Geoffrey

With the total engine management systems available today, you can dial in timing properly for every RPM/Load site, something you cannot do with the stock system, nor could they on the 935. On an engine dyno, or a chassis dyno that can control the rpm of the car, you can adjust the timing and watch the torque rise and fall. You then can use the minimum amount of timing that produces peak torque. Within the 7:1 - 8:1 compression range and with both 3.4l and 3.6l engines, I've seen that 24 degrees is about the least amount of timing that will produce the most torque under boosted conditions AT PEAK TORQUE. Between 24 and 28 degrees the engine does not make any more torque, but also does not appear to detonate either (on race fuel).

At cruise, you can run much more timing and a much leaner mixture because the cylinder filling is INEFFICIENT and detonation will not occur (within reason). If you plug a scanner into a 993TT you'll see 44 degrees of timing under these conditions and very lean air fuel ratios.

The ability to manage timing (every 250rpm with MoTeC) gives you a significant advantage in that you can build an engine and manage it properly which will get good torque and great throttle response under light load conditions. The difference is staggering!!!

The only reason I can think of for not twin plugging is the issue of cost. Not only do you have the cost of the head preparation (500-600), but you have the larger issue of how to manage it (Electromotive HPX, Stock Porsche dual rotor, 964 dual distributor, or total EFI) which is the expensive component.

I read recently that the 993TT was not twin plugged because of issues of time to market/cost considerations to work out the packaging of the 993N/A distributor. I dont' know if that is true, but it sounds reasonable. Remember that a stock 993TT runs minimal boost levels - .8bar and produces a modest 400hp. Many of us are 25 to 50% above that range and running boost pressures 25% higher. Different goals, different requirements.

Steve Weiner-Rennsport Systems

Geoffrey:

Don't forget that the 993TT gets away with 44 deg and ultra-lean cruise mixtures by using knock sensors and VERY sophisticated management. Those engines walk a very fine line.

MoTeC cannot do that so one must be much more conservative to avoid broken rings or worse,...

Porsche elected not use twin-ignition on the 993 TT due to cost and the ability of that Motronic to prevent detonation while maintaining peak performance all the time. Those engines are very fuel sensitive.

Geoffrey

Steve,

I agree with you regarding the sophistocated nature of Motronic and as you know, that is why tuning ignition maps on a dyno is very important, especially if you are wanting to be close to the edge.

Knock control is a very grey area even for the OEM's. The current state of the art in production vehicles is done using piezo-electrics which are also known as a microphone. The thresholds of knock that cause damage
are in the inaudible regions of frequencies so, by the time the microphone "hears" the ping, knock has been going on for several revolutions. Knock strategies are designed by the OEM's as a stop-gap measure against different fuel grades available in the mass consumer arena. Contrary to popular belief, knock sensors are not used to bring the engine to the edge of performance, then back down. Motronics still rely on an igniton map similar to MoTeC or other EFI systems. The knock sensors are used primarily for protection against poor fuel.

Knock sensors are not appropriate for usage in performance vehicles when used to control timing at the edge. They simply aren't fast enough nor are they reliable enough. They are meant for the consumer market.

So, unless you have are rich and have "in cylinder pressure" measuring devices (at about $6000 per cylinder) and about $100,000.00 worth of DSP dyno equipment, it is best to tune the engine at a proper A/F without being on the "ragged" edge of spark advance. Also keep a close control of the fuel grade that is being used.

KeithC2Turto

I love it!

Thanks guys.

I appricate learning for some of the best Porsche tuners!!

Sounds like twin pluging is diffenetly a big advantage, makes a motor run cooler & more effecently.

The cooler must be why it fights knock to some degree.

So, from this can we accept Bruce Andersion's inference that we should be able to run one point higher compression on a 930 motor at the same boost.

Thus an 8 to1 CR at 1 bar with twin plug?

(Kind of back to Brent's original question.)

If I do the math right this would be equal to an effective compression ration of near 12 to 1 compaired to near 11 to 1 on for a 7/1 CR motor runing 1 bar.

