Starting, initiating, more than one flamefront simultaneously does get you closer to the dieseling torque level, peak cylinder pressure arriving a lot quicker, so it makes sense that a before TDC spark should be more delayed.

"..100mj there's more than 3X..."

With an O'scope at hand for analysis will find that after the spark extinguishes much of the energy now left stored in the coil inductance will be returned to the charge capacitor as the inductive field collapses, perhaps as much as 50-60%

Let me get this straight; an MSD makes THREE times the the spark energy that's required to light a plug and coil, but the argument remains that it can't fire TWO coils. Seems to me it would (and does) fire TWO with a third left over. I'm just a simpleton though because of the Weber thingys so what do I know. From a peak horsepower aspect, carburetors make just as much as EFI.

yes carbs work the same as FI at full throttle and the 3 spark cuts out over 3000 rpm. on the MSD,s, voltage to the coils is 12 volts, the CDI voltage from the bosch box of around 300. so you need 2 CDI's for a dual plug set up but you can run dual plug set up with one MSD box and 2 coils.
for what it's worth..

now i am a bit confused. here are some quotes from an article from a guy named ALLEN CLINE, designer of the northstar engine. the article is about preignition/detonation. i understand him to say you want to design an engine to need less advance, and that once peak power is reached, any more advance has no benefit, or no more power is produced:


Production engines are optimized for the type or grade of fuel that the marketplace desires or offers. Engine designers use the term called MBT (Minimum spark for Best Torque) for efficiency and maximum power; it is desirable to operate at MBT at all times. For example, let's pick a specific engine operating point, 4000 RPM, WOT, 98 kPa MAP. At that operating point with the engine on the dynamometer and using non-knocking fuel, we adjust the spark advance. There is going to be a point where the power is the greatest. Less spark than that, the power falls off, more spark advance than that, you don't get any additional power.

Another thing you can do is increase the burn rate of the combustion chamber. That is why with modern engines you hear about fast burn chambers or quick burn chambers. The goal is the faster you can make the chamber burn, the more tolerant to detonation it is.

Another thing you can do is increase the burn rate of the combustion chamber. That is why with modern engines you hear about fast burn chambers or quick burn chambers. The goal is the faster you can make the chamber burn, the more tolerant to detonation it is. It is a very simple phenomenon, the faster it burns, the quicker the burn is completed, the less time the end gas has to detonate. If it can't sit there and soak up heat and have the pressure act upon it, it can't detonate.

If, however, you have a chamber design that burns very slowly, like a mid-60s engine, you need to advance the spark and fire at 38 degrees BTDC. Because the optimum 14 degrees after top dead center (LPP) hasn't changed the chamber has far more opportunity to detonate as it is being acted upon by heat and pressure. If we have a fast burn chamber, with 15 degrees of spark advance, we've reduced our window for detonation to occur considerably. It's a mechanical phenomenon. That's one of the goals of having a fast burn chamber because it is resistant to detonation.

There are other advantages too, because the faster the chamber burns, the less spark advance you need. The less time pistons have to act against the pressure build up, the air pump becomes more efficient. Pumping losses are minimized. In other words, as the piston moves towards top dead center compression of the fuel/air mixture increases. If you light the fire at 38 degrees before top dead center, the piston acts against that pressure for 38 degrees. If you light the spark 20 degrees before top dead center, it's only acting against it for 20. The engine becomes more mechanically efficient.

T77911S, in a word, Yes.

Loren, i believe a 3.2 will make more and more power until detonation.

However, if the 3.2 engine is dual plugged and the compression raised to a level greater than a single plug could handle at a reasonable advance #, I could almost guarantee it would make peak power at a lower# than the threshold of detonation. I'm not saying it wouldn"t make similar power at the threshold of detonation, just that it will also make that power at a lower adv. #.

The flame travel time cut in half which is a lot of time on a large piston and also why you don't see small bore dual plug setups, they just don't need them.

Comparing a factory designed twin plug engine (993 964) with anti knock to an engine modified to twin plug without anti knock is apples to oranges. The reason Porsche chose to run advance #'s at the threshold of detonation is because thats how an anti knock system works !

"Comparing a factory designed twin plug engine (993 964) with anti knock to an engine modified to twin plug without anti knock is apples to oranges."

Hardly!

more like apples to pears

How come the full throttle timing maps i see for stock 964/993 are around 17 degrees at max power (5000 and up) and the stock 3.2 single plug full throttle maps are around 23 degrees at max power (5000 and up) ???

These maps exactly support my real world dyno experience and seem to contradict your above statement.



So you recommend advancing the timing on 3.2 engines over the conservative stock timing #s for improved performance?

Firing twin plugs would undoubtedly create 2 advancing flamefronts resulting in reaching PEAK cylinder pressure somewhat earlier than a single plug system. So everything else remaining equal the timing advance for a twin plug motor will always be less than that with a single plug motor.

With enough "swirl" within the cylinder near TDC multiple firings of a single plug might actually accomplish the same gain.

