1979 911 SC Twin Spark Plug conversion
 

Hello All,

I am considering converting my standard 911 SC to a twin spark plug set up. Here are some questions: -

1) Is it even worth doing for a road car, never, nor will be used on the track?
2) Along with the additional tapped hole for the additional spark plug, will I have to modify/get different pistons?
3) Apart from a new twin distributor, what additions/modifications do I need to make to the ignition system?

Many thanks for the help.

It's only worth doing if you are modifying the engine in a way that requires twin plugs. Are you rebuilding the engine and increasing the compression?

Just a top end re-build, but not modifying CR

Really not worth doing. The point of twin plugging is to ensure a strong flame propagation usually associated with higher compression pistons. Or maybe for turbos. But that's basically why turbos have flat headed pistons so the flame can reach all the way across. Boost is what increases the compression ratio not the shape of the piston like on N/A cars.

At least from what I understand.

Would twin sparks improve fuel consumption though?

Yea, technically it'll make things more efficient and get you better gas mileage. By how much, I don't know. Probably negligible. ??

It may also run a little smoother.

Is it worth the price to do? I wouldn't spend the money if that's all I was doing it for. But that's me. It looks cool, though. :)

Only if the twin plug setup improved efficiency AND you could tune the CIS to to take advantage of any efficiency gained...

Don't even bother...it's not worth it on a stock 911SC engine....

I was told by a highly respected engine builder that one plug is barely sufficient for the stock 84mm bore. He recommended dual-plug if you increase the bore (i.e., a short-stroke 3.2). I wanted short-stroke, higher compression, big cam, and carbs, and he wouldn't touch it without dual plugs.

Remember, the CIS system is pretty crude. At best, it was "close enough" and is not suited for "dialing in." I suspect it is the primary limiting factor if you are trying to find maximum efficiency. If I was doing it again, I'd go with a full-blown engine management system and EFI.

You will also need to have the lower valve covers machined. I had to have custom plug wires made. Also be prepared to buy retainers to hold the wires on the lower plugs. I used a dual-head distributor from a 964, which meant that I needed two MSU ignition units and two coils as well.

Sounds like you need to do it and then report back

Here is a doc that I post whenever someone asks this question:

http://forums.pelicanparts.com/uploa...1674477379.jpg

An engine can operate with more ignition advance with the new higher compression than without twin plugs, resulting in a reduced probability of detonation.
Without twin plugs, the additional power gain from the additional compression ratio would be offset by a required lower advance curve to prevent detonation.
Additionally, a stock compression engine will be able to have a more advanced timing curve without the probability of detonation by having twin plugs.

The twin plug machining isn't cheap - it is hard to bore through the head's fins without breaking some of them. A dual distributor is also expensive, so perhaps a better alternative would be some sort of crank fire setup, selling your distributor and blocking the hole in the case. Plus you need the extra (longer) wires, and a way to feed them through the engine tin to the lower plugs. Now spark plug changes are twice as time consuming, especially since you are now down under the car (and even with a lift a stock exhaust has to be dealt with). I rate the likelihood of power gain at zero, and economy at very little. Twin plugging is for highly modified engines (at least for those with significantly higher compression ratios. It might be worth it if you had a later Euro SC, which can be borderline on US gasoline at a nominal 9.8:1 CR.

If you have come up with a nice budget, and are determined to spend it, you might look into some kind of single plug crankfire system and sell your distributor to offset some of the cost. Those are said to smooth out the idle some. Or switch to one of the various electronic fuel and ignition systems, which you can tune for maximum economy if that's a benefit. If you have an inner geek, you can spend profitible time tuning the programming.

Sounds like you are happy with the car's street performance, which realistically is all any sane person really needs for street/highway/even curvy mountain roads.

there is even a twin plug distributor for sale here on Pelican.

https://forums.pelicanparts.com/porsche-911-used-parts-sale-wanted/1133361-nos-genuine-bosch-3-0-rsr-twin-plug-distributor.html

What's your current mpg? How do you use the car? (track, inspired drive at weekends, ...?)

I suppose that your CIS is misadjusted/defective. This results in poor performance combined with a way too high fuel consumption. You described in 2021 a smoking issue here, this could hang together. Has this been solved already?

Thomas

Ha! Don't scare him Bill, that's a NOS race distributor for $12k. A regular twin plug distributor should run 1-2k.

Sorry, I'm new here. I don't know these things (LOL)

I would do a full 3.2 SS with twin plug or nothing. I have one in my RSR Frankenstein car and love it. Revs like a motorcycle, 246 at the wheels and sounds amazing.

Correction to some other posts: a twin-plug system allows LESS timing advance. Because it burns the fuel faster, less advance is required to burn the fuel by the time the piston gets slightly past TDC. Less advance reduces lost power pushing the piston up against combustion pressure, and saves fuel. It also allows more compression to be used. Compare the CRs of the 963/993 engines to the previous versions.


Is it worth it? Your wallet, you decide. It's always easy to spend other people's money...

And don't diss the CIS. It works very well when correctly set up. If you plan to go to a significantly higher RPM cam then consider using an ITB FI system. Throw the carbs in the trash. Or build a coffee table to display them.