someone told me thath 2 or even 3 plugs in the head help combustion with high % nitro( 30-40%) make sense?

Several manufacturers over the years have used two or more plugs. I have an OS 80H rear induction engine with two plugs, and I think Fox had a dual plug 60. Saito has some two plug engines and a Yamaha rider has added more plugs to his 80 hp Raptor engine. But, do they help? Maybe someone can respond here with real information, or at least plausible theory.

Sincerely,

Richard

We have them in helis though depending on the engine they are more a nusance than a help. On the 91's though, the low and mid range noticably smooths out when the second plug lites off.

They are dual plug heads called Viper heads.

Though most of us run 30 percent every weekend with single plug heads just fine. If its acting up on high nitro the best thing to do is play with shims and lower the compression ratio.

Two plugs are used on engines for many different reasons. For aircraft engines it is for reliability, if one ignition system quits then the airplane keeps flying. Many pilots think it increases power, because the engine RPM drops when you check the mags by turning off one mag. But actually using two plugs changes the timing. Because there are now two flame fronts the flame spreads faster and it has the effect of advancing the timing. You can regain most of the power by simply advancing the timing on one cylinder, there may or may not be a very small power loss after adjusting the timing. On racing engines it is usually to increase timing on very high RPM engines, to a point that cannot be done with one spark plug. On nitro engines it is to allow a richer mixture of nitro, and to keep it running if that mixture is off.

But for the old glow engines it was to allow two heat range of glow plugs, one for idleing, one for power. Not sure it worked very well. It often improved idling, but still lost power at full speed.

Twin plugs are usually used for the reason Sport_Pilot touched on, two seperate ignitions for a wider flame front. The idea being that, in larger engines having a wide combustion chamber, the flame front is more evenly spread because each one has less distance to travel to spread across the piston area. Pressure rise is then faster so the ignition point can be set closer to TDC to make better use of the pressure where it's needed after TDC. Any pressure rise before TDC works against crank rotation so it's wasted power. Twin plugs are most effective in large, high speed engines where there isn't much time for the pressure rise to happen.

but the use of two or more plugs with high nitro seems to me that nitro slow down the speed of burning front, is it true?

Nitro has double burn, the flame front is fast, butthen after theflam frontit burns slow (so slow it is still burning out of the exhaust pipe). Because of this and the very rich mixtures they use the race cars use a high advance at full throttle of about 60 degrees. And another reason for the twin plugs. But when you get the ratio below 1:1 or 50% nitro (not including oil) then the timing is similar to alcohol. But because of detonation with the leaner amounts of nito fuel mixture we use we need colder plugs and cannot use the high amount of advance.