Alternately it seems that if one kept the 7 to 1 CR we should then be able to run and addional 3.7 lbs of boost instead (as long as we have enough fuel) with a twin plug system, and still stay with in the 12/1 effective compression ratio.

Thanks again for the everything.

Keith

Steve Weiner-Rennsport Systems

Hi Geoffrey:

LOL,...With all due respect, I must disagree,.... knock sensors are used in all Bosch (and other) racing engine management systems and indeed are not just "consumer market" items.

They are an integral part of modern engine management to maintain peak torque under varying temperature (coolant-oil-ambient) ranges during competition. Just look at the latest GT-3RS and GT-3RSR systems to see what I'm referring to.

The latest most modern engine management systems found on these Porsche racecars as well as most F1 engine manufacturers use knock sensing and you'll see more of this in the future,....

I'm sorry to say that there is little detailed information about these systems since they are proprietary in nature and the whole professional racing world is extremely competitive. What little I've gleaned is from my partner's involvement as one of the Crew Chiefs on the Alex Job Race Team. The Factory Porsche software engineers that assist AJ Racing and do all the engine-related things do not share much,.....

Steve Weiner-Rennsport Systems

Keith:

Here's an oversimplified rule-of-thumb:

All things being equal, twin-ignition permits the compression ratio to be 1 full point higher on the same fuel without detonation.

This mainly applies to N/A engines. There are too many other factors at work in turbocharged ones,....

Geoffrey

Steve,

Ahh, yes, I stand corrected. I would venture to guess though that the knock sensors are used as safeguards rather than an active mechanism to run the most timing. By the time the knock sensors sense knock and react, detonation has occured. I'd imagine that the design is to keep the knock sensors from ever being used by properly managing the engine temp, air temp, cylinder head temp, etc. compensation for timing and fuel and then use the knock sensors as a safeguard when something causes the engine to run out of these ranges.

Since 930s didn't come with knock sensors and we're limited to aftermarket ECUs, knock sensors are pretty much a moot point. I've yet been able to get MoTeC to even discuss knock sensors at a detailed level.

Geoffrey

Its also hard to really discuss what is proper and what can be done without first considering how the car is used. I've personally seen programming that while on the street with limited time in boost run fine. On the street, having the turbo spooled up completely for more than 5-10 seconds is a hard feat, and not repeated with short intervals. However, when tracked where the turbo is constantly spooled up and down, the same program destroy an engine. While running on the street the programming was not correct, it ran ok and the potential damage was unnoticed, but while on the track the incorrect programming caused damage.

This is why I strongly suggest finding your tuner first, then building the car.

Steve Weiner-Rennsport Systems

Good advice!!!!

Usage defines everything your should/will do to the car.

Its really too bad the MoTeC people are not incorporating effective knock sensing into the top-end products. There is no question that it will add several dimensions of cost to the product, but it THE only way to go on any high-compression or turbocharged engine when optimal performance and longevity are required.

Geoffrey

Steve,

My understanding is that while MoTeC, and others have channels that can be used as knock inputs, the detection of knock, or the personalized signature or frequency that the knock emits differs from engine type to engine type and requires expensive equipment to determine that signature. It gets even more expensive when engines are destroyed for the purpose of finding that signature.

I guess what I'm saying is that knock sensing is not plug-n-play, not due to limitations of the electronics, but due to the mechanics of finding the knock signal.

KeithC2Turto

Didn't Porsche have to change the cam chain / drive design to quiet it down so they could use knock sensing.

Seems like this would have be inportant if trying convert a 930 to this.

K

Geoffrey

I believe it was part of the reason to move to Hydraulic tappets in the 993.

Steve Weiner-Rennsport Systems

Geoffrey:

You are right,.... Its not a plug-and-play proposition. Each engine design has its own accoustic signature and this must be measured and programmed into the knock-sensing software. Porsche & Bosch spent millions to perfect this.

Hydraulic lifters were primarily used to reduce maintenance costs; a major marketing tool.

Keith:

The cam chain drive was redesigned to make it more durable than its predecessors as well as quieter.