of course it is not as easy as Jean is trying to explain...simply...air, fuel, spark = combustion. Air varies (as well as air density), fuel varies (octane and additives), spark varies (temp.)...theoretical is a nice start, but does not account for the variance (due to several factors). I know the limiting factor in my system right now is fuel...to get any more means injectors & perhaps rails. Not sure I need any more power though.

What is the optimum AFR @ WOT? I recently had a wideband AFR gauge installed. I was under the impression 12.5 was the sweet spot. My car shows a richer mix on 94 octane @ 1.5 bar than what I had thought was optimal.

Around 590hp at 3.8 bar and 100% duty cycle.

Chris is correct. Even Bosch Motorsports injectors are only rates to 85%.

Most 996tt's I tune that are making less then 700 chps are no leaner then 12.5 and no richer then 11.9. This is a blanket statement, and every cars is different. This is where custom tuning comes into play.

91 octane cars I run on the richer side which allow you to get away with 1-1.5 degrees more ignition timing. 100+ octane cars I run alittle leaner.

Every car is different....

Tony and Jean! This is very imformative discussion! Thanks for great writhe up!

my car ARF's WOT are 11.5
my car with hybrid 24's rwhp dyno'ed at 525 with a 5 bar FPR rest was stock and injector duty cycle (stock injectors) was at 85%
for some comparison.

And no one is saying the contrary Tony, this is very true.

85% should be a maximum generally, it does not mean that going to 90% will blow up your engine, just that it is not optimal. I have not read evidence anywhere that would indicate a flow disruption at 90% duty cycle, but I would not say it is not true and certainly would not say that it is wise.

To cut to the chase, this is not rocket science. For some this might sound too technical but it is much simpler than it looks like and it is the only way to answer the question of injector size vs. HP target.

The formula for calculating maximum HP for a certain injector size (or injector size for a certain HP requirement) is simple:



Fuel injectors max HP (lb/hr) = Injector flow rate x Number of injectors x Duty cycle

Choose what duty cycle you want to target and the formula will give you the resulting max. HP. All injectors flow rates are rated at 3 Bar unless otherwise stated, if you want to check at 3.8 bar, you add around 12.5% and if you want to check at 5Bar you add another 15% on top. So the difference between a 3 Bar FPR and a 5 Bar FPR is an incremental 29% approximately.

The only other uknown is BSFC. BSFC is the amount of fuel needed to produce 1 HP for one hour, the lower the number the more efficient the engine. Typically a great efficiency is a value of around 0.5 (1/2 lb of fuel is consumed to produce 1 HP for one hour), however on our Porsche turbo engines you will see 0.55-0.6 most likely if you have a great tune, how do I know, I have the datalogs of my engine dyno tuning sessions. A chassis dyno can also give you the readings obviously with the right sensors mounted. This is the only way to get an accurate BSFC number, but 0.6 is a good enough guess.

So back to the original quesiton, stock injectors on a 996TT with a 5 bar FPR, will give you approximately the following values (and I correct my previous numbers because I used 0.5 BSFC instead of 0.55 which is more accurate)

With the 3.8 Bar FPR and 0.55 BSFC:

At 80% duty cycle: 432 fwhp
At 85% duty cycle: 460 fwhp
At 90% duty cycle: 486 fwhp
aT 100% duty cycle: 540 fwhp (theoretical maximum)

With the 5.0 Bar FPR and 0.55 BSFC:

At 80% duty cycle: 495 fwhp
At 85% duty cycle: 526 fwhp
At 90% duty cycle: 557 fwhp
At 100% duty cycle: 620 fwhp (theoretical maximum)


Amazing how these things never get posted, answers are very straightforward. Use 80% duty cycle and you know you are within very safe parameters, use injectors too large and you will have problems maintaining stable AFRs at low RPMs and drivability, all of this is public knowledge.

There is no such thing as best AFR, each tuner and manufacturer has his own theories depending on many parameters, and one of them is fuel consumption, the other is BSFC and BSAC (or BMEP) and the objective of each engine. My engine runs rich on purpose because my tuner knew that I track at 120F, so more fuel helps with the heat.

This is not theory, this is how these things actually work and get calculated.

If your dyno is showing much higher numbers than these limits with stock injectors, now you know what the reality is

Tom, whatever numbers you have seen above the ones posted above are dyno optimism ( at least in large part) Race fuel improves BSFC.

very informative post.. subscribing

May I ask some questions?
Should the lambda values vary for dfferent gears and different rpms?
Which will be better for perforance at higher gears, richer or leaner?

The fuel system consists of a garden hose (fuel rail) and a nozzle (injectors) with an on/off switch.

The computer can only control how long the nozzle is turned on or off. from 0% of the time to 100% of the time. AKA Injector Duty Cycle.

A higher pressure regulator does just that. Put the fuel under higher pressure so that, for example, at 10% duty cycle (the time the injector is open), you get more fuel coming out of the injector during the time it stays open.

A question of the guys that know about the TT in specific. So... a 5 BAR FPR is not enough for 650 Fly Wheel HP? I t would have to be above 100% duty cycle at that HP level. Which is not possible, unless the BSFC is higher...

K24 cars have a slightly different fuel setup. Is that a second fuel pump? How much fly wheel HP can K24 cars flow in comparison to K16 cars stock?

Lenny, higher fuel pressure allows more fuel to flow at a certain duty ceycle (teh amount of time the fuel injector is open). The flash must compensate by lowering duty cycle to maintain the engine's fuel requirement, but frees uop up soem overhead on teh top end for more fuel. Make sense?

Lambda values are not relevant which gear you are in. This may help: http://en.wikipedia.org/wiki/Air-fuel_ratio

Leaner will make more power, but can detonate. Richer makes less power but runs cooler and is generally safer. Really rich leaves a lot of gunk that can clog up your engine with carbon deposits. It's a balancing act.

I'd be very interested to hear if X50s had altered fuel delivery (other that programming).

uhh...NO