I, obviously, disagree.....

Loren loves to bash any solution he does not sell or rebuild. MSD is his most common target.

Scott

Modern cylinders and head design have less of a flame shadow where unburned fuel can sit in the corners compared to earlier designs.

Big cylinder flat heads with a central spark plug was really bad. Pistons with squish bands helped a lot, as did hemi-shaped heads with a spark plug pointing in from each side.

In the 80s lots of more pedestrian cars and trucks started sprouting twin plugs in order to overcome emissions before better cylinder & head and fuel system designs came along.

Nowadays twin plugs are less common and (just the last few years) they use direct injection to take it to the next level of efficiency

All right I have been trying to stay out of this but here's my 0.02.

First of all, the primary reason that GA aircraft have two plugs is redundancy.

The second reason is that direct-drive aircraft engines are limited by prop RPM. If you try to spin a prop faster than about 3000 rpm you will end up with huge vibration issues and possibly mach overspeed of the tips which makes a hell of a racket, just listen to a Cessna 185 ampbib on floats to see what I mean. That means you need to make rated power at lower RPM and the only way to do that is by increasing the displacement. A Lycoming o-360, which makes about 180 HP, does this at 2700 rpm with a 130mm bore. Contrast this with a 3,0 SC motor which makes the same 180 HP at 5500 rpm with a 95mm bore.

With a bore that big you would have to ignite the fuel WAY before TDC to get the peak pressure point at the optimum point after TDC for optimum power. But the earlier you light the mixture, the more pressure you get before the peak power point, which ends up reducing your power. So by using twin plugs, you can light the mixture later in the cycle and end up with a peak pressure point closer to optimum.

But here's the point: twin plugs do not by themselves CAUSE increased performance-- for a particular design they avoid a performance decrease. In an otherwise optimized engine configuration for a particular displacement adding a second plug won't really help you, such as in a four-valve, pent-roof, watercooled motor with a central spark plug, like a 996.

Also, we read above that more ignition energy causes more performance. And here again, the same principle is at work. Why do you think that nitro dragsters have to use magneto ignition with leads the size of welding cable, with enormous output from the mags? It's because the compression ratio is so high that anything less wouldn't ignite the spark. But here again, the ignition system doesn't CAUSE greater performance, it merely allows a high-performance design to function.

Both points above are somewhat moot anyway in the context of MSD vs. Bosch because the spark plug doesn't move around, it stays in the same place, so multiple sparks once the mixture is already burning accomplish nothing!

As our departed friend Warren said years ago on this Forum, the only performance benefit achieved by an MSD vs. a Bosch is that switching to the relatively inexpensive MSD from a poorly maintained factory CDI gives the "perception" of a performance increase, merely because of the substitution of new components for aged, life-limited, timed-out ones!

Also Loren is exactly right with respect to spark duration. The CDI system in general suffers from the limitation of extremely short spark duration as well as complexity and higher cost. By moving to inductive coil-on-plug, the manufacturer achieves many great things: a) more accurate spark timing and reduced "spark scatter" b) elimination of moving parts in distributor and advance/phasing mechanism c) inductive spark duration vs. capacitive spark duration and d) superior packaging, the pencil coil is ideally suited to pent-roof, single central plug as in the modern combustion chamber design.

All that said, I have a 6AL box that's in my 911E, it works just fine, but the factory CDI works fine too, even though it doesn't have a rev limiter.

Loren

If Bosch did not use electrolytics (Warren claimed they were non-polarized electrolytics and you disagreed with him) then what are they? Polyester?

"Rather than hypothesize based only on Labortory experiments where real world factors can not all be accounted for... why dont YOU link up with a professional with the tools needed (dyno, gas analyzer) and answer the question definitively."

The purpose of the thread was to present the results of a comparative evaluation
of the MSD ignition unit versus the Bosch ignition unit. The intention was not meant
to be super robust, but just evaluate both from some key factors of an ignition system.

The results basically concluded that the MSD ignition provides no real benefit over
the Bosch ignition and in some cases my be less desirable, i.e. the overly complex
design, the use of electrolytic capacitors, a short spark pulse width, a very high pulse
voltage combined with unnecessary multiple sparks (beyond 2) potentially shortening the ignition components' life. If there are benefits to the MSD, then it's the responsibility
of MSD to prove such and provide the appropriate data to support the claims if there
are any. But from an electronic evaluation, the MSD is just another capacitive discharge
ignition (like the old Mark 10 from the '60s) with another 'face'.

I don't get what all the fuss is about. I've been using an MSD in my 911 for about 10 years and it's been nothing but reliable.
When my Bosch unit became faulty it was a no brainer to go with the MSD 6AL. The cost was the biggest factor. I still have my faulty Bosch unit stored away in my garage.

