Air Filter Mounted In Direct Air Flow On Front Bar

[Guilt-Toy]

I had my air filter mounted in the front gaurd right infront of the front bar for cold air flow and i have noticed issues when driving over 70ks an hour in top gear the engine would cough and pop and miss all the time especially at over 110ks an hour.

I just upgraded my air filter to a Apexi power intake model which must flow much more then the other ones i was using (long story) and noticed the issue got worse - then i realised it could be air being forced into the filter and going straight past the AFM.. so air was being forced through the afm and the engine does not need to use it.... (kinda like a boost effect through the air filter intake pipe)

So this morning i put a piece of cardboard and covered the gap on the front bar and the drive to work was nice and smooth - perfect as a matter of fact. no missing or coughing or spluttering and the PFC was showing around 3 - 4 less % injector duty.

Going down a big hill i turned the engine off and turned the computer on and the AFM was measuring about 1800 - 2200mv at 110ks an hour with the engine OFF (before i blocked the air flow) - at idle it sits at 1050 - 1100mv - have not done a test after the air flow was blocked yet...

So causing a ram effect on the air filter for cold air feed can cause issues....

I am going to build a box in the front gaurd / front bar and enclose the air filter .. if that dont work i will have to move the AFM to the boost pipe.

Just wanted to share this

[xRHETTx]

Alot of people have always said this would be a problem, its good to have some proof.

Also, you turned your engine off at 110km/h? Thats brave... no power steering etc.

[Guilt-Toy]

Alot of people have always said this would be a problem, its good to have some proof.

Also, you turned your engine off at 110km/h? Thats brave... no power steering etc.

yeah its a big hill.. goes for about a km and its three lanes wide... was just a test!!

[Sydneykid]

Simple solution, put a one way valve in between the filter and the AFM, it will "blow off" any excess air flow.

Cheers

[Busky2k]

^^ Is that sarcasm?

But yeah any direct airflow I've found results in unstable readings for the AFM due to turbulence. Ie cruising at 60kmh the voltage will be +- 50mV. Do the same at idle by raising the RPM to 2000rpm and its pretty much steady (+-10mV)

EDIT - forgot to mention the above was with a CAI feed from the front with a 3" hose.

Edited October 18, 2006 by Busky2k

[Guilt-Toy]

hahaha SK. what the ? the air is only going one way.. its just being rammed in

[TurboX]

Noticed this on the Dyno at Dr. Drift with my CAI

- had to reposition the DynoFAN to stop it causing problems.

Never considered the on road effects before.....

[Sydneykid]

hahaha SK. what the ? the air is only going one way.. its just being rammed in

OK, either you are having a dumb day or I didn't explain it very well. I will assume its my fault and have another go.

What is happeing is the air is being rammed in

1. through the filter

2. into the inlet pipework

3. through the AFM

4. through some more inlet pipework

5. into the compressor

6. the compressor "rejects" what it doesn't need

7. some of this air goes back out through the AFM (ie, gets measured twice)

8. this is air that the AFM has measured (once and/or twice) but it hasn't gone into the engine

9. so the ECU is squirting in more fuel than the engine needs

So what you need to do is stop that reverse airflow. You do that by sticking a one way valve in the inlet pipework (#4 above) between the AFM and the compressor. When the compressor is using all of the airflow, there is no pressure build up, so the valve stays closed. When the compressor is rejecting some air flow, there is pressure build up, which opens the one way valve. A simple PCV valve works OK.

We bleed off pressure from the airbox on N/A race cars (even V8 Supercars) for exactly the same reason.

Does that make sense now?

cheers

[mad082]

what sk is talking about is basically a pressure valve. it is kind of like the pressure valve in a hot water system, or pressure cooker. when the pressure gets to a certain point the extra air will go from inside the plenum, out the valve, and into the engine bay.

could it possibly also be your plugs? what gap, how old, etc are they. if the extra air is getting into the motor it could be blowing your spark out.

Edited October 18, 2006 by mad082

[Guilt-Toy]

I never considered that the air could be rejected by the gt35r compressor... Interesting.. i think in this case because the AFM is connected straight onto the air filter sitting right in the front bar the air is just being rammed through the AFM and telling the computer that it using more air then it needs...

On my AFM it is connected to a 4" pipe which feeds up into the engine bay and then onto the compressor housing - probly around 70 - 80 cm of piping or more by the time it hits the compressor - i assumed it would just compress not surge back down the pipe.

Can you get a 4" pcv valve ? where would you get it from??

If i enclosed the Air filter / AFM in a aluminium enclosure and had a smaller entry to the box so air does not directly hit the air filter would that fix the issue as well as the pcv valve?

OK, either you are having a dumb day or I didn't explain it very well. I will assume its my fault and have another go.

What is happeing is the air is being rammed in

1. through the filter

2. into the inlet pipework

3. through the AFM

4. through some more inlet pipework

5. into the compressor

6. the compressor "rejects" what it doesn't need

7. some of this air goes back out through the AFM (ie, gets measured twice)

8. this is air that the AFM has measured (once and/or twice) but it hasn't gone into the engine

9. so the ECU is squirting in more fuel than the engine needs

So what you need to do is stop that reverse airflow. You do that by sticking a one way valve in the inlet pipework (#4 above) between the AFM and the compressor. When the compressor is using all of the airflow, there is no pressure build up, so the valve stays closed. When the compressor is rejecting some air flow, there is pressure build up, which opens the one way valve. A simple PCV valve works OK.

