sumpcracker

Is 320bhp really the limit fot the AFM?

I really like to have my engine bay period correct, as I have done with the whole car,
I know with my current spec the AFM is a hold up, the car is really fast and already spools fast enough for the 265s to break traction in 4th gear on a cold road, I need to get the car on a dyno, but I am sure from drags with friends cars it is more than the quoted 320hp.. maybe lots of torque..?

Is it that the afm cannot meter the fueling past the 320hp point? Or can it not flow enough air?
My AFRs are good a 1.5/1.6 bar,

I assume back in the day others did my level of tune and retained the AFM?

sumpcracker

This is the spec of the car (easier than typing it all out)

sumpcracker

This is the turbo, Its a Borg warner from a bin lorry with a modified #8 hotside



And this is the WG set up





The intercooler is now a front mount, good for 500hp, the turbo is good for 500ish also.
I believe the 55lb injectors (3 bar FPR) and Walbro 255 pump are happy to 400?
I have had various exhausts but find little gain as the waste gate duping to atmosphere seems to get rid of enough gasses.

My next question is, with larger injectors and a MAF or map sensor set up what sort of gain am I likely to see?

odonnell

931 trans and the FOHA lip.... sweet car!

There are a few other threads around somewhere with a similar question, but to summarize, here are some facts about the AFM. Key to deciding what YOU want to use is to keep in mind that the AFM did it's job pretty well back in day and to be fair, is still pretty reliable and has been a known quantity for tuning for decades. If you migrate away, you are looking essentially at 3rd party or DIY approaches. Luckily there's a ton of support and documentation for those now, too. But here are the main areas you can improve on it from my personal experience (with AFM, MAF, and MAP in my car over the years) and the experience of others.

1) The physical throat of the AFM is smaller than the surrounding tract. Relative to other parts, it is most certainly restricting flow due to that alone. Also, the flap is simultaneously decreasing the throat area even more when it isn't 100% open.
2) The spring loaded flap inside also hurts flow by nature... you have to remove kinetic energy from the air to store potential energy in the spring. Academically, this is the force required to keep the flap depressed being countered by an equal and opposite force... flow dynamic pressure acting on the frontal area of the flap. Read: less dynamic pressure in the flow....less optimal cylinder filling.

That's physics 1401 for it, but I'd posit that 75% of your gains would come through improved partial throttle response, more accurate/tunable fueling and spark advance, and better control of WOT tuning.
so to continue that list...

3) There's physical signal dampening when you are relying on the angle of a barn door as input. So partial throttle suffers due to physical reaction time, plus the energy being taken out of the flow (see #2). This is more of a "sharpness" thing you feel driving around town rather than absolute power. The actual measurement (a voltage) that the DME receives seems to also be subjective, Rogue Tuning has shown different AFMs give similar but somewhat different signals under the same conditions, probably due to wear and age.
4) The flap in the AFM, specifically the angle it is pushed to, is the only way the DME can determine roughly how much air is entering the engine. At a certain RPM (I think it's 4500? for the NA cars) that flap is pushed 100% open. So the DME is basically blindfolded with regard to engine load before you even hit peak power, and dumps in tons of fuel and pulls conservative timing for protection against knock and lean running. This is a primitive solution and is obviously not ideal. LINK
5) WOT tuning is basically the same issue... see link above...the DME can't get the data it would need to calculate an ideal mixture and advance, so it plays it safe.

The alternatives solve for all of those, and can do a lot more, depending what you end up getting. The main perk is tunability. sure you can use a chip with your existing AFM but you still run into the same issues as above if the AFM itself is limiting your power. The Rogue Tuning products, or any standalone, will net you real modern tuning without those compromises. Plus any MAF or MAP setup will obviously mitigate the issues with the AFM impeding flow and relying on a mechanical signal.

In sum, you can definitely improve peak power if that's what you're after. But your real gains will be everywhere else in the power band and partial throttle transitions.

V2Rocket

the AFM door isn't fully open at full throttle until a given RPM...on NA cars it is around 4500 rpm, I suspect it it somewhere similar on the 951.

this poses two problems...
1) part throttle power is severely hampered due to a literal wall in the way of the airflow
2) at WOT past the "open" rpm, the DME is no longer measuring the actual quantity of air coming into the engine - it is just guessing at that point and dumping fuel to its assumed air mass. if you dyno a 944 on stock chips, in the high rpm it goes rich, to the point it may kick black smoke out the tail pipe...

Arominus

Get the rogue maf convertion from Lindsey racing and never look back.