Air box velocity stacks

[Pondo]

After some advice, will be fitting the Z club airbox soon and have started thinking about velocity stacks and what people recommend or are running?

I have triple Weber’s which had k&N filters previously fitted. I did purchase from kameari some 50mm stacks a while ago but never got around to fitting them, they do have the grit guards on though.

 

[jonbills]

stick them on inside the box, they'll be good.

 

[Pondo]

stick them on inside the box, they'll be good.

You don’t see them as being restrictive Jon, especially as I was thinking of fitting a filter the other side of the radiator?

 

[Turn & Burn]

You don’t see them as being restrictive Jon, especially as I was thinking of fitting a filter the other side of the radiator?

I don’t think they’ll offer any benefit down stream of a filter. Just go with the ram pipes.

 

[johnymd]

If you look at how cosworth designed some of their airboxs, the end of the trumpets were quit close to the back of the airbox. The air was supposed to enter each trumpet from behind and travel around the curved tip.

 

[Pondo]

I think the only way I can get the guards off on these will be to cut them off....which seems a shame, but I agree I can’t see them being of benefit and I don’t want to buy more when I already have these.

 

[Pondo]

If you look at how cosworth designed some of their airboxs, the end of the trumpets were quit close to the back of the airbox. The air was supposed to enter each trumpet from behind and travel around the curved tip.

Are you saying longer trumpets would be better? We’re not quite at airbox stage for a week or two and as a result I’m not sure on how much room I’m going to have when fitting everything yet.

 

[Turn & Burn]

Are you saying longer trumpets would be more better? We’re not quite at airbox stage for a week or two and as a result I’m not sure on how much room I’m going to have when fitting everything yet.

That totally depends on your engine, inlet manifold etc, but I would say your in marginal gains territory. If you go too long you may find you can’t get the top off the airbox without it clashing with the suspension tower. My stacks are 60mm but I’ve had to remove the carb insulator which was 10mm thick. ( I wouldn’t recommend this unless you’re on injection and not carbs.

 

[jonbills]

You don’t see them as being restrictive Jon, especially as I was thinking of fitting a filter the other side of the radiator?

If the mesh was easy to remove, I would, but if you have to cut them out, I'd leave them in. You need the ram pipe bit and I doubt the mesh will be restrictive.

 

[AliK]

So, I’ve been researching and vexing over this for some time. Let me be controversial!

The whole point of trumpets is to enhance airflow into the carb and smooth out turbulence before the jets. Turbulence at the edge causes eddys and effectively reduces the opening.

IMHO the worst thing you can do to trumpets is to add a mesh in front, add lots of sharp edges and undo their effect. Also that mesh covers a lot of cross sectional area!!

Vizzard proved that by far the most important factor was the radius at the mouth. Go for fully radiused if possible. As Johny said already.




Every time the valves close, they send back a shockwave at the speed of sound back up the carbs. Every time the shockwave finds a difference in diameter or a step, it reverses (or some of it does) and bounces back and forth down the inlet. The real world effect of this (when in tune) is felt when you feel the engine really getting on cam. A wider taper from bottom of trumpet to mouth, spreads the effect over a wider RPM range but the engine will feel less peaky.

Tuning the the entire inlet tract length, from valve to rolled tip of the trumpet has quite an effect and depends on where you want the torque to be + dependent on so many factors such as your cam timing, overlap LSA etc. Generally, longer trumpets work better for lower down the band and shorter ones at the higher end.

Here’s a good read ....

https://www.emeraldm3d.com/articles/emr-adj-length-intake/

 

[TimFZ]

This makes sense. I am heavily involved in loudspeaker and cabinet design and much of thus relates to quarter wave resonances (like an organ pipe or Transmission line and horns) and also Helmholtz resonances (such as a bass reflex speaker or blowing across a bottle).


Much of this is predictable but will depend on a combination of both the above systems in practice including the intake tract volume, length and open area.

