JohnM

Hi John,

Generally injector sizing for race engines starts with working out the flow required to achieve an injection duration of 360 degrees crank angle at full load and max rpm. Knowing that and the fuel rail pressure that is to be used means the flow rate is defined, the particular injector that is chosen (more specifically, its minimum linear pulse time) will then determine whether there is sufficient dynamic range to give reasonable accuracy for part throttle fuelling. If not, then a twin injector setup generally comes into play, with one injector used for part load and the other or both for full load. The part load injector can be lower or the same flow rate as the full load injector. Twin injectors can also be considered even when dynamic range is not a problem, because all other things being equal a lower flow rate injector will have better atomisation, so you can get improved mixture preparation from twin injectors compared to single. In either case the engine management system needs to be carefully set up to properly manage the transient effects during the transition between single and twin operation.

fusionsport

twin injectors is generally a solution when the injector size needed to deliver enough fuel flow is so large they do not respond well at idle- in plain terms they operate effieciently at say 35lbs/hour at 810ms, but when asked to idle and deliver 11lbs hour at 410ms they cannot respond fast enough to not flood the engine-in which case a second set of injectors is installed. There are two schools of thinking here- A) to use small idle only injectors with the reasoning the car only idles in the paddock
B) To use a large injector as a main, but with a stable idle-, and use a secondary "extra" injector when on boost or at max load/rpm.
A lot of small displacement turbo engines have this issue, as to get the mixture at max boost they have to use ungodly huge injectors, but the engine is soo small that when on idle it floods itself out. There are a couple of different ways they try and get around it, like using rising rate fuel pressure regulators, extra injectors in the intake,etc. fortunately with most Porsche cars this isnt neccassary at all, as we can use large enough injectors to supply the fuel requirements without sacrificing much idle, and our engines require a bit more fuel at idle, which also helps.
Nitrous applications also often require a second set of injectors, but that is another issue

johnfm

Ok, so here is my thinking for my C2 tip:

Cat bypass, primary bypass, big bore throttle to improve volume of air getting into the beast.

Unichip piggy back and either MAF OR use the throttle pot that is already part of the tiptronic throttle assembly to run things using alpha-N.

Either larger injectors OR rising rate fuel pressure regulator to provide extar fuel at WOT, without compromising idle & part throttle feul delivery.

This MAY be an economical route to 300-315 HP. I say MAY, as MoTeC with big injectors, throttle pot(for alpha-N) etc can give 315-320.

Now, MoTec doesn't have knock control as fas as I know, BUT unichip may not have correction maps ability??

I'm still confused!!!

Tim

We built a turbo engine with 2 injectors per cylinder, throttle bodies, a plenum , and a tremendous intercooler

The engine management was set to run the first set of injectors until 1 bar, then the second set was fired at pressures over 1 bar. We have seen some incredible power outputs from this engine. And have seen the other side of 200mph on the IMSA course at Pocono

It seems like the hp of this engine is limited by the amount of fuel that can be moved through it..We havent run above 1.5 bar yet

fusionsport

quote-
Ok, so here is my thinking for my C2 tip:

Cat bypass, primary bypass, big bore throttle to improve volume of air getting into the beast.

Unichip piggy back and either MAF OR use the throttle pot that is already part of the tiptronic throttle assembly to run things using alpha-N.

Either larger injectors OR rising rate fuel pressure regulator to provide extar fuel at WOT, without compromising idle & part throttle feul delivery.

This MAY be an economical route to 300-315 HP. I say MAY, as MoTeC with big injectors, throttle pot(for alpha-N) etc can give 315-320.

Now, MoTec doesn't have knock control as fas as I know, BUT unichip may not have correction maps ability??

Knock control is worthless- it is something people who know little to nothing about engine management talk about when trying to act like they do. Why? Because all a knock sensor is is a simple transducer/microphone and it is s-l-o-w in relation to what is happening inside any racing engine. Production units can use it cause they loaf along and run conservative compression/cams/mixture settings. They run on the knock sensor becuase it is the only way they can get the cars to pass ulev and still make any power at all. Trying to tune to knock sensors is garbage, almost like letting the lambda make your map- yes it is kinda safe and easy, but it is nowhere near optimal. In my engine management set-ups i have a switch the driver can use to enable Lambda tuning, and the ONLY reason it is there is to get the car to run IF he had to swap ecus and lost the maps, or he sucks in half the track and clogs the air filter, etc- things that will in actuallity never happen, but it is a safety net to keep the car running until a map can be reset or the car fixed.
The unichip looks very very promising, but I decided three years ago to not use or recommend it even though I was on the verge of being a dealer for Unichip. The reason is that it is a false economy compared to MOTEC, or DTA, and I have come to not believe at all in piggybacks or other stopgaps.
The Motec might be able to deliver the fuel needs using stock injectors for 315 hp without the need for a rising rate regulator. The problem with trying to tune to the rising rate is that it can screw with the mapping if you have a inexperienced mapper.
The rest of the set-up sounds good, although if you go through so much expense I would high;y recommend swapping cams and headwork by someone that knows what they are doing.
Feel free to give me a yell if you have any questions-

fusionsport

Tim-

how much power to the wheels? We got a touch over 780 atw with a twin turbo MOTECed 911, about 3.5 litre if I recall correctly. It clocked 194 through the traps at Daytona at the 935 challenge a few years back. And that was a single injector system. I am just curious as to you r specs as I would assume our injectors on that car were good for more power at higher boost levels.

JohnM

Whether you can get useful info from a knock sensor on a road or race engine has nothing to do with cams or mixture settings, nor is the knock sensor "slow". The main problem on race engines is that vibration levels are so high that the signal-to-noise ratio from an acoustic sensor is too low to distinguish the characteristic frequencies of knock occurring. Engines that rev very highly can have additional problems in that there can be other sources of vibration freqencies that match the knock frequencies (especially from parts of the valve train). It is worth bearing in mind that the exact frequencies that characterise the knock are specific to the particular combustion chamber, so modifications to the pistons or heads would invalidate the use of a sensor tuned to the frequency of the original engine design. Various kinds of combustion pressure transducers are, on rare occasions, used to detect knock in racing applications with relative immunity from vibration issues. In practice, knock does not occur in racing engines with well designed combustion chambers and correct fuel.

Tim

We ran out of dyno on a 750 hp mustang dyno

Ill have to check the other details, but I think the primary injectors were around 80 lb, and the secondaries were about 50 lbs..

The engine is a 3.3, lots of head work, single(huge turbo) -8 fuel lines. two 100 paxton pumps

The cam grind is custom by web-cam, I guess only Laurie and my friend know the specs

The fuel is usually c-16

We built this 2 yrs ago, so Ill have to see my friend and get the details.

fusionsport

The batmobiles(every car had a nickname) set-up was two Bosche hi-output pumps, cant recall the exact injector sizing but only needed one each cyl, two t-28s(I think- we tried several before getting a version we liked, seems I recall it was two ball-bearing t-28s, mighta been a t-3 or t3/4 hybrid) -8 lines, running a wicked fuel supply setup with a surge tank and a holley low-pressure feed pump, never had much issue until the 964 twin-plug distributor(will never use another spinner in any performance engine) jumped a couple of teeth on the belt and melted a piston- this was with over 70 race hours on the motor, not including dyno time, cams were either SC or custom, again we tried both. The shop that I was with that did the work now has a 2.5 litre twin-turbo 914-6 dropping 684@the wheels on 17lbs boost