re-curving a distributor

[dsiegel360]

The mods I've done to my 88' 930 have been toward lower end torque. SC Cams, K27 7200, SSI, Rarly's muffler, adjustable WUR, Kokeln I/C, and a 1 bar tial. While timing needs to be set at 26 BTDC at 4000rpm's (vac disconnected and plugged), idle then lands at TDC. The car still has less low end torque than I'd like. Has anyone had their distributor re-curved? I'd love to know who you recommend and what numbers to set the distributor at. Thanks.

[NathanUK]

What we need for our cars is boost retard.

The dizzys we have don't do this. All they actually do is vacuum adavance.

So, when you get on the gas with no boost the ignition is too far retarded. That is one of your problems and why going with an EMS works so well and gives much better drivability.

Let us know if you can get this done.

The other way of doing it is with an Electromotive XDI, but it's not cheap and doesn't look very sophisticated at all.

[dsiegel360]

I've decided to get the stock dizzy rebuilt. The vac diaphragm is in working order. I'm gonna wait on the electromotive for a while. When I go twin plug I'll reconsider...or when the diaphragm breaks.

[RarlyL8]

You can recurve the distributor just like in the old timey muscle car days. Anyone who was a shade tree mechanic back then knows how to do it.
I am planning some changes to my engine soon that will include a custom dizzy curve. I can share the details afterward.

[NathanUK]

Will it offer boost retard?

[RarlyL8]

What I had planned for mine was to change the spring and lock out the idle retard. For my purposes I don't need boost retard but a more agressive mechanical advance with the stock vac advance. I'm sure there is a way to add boost retard but I don't plan to do it myself.

[les_garten]

Originally Posted by RarlyL8

What I had planned for mine was to change the spring and lock out the idle retard. For my purposes I don't need boost retard but a more agressive mechanical advance with the stock vac advance. I'm sure there is a way to add boost retard but I don't plan to do it myself.

When you say aggressive, do you mean how quickly you advance or more total advance, or both?

[356-930]

Rather than attempt to re-curve the distributor, switch to MSD ignition with boost retard control or if your system is already an MSD ignition, buy the MSD boost retard module. It will pull out a maximum 15-degrees at a max rate of 3-deg per psi. Onset of retard is adjustable from 0 to 5-psi.
This scheme allows one to run substantial initial advance and not grenade the engine under boost.

[RarlyL8]

No Les, I just want to get the timing in a little quicker for good throttle response. Nothing radical at all, but it is part of the equasion to eleminating boost lag.

+1 on the MSD, especially if using EFI. Tuning the ignition reguires attention to the A/F ratios which can become a tedious back and forth process with CIS.

[NathanUK]

Do you guys think the MSD is a quality product and would trust your $10,000+ engine with it?

[mb911]

Originally Posted by NathanUK

Do you guys think the MSD is a quality product and would trust your $10,000+ engine with it?

I don't personally use this but a bunch of drag racers use them and their engines are way more expensive then 10g's but again it would not be for an all out drag car either..

curious though doug why you are recurving the dizzy? You will not notice much difference as I have been through this for the past 3 years and finally have it worked out very well with a 12 plug dizzy from supertec..

[dsiegel360]

Ben,
It seems odd to me that the car idles at TDC and since the car lacks power down low, I figured a recurve would help drivability. I've been trying to diagnose a couple of problems and decided that a distributor rebuild was called for. Then I thought, well, while I'm at it do a recurve. But since then I've decided against it. Until I go twin plug and down the rest of this slippery slope I'm gonna stay with what I got.

Seems there are differing opinions about distributor vs non-distributor. What's the deal? Ben likes the 12 plug dizzy and 356 likes the MSD. What are the positives and negatives?

[356-930]

Originally Posted by NathanUK

Do you guys think the MSD is a quality product and would trust your $10,000+ engine with it?

