Please educate my on Twin Plug ignition

[Geronimo]

It seems from the little reading I have done that a Twin Plug ignition can really wake up a good hot motor. Almost like it's the last part int he puzzle to get pretty good power out of older 3.0L or 3.2L engines?

[Trakrat]

There's more to it than just twin plugging.
If it were that easy, everyone would just twin plug their completely stock engine just for a performance boost.

[macssc911]

Twin plugging does make quite a difference, depends on what you consider a hot motor. You need at least 10 1/2 to 1 pistons for a start. If you don’t have that save your money. Of course cams want hurt ported heads for sure. Carbs a plus. Check out my garage will give you ideas
Good Luck

[Trakrat]

Originally Posted by macssc911

Twin plugging does make quite a difference, depends on what you consider a hot motor. You need at least 10 1/2 to 1 pistons for a start. If you don’t have that save your money. Of course cams want hurt ported heads for sure. Carbs a plus. Check out my garage will give you ideas
Good Luck

LOL! sounds more like higher compression and cams make the difference...

[macssc911]

Well if you want the twin plug benefits, you need high compression pistons

[wayner]

Plug location and the design of the combustion chamber limits the ability to use higher compression ( or on the verge, produces less performance than a high enough but inefficiently burning combustion would otherwise produce).

Twin plug fixes that or let’s you take compression even higher

[NICE 69 S]

Give Aaron Burnham a call. He makes twin plug distributors. Very nice guy, great engine builder. He should be able to give you some valuable info.
Bob B

Google "burnham performance"

[macssc911]

Wayner Please technically explain your post
Thanks

[Jeff Higgins]

It's all about flame propagation across the combustion chamber. The spark plug in these motors is placed quite far off center, in the "crotch" of the intake and exhaust valves on one side of the combustion chamber. Adding a second plug, placed opposite of the first, in the other "crotch" formed by the intake and exhaust valves, aids in speeding up flame propagation by lighting the charge from both sides.

One of the things that high octane fuels do is to burn more slowly than lower octane fuels. This allows the spark to light them sooner, starting combustion sooner, giving it more time to occur and extracting greater power from a given fuel/air charge through a more complete burn of the charge. Light it too soon, however, and peak pressures rise to the point where the charge detonates, causing harm through dramatically increased pressure. High compression motors add further to the problem by also serving to raise combustion pressures.

It's a balancing act - how soon can you light the charge without exceeding maximum allowable cylinder pressures. Higher compression or lower octane raise pressures sooner, so either will require that the charge be lit later by the spark plug. At some point, the flame front will not have enough time to travel fully across the combustion chamber before the piston has gone too far past TDC to provide any meaningful power.

With a second spark plug on the opposite side of the combustion chamber, we can light the charge later and still get full combustion. This is important in a high compression engine, or in an engine running lower octane fuel, both of which raise cylinder pressures faster. Combine the two - high compression with lower octane - and the charge must be lit very late to avoid detonation, resulting in a less than complete burn, and very poor power. Twin plugging allows us to light it later, avoiding detonation, but still allowing for a complete burn.

In my own twin plugged, 10.5:1 compression 3.0 liter, I run total timing at 22 degrees when running pump gas. I get very good power while avoiding detonation. Running the timing this late with a single plug would result in incomplete combustion, dramatically reducing power. Trying to advance the timing with a single plug enough to achieve complete combustion - say 32 degrees advanced - would result in cylinder pressures peaking too early and too high, resulting in detonation.

Bottom line is, twin plugging in and of itself will not do much at all unless compression has been increased past the point where the fuel being used in the given motor will not allow enough ignition advance to burn it properly. You can run a single plug and 40 degrees total advance on a 12:1 compression motor on the right gas - very high octane race gas. But, if you want to run even moderately high compression, like the 10.5:1 I'm running, on our 92 octane pump gas, you will have to back the timing off so far that lighting the charge from just one plug, on one side of the chamber, will result in incomplete combustion, lost power, overheating, and erectile dysfunction. Lighting it from both sides cures all of that. I promise...

