3.6l with PMOs different spark plugs top and bottom

[midnightrider36]

Hello all,

working on a customers 91 964. 3.6L with 50mm PMO's, Fabspeed headers and dual outlet muffler, single MSD 6al triggered by a 3.6 dual distributor with what I think is a 930 trigger set up. I don't know anything about the engine internals, if anything was done such as cams or compression.

Car came to me running poorly. I found the float levels all outta whack. Disassembled and cleaned the carbs, set the floats and synced the carbs. It's running much better now. Recommended plugs, oil change and valve adjustment. Pulled plugs and theres different type plugs for the uppers and the lowers.

Upper plugs are Bosch FR6LDC dual electrode. Lowers are Bosch F5DP0R platinum plugs.

I've never seen someone run this setup, but that means little in the grand scheme. Any idea why one would or opinions on whether I should continue? The uppers looked sooty when I first checked, but after driving with the carbs set up correctly, they look to be lean. Also, I usually see the triple electrode plugs in the 3.6L engines. Any thoughts are appreciated.

Cheers,
Jesse

[Uwon]

My RST hot rod has a 3.2 converted to a 3.4 twin plug turbo with an Eletromotive tec3r engine management system programmed for sequential firing. The few times I have experienced fouled plugs it’s typically one or more of the bottom plugs so I immediately change those first and then go for a test drive. If all is well that is as far as I go until I drop the motor in the off season. I can recall one year when I could not get the right made in Germany (not Russia) triple electrode plugs anywhere so I ran with the single electrode alternative for much of the summer which ended in frequent fouled plugs.
So my long winded story may have your answer. Bottom plugs may simply foul easier and the triple electrode solves it. My guess also is that sequential firing may have something to do with it.
Also, changing the bottom plugs is so much easier if it solves a fouling problem.
Hope this helps.
Johan

[phunt]

You say you are using one msd box to drive two coils? I am not debating it has been done to death. My opinion is that it is not optimal. With one box I would for sure run the same plug. I would want to eliminate the chance of side (top or bottom) having less resistance. Electricity travels along path of least resistance. One coil charges easier one set of plugs fires easier. May cause a weaker spark on the other side. Could lead to fouling. If you had two msd boxes then I would not think different plugs would be as much of a factor. I would still run the same plug top and bottom. If I was worried about fouling on the bottom I might use a hotter plug of the same. But why is it fouling? If you are running sequential injection then there is minimal fuel left in the cylinder. Fuel fouling is less common than oil. If it is oil then you have something else to figure out.
I am building a 3.4 from a 3.2 as well. Twin plug sequential efi. I have two msd boxes jb twin plug dizzy. I will use the same plugs. I like the old school cooper core plugs. But they can be noisy and do not last as long.

[winders]

You can address the plug fouling problem by getting them optimum current…you don’t get that with one MSD unit powering two coils.

[Henry Schmidt]

The different electrode situation may be the result of piston to plug clearance.
Quite often in twin plug configurations with after market pistons, the projected tip plugs will hit the pistons.
Although there seems to be mismatched heat ranges top vs bottom, neither seems to be foiled.
Plug foiling is generally the result of poor fuel mixture or incorrect heat range on the plugs. Running a single MSD with two coils is not an issue.....no matter how much the internet "experts" argue the point.

[Henry Schmidt]

In twin plug applications like this, we had great results with the copper core NGK BKR6EKUB. They are reasonably priced from our host as well.

https://www.pelicanparts.com/More_Info/99917020791.htm?pn=999-170-207-91-M47&bc=c&SVSVSI=

[winders]

Originally Posted by Henry Schmidt

Running a single MSD with two coils is not an issue.....no matter how much the internet "experts" argue the point.

It’s not an issue if you are okay with the spark plugs getting less than half the energy they are supposed to be getting.

Henry may be okay with that but I can’t find a reputable air-cooled 6 cylinder Porsche engine tuner that is. Ask William Knight. Ask Peter Dawe. Ask Jae Lee. Ask Mat Lowrance. You will get the same answer: each coil should have its own ignition box.

