If I understand correctly what you are collectively saying about the euro distributor, is that boost will cancel out the vaccum advance mechanism (which is added to the centrifuginal advance of the distributor).
A very early spooling turbo (like modern turbos tend to be) will basically never benefit from vaccum advance under acceleration, but on only in cruse operations "off boost". As soon as the engine accelerates and the turbo has the ability to build boost, ignition will be set at -18 or -19, no more. Clearly not the most efficient timing curb... Ideally you would want to see retard being applied progressively as boost builds, and not all additional advance being totally cancelled as soon as it sees boost.
I understand that it is a very safe mode!

We are trying to connect the dots.

That is my belief so far. Vac-advance is used at low load to increase efficiency. This makes sense as a motor under a light load seems to take more timing even without a turbo.

WOT from idle would probably be mechanical advance taking us to about -16 through -19 by 3200rpm. For most modern quick spool turbos they have boost onset by about 1500rpm.

With my K27-7200 and using some tricks, I used to pull .2 to .3 bar right off idle under WOT.

I agree however, there might be some opportunity to improve the pre-boost and early boost curve.

The Vac/Boost-retard pots distributors reach full retard with about 5 lbs of boost.

I like the Euro from a safety point however it is not getting that much more advance on cruse. I am starting to think a Vac-Boost retard pot might be better tuned for performance with a re-curve, limiting the amount of mechanical advance, and abandoning the Vac-Retard side & function. If we added a spring to the boost-retard function we might be able to get a more progressive retard function if desired.

Keith & 1100 Sport,

First: If we look at the "so called" Vacuum Retard Curve is actually a vacuum advance curve that advances the timing just off idle and as soon as the manifold pressure goes positive the advance springs back which effectively retards the timing back to static timing plus any centrifugal advance. Also If you look at the scale of the graph it is scaled between 5* BTDC and 15* BTDC. Therefore the advance / retard? is not 11*. The first trace is aprox. 4.5* and the second is Aprox. 5.3*. If you think about it may be used during cruise to simply smooth transitional surge caused by light accelerator movement or load change as the engine moves between pos. and neg. manifold pressure.

If we look at the history of distributors; originally plain static timing worked just fine. Once the "I want to go faster" bug bit that meant more rpm which required a way to get spark to keep up with the more rapidly moving piston that approached TDC faster as RPM increased, hence the development of the centrifugal advance. The natural progression from there was more displacement, multiple cylinders, more compression , and better fuels, and the need for better power and efficiency over a broader range. Hence the development of vacuum advance to deal with the adverse effects of more pressure in the cylinder.

The answer for us is ideally: A Crank Fired Digital Programmable Ignition System. But once you have it installed you have gone 360* and your right back to
our original question "What is the Ideal Timing Curve for a 930" Well it turns out the ideal curve is going to be different for every engine. " THAT SUCKS" so lets take what we know and work with it.

As an engine speed up the spark pulg needs to fire earlier (timing should be advanced) for the simple reason that there is less time for the cumbustion to happen as rpm increases. Spark should happen at a point so that the flame front travels through the mixture and burns it completely, and the expanding cumbustion products reach a maximum pressure when the piston is about
15*-20* after TDC. In the case of the NA engine that makes timing relativly simple because combustion chamber pressure remains relativly constant. In our case it is well to remember " Pressure has the most effect on engine timing" hence forced induction causes more pressure which makes the flame front move through the mixture much faster due to the excess compression of the mixture. Since the increase in pressure results in decreased combustion time, the ignition timing must be retarded as pressure increases to avoid detonation. So we have the two main variables that determine timing. RPM is pretty easy as it usually is controled by our foot. Pressure is another story. Pressure is easily affected by mixture, temperature, humidity, and all effect charge density which influnces combustion speed. We can moritor pressure with MAP & MAF sensors. But all this is controlable and perdictable if you understand what happens inside that cylinder
and the effect the pressure changes have on combustion. From there we can put in place known counter measures to control the effects of pressure. Good intercooler, freeflowing exhaust, correct functioning fuel system with monitored afr's, safe boost levels, retarded timing on boost, etc. If you've been paying attention all this discussion about pressure is because of one word "detonation.

The best thing to do is put the sled on a dyno and let a good tuner dial in your timing and map it so you know what the correct timing is. That's $200.00 an hour for the Dyno. The other option is to play with it. To play with it safely install a Lm1 or similar device and be able to monitor just RPM,AFR, and MAP you know you advance with RPM and Retard through boost. You know the factory max static timing specs are a safe range to fall back on. Do everything you can to prevent detonation and have fun but always listen for that strange ratteling sound.

