airbox design and airflow

[Kev_205]

I want an airbox on my throttlebodied 205. Ive been told you can build a pressurised airbox for a N/a car but what are the design principles behind it.
As big as possible to get maximum air to the engine???

[Cameron Winton]

I intend on doing the same. The design principles are based on Bernoulli's equation. You use the velocity of the air from from forward speed to increase the pressure and hence mass of air at the inlet to the throttle bodies. Airspeed and pressure are inversely proportional. ie for a fixed volume (Your induction box), you slow the air down, you will get a pressure rise. As the volume is fixed, higher pressure = higher air mass. Inlet depression from the Induction stroke draws the high pressure/mass air into the engine. More air mass per induction stroke, more power if your fueling is good.
A good airbox design will have a reasonable sized throat but not too big. The compromise is that airflow is not too restricted at low speed but is small enough that you don't get flow reversing back out again. You should have a reasonable sized volume over your inlet trumpets fed by a divergent duct from your inlet (like a big inlet trumpet reversed. This ensures you maximise the pressure rise from the inlet velocity. Sharp corners, sudden openings in area etc create eddies which induce losses. Too small a volume relative to the inlet does not leave enough to be drawn into the engine and the induction stroke tries to draw air in through the inlet.
On a 205, remove the pressed steel bolt on cross member and use the gap between the radiator and bonnet as your inlet. I have cut about the radiator grill. Most people use sheet alu to form the airbox. If you want to look super professional, create a mould and get it made in carbon fibre. Another technique is to use the gap where the original inlet was and have it feed a box along the front of the t/bodies. Look at a S1600 rally car or one of the Castle Combe Saloon cars for ideas
Long winded?

[Cameron Winton]

ps, try and have it fed from the radiator grille area. the bluff nose of the car creates a little higher pressure without much airspeed loss. Other FWD cars like Vauxhalls have the inlet side of the engine facing the bulkhead. You can get a similar effect with a scoop on the bonnet in front the windscreen. Looks a bit boy racer though

[graham bahr]

forget trying to get a pressurised air box, you wont be able to go fast enought for it to do anything, some of the bikes i used to ride such as the kwacker zzr1100 had such devices and it was said that you had to be going in excess of 160mph before they started working. i remember reading that the nissan primera touring cars made a tiny bit of pressure 125mbar, at 125mph which is basically nothing and if they had another car in front of them the pressure became zero, an air box is a good idea though if it gets your engine fed with nice cool air.

[tzei]

We did an adjustable air box from sheet alu. Booked dyno and by trial optimized the volume. Then started the fiber glas - fun

[zefarelly]

I'm thinking of making one just to help feed cold air in and separate it from the hot air rising from the exhaust below . . . .

[graham bahr]

go for it, it should pay good dividends on a pre x/flow especially if you can also fit some insulation between the underside of the carbs and exhaust, be prepared to rejet your carbs a size or two, we had to go up a couple of sizes on the mains when i tried it some years ago

[zefarelly]

well I have a completely new engine so I'm using previous settings as a base then off to the rolling road, I've wrapped the new manifold in thermoteck stuff, twice, I've fitted the back plates of two old K&N's, clamped by the trumpets, now I'm forming an aluminium case to direct air from the front grille stright to them, I may even try and insulate the box at the bottom slightly as well to keep it cooler than the manifold below, hopefully every little will help.

[dtype38]

Originally Posted by graham bahr

forget trying to get a pressurised air box, you wont be able to go fast enought for it to do anything,

Yeh, that supports what I've heard as well. Haven't done the calculation myself, but I was once told that the inlet airspeed of the carbs for my Jag engine is 90mph+ at full power so even if I pointed them straight forward above the bonnet, most of the time they'd still suck faster than I'm going. And I'm only tuned up to around 60hp/litre

[graham bahr]

Originally Posted by zefarelly

well I have a completely new engine so I'm using previous settings as a base then off to the rolling road, I've wrapped the new manifold in thermoteck stuff, twice, I've fitted the back plates of two old K&N's, clamped by the trumpets, now I'm forming an aluminium case to direct air from the front grille stright to them, I may even try and insulate the box at the bottom slightly as well to keep it cooler than the manifold below, hopefully every little will help.

on the basis of every little helps, polish the OUTSIDE of your inlet manifold to a mirror finish to reflect heat and help keep the inlet charge cool, sounds daft but david visard used to reacon it worked on A series engines

[dtype38]

