Airbox - can you simply have one to provide clean air?

[trialsrich]

Hi all,

Simple question that google and my searches on here can not seem to provide an answer to.

I have a rear mounted redtop that will shortly be propelling an off road racer and be equipped with DCOE type ITB’s, I’ve already been advised (by ITB manufacturer) the best solution to provide air is with minimal obstruction and a large sausage type air filter.

The issue I would like to overcome is, as it’s rear engine, 4WD and there’s going to be plenty of muck and most damagingly dust around any air filters mounted directly to the throttle bodies (16” MT tyres and a low mounted engine) will be clogged or worse in pretty short order.... but all “air box” related searches guide towards best design etc... getting clever might be for a date in the future when engine development is suitably progressing.

Essentially is there a point when an airbox has suffices clearance from trumpets it is no longer considered an airbox but simply a route to clean air?

Cheers

[Graham]

yes, but the air box needs to be MASSIVE and have a huge intake

[trialsrich]

Hi Graham,

Thank you for your reply, I suspected that may the case be but what constitutes massive and huge I wonder?

I’m not looking for performance gain but would like to avoid any detrimental effects.

Clearance from bellmouth to filter plate (k&n style) is commonly held at min 30mm... what multiple should be applied to ensure there’s no significant effect? - 5 times - 150mm around?

Might just have to fabricate the largest I can and hope for the best as a lot of clever science says the side entry offerings could be actually detrimental.

[Graham]

its not just about distance from the bellmouth to the back plate, intakes pull air in from the sides so you want volume all round them

[trialsrich]

I’ll do some measuring tomorrow and report back in the evening for your opinion - being rear engine, tubular frame and with a flimsy fibreglass clam shell rear cover there could (should) be some available scope for clearance... 350mm tall, 150mm deep and 600mm long should be achieved

Thanks again, much appreciated

[modifyitmike]

Could you not design something along the lines of the old RS200, a long roof mounted intake pod, as per minimal obstruction and a large sausage type air filter

[trialsrich]

So a quick bit of measuring and I was a little optimistic, generally I have loads of space but there is a key tube just off line with no 1 cylinder bellmouth that for a short distance is going to restrict depth to bellmouth to around 130mm.... elsewhere else stacks of space.

Regarding inlet - absolutely I have good space to get clean air in, the rear shell sits 50/60mm above the roof full width and it has openings to the sides, it’s also to aid cooling as the rad is rear mounted so ‘ducting’ nice clean (as in not full of dust) air from on high is the plan.

With the restricted space I may have resort to as large as possible and a centrally mounted airbox inlet to try and minimise over feeding feeding (lean) or under feeding (rich) with air any cylinder

[katana]

If the 'airbox / plenum' volume is big enough ie 2 - 3x engine capacity it can be treated as a 'still air' box and a NA engine can't under or over draw from it - it will only take what it requires IMO. Inlet restriction would be the worst problem.

[trialsrich]

A 200mm cylinder, 600mm long has a volume of around 19ltrs (by my phone calculator and memory of formula maths) so 2-3 times the engine capacity is easily achieved, 19 would equal over 9 times, two 70 or 100mm ducts to supply the clean air mounted centrally above the bell mouths should be able to provide adequate air without restriction.

Does this seem like a proposal with ideally no detrimental effects?

[katana]

Sounds plenty to me - glad you've got that much 'free' space LOL!

[apmonty]

Reverie has a lot of data and designs for airboxes all in carbon fibre so not cheap, maybe of use. https://www.reverie.ltd.uk/carbon-airbox.php

[trialsrich]

Perfect and plenty of food for thought, time to get some duct and the tin snipps out and see what can be done....

Thank you all