Cole

Cole,

Good to hear from you.

Yes, dyno tuning is the best way to achieve the most from our motor no matter what we do to make the system better (noted in early part of thread).

Yes, a programmable ignition is even better. The MSD fully programmable looks like a possible winner for us and I think we can use the signal off the distributor or a crank trigger.

Total Boost retard. The graph is a little misleading as it dose not have "0" deg's printed between the top and bottom of the Advance-Retard curve.

Each gradient is one deg. If you count the total of degrees by hand between the two lines at the top and at the bottom there is 10 deg of retard.

This fits with Chris T/930-365's findings from tests on his dist.

Just to add confusion my Euro/ROW car has two hoses on the dizzy pot.

Does this mean it is not an original and someone has replaced a single hose pot with a 2 hose pot? Also no solenoid on any of the hoses.

Ben,

Later US and CA 930's had dual hose pots.


Cole

Thanks Cole,

Mine is a Euro model with a Euro engine however I have the 2 hose pot on my distributor. I was just wondering if all Euro models had a single hose pot which would mean mine has been replaced/changed over the years. If this is so then I presume I can just follow the timing specs for a US car instead?

Ben,

I did not think later ROW cars had dual hose pot distributors. But I would not swear to it. But following the US/Ca timing procedure would be correct.

Cole

Keith,

If I remember correctly Chris' distributer was a 79 with a dual hose pot not the single hose pot ROW therefore he would have more retard.

Cole

I would think some of the newer euro's would have the dual pot distributors but I do not know that to be the case.

If you have a hand pump you could tee it to both sides of the pot at the same time. This would validate. If the motor retards with pressure you are getting your boost-ret. You could test for Vac-ret at the same time. When you disconnect both sides for the test, rpm should increase as retard is lost. With pressure or vac to the pot it will retard and rpm will go back down. Or, you could use a timing lite.

It just makes sense that equal pressure on each side of a diaphragm would equalize and keep the pot from retarding.

If so, I see no reason why you can not just remove and plug the low-pressure/vac-retard side and reset the idle.

I have seen several EURO grey market cars in the USA that had double acting distributors. I think it must have been common practice for the companies that federalized the grey market 930's.

Check these calculations:

From the manual, the 1978 California cars have the initial timing set at 31 degrees and 4000 rpm. From the charts in the manual, at 4000 rpm the centrifugal advance is 16. The vacuum advance is at 10, so the total from the distributor is 26. 31 - 26 = 5, which is the static timing. At 5000 rpm and full throttle, the charts indicate a centrifugal advance of 16, and a pressure retard of 6. 16 + 5 - 6 = 15 degrees advance at full throttle.

For Euro cars, the initial timing set at 0 degrees and 1000 rpm. From the charts in the manual, at 1000 rpm the centrifugal advance is 0. I think that the vacuum advance is also 0 because the vacuum port is above the throttle plate at 1000 rpm. So the static timing is 0 for Euro cars.

At full throttle the charts indicate 11 degrees of centrifugal advance, and 0 vacuum advance, so 11 + 0 - 0 = 11 degrees advance at full throttle.

So, someone who is providing a warranty for an engine at ambients for -20 to 30 C and -100 to 2200 meters in altitude (Porsche) says that you should run no mor than about 15 degrees advance in a stock engine.

Chris did a lot of testing on a Sun tester and did have a dual pot. If I recall correctly he found that he retarded about 8 to 10 deg with vac and with boost.

That he also found he had 22 deg of mechanical-advance.

He also found that with vac tee'd to both sides he saw retard. That is against what I would think makes sense. He did not test boost to both sides at the same time.

I really applicate Chris's helping me to better understand things.

Speedy S,

You have this stuff down, excellent!

Makes a lot of sense.

Do you think we need a solinoid on the Vac-Ret line to the dist for boost retard to work?

You said Vac-Advance, do you mean 'loss of vac-retard' or dose it have vac adv?

Dose it look like the Calif cars were just US dist with the timming advanced?

It looks like timing on boost would then be -21 which seems high or dose it?

16 mechincal-adv is close to the 18 Chris found on the Sun Dist machine with his dual pot.

I agree with your thought that there is probably no vac at idle (ported?) with a euro Vac-Advance dist.

