Throttle bodies and airflow meters

[dontfeelcold]

was wondering if running the airflow meter after the throttle body would work properly.

when running a bov with an afm, i have heard of engines stalling due to being flooded because air is still going through the afm (and out the bov) when the throttle body is closed. you could run no bov but that puts strain on the compressor blades.

just wondering if this would work.

[RB Fairlady]

A post-TB AFM would not work. When the TB is closed, there would be no air flow. And #2, the AFM isn't calibrated to work in that location - Most AFMs are supposed to be before the turbo i.e. non-pressurized.

The major stalling problem is when your dealiting with venting BOVs. THe ECU sees air going into the turbo/intake track, but when the BOV vents to the atsmosphere, the ECU looses track of the air, so at the subsequent idle, the ECU thinks there is way more air, runs pig rich and drowns itself.

A remedey of that is going to a different AFM type, or getting a computer to mount the hot-wire afm in a new place. Most aftermarket computers replace the stock AFM with a MAP (manifold absolute pressure) device that doesn't read any airflow in the track, but regulates fuel by intake pressures.

[dontfeelcold]

afm before the turbo, didnt think of that.

is the r33 rb25det afm before the turbo?

[redline2003]

yeh i think ALL turbo cars hav the AFM b4 the turbo

[mambastu]

I'm running a Z32 AFM on my CA18DET in a blow-through arrangement at the moment. A GReddy E-manage controls the fuelling.

Heres a picture of the installation. Its bodged in at the moment because I just wanted to check it worked before trying it on the RB25DET I'm installing in January.

Seems to work well now its mapped and it overcomes the overfuelling problems I used to have with atmospheric BOV's.

Heres a picture that somebody posted recently of one fitted to an RB too.

[Clicks]

hmmm - plastic afm.... warm compressed air.... plastic afm.. warm compressed air... Dosn't sound like to good of a combination to me... Good luck with it though by all means!

[Sydneykid]

Hi clicks, "plastic afm.. warm compressed air..." How about plastic AFM, 35 degree compressed air (after the intercooler, so not too warm) compared to 80 degree engine bay. I think the plastic can handle the temperature pretty easily.

I have seen quite a few drag GTR's in Japan with a Q45 90 mm AFM plumbed in between the intercooler and the throttle body. Just align the ID of the intercooler pipework with the ID of the AFM, no restriction there. They seem to work fine, but I must admit the first time I saw I was a bit perplexed.

When you think about it though, it's not such a silly idea. It fixes the atmosphere vented BOV problem, removes any possibility of restriction on the turbo inlet and it eliminates the air reversion problems of running a big turbo/turbos close behind the AFM. Best of all you get to run an AFM and not have to resort to the less precise MAP sensor driven ECU.

So mambastu, let us know how it works out. I am particularly interested in what the voltage ramp table looks like.

[Clicks]

Sydneykid. I never thaught of it that way. I guess it was just a first idea that came to my head and set off the alarm bells.

Given the oppertunity would you do it?

[mambastu]

Sydneykid. I never thaught of it that way. I guess it was just a first idea that came to my head and set off the alarm bells.

Given the oppertunity would you do it?

When I fitted it I wasn't too worried about heat but I was worried about the boost pressure as the RB with the blow-through setup looks like its got a metal cap on it. Did you notice that on the cars you saw in Japan Sydneykid ?

I have noticed that the MAF is very sensitive to throttle position in its new location, presumably because its so close to the throttle body. When the RB goes in I'll use a GTR34 intercooler and move the AFM much further away from the throttlebody so hopefully the airflow will be a bit more stable.

Does anyone know if a hotwire AFM can cope with water vapour OK ? I was planning to use water injection at the intercooler outlet end of the inlet pipe which means that it would be blowing a water/air mix through the AFM. This seems (to me anyway) to be the best way of doing it as the AFM would compensate slightly for the cooler charge but if it will cause reliability problems I'll run the jet after the AFM. The WI system I'll use will be mappable anyway.

[riceline]

I realise im bringing up an old thread here..

But how have people gone with this?

[mambastu]

I realise im bringing up an old thread here..

