j.kenzie@sbcglobal.net

Thanks, Carl. Thanks everyone else too.
I have taken advance out of the car and run at the low end of stock (4 degrees at idle and 18 degrees at 3000 RPM w/o vacuum advance).
I do run 93 octane, and I am intercooled, also. Most important I am running in the rich range on both cylinder banks AFR's = 12-14, and that seems to have eliminated my off idle stumble.
I think I knew that timing was dangerous with boost, but forgot. Thanks to you guys for protecting me before I did any damage.
Dave

j.kenzie@sbcglobal.net

Thanks again to everyone.
I would like to ask again how much timing advance should I be running on my 84 L-jet SC? The answers that I have received so far are essentially stock or less. Let me summarize what stock is for my vacuum advance only distributor: (per WSM 28:29)
without vacuum advance: 4-8 degrees at idle, 20 degrees at 3000RPM, 24-28 degrees at 5000RPM.
14-17 degrees of vacuum advance at 400 mbar vacuum.
Is this really what I should be running? Should I retard it? How do you come up with a correct answer?
Thanks again in advance.
Dave

hacker-pschorr

You first need to know what maximum safe timing was without the supercharger.
Once that is set you tune the engine to pull back timing under boost. The old rule of thumb is one degree per pound of boost.

How do you do that? With your distributor you can't, at least I'm not aware of it working.
As I stated above (or one of your other threads) you need an 80-82 distributor with a vacuum retard port As luck would have it, removing the vacuum port (or adding a check valve to allow pressure out and still make vacuum as stock) will retard timing under boost.

How much does it retard? I never got around to measuring timing on my car before scrapping the whole system. From memory it was estimated to be a bit over 1 degree per pound of boost. I never had any issues with 17psi of boost with this configuration but I was never happy with the arrangement.

There are other piggy-back devices that go between your ignition brain and distributor to pull back timing under boost. I never played with any of those and I've heard mixed results from those who did.

j.kenzie@sbcglobal.net

Thanks, Erik. If I may ask, how does a vacuum retard actuator retard the spark under boost. It sounds like a great idea, but I'm just not catching on to how it works. For now i have retarded my distributor about 4 degrees across the board from the stock setting, but that is not ideal for all conditions.
Thanks again,
Dave

hacker-pschorr

The dist with advance & retard ports move the arm in both directions depending on which port is generating vacuum drop.

If you leave it hooked up stock, both ports will see boost and nothing changes. If you disconnect the line going to the retard port, now when the advance port sees boost, it pushes the arm in the retard direction.

I used a check valve on the retard port so that side would still see vacuum as stock, but still allow the boost to bleed off and retard the timing.

j.kenzie@sbcglobal.net

That is very clever. Were you pretty happy with the timing across the RPM range?
Thanks,
Dave

hacker-pschorr

No idea, as stated above I never got around to measuring it. By the time I added a stock timing ring and crank sensor, I decided to scrap L-Jet.

It's a "better than nothing" setup. If you really want to delve into fine tuning your car, time to ditch L-Jet and get something you have complete control over.

I've had a few moments of insanity of re-installing the "stock" setup on my 80 just to see what I can do with it.

j.kenzie@sbcglobal.net

Thanks again, Erik. I will give it some more thought. I see a used 80-82 distributor for $100-200.
BTW I'm planning to max out my boost at only 8 PSI. Your 17 PSI must have been scary.
Dave

hacker-pschorr

The thing is, I have no idea what happens to the advance "arm" in the 83/84 dist under boost. Maybe it pushes the arm back to "neutral" but again, just a big guessing game.

The theory is since the earlier unit is designed to retard timing past "neutral" you can pull out more timing with them under boost.....

You'll most likely take out more than necessary but with such a crude setup, that's not really a bad thing.

j.kenzie@sbcglobal.net

I was thinking along the same lines Erik. You set the base timing with the vacuum advance line disconnected, ie 0 PSI. Even if boost does not move the arm past the 0 PSI setting, it has taken maybe 15 degrees of advance away. If it took a little more, even better. Timing then is just the base setting plus mechanical advance. I still bet the 82 distributor would be even better though. I may be able to play with my 84 distributor using a Mity vac.
thanks,
Dave

hacker-pschorr

I'm having flashbacks of testing these things. I seem to recall we did confirm that applying pressure to the advance side (with nothing on the retard side) lowered timing - confirmed by engine rpm, test done at idle.

Jeez.... that was all 15+ years ago

j.kenzie@sbcglobal.net

I think it's a great idea. Since I have a throttle body designed for only distributor advance, I don't have a retard port on the TB. That being so, I think I would just leave the retard line on the distributor open.
It is interesting how your memory works. You probably haven't thought about this in years, but it's still in your head.
Thanks,
Dave

hacker-pschorr

To jog my memory, I stick my tongue in here:

j.kenzie@sbcglobal.net

I ordered an 82 distributor. I really like your method because instead of pulling out 4-5 degrees of timing across the board , this will retard only under boost where I need it. Losing timing at low throttle robs the car of pep around town. I'll let you know how it goes.
Thanks,
Dave

hacker-pschorr

Something else I mentioned above.

There was some debate a LONG time ago about the merits of vacuum retard. Porsche stopped using it in 83 and capped off that port on the TB, but nobody really knows (or shared) other possible differences with the various distributors.

Point is this: If you want that 82 dist to work as stock, you will need a vacuum valve on that port so the retard side will still see vacuum but allow boost to bleed off which will create a pressure drop across the diaphragm and retard timing.
Something like the valve next to the brake booster, same thing.

Also need to dig up the 80-82 WSM photo of the TB connection ports to use. I'm 98% sure all the TB's are the same, they just put a cap on the port which goes to the retard side. The holes which feed these ports are very close together inside the TB itself..... it seems Porsche wanted the timing to vary with any kind of vacuum differential between these ports with that distributor.