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Old 22 Dec 2008
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I think there are multiple reasons why people have various experiences with dual plugging. The technical differences can be:
  • Modifications of top (squish-band, porting, polishing ++)
  • Type of heads (valve size etc)
  • Type of petrol used
  • Type of carburetors used
  • Setup and state of carburetors
  • Type of exhaust/air filter
  • Compression
  • Types of pistons
  • Ignition timing (static firing point, firing-curve, coils etc)
  • Driving style
  • ++++

When people change to dual-plugging they often change some of the other factors, maybe they “only” overhaul the carbs which often gives great improvement. Maybe some modifications where done to the bike prior to the rebuild, maybe the cleaning of the tops alone has changed the way the bike worked.

It’s true that the old Paris Dakar bikes didn’t use dual plugging; the compression for some of the years was like this:

1980 9.3:1, 798cc, 55hp, HPN
1985 8.5:1, 980cc, 70hp, HPN
1987 9.0:1. 1050, 85hp, HPN
But what type of petrol did they use? What other modifications were done to the engine?

Today HPN recommends the use of dual-plugging, at least with high compression engines, but dual plugging is only one of the factors which are changed. These engines also have different camshafts, different pistons, rebuild heads and a completely different ignition-system and a few other changes.

Personally I believe in dual-plugging, but I think that if you want a good result you have to change a lot of other factors.
So I haven’t done it and there are a lot of things I would like to change with my engine before I do, it will probably never happen.
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Old 21 Dec 2008
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Quote:
Originally Posted by oldbmw View Post
I am curious, How did he know he had a 10 mpg improvement if he bought the bike dual plugged?
Maybe he did and maybe he didn't buy it dual plugged. Or he may have had an identical bike previous to that one to compare it to. Its been quite a few years. I know him and trust his numbers...we work in a test lab so he is quite analytical.

I do remember going over the timing and advance issues with him over coffee in the cafeteria.

Fact is, if the burn is incomplete, you are wasting gas...less power...less MPGs. And when you have a large diameter piston coupled with certain shaped combustion chambers, you will probably not get a complete burn. So you have two spark plugs to initiate two flame fronts. That speeds up the combustion process which is why you need to alter the idle timing and advance.

Ever wonder why the RPM decreases by about 150 RPM in a piston engined aircraft when you switch the magnetos from BOTH to L or R during your pre-takeoff checks?
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