Air intake - from low or high pressure area???

[jonners]

Please take it as read that I'm an idiot - or to be more polite, don't assume that I know or understand anything...

Right then, twin webers on a saloon. Currently, we use foam socks tie wrapped around the intake trumpets (or whatever they're called). We want to improve on this because at high revs the socks start to collapse into the trumpets and they're generally horrible.

We are going to fit an air box with trunking so that the filter/inlet is out of the under bonnet area.

To my uneducated and untechnical mind it seems to make sense to fit the 'mouth' of the inlet tract in the air stream so the air comes rushing in at speed.

However, I remember reading in CCC (RIP) years ago that it's better to put the 'mouth' in a low pressure area out of the air stream.

Anyone understand this? Have I muddled things up?

Thanks in advance

[dtype38]

I think you're probably mis-remembering what you read. Ideally the "mouth" should be facing the oncoming air at the very front of the car. It can be to the side of the rad grill, in place of a headlamp if that's allowed by your regs, or it could be stuck up out of your bonnet as a big scoop. You could even stick an F1 style intake out your roof if you were inclined, but remember the longer the duct you have, the bigger a diameter it will need to keep the resistance down. Oh, I also seem to remember that the base of the windscreen is a high pressure area on many saloons, so if you aren't allowed protrusions, and you cant get a tube down to the front of the car, then that's a possibility.

[Chris Y]

Originally Posted by dtype38

Oh, I also seem to remember that the base of the windscreen is a high pressure area on many saloons

Are you sure? I always thought it was low pressure, as most of the air is trying to avoid the area and get up the windscreen.

I figured that was why some guys prop the bonnet up at the back, so that the low pressure area would take some of the under bonnet air out, thus aiding cooling.

Now I'm confused

[ubrben]

Originally Posted by Chris Y

Are you sure? I always thought it was low pressure, as most of the air is trying to avoid the area and get up the windscreen.

I figured that was why some guys prop the bonnet up at the back, so that the low pressure area would take some of the under bonnet air out, thus aiding cooling.

Now I'm confused

Why don't you get a water filled u-tube manometer and find out.

Ben

[greenamex2]

A high pressure area is better for maximum power

BUT

It could also cause you engine to run weak and, in the worst case, melts plugs and pistons.

I suspect this is why CCC recommended putting the feed in a low pressure area. This would still (probably) be better than than your current arrangement.

On the subject of raising the back of the bonnet. Certainly on my car it helped cooling, especially on the start line where the pressure doesn't matter. Whilst it may not be the most efficient way of doing it, most saloons don't have too many other options!

[greenamex2]

Originally Posted by ubrben

Why don't you get a water filled u-tube manometer and find out.

Ben

That could be an interesting discussion with a scrutineer!

Definately one best left for a track or test day unless your car is still road legal.

[JamesH]

Originally Posted by Denis Bassom

On the subject of raising the back of the bonnet. Certainly on my car it helped cooling, especially on the start line where the pressure doesn't matter. Whilst it may not be the most efficient way of doing it, most saloons don't have too many other options!

I think that some MSA reg from last year means any gaps like that on an open car means that the gap needs to be filled with 1 grill of 1mm holes, in case there is a petrol leak/fire int he engine bay. Certainly in Locost we have to ensure that any rearward facing holes in bonnet (ie louvres) over the top chassis member must be filled in this way. Not sure what it would help with though - apart from getting any leaking petrol into more of a mist, so it makes a bigger fireball!!

James

[greenamex2]

Great, another thing to fail scrutineering on.

[jonners]

Thanks to all for the input.

The answer seems fairly clear and I'll bear in mind the point about running lean - one of the reasons why now is a good time to sort this mod is because the car will go on the rollers before next season although I'm dubious about how effectively the cooling fans will replicate the real ram effect.

