Tomas L

Because of the logaritmic shape of the flow/voltage curve of the VAF (this also applies for a MAF) it's hard to tell the flow limit. The higher flow, the lower the measuring accuracy will be since the flap movement per cfm will be lower at high flow rates. The limit where the VAF bottoms out can therefore be at quite high flowrates, it's just that you have lost most measuring accuracy long before this.
I'm sure the VAF does a good job under the circumstances it was designed for. I just don't think you should expect it to perform well when you try to flow 50% more air than it was designed for through it. And I'm more concerned with the accuracy than with the pressure drop, although lower pressure to the turbo inlet will force it to work harder with increased charge heating and backpressure.

Tomas

TurboTommy

"although lower pressure to the turbo inlet will force it to work harder with increased charge heating and backpressure."

Exactly,
like I said , you can make higher power through the AFM, but the law of diminishing returns will apply.
For example, if you have achieved 375 crank hp with the stock AFM, you'd probably get a little over 400 hp just by changing to a MAP or MAF (changing absolutely nothing else except proper tuning). This difference will increase drastically the more power you wanna make.

Laust,
I believe this to be true: if the barn door is fully open, it already took quite some work from the moving air to flow through the opening and overcome the spring. Therefore the pressure drop is already happening in order to get the flapper to be in that position.

Laust Pedersen

Tomas, I don’t hope you are referring to the graph in post #2 as a flow/voltage curve. If you do, the logarithmic or asymptotic behavior could very well because the HP is constant or dropping slightly above 4500 rpm.

TurboTommy, Good understanding of qualitative relationships are necessary, but only the first step. Quantitative analysis/relationships are often needed to make go/no-go decisions. So, do you want to venture numbers for the pressure drop caused by the “barn door” only? If you do I will post my guesses with some justification as well.

Laust

Weston Dillard

Interesting topic. I have a running argument with a buddy of mine about this. My car made 328 HP to the wheels through the stock AFM. This is not a stock car, BTW. The HP number was way below the torque number though, which indicated to me some sort of limitation at high revs. I was using a Autothority J pipe at the time which has a restriction just before the turbo, presumably to do something clever when using the stock turbo. Anyhoo, I think the restriction was in the J pipe and my MAF-running buddy thinks it was the AFM. I've changed the intake setup and will see what the dyno says next time I get over there.

West

Tomas L

Laust, no I don't mean that curve. I mean the sensor volt/flow caracteristic. Both our VAF and all MAF I've seen graph's on have a logarithmic volt/flow curve. High resolution at low flow and much lower resolution at high flow rates.

Laust Pedersen

Tomas, I see, and believe you (without having seen those graphs). Odd though logarithmic/exponential potentiometers have been around for a long time and custom characteristics should be easy to make on a thick-film, which I am guessing is the circuit substrate on the AFM.

Do you know what the flow is at 90% voltage?

When getting my black car it was obvious that the chips from my white car didn’t work well. It needed much less fuel at low flow including idle. I finally decided to switch AFM’s and accidentally discovered that the barn door was much easier to move on the installed AFM and crudely measured the difference to be a factor of 3 . Somebody must have been looking for more fuel without changing chips and played with the return spring (at the cost of max flow registration).

Tomas L

I have no specific knowledge of the flow capacity/output level for the VAF. I've just seen a general description of this type of flowmeter.
Here is flow voltage curves for Bosch HFM5 MAF:s.

hally

So what are peoples opinion on this bang for buck hypothesis: spending 850 on a TO4E Turbo and hooking it up with the stock j-pipe & AFM should be able to achieve ~310rwhp and more area under the torque curve than adding a MAF / MAP kit to a stock turbo.
In terms of tuning i was thinking that i should be able to use the same fuel and timing maps that i have for the k26/8 other than the WOT fuel map which i could easily dial in with the wideband eprom burner combo.
Is it easy to properly secure the stock j-pipe to a TO4E, what adaptors, clamps do you need? I would be borderline needing 55# injectors too I guess, but that part of the problem is easily solved and exists either way.

FSAEracer03

The j-boot goes down to 2" for the K26 housing inlet. The TO4E, however, has a 3" housing inlet.

Bengt Sweden

hally,
I have seen cars with the J-pipe just cut down to fit 3" inlets. Any properly sized hose clamp would do.
You ar probably ok with your mapping idea but I would recommend the larger injectors.
Bengt

FSAEracer03

Bengt, I was considering the very same idea... but how much of the J-boot is cut off in order to get to the 3" I.D.? If it's a pretty good amount, then a spacer would obviously be needed. How ghetto-rigged does this get, I wonder. I don't have my car with me to inspect the J-boot, unfortunately.