Hell, what did Porsche do on ALL of their twin plug engines? One ignition for each coil.

[midnightrider36]

Thanks for all the replies,

AFAIK, it's a stock 3.6 L, so stock twin plug heads. I don't have any build info on the engine, but I am assuming it's stock internals with the fuel injection removed and the PMOs installed.

The customer bought this as it is, from possibly the second or third owner. He's not interested in maximum performance, so I'm not going to mess with the MSD box. I appreciate the opinions, tho.

Henry, thanks for chiming in. Again, i'm not sure, but i would guess stock pistons so plug to piston clearance shouldn't be a issue. I suspect this is more of a build/people futzing with it thing. Throw the PMOs on a stock engine, futz around trying to get it to run well, put different plugs in the bottom as opposed to tuning it well. No signs of fouled plugs after cleaning the carbs and setting the floats correctly. The oil was very black, probably full of oil.

I do generally prefer NGK plugs to Bosch (no particular reason, really) so I'll try to grab a set of the ones you mentioned.

Interestingly, the modified the distributor housing to give some timing adjustment. Oh, and the engine has 1900 miles on and leaks oil. Anybody can "build" these cars and charge a lot of money for them.

Cheers,
Jesse

[Henry Schmidt]

Originally Posted by winders

Henry may be okay with that but I can’t find a reputable air-cooled 6 cylinder Porsche engine tuner that is. Ask William Knight. Ask Peter Dawe. Ask Jae Lee. Ask Mat Lowrance. You will get the same answer: each coil should have its own ignition box.

That is an incredible list of very qualified Porsche engine builders. They must know.
Let's ask MSD what they think about one MSD CD with two coils?

Form MSD literature..... I guess their "experts" just don't know or maybe they don't want to sell two CDs. Wait, I know, MSD doesn't care if your engine runs properly....yeh, that's it.




Quick question for Jesse: you said it wasn't running well but with some minor tuning it runs much better. Do you believe the previous owner(s) didn't care how it ran?

As for oil leaks with just a few miles....leaks are always a challenge. That is why I came up with the sealing system sold right here on Pelican.

[Cloggie]

Originally Posted by Henry Schmidt

That is an incredible list of very qualified Porsche engine builders. They must know.
Let's ask MSD what they think about one MSD CD with two coils?

Form MSD literature..... I guess their "experts" just don't know or maybe they don't want to sell two CDs. Wait, I know, MSD doesn't care if your engine runs properly....yeh, that's it.




Quick question for Jesse: you said it wasn't running well but with some minor tuning it runs much better. Do you believe the previous owner(s) didn't care how it ran?

As for oil leaks with just a few miles....leaks are always a challenge. That is why I came up with the sealing system sold right here on Pelican.

Nice post Henry, most definitely up to your usual standard!

For those who frequent these forums, one learns that challenging Henry on Porsche 911 facts is a losing proposition.

It's one thing to disagree and prefer an alternative solution, but I do not recall him stating facts in error...which is why I read his posts very carefully.

But thanks to all who contribute their knowledge and insight, us lurkers and consumers of your knowledge truly appreciate it.

D.

[phunt]

Hey Henry happy new year. I knew mentioning one box was a mistake. Where did you get that diagram? I could not find it on holley/msd website. No where do they mention twin nor dual plug. While planning and gathering for my build I read and researched everything I could find. Even talked with you about the box you are or were? Developing. I talked directly with msd and had the technician verify with others there. Didn’t want just one opinion. MSD does not recommend one box for two coils. Will it work? Obviously. In the higher rpm range where the box is single spark 20* of spark can not be guaranteed. The magnitude nor strength of the spark either. Will it fire and run with a shorter duration and weaker spark. Yeah. I feel it takes away the benefits that the cdi boxes give. I am also a copper core guy and I will be using copper plugs. Wish they lasted longer. The question of different plugs to and bottom? I think same plugs is the way to go like you said fouling comes from other factors. Henry I appreciate you and I am grateful for your help and all of the things I have gotten from you. Just because I disagree doesn’t mean anything.

