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Sat Dec 12, 2009 5:52 pm

Quote: Their use is more of a requirement on a higher compression engine, which needs the extra gap/spark?

The larger gaps (read higher voltages) are used to ignite leaner mixtures on water cooled motors. Leaner mixtures are harder to ignite. If you lean your ACVW out to those mixtures that require a larger gap (read higher voltages) you will overheat it. Use the stock gap as it is fine on these engines.

Under higher compression the spark will not jump as far. Rather than lower the gap the preferred method is to raise the voltage and use a plug with a racing style tip. These are not recommended on ACVW.

Modern cars have multiple coil packs. We started seeing these in the early 1980' first on high RPM Mercury racing outboard motors. They are used because a coil saturates (the magnetic field fails to collapse) as the rpm's increase. This causes the coil to lose strength (voltage) as the rpm increases. But at 4,000 RPM on a VW you won't saturate a coil with 4 cylinders. On a 4 cyl VW, 4000 RPM is 2000 cycles X 4 cylinders per cycle that the coil fires or 8,000 times. On a V6 at 4000 RPM that is 2000 x 6 or 12,000 times a minute the coil fires. So to reduce coil saturation the load is spread over multiple coil packs. My Acura has one coil per spark plug. The 4 cyl VW has to turn 6,000 RPM to equal the same magnetic saturatation as the 6 cyl at 4000. We used to turn close to 9,500 rpm in the 4 cyl F and G Production cars before we ran out of coil.

Air Cooled Nut. Jake is right in that if you are going to build an engine for high RPM's like the F Production 914 they have, you will see HP gains but they will be at the top end of the runs, not at generic stock RPM's. But that is the game with racing - to keep pushing that top end up for a few more HP without losing the torque on the bottom end.

Sun Dec 13, 2009 1:22 pm

Jeeeez!....you guys are rough on each other! There are a whoel lots of "rights" in here and a handful of incomplete explanations that make it sound like a whoel lot of wrongs.

(1) Yes....a larger coil will NOT hurt your electronic ignition as long as the coil resistance is correct...and you have a stable power supply to that coil to cut down on creeping current rise and excessive coil heating (a common problems)

(2) A one mentioned here.....on a bone stock car with low compression and what is considered quite rich mixture by modern standards ( anything under 8.0:1 and sporting a carb)......having extra voltage over stock will do largely nothing for you other than making it a more reliable start in very cold climates. In this case...what you are looking for is spark temperature. It can make a difference.....but is not a horsepower tool....its a reliability tool.

(3) All multiple exlectrode plugs are not created equal and there are a lot of different and well proven technologies out there. The Bosch W7DTC and NGK BP6ET for example......are top notch...and make a HUGE difference in cars that require them. The object with these plugs is that all electrodes are gapped exactly the same. Never in any case will more than one electrode ...EVER...fire at one time. The object is a larger initial flame Kernel.....a job they do admirably. They do this by having a cage area wherin fuel mixture is interspersed. At any given time...the resistance of any one electrode.....and hence its lower resistance to ionization point (meaning the ability of current to arc) will be better than the others due to heat, compression, location of fuel and air adjacent etc. This makes for a very reliable and consistent spark. They also have the added benefit of not having any electrode shading or index issues compared to regular plugs. They are and were designed for high compression engines.
If you don't believe they work....try taking a perfectly tuned Sirrocco 2 and simply swapping to normal plugs of the same heat range. It becomes and instant "dog" off the line.
They also require quite a bit of guts out of the coil to really operate well and have any noticable difference.
Again....they will not gain you one iota of power on a stock low compression carbed engine. But...the ommission of them can lose you HP on a high compression lean burn engine that is tuned around them.

The later models of 3 and 4 prong platinum plugs are a different animal. Those use a very easy sparking platinum temp that is great for high temp lean burn. The multiple grounds make it a very long lasting plug. It creates smaller sparks but requires less current. The platinum plus 4's are great for electronically controlled lean burn engines.

All that being said....the stock Bosch ignition was just fine for just about everything up to the fuel injected higher compression type 4's. Those enegines and most of the EFI vans....are highly underignited. This is also why the vanagons quickly went to electronic ignition with 55Kv coils.

