AFM Questions.

[GTR-Ben]

Ok we have been having a discussion on SDU about AFM's, inline, increasing the ID etc to change the amount of power they can handle.

I thought I would make a post here and get some other opinions.

Basically for my RB30 being in a S13.. I would like to save some space. My turbo has a single 4" opening, and I am running a Power FC so need to keep the AFM's.

Now instead of running two RB26 or Rb25 AFm's as a stock setup I wanted to consider running a single AFM in a single pipe, but double the physical ID so the overall flow would be the same as if there were 2 AFM's. Then wire it so both AFM wire get the same signal to the PowerFC.

The ID of a RB25 AFM of 76mm when the area is doubled would come out close to 4" the same size as my turbo inlet.. which makes life easy.

I have a feeling, that by installing the air flow meter probe into a pipe that has double the surface area.. this may mess things up. First of all the lower air speed effecting idle tuning? Will the resolution be halfed or doubled?

Now I know there are cars running fine with AFM's Post intercooler pre throttle body. But these keep the original ID.

Any one have any more "physics" related answers.. or have actaully tried this?

Thx

[Abo Bob]

I can't believe no one has bitten yet Ben.

This sounds quite interesting.

Thought it deserved a bump.

[paulr33]

well i guess logic would prevail, if the diameter of the intercooler pipe to throttle body is 3 inch then using a 3" afm technicaly would be fine, i think running the vh41 afm would suit it fine as its 3" and is in the same range, and powerfc has support for it. would you honestly see yourself running a 4" intecooler to throttle body piping?

[Cubes]

The ID of a RB25 AFM of 76mm when the area is doubled would come out close to 4" the same size as my turbo inlet.. which makes life easy.

I'm not so sure it would work too well for low rpm as AFM's are calibrated non-linear.

What I have observed is that the RB20 and Z32 AFM's are calibrated so low airflow levels have a high resolution.

The difference between the resolution with the RB20 and Z32 afm was at the higher airflow levels.

However, there is a very slight difference in the start of where x amount of air begins to read y volts, I put this down to the z32 coming from a 3ltr hence it does not need the lower airflow resolution as the larger motor never sucks so little air.

In other words, I don't think your setup would work well for low rpm potting around, fuel economy and light throttle or even full throttle @ low rpm (not sucking much air) may suffer.

I really have no idea but thats what logic tells me.

In my web travels I remember coming across a little trinket that converts a MAP signal to an AFM signal for the ecu to understand.

Maybe worth a look in to.