A horn speaker tries to avoid the reflection by gradually tapering to a large open mouth and modern ported designs also tend to use parallel ports (non tapered) apart from a small bell mouth entry (and exit for a loudspeaker as air moves both ways!)

Assuming the cross sectional area of the intake is fairly constant except for the bell mouth, you need to consider the entire length from valve to bell mouth plus about 10% of the mouth diameter additional length.

The complexity of an intake also relates to sound speed of the moving air, going in its 343m/s @20c plus the average velocity of the moving air and in the opposite direction, its 343m/s minus the intake velocity. The air density also changes when fuel is added and due to vaccum increasing closer to the valve.

Inevitably, its predictable by computer design and CFD but will still require fine tuning by testing small length changes (or volume of an intake) by the manufacturer. You know when they have got it wrong or cut corners when there are flat spots (dips) in the torque curve.

As stated, it's a balancing act to get drivability and also impressive marketing power figures yet retaining economy at light throttle. Plastic moulded intakes on modern cars are used for fine tuning the engine characteristic to allow all the above and is also partly responsible for the unresponsive dead feel of many modern cars (well that and all the electronic management gubbins!). Choose a responsive modern engine and it won't be economical! (and vica versa).

 

[AliK]

Loving the knowledge base on this club! Liking your explanations Tim! It just shows fluid dynamics is the same whether you are talking engines, instruments or subwoofers.

I started to look at a potential Helmhotz resonator and a 1/4 wave J pipe for my old exhaust + the hemlmholtz as a bolt on to my balance bar. Unfortunately, without some serious computer modelling or tonnes of trial and error it’s very difficult to get it right - especially as the twin SU manifolds share so much space across 3 cyls, have so many shape changes and slightly unequal between cyls.

the first time I came across inlet tuning in modern cars was the Porsche Boxster with it’s 3 stage inlet controlled by two valves. I guess this stuff has been around since the 50’s F1 days but modern electronics can really make it versatile across the rev range / ambitions for fuel economy / improving BSFC etc.

I can’t find the original diagrams I saw with Porsche but lots of manufacturers do it using snail shell shaped intakes.

Here’s a good video that explains things in simple terms ...

 

[TimFZ]

Even if you model it correctly, you will need to place it at one of the multiple quarter wavelength points along the exhaust. These points change with velocity and temperature so your power benefits will alter in rpm slightly depending on temp and flow, which in turn also depend on increased power out and then this causes additional temp rise.

Your model needs to include accurate flow modelling for WOT and at full racing temps if it is for the track.

One issue with 90 degree placement of helmholtz resonators is the flow itself can disrupt the end condition for the resonator neck and prevent it working well. For example blow across a bottle to make it resonate and then blow very hard, it often changes. Or use a penny whistle, gentle blowing over the holes produces the fundamental tone, a harder blow increases the pitch from 1/12 wavelengths (odd harmonics) so at the frequency desired, you get no effect but it could help at 15000 rpm lol.

You can minimise this effect with a fine mesh over the neck as it makes it more consistent but likely to be compromised overall. If you are modelling in CFD, you also should use acoustic modelling which is sympathetic to changes in flow or your model will always be inaccurate and bench testing is inevitable.

Good luck, it's complicated!

 

[tyroguru]

I love EE and have wasted way too much time looking at his videos. He has several more that are really interesting in this area (which Ali has no doubt seen already!) including how Porsche have gone about their variable inlet manifold system(s):

BTW, I think the patent that EE refers to in the above video ("Resonance charging combined with oscillating pipe charging") is https://patents.google.com/patent/DE102007014447A1/it . It's in German but Google translate does a pretty good job with it.

 

[Rob Gaskin]

Clever stuff guys. When I first modified my trackday car I went to GDS exhaust systems in Brackley. Graham (the G of GDS) told me that he and Spike Anderson understood airflow because they were in a band together and played wind instruments. Perhaps it was just BS
I have wind sometimes and can make a noise like an exhaust but I don't think that helps here.