I suspect every dragster in the USA is using an MSD ignition system, and $10,000 wouldn't even buy a good pair of heads for the typical blown dragster engine.
Some MSD ignition systems are so sophisticated they can advance/retard timing cylinder by cylinder.
Definitely a reliable and quality product - I trust my $10,000 engine with it.

From the MSD wesite:
MSD 6BTM, PN 6462
The 6 BTM is ideal for engines with a turbo or supercharger. Not only will the engine benefit from MSD’s full power CD sparks, but there is also an adjustable boost/timing retard circuit to prevent detonation. . . .The 6 BTM lets you dial away detonation by retarding the timing in relation to the boost pressure. By electronically controlling the ignition timing your engine can run on the threshold of detonation to obtain maximum power and efficiency.
The BTM features a dash mounted control knob that lets you adjust the amount of timing retard. It can be adjusted from 0° per pound of boost to 3° per pound. The boost pressure is monitored by a sensor built into the MSD.
The 6 BTM also shares the Soft Touch Rev Control of the 6AL Ignition for overrev protection. The BTM is supplied with rubber shock mounts and rpm modules for 3,000, 6,000, 7,000 and 8,000 rpm.

BTW, Rev limit modules can be special ordered in 100 rpm increments i.e. 6,500, 6,600 etc.
Lastly, a track racer’s tip. Mount the ignition module upside down as the rev limit module is a simple 2-prong plug in device that is held in with simple pin/socket force. If it were to vibrate/fall out, there is no rev limit, period.
(I did not mount my MSD upside down - sort of like running with scissors - hope I don't trip.)

[356-930]

Originally Posted by dsiegel360

Ben,
It seems odd to me that the car idles at TDC and since the car lacks power down low, I figured a recurve would help drivability. I've been trying to diagnose a couple of problems and decided that a distributor rebuild was called for. Then I thought, well, while I'm at it do a recurve. But since then I've decided against it. Until I go twin plug and down the rest of this slippery slope I'm gonna stay with what I got.

Seems there are differing opinions about distributor vs non-distributor. What's the deal? Ben likes the 12 plug dizzy and 356 likes the MSD. What are the positives and negatives?

Twin plug trumps an MSD but then that twin plug update trump card is probably 10X the $$$ for the right MSD system.
Non distributor trumps distributor but again its the $$$.
I guess that's the cool thing about Porsche's - we can spend all the money our wives will let us spend on em or, all the money our wives don't know we are spending on em.

[JFairman]

If there is still a small vacuum solenoid installed in the vacuum advance hose line then maybe you still have some boost retard as it was from the factory.
I've been told that solenoid makes it so the vacuum advance and boost retard don't fight each other at full throttle.
Vacuum retard and vacuum advance change places or something like that when boost comes on since manifold vacuum becomes pressure then.

I think the vacuum advance signal from the throttle body is disconnected by that solenoid under boost possibly by the throttle position switch on the throttle body at full throttle, but I'm not sure, it may be the other way around...

[356-930]

Originally Posted by JFairman

If there is still a small vacuum solenoid installed in the vacuum advance hose line then maybe you still have some boost retard as it was from the factory.
I've been told that solenoid makes it so the vacuum advance and boost retard don't fight each other at full throttle.
Vacuum retard and vacuum advance change places or something like that when boost comes on since manifold vacuum becomes pressure then.

I think the vacuum advance signal from the throttle body is disconnected by that solenoid under boost possibly by the throttle position switch on the throttle body at full throttle, but I'm not sure, it may be the other way around...

The term "Vacuum Advance" is misleading. With the conventional mechanical 'vacuum advance' distributor, the harder the vacuum, the greater the retard. Timing "advance" occurs with loss of vacuum.
I tested my dual diaphragm/dual port '79 930 distributor on a Sun distributor machine. The machine's vacuum retarded timing -20 degrees. Applying up to +15-psi on the vacuum port had no effect on timing. With +5 psi applied to the boost retard port, timing was retarded -8 degrees. Increasing pressure on the boost retard port up to 15 psi had no further effect on timing.
Not having the specified solenoids may not affect performance but they may affect longevity of the diaphragms. Porsche must have had a good reason to use em.