[dannobee]

I have no dog in this fight, but the second diagram is how we wired them up in nascar. The driver had access to the "A-B" switch and could select a completely different ignition system, except for the distributor cap and rotor, which rarely failed. MSD boxes at the time were notoriously failure prone.

When I see "leading" and "trailing" on a diagram, I usually think of Mazda rotary engines, FWIW.

I think Patrick Motorsports has a wiring diagram for two separate MSD boxes one a twin plug setup. If I had a choice (and was wary of COP ignition), I would likely step up and buy two MSD boxes and wire them up like that.

https://patrickmotorsports.com/products/ignmsd6421

[Neil Harvey]

Originally Posted by Henry Schmidt

That is an incredible list of very qualified Porsche engine builders. They must know.
Let's ask MSD what they think about one MSD CD with two coils?

Form MSD literature..... I guess their "experts" just don't know or maybe they don't want to sell two CDs. Wait, I know, MSD doesn't care if your engine runs properly....yeh, that's it.




Quick question for Jesse: you said it wasn't running well but with some minor tuning it runs much better. Do you believe the previous owner(s) didn't care how it ran?

As for oil leaks with just a few miles....leaks are always a challenge. That is why I came up with the sealing system sold right here on Pelican.

The new year starts off the same as the old year. I posted something about opinions. Right or wrong, it comes down to what the engine does. Does it start and run without misfiring? If it does, then its about an opinion of what is better, not what is right or wrong.

What Henry posted is basic high school physics. The image shows the coils in series. What is the result of a series electrical circuit? Maybe answer this question first.

You can always find fault in another's work when your own opinion drives the narrative. Often totally unfair.

How would I do this. Opinion based, I would not use MSD, I would use two CDI boxes and two coils. Reason, I want the max energy released in the cylinder. Ignition is the last function to happen before combustion.

Getting back to the spark plug question, the reason one plug looks more lean than the other is, fact based, because its projected out into the chamber further absorbing more temp.

Typically, the more projected the plug, the warmer it runs and the more its construction is about removing that heat. A colder heat range is required. Conversely, the more retracted the center electrode the warmer the plug can be. Two factors change this reasoning. Boosted cylinder pressures or Piston clearance distance. If you have to use a more retracted plug due to the piston clearance, it should always be done only when the piston cannot be remodeled.

[Henry Schmidt]

Building these early 911 engine can be a money pit. The "while you're in there" syndrome can escalate a simple repair into a second mortgage on your house.
The question from the OP was, "I have a poor running issue, here's what I have, look what I found".
Someone suggested that his problem was triggering a twin plug system with a single MSD CD. As stated by me and others for decades, this applications works effectively.
Over the years, dyno tests have proven that in a non-racing application, single CD will perform exceptionally well.
Like any other proposition, how much do I need is different than how much can I spend. If the budget is unlimited, spend away.
One of my favorite examples of the "I want the best" non-sense is CARR bolts in Carrillo rods.
Carrillo rods come with two types of bolts WMC and CARR. CARR bolt adds about $ 300 to the project.
WMC bolts are rated for engines making 1100 horse power and can be reused 8-10 times.
The Carr bolts are rate at 1500 horse power and can be reused infinitely. Cool but is there a good reason why a 400hp engine needs CARR bolts?
Add enough +$300 options and pretty soon you're talking about real money.
Your budget may be finite and buying an extra CD and wiring not to mention a clean place to locate a second CD, just because it has perhaps unneeded potential is your call.

[midnightrider36]

When the car first came in, I saw the single MSD box and distributor set up. It seemed a bit funky to me, but I haven't seem too many 3.6L with carbs. I searched the forums and read a few threads on the single vs dual box setups.

I agree with both Neil and Henry. The single box works well for this application. I don't have a triggering or spark issue so how much better one is vs the other is a bit moot. This car isn't built for all out performance (964 cab backdated to RSR vibes, with full interior and A/C and alot of fiberglass) and the customer isn't interested in that either. He wants it to run better than it did when he brought it in. He heard me syncing the carbs the other day and thought it sounded great.

