Geoffrey

M42Racer,

There are two types of batch firing of the injectors. One method fires all six injectors every revolution of the crankshaft or 360 degrees. This means the injectors fire twice per 4 stroke or 720 degrees of crankshaft revolution. The second method links the fuel injection firing to each pair of cylinder ignition events 1/4, 2/5, 3/6 in our engines where one piston is TDC on compression and the other on combustion.

As I said above, given a choice, I would always use sequential injection over batch because the merits of sequential outweigh batch. However, there is nothing wrong with batch, it works very well, is simple and the differences show up mostly in fuel economy and a small amount of torque.

In a batch fired engine, the injectors can be smaller since they are fired twice per 4 stroke while in sequential they are fired only once. There are many formulas out there for fuel injection sizing. You CAN run injectors to about 85% duty cycle although with sequential injection 65-70% is considered maximum for maximum power. As I mentioned above, I have found 55lb/hr inejctors will run to 550hp, 72lb/hr will run to about 700, and 82lb/hr injectors will run to about 850. For proper atomization, an injector needs to be on for a minimum of 1.8-2ms.

The vaporization of fuel on the inlet valve cools the incomming air charge, making it more dense.

Again, what is being discussed here is theory. In practical usage here, either batch or sequential works very well provided it is engineered properly.

Geoffrey

Also I'm curious on the method of turbocharger sizing that came up with a GT35 turbo for your car. Is that something that PD did, or some other shop? I've seen those turbos used in water cooled applications and in fast n furious cars, but never in porsches except for maybe some of the newer 996TT applications. The GT series turbos seem to have huge compressor wheels, but in reality don't flow much air. Garrett has yet to publish the full compressor maps on these turbos, so one is only to guess.

m42racer

With respect to the Turbo choice, I choose the turbo, the specs and configuration. I have somewhat of an inside track on turbo's. There are many different Comp wheel sizes, trims, available for both the plain bearing and ball bearing turbo's. We are oil cooling this turbo, and so far so good. I am benifiting from the ballastics of this turbo design, and will hope that we have good reliability with the oil cooling.

As for your Injector sizing methods I cannot comment on this. It appears to be somewhat ambiguous from your post. I'm not sure I understand you here. You say with batch fired systems, the Injectors can be smaller, but at high RPM's where the time between firing is very very small, should not the sizing almost become the same. Is not the size of the Injector based upon the max Hp required. Again, this is not my area of expertise, but I was always under the impression that Injector sizing was the same for batch or sequential. I will ask Neil @ PD for his opinions.

Kevin

Simon;
Since your post I'm going to guess that you work for Garrett, good ol Honeywell. You and I share the same day jobs, however you mentioned the OE side, is it automotive or diesel? I'm curious. I do admire your position, while we have you on this forum, maybe you and I can agree or disagree over a few things. As you know, if you are on the OEM side, you see amazing applications. I'll get to the chase. I have not seen reliable ball-bearing outcomes on our 930, or 993 or 996 applications. Infact, once we have a sealing failure with the ball-bearing units they are pretty must useless. On larger aircooled engines I have not seen any GT series turbo's last over a year, this includes Drivers Ed events but no Pro racing. I understand that you are cooling the H2O ports with oil. I feel that it is short lived.

Another concern & observation that I have is the short sided marketing of people wanting a ball-bearing unit for it's anti-friction abilities. As you know when the bearing housing is running at 3-5bar, the rotating assy bogs down. You should try it in the lab (in all fairness it still does rotate longer than the bushing design, but not by much).... The main and most important design element of a ball-bearing unit is it's ability to keep concentricity or the shaft centered in the bore. And with that we are able to run tighter tolerances between the compressor wheel and compressor housing. Running gaps of .006 increases the efficiency greatly.

In high hp applications and large wheel applications, I wish that ball-bearing units would last, and be repairable. Explaining to your customer that they need to spend another $1,800-2,500.00 every 6month isn't fun. Yes, I see and sell the units for Supras and Honda's, but they are failing, from heat and hot shutdowns....

Geoffrey

M42Racer,

Fuel injector sizing is not an exact science since there are many factors that affect size, engine effeciency, fuel pressure, max engine rpm and max duty cycle. However, here are 3 formulas you can use:

1) Fuel injector size (lb/hr) = HP / 10

2) Flow rate (lbs/hr) = Max Hp x BSFC / no. injectors x duty cycle
BSFC = 0.45 - 0.55 for naturally aspirated engines
0.55 - 0.65 for forced induction engines

3) Req flow rate (cc/min) = hp x k (k=5.6 for turbos, 4.6 for n/a)
---------
C (c=number of cylinders

Then find the static flow rate = req flow rate x 100
------------------------- (m = max duty cycle)
n x m (n=number of cyl)


You can then fine tune the equation by determining if you can alter the fuel pressure and use a different size.

revised flow rate = static flow rate x the square root of new fuel pressure / rated fuel pressure which is usually 2.5-3bar depending on mfg.