Alternatively, heat tape wrap the exhaust manifold which is an excellent way to generally keep everything under the bonnet a bit cooler (supposed to improve performance as well). Or I can highly recommend the heat matting with a reflective foil face on it, for the underside of the manifold. I've used that stuff to protect grp from an exhaust only 5-6mm away and it really works. :-)

[zefarelly]

Originally Posted by graham bahr

on the basis of every little helps, polish the OUTSIDE of your inlet manifold to a mirror finish to reflect heat and help keep the inlet charge cool, sounds daft but david visard used to reacon it worked on A series engines

unfortunately I'd like to drive it before I die, no offence but that has to be one of the daftest things I've read on here for . .. ooh . . .24 hours ! polishing the inside is bo11ocks as well, if anything it has a detrimental effect to gas flow.

exhaust heat wrap is in place, and seems to help . . .

[dtype38]

That was a bit harsh Zef. Don't stress Graham, Mr. Visard has lots of excellent ideas about making an engine perform. Unfortunately he also has a few which have a little more basis in his own mind than in the real world. That was one I'm afraid.

[Tim Falce]

I know the idea is to keep the airflow into the engine as cold as possible to make the air dense so why do some inlet manifolds have hot water from the engine flowing through them?

[dtype38]

It makes the engine more fuel efficient and is used whenever economy is more important than maximum power. On some cars the inlet mainfold is cast as one piece with the exhaust manifold instead of a water jacket for the same reason.

[johnh875]

I agree re the comments on polishing the outside of the intake manifold – technically it works however the amount of heat absorbed by radiation would be insignificant compared to the amount absorbed by conduction. Better bet would be to find/make a phenolic spacer for the manifold, or a physical heat shield between intake and exhaust if appropriate.

Falcemob, heating the inlet is also related to cold running so icing of carbs or condensation of fuel does not occur.

[zefarelly]

no offence intended . . . . .it was a weekend post and I think I'd had a celebratory beer or two after getting the car running !

the STD GT set up on a MK1 Cortina uses water heated inlet, to aid vaporisation in winter I believe, and as has been posted above, it also uses a spacer betwixt carb and manifold which I thought was to prevent foaming of fuel /vibration ?

[listernoble]

a pal of mine who hillclimbs a Ginetta G27 ,5.3 litre with Webbers,made up a box that drew air in around the carb bodies,so the air cooled the carbs and it gives an incredible power lift,but then he went and supercharged it!
He is a boffin at Land Rover explained all the theory to me,if you are interested i will get him to explain u=it all agin and post it.
Got to go,the cat's on the keyboard!
Lister

[Tim Falce]

Originally Posted by zefarelly

it also uses a spacer betwixt carb and manifold which I thought was to prevent foaming of fuel /vibration ?

Apparently Webers suffer from fuel foaming and have these rubber mounted spacers to cure the problem although I have the same thing fitted to a set of SU's I have on another manifold.

[dtype38]

There is a company (who's name escapes me at the mo) who are marketing a liquid nitrogen cooling system running off an onboard cylinder. It's aimed at race series where restrictor plates are required in the intake and it uses the nitrogen to pre cool the intake air to increase the mass flow through the restrictor plate.... might need a water heated manifold for that one

[zefarelly]

liquid nitrogen . . .you'd end up with solid fuel ! its not a train is it ?

the side draught webers use O rings to alleviate vibration/foaming but the twinchoke downdraughts use a kind of spacer of paper/plastic/paper, or even a kind of fibreboard/wood on old ones, the same as the spacer gasket on the fuel pumps (kent block engines) ? ? ?

[dtype38]

Yeh Zef, seen that sort of thing on BMC A-series as well. The spacer to the single SU is about 3-5mm thick on a Metro and made from a fibreboard/bakelite type material, but on the Metro Turbo I think its more like about 10mm thick. It looks like a heat barrier from the manifold.

[zefarelly]

this is what I've just done . . . .we'll see if it performs in the next few weeks

http://www.throbnozzle-racing.co.uk/...ing/airbox.htm

[Peter Mallett]

Cool.

I've looked at doing something with mine but unless I rase the bonnet or cut away the front panel I'm stuck.

So no chance of that. Might look at fuel coolers instead.

[listernoble]

Originally Posted by Peter Mallett

Cool.

I've looked at doing something with mine but unless I rase the bonnet or cut away the front panel I'm stuck.

So no chance of that. Might look at fuel coolers instead.

Serious stuuff,what series do you run in?