distributor basics

[jpearson]

I am a noob trying to learn some distributor basics. Can someone explain (or recommend a good source of information) why and in what situations it is advantageous to lock the distributor advance? Thanks for the help.

[hcoles]

what is the context?
on a dragster we ran the timing was set and didn't move.. I think something like that, it was long time ago.

[jpearson]

My car is a '75 911s and the vacuum pot was removed from the distributor (please correct me if I'm wrong but I think that in 75 it was a vacuum advance only). The car had a rajay turbo system that I just removed and I am restoring a stock configuration to the CIS. I started trying to learn about this when I was deciding whether or not to mess with adding the vacuum back to the distributor. I am new to all this so I started by reading about timing curves and distributor advance, just trying to understand the basics. I have heard people talk about locking out the advance for racing (I assumed both vacuum and mechanical) but I don't really understand what the advantage is or what the advantage of removing the vacuum advance in my particular car was.

[hcoles]

others can provide a better specific explanation if one exists for your car, but

The general idea of mechanical and vacuum advance is to attempt to provide the optimum timing for all speeds and load without the use of electronic controls. At high load the timing in some cases needs to be retarded, this is why a vacuum advance mechanism is used, at lower loads the vacuum is higher and advances the timing. The mechanical advance increases timing as a function of rpm. These two timing adjustments are somewhat separate and additive. Of course modern controls allow timing adjustment using all available and pertinent inputs and any algorithm needed. The Motronic cars have a mechanical advance mechanism to keep the rotor tip across from and near the spark plug wire contact pole throughout the range of advance angles. This feature is probably on other cars as well.

[Flat6pac]

The 75 is a vacuum retard for emissions only.
Bruce

[RoninLB]

my stock 1977 is a vacuum retard for smog

[jpearson]

Thanks guys -
hcoles - I think I have the basic concept of advance but I don't totally understand why it's ever advantageous to lock it out.

If the vacuum retard is mainly an emissions thing does it have any noticeable effect on drive quality?

[hcoles]

I don't remember, back in the day people would lock out the vacuum advance or maybe they locked out the vacuum retard or plug the vacuum advance and advance the timing overall without having the low load affected. That's a guess.

[RoninLB]

Originally Posted by jpearson

If the vacuum retard is mainly an emissions thing does it have any noticeable effect on drive quality?

In my 1977 it scrambled the whole curve when opertional. The easy fix was to disconnect it permanently and just set total advance at 35deg.

different year 911, different strokes

[1976 TN Porsche]

My 1976 had a vacuum retard. contact Barry Hernson at IAE INC He can fix your distributor.

[psalt]

I think I have the basic concept of advance but I don't totally understand why it's ever advantageous to lock it out.

If the vacuum retard is mainly an emissions thing does it have any noticeable effect on drive quality?


Josh,

There is no such thing as a "vacuum advance only" distributor on a pre Motronic 911. They all have mechanical advance because the engine needs different timing at different speeds. The best way to think of this, is that to do the most work, the engine needs the cylinder pressure to peak around 14 ATDC. The ignition needs to start the burning earlier and earlier as rpm increases because there is less time for the burn to reach peak pressure. The 911 is a knock limited design so there is a kink in the curve to avoid detonation when VE is highest.

Vacuum advance is a fuel economy device. At load load part throttle cruise, when VE is low and the probability of detonation is low, the engine can safely run another 10 degrees of timing which increases economy around 5%. It uses a ported vacuum source, no vacuum at idle, because you do not need extra advance at idle. Vacuum advance is irrelevent to measured performance and it is a generally a good thing on a road car.

Vacuum retard is strickly an emissions device, it makes the engine run hotter and reduces HC at idle for the early curb idle emissions test. It uses manifold vacuum, full retard at idle. It mostly causes overheating in traffic and poor cold start/warm up idle, and you are better off without it. On a converted turbo engine you would plug it so that the boost pressure does not push the diaphram the other way. Most turbo engines need a different curve with boost retard.

Road cars need to start cold and hot, idle cold and hot, run at part throttle cruise, etc and have a wide range of timing requirement. A race car never cruises, seldom idles, etc and some race cars, oval track and drag race, have locked distributors at the full advance timing to eliminate one source of failure. The only reason to lock a distributor on a road car is to fit a programmable ECU. In this case, you lock the distributor at full advance and program in a retard curve that decreases with rpm.