To conversion from lbs/hr to cc/min = lbs/hr x 10.515

Also, don't forget that the higher the max rpm, the larger injector you need since you have less time to inject fuel.

Maximum pulse width = max duty cycle x 60,000 ms
-----------------------------------
RPM (batch fired = rpm, sequential = 1/2 rpm)

So, with sequential injection at 7000rpm and 70% duty cycle, you have 12ms for fuel
at 8500 and 70% you have 9.88ms for fuel and would most likely require different injectors.

Does this answer you questions regarding fuel injector sizing formulas?

Kevin mentioned that you work for Garrett, that would explain your inside track. However, the vagueness of your answer doesn't help. Would you explain about the GT series turbos and why they have such minimal flow rates, but yet large compressor wheels?

EDIT: sorry about the spacing on the formulas, the editor looks fine, but when posting as a webpage, it looks like the board software is ignoring all of the spaces. Let me know if you can't read them.

m42racer

Kevin,

Do you work for Garrett also? Maybe you work with Chuck Miller. He left LA and went Northwest some time ago.

My choice was to find out the difference dynamically (in car) between the plain bushing and the Ball bearing design. To this day, so far so good. As for the oil cooling, I have no other choice. If I have a failure, somewhat expected, conversion is not really a problem.

Although I have many designs and config's available to me, I cannot offer any of these to fellow rennlisters, as they are unavailable to the aftermarket. If you work for Garrett, you will know what I mean. I did see your post the other day. I could not post such information or pic's.

m42racer

BTW,

We all have gotten off topic here a little. The original topic was about the modification to the CIS system. Sorry Rob. Please have your post back.

Geoffrey

M42Racer,

Nice...thanks for sharing.

Rob S

Folks, no problem hijacking the thread. I've thrown in the towel on CIS and am going electronic. All this banter is very interesting to me and to many others, I'm sure, who are either executing (or contemplating) a switch to EFI. Throttle vs. engine response, oil cooling, injector sizing, batch vs. sequential, the merits of different manifolds, durability of ball bearing turbos, and discussion about different ignition systems is all relevant to this forum. Of course, there's still lots of interest in following the progress of those who are eeking out every last bit of performance from CIS.

Something that's relevant to both CIS and EFI is the topic of inductive vs. CD types of ignition. I think there's been a lot of debate and misunderstanding about this historically. M42Racer (or others), would you care to enlighten us further as to your thoughts on choosing CDI vs. inductive ignition for 930s? What's "the best way to go" and what should be avoided?

m42racer

Rob,

This is not my area of expertise, but form what I have been told, there is no better Ignition type for our engines than CDI. There is a formula for Inductive and CDI Ignition. I do not have it to post. Although we typically do not use high rev's, we are limited to a very large hemi combustion chamber. The reasons given to me were Combustion chamber design and size, generally poor fuel mixing, low Injector placement, sometimes large Injectors, low fuel pressure, to name a few. Many cars have had Inductive ignitions fitted, and have given good results. Most new production systems are now Inductive. Multi coil, sequential Ignitions along with lower interference are the reasons I'm told. They are also less expensive to produce and fit.

I have CDI fitted to my engine (6 channels) and the difference between this and the old Inductive which was originally tried, is huge. Even more so if you are to use 1 or 2 coils only. The spark energy is way higher. My CDI units generate 150mJ of energy. This is a short and very hard spark in comparision to Inductive which is a very long spark generated at lower energy levels. There are some CDI which are multi spark, which generate muliple sparks for starting etc. The idea is a good one, but typically these do not produce any better perfromance than good standard CDi units.

So which is better. My good friend Geoffery will no doudt disagree here, but tests have proven that on our engines CDi Ignitions are far better. Most Inductive systems fitted to our engines are done so because of lower cost and or it is thought that they are our only choice. Not so. There are replacement CDI units for our engines, some really good, some with poor reliability reputations. You must make your pick based upon price and who you talk to. There are other Ignition systems which can be fitted with other pick up systems, but spark distribution is always a problem. For NA engines the factory twin headed distributor is a good choice, but on Turbo engines we cannot fit these. Typically this is why the Wasted spark coil assy's are used. These are ok, but given a choice, a CDI system is way better.