Contrary to everyone's advice I'm sure CCC were adamant about the low pressure point. Again, the lean mixture point is a good one but proper tuning should dial this out. The reason it has stuck in my mind is because they even advocated putting the inlet in the front wheelarch and I remember thinking that's a very challenging environment for an air filter...

Perhaps they were just wrong. I've discovered several times (i bet we all have) that the comics can lead you up blind alleys occasionally...

[greenamex2]

Unfortunately the leaning out will be speed related so if you get it right at one speed it will be progressively richer at slower speed and progressively richer at higher ones.

In theory.

One way to partially compensate is to feed a 'pressure line' from the same pick up point into the carburettor float bowl. This is similar to what is done on some carburettored turbo installs.

[jonners]

Oh no!! The low pressure idea is starting to look appealing again...

[zefarelly]

johnners, have you seen my airbox/scoop . . .seems to work well

the inverse of this is . . . .I have seen some down draught Anglias with airboxes that drew from the base of the screen, through the louvred panel perhaps ?

that should confuse the issue

[terence]

Jonners, go the low pressure route ,get a good sized filtre,the socks would have cost you heaps of power ,one thing to remember ,make sure that the low pressure is a constant supply,and yes it could well be speed related,in other words the air pressure does not alter with speed,if you check it out on the rollers then it easy enough to check.

[Cameron Winton]

On a saloon, there can be two reasons for having an air intake of some kind.
But ultimately they come down to the same need, more air mass at the inlet. So, the 1st reason is to get cooler air (More dense therefore more mass for a given volume) from somewhere outside the engine bay. 2nd is to get what is known as ram effect. This is where you use the forward speed of the car to increase the mass flow, then stall it in a plenum to increase the air mass presented to the inlet.
A misconception is that you use the speed to "force more air in".
I would choose a high pressure area as the shape of the car has already stalled some of the air flow already but there will be enough flow to keep it moving. Whether the base of the windscreen is high pressure or low pressure depends on the shape of the car. The transition from bonnet to windscreen angle could have an effect. If it is such that "separation" occurs then you may have low pressure. If it is such that "stall" occurs it will give high pressure. The greater the transition (ie old car with vertical windscreen!) then the more likely there will be stall.
On propped up bonnets for cooling, you might find that the less dense hot air rising finding an easier way out might be what helps. At speed, there will be a higher pressure under bonnet due to there being few places for the air coming in to get out & it may help.

[Locost47]

That second-to-last bit isn't quite right. Separation occurs in a concave corner like the bonnet/windscreen transition *because* of the increase in pressure there, not the other way around. The 'stall' that is referred to is stagnation, which isn't the same thing.

As far as i'm aware the general rule for engine intakes is to put them in a region of high pressure, to ensure a healthy mass flow of air. Once you get near to the actual intake trumpets though it all gets ridiculously complicated as you then have to consider the pressure pulsations within the system and try and optimise them for peak air-charge conditions. Shape & length of the intake plenum then become more critical for maximum performance than the actual average static pressure (especially for 2-strokes), but getting into all that is a dark path that i wouldn't recommend for anybody who values their sanity!

[terence]

One question,how do you get forced air though a filtre??These filtres are "flowed" by the manufacturers for the optimum air speed at all vehical speeds ,are,nt they?

[R59]

Those socks are half of the problem too. Dreadful things. They stop the bellmouths from working properly.

Dave Vizard wrote a good article on them many years back, and a "saucepan" design was better than a trumpet. The full radius on the ends of the trumpet bit is important too.

The subject of lifting the rear of the bonnet - Denis - it doesn't matter, there's a firewall between you and the engine bay. And what's more, the way cars are designed, for the internal ventilation to work, there needs to be a positive pressure at the base of the windscreen. So in that respect, how is lifting the rear of the bonnet going to improve airflow under the bonnet? If anything it could cause a null in pressure under the hood where you want to see a bit of a pressure increase.

I remember again, in the dim past, some comments from John Cleland about the reverse head on the Vauxhall BTCC engine - how it was worth Xbhp because it allowed the engine to breath cold air better than a rear of engine intake.

