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How to roughtly calculate the air box volume with panel air filter for a 4 stoke engine ?

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I am trying to build an airbox for a NA engine? Set up is with a single throttle body going to a plenum with a RF.

The engine is a Porsche 3.6 litres

I wish to make an airbox with a panel filter then from inside the airbox run a 200mm long pipe with a trumpette connecting the throttle body and of course with the airmass sensor connect to the pipe.

My guess is a air box of about 6 litres volume.

Would anyone know a way to calculate the volume ?

Actual power is 295 bhp, with a few mods, improvements and a remap, I am hoping + 10 to 20 bhp increase.

Thanks a lot

Great question. I've worked on projects where this was done with engine analysis software, or by an aerodynamicist in conjunction with me as the calibrator offering details on expected max mass flow.

How the air gets into the box is a key factor. Weighing volume vs. inlet pressure is an important consideration. For example 4 liters with 1 psi inlet pressure would be my preference to 6 liters with 0 psi inlet pressure, if making the box smaller allowed you room for better inlet ducting.

Others will hopefully chime in with calculations they've found success with, which can get rather complex. For now, I'll offer a pragmatic solution. In a situation where max HP is the goal rather than OE production which has far more considerations, I've not encountered a situation where an airbox was too big and it hurt performance. Bigger has either been better or offered no gain as long as inlet pressure pre filter was constant. I've seen a 5" intake with a big box make more power than a 4" intake with a box that was still 3x stock size on a 4 cylinder half your displacement, likely making half as much power. Tiny improvements in air intake pressure can really add up on NA engines.

What usually makes our mind up is:

How much space do we actually have for this airbox, leaving room for good inlet ducting?

How annoying is it going to be to get to everything else once we put this massive airbox in?

How hard will it be to get the airbox and intake tube in and out?

What sizes can I reasonably get a panel filter in, so we don't have to deal with a custom filter we have to wait for and can't easily get again when we need one?

Hi Mike,

Thanks for your advise, It is helping me understanding I am going in the right direction.

I think at my level of knowledge and with the information I could gather, for now as you said a pragmatic approch would be the way to go.

In the mean time I will try to found out the inlet pressure.

I have quite a lot of room to fit a box between 4 to 6 litres, My car has a fixed spoiler witch gives a lot of room and I have turned arround the throttle body, so the feeding is towards the rear of the car, the engine is not back in, but I guess there is plenty of room and also I can tilt the throttle up around 30 degrees witch may help downdraft.

There is a company in UK (no afiliations) called Reverie that make universale air boxes between 4 to 6 litres capacity, They could also make a few special parts, I will use their inlet 75 mm trumpet (witch is spot on the size of my throttle body) and probably of a lenght of 5" , 200mm, 50 mm inside the box with the trumpet and the 150mm straight part to the throttle with the air mass sensor conected.

I will try to disscuss with the guys at Reveries if they can modiffie and make me a sort of flared lid that clamps the panel filter, to smooth the edges and help the air going in the airbox.

Then will be left to position the airbox and make some brackets to secure it well for any driving situations.

I will try to post a few pictures later on;

Many thanks



If possible, a velocity stack/trumpet inside the airbox would improve the quality of airflow reaching the MAF in addition to potential performance benefit. Air straighteners can help even more.

Anything that promotes laminar airflow as it reaches the MAF greatly improves the accuracy of MAF readings.

150mm of straight with the MAF in the middle of it should work pretty well. Shorter straight sections tend to cause MAF accuracy issues, again due to flow not being laminar and consistently distributed in the tube.

75mm diameter could be larger to improve flow, but you'd likely run into issues with the MAF reading if you went real big and had to step down to throttle body size right after the MAF, so 75mm likely makes sense.

Hello Mike,

Yes planing to fit the trumpet inside. The actual throttle size is 72 mm, the inlet is 75 mm therfore the need of the 75 mm pipe.

Here is a few pictures of the rear of my car showing the space under the spoiler.

2 pictures of the Interlagos Reverie airbox with the panel filter measuring 4.4 litres , I am not sure if the second airbox with a cover with 127 mm oval ram, or tho it will need fittement of a trumpet that they could provid too, will be efficient as there will be not much air stress inside by the speed of the car, like in a front engine car where a ram can be place facing the wind

Many thanks


Attached Files

I look forward to seeing what you put together!

No specific recommendations, but I would suggest you give some consideration to what you are going to use the vehicle for, and what you expect from the vehicle's engine response?

What are you using to measure the air mass entering the plenum/engine? As you increase the air-box/plenum volume you're going to dampen the response of the air flow/mass metering to the change in air demand fom the engine.

The other thing is that as the throttle body is increased in diameter, the low throttle opening control is also going to be compromised, and this can make low rpm/speed driving difficult, and there may be mapping issues if the cell count is low for the TPS mapping - even more so it it can't be biased to the lower opening position.

The thing to remember with a MAF sensor is that they are calibrated to work within the specific pipe that they are fitted to from the factory, changing this, and thus changing the flow through that pipe can change the way that the MAF reads the airflow through it. Even changing the pipework leading into the calibrated pipe section can change the reading, having a laminar flow conditioner fitted can alleviate some of this. Post MAF pipework can also have an effect on the reading, especially if there is a large change in the area that promotes the creation of a pressure differential through the MAF.


Thanks for everyone imput.

The existing throttle is 72 mm from 68 mm for many years and the car was mapped with a standlone ECU for over a decade.

It is runing find on public roads and even in cities with its LWF and AC on, it was also maped on roads for this purpose.

I am now restauring the car, and making some upgrades to the engine, updating the ECU, fitting a better exhaust, a shaftless throttle dody, DBW, COPs, performance camshafts (from 6.2k) to 7.5 k, and this is where my though was to try and improve the air intake.

It has a quite long double bend aluminium pipe of 75 mm ID, going into an ITG JC60 foam filter, honestly not sure its the best .

I have turned around the throttle to be able to fit a 200 mm stright pipe, hoping that alone will improve flow.

Next though was should I keep the ITG J60 cone filter or fitting an airbox.

The car will be back on the dino for a remap. My existing maps will be loaded to the new ECU.



All sounds good to me.

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