ignition advanced with twin plug

[MaRu]

Hello everone,

my engine is a 3,0 with 10,5 cr. In the past it was 10,3 with single plug. It ran perfect with 35 degrees total advanced. (We have 102 octane pump gas, so no worries with deternation) Now with the twin plug I got the advice to go with 25 degrees. I haven´t been on the dyno yet (with twin plug) but I have the feeling that more advance is more power. So why does everyone advice to go to a lower rate?

Thanks Malte

[MBruns]

Depending what you have for an ignition system we run 28 to 30 degrees total with twin plug and the info you gave, there is a good bit of power to loose at 25 deg vs. 30deg. depending how developed your engine is.
Mike Bruns

[otto in norway]

Really?
That sounds to be a bit over the top to me..?
Other experienced guys have quoted 25-26 degrees.

Is this on race fuel?

Why does it matter what kind of ignition system you got? I thought that advance in degrees is the same, regardless of what made the high voltage?

I got a twin spark 3.4 liter (3.2 based) running 25 max at WOT. (using TEC3r and their coil packs) I fear taking it any further than 25 would do damage.

[Steve@Rennsport]

Malte,

You can read some information here on this subject.

There are lots of variables that affect optimal timing in a twin-plug (or single-plug) engine that include: compression ratio, camshaft profile, piston design, air-fuel ratio and octane. Its not a one-size-fits-all proposition.

One would need all the details about a particular engine as well as the fuel in use to know what range of timing will result in best torque & power.

[kerthunk]

Steve told me to go with 25 degrees for my setup (3.2ss, twin plug, mod-s cams, 9.8:1, 92 octane pump gas). While I had the car on the dyno, I tried a couple degrees more and a couple less. 25 was the sweet spot and produced the most HP. That's how it worked out for me anyway.

[fred cook]

I am using the XDi direct fire ignition on my 3.3SS twin plug engine. The recommendation for this system was to use 22-24 degrees for the max advance. Of course it is dial adjustable so when I get a chance to put it on a dyno it will be easy to look for the "sweet spot". Like Steve W. says, it all depends on the build details of your engine.

[carrerarsr65]

hey kerthunk how was your dyno ###?

[Aurel]

Since advance is designed to account for ignition propagation speed of the gases, it makes sense that with twin plugs, less advance would be needed. That's because the flame has about half the distance to cover with twin plugs. Roughly .

[kerthunk]

Originally Posted by carrerarsr65

hey kerthunk how was your dyno ###?

The car did 228.4 at the wheels, so somewhere around 262 at the crank.

[MaRu]

Hello Mike and Steve,

thank you very much for the infos!!

Regards malte

[Steve W]

Twin plugging completes the burn of the charge faster. The timing advance must be reduced or the effect will be as catastrophic as running too much advance with a single plug motor - hammered bearings, cracked rings, blown pistons, damaged heads. Ideally you want the peak pressure of the combustion to come in 8-12 degrees after TDC. Timing the burn earlier will cause the peak pressure to come in before TDC, like pressing down on a bicycle crank at 11:00 while it's still rising, vs waiting till 1:00. On Porsche motors we've tested many times over the years, for the same engine configuration, a reduction of 8-9 degrees of advance for a twin plug vs single plug configuration.

[Walt Fricke]

Timing seems to be a balance. If flame spread/combustion were instantaneous, you'd want it to happen a bit after TDC. Like Steve W's bicycle pedal example. The pressure build-up before TDC is pretty much all negative work - something to keep the piston from wanting to come up, requiring energy to overcome it. Which I think is why a Model A had you retard the spark manually to start it up. Once things are running you have flywheel inertia to help keep the crank spinning.

So up to a point the less advance, the better. That point being if you wait too long, the piston gets too far down the down stroke for the optimum peak cylinder pressure to develop.

The beauty of adjustments and dynos is that you don't have to try to quantify this qualitative understanding. You just get ball park and safe figures to start with (as given here), and then watch what happens. My understanding is that you start with a fairly conservative advance, and then adjust it upward. Torque should rise, peak, then fall off. And the trick is to catch where it falls off before going too far, and then retard back.

Where theory can get a bit tedious is doing this over the whole RPM range, and at different loads. But it seems it isn't really necessary to tweak on a very fine grained level. The HPV1 gives you only three real settings: Initial advance (also settable by how trigger wheels are set), 3000, 6000, and 8000 RPM. Straight line interpolation between these points. Fancier systems (like my Tec3r) have a matrix (mine is 16x16), again with interpolation as the RPM changes. But for a race motor you don't worry much about part throttle efficiency, so you can use basically a single value for each column and be fine, as you don't worry about the higher vacuum part throttle areas. And get it all in at a certain point, and maybe pull a little back up high. The dyno (after working with the fuel injection side of things also, and the A/F rations) will tell what the engine likes with the fuel supplied.