But how have people gone with this?

I'm now running it on the newly fitted RB25DETT in my S13

It works fine, in fact a lot better than it did on the old CA18DET. The AFM is level with the O/S/F wing so theres a reasonable amount of pipework on either side and the airflow is now a lot more stable. Haven't had the car on a dyno yet but I have used an AP22 perfomance meter on it and over a few runs it was averaging a peak of 330bhp at the rear wheels on 14psi.

I've got a water injection kit waiting to go on before I raise the boost a little more and finalise the mapping. I haven't touched the ignition map yet.

[Warpspeed]

I agree with Sydneykid, it is a very good idea, but some caution is needed.

Provided a decent intercooler is being used there should be no problems, but there might be if there are very sudden temperature spikes reaching the airflow meter. Hot wire airflow meters will track air temperature changes very well, but they were never designed to do this extremely rapidly.

[GunMetalR33]

this might be a stupid question, but what is a Hot wire AFM?

Can I have this setup using the standard AFM?

[Warpspeed]

A hot wire airflow meter is what you already have in your Skyline.

It works on the principle of heating a fine wire to a temperature that is higher than the air flowing through the airflow meter. The air flowing through the flowmeter tries to cool down the wire, so the electronics in the flowmeter bumps up the wire heating current to keep it at the correct operating temperature. That is how they work.

As more air flows into your engine, the airflow meter wire requires more heating current to keep it hot, that current also provides the flow signal back to the ECU.

The circuit that decides how much current to feed into the hot wire, also needs to know what the air temperature is going through the flowmeter so it can adjust wire current to keep the wire at the correct amount hotter.

This usually uses either another wire, or a bead thermistor to measure air temperature. This usually just monitors entering ambient air so it does not normally have to respond very fast to outside air temperature changes.

If you installed the flowmeter right at the turbo outlet, the massive fast changing temperature swings would give it a mighty hard time trying to work out what was a temperature change and what was a flow change.

A decent intercooler SHOULD keep the inlet air temperature to the engine reasonably constant, or at least it is not going to go up and down by 100C in one second when you change gear.

[Steve]

How would the chill factor of spraying water through an AFM effect its accuracy? sounds, form what you posted above Warpspeed, that this may cause troubles?

[GunMetalR33]

So basically if you situated the AFM right after the intercooler and you have a decent intercooler this theory should be sound...

I would agree with you steve, I think from what Warpspeed said that water injection right before the AFM will cause troubles!

Thanks for the info WarpSpeed

[Warpspeed]

I had never thought about upstream water injection with this setup.

Thinking about it, it may very well cause some problems. Best ask one of the heating ventilation and air-conditioning engineers (HVAC) about this, any of you guys out there ?

How does sensible cooling vary with changes in humidity ?

If for example, you blow 50C relatively dry air, as compared to blowing 50C 100% relative humidity air over a hot dry object, how does that effect the heat transfer into the cooler airstream ? ? ?

Warpspeed who knows nothing, is always eager to learn something new......... !

My gut tells me that water has a far higher specific heat than dry air, and more of it is going to suck out more heat, but I would like to hear from someone that really knows about this stuff.

And if it's true, how does a standard hot wire AFM cope with large changes in relative humidity ?

[riceline]

one thing i would like to know is what about oil blow through geting onto the afm?

how does this affect the afm's ability to work correctly?

[ylwgtr2]

havent people been doing this on VL commodores for years?I remember seeing it a number of times and i questioned the life span on the afm considering that they had enough trouble in there original position let alone down stream....they seemed just as happy down stream though

[R31Nismoid]

havent people been doing this on VL commodores for years?I remember seeing it a number of times and i questioned the life span on the afm considering that they had enough trouble in there original position let alone down stream....they seemed just as happy down stream though

i totally agree.

more than 70% of the VL turbos ive seen (i seen a lot) down here in Vic have the AFM after the I/C and before the T/B.

They run fine, infact, they run extremely well with no problems at all.

Seems to be a standard/accepted mod as most of them run no BOV to get the ful-sik 'choo choo'. So it seems to stop the stalling etc etc being close to the T/B