Air box, volume calculation

[jack&mle]

Hi

I'm looking to built an air box for my engine a 1.8K vvc.

The air box will be connected to the air filter by a 120mm diam tube 400mm long and the air intake of the air box is about 180mm diam (it will be based on air box from an Exige)

What do I need to be aware to design an air box?
Is there some special calculation I need to do to create a perfect air box, air flow, volume pressure, etc...?


Thanks for your help

Jack

[busa_rush]

As big as possible but at least the same size as the engine capacity, so really at least 1.8 litre x 2. Keep the sides and top well away from the trumpets, at least d/2 where d is the diameter of the trumpet.

[GavinPearson]

You need more like 3x engine capacity if you want best top end power.

As a guide the S-Type Jag (99 to 2001) had a 9 litre inlet manifold assy on a 3 litre engine.

[CombeMarshal]

Take it off and use the Earth as your airbox!!!!!
(I'm not being serious!)

[Jack&MLE]

The problem is using the Earth as my air box, is that it won't fit under the bonet

[CombeMarshal]

You don't need that either!!!

[HarryW]

GavinPearson said:

You need more like 3x engine capacity if you want best top end power. As a guide the S-Type Jag (99 to 2001) had a 9 litre inlet manifold assy on a 3 litre engine.

Gavin you seem like you kow what you're on about,some burning questions for you on this subject.
I thought that the ratio was different for differing engine configurations as well as capacity .
What about optimal plenum roof height above trumpet, is it a function of trumpet diameter (not flare diameter).
Do you know the colume of the standard Rangie 3.9 plenum off the top of your head
Is there any guidlines available on-line for this sort of thing .
:thumbsup:

[thong]

[quote=jack&mle]Hi

I'm looking to built an air box for my engine a 1.8K vvc.

The air box will be connected to the air filter by a 120mm diam tube 400mm long and the air intake of the air box is about 180mm diam (it will be based on air box from an Exige)

What do I need to be aware to design an air box?
Is there some special calculation I need to do to create a perfect air box, air flow, volume pressure, etc...?


Thanks for your help

Jack

well how high you going to rev it,dont make the throttle body the size of a man hole it wont work,the engine dont suck never forget that its charged by atmo,where on the car you going to feed said airbox,its all fluid mech good luck.
[/quote]

[GavinPearson]

HarryW said:

GavinPearson said: You need more like 3x engine capacity if you want best top end power. As a guide the S-Type Jag (99 to 2001) had a 9 litre inlet manifold assy on a 3 litre engine. Gavin you seem like you kow what you're on about,some burning questions for you on this subject. I thought that the ratio was different for differing engine configurations as well as capacity . What about optimal plenum roof height above trumpet, is it a function of trumpet diameter (not flare diameter). Do you know the colume of the standard Rangie 3.9 plenum off the top of your head Is there any guidlines available on-line for this sort of thing . :thumbsup:

Most engines are driven by package so there is no best example out there on a road car, but if you think about a car like a BMW M5 that has trumpets in a plenum they place them to get maximum space above the trumpet. Probably about 2x to 3x diameter. Bigger is better.

The thing to remember about a BMW M5 engine vs a Plenumed Rover engine is that the BMW uses port throttles and the Rover uses a single throttle opening. When the BMW throttles are opened, there is very little additional air going through that needs to be compensated for with enrichment, whereas in a plenumed engine the mass of air has to be calculated and then enriched. The higher the intake volume the harder it is to get right, and so big volume plenums with one opening are not great for driveability. In most cases people will sacrifice top end power for the driveability and go for a lower volume. In a Range Rover where top end power isn't that important, plenum volume I would imagine is 1.5 to 2x engine capacity.

I don't know of any guidelines, you will find that most manufacturers do a design of experiments to tune the lengths, and then package what they need to get the most out of the engine, then try a few iterations of the packaged design to optimise it before production.

[Marquis_Rex]

There’s some confusion here between AIR BOXES and inlet manifold plenums/throttled volumes.
The Jaguar S type V6 is a special case because it is an example of a V6 RESONANCE inlet manifold. The earlier X200 V6 has a throttled volume of close to 10 litres where as the later X202 (1999 onwards) was reduced down to 8 litres. I know, because I was involved in designing them. Don’t let the word resonance intimidate you, like many modern V6s and straight sixes it uses the phenomenon of resonance in tandem with variable cam phasing to bolster various regions of the torque curve. This is why this under rated engine achieves over 12.6 bar BMEP and 81 Bhp/litre (higher than the competitive BMW 3 litre at the time). The throttled volume was reduced for better idle speed control and throttle response- there’s a compromise, but it’s tougher on a resonance type manifold- as you are USING the volumes to bolster the torque curve up but too high a volume can hurt your throttle response. These volumes and the runner lengths themselves are optimized using a sophisticated engine simulation code such as GT Power or Ricardo Wave. Some DOE work is done, depending on the variables and design space but I knew the engine as a system very well and this wasn’t required. In the case of a resonance system manifold it’s not as simple as needing more volume for more top end power, the sizing must be carefully carried out and matched. Nearly all manufacturers use these engine simulation tools these days. You can do it by reducing the inlet system into an analogous system of masses and springs but this takes longer!

