Kevin

Why throw a OBDII street legal system away, when the Motronic outputs to 2 ignitors.. If you are going to go to a standalone system you can add the same components in conjuction to the factory Motronics.. I don't want this thread taken away as a standalone Race application. I request that you start another thread.

Geoffrey

How does outputting to 2 internal ignition drivers help you?

K24madness

I would hate to apply that same logic to many of my other hobbies. All of my toys would be gone!!!

Kevin

I'm not suggesting that you run the factory coils, with the dual ignitors one can choose his (dual) coil packs and run a 993 or 964 distributor.

Red rooster

Geoffrey has run through the argument pretty well but maybe not finished it off ?
At 8500 rpm there will be still a bit over 2ms charge time for these high energy coils . I think it is very debateable that at this 60% of maximum possible energy the spark can be considered weak .
Ah well, time will tell but I dont think this represents a serious problem.

On the old points cars where CDi was introduced , whole different ball game.

All the best

Geoff

Geoffrey

It was not my intention to argue or start an arguement and therefore have not gone on a diatriabe about the merits of an 8500rpm engine or the value one places on experimentation or the value of one's monies spent. I am not in a position to argue given my propensity to sink endless money into the pursuit of my own interests and certainly support other's doing the same. I am not the one who brought up the ignition system, only answered a question pointed at me.

My original post was an attempt to clear up the fallacy that single plug ignition in a hemi style head with no squish area could be as efficient or better than a twin plug design. You cannot fake the facts and in this case, twin plug is a better, more efficient design that will produce more power. The math speaks for itself.

If one wants to discuss spark energy, we can do the math on that as well. The ignition drivers in the Motronic ECU are current limited to about 7.5A, so at 12v, there is a finate amount of energy that can be stored in a given coil with a given primary and secondary resistance and required time to fully saturate. It is weak, will always be weak regardless of the coil used and will also be an inefficient ignition system in the RPM ranges being discussed. It will likely misfire especially if the engine does produce peak torque in the 7200rpm range which is what you'd see in an engine with 8500rpm redline. In the case of a Porsche dual distributor that Kevin mentions, there would not be coil packs used, only single pole separate coils. Coil packs are used in a direct ignition, waste spark environment which is not the case here. I've done a number of 8000-10000 non-Porsche engines to have seen my share of ignition system issues.

K24madness

Geoffrey,

Can you be more specific??

JK

Is there really no squish band on the 993tt style head/piston combo? Would a CDI unit be 100% charged if used in the above application?

Peter S 993tt

Kevin - I have a question - if you back up to the clame rings when you were preparing the heads, won't the dissimilar metals lead to corrosion - copper and aluminum? Wasn't that the problem with Lucas electrics?

Kevin

There is a squish oring placed in the cylinder liner.. However the EVO style flame ring is stepped to allow for a groove that is cut into the cylinder head.. This locks the head and liner together and at the same time provides sealing similar to traditional factory oring.

There won't be corrosion taking place that will effect the integrity of the aluminum liner or cylinder head. Copper head gaskets and orings have been used for many years to seal heads to cylinders or engine blocks (cast iron or aluminum)..

AVoyvoda

It was not my intention to start an argument either. Or talk about race cars. Simply that I recently had the same discussion with a UK uber-tuner (can one get a street engine to 8,500 rpm?) who mentioned precisely the same points as Geoffrey. When contrary to all that I was told, I read that Motronic and standard ignition will be retained on this car, I thought it would be worthwhile to examine whether it is, indeed, feasible. Thereby avoiding considerable expense, maintenance and emisions issues. If, on the other hand, it isn't feasible, then it's better that the member who is planning to get his engine to run reliably at 8,500 rpm know of potential problems beforehand.

