Opinions on twin plugging with race gas

[nein14-6]

I realize that twin plugging is a common practice among the Porsche enthusiasts. I also realize that twin plugging allows the flame to start from both sides of the combustion chamber so that the flame doesn’t have to travel across the top of the piston.
On another note, on early, low displacement (2.0) motors, with lots of compression, you need to fire the plug with a ton of advance in order to make the motor work and the twin-plug modification is supposed to make it so you don’t have to run as much advance.

Here’s the issue. Recently a very well respected engine builder that I know has told me that on a 3.0l race motor using race gas, that twin plugging doesn’t make any more power than a single plug motor.

I also understand what octane in fuel is and how fast/slow it burns as well as resistance to premature combustion.

Can I see your thoughts/suggestions/research/findings on this subject?

[jluetjen]

that twin plugging doesn’t make any more power than a single plug motor.

I've always understood that to be true, even on the earlier motors. But what it does do is make the engine run much cleaner and stronger under part throttle conditions when the speed of combustion in the camber is much lower. It also allowed a much higher CR to be used on the various race fuels used in Europe then the combustion chamber would normally allow.

[Grady Clay]

Nein14-6,

Check out; Twin plug
twin plugging question

There are several other threads on these 911 forums that address various aspects of this issue.

To answer some of your questions:

My experience is that twin plug actually raises HP. While I don’t have technical proof, just dyno runs, I think the reasons are:
1. The ignition doesn’t have to be advanced as far which reduces area under the cylinder pressure curve after the intake valve is closed and prior to TDC compression. This reduces the power loss required to compress the mixture.
2. That mixture/pressure is now available to provide power after TDC compression. It moves the overall curve slightly from compression up-stroke to power down-stroke.
3. These are detonation limited engines. Any thing you can do that reduces detonation will allow that mixture to provide power. Detonation robs power by using part of the mixture to go “bang” and not burn completely.

Your description of the flame propagation of a two-liter is correct. A 2.0 has a significantly different combustion chamber than a 2.2, 2.4 and later. Its piston profile is much “taller” when comparing the height of the piston pop-up and depth of the cylinder is much “deeper”, particularly when compared to the cylinder bore. The higher the compression, the worse the problem, more benefit from twin plug.

You say a 3.0 race engine doesn’t benefit from twin plug. What are the details of this engine? What pistons? What cams & induction? What CR? What is the squish clearance? 911 or 914-6 application & what exhaust?
I think any SCCA legal race engine would benefit from twin plug by lessoning detonation propensity. You just need to lobby SCCA for twin plugs. E-mail me and I will help you make your case.


John is right on.
At part throttle almost any engine has an occasional plug misfire. I think this is usually due to not perfect mixture at the plug. Two plugs double the opportunity to fire that cylinder charge.
Again, twin plugs reduce the octane requirement by insuring better and more consistent combustion.

BTW John, The exhaust plug connectors (not OE) and wires do not have to be exposed. They can be under the exhaust valve cover. The only disadvantage is you have to remove the valve cover to change those plugs. No big deal. You can’t tell its twin plug, “only your hairdresser can tell for sure.”

Best,
Grady
gradyclay@hotmail.com

[afterburn 549]

you can proove twin plugging is more pwr with this exp.
Go jump in your buddies airplane throttle up to almost full load and drop one mag.....What happens? you loose 15% or more RPM. Whats that tell you??

[Steve@Rennsport]

Grady is spot-on.

Twin-ignition has added HP to each and every engine that I've seen since 1976, all things being equal.

The 2.0 litre's deep chambers and high-domed pistons really need this to make power at 10.5:1 or over; its 15 HP upgrade with far better drivability.

More info can be found on this subject at http://www.rennsportsystems.com/2.html

Its one of the "prime ingredients" for any performance 911 motor,...

[Mark McClure]

Afterburn.....I think that you loose RPM's when you drop a mag due to the fact that the effective timing has changed. My understanding is that twin plugs are more effective at burning the fuel, not creating HP, therfore you retard the ignition. When you drop a mag you are effectively running a single plug so retarded that it reduces the effectivness. If you advanced the timing back the power would increase to were it was.

Steve......would twin plugging alone increase power or is it an activator that assists other fundamental designs.

Cheers

Mark.....

[campbellcj]

What are ballpark machining costs to do a twin-plug conversion?

Also, are there currently any affordable/practical twin-plug distributor options for the early/small engines? Do you run modified 964 dizzy's? The GCR's I do/will run under are highly restrictive re. using electronic engine mgmt so I would need a conventional system.

[Steve@Rennsport]

Hi Mark:

Twin-plugging does increase power to varying degrees, depending on bore size and combustion chamber shape and how homogenous the air/fuel mix is. Remember, hemispherical chambers are inherently "lazy"; they have very little natural swirl and quench. This makes this design very detonation prone unless you light the fire on both sides of the tall piston domes required for decent compression.

Power increases are all due to a better, faster, and more thorough flame front progression. This requires less ignition advance to achieve peak cylinder pressure JUST after TDC.

Many years ago, we did a custom set of RSR twin-plug heads that only required 15-17 deg of advance for best power. That 3.5 slide-valve, flat-fan motor made around 412 HP,...


Chris:

Cost vary with how you do this. The cheapest way for most engines is using the 3.6 dizzy as the foundation, but those cannot fit the 2.0 and 2.2 engine cases. Those early cases will use a custom made billet distributor that mates to the RSR cap & rotor. Although not cheap, its the only way to make power in those things.