[Canada Kev]

Originally Posted by Jeff Higgins

Bottom line is, twin plugging in and of itself will not do much at all unless compression has been increased past the point where the fuel being used in the given motor will not allow enough ignition advance to burn it properly. You can run a single plug and 40 degrees total advance on a 12:1 compression motor on the right gas - very high octane race gas. But, if you want to run even moderately high compression, like the 10.5:1 I'm running, on our 92 octane pump gas, you will have to back the timing off so far that lighting the charge from just one plug, on one side of the chamber, will result in incomplete combustion, lost power, overheating, and erectile dysfunction. Lighting it from both sides cures all of that. I promise...

So 10.5 on 92 octane may require twin plugs and/or

[wayner]

Originally Posted by macssc911

Wayner Please technically explain your post
Thanks

In my post I was agreeing with you.

I was just stating it in revers.

If you want higher compression pistons, your gona need twin plug (which is what I would say if someone asked about increasing compression, but since they asked about twin plug, your opposite response is appropriate.

To support your point to the other person, here is the technical


TECHNICAL:
With relatively low compression the chamber is still pretty wide open for flame propagation.
With higher compression the flame path gets blocked as the piston dome encroaches into the flame path

the post that followed did a good job of technically explaining some of it.

In support of your post, here is a picture of an RSR racing piston from back in the day.
given that the spark plug is located on one side of the dome, its pretty obvious that a spark plug on the other side would help in a high compression engine where there isn't much clearance between the dome and the top of the combustion chamber




Below is the max moritz style piston, designed to open of the flame path while boosting compression (but still relatively low compression compared to the one above, just a better burn for single plug applications )

[kent olsen]

Jeff Higgins and I have the same cam in our 3.0L. It was ground to give lower peak torque and I love mine. Jeff ended up with 10.5:1 pistons while I ended up with 9.5:1 pistons. Not sure what years his is but my is from an 81 so it has smaller ports.

After my build we ran the engine for 30 pulls on a engine dyno adjusting the 40mm webers and using first a single plug distributor and then a twin plug one. As I remember the twin plug gave me about 10 more HP. I got a nice flat peak HP of 245 between 6200-6900 rpm and the peak torque was 225 between 4600-5100 rpm. I did have Paul Abbott open up my webers to 42 mm just to optimize the flow.

[darrin]

One point that doesn’t seem to have been made yet is that twin plugging also helps a lot when increasing an engine’s bore. Flame propagation issues caused by off-Center spark plug are exacerbated as piston dome gets wider - helpful for more power on a 98mm wide cylinder (eg 3.2L to 3.4L) necessary on a 100m+ wife cylinder (eg 3.2L to 3.5L or factory 3.6l)

[Jeff Higgins]

Originally Posted by kent olsen

Jeff Higgins and I have the same cam in our 3.0L. It was ground to give lower peak torque and I love mine. Jeff ended up with 10.5:1 pistons while I ended up with 9.5:1 pistons. Not sure what years his is but my is from an 81 so it has smaller ports.

After my build we ran the engine for 30 pulls on a engine dyno adjusting the 40mm webers and using first a single plug distributor and then a twin plug one. As I remember the twin plug gave me about 10 more HP. I got a nice flat peak HP of 245 between 6200-6900 rpm and the peak torque was 225 between 4600-5100 rpm. I did have Paul Abbott open up my webers to 42 mm just to optimize the flow.

My motor is also based upon an '81 small port 3.0 liter, but I opened up the intake ports to 38 mm. My induction differs from Ken's as well, in that I'm running MFI and he is running carbs.

Our motors, being quite similar other than my higher compression, illustrate just what compression buys you. Not necessarily more horsepower, as my motor makes somewhere between 245-250 as well. Horsepower is a function of a motor's ability to move air. Torque is a function of compression and displacement. My single point higher compression results in a worthwhile increase in torque - my motor just touches 240 ft lbs at the crank.

IMHO, that is the real gain to be had with twin plugging. Horsepower is made through other means. Big cams, porting, higher RPM's, that sort of thing. Moving a lot of air. All of that happens up up high in the rev range. Increasing compression works everywhere, adding useful torque across the range.

