BTAL

Am working on setting the timing on a 78 L-82 4speed with air. All stock with the exception of a "stock" HEI replacement dist. from Summit.
Specs here And new BorgWarner plug wires. Carb has not been sent to Lars yet as was wanting to give him some recovery time.

Also, have a Mr Gasket HEI Advance Curve Kit with springs and weights along with the original dist. springs and weights (Albeit, the original and MrGasket weight bushings don't fit the new dist. posts. Posts are slightly larger.)

Also have the original vac can, which has less travel than the new can. The number on the original is 626 10 which I can't find in any of Lars' papers. The new can doesn't have any numbers. Both start advancing at about 5-7 HG and are "all in" at 10-12 HG. Am pulling manifold vacuum from the port at the bottom passenger side of the QJet. That source is producing 15 HG at idle and maxs. out at about 20-22 HG with rpms. The original can bumps the base timing at idle from 12* to 18-20* The new can bumps the base timing at idle from 12* to 36* - Yes, 24* vac advance. My opinion is that this is too much and limits my tuning options?


With medium springs and the new weights and original vac can here are the numbers:

Idle rpm 540-550
Base timing no/vac and rubber bands on the weights: 12*
Base timing with vac and rubber bands on the weights: 18-20*
Timing steps based on rpm with no/vac:

1000 - 14.2*
1500 - 24.2*
2000 - 26.9*
2500 - 29*
3000 - 32*
3500 - 32.1*

After reading the BarryK's complete sticky several times and Lars' papers - Finally, now for the questions.

1. Is it safe/wise to advance the base no/vac timing above 12* to get that last 2-3* "all in" at 3000+

2. Based on the instructions, I should be ending up with a total/all in max timing of 40 (32 + 8 vac). This seems way off the mark from the "optimal" 52? If I try to use the new vac can, I blow way past 52.

3. The car is running much better than it was prior to starting this effort with the normal issues of poor power, running too warm, etc. I hope, this isn't a replace the diff or send it out to get re-curved (a concept I really don't understand).

4. How much of this will change next week with installation of headers and the Pypes true dual exhaust system?

Thanks in advance for any help
Brad

PS: A very hugh thanks to BarryK and Lars for the wealth of knowledge that helped in getting to this point. Worth every minutes reading time.

BarryK

most HEI's come with a 20º mechanical advance which as you can see from your timing numbers your new distributor is doing the same (initial timing at 12º and all in at 32º@3000rpm = 20º mechanical advance.
Without sending it out to be recurved, which, in the simplest terms means changing the amount of the mechanical advance, this is what you have to work with. To get your total timing (before vac. advance) to the optimum setting of 36º at 3000rpm this means you will need to advance your initial timing 4º to bring it up to 16º initial timing. This will than give you 16º initial + 20º mechanical advance = 36º total timing at 3000rpm. The 16º initial should be fine for your L82 and after setting it just go for a nice cruise (with the vac. advance still disconnected and plugged) and check to make sure she starts fine, has good throttle response, and does not ping or knock from detonation. If all checks out ok your timing is good for you.

Next, for the vac. cannister, if the stock unit is giving 24º advance that's way too much and i'm surprised since the unit on my '78 L82 doesn't give nearly that much advance and it's the original stock unit.
You need one that will give you 16º of vacuum advance and that is "all in" at least 2" below what your manifold vacuum level is at idle. Since you report that your vacuum is at 15" you need a can that is all in with the 16º advance by 11" or 12" of vacuum.
Looking at Lar's vacuum unit chart I'd say the AR12 (VC1838) or the AR23 (VC1853) would be good units to go with. These would bring your total timing plus vacuum advance up to 50º or 51º.

Of course if Lars is around and replies to this thread and says anything different than listen to him and disregard this response completely.

BTAL

Barry,
Thank you a ton for responding! Actually, it is the newer Summit vac can that is giving the over the top 24* advance. So, I pulled it off and went with the lower/more conservative albeit lower than the "normal" stock advance. As long as upping the base from 12* 16* is safe, then it overcomes the lower vac advance (until I get around to getting another).

Any input on the last item of expected changes with a header and true dual exhaust install? I guess though, looking at my numbers, I'll have some room at the top end of the curve if either of those advance the timing in anyway. If they could possible retard the timing, then the new vac can will be needed sooner than later.

Thanks again!
Brad

BarryK

Brad, no, headers and exhaust changes won't affect your timing.