If you are going to build some sort of scoop affair, with a plenum which will see a small increase in air pressure, maybe one PSI over atmospheric, then you will need to increase your main jet size a tad to be safe. If you're using EFI, then closed-loop Lambda helps as it will add the extra fuel to keep it all happy (like wot the DTA brain will do if used with an oxygen sensor)

[Notso Swift]

If you are using EFI you could also use a MAP sensor for correction.

[R59]

Absolutely true too.

[dtype38]

Originally Posted by racing59

The subject of lifting the rear of the bonnet - Denis - it doesn't matter, there's a firewall between you and the engine bay. And what's more, the way cars are designed, for the internal ventilation to work, there needs to be a positive pressure at the base of the windscreen. So in that respect, how is lifting the rear of the bonnet going to improve airflow under the bonnet? If anything it could cause a null in pressure under the hood where you want to see a bit of a pressure increase.

Interesting... I've always thought of the issue of underbonnet airflow (particularly through the radiator) not so much a problem of getting the air in as getting it out. Intake through the rad grill or above a front splitter is obviously easy, but if it can't get out of the engine bay, then it won't enter it in the first place. I agree with you that the base of the windscreen is positive pressure, but it ain't nearly as positive as at the radiator grill. Hence the improvement in underbonnet airflow. From personal experience, adding a row of louvres right across the back edge of the bonnet of a saloon I used to race dramatically increases radiator airflow (as seen by lower water temp). On my current car I've not only got louvres behind the front wheel arches, but I've also opened out the transmission tunnel as far as I can to let some of the air "through" the car to cool the rear brakes and diff.

There are, however, a couple of disadvantages to lifting the back of the bonnet... If you have a normal ventilation system with its air intake at the base of the windscreen, then the intake air is always pre-warmed by the engine. Nice in the winter, but a major pain in the summer! The other is that I once had an oil hose blow off my oil cooler at the front of the engine. It filled the whole engine bay with a hot mist of oil.... a split second later I couldn't see a bloody thing as it completely covered the windscreen! I was just turning into Cascades at the time

[Cameron Winton]

Apologies to Locost47 - I use the term stall for stagnation because I work on gas turbines as in compressor stall.

[crogthomas]

Whenever im considering constructing a air intake, I find myself stareing at this picture for hours for some reason...

[jonners]

Again, thank you all for the input - plenty of food for thought.

Have to confess that am still confused albeit on a higher plain. Some of the comments made touch on points I've either read or wondered about already - for example there is a web page that suggests that a non vented bonnet will generate lift but ours isn't very well sealed and the bonnet pins still look okay...That's a new trick for the max power boys - seal your bonnet area and pull wheelies at 100mph....

As for little air getting in to the engine bay because there's no way for it to get out, that seems to make sense but if it was a real problem you'd be over heating wouldn't you??

Just about all performance cars (as manufactured) seem to take air in from the usual high pressure/grill area but I suppose (again, as manufactured) they are tuned for a wide envelope of operating conditions rather than optimum performance so rich/lean depending on road speed temperature is catered for by the basic set up and in today's world there's the ecu as well.

So, given that the aim on a highly tuned engine is to keep the rich/lean envelope as narrow as possible, perhaps the low pressure intake is best? But which low pressure area - where's a nice cold and clean one?

I suppose the real answer (as usual) is there's no right answer without proper experimentation for our particular car and other installation factors - but from what everyone says the high pressure route is best provided the mixture's ok, and the trunking is big enough.

Maybe we'll stick a tiny fan in the trunking and hope the scrute doesn't apply the 1.4 equivalency factor....

[jonners]

Originally Posted by zefarelly

johnners, have you seen my airbox/scoop . . .seems to work well

I did have a snoop at Castle Combe last Easter but your bonnet was down. Have also checked your website but no under bonnet pics - can you pm one??

[retro_msport]

Jonners.. what car is it?