Twin plug heads.

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mikeG15
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Twin plug heads.

Post by mikeG15 » Thu Aug 24, 2006 6:41 pm

OK
I was reading Motorcycle News ( UK motorcycle magazine ) today - a brief tech article on twin plug heads. They said that with high revs BMW and Suzuki delayed ignition on the second plug so the second plug did not fire. The view is that with large bores twin plug systems are only useful at low revs.
Comments ?

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Unkl Ian
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Post by Unkl Ian » Thu Aug 24, 2006 7:14 pm

What rpm range are they working in ?

And what bore/stroke do they use ?

76bug
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Post by 76bug » Thu Aug 24, 2006 7:53 pm

stan pobjoy in australia has developed twin plug heads for the type1 vw head, he runs them on 1916's. alot more low down torque

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Wally
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Post by Wally » Fri Aug 25, 2006 3:41 am

76bug wrote:stan pobjoy in australia has developed twin plug heads for the type1 vw head, he runs them on 1916's. alot more low down torque
Limbach (VW aircraft engines) had these developed also, but already long time ago. I had a set. Reasons were safety-regulations made them necessary for aircraft application, they were not developed solely because of performance reasons, tho it can never hurt to have it.

Porsche reseached that as from a 100mm bore onwards, they were a noticeble benefit for street engines, hence twin plugs on later 993 3,6 engines with the 100mm bore.
Although some 356 carrera (race) engines with small bores also used twin plugs...
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MASSIVE TYPE IV
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Post by MASSIVE TYPE IV » Fri Aug 25, 2006 5:33 am

All the 547 and later Carrera engines had twin plugs...

mikeG15
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Post by mikeG15 » Fri Aug 25, 2006 11:30 am

My understanding is that twin plugs allow higher compression ratios without detonation issues ( on two valve bathtub or hemi shaped chambers ) . What would be the reason to have different timing for each plug?

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Unkl Ian
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Re: Twin plug heads.

Post by Unkl Ian » Fri Aug 25, 2006 3:54 pm

mikeG15 wrote:OK
The view is that with large bores twin plug systems are only useful at low revs.
Comments ?
The benefit of dual plugs is more noticable at lower engine speeds,
when flame travel is the slowest.

Flame speed increases with rpms,as will turbulence due to squish,
so the benefit at higher speeds would not be as drastic.

Of course,the shape of the combustion chamber and dome will influence to need for a second plug.


I'd love to do a set of heads with as many spark plugs as I could
possibly squeeze in there,and test different combinations. :idea:

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Piledriver
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Post by Piledriver » Fri Aug 25, 2006 6:24 pm

Most bikes don't have ~4" bores, and the ones that do aren't generally "high performance".

(Twin plugs in very radical Harleys is pretty common)

Perhaps they are reducing plug wear--- I imagine at 10K the second plug timing difference would be on the noise.

I believe a chap in India was using 6 plugs... per cylinder.(2 stroke IIRC)
Little hard to cool perhaps...
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mikeG15
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Post by mikeG15 » Fri Aug 25, 2006 6:31 pm

Combustion propagation rates should have no relationship to engine RPM
At low revs the mixture is denser than at high RPMs unless there is a very effective harmonic ram effect.
Therefore at low RPMs the flame front actually propagates faster.
Does this mean that at high revs ( as used in competition ) twin plug setups are un-necessary even with high compression ratios?

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Unkl Ian
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Post by Unkl Ian » Fri Aug 25, 2006 6:47 pm

There was a recent thread on the subject of flame speed
on the Speedtalk.com engines forum.

http://speedtalk.com/forum/viewtopic.ph ... 132825e977

mikeG15
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Post by mikeG15 » Fri Aug 25, 2006 7:03 pm

Great link!!!
Your comment on number of spark plugs per cylinder reminded me of on of my own posts
http://www.shoptalkforums.com/viewtopic ... ed+circuit

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raygreenwood
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Post by raygreenwood » Sat Aug 26, 2006 4:31 pm

The conversation concerning bisection of teh flame front is spot on. I was watching my brother build a custom engine for circle track a couple years back that had very advanced combustion chamber techniques. It had...as we would term it...."squishies"......which have been in use in teh V-8 racing world for quite some time. It was a 327 making in teh neighborhood of 500 hp @ about 13:1. The piston domes were near mirror replicas of the combustion chamber.
On teh dyno he found much better efficiency and power by literally cutting a "trough" through the dome of several mm deep to allow linkage of both sides of the combustion chambers which were cut off from each other as the piston approaches TDC. The flame front is split...and or the flame on teh side of teh dome opposite the plug was underignited (wasted fuel and power loss). Short of a second plug....this was one method of connecting the spaces that are full of fuel air mix and under compression. I beleive in the link in that discussion they may have been speaking about putting the trough in the head itself. Ray

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