[afterburn 549]

I am surprised at this conversation.......but its a proven fact that twin plugs burn better there for release more energy therefor more hp.
It comes dwn to basic BTUs........heat /energy It works on Harleys and airplanes( All thoe thats not why it is done on aircraft)
Someone will step here and produce a dyno report I am sure.

[nein14-6]

Thanks for all the info.

The rennsportsystems page was also informative. It was interesting to see that the electromotive-type coil packs don’t put out as “Hot” of a spark as the MSD/dizzy setups provide and therefore are more prone to fouling the plugs. I didn’t realize that. Do many people on this post use the electromotive setups?

Would everyone here go as far as to say that on a 3.0L with 10.5:1 cr, that twin plugs would not be as important when using race gas (110 octane) as they would be when not using race gas?

[Grady Clay]

I have a 2.0 that I added a second distributor on the end of the left cam where the Gilmer toothed pulley is for the MFI pump. Hey, if it works on an early 547 ….

Another issue in the case for twin plugs is that it slightly lowers the octane requirement of the engine. Important for street use. Twin plugs would be slightly more important when using pump fuel. Having 10.5:1 CR is definitely pushing the limits with pump fuel. If I were in that situation I would get to know my fuel supplier very well and keep some leaded race fuel at home. Adding one gal of leaded 112 octane race fuel to 20 gal of 98 octane unleaded premium has a disproportionate positive effect.

Best,
Grady

[neilca]

Grady,
I would love to see a picture of that distributor setup. Sounds very interesting. I was also thinking you wouldn't need two triggers, just the one in the regular distributor so your slave distributor would only distribute the fire.
Interesting.........
neil

[Tim Walsh]

It would also be perfect for that "sleeper" look.. a set of webers, black shroud, "single" plug.... with something like 3.5L of displacement in a narrow bodied car... :-D

[Grady Clay]

Neil,
Those engines are not easily accessible. I have two 45’ tractor trailers full to the top, resting quietly until “the kid” graduates from college in about 5 years. Then he gets to play with p-toys.
Of course he has been terribly deprived; competitive racing in karts since 10 including shifters, Spec Miata last summer, and currently testing in Formula Mazda.
I gave him a daily driver that is the same age as he is; ’85 Toyota Camery, 4-door, 4-speed automatic, 4-cylinder, original paint, 70K mi, new tires, new cam belt, shocks, brakes, $950 – a trade in from a Toyota dealer friend. Perfect for a teen boy. He took one look and said; “Dad, It’s not a Boxter S.”

Back to twin plug.
My first attempt with dual distributors was just to put just a cap and rotor on the end of the cam. With the small (early) cap you need the advance mechanism to keep the rotor pointed at the correct terminal.
Yes, I use two CDs that only require only one set of points. This insures that both plugs fire simultaneously and you don’t have two advance curves chasing each other.

Best,
Grady

[Grady Clay]

Tim,
Right on. I used short plug connectors that fit inside the exhaust valve covers. I notched the inside of the covers to route the wires out and behind the intakes. It is a ’68 normal (E cams) with ‘69S pistons at 10.8:1. It even has the OE smog pump. Most would be hard pressed to tell it isn’t virgin stock. I ran it several times with 50-50 gas-methanol and a little nitrobenzene. Great fun to run with the 2.8s with a 2.0. We had (have) the Colorado “Das Schwartzgruppe” made up of mostly instructors and lots of outrageous cars.

Best,
Grady

[cstreit]

I use an electromotive and initially experienced some fouling. However I went one range hotter plug and never had a problem. My car idles VERY rich but still doesn't foul plugs even idling on the grid for long periods.

Steve@Rennsport has vastly more experience than I do, but I personnally believe that the issue can be solved with the right plug and mixture.

However VERY high compression motors might have a bigger issue because of more difficult combustion, but the electromotive spark is still much hotter than stock. From that aspect MSD may be better, but for lower compression, say 10.5-11.0:1, I think electromotive in my experience and observation is fine...

[Grady Clay]

Chris,

I assume you have 14 mm plug in the original location and the second (added) plug is a 12 mm. Specifically what plugs are you using? Describe their relative heat range and type (recessed, extended, Platinum, etc.)

Best,
Grady

[ChrisBennet]

Grady,
Hiding the 2nd distributor and the 2nd set of plugs. Very cleaver!
-Chris

[Grady Clay]

Chris,
The original idea was to keep the wires and plug connectors out of the trajectory of stuff coming off the tires. This 911 ran Goodyear 22.5x7.5x15 slicks on 7x15 911R wheels. I took 2nd overall at a Seattle Parade with it many years ago. There is a photo of it on two wheels in the turn coming out of the gulch up toward the drag strip. I was lucky, the LR tire was almost off the rim as the 911R wheels don’t have a “safety bead.”

I agree that this is a great solution for “stock appearing” twin plug installation. So is the system you are using because it can hide behind the air cleaner assembly.

Back to the important stuff.
Specifically what plugs are you using? Describe their relative heat range and type (recessed, extended, Platinum, etc.) I assume 14 mm upper and 12 mm lower plugs.

I think there are significantly different requirements for the upper and lower plugs. What is your experience?

Please, others chime in.

Best,
Grady

[cstreit]

Grady,

I actually use the 14mm plugs top and bottom, but with the smaller socket size.

I'm using the NGK race plugs, but not 100% sure the part number.

What I remember is that I started with the 9 heat range, and then ended up going 1 step hotter to the 8's to solve my fouling problem.

I'll try and look them up when I get back into town.