Geoffrey

There is no squish area in a 911 head. If you look at the pictures the combustion chamber looks like 1/2 of a tennis ball. There is a lot of volume in the head - over 90cc and without the squish area, the air fuel mixture will be homogenously dispersed throughout the entire 90cc and not concentrated near the spark plug or spark plugs. If you look at a Harley Davidson Evolution engine for example, it also has a hemi head with a single spark plug, however, the head has a squish area to force the air fuel mixture near the spark plug. This effectively reduces combustion time and adds some amount of efficiency. In the 964/993 N/A application, the piston dome is designed to create a squish area which concentrates the air fuel mixture near the two spark plugs rather than in the valve pockets (the problem with JE Piston design) so it also takes less time to burn and is more efficient. With the 993tt there is effectively no piston dome, it is almost a flat top piston. You can see the difference on a dyno when setting ignition timing. The fact that the cylinder head is so large is a contributing factor to the weight of the piston since more material must be used to maintain compression ratio. Take for instance a 100mm 993tt piston which has a net dome volume of some what less than 10ccs and compare that to the same 100mm 996tt piston which is a dished piston of about -25cc. The 996tt piston is far lighter and can more easily be spun up to 8000rpm than a heavier 993tt piston.

JK. Yes, a digital CDI can charge and fire a coil repeatedly in less than 1ms. If I have time this weekend when teaching an EFI Advanced course, I'll try to capture an oscillosope trace of an inductive coil.

K24madness

Kevin,
Sorry to streer your thread of track.

Geoffrey,
I am amazed by your statements. I can't believe there is no squish band. I will try to get my hands on a 993tt head and piston and send it to one of the many local harley head porting shops. The have welded combustion chambers in various methods. Bath tubbing or 30 degree dual sguish bands may work. This may also streathen the head from the inside instead of welding fins.

These guys have LOTS of experance with hemi style heads. They are able to add squish bands AND increase flow.

If either you of Kevin has a spare head PM me off line and I will send it off to be looked at.

Tom

Red rooster

Geoffrey,
Interesting observations. If Kevin was attempting 10,000 rpm then pretty well no inductive discharge system would be satisfatory.

I cannot get my head round the observation that the Motronic coil drivers are weak . With 7.5 A available, a quick 0.5 LXI squared sum shows big energy levels while the L/R sum shows a reasonable time constant.
How are the Motronic drivers inferior to pretty well every duration controlled ,constant current , driver found in every modern OE, ECU ?

All in the interest of science !!!

Geoff

Kevin

We all know that these engines have been raced successfully.. Without the technology in metallurgy, and precision machining that we have today. The 3.8ltr RS pistons (Mahle) weigh close to 474 grams.. More than the stock 100mm and 102mm 993 turbo piston. From memory the 996TT piston weighed only 18 less than the 100mm 993TT piston.. I realize that you can't install stock components and expect to spin this engine at these RPM's.. This engine will have a weight reduction program, the rods that are being used for this project with the lightening of the Mahle pistons and other fine tuning will knock over 150 grams off the mass.. For the record at 8500 RPM (at stock weights) the tensile load on the rod is 2.40 Ton/F produced by the piston acceleration @ max RPM (8500RPM) is 23.94 KN.. When I have the final weights I will print the "net" load reduction.. One also must state that even though these calcs are applied to a Normally Aspirated engine, the shock and stress placed on the large end of the rod, is dampened on a turbocharged engine. The loads are considerably less. A few of the rod engineers has stated 18-24% reduction. This is dependant on your camshaft grind.. Having a rotating assy that will be engineered to survive requires other areas of concern, such as valvetrain components to survive and these RPM's and yet stand up to the EGR's from the turbocharged engine. I will be posting the custom valves hopefully next week.

There are some bright individuals around this world that have worked on these engines for many years. Individuals that "Thrive" on stepping the game up against all doubt..

Geoffrey

Geoff, You are making assumptions about statements I didn't make. If the coils are not fully charged, then the spark will be weak. In and around the production envrionment they were designed for, they work fine. Increase the cylinder pressure and they are not enough. TT owners who have increased boost pressure or added larger turbos have experienced the ignition system not keeping up. Additionally, the factory production inductive ignition is not as powerful and nowhere near as fast compared to a rhigh quality digital CDI unit. It really isn't fair to compare the two.