Twin-plug ignition timing?

[zuffenhausen]

Hello!

I'm getting ready to do the run-in on a newly-built twin-plug 3.4 liter race engine and I'm wondering what would be a resonable starting value for ignition timing? I've read where twin plugs allow you to set the timing up to ten degrees less advanced than normal, but since this is a new engine combination I'm not really sure what a good starting value would be. Would 25 degrees be good place to start (typical 35 degrees of advance for a single-plug engine less 10)? If it helps, the engine is using GE80 cams, 46IDA Webers, and is running a C/R over 12:1 (on race fuel, of course).

Best Regards,
zuff

[Formerly Steve Wilkinson]

I've got basically the same engine--twin plugs, 3.4, S cams, PMOs, but about 10.5/1 compression--and am running it at 26-28 degrees.

Stephan

[MMARSH]

Sounds like that will be a great motor. I think you are right about running ten degrees less timing, but I would ask Steve at Rennsport (http://www.easystreet.com/~porsche/) or someone Johhny Walker to be sure.

Post some Dyno numbers when you get it together.

[Roland Kunz]

Hello

Your timing seting depends on the real compression, the used fuel and the distributor curve.
Normaly you drive the engine on a bench to find the optimum curves.
Untill then you use the stock setings to run in the engine ( as long the stock setings are working, if not you have to find out the new base seting while the run in period as well )

Grüsse

[Wayne 962]

Agreed, normally, you could set it at about 25 degrees versus 34 (we're talking total advance here of course).

As for break-in, I would make sure that you set it conservatively. You're running a very, very high-compression ratio, so you're like to get some detonation while you're figuring it out, which is always a wise thing to avoid when breaking anything in.

I would play with it a bit right away, but start with the stock values and play it conservative.

-Wayne

[zuffenhausen]

Thanks guys. I've currently got it set at 26 degrees BDTC total advance and it sounds fine. Of course, that's without any load. Unfortunately I can't afford a day on the dyno to fine-tune the engine right now, so the only thing I know to do is take it to a test & tune day, run a lap, and then pull the plugs to see if there's any evidence of detonation (unless you have a better idea?). The advance curve is whatever ANDIAL uses when you have one of the twin-plug distributor conversions done. Now that I think of it, I should've requested advance curve information from them. They certainly didn't volunteer any with the distributor!

Regards,
zuff

[dean]

Why don't you ask Andial?
Dean

[Wayne 962]

Once the engine is done, you should be able to run it around 25-27 degrees. I wouldn't risk it before the rings have seated...

-Wayne

[Roland Kunz]

Hello

A good enginetuner should calculate your needed distributor curve and your base setings.

Hope you cc´d your pistons an heads perfect.
If not stay on the conservative side.

Not sure what distributor you use but you can modify them for quick adjusting if you run out off aviation gas.

Grüsse

[Wayne 962]

A related question. Does anyone know of any rationale of having the plugs fire at different times (like one slightly after the other)? I have heard of this being done, but I can't remember where I heard it, and why it was done...

???

-Wayne

[HawgRyder]

Firing at slightly different times was done in some engines to account for the flame front travelling accross the cyl.
If memory serves...the flame front travels at 3000 fps...or there abouts....and you measure the distance from the first plug to the second...and the distance form thee second plug to the far side of the cyl wall and work the relective ratio backwards to get thee detonation timing.
The idea of thee second plug id to prevent the reflection off the far side of the wall form hitting the main front in the middle of the chamber.
Ideally you would like the two fronts to blend when meeting...so no big explosion (detonation).
Bob

[CamB]

I can't answer Wayne's question, but I have three thoughts to add:

1. How does an RSR 12 wire distributor cap work? Are the twin plugs fired sequentially as Wayne suggests? (ie it fires 1,1 6,6 2,2 4,4 3,3 5,5) (won't I look stupid if that is the wrong firing order ) - I can't see how not (ie how it could be simultaneous).

2. Do 964s (etc) fire simultaneously or is the second distributor timed slightly behind

3. Wayne - Alfa Romeo twin sparked plenty of production 80s and 90s cars (and still do, I think). I have vauge recollections of them timing the spark differently (and also of one plug being smaller than the other????)

I hope someone can answer - I am very interested.

Cam

[zuffenhausen]

CamB,

The RSR distributor fires both plugs simultaneously. The distributor has two "rings" of plug connections, one inside the other, along with two coil connections. The rotor contacts a plug connection on the outer ring and a plug connection on the inner ring at the same time.

I believe the 964 distributors fire simultaneously as well.

I know that Mazda RX-7 rotaries used two plugs per rotor with lead and trail timing, which makes sense for improving combustion as the rotor slides by each plug.

-zuff

[CamB]

Ahah - makes sense now - this is why I am not an engineer because I never would have thought of such a simple solution. The two coils bit was where I wasn't thinking:

http://www.nosubstitute.org/events/Dunkels11Feb01/B1.jpg

Thanks again,

Cam

[Oldporsche]

According to the information I have about the twin cap distributor, ala 964, etc. One plug is fired slightly before the other. Which one? I don't know. Which one is supposed to be fired first the upper or lower? Does it make a difference?

What I can tell you is that, some time later this summer I will know. I have one in a box under the bench. I intend to modify it to work as a twin plug distributor for a pre-SC motor.

Good luck,
David Duffield

[MMARSH]

Originally posted by old_porsche


What I can tell you is that, some time later this summer I will know. I have one in a box under the bench. I intend to modify it to work as a twin plug distributor for a pre-SC motor.

Good luck,
David Duffield

David, I could be wrong, but i think the firing of the 964 distributor is controlled by the DME. So when you us it on your pre SC motor it both will fire at the same time. When used on my 3.2 twin plugged motor the also fire at the same time.

[porsche74]

David:

Michael is right on the money; the 964 distributor, as does the RSR one, fire both plugs simultaneously.

The only way it wouldn't, is if the distributor was incorrectly phased when the belt was replaced or when it broke,......

[Wayne 962]

Not necessarily true. The pickup controls the firing. If you set it so that there is a delay, or use two pickups, then you can fire at different times...

-Wayne