Another Twin Plug Question
 

Is it best to use a distributor or a coil per head setup on a twin plug system?

That depends on the rest of the Engine Management and/or fuel system. :)

Are you using EFI? carbs? MSD's?

Asthetics aside, what are you trying to do? :)

Subscribed.

I am building a 2.7 RS motor, 10.5:1, extrem grind on cams, carburetors. I will track the car but it is mostly a performance street application. I just figured since the car is totally rebuilt that I would do waht I could to the motor. Everything I have been reading and posted here seem to be pointing me in the twin plug direction. Looking for good advice.

Dave,

At 10.5:1, you MUST use twin-ignition unless you use race gas. If you wish to use pump premium, twin-plugs are mandatory to prevent piston, ring and bearing damage.

If its carbureted, you could use either a twin-plug distributor with MSD's or a coil pack system. There are performance as well as prices differerences between the two options. :) :)

Thanks. I am getting a quote right now for a distributor setup with MSD coils.

I do have those, as well. :)

I was just over at J&B racing in Taveres Fl, next to Alex Job racing. He has manufactured a nice looking twin plug distributor for around $1500.

I was thinking of going that way on the 3.0L I'm building but I only have 9.5:1 pistons so probably don't need it. However I might drive the heads anyway so I have that option down the road.

Steve, would I gain anything with twin plugs on 9.5:1 pistons and John Dougherty's GT II turbo cam?

I suppose one could say future gas quality might be an issue and then twin plugs might become a requirement.

Is the need for a twin plug set up indicative of a poorly designed combustion chamber? Not trying to be a wise guy. I just don't know why this is required sometimes.....

Steve,

I would love to get a quote from you. I need everything to make mine a twin plug setup to include the head work.

Thanks


davemc@itinet.com

Not poorly designed combustion chamber- per se...

The spark plug is on one side of the hemispherical head. High dome pistons for high compression tend to shround one side of the cumbustion chamber from the spark plug and hinder flame front propagation. This causes detonation and requires more spark andvance than is optimal for power. The hemispherical head shape is efficient at harnessing the combustion, though. If the plug was in the center of the chamber, you would be fine with 1 plug. Twin plugs are needed otherwise, in order to light the mixture on the other side of the piston dome and start 2 flame fronts.

Let's call it "a Legacy app."

Modern engines do not require twin plugging -- at least not this kind of twin-plugging. Old, air cooled motors require it at a certain point....

With water-cooling, you can put in 4 valves and get better gas flow AND a better plug location.

That may not be true.

The last generation of MBZ motors used two intake valves to create a swirl, one exhaust valve, and two spark plugs for faster burn.

This combo is subject to a patent and I believe was on license to MBZ and maybe Ford but not sure.

I belive the faster burn from twin plugs allows combustion to begin in the compression cycle for less resistance on the way to the power stroke. Plus, the double flam paths may make for a more complete burn.

Kent,

Yes, you would see noticable gains with twin-ignition,...:)



Gents,

Twin-ignition isn't new: Hudson used it on their old straight-eights back in the forties and fifties,.......:)

Ford used it on their 4 cylinder engines for emissions compliance as does MBZ.

Hemi-headed engines have "lazy" combustion chambers with little natural swirl that leave residual gases at the edges of the chambers at TDC. The tendency for spontaneous combustion at the edges is why these motors are susceptible to detonation and the offset plug positions makes it more so. By starting two flame fronts simulataneously with two plugs, the flame paths are shortened sufficiently to make large reductions in ignition advance for complete combustion.

Read this for more info on the subject: http://www.rennsportsystems.com/2a.html

That is why I said "-- at least not this kind of twin-plugging."

MB used 2 plugs in a motor series - it is the 1999 ML450 (which I used to own).

BUT - the plugs do not fire at the same time as in a 911. Instead they are under computer control and the firing sequence is altered based on what the sensors tell the computer....

MB no longer uses this system AFAIK.


Everyone should read Steve's article

AND, AND! -- it has pics!

Go get 'em

Thanks for the explanation. I understand it a lot better now. I'm off to read more at link steve provided.....

Well I'm in too. As I get to the assembly part of this 3.0L rebuild I'll have the heads drilled for another plug. Hmmm, my wife keeps asking me what I want for Christmas, I wonder what she'd say if I said I wanted a twin plug distributor. "That mistress of your's is not worth that much, try again!!!"

remember -- the machine work is the cheapest part of this

you can do that and then run it 1-plugged while waiting for piles of dollars to be wheeled to your (garage) door by Ed McMahon...

Well how does J & B racings manufactured distributor at $1500 compare to other options?

Not trying to hijack, but...

Steve, when you say:

Is this specifically directed to Dave and his build, or is this a general statement? And, if twin-ignition is a MUST at 10.5:1, where would you say the threshold is? (ie, highly advisable) I ask because my RoW '81 started at 9.8:1 and allegedly had compression increased on rebuild... to what I don't know (no documentation.)

+1 on that article, even though it's quite a bit of info to take in.

You reference the relationship between flame front and timing - is the delayed combustion (caused by spark plug location) the main reason for advanced timing? Or is it just that they are related to each other? Are the higher cylinder head temps seen with advanced timing the result of compressing already-combusted air/fuel? Or some other cause?

Thanks.