... I liked it so much I installed an MSD DIS-4 in my VR6 Golf. (I'll post a picture if requested)
The OEM coil pack in my car is notoriously unreliable and often needs replacing.
The MSD DIS-4 works perfectly and the whole set-up was about the same as buying a coil pack from the local dealership. (MSD DIS-4, MSD coils and spark plugs wires)

"If Bosch did not use electrolytics (Warren claimed they were non-polarized electrolytics and you disagreed with him) then what are they? Polyester?"

You are correct. There wasn't any form of an electrolytic. They were all polyester metal film.
The key one being the energy storage cap which 'sees' all the energy transfer:

L X I^2 = C X V^2

for V = 400, C = 1.5 uf, L = 1 mh, then I (peak) = 15.5 amps (which requires a special cap for long life)

My statement was based on a twin plug set-up used with one MSD vs two; Read the entire post Loren and respond to the entire post not just one sentence taken out of context. I have nothing to sell here just calling BS on most of your "facts and data"... What you have posted is your interpretation of data and your opinion, nothing more.

"My statement was based on a twin plug set-up used with one MSD vs two; Read the entire post Loren and respond to the entire post not just one sentence taken out of context."

Ok. Let's try again and the statement is still wrong.

"In a twin plug engine running two coils and one MSD box, effectively cutting the current to the coils in half, a chassis dyno shows a 15 hp loss compared to the same set-up with two MSD boxes driving the coils at full current. There is a correlation between spark voltage and power produced. Mike Bruns has the details and the testing."

1. The total current from the CDI splits but not necessarily equally between the two
coils which depends upon how well matched each coil is. The voltage, though, is the
same across each coil, but the energy transfer is basically half of the initial, again
assuming matched coils. This assumes the coils are in parallel. If in series then
the voltage divides between the coils and mostly likely unequally, and this being
the worst hookup.

2. Again, there is no correlation between voltage (in this case) and power (HP) since
the voltage is the SAME for coils assuming a parallel setup. In any case, connecting
two coils to one CDI is a Mickey Mouse setup whether series or parallel.

Bottom line: So to make a statement about a correlation between spark voltage
and power is totaling incorrect, especially since the thread discusses the abnormally
high voltage of the MSD versus the lower and more normal Bosch spark pulse voltage.

"..twin plugs do not by themselves cause an INCREASE in performance.."

Is that why the engine RPM drops as I do the mag check on the T210...?

I agree that they do not increase performance as much as they decrease non performance present in less efficient engine designs.

Some engine designs benefit from twin plugs, and some do not.

Some ignition systems on some cars are worse than MSD, and others may be better and show no benefit.

My gut tells me that if you replace a 70s GM ignition with an MSD setup, there are likely some benefits. On the other hand, I'm not sure that replacing a modern formula one ignition with an MSD would do anything more than slow it down.

My question would be "what problem are you trying to solve?"

My problem was that my 73 didn't come with an ignition so I bought an MSD. On my car, the MSD gave a huge performance increase. (Now it runs :-)

There is no problem to solve. Those that prefer Bosch use Bosch. Those that want to save some money buy an MSD for a generally reliable product. No one here is claiming that there is a big performance gain with MSD over Bosch.

Ask yourself what is happening when you ground one mag out.

The engine is now running on either the top or bottom plug in any particular cylinder.

Either way, the combustion process gets initiated 1) on the far side of the cylinder, which means that the flame front has to travel all the way across for complete combustion to occur. Because pressure rise in the cylinder is proportional to temperature, and the temperature increases as the fuel burns, the pressure rise ends up occuring later relative to design;

2) This "late peak" of the pressure is compounded by the fact that your magnetos are timed to initiate the combustion event closer to TDC than if the engine were single-plug. Your TSIO-520-R (or whatever) has ignition timing of 22 degrees BTDC. If the pressure peaks later than about 20 degrees ATDC, by that point the piston is already moving away from the head and the pressure rise ends up "chasing" the piston down the bore.

This is why you lose power.

Summarizing: adding a second plug allows the designer to compensate for the limitations of RPM inherent in direct-drive aircraft engines. If, on the other hand, you had a Porsche engine that you twin-plugged and otherwise didn't make ANY changes, then you would probably end up losing power because combustion pressure would peak ahead of the optimum peak pressure point. If you then retarded the timing, you could put the peak pressure point back where it belongs, but you still wouldn't make any more power-- power is a function of the compression ratio, the displacement and the AFR.

If you DID want to make more power you could increase the compression ratio to, say, 10,3 to 1, and then use twin plugs to prevent the detonation that would otherwise result.
But an MSD box doesn't increase your compression ratio.

Obviously there are too many variations to be able to make much in the way of "carved in stone" specific claims about single plug verses multi plug but 304065 has covered a lot of it very factually but some more detail might help.
(There are always more details when it comes to cars...)