Now that you have decided to use CDI, how do you get control? Again, this is another reason why the Inductive coil pack systems are used. They offer 2 solutions. Unfortunely, they still lack the power of CDI. There is alternatives available to us. I have told of the repaceemnt CDI units available thro Performance Developments before, and of the Ignition system which can be programmed via a Lap top. This system can drive either CDI units or Inductive. The really neat thing they have is a 6 pole distributor driven off the camshaft at # 6 cylinder. This makes for a really low cost alternative for twin plugged engines. You can use the trigger inside the factory distributor and either 2 single coils or a twin output coil dive 12 plugs. Not sure of the cost, but call them and ask. For those who wish to have just a way better CDI for a stock system, they also have a awesome CDI which plugs right inot the factory harness.

Hope this helps. From my own experience, the CDI made the engine run way better.

Geoffrey

m42racer,

I wouldn't want to disappoint you so here it goes...I DISAGREE with your usage of the adjectives "no better", "far better", "always a problem", "cannot fit these" as I think they are too absolute. Actually my problem with your posts from the beginning...

We've used both CDI and inductive ignition systems on N/A and turbo cars with suceess with both methods. I'm not sure it really matters which method you use, but as m42racer suggests, pick your components carefully since there are some unreliable pieces out there.

PS. If you run a twin turbo setup or move the single turbo towards the center, you CAN use the factory 964 twin distributor which works very, very nicely.

Rob S

I wouldn't want to disappoint anyone without probing yet a bit further. It seems that the merits of CDI versus inductive would be (or could be) empirically determined, but unitil we see some real data from side by side comparisons on 930 engines, which may never happen, maybe this will remain too academic. I suspect that there are elements of truth to both arguments, but that the circumstances that led to conclusions on each side of the argument were not equivalent (the old apples and oranges phenomenon), and that for practical usage, one system is superior to the other *for track-oriented 930 application* and one is not as good. But I still don't know which, or why.

For starters, look at this segment taken directly from the Electromotive website, touting their inductive system as superior to CD-based systems:
___________________
Every Engine Control system from Electromotive uses multiple ignition coils and advanced, automatically adjusting dwell circuits to assure the coils are fully charged every time. The powerful spark of this patented system delivers this full spark energy directly to the plugs without misfires. Unlike multi-spark CD systems that from around 3000 RPM just give you a single very short duration spark, Electromotive puts a full 150mJ of spark energy to the plugs with a spark duration more than ten times the duration of a CD spark from idle to 9600 RPM. This long duration spark makes more power!
FIRST SOLUTION: the C.D.(Capacitive Discharge) Ignition. This Ignition does not CHARGE the Ignition Coil, rather it uses the 1:100 Winding ratio as a TRANSFORMER. First the 12 volts of your electrical system is converted to 200-500 volts and stored in a CAPACITOR, when the SPARK is needed the CAPACITOR is DISCHARGED into your Ignition Coil, Instantly producing a SPARK of 30,000 to 50,000 volts with a DURATION of only 0.1 to 0.3 milliseconds (0.0003 seconds)... this is NOT A LONG SPARK !
SUPERIOR SOLUTION: Multiple Coil Ignition Systems. By using an Ignition Coil for every pair of companion Cylinders, the TIME available to CHARGE an Ignition coil goes up by a factor of 4 on an 8cyl Engine. This allows you to use the advantages inherent in an INDUCTIVE SPARK... this uses only enough voltage to ARC the gap of the Spark Plug and dissipates the rest of the SPARK ENERGY in DURATION ! up to over 2 Milliseconds (that's over 90 degrees at 8000 RPM) This MAKES MORE POWER !
So, no matter which of our Products you choose, you will always know that the Ignition System is STATE OF THE ART and READY FOR ANYTHING !
______________

The question seems to be whether it's better to have an inductive ignition, with a spark of longer duration, relatively low voltage and relatively high current, or a CD ignition, which has a relatively short duration (though sometimes multiple), relatively high voltage and low current spark. The spark energy appears to be the same -- Electromotive claims 150 mj for their inductive system; M42Racer claims 150 mj for his CDI. If both statements are true, then performance gains from one system or the other can't be attributed the energy output alone. Is one system "better" and if so, is it always superior, or is it only better under certain circumstances (such as high rpm, high boost, fouled plugs), and worse in others, which is the classic engineering dilemma. And if one is better under, say high rpm, how high are we speaking before it makes a difference? Our engines generally don't rev beyond about 7000, and likely, a shift would be made well before that.

Is there something unique about the 930 engine that would tend to tip the balance one way or the other? The above segment from Electromotive may well be true for a big V8 on a dragstrip, but what about a 930 on a road course or on the street?

m42racer

Rob S,

Its just as well Electromotive sell Ignitions and not Real Estate, or we all would be living out in the desert.