BTAL

Barry, thank you.

Just made another setting run (without the test drive as it is storming quite bad here). Upping the base to 16* resulted in the following:

No vac connected:
Base 16*
1000 - 16.5*
1500 - 27.2*
2000 - 30.0*
2500 - 31.6*
3000 - 33.9*
3500 - 35*
4000 - 36.4*

For an inexpensive dist., weights and springs, that is probably the best am going to get out of it. At least, am getting a significant adv. starting at 1500 and above but with a very flat curve from that point.

With the vac connected the results were:
Base/Idle 24.9*
1000 - 25.9*
1500 - 35.4*
2000 - 38.9*
2500 - 39.9*
3000 - 42.4*
3500 - 43.9*
4000 - 45.6*

Is there a "too much" base advance? Meaning, with this consistent curve, if the base was bumped up more where is the safe and no-go point for the base number.

Thanks

BarryK

ok, your total timing isn't coming in until 4,000 rpm (did you check to make sure nothing is still coming in after 4000?) so you should try some looser springs to get the timing all in by between 2800-3000 rpm.

C3 4ME

Just as an FYI, as per Barry and Lar's instructions, I did the whole timing drill on my 79 L-82 4 spd just the other month. The only parts change I made was the advance springs in the distributor. I now have the 36 degrees coming in at about 2800 rpm, which in my case results in 18 degrees initial timing. This works fine for my setup, no hard starts at all and I can stand on the gas at 20 mph in 4th gear and not hear any pinging. I intend to bump it up to 38 degrees and see what difference that makes. I burn 89 octane in my car. My mods include a bigger unknown cam, headers with true duals, and an Edelbrock performer intake. I believe the heads are bone stock as too is the compression. Ive' said it before and I'll say it again, a big thank you to Barry and especially Lars for all the help along the way.

BTAL

Barry, no I didn't push it past 4000 to see if there was anything left.

As for springs, I have one set of looser springs from the kit and have tried them and they seem to throw all in even sooner but, I need to go back and verify that. Those springs are VERY loose however, if I understand correctly, it isn't too wrong to have it come in slightly early in the rpm ranges?

Thanks too, C3 4ME. Supposedly, there isn't much difference between the 78 and 78 L-82s, so will also give the 18* base setting a try. A couple questions for you. Were you experiencing any running warm issues or just wanted to get the best timing? Also, how loose of springs did you end up going with - meaning, the loosest in the kit?

Brad

427V8

did you remove the springs to determine what your total timing really is? Or at least use only 1 really light spring?

It's very possible that you really arn't ever reaching max timing...

BTAL

Am a bit reluctant to try it sans-springs as the weights will immediately kick in - probably to "all in". Next step is to use the loosest springs and see what the curve is.

Went to a car show today that involved about 20 miles Interstate speeds and some local driving. No ping and much improved throttle response. Still getting some too high temps, but will deal with that as a true cooling issue (original rad and water pump and hoses). Also, the carb has not been touched at all.

BarryK

try it with the looser springs to get the timing all in by 2800-3000rpm and as long as no pinging leave it.
Get the right vacuum advance unit to get the extra 8º of vacuum advance in there.

After all that you can set the correct idle fuel mixture on the carb with a vacuum gauge - just plug it into the carbs full manifold vacuum port and at the correct idle speed adjust the mixture screw to achieve the highest STEADY vacuum reading. When adjust just make small changes at a time, no more than a 1/4 turn, and than let the motor stablilize for 15-20 seconds before taking a reading that keep going until you get the highest steady reading. As you make adjustments you may need to readjust the idle to keep it at the correct level

BTAL

Barry,
Changed to the loosest springs (man there is not a lot of spring in those) and was able to bring the base back down to 12* and she goes all in at about 2400-2600 with a big jump at the beginning between 1000-1200/1300 then the curve smooths out.

Thank you for both your original sticky posting work and the responses here.

She is running like a raped ape with responsiveness that wasn't there before. I let my younger brother driver her for the first time and he just about scared the crap out of both of us - especially now that the performance brought the needed rear spring/TA work immediately to the forefront. She just flat runs great and will probably truly be scary once I send the carb to Lars.

Thank you again sir!

BarryK

Brad

having the timing all in by 2400-2600 is kinda early so double check that you aren't getting detonation at all. If not, than enjoy the driving but if you are getting any detonation I'd go up to the next spring rate to bring it in just a bit later.
Enjoy

Barry