Originally Posted by darrin

One point that doesn’t seem to have been made yet is that twin plugging also helps a lot when increasing an engine’s bore. Flame propagation issues caused by off-Center spark plug are exacerbated as piston dome gets wider - helpful for more power on a 98mm wide cylinder (eg 3.2L to 3.4L) necessary on a 100m+ wife cylinder (eg 3.2L to 3.5L or factory 3.6l)

Absolutely. Even modern four valve, pentroof combustion chambers with a single, centrally located plug over a flat top piston run into trouble. Ducati reached the limit of this with their 1299 Panigale twin, with its 116mm bore. They simply could not go any bigger and still achieve efficient combustion across the whole combustion chamber in the time allowed. That's when they finally had to break tradition and go to a four cylinder motor.

[emcon5]

Originally Posted by kent olsen

Jeff Higgins and I have the same cam in our 3.0L. It was ground to give lower peak torque and I love mine.

What cam is it?

[Jonny H]

There are some knowledgable posts here, thanks gentlemen.

We have a a very rapid dual spark system in our CDI+ units. 300us between sparks. It's not a twin plug, but two sparks per combustion. We get consistent and repeatable dyno proven gains over the stock ignition.

The effect is more pronounced on the earlier engines with the domed pistons, typically 7 ft lbs improvement across the board without timing changes on a stock 69S for example. AFR shows a more complete burn.

We have a couple of theories as to what is happening here, but I'd like to get your opinion.



.

[Nux]

Originally Posted by kent olsen

Jeff Higgins and I have the same cam in our 3.0L. It was ground to give lower peak torque and I love mine. Jeff ended up with 10.5:1 pistons while I ended up with 9.5:1 pistons. Not sure what years his is but my is from an 81 so it has smaller ports.

After my build we ran the engine for 30 pulls on a engine dyno adjusting the 40mm webers and using first a single plug distributor and then a twin plug one. As I remember the twin plug gave me about 10 more HP. I got a nice flat peak HP of 245 between 6200-6900 rpm and the peak torque was 225 between 4600-5100 rpm. I did have Paul Abbott open up my webers to 42 mm just to optimize the flow.

Yep. According to Steve Weiner (Rennsport), twin-plugging should boost your engine's performance about 6% - or in this case about 10HP. Without TP, this particular engine should deliver approx 233hp or so.

Personally I don't think the cost of TP corresponds to the gained power in relatively low-HP engines without high CR (that requires TP). You would be better off with the usual stuff like cams, carbs/EFI, exhaust etc. Unless of course you're rebuilding and going electronic/sequential ignition anyway, then the extra cost for drilling the heads, adding sensors and coils is really a minor deal.

[Fred Winterburn]

Larger flame kernel every time. What else could it be? A great help for sure but not a replacement for a second spark plug. The longer effective spark duration from the second spark close enough to the first strike must help. I do not have any scientific data to support any of this other than satisfied customers with my own CDI that produces the extra sparks differently than yours. Throttle response is even better than the original CDI design my father designed that I used as the basis for this one. It's made a real difference on several Porsche 356 cars (the 6V version), but it doesn't take much to beat the original 6V Kettering system, or any of the lower powered inductive switchers. It also doesn't take much to beat a 6V Permatune CDI (horrible). Got a CDI going on a 1971 911 soon in England, but that is not an application I build for normally as the 911 CDI coil is not suitable with the switch on the CDI box in 'STD' (Kettering mode). The switch could burn up as well as the points and coil in 'STD'. Fred

Originally Posted by Jonny H

There are some knowledgable posts here, thanks gentlemen.

We have a a very rapid dual spark system in our CDI+ units. 300us between sparks. It's not a twin plug, but two sparks per combustion. We get consistent and repeatable dyno proven gains over the stock ignition.

The effect is more pronounced on the earlier engines with the domed pistons, typically 7 ft lbs improvement across the board without timing changes on a stock 69S for example. AFR shows a more complete burn.

We have a couple of theories as to what is happening here, but I'd like to get your opinion.



.

[kent olsen]

The Cam is from Doughery racing, Shingle Springs, Ca. 530-672-2286

GT2-102 profile

[emcon5]

Originally Posted by kent olsen

The Cam is from Doughery racing, Shingle Springs, Ca. 530-672-2286

GT2-102 profile

Do you have the specs, or a cam card? I don't see that profile on their site.
Dougherty Racing Cams Porsche 911, 930 and 964 camshaft profiles

Do you have a dyno sheet?

Thanks,