[les_garten]

Originally Posted by 356-930

The term "Vacuum Advance" is misleading. With the conventional mechanical 'vacuum advance' distributor, the harder the vacuum, the greater the retard. Timing "advance" occurs with loss of vacuum.
I tested my dual diaphragm/dual port '79 930 distributor on a Sun distributor machine. The machine's vacuum retarded timing -20 degrees. Applying up to +15-psi on the vacuum port had no effect on timing. With +5 psi applied to the boost retard port, timing was retarded -8 degrees. Increasing pressure on the boost retard port up to 15 psi had no further effect on timing.
Not having the specified solenoids may not affect performance but they may affect longevity of the diaphragms. Porsche must have had a good reason to use em.

Do you have the SUN curve in a form you could post? I wold love to see it, especially with the diaphragms manipulated.

[356-930]

Originally Posted by les_garten

Do you have the SUN curve in a form you could post? I wold love to see it, especially with the diaphragms manipulated.

Sorry Les but no. I was lucky just to find a working unit in SoCal (Superior Automotive Engineering, Anaheim, CA). These machines are no longer in production and the one I found was over 50-years old.
The owner tested the distributor while I watched and recorded the changes. We made no attempt to correlate timing to vac or press. My mission was simply to find out what happened when I put pressure on the vac port and just how much timing the boost retard pulled out.
I had been running my little car without any vac advance and was running +35 deg. of advance at idle in order to get it to come off the line. Running 1-bar boost without an intercooler and reading this forum I was beginning to get a bit nervous about the grenade in the back seat.
If I pull the distributor for any reason, I'll return to Superior, get some data for posterity and post it.

[RarlyL8]

As you have seen the 930 dizzy does not have a vacuum advance. It also does not have a boost retard. For the 2 pot units one pot is a vacuum retard (at idle) and while the second pot is actually a vacuum advance, but the performance function does not allow advance due to the signal being positive pressure. What you end up with is vac retard at idle, vac/mechnical advance at cruise, and mechanical advance only on boost.
You can test all of this with a vac pump, an air compressor, vac/boost gage and a timing light while your engine is running from 1000rpm - 4000rpm.

[356-930]

Originally Posted by RarlyL8

As you have seen the 930 dizzy does not have a vacuum advance. It also does not have a boost retard. For the 2 pot units one pot is a vacuum retard (at idle) and while the second pot is actually a vacuum advance, but the performance function does not allow advance due to the signal being positive pressure. What you end up with is vac retard at idle, vac/mechnical advance at cruise, and mechanical advance only on boost.
You can test all of this with a vac pump, an air compressor, vac/boost gage and a timing light while your engine is running from 1000rpm - 4000rpm.

I respectfully disagree that the dual diaphragm distributor has no “boost retard.”
In the Sun test, we held the distributor at a constant speed of 800 rpm. Applying +5 psi to the distributor’s “boost port” produced a negative -8 degrees of timing. Yes there is a mechanical advance component in these distributors but only with a positive change in distributor shaft rotation speed. Given the constant mechanical rotation speed during test, the reduction of -8 degrees of timing can only be attributed to boost pressure.
Rhetorically, if there was no “boost pressure retard,” what is the purpose of the dual diaphragm?
If the ‘second pot’ was indeed a "vacuum advance," plumbing the two ports in parallel, which I have done, would result in no advance or retard for the two diaphragms would act against each other. As evidenced by test on the Sun, application of positive pressure to the “vacuum advance” port has no effect on timing.
We did not apply vacuum to both ports at the same time during the Sun test but in actual use on the engine, with the two ports plumbed in parallel and -12.5 inHg vacuum applied, timing retards -20 degrees evidencing that vacuum on the "boost port" has no effect.
Like I said, I respectfully disagree. Can't tell you how much I have learned reading your posts. What am I missing here?