Running twin plugged heads with only one active sparkplug ?
 

I'm in the middle of a top-end rebuild on my '82 Euro SC 3.0L 204bhp engine (was making 216bhp even with 'very' damaged cylinders !)

The heads are currently in a big queue at the machinists and I am considering having them drilled ready for twin plugging whilst they're in there having the other work done (valve seats,guides etc).

However, I will not actually be running a live twin plug setup yet, just using an additional spark plug in the hole (whilst I save up !)

My questions are:

• Is it ok to run like this ?
  
• How much compression will I lose by having the extra spark plug hole machined in the heads ? I currently have 9.8:1 compression and have heard that I could lose 2cc on each head due to the machining, resulting in a compression drop to 9:5.1 !
  
• Would I be better off leaving the machining of the additional plug holes until I actually install a functioning twin plug setup ?

Any info greatfully received.

Get the machining done while the heads are at the machinist. You dont want to tear the engine down and send the heads out to be machined in the future.

Its not a problem to not fire the second plug.

I don't think the 2nd set of plugs produces an increase in performance by itself. It just allows you run higher compression.

In my (much less learned than Tim's) opinion, if you don't intend to change your pistons I wouldn't bother with twin plugging the heads. If you intend to bump the compression and run twin plugs "someday" I would wait and have the extra plug holes done "someday" also (along with the stiffer valve springs to go along with the upgraded cam you'll probably be doing then too).
-Chris

Cheers Chris.

I already have 964 cams/race springs and TI retainers going in. Will definately go 'twin plug' and higher compression in the (not too distant) future. This rebuild came at a very inopportune moment financially so I can't do exactly what I want to 'right' now. So I'm getting a good, reliable 'base' engine for the 2008 trackday season. Just trying to make the right decisions now to enable a smooth upgrade path when I get the funds sorted and not have to duplicate too much effort.

Anyone with any info on the potential compression loss problem ?

I know you will loose some compression, but I forget how much exactly.

as for plugging the holes, I am doing just that right now, until i decide on how I want to fire the twin plugs.

A little trick a very experienced 911 guy once told me on this subject:

If you are thinking about twin plug in the future but don't have the funds to splurge for the full set-up right at the moment.... ...Have the heads machined for twin plug and the higher CR to take advantage of the extra plug. Build the engine with two stacked cylinder base gaskets to reduce the CR to an acceptable single plug level. When you are ready to spend for the twin plug, remove the heads and cam towers, lift the cylinders out of the spigots enough to clip and remove one base gasket and then reassemble. CR will come up and you're ready to go.

Apparently, his shop has done this in the past for customers with good success.

I remember reading that a twin plug would drop a 9.5 squeeze down to 9.3 or so. Don't think it's a problem though as the performance differences would be negligible.

Going Tom's route seems to make the most sense since 10.5 slugs run $900 and cracking your engine back open in a season or two will cost twice that easy

My concern over the compression drop is due to the 964 cams I'm installing. With the lower compression they'll push the power band up by 300/500rpm and I don't want too much of a 'hole' in the curve. Must admit I'm not really sold on the extra gaskets idea. No technical reason just a kinda nasty gut feeling !?

Your gut feeling goes against standard practice and convention. From what basis do you have your feeling?

It is absolutely common practice to add gaskets, spacers, etc. to adjust compression. Besides Porsche, I'm aware of folks who use a similar trick with BMW 6 cylinders when they add turbocharging.

The need to reduce compression based on a given cylinder head volume and piston is not new.

There is merit in not sending the heads off to the machinist twice an not breaking the engine down to individual heads.

OR
Why not just buy a used engine, install it and rebuild yours as money is freed up?
SC engines are not hard to come by.
Doug

HI Gary
If you twin plug with STD pistons, check the plug to piston gap when you rebuild, as depending on the plugs you use it will be close.

regards mike

>Must admit I'm not really sold on the extra gaskets idea. No technical reason just a kinda nasty gut feeling
Perhaps your gut is right. Adding gaskets will raise the deck height and reduce the compression. But it may also increase the tendency for predetonation because your new taller deck height will leave a larger "pocket" of air+fuel out at the edge of the piston. Steve Weiner has spoken of this.
-Chris

I'm currently running this questionable setup in my stock-pistoned engine with DC20 cams and 'racing' valve springs. The plug I have installed is a 12mm E46 M3 plug. It's an Autolite 4164- I bought the cheapest M3 plug they had at Advance Auto.

Had I thought about this a little more when I did it, I would have searched for a shallower plug. The E46 M3 spec plug has a 12mm thread (necessary to fit in the tight reaches of the second plug location), a 3/4" reach and uses a 11/16 (18mm) hex for installation. A plug spec search on Autolite's site tells me I could have instead gone with the 4173 which has a shorter 1/2" reach and smaller 5/8" (16mm) socket. :mad:

That extra 1/4" of reach is sounding like a good idea after what MB Engineering mentioned. Thanks Mike! I'll pull the plugs, since the engine is currently out anyway, and probably find bent electrodes! :eek:

Kevin, Your heads were set up to use the NGK DCPR7E Plug. Thats 1" from base to electrode tip and also 5/8 socket which makes it easier to get in there.

Well at least the 4163 and the DCPR7E have the same reach. I'll have to go to the store and get the NGK. Can't remember why I decided to go against the recommended plug number you gave me last year. Cost? :rolleyes:

I got around the bigger 11/16 socket issue by "chucking up the socket in my lathe." Translation: I ground the socket thinner with my bench grinder!

I've got a second set of plugs in my heads that have never been fired. I've run this way for about 5-6 yrs or so. Yes you do change the C.R. by cutting the second hole but then you've got a plug filling it up. ...not sure what the c.r. change is.