One note I saw indicated:
Euro 2 +/- 2degs at idle (950rpm +/-) Vacuum Hose CONNECTED
And to adjust timing:
EURO 29 +/- 1deg BTDC at 4000rpm Vacuum Hose DISCONNECTED


If there is only 11 deg of mechanical advance, somthing is not adding up.

If we have 11 deg of mechanical plus 10 deg of vaccum advance, that is only 22 deg of advance. Somthig is not adding up unless idle timing is -7 to start.

I think full advance with no load on the euro is suposed to be -29. Then when vaccum, drops under acceleration an on boost, timming drops 10 deg with vac-adv loss and becomes -19.

That would mean if timming starts at -2 we need about 17 deg of mechinacal advance??? This is the same the US dist's have. However this dose not match the book referance you site and as posted above.

Somthing is up??? I am thinking there is an error in the book, it happens. Makes more sense that Porsche adapted one dist by putting different pots on it.


Please explain further, I am not following.

OK, let me add my contribution to the confusion. I have an 87 US model dizzy. To get a better understanding of the system I tested both sides of the vacuum pot under pressure and vacuum. I found that the advance side of the dizzy responds quite a bit to vacuum and retards a little under pressure. The retard side only responds to vacuum. So at idle the timing is retarded by vacuum, when you push the pedal that vacuum is gone advancing the timing. when you accelerate the advance side sees vacuum and advances the timing with both vacuum advance and mechanical advance. when under boost the vacuum turns to pressure in the line, eliminating the vacuum advance and starts to retard the dizzy with pressure. Am I understanding this correctly? 911st mentioned a solenoid plumbed into the advance side, this is confusing to me. Which solenoid are we talking about. is it the one that looks identical to the one on the retard side, that is closed for about a minute to give you a high idle at startup? I thought the other one was used only for emmisions air pump etc. or are we talking about the thermal valve up by the crankcase breather hose. I removed the solenoid on the advance side, hope that wasn't a mistake.

Eric

Eric,

We need all the data point we can get. Especially actual presser/vac tests at the pot.

From your point of view, which side of the pot is which.

The side facing the dist I believe is the Vac-Retard or low pressure section.

The side furthest from the dist is the Boost-Retard.

Both sides are supposed to only retard timing. One with boost and one with vac.

With this I will try to understand what you are saying.

Thx

Ben,

I was digging through some old posts and found a dual port pot on a 79 Euro car so at this point I give up. I think if you want to know what you really have, and what it does, our only chance is to find someone with an old Sun Distributor Machine and run it to find out what it does. From what we're seeing from all the input here things are all over the place and we have nothing definitive, even with the manuals. I've got an old friend that will find a old sun someplace and then may be we can get some actual proven information. If I find one I'll get mine done, then maybe we can get others sent their's to me and we can start logging maps by Bosch part number and build a reference library.

Cole

Thanks Cole.

It makes it hard when the manual says one thing but because of the age and differing parts we have on our cars I don't quite understand the procedure for setting the best timing. eg. my Euro/ROW car with a dual pot distributor and no solenoids - how to adjust timing? euro method or us method?

I will take some pics when I get home tonight and also try some vac and boost testing when i get my timing light to work again (or buy a new one).

I also have this picture a previous pelicaner posted. Same as my setup.

Sorry for going a little off topic but this has been bugging me for a while.

http://forums.pelicanparts.com/uploa...1254197574.jpg

Cole,

That would be cool.


I suspect we might be just seeing one 3.3T distributor and it has about 18 deg of mechanical advance.

Depending on the need it is pared with one of two Pots.

The euro pot is an advance pot that only advances with vacuum.

The US pot is a Retard -Pot. It retards with Vac or Boost.

Gross timing is changed by country or state depending on fuel quality or smog needs.

With either pot, timing is modified about 10 deg. 10 deg advance with one, and about 10 deg of retard with the other.

Maybe someone can look part numbers for the pots and see if they are only two numbers.

I could be wrong.

We are still connecting the dots.

I am pretty sure all timing is set at over 4000rpm to ensure mechanical advance is fully engaged.

Single port pot needs to be hooked up (to get vac advance).

Dual Pot needs to be disconnected (to ensure there is not retard).

Safe timing being around -26 for .8 bar.

If one has good fuel or race fuel, -29.

If running 1 bar on todays gas, -25 is probably a better target.

Can anyone confirm this?

This looks like a home made drawing to show where an RPM boost clamp is installed to delay on boost enrichment.

It is simplified and may not be correct as it dose not show the cold start clamp or the solenoid that should be on the Vac-ret side.