so why (and how) twin plug conversion?

[turfclubroad]

This (by now) mistery 67S from NJ talked about in other threads (the one which was listed on ebay, pulled, relisted, pulled again) made me curious about the twin plug conversion for a road car, its pro's and costs...... I did read in a (I believe rennsport) site that especially for the 2.0 (or may be it was what I wanted to read) the upgrade would be beneficial in terms of drivability (better combustion, more regular, more torque?) but there was no quantification. Can someone educate me what a conversion involves (assuming it would have to be period correct), what it would cost, and what it adds?

thx Turf

[kenikh]

Your little question has a lot of long answers.

The 2.0S had highly rounded piston and cylinder domes. On the compression stroke, this caused the piston crown to hinder flame travel due to interference across the flame propagation zone by the piston. Twin plugs solved this by allowing the fuel to be detonated on both sides of the piston, improving the knock sensitivity of the early motor.

Twin plugs were added to the 3.6L motors from the factory due to piston size for similar reasons. The large pistons resulted in very slow flame propagation across the cylinder due the larger diameter. By adding twin plugs, the motor was able to combust more efficiently allowing more aggressive ignition timing and higher compression.

The reason to dual plug any given motor is typically because the owner wants to increase performance by bumping compression. The twin plugs help in this endeavor. If memory serves me correctly, merely twin plugging a stock motor results in minimal performance gains. The real performance bump comes in increasing your compression to the max that twin plugging allows without detonation on pump gas.

As far as conversions and what is correct vs. functional, check these threads out on Pelican:

http://forums.pelicanparts.com/showt...ight=dual+plug the post by emcon5 is especially good.

http://forums.pelicanparts.com/showt...ght=twin+plugs

http://forums.pelicanparts.com/showt...ght=twin+plugs

http://forums.pelicanparts.com/showt...ght=dual+plugs

(edit) spelling

[roblav]

On engines with 4 valve heads and the plug in the middle, it is not so much of an issue.

[kenikh]

But we don't live in the watercooled world.

[Steve@Rennsport]

This (by now) mistery 67S from NJ talked about in other threads (the one which was listed on ebay, pulled, relisted, pulled again) made me curious about the twin plug conversion for a road car, its pro's and costs...... I did read in a (I believe rennsport) site that especially for the 2.0 (or may be it was what I wanted to read) the upgrade would be beneficial in terms of drivability (better combustion, more regular, more torque?) but there was no quantification. Can someone educate me what a conversion involves (assuming it would have to be period correct), what it would cost, and what it adds?

thx Turf

Turf:

You'll find lots of information on this subject at http://www.rennsportsystems.com/2a.html.

Converting an engine to twin-ignition requires:

1) Machining the lower plug holes in the heads and lower valve covers.

2) Installing either an RSR/906 type distributor, a 3.6 dual distributor (suitably modified), or a crank-triggered coil pack ignition.

3) Installing an additional CD box and coil (when using a distributor).

There are certain limitations here such as the early cases cannot use the 3.6 dual distributor and need either a custom billet RSR unit (we have those) or an original Marelli 906/911R unit.

Other issues are matching distributor and crank drive gears as well as adding 2 holes to the engine sheet metal for the 6 lower plug leads.

For carbureted or MFI-equipped engines, a distributor and dual MSD setup runs FAR better than the crank triggered, coil pack type ignitions.

Costwise, one should budget between $ 2200 and $ 3000 for the whole thing.


Hope this helps,

[Eric911S]

Hello; I would have to say that crank fire is alot better than a distributor no matter what type of fuel delivery. The distributor has too many moving parts. The shaft could wobble, the cap and rotor could wear, play between the gear drive and distributor gear. The last thing is the fly weights and springs, do they move correctly. One last thing to think about is that all new cars don't have distributors any more. The cost for crank fire is less than any conversion distributor and you can still get parts. I have a converted distributor with twin plug Bosch cap and rotor. I also have a twin plug Marelli cap. Thanks Eric

[Steve@Rennsport]

Hi Eric:

Well sir, we all are the result of our experiences and mine is that the DIS systems simply do not have the drivability and HP that a good distributor/MSD setup has on these engines with their less-than-ideal A/F ratios.

Crank triggered systems do well on engines with worn distributors, EFI, or where the distributor is not driven directly off the crankshaft.

If a distributor is in good condition, the timing signal is very stable and with MSD's, they provide a FAR better (high current) spark than inductively fired coil pack designs. Further, the MSD's and matching coils will easily fire plug gaps at .040 and beyond which are well proven to make more overall power and improve idle quality.

I ran RSR's, 935's, and a 917/10 for some years and the MSD's gave these cars a level of drivability and acceleration that the OEM ignitions and HPV's never matched,....

If I ever meet you sometime, I'd be happy to share some further details and experiences,...

[turfclubroad]

Turf:


Costwise, one should budget between $ 2200 and $ 3000 for the whole thing.


Hope this helps,

Steve, your estimate for what kind of conversion is that? I have a 67 911s, meaning that I would have to start with first finding a proper distributor and that alone would set me back 2500 plus if I am lucky.

Another question: My interest in this conversion started when I read about the improvement in drivability of the early 2.0S's and is not so much driven by a lust for extra hp's. If a single plug 2.0S engine was converted just to having dual plugs, all other things being equal, what would that actually accomplish? How noticable is the difference? Or maybe another way of asking would be: if you had a stock 67 2.0S and 3000 dollars to spend, would the conversion be the first thing you would do (the conversion having the biggest marginal improvement per dollar spent)?

thx

Wiebe

[kenikh]

Look up the thread by the user 'BURN-BROS' on Pelican Parts. He is developing a conversion around the Bosch distributor using a Jaguar cap and custom dizzy to get dual plug w/ an authentic look, but at a way cheaper price than the RSR or 935 untis.

[ttweed]

My interest in this conversion started when I read about the improvement in drivability of the early 2.0S's and is not so much driven by a lust for extra hp's.

I am just curious about what your "driveability" problems are with the 2.0S? The only beef with my '67S was that it would load up (foul) plugs quickly in its original configuration. My father bailed on the expensive OEM platinum plugs and just used to change out standard copper plugs every 3-7,000 miles to deal with that. It has been a daily driver for over 200K miles. After converting to a pointless ignition and MSD box and coil, I have had no problems with the car at all, and it is eminently driveable, although it has very little torque below 5000 RPM, which is typical of the S cams in a 2-liter. Carbs have been re-jetted slightly to deal with today's highly volatile fuels, but IMHO, there is no need to twin-plug a stock 2.0 engine.

TT