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Yes, that's when I did it too. "While it's open" type of thing. Wouldn't do it just for itself, but then again, I don't know anybody who twin-plugged a 930 motor JUST to push the timing and get a few more hp.
Old 03-06-2008, 10:06 AM
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Quote:
Originally Posted by Bruce M. View Post
Yes, that's when I did it too. "While it's open" type of thing. Wouldn't do it just for itself, but then again, I don't know anybody who twin-plugged a 930 motor JUST to push the timing and get a few more hp.
Refer back to the original Post. All of this references directly to it.
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Old 03-06-2008, 10:19 AM
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My turbo is twin plugged, with am electromitive crank fired ignition. When I bought it, it was runnin 1.1 bar worth of springs in the tial without any detonation.

I will say the lower valve covers become a beatch to seal, and the removal of the distributor adsd another wrinkle of potential problems.

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Old 03-06-2008, 11:20 AM
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What would you say the the advance should be on a Andial built twin plug w/ 934 type 12 lead distuibuter . Compression 7.0 boost, 1.2 on 100+ leaded fuel now set set at 29 btc @ 4000 RPM

Last edited by voitureltd; 03-06-2008 at 01:48 PM..
Old 03-06-2008, 11:39 AM
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JFairman said:

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The advantages and theory of twin plugs and flame front travel vs igniton timing has all been said, but there's another VERY big advantage to it for a race car or aircraft.

Ignition systems all eventually fail and if you have twin ignition sytems and sparkplugs and one fails in service the plane won't crash and burn, and the race car will still finish the race.

Thats the biggest advantage for some folks.
I agree with this for aircraft, but the problem with our cars is that we rarely have the ready ability (or foresight) to check that both banks of plugs are actually firing. I've heard of two instances with twin-plug 911s where an entire bank of plugs was not firing (for some reason), unbeknownst to the driver. The engine can seemingly run okay on one set of plugs, but the timing would be set wrong and it may have serious consequences. Before track days, I always unplug one bank of coils, then the other, to make sure my engine is actually firing both sets of plugs.
Old 03-07-2008, 09:27 AM
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Why didn't I think of that,thanks,great idea
Old 03-07-2008, 09:40 AM
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Quote:
Originally Posted by copbait73 View Post
You can use twin plugging to increase your safety margin against knock or you can take a power increase up to the same margin with increased boost. That is how you get increased power. At the the risk of repetition, twin plugging allows the equivalent HP increase as 1 point C.R. increase before onset of knock. This is not about timing.

In the N.A. 911 forums this benefit is readily understood and accepted.
If we were talking about non turbo 911s would you dispute a 3.0L 11:1 C.R. (made possible by twin plugging) makes more power than one at 10:1 C.R.? Turbocharging does not change the impact of fixed and equivalent C.R. (fixed plus boost) on power output.


This is another very interesting chart coming directly from Porsche engineering SAE technical paper, Hans Mezger, Aug 7, 1978.

Interesting, the presentation is discussing geometric compression ratio (fixed C.R.) in terms of.....hold on to your hats power junkies...not throttle response, not boost response, not ultimate power production......FUEL ECONOMY.

Posted fuel consumption values are set at low power production (20-30HP ? for a 930 on the interstate). They wanted to maximize engine efficiency at low throttle angles, off boost. High fixed compression delivers this.
Old 03-07-2008, 10:22 AM
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I had never seen that graph before but it explains why Porsche picked their CR and Boost pressure settings.
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Old 03-07-2008, 11:53 AM
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I had never seen that graph before but it explains why Porsche picked their CR and Boost pressure settings.
Les, I'm expanding this chart. Please give me a data point. What is your HP and boost level?
Old 03-07-2008, 12:44 PM
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Quote:
Originally Posted by Rob 930 View Post
JFairman said:



I agree with this for aircraft, but the problem with our cars is that we rarely have the ready ability (or foresight) to check that both banks of plugs are actually firing. I've heard of two instances with twin-plug 911s where an entire bank of plugs was not firing (for some reason), unbeknownst to the driver. The engine can seemingly run okay on one set of plugs, but the timing would be set wrong and it may have serious consequences. Before track days, I always unplug one bank of coils, then the other, to make sure my engine is actually firing both sets of plugs.
This is the first I've ever heard of an entire coil pack/bank not firing, but good idea on the check. My LINK EFI system has an alert for injector and ignition failure, but you never know....

On smaller aircraft engines, like a 540 (cu in), in addition to safety, the twin plug ads to the flame propagation which improves torque. Since the motor red lines at 2700 rpm, you need torque right from idle. In our bigger piston motors (1820 cu in), you need twin plugs just to light the fire without reverting to 45 degrees of advance.

Last edited by DonE; 03-07-2008 at 07:53 PM..
Old 03-07-2008, 07:45 PM
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Don, your high mod engine at 8:1C.R. is another data point. What boost and HP are you running? Are you running dual plugs? Thanks
Old 03-07-2008, 07:51 PM
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I run .8 bar at 500 rwhp. At 1 bar, 525 rwhp. My ignition is twin plug, wasted spark, Bosch coils, LINK ignitors.
Old 03-07-2008, 08:01 PM
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500HP @ 0.8bar, damn Don that is sweet! If memory serves you're using a GT35. Looks like they work well!
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Old 03-07-2008, 09:04 PM
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Quote:
Originally Posted by Rob 930 View Post
I agree with this for aircraft, but the problem with our cars is that we rarely have the ready ability (or foresight) to check that both banks of plugs are actually firing. I've heard of two instances with twin-plug 911s where an entire bank of plugs was not firing (for some reason), unbeknownst to the driver. The engine can seemingly run okay on one set of plugs, but the timing would be set wrong and it may have serious consequences. Before track days, I always unplug one bank of coils, then the other, to make sure my engine is actually firing both sets of plugs.
Interestingly enough; this a required 'pre-flight' procedure for every reciprocating engine aircraft prior to take off. Cycling the magnetos on and off on the run up will determine whether or not that particular mag is firing or whether it is faulty.

I am huge proponent in doing pre-flight to my car or motorcycle each and every time I take it out.

Of course, imminent power failures are far easier to deal with on four wheels than on two wheels,, let alone being in the air.

Good tip, Rob.
Old 03-07-2008, 09:55 PM
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