Engines like the M5 and M3 Do NOT use resonance, and as Gavin correctly says, the use of port throttles means that the small volume constraint for better throttle response is no longer there. Port throttled engines inherently have a small throttled volume. If this philosophy is pursued then bigger is certainly better- the E36 3.0 M3 has a 14 litre plenum volume!
Air box volumes are another matter. Air box volumes on conventional V8s and V6s are sized using a combination of package and NVH guidelines. Just like on the exhaust side, the Air boxes can act like a silencer/system damper and can also be used to get rid of nasty boom periods in the rev range (typically in the 230-300 Hz range) or even tailored to give a certain sound characteristic. Some Jaguar V8 and V6 share the same air box and zip tube arrangement and have a similar sound characteristic in the lower frequencies, but this is more dominated by engine order effects so that the “V8’ey” and “V6’ey” sound later shines through as you rev them. The good thing about conventional V8s and V6s are that they are not that sensitive to TUNING effects as far upstream as the airbox. I know this because lots of plumbing and NVH devices were played with on the zip tubes of various engines I’ve worked on over the years and I’ve had to evaluate their effect on performance. Obviously removing the airbox itself will reduce pressure loss and change performance, BUT the shape of the torque curve is little effected- hence not significant in terms of tuning effects. The other requirement for an air box is low loss, and this is achieved through detailed design, using a combination of Computational Fluid Dynamic techniques and a steady state flow rig.

Unfortunately on a 4 cylinder (and a flat plane crank V8- notice I said “conventional V8” even playing with the volumes as upstream as the air box (and it is the AIRBOX I think you mean??) and zip tubes has a marked effect on the torque curve of the engine. This must be carefully matched to the engine, using either empirical experience or one of the aforementioned engine simulation packages. Just look at the air intake system of a Honda S2000 and you’ll see what I mean! A lot of attension has been put at sizing the air box, no doubt with not only the factors I outlined previously (pressure loss, NVH) but also TUNING effects. There are no special calculations for the air box sizing, but my advice to use see what works, perhaps in competition or contact someone who races that engine. Or you could find another suitably high HP/litre application of a 4 cylinder engine or similar capacity and copy that system.








>> Edited by Marquis_Rex on Monday 27th February 09:17

[jack&mle]

Thanks Gentlemen for your input

What I did not mention at the beginning is the VVC will be fitted with DTH TB.
This is why I need an air box.

The TBs are from a Exige.

I have an air-box from an Exige which will need to be severely modified to fit under the bonnet.

As I will be looking for torque rather than top end power, then from my understanding and believing the air box was design for top end power for the Exige (?) , if I reduce the volume of it would not matter too much.

Do someone knows the volume of an airbox form an Exige?

Thank a lot for your input

Jack

[Marquis_Rex]

HarryW said:

What about optimal plenum roof height above trumpet, is it a function of trumpet diameter (not flare diameter). :thumbsup:

The distance from the bell mouth to the flat plate/wall.
I tested a high horse power per litre (90 bhp/litre) 3 litreV6 on an engine dyno a while back for a production application. In this case this was NOT a resonance type intake system. On this engine one bank had a plenum wall about 40mm next to the 3 runner bell mouths the other had lots of room. On test there was no dicernable difference in bank to bank AFR –therefore the air flow per bank was not particularly different.
Analysing it, it’s not difficult to see why. Assuming you have a good peripheral area around the bell mouths you can get quite close.

The engine in question had 64 mm diameter runners at the plenum end. You want to make sure that at peak gas flow demmand this area around the bell mouth-close to the wall doesn’t represent a restriction.
A initial funademntal way –that simply makes sure that this periphery area doesn’t represent a restriction is to equalise the entry area of the intake runner to the periphery “cylinder” area that the runner will draw from:
In my engine example for instance we had
Runner entry area needs to be greater then or equal to the periphery cylindrical area
or
Pi* (R)^2 = 2*PI * (R) * X
Where R is the radius of the entry to the runner
Pi is 3.14159
X is the distance to the plenum wall

In my example we fine
R was 32 mm and X was 40 mm
And 2*PI*( R) * X was far greater then pi* (R ) ^2 hence there was no retriction there. In fact if we cancel the above formula we arrive at a wall distance X of R/2!!

However this is only a fundamental initial way at arriving at the distance of the plenum wall /flat plate proximity to the bell mouths, as in reality it’ not a steady state problem but a transient 3 D problem: There could be 3 D wakes, and turbulence that further restricts the flow, so if you’re not package limited it’s always good to try to get as much distance from the runner bell mouths as possible.