Airbox and airflow question...

Right, one for the engineers. I recently replaced my old (heavily cutout) airbox with a standard G60 one, just to see how much of a difference in noise there was. Now I quite like the standard box for the tone it makes. But I noticed that there is very, very little clearance between the front of the helmoltz resonator where the air comes in and the back of the headlight...
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Now I'm presuming this flows less than my old box given the tiny gap. So my question is - how would I know whether this is flowing less air than my old box without using a AFR gauge for example? My though was that if it was flowing less at full throttle then the boost pressure would be slightly lower than my heavily cut-up box. Does that make sense?

You could place something hot over the entrance to the snorkel (helmotz resonator?) and see how quickly it cools down.

Not massively technical, but if the air is moving slower then the hot thing will stay hot for longer.

God, I should study engineering...

otherwise a water to air manometer is easy enough to rig up :-) I thikn you probs know what one is though.

There is some simple theory that can be applied here. Look at the diameter of the intake of the charger and calculate the cross sectional area.

Next look at the intake of the air box and calculate the cross sectional area. This needs to be the same/larger than that of the charger in order not to be a restriction.

Lastly measure the distance from the air box inlet to the back of the headlight and work out the skirt area (circumference x distance) The area of this again must be the same as the intake of the charger or greater.

Long and short of it if the charger has the smallest cross sectional area then the standard airbox doesn't offer a restriction in theoretical terms.

Obviously in practice there will be variations based on intake air temps, if these are lower then you will make more boost to air density rules, subsequently the extra holes if correctly placed will provide a cooler and therefore denser intake charge and potentially slightly higher boost.

Now there is some other theory you need to look at which is the velocity of the airflow. Every hole has something called a choke factor in respect to airflow, at a specific velocity the intake of the airbox will hit a stage where it physically can no longer flow anymore air even if you tried as the velocity of the airmass entering it exceeds choke factor.

Providing the airmass does not exceed choke factor in respect to it's velocity you can get away with feeding the charger with a smaller cross sectional area than the intake of the charger itself without adverse power loss.

^^^^ With respect to the manometer idea that's not a bad one. I use a digital one on my flow bench and have thought many times of how to utilise it correctly for use on a car in real time. I wouldn't use an atmosphere manometer as reference though as pressure drop is not an accurate measurement for airflow as pressure drop is linear and airflow is not.

The way I would go about it is measure pressure drop as reference and then concentrate on velocity using a pitot tube. That way you can actually check whether the intake of the airbox is restrictive.

Putting all that aside there is a much simpler way though then all of the above , providing you hafe a MAF fitted to the car. All you do is tap into the MAF 0-5v signal and datalog the infromation through the rev range on WOT. If you have a lower voltage you have less airflow and visa versa.

/\ Great info!

JMC, what's the inside diameter of that rubber bellows in the picture?

I need something exactly like that, 83-85mm ID!

The small gap between the intake and headlight is a puzzler. They did the same thing with MK2 Golfs. Somebody at VW must have looked into it and deemed it adequate. Mind you, you're running more boost than standard

If your G60 box is like the VR6 one underneath, then you can graft in a nice 3" guttering elbow that pokes directly down into the carbon can hole.

Yes thank for the info guys - helpful as always.

I'll take some measurements tonight for you Kev. On my old air box iIbasically removed the bottom right hand side and I have a tube coming up through the carbon canister hole which ends up behind the lower front grill. Also I relocated the outside air temp sensor into that tube to get a feel for incoming air temps. The reason I have tried the standard box is because I wondered what it would sound like, and to be honest I prefer it over the cut one. My main concern though is around air flow - even though I'm running less boost I'm flowing more air, or trying to anyway.

Thanks JMC, I'd appreciate that. If a G60 rubber elbow fits, it will save me a lot of trouble trying to hunt an alternative down on the net!!

Does the car feel appreciably slower with the standard box?

No it doesn't Kev, although to be honest I need to get it out on the open road, which I should be able to do tomorrow. I'll get the measurements sorted this eve...

Encouraging so far then. I suspect if there is a restriction, it won't be noticeable until the upper end of the rev range.

I'm especially interested in this because I'm also reverting back to the standard VR6 airbox on my VRT.

The corrugated tube is 69mm internal diameter where it joins on to the airbox Kev. So too small for you Mid range it seems fine, but as you say it'll be at the top end where it struggles if anywhere. I feel one of my rolling road sessions coming on...

Something that JP mentioned about the MAF signal - this is going to show my ignorance, but does the G60 even have a MAF ? All that comes to mind when I think of thing monitoring air flow and pressure is the CO pot and MAP sensor in the ECU.

Have to say haven't looked at a G60 setup properly, I know the digifant mk2's use a primitive MAF setup build into the airbox. Not sure if the G60's do. ECU's are not really my forte. Doesn't stop you fitting one though and only using it as an external measuring device

No they do not have a MAF, just as you said it has those.

You need to buy your own rolling road Jonathon with the amount you spend, some people do thousands of miles on the 1/4 mile, you could do them on the rolling road, think of the advert when you sell, never used in the rain, only used in the garage, rolling road miles only lol

jmc wrote:The corrugated tube is 69mm internal diameter where it joins on to the airbox Kev. So too small for you Mid range it seems fine, but as you say it'll be at the top end where it struggles if anywhere. I feel one of my rolling road sessions coming on...

OK, back to the drawing board then! Thanks for checking!

Yeah get it on the rollers again, I'm keen to see the difference in the plot at the top end