123 Install Today - GETTING CLOSER..., PROGRESS: BAD MPS Options

[FlacaProductions]

74 2.0 D-Jet

Heading to the garage now to start the install and my main, unresolved-in-my-brain question is if i need to hook up the vacuum line that is on my current stock distributor, on the 123.

[technicalninja]

On my '74 2.0 with 123 ignition, I've connected the disty to manifold (not ported) vacuum. But I've not been able to program a MAP curve into the 123 that works well (provides extra advance just when the engine is under load). If anyone has successfully done this, could you share the MAP curve set points you're using?

Gotta say though, car seems to run just fine without the vacuum line attached to the disty.

And this is where it is counter-intuitive...

Distributor advance is MOSTLY a function of RPM. As the engine speed increases we need to "start the fire" earlier to achieve MBT (mean best torque-highest cylinder pressure) about 10 degrees AFTER top dead center. At a specific RPM advancing further is fruitless as the time available to complete the burn becomes too short.

So mechanical advance usually starts just off of idle (800-1200 rpm) and is "All In" by 3K-4K. This IS the reason that 914 balancers are marked at 37 degrees. You are setting "total" timing by hunting the 37 degrees mark at 3K RPM.

Load is also a factor with timing, but it is the OPPOSITE of RPM. As load goes UP timing should move down...

A timing curve that will make any gasoline-based engine function is as follows.

Mechanical advance 25 degrees total at 3.5K rpm.

Initial timing 10 degrees (I call this static timing) this is what will show with both the mechanical and the vacuum advance disabled. This is where MOST adjustable timing vehicles are timed with a light.

Vacuum advance 10-15 degrees at 17-20" of vacuum.

At idle this set up will show initial + vacuum and give you a sum of 20-25 degrees.
The engine will idle fine with just 10 (no vacuum) but if you add the vacuum, you will notice the engine is idling 400 rpm higher and is more stable requiring a reduction of both air and fuel to idle at the same rpm as the 10 degree set up.

At 4000 rpm WOT you will be at Initial+ Mechanical or a sum of 35 degrees and this works fine.

But at 4k rpm CRUISE you will have initial, mechanical, and vacuum (throttle blades will be at a low percentage and manifold vacuum will be high) for a sum of 45-50.

This "strangling" of the engine via the throttle blades produces an incomplete fill of the cylinder and is harder/slower to burn requiring additional timing to achieve MBT at 10 degrees ATDC.

So, timing goes up with RPM and reduction of load...

A race car that is ON throttle most of the time does not need vacuum advance. This is why a lot of you say "works fine without vacuum" because it does, especially when the throttle is hammered which is how most folks "test" their performance.


A member on here (I think it was emerygt350) did some testing with a good cylinder head temp gauge and posted some very interesting data IMO.

When he added vacuum advance his head temps dropped dramatically!

Running too little advance means some of the charge has not be consumed in the cylinder and the burn continues into the exhaust system. This is HARD on head temps IMO and his testing showed how very easy it is to achieve cylinder head damaging temperatures.

Correct timing fixed his shit up fine...

To me his data ment...
Oh my god! an accurate cylinder head temp gauge is an ABSOLUTE must for tuning.
Be damn careful with the timing on these, even stock ones can suffer catastrophic destruction (over 425 temps) easily.

Running a vacuum-based timing curve will improve cruise to WOT acceleration, fuel economy, and keep cylinder head temperatures at bay. It will make idle more stable and (if you're a greenie!) it should reduce emissions...


Of note : Pretty much ALL ported vacuum is gone now. I cannot remember a modern car that uses ported vacuum for anything at all.