On a stock 411/412 or 914....the stock 18-22 Kv Bosch coil ran hot and produced relatively worthless spark over 3000 rpm....which means it was barely adequate for crusing speed on the highway. The first sign of plug gap increase due to wear...or point surface or rotor erosion....was intermittent misfire at highway speed. A large improvement even when using better than stock points (like Blue streak)....is a 42Kv Pertronix flamethrower coil and a set of Bosch W7DTC plugs (triple electrode copper nickle). An even better set-up.....was adding a pertronix point replacement module or a crane Xr700 trigger to replace the points.

The real issue here is what the sensitivity of your enegine is. A carbed type 1 with a basic coil has few sensitivities. However...D-jet and L-jet....even with low compression has a lot of sensitivity. A single spark related misfire...causes a change in airflow that causes a metering spike in the fuel. That single misfire creates about 10-20 more misfires in rapid succession before it rights itself. That whole cycle lasts about 1-2 seconds....because at 3000 rpm you have roughly 25 spark intervals per second.

That last statment is also why a larger coil helps when you have higher compression....because at that speed the coil saturation time is really short.

Ray

Sun Dec 13, 2009 2:34 pm

Ray -this thread began as a statement about a stock VW bus engine. If it were in the performance section about a drag engine, a formula Vee engine or a SCCA FP 914 engine my answers would have been tailored differently but the science and engineering are still the same.

The plug gap combined with compression sets the rise of the coil voltage before the spark occurs. If the plug fires at 12KV then it will fire at 12KV whether the coil can produce 12KV or 100,000KV. The extra energy in a larger coil will be turned into a wider spark and a longer spark but not additional voltage. The L-Jet ECU uses the coil pulse length as the base pulse to drive the injectors. It then adds for temperature and flap position, but it never subtracts. The flap has a damping chamber to slow changes from misfires etc. Adding a coil that takes substantially longer to fire will change the mixture, whether that is desired or not. And, on a stock T1 or T4 with a carb, you won't benefit from a bigger coil unless you are going up in rpm or compression ratio. If you current coil is suspect or bad, a blue bosch coil will be more than adequate to replace it.

I am done commenting on this and suggest if you want definable proof take the car to someone with an ignition analyzer scope that yields traces like those below. And yes, electronic ignitions are really great vs points except in the middle of the night when they quit working and then they are a pain to troubleshoot for the average mechanic who has trouble determining the difference between a bad fuse, burned out bulb, troublesome relay or defective ground.

Sun Feb 07, 2010 2:18 pm

Quote: OK, so a high output coil allows for a larger gap, which creates a larger spark, which is NOT beneficial to the performance of a stock 1600 motor.
Their use is more of a requirement on a higher compression engine, which needs the extra gap/spark?

A larger gap will ignite leaner mixtures also. That is why smog cars run wider gaps and hotter coils. Do you want to run a leaner mixture in an ACVW? Or very high compression? If so use a hot coil. If not just use a stock or blue coil.

One big disadvantage of a hotter coil is that the magnetic field takes longer to build and collapse each cycle. This property is called coil impedance. This is essentially resistance to changing pulses of electricity. The greater the impedance the the longer the coil takes to collapse and the lower the RPM the coil saturates at. This is why late smog engines have gone to multiple coil packs as the coil is only handling 1 pulse per cycle instead of all the cylinders. The larger and hotter the coil the lower the RPM it will saturate at.

Sun Feb 07, 2010 3:28 pm

I kind-a like my stock 205Q dist, points and a bosch blue coil. It's like my visa card (get's me where a want to go) discuss among yourselves.

Sun Feb 07, 2010 9:42 pm

Air_Cooled_Nut wrote: The stock system is fine, I agree, I've used it for years. But like many things it can be improved with the advancements of technology.

Sun Feb 07, 2010 10:18 pm

Malokin Martin wrote: Air_Cooled_Nut wrote: The stock system is fine, I agree, I've used it for years. But like many things it can be improved with the advancements of technology.

What's your point? Because that isn't advanced technology :roll: