Thread: Twin Plug?
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JakobM JakobM is offline
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Join Date: Sep 2010
Posts: 252
Reason for twin plug is not really power in first place IMO...The reason is safety. To say Twin plug gives power is not nessecarely true IMO - it depends on your obstacles (detonation) on your way of tuning for highest tourge on your fixed rpm vs. boost "load points". If you have build a big bore high CR forced induction engine then twin plug helps you keep normal/high boost levels.

Twin plug on these aircooled engines is due to the combination of huge bore diameter and a plug that sits as only option in one site of a the champer. Add then forced induction and we are in for a challenge if without EFI or old faschient ineffecient low CR. As BORE diameter increases (i.e. 100-102mm) the longer the flame needs to travel from the plug to the other site and thereby "sits" in the cylinder longer waiting for detonation to happen. Detonation ... self combustion of "leftovers" of still not yet burned gasses after the plug has ignited. Preignition ...next obstacle... uncontrolled self combustion before the plug is to ignite. Preignition can actually turn a 4cylinder carburated engine backwards when trying to turn it off. Ever tried it? ...on its attempt to stop it keeps going, lots of bangs and it may roll backwards to stop. Imagine the beatings the pistons/rod/crank take just on that stroke when it turns direction at idle. Then imagine on full load rpm. In an ideal world combustion would happen on the speed of light. We could then run a i.e. fixed +14 degree of RETARD ATDC through out the hole RPM scale and have maximum tourge on fixed timing (14% is just the theoretically generel thumb rule for correct angle, it varies depending on rod lenght). Makes good perspective I think, to the fact that it is not numbers of degree before TDC that is important, it is the exact spot ATDC that is in focus. EXAMPLE: Say we need 30 degree advance (just an example) to have the combustion to evolve just in time to have its maximum power at the 14 degree ATDC. We reached 20 degree before "pinging" detonation. Combustion evolves now too late...it hits with maximum power on 24 degrees ATDC giving less power and more heat. In that case a twin plug setup would give benefit of shortened flame travel, giving us the possibility to tune with less advance to reach maximum tourge (all other aspects held equal in this example). However, if we do not experience "pinging" on our way to this point, well then in regards to POWER, we dont really need the twin plug. We then only have a comparable minimal loss to the extra down force the piston is heading on its way up to TDC.

As CR increases the obstacle of detonation will come much earlier when tuning ignition advance (all other aspects set equal). So Twin Plug for me is more a way to get easely and safe on target and eliminating the risc of sacrifing CR or boost if heading into detonation issues.

Jakob

Last edited by JakobM; 11-24-2011 at 08:42 PM..
Old 11-24-2011, 08:02 PM
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