I put Bosch FR6LDC copper plugs in. No plug clearance issues with the lower plugs. The engine runs well, or at least well enough for the current mission.

I have found that the most important question in this industry is "what do you want, what are you trying to achieve and how much money do you want to spend?" I understand the pull of THE BEST, but I feel that it is rarely achievable, let alone affordable.

Thanks for all the input, everyone who replied. I really do appreciate it.

Cheers,
Jesse

[phunt]

I took the question of the op to be ... is different plugs top and bottom ok?
I feel that if you are using one cdi box then as much as you can make it the same. so same plugs top and bottom
I feel that if you are running two. then different plugs is ok. two separate systems.
I did not say that one box was the problem nor that one box would not work.
I guess to me it comes down to ... works well enough ... or ...works effectively ...
it is not about deep pockets or unlimited budget. it is crazy expensive to build one of these engines. crazy. so for me the additional $349.99 for a second msd6a was worth it just encase it is more effective.

[winders]

Originally Posted by Cloggie

Nice post Henry, most definitely up to your usual standard!

For those who frequent these forums, one learns that challenging Henry on Porsche 911 facts is a losing proposition.

It's one thing to disagree and prefer an alternative solution, but I do not recall him stating facts in error...which is why I read his posts very carefully.

But thanks to all who contribute their knowledge and insight, us lurkers and consumers of your knowledge truly appreciate it.

D.

Sure, MSD has a diagram on how to wire up a single MSD for two coils. But they don't recommend doing it that way.

Henry is certainly experienced and knowledgeable. But that doesn't make him infallible. He is right in that MSD provides a wiring diagram for using a 1 MSD box with 2 coils. He is wrong is suggesting it is the recommended solution by MSD for 2 coil setups.

Henry is also wrong about the 993 Turbo dilivar head studs being junk.

Hell, he was wrong for suggesting to someone to stamp a blank case to keep a car numbers matching. By the way, that topic is where Henry's animus toward me started back in 2013.

But hey, you keep putting him up on that pedestal of infallibility if you want. I suggest that you should questions things that don't make sense....

[safe]

Originally Posted by Neil Harvey

What Henry posted is basic high school physics. The image shows the coils in series. What is the result of a series electrical circuit? Maybe answer this question first.

Well, its not in series, its in parallel.
So each coil would see the same amount of voltage at the same time. The amperage would be a question. Each coil has a need based on its impedance, putting them in parallel halves that impedance and doubles the current. If the MSD unit can support that current it will be just fine... The unit doesn't need to be able to supply more than the coils need.

My opinions:
The better, modern, option would be to run a coil per plug and have the timing controlled by an ECU, along with the fuel... Carbs are very pretty...on a shelf.

[winders]

Originally Posted by safe

Well, its not in series, its in parallel.
So each coil would see the same amount of voltage at the same time. The amperage would be a question. Each coil has a need based on its impedance, putting them in parallel halves that impedance and doubles the current. If the MSD unit can support that current it will be just fine... The unit doesn't need to be able to supply more than the coils need.

My opinions:
The better, modern, option would be to run a coil per plug and have the timing
controlled by an ECU, along with the fuel... Carbs are very pretty...on a shelf.

https://forums.pelicanparts.com/porsche-911-technical-forum/231256-msd-twin-plug.html

[Neil Harvey]

Originally Posted by safe

Well, its not in series, its in parallel.
So each coil would see the same amount of voltage at the same time. The amperage would be a question. Each coil has a need based on its impedance, putting them in parallel halves that impedance and doubles the current. If the MSD unit can support that current it will be just fine... The unit doesn't need to be able to supply more than the coils need.

My opinions:
The better, modern, option would be to run a coil per plug and have the timing controlled by an ECU, along with the fuel... Carbs are very pretty...on a shelf.

Yes you are correct. My mistake.