Twin Plug?

[911TT33]

At what point does Twin Plugging become an absolute requirement?

I noticed the RUF CTR wasn't a twin plugged motor, and even some of the big HP boys here don't even run twin plugs (i.e. Juan etc).

Is an aftermarket, high energy CDI ignition with larger plug gaps enough to improve ignition under extreme boost?

Discuss?

[RarlyL8]

Absolute requirement? When you are running on the ragged edge of detonation and must have a complete burn that never skips a beat.
Twin plug systems are a wonderful wish list item for less stressed engines as well. The idle and power delivery curve are smoothed and you have detonation insurance for those times when experiencing heat soak or lower octane fuel.

[911TT33]

Thanks Brian

[911nut]

Twin plugging gets the flame front burning from two directions at once (because the plug is offset to one side of the chamber) so you get the pressure peak from the expanding gasses to occur over a shorter period of time. The piston will not have as much counter-force on it as it moves to TDC. This in turn allows the ability to run less ignition advance to take advantage of the shorter peak pressure. In the end you gain a little torque. It's something to have along with crankcase main web boattailing, crankshaft counterweight knife-edging and cylinder half-mooning if you want every last bit of power out of an engine.

[911TT33]

Originally Posted by 911nut

It's something to have along with crankcase main web boattailing, crankshaft counterweight knife-edging and cylinder half-mooning if you want every last bit of power out of an engine.

What on GOD's earth are those mods? Care to elaborate?

[Flieger]

Basically you make everything that moves in the case sharp and smooth so that there is less air resistance. This is why the GT3s use a huge oil pump and a special crank seal. They suck most of the air out to reduce the power lost to pumping air.

[copbait73]

In general it is an expensive item that adds little bang for the buck. Usually installed on a turbo engine to overcome having too high fixed compression ratio and or boost for the octane of available fuel. Limit both of the excesses mentioned and it is not needed.

The HP gained - money spent - by all the racer mods mentioned above can be overcome by simply avoiding the urge to increase the fixed compression ratio during engine build.

[Ken911]

Originally Posted by copbait73

In general it is an expensive item that adds little bang for the buck. Usually installed on a turbo engine to overcome having too high fixed compression ratio and or boost for the octane of available fuel. Limit both of the excesses mentioned and it is not needed.

The HP gained - money spent - by all the racer mods mentioned above can be overcome by simply avoiding the urge to increase the fixed compression ratio during engine build.

too late for that!!!

[JakobM]

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

[911nut]

I think we're confusing the needs of a 911 n/a engine vs. a 911 turbo engine here.

As compression ratios are increased in a n/a 911 engine, the piston dome gets high enough so that it divides the chamber at TDC, which impedes flame front travel (since the spark plug is offset). Twin plugs insure that the mixture will be completely combusted which will suppress detonation. Pistons designed to provide a compression ratio above 9.7:1 (the Euro SC engine spec) need the twin plug arrangement.

Turbo pistons are flat so flame front propagation is not an issue, even at an 8:1 static compression ratio. Temperature rise due to increasing boost pressures is a much bigger threat. Also, atomization of the mixture on a turbocharged engine is superior to a n/a engine, so the case for twin plugs to yield complete combustion can't be justified as a prevention for detonation. Will twin plugs provide a margin of safety against detonation on a turbocharged engine? Sure, since combustion will be improved slightly nonetheless. Is it worth doing for this reason alone? IMO, no.

Yes, twin plugs are great for "huge bore diameters" but if we look to the factory as a guide, the engine designers and calibrators at Porsche did not feel that the size of the bore diameters (even the 100 mm bore diameter of the 3.6) warranted the added expense and complexity of twin plugs.

Therefore, why would anyone twin plug a turbo 911 engine if it has little to no benefit as detonation protection? Because the ability to run less advance yields a little more torque. It's all about time. As the engine speed increases, there is less time to do work to the piston. Flame front propagation time is fixed. That's why the mixture is ignited before TDC. The trick to obtaining maximum power is to get peak cylinder pressure to occur just a little after TDC. Problem is that as the mixture is burning and pressure is rising, the pressure rise is applying a counter force to the piston and crankshaft. The faster the engine speed, the earlier the mixture must be ignited and the more counter force gets applied to the crankshaft. So if we can burn the mixture faster, there's less counter force that the engine has to work against. Hence, more torque is obtained.