Multi plug systems in Porsches came about to allow the combustion to complete in less time. This became more important as cylinder diameter increased which is why twin plug is considered a good mod when you go to 98mm cylinders or larger. You can light the fire later (retard) and still get the combustion done earlier. This can allow you to run higher compression ratios. This was used in some earlier racing motors simply to allow higher compression ratios even though it added a lot of expense and increased complexity.

One of the notable features of an aircooled Porsche is it has 2 valves. This means that the plug has to be set to one side of the chamber. Once water cooling came along it allowed 4 valves and then the only logical place to put the plug is smack dab in the center. And there were no more dual plugs.
This played out in lots of other cars too. In the battle to get the combustion done quickly in some cases they went to dual plugs. When they went to 4 valve heads they went to single plugs.

...

But all of this has nothing to do with the merits of an MSD system. Trying to light a flame a second time in a location where you know that you already lit a flame is WAY different then lighting two flames at the same time as different locations.

This doesn't mean that MSD doesn't work at all. If you ask Loren he will tell you that an MSD system will fire the plugs. That isn't disputed. The point is the additional sparks are just a along for the ride kind of like all the stickers on the side of a ricer's street machine. Then you have to look at the cost of the mod and the reliability. (BTW - Having used one or two units that haven't failed is not a statistical base for any sort of reliability claim.)

The only place where I would expect to see any improvement with the MSD is in a system that is misfiring. If you are running carbs and the mixture is out to wack in certain conditions (like a hole in the fuel curve or problems with float control under G...) or if there are degraded components in the ignition system the second spark might be the one to light it off.
If you have degraded components... Replace them!
If you have fuel delivery problems then I would expect you to be working on them but with the difficulty of ironing them out with carbs and MFI it might be of some use to get a late ignition instead of none... but you should still work on the fuel delivery!

----
BTW as far as the personal pot shots taken at Loren among others, saying that he "loves to bash any solution he does not sell or rebuild."
Loren is VERY capable of repairing an MSD unit. I bet if you gave him a week he could come up with a number of nice mods to them that people would pay $$$ for. The thing is you will never get him to see the reason to spend any time on a sow's ear when you already have a silk purse.
Loren has plenty of work and when I last checked Loren's price for repairing a DME was about $100 less then the price of a "competitor" that is very popular on this board and Loren knows a lot more about the DME.

"If you DID want to make more power you could increase the compression ratio to, say, 10,3 to 1, and then use twin plugs to prevent the detonation that would otherwise result."

That's the key point and why Porsche with the pre-996 head design went to twin plugs,
which also allowed for more advance timing before detonation. As many of us know,
a more advance timing always yields more torque, i.e. up to the point of detonation.
It has been demonstrated with a 911 3.2 on a dyno at peak torque that for every
one degree change in timing, a 3-4 ft-lbs of torque (or 3-4 HP @ RPM = 5252) change
results (the so-called 'performance' tuning), whereas tweaking the AFRs once near the
ideal (12 - 14) yields nothing in the way of any significant torque change.

I had an MSD 6AL in my old 5.0 Mustang, one thing I really liked was the rev limiter -- it was silky smooth, dropping individual sparks - you could drive it right against the rev with no bucking whatsoever, power would just drop off at 6000 RPM.

Of course this method is putting unburned fuel into the exhaust, so it's not practical for CAT equipped cars.

NASCAR builders are so fond of MSD they put two of them in every car :-)

Chuck.H
'89 TurboLookTarga, 334k miles

"I had an MSD 6AL in my old 5.0 Mustang, one thing I really liked was the rev limiter -- it was silky smooth, dropping individual sparks"

Not a good method of rev limiting, i.e. cutting the spark at the cylinders and have the exhaust fill
with unburned fuel waiting for the spark to return. The best approach is reduce the fuel pressure
or by shutting injectors to certain cylinders.

"NASCAR builders are so fond of MSD they put two of them in every car :-)"

And what is the implied motivation for that, maybe sponsor dollars?
Could it be like any other sponsor's, i.e. use our free product and how much more in $$$$?
And why two, maybe a reliability problem?

Not even close...Steve W is enemy #1

My understanding for this was for redundancy in case of failure. A few of the chaps that I drive with also have 2 MSD units installed (only one system active) and a switch to activate the other in case of failure. Don't know how often they have to flip the switch... So can't speak to reliability.

Only thing I can say about MSD is of all 3 I've wired up on other makes, they seem to never work until you change a wire around that doesn't reflect their schematic. Of course, this was 15 years ago and things probably have changed.

MSD was also required due to the factory stuff not being able to work well under boost (other makes, not Porsche). Once MSD installed, all "flame outs" stopped.

Never had one fail on me but never had the cars longer than 5 years either.