I'm no expert here, but I did discuss this with some other people and they were very interested in what the offical line is from Electromotive. Their comment about the dwell time is meaningless unless compared to another engine configuration. Yes, a 8 cyl engine running at 4000 RPM, is the same as a 4cyl engine running at 8000 RPM. So what. If this is what they mean, then they are just offering up "apples" when we all are discussing "oranges".

Many factors contribute to making power, when talking about Ignitions. It is true that Inductive ignitions generate a longer spark, (time), than does a CDI system. But never does a Inductive generate the same amount of energy, given the same Coil spec's. As for their claim about 150mJ of spark energy, this is completely untrue. The coil packs they supply do not have the amount of Primary Inductance to produce this sort of energy. Another factor here is, when Wasting spark, the cylinder unloaded always provides the least resistance, therefore the spark energy wants to go to that cylinder. This is why resistor Plugs, wires are required. What has this got to do with anything? Well, the energy produced from 1 coil is shared between 2 cylinders. Try to start an engine with fouled plugs with an Inductive system. It will never happen. Thats all POWER. The biggest gain that CDI offers is really the energy supplied over a greater RPM range. 930 engines do not rev this high, as you have stated. CDI's still offer greater energy over the same RPM range. 930/911 engines have very large hemi combustion chambers. At low RPM's non boosted areas, the fuel mixture is required to be alot leaner. The Inductive system will not light off this leaner mixture as well as CDI Ignitions. At the higher RPM's, and boosted, the cylinder pressures are higher, the energy required to produce more complete combustion has to be very high. Inductive Ignition will generate a longer spark and this extended time will produce very good combustion. But it will not be at the same energy level.

All of this is well proven. Inductive Ignitions are simple, very inexpensive in comparision, and produce lower levels of interference. As for producing more HP, not in a million years. The very best system for the 930 would be 6 individual CDI coils. The problem here is $. Each coil typically requires its own channel of Ignition. Then we have to add in a 6 channel CDI. Heres the rub. Its all about $. I went this way, only because I had originally used 6 coils with 6 channels of Inductive Ignition. It was tried to see if we could shirt around the energy level problem, by having individual coils. We didn't succeed. So 6 channels of CDI was used. Wow, what a big difference. I'm told, that 930 owners should use 1 channel of Ignition(CDI) and run either a single coil or dual output coil, via 1 or 2 distruibutors. It could also be done with 2, 1 channel CDI units driving 2 single coils. The problem with 2 channel CDI units, is that each channel cannot be driven at the same time. This keeps everything very simple. The kit is available from PD, to drive the second distributor off the 1,2,3 Camshaft. You have to lose the air pump, but the scavange pump is kept. This makes for a very neat and cost effective solution. Stock Distributor caps, and Rotors are used, and the thought of those expensive RSR parts is gone.

You have to make a decision, and unfortunately there is so much out there, me included that tell a different story. I can tell you first hand I tried both, and the CDI made a huge difference. I'm sure there are those out there that will disagree with me. That maybe my engine was not built right, the EFI system was poor, etc. Well, on my engine with the EFI system I am using, the CDI made a big difference. I would say to those skeptics, maybe you have never experienced the difference.

Geoffrey

M42racer,

Since this made a "huge" difference in your car, would you have any dyno sheets you can post comparing the power differences between the different types of ignitions that you tried on your car as well as the test scenarios? It would be interesting to see really what we are talking about here.

twinplug930

I've noticed some of the great dyno runs that have been posted here, and have a few questions prior to running mine on a dyno. I have a 1981 930 I recently bought, with these mod's: 3.4 ltr pistons and cylinders, Groupe B cams, larger intercooler, K29 turbo, dual exhaust, twin plug heads, adjustable turbo boost. It's a stock 4 speed, with engine rebuild about 9,000 miles ago.

Questions (in terms of all questions, I really want the car to stay in good shape, and am willing to sacrifice a bit of hp and torque numbers on the runs to protect the car):

a. which gear should the car be dyno'd in? (I'm not aware of any mod's to the stock transmission)

b. how much boost should I run for 3 pulls of the test?

c. where should I let them take the car up to in terms of RPM?

d. I think the dyno place in town just has one fan to blow on the back of the car, and it's not huge. Will this fact possibly damage the car's turbo/engine/or anything else due to heat issues? The temp guage usually runs at about 180 degrees and I think I'll tell them I don't want any more runs past 210 degrees.

e. test with the engine cover up or down, or does it matter?

f. the car has an installed temp guage showing outbound air temp from the turbo unit. How high should I let this get? It normally runs about 70 degrees.

Also, on a related note, this car has an installed, not fully connected, HKS additional injector control unit. Any guesses how much extra performance would be achieved by getting it hooked up? Thanks.