So, if someone is building a competition engine where twin plugs are allowed and every hp must be obtained, they would do it. If I were rebuilding my engine and switching to EFI, would I add twin plugs? Sure, why say no to more power?

[Tippy]

As I've had engine theory on auto and aviation engines, I have a question.

On a recip aviation motor that is twin plugged, when you drop one magneto, the engine loses RPM.

So, is it power lost or the lost effect of timing causing slight loss of power?

This would explain if twin plugging adds power or not......

[drmatera]

The last 3 responses were very informative.

I will add this to the comment " the engine designers and calibrators at Porsche did not feel that the size of the bore diameters (even the 100 mm bore diameter of the 3.6) warranted the added expense and complexity of twin plugs. " if you are referring to what Porsche did to the stock production 911 Turbo vs. the guy building a modified 911 turbo, I see no comparison to be drawn. Porsche was looking at the bottom dollar, and at 300hp saw no need for twin plug, and we can all agree that it is not money well spent twin plugging a stock 911 Turbo. But, since most of the guys considering twin plugging their cars are modifying them I wouldn't use what Porsche did as an example. Even when comparing to the race engines Porsche built back then, technology of today forces a new way of thinking.

Since I plan on 9.5:1 comp when rebuilding my 3.3 I will most definately twin plug the heads

[911nut]

Originally Posted by drmatera

if you are referring to what Porsche did to the stock production 911 Turbo vs. the guy building a modified 911 turbo, I see no comparison to be drawn. Porsche was looking at the bottom dollar, and at 300hp saw no need for twin plug, and we can all agree that it is not money well spent twin plugging a stock 911 Turbo.

Here's why I say that.
Porsche, like any manufacturer, must balance manufacturing cost, warranty cost and customer satisfaction against performance targets. We can assume that they did enough testing to where they said "ok, we've got a powerful and reliable engine here. We don't need twin plugs to make it live to 100k miles". So, the 3.3 engine and the 1994 3.6 engine didn't get them for longevity reasons. That's my point. I'm saying don't twin plug for detonation protection, do it "while your in there". Do it for performance.

[drmatera]

ok, we are in total agreement. well put

[911nut]

Originally Posted by drmatera

Since I plan on 9.5:1 comp when rebuilding my 3.3 I will most definately twin plug the heads

So what will be the max boost that you will be able to run at that cp?
I was told once that the magic number where crankshasts break and rods buckle is at a cylinder pressure in excess of 10,000 psi. I can double check that if you like.

[drmatera]

I am hoping the Carrillo rods and 3.6 Turbo crank will handle the power. I expect I'll need about 20psi to reach my goal of 650hp give or take a few psi with the above parts, Extreme cylinder head porting and twin plug, GT2 Evo cams and roughly 3.5 liters if engine. Not to mention the car lives on E85 fuel

[Ken911]

Originally Posted by drmatera

I am hoping the Carrillo rods and 3.6 Turbo crank will handle the power. I expect I'll need about 20psi to reach my goal of 650hp give or take a few psi with the above parts, Extreme cylinder head porting and twin plug, GT2 Evo cams and roughly 3.5 liters if engine. Not to mention the car lives on E85 fuel

Did you read the spec sheet on the 044 pumps over on the other thread about 044 pumps it says on that sheet that the run time for one of those pumps is only 500 hours on e-85. have you heard any thing else on that subject? i was planning on swapping to it with my efi swap but kinda afraid of the fuel system crapping out on me after a few months. If i take the two hours a day driving to and from work every day that means it will only last a year.

[911nut]

Originally Posted by drmatera

I expect I'll need about 20psi to reach my goal of 650hp give or take a few psi

Here's the formula for calculating effective compression ratio:

effective comp. ratio = [boost/14.7+1] x static comp. ratio

Twenty psi with a static compression ratio of 9.7:1 yields an effective compression ratio of 22.90:1.

I think temperature rise in the cylinder at peak compression will yield uncontrollable temperature with resulting detonation. Also, will the structure of the engine survive these forces?

There are others who have obtained 650 hp using a 7:1 compression ratio and 1.4 bar of boost on a 3.3L engine, which is a more cost effective alternative.