Dual Plugged BB Heads and Timing Question

[NCAmother]

I’ve read dual plugged heads equals less initial timing, what is the advantage of less initial timing? Lower idle rpm? What about total timing?, does that change as well?  Thanks

[Alfetta]

non pro answer...
there is a specific point in time when you want the mixture to "explode".  If this happens too soon, the engine will knock and if it's to late, you are not efficient and are leaving power on the table...

Single plug engine takes some time to burn the fuel from the initial spark. This time must be accounted for (thus spark time is advanced).  If the bore diameter gets bigger the amount of time gets longer...  so more timing advance will be required.

Dual plug engines take less time to burn a fuel charge (as they "burn the candle from both ends". there for they typically do not need to have as much timing advance.

just my 2c....

[Frenchfrog]

Benefits of dual plugging are faster and more complete burn...so engine efficiency goes up ,pollution goes down.
On Guzzis ,the full advance for a single plug is around 34 degrees for most of the Tontis. Dual plugged , it should be brought back to 26/28 degrees according to most experienced sources. That eliminates potential pinging /pre detonation issues too.Much more of an issue with high compression engines.

[nsmith]

I have a dual plugged 850T, as far as seat of the pants power increase, I'm gonna say zero. However the starting is quicker and the top end while, not being any higher, the engine seems to run better, more smooth if you will. And yes you will bring the timing back a little.

[Wayne Orwig]

In my humble uneducated opinion.

Dual plugging starts two flame fronts. So the fuel is consumed faster. So the peak cylinder pressure occurs sooner. You do not want peak pressure too soon, or you lose horsepower and stress the parts. So you delay the timing some when dual plugging.

I think one of the main benefits is that you can safely use lower octane since the fuel is burned faster and has less chance to detonate.

Maybe.

Maybe not.

[DAMMAG]

I've got a twin plugged LM1000 motor in my LM3.

If I recall correctly I modified the distributor to advance only 16 degrees, with timing at 12 degrees BTDC initial and 28 degrees BTDC at full advance as per the Raceco website:

"At the same time as having your heads converted you will probably have had your distributor recurved. The new approximate timing figures are as follows: 12 degrees initial advance, 28 degrees full advance. Final full advance ignition figure will have to be determined either by seat of the pants testing (i.e. advancing the ignition timing until the engine pinks at high rpm and then backing off 2 degrees) or dyno testing. If re-marking your flywheel, remember that each tooth on the ring gear is 3.75 degrees, so your initial timing mark will be in the region of 3 teeth before tdc and your full advance figure will be in the region of 7 1/2 teeth before tdc. When setting your igntion timing it is important that you strobe it at full advance as this is the most important figure."

Damian

[lazlokovacs]

I agree with the above statements,

however it's interesting to note that dual plugged guzzis (eg Cali Vintage and CARC 1100s) left the factory with the same timing as the earlier non dual plugged 1100s

At least a good percentage of them also had a marked tendency to ping on all but the best fuel

Beats me!

[moto-uno]

  Another advantage of dual plugging the heads , is that they run better at lower engine speeds , something over-carbed
and cammed big bore Le Mans 2's were not particularly great at :) !  Peter

[NCAmother]

The heads I’m going to run are Raceco.  Where did you find that? (Can’t find the website) This info is great and makes selecting a curve so much easier(I have a Sachse ignition with 10 different curves

I've got a twin plugged LM1000 motor in my LM3.

If I recall correctly I modified the distributor to advance only 16 degrees, with timing at 12 degrees BTDC initial and 28 degrees BTDC at full advance as per the Raceco website:

"At the same time as having your heads converted you will probably have had your distributor recurved. The new approximate timing figures are as follows: 12 degrees initial advance, 28 degrees full advance. Final full advance ignition figure will have to be determined either by seat of the pants testing (i.e. advancing the ignition timing until the engine pinks at high rpm and then backing off 2 degrees) or dyno testing. If re-marking your flywheel, remember that each tooth on the ring gear is 3.75 degrees, so your initial timing mark will be in the region of 3 teeth before tdc and your full advance figure will be in the region of 7 1/2 teeth before tdc. When setting your igntion timing it is important that you strobe it at full advance as this is the most important figure."

Damian

[fotoguzzi]

  I think one of the main benefits is that you can safely use lower octane since the fuel is burned faster and has less chance to detonate.

Maybe.

Maybe not.

yet the Norge 1200 2V dual plugged calls for premium in the service station manual.
I might have ask this before, does the Norge really need premium fuel?

[DAMMAG]

The heads I’m going to run are Raceco.  Where did you find that? (Can’t find the website) This info is great and makes selecting a curve so much easier(I have a Sachse ignition with 10 different curves

I think the Raceco website is now gone.

The post I made is a copy of a post I made to another forum years ago.

The motor I built has 0.9mm squish and I got 0.8mm machined off the heads and 0.7mm machined off the barrels so it is quite high compression. With that timing it would still ping a bit so I have backed the timing off a touch further and it now runs well on 95 octane.

I modified the distributor by making a nylon ring to sit over the distributor shaft to limit the travel of the advance weights.

Here's a couple of links to dual plugging BMW's if you haven't come across them.

http://www.gunsmoke.com/motorcycling/r100gs/dual_plug/

http://rubberchickenracinggarage.com/Downloads/TomCutterDualPlugIgnition.pdf

Damian.

[Huzo]

In my humble uneducated opinion.

Dual plugging starts two flame fronts. So the fuel is consumed faster. So the peak cylinder pressure occurs sooner. You do not want peak pressure too soon, or you lose horsepower and stress the parts. So you delay the timing some when dual plugging.

I think one of the main benefits is that you can safely use lower octane since the fuel is burned faster and has less chance to detonate.

Maybe.

Maybe not.

(Uneducated, my arse…!) 

[Huzo]

yet the Norge 1200 2V dual plugged calls for premium in the service station manual.
I might have ask this before, does the Norge really need premium fuel?

I’ll have a running stab at that one..
It’s generally accepted that the a/f ratio is targeted to be as lean as buggery to meet emission requirements. The higher octane level will reduce the tendancy to ping under high CHT and cylinder pressure conditions.
I’ll bet a 3 cent coin on that one…

[s1120]

You know I know this is true, and it makes a lot of sense that having two flame fronts, you would not need as much advance..  But it was strange that in trying to get my 92 stone running right when I got it, I was swapping a lot of maps into it to get it to be rideable, and one that I tried ended up being from a V11 sport. It ran AMAZING in the bike, but was told that due to the advance curve I should not run it. I didnt think much of it at the time, but I wonder why, knowing the dual plug heads need less timing, that this map would be so much more aggressive? Maybe cam specs?