We bleed off pressure from the airbox on N/A race cars (even V8 Supercars) for exactly the same reason.

Does that make sense now?

cheers

[Guilt-Toy]

Well if i did that then the AFM would be reading air that would be escaping from the valve which would make the engine run rich. Yes ?

I use copper plugs and change them every 2 months. i run .7 .8 gap.

what sk is talking about is basically a pressure valve. it is kind of like the pressure valve in a hot water system, or pressure cooker. when the pressure gets to a certain point the extra air will go from inside the plenum, out the valve, and into the engine bay.

could it possibly also be your plugs? what gap, how old, etc are they. if the extra air is getting into the motor it could be blowing your spark out.

[Sydneykid]

Well if i did that then the AFM would be reading air that would be escaping from the valve which would make the engine run rich. Yes ?

Yes, but it would stop the AFM "reading" the air flow twice, which is what is happening now.

Plus it sounds like you have the AFM located in the rammed air tubulence. It may be worthwhile moving the AFM a bit further away from the filter. I understand why you have it as far from the compressor, it helps with any reversion problems. But somewhere in between might be beneficial. At least it would move the AFM out of the turbulance and it would be getting only straightened airflow.

cheers

[Spook's_Skyline]

i had my pod mounted directly in front of my left front wheel where standard intercooler used to be for a few years, well, it was right behind a vent in the front bar

never had any issues with it at all

only removed it coz the pod would get so dirty, especially in the wet

Edited October 18, 2006 by Spook's_Skyline

[Guilt-Toy]

i think rather then moving the AFM (unless i move it to the boost pipe on other side of engine bay) i will just enclose the filter and and afm away from the direct air flow.

[assyrian4evaa]

intresting...

[R31 POWER]

I've wondered the same Guilt-Toy. Even tho i dont have a cold air intake i have noticed when backing off light throttle the afm volts go back and forth higher then before i backed off LOL. When it does and i change gears it wants to stall, pisses me off.

I think i might get rid of the PFC as it's shitting me.

[GeeTR]

Interesting indeed’ally, iv been playing around with how im going to make a CAI for a BNR32 running a single turbo and twin AFM’s.

I’ll be pushing most of the air that goes into the passenger side front bar vent into a effectively sealed box containing the filters. My thinking gets me as far as, if it’s the speed of the air going into the vent, that creates a higher then normal (read, higher then the car was tuned) pressure at the turbo mouth then I could as easily vent air off at the air box. I reckon a large 250x250mm spring load flap, on top of the CAI would relieve enough air at higher vehicles speeds to prevent the pressure spike AFM double reado thingo. Seems safer then one way valve, introduces another possible fail point.

All this, and a cossie bonnet vent over the CAI, (mounted the right way :0 ) that seals hard against the bonnet might do.

I guess I could record AFM outputs, injector times, some air box pressure readings, switch on the flap lid cross match vehicle speeds blah blah and write a super nerdy write up… but could one be f*ked  Hahahaha

- M

Edited October 18, 2006 by GeeTR

[Guilt-Toy]

thats a interesting idea.. i am just going to make it so air can go in and out of the box easily so its allways got fresh air in it and have the air filter sitting below the air flow

[khunjeng]

so if I'm reading correctly too much air ramed in is not doing you any good.

I setup a box over the pod and AFM and took away the stock intake. I used a 3" pipe from the grill which comes in under the pod - using the stock IC hole..I havn't noticed any issues... I would assume this setup is basically the same as stock with more air and a more direct flow into the CAI area however the differnece is that its not in direct airflow at the bar...yet I would assume the 3" pipe would allow a lot of air to get in...maybe its enough of a baffle to avoid the issues being discovered.

Its not totally sealed...it has gaps on the sides..I never worried as I assumed so much air was getting in there it would keep away the hot engine air...after all that was the aim of the box...I wont bother to seal it up perfectly then

Myabe you don't need such a good flowing filter under such circumstances...as you have so much air its not really a flow issue. I have been using a UniFilter which has less flow than the pod mentioned but filters much better...just another thing to consider IMO...

[Guilt-Toy]

Thats what i was thinking.. a box that is not sealed so it can get cold air as well as not have so much pressure at the same time.

See how it goes at the moment the piece of cardboard is working wonders

so if I'm reading correctly too much air ramed in is not doing you any good.

I setup a box over the pod and AFM and took away the stock intake. I used a 3" pipe from the grill which comes in under the pod - using the stock IC hole..I havn't noticed any issues... I would assume this setup is basically the same as stock with more air and a more direct flow into the CAI area however the differnece is that its not in direct airflow at the bar...yet I would assume the 3" pipe would allow a lot of air to get in...maybe its enough of a baffle to avoid the issues being discovered.

Its not totally sealed...it has gaps on the sides..I never worried as I assumed so much air was getting in there it would keep away the hot engine air...after all that was the aim of the box...I wont bother to seal it up perfectly then

Myabe you don't need such a good flowing filter under such circumstances...as you have so much air its not really a flow issue. I have been using a UniFilter which has less flow than the pod mentioned but filters much better...just another thing to consider IMO...