I get the advantage of twin plugs ... actually lighting the charge from two opposite locations, so the time to burn the charge is faster/better. I guess what I miss with the single-plug, multiple-spark concept is that IF the first spark lights the charge, then it's lit and travelling away from the plug. How does re-sparking help? Is there still an un-burned part of the charge lingering around the spark plug? It seems that the unburned part of the charge is physically no longer near the plug. Now, taking the case of a charge that DIDN'T fire with the first spark, sure a multiple spark makes sense.

We need a time-lapse image of what's happening in there!:D

Let's talk about neon indicator bulbs. These have 2 electrodes spaced a specific distance apart inside a glass encapsulation filled with neon gas. Let's pick a model that will fire an arc between the electrodes at 90 volts +/1 10%

connect 2 of these in parallel along with the typical resistor to limit "fired" current. Now need a method to slowly increase the applied volt until the first one fires....Now, will the second one ever fire with the first one providing a current flow load and in doing so also provide a "clamp" of the applied voltage to prevent it form rising higher...??

That's what would happen with ANY CDI driving 2 ignition coils in parallel, if the 2 plugs do not fire at EXACTLY the same time then the "reflected" short to the primary of the coil/plug that first fires will "clamp" both primary voltages to only the level needed to sustain the ONE arc. That would not be enough, in the general case, to initiate an arc across the second plug.

Installing a resistor in series with each coil of a size selected to drop 150 volts (maximum at peak current flow) out of the 450 volts the MSD supplies might prove to be more reliable insofar as ALWAYS firing both plugs.

What we see here is the coil primary voltage rising fairly rapidly to ~350 volts and then when the plug fires the coil primary voltage falls, with ~2 microseconds, to ~250 volts.

Not that I am embracing the theory.

The theory seems to be that you have enough A/F mixture "swirl" within the cyclinder at TDC to class it as a tornado. The initial flamefront has moved away from the plug by the time the plug is fired again...and again.

This exact scheme had been suggested to me on a 962 that has a tuning issue that people were having trouble chasing down. It was suggested that the guts of the 1.7 DME (about $45k) be removed and that a Motec be put inside so anyone could tune it.
When you have an extremely original car it cocks the ear when people start out with the "just scrap everything" solution instead of making a serious effort to go work through the problem.

This thread cracks me up.. I had a permatune, then a normal MSD (both failed) and now a $50 streetfire MSD and its working fine.. Im WAY ahead of buying a bosch and my street car runs fine... If a Bosch has any advantage at all Id not know about it on the street, only a dyno and I frankly dont give a crap about 5 hp on a dyno..


This Loren gentleman enjoys this stuff.. wouldnt be the first thread he created all sorts of ill words between members.. seriously WHO CARES? this is a 30 minute swap... choose your weapon based on your porsche religion and wallet.. right now my wallet requires a MSD street fire.. love it.. its WAY smaller than the full MSD.. I could carry 20 spares for the price of a Bosch.

BTW.. my permatune is the open type.. (repairable).. however, even with it IN THEIR HANDS they told me they couldnt fix it.. they dont EVEN HAVE THE SCHEMATIC FOR THEIR OWN ORIGINAL DESIGN.. Hahahahaha.. that is ridiculous.


They blamed the failure on my coil (which I sent them) and frankly, I believe them as my MSD failed as well... its was a cheap ass coil (not bosch).

Oh and all this talk of a late secondary spark on the MSD.. again WHO CARES? the primary is all that matters... at idle with an old oil burning, rich running beater the dual spark may just keep your car running long enough to grenade the motor really good and require a nice new rebuild. Which is good right? :)

Boys and girls, I turn your attention to more important subjects.. like.. hey where did you get that cool mirror? :)

JP

Is this the one you referring to? Its listed at $150

Street Fire CD Ignition by MSD Ignition

Sorry, I just realized I paid $50 for the coil (msd blaster) and about $120 for the street fire.. my apologies.. however, I think my point (if I had one) stands.. its cheap and works fine on the "street"... I can tell no difference between it, and the permatune, nor the MSD "red box"... My buttometer is pretty calibrated.. ;) Not sure but I think I posted a picture of it after I installed it on here somewhere..

Most of them on the car and motorcycle forums I belong to, do. This thread wasn't one of them.

Technical threads and posts by weekend warriors without any basis that are at best, lucky guesses, instead of based on measurements. Yes, those threads do crack me up. I can't believe some of the complete BS that gets passed on.

And if the little nitty gritty is a matter of "who cares" why do owners go through the trouble of adding so many aftermarket upgrades to gain very little in a world where it hardly matters...like street use...as if our Porsches were such crap as delivered from the factory. It reminds me of the motorcycle crowd where it isn't about the motorcycle or the touring, but about the farkle and bling.

Doubt any of us use MSD because it offers superior performance in real world driving nor do we use it because of company hype or bling.. Many of us use it because we had a bad Permatune, dont have a bosch core, and dont want to spend $2000 on a remanufactured Bosch when we can have something new for 1/10 the price that is equally reliable at 5k miles per year.... Dont read into the Loren Hype. I could care less about the "hypothetical" performance gains.