about 900 would be a good choice.

If you can achieve 14"@900 replace the VC1810 with a VC1765 and do some test driving.

Finally, file out the slot to achieve 24-26 degrees centrifugal. This will require distributor disassembly, which is a good opportunity to overhaul, clean, lubricate, and adjust end play to 2-7 thou. This will help TI distributors, too by eliminating timing scatter due to shaft bounce. If the limit bushing is not in good shape, replace it.

What fuel grade are you using?

Duke

[Duke W.]

Re: C3: Vacuum advance curve

I believe the LT-1s have OE ported vacuum advance because they were all emission controlled engines.

Maybe I'm wrong.

If they don't have OE ported vacuum advance, that's fine.

As long as one doesn't have to have their car emission tested I recommend that ported vacuum advance be changed to full time along with installation of a new vacuum can that is suited to the engines manifold vacuum characteristics - it should provide about 16 degrees of crankshaft advance at no less and 2" less than typical manifold vacuum with 24-32 degrees of total idle timing.

Total idle timing should be in the low to midpoint of this range with low overlap cams and the mid to high point with high overlap cams.

[Robert E.]

Re: C3: Vacuum advance curve

Duke,

Thank you very much. I measured 14" of vacuum at idle. I'll give the VC1765 can a try and advance the Initial timing to 14 degrees. I'm a little apprehensive about yanking out the distributor at this time. I think I'll need a hand with that.

Robert.

[Jack H.]

If memory serves...

the original setup for '71 LT1 was vac taken from carb below throttle plate and routed to the CEC solenoid. CEC solenoid either passed vac to the distributor or denied vac based on controlling relay (engine just started and 15-second timer hasn't expired, engine temp is cold/normal/hot, as well as transmission position low gears: no vac, hi gears pass vac)... So, regardless of vac source, there'd be no vac advance active at idle on a warmed up engine.

[Duke W.]

Re: C3: Vacuum advance curve

I've always disassembled single point and TI distributors to change the vacuum can. Others report it can be done with the dist. on the engine, but it's probably a little tricky. It's no problem changing the vacuum can on an HEI on the engine.

You still haven't told me what fuel you are using.

Are you using regular? Midgrade? Premium?

Duke

[Mark #28455]

20 degrees total mech advance

This was a common modification in the 1970's when the vac advance can was disabled - if the idle vacuum was too poor or the "experts" thought it was better. BUT, you are then supposed to crank up the initial (static) advance to about 16 degrees to get a stable idle and still get the 36 degrees total at WOT. Another unexpected byproduct of this arrangement is that the car is much harder to crank over - go figure! For street use, go back to the original system as noted by others.

The person who did this modification likely brazed or welded the slot on the doohickey (oops, I forgot the real name - guess I'm getting senile) that has the cam that actuates the points, or the rotating pole piece in a TI distributor. If you are going to try to grind the slot, make sure you grind the correct end because if you grind the idle end it will make it virtually impossible to get a stable idle because the advance will start to come in way too early. You need to grind the end for the high RPM limit.

What do you really do? If it's a points distributor, find ANY 1960's or 1970's pass car and get a replacement non-Bubba'd part. If it's TI then that would be a LOT more expensive - sorry!

Good luck,
Mark

[Robert E.]

Re: 20 degrees total mech advance

Mark,

Thanks for your advice. I can't account for what was done to my car in the distant past, however the vacuum advance was attached when purchased a couple of years ago. I was wondering if by following Duke's advise by cranking up the initial timing to 14 degrees would make it hard to start, but I am willing to give it a try. I am using the original TI system and use Sunoco 94 fuel.

What I'd like to accomplish is getting the most of what my LT-1 motor has to offer. In addition, I did have a higher temperature at idle than when I had it hooked up the summer before. I hope it will also take care of this as well.

Is it more advantageous to pull the distributor and send it off to Dave Fiedler for a thorough rebuild and re curve, or is there anything that I can do to fix this problem? (purchase a re curve kit and play with the weights and springs?

I appreciate all and any advice,

Robert

[Mark #28455]

Re: 20 degrees total mech advance

I think about all the TI rotating pole pieces gave 28 or 30 crankshaft degrees mechanical advance. Although you can try boosting the initial (static) advance to make up for the decreased mechanical advance, this may lead to an over advanced condition at cruising speed (low to mid RPM with high vacuum). It's certainly your cheapest option so I would agree it's reasonable to try especially if your engine has about 9:1 compression, but you'll have to listen closely for knocking.

If that doesn't work, you'll likely have to either grind the limiting slot or replace the pole piece. For a small block, I don't think 14 degrees initial advance will give a good starter too much trouble, my big blocks didn't start getting obnoxious until about 16 to 20 degrees initial.

P.S. in my past, I did a lot of "Bubba" things to cars, now I have to go back and fix what other "Bubbas" did to mine - I guess that's a fair pennance!

Mark

[Duke W.]

Re: 20 degrees total mech advance

It would be better to work with some real data rather than speculating all night.

Do you have a CSM or AMA specs for your year that lists the vacuum and centrifugal specs?

How about posting them?

Pre emission mechanical lifter engines had 24 centrifugal with an initial timing recommendation of 10-14 degrees. With super premium you could run 16-18 and squeeze a little more power out on grudge night.

The actual CR of your engine is about 8.5:1 (not the 9:1 advertised) unless the engine has been rebuilt with steps taken to raise the CR. There are LT-1 cammed engines running around at 10.5 that don't detonate on unleaded premium, even with aggressive timing maps.

Duke

[Chris D.]

Re: 20 degrees total mech advance

Robert,

From an old Chilton's (I know, suspect source BUT has shown good cross reference on other known applications) I find the following:

For '71 LT1 (330hp) distributor,
PN - 1112038
Centrifigal - 0@1300, 16@2250, 24@5000
Vac - 0@8"HG, 15@15.5"HG

Initial setting, 8 BTD

Any chance the distributor ID tag is still attached? 20 degree centrifigal units show up a few years later when tuning was even more emissions targeted.