Bonnet cutouts

[Symbiotic_Caterpillar]

Another way to reduce the intake air temperature is to fit an air box and air scoop.

Of course that is a bit more expensive than cutting a big hole in the bonnet. I've also heard that they can need a lot of modification to fit under the bonnet. See Westfield World issue 1 2007

[pistonbroke]

I think you will find that longer intake trumpets give a slight improvement in low speed torque , at the expense of a slight power loss at high rpm  

[stu999]

You could of course follow the route taken on my car. 36 x 1" holes in 4 rows over the area of the air filters, no loss in bonnet strenth, plenty of cold air plus the weight loss.      

       

In reality, when you are driving there is probably no cold air whatsoever getting to your filters - all that has happened is that you are allowing hot engine/radiator air another escape route out (via your filters!)

Sticking trumpets and filters out of the bonnet to allow them to gulp cold air is in theory good. Until the same trumpets are trying to gulp cold air that is in effect rushing past at XX(X) mph. Methinks there isnt as much benefit as people would lead you to believe...

[nikpro]

Sticking trumpets and filters out of the bonnet to allow them to gulp cold air is in theory good. Until the same trumpets are trying to gulp cold air that is in effect rushing past at XX(X) mph. Methinks there isnt as much benefit as people would lead you to believe...

As stated in previous threads on this topic I'm with Stu; pressure waves around car bodies when they are moving make this a very complex dilema.

[Tigger]

Sticking trumpets and filters out of the bonnet to allow them to gulp cold air is in theory good. Until the same trumpets are trying to gulp cold air that is in effect rushing past at XX(X) mph. Methinks there isnt as much benefit as people would lead you to believe...

As stated in previous threads on this topic I'm with Stu; pressure waves around car bodies when they are moving make this a very complex dilema.

I agree to a point that it may be more complex that just sticking a filter out of the bonnet but surely common sense would tell you that if there is air passing the end of an open sucking object (Filter) then it would suck that passing air in to a degree thus making it more efficient.

It's not as if the filter is going to say "whoah man, that air it too fast for me, I'm not touching that with a bargepoll, I'll just suck in that nice slow moving warm stuff behind me ...!!!"

Tigger

[mike150]

Had been thinking about doing the bonnet cut out thing for a while, but had got to the same position as Stu - in thinking that the hole would probably allow hot air to escape but probably wouldn't allow the filter to suck in cold air (unless it's extended out of the bonnet line and into the forward air flow). As a result, decided not to bother but (and hate to say it) took a lesson from the cateringvan fraternity  which have a low level air scoop as an option on some of the R cars.

Now have a sccop sitting under the steering column  and below the nose cone(as low as the sump) with a 60mm ali hose feed to directly adjacent to the filter - which feeds lots of lovely cold air directly onto the trumpets at speed. I know this works, as I've also got an under bonnet temperature readout on the dash (sensor next to the air filter) which reads significantly lower with the scoop on than it does off . Entire set up came to less than £25, providing you're happy to utilise some resoures imaginatively!!!

Can post pics if anyone is interested and I can figure out  how to.

Mike

[stu999]

Now have a sccop sitting under the steering column  and below the nose cone(as low as the sump) with a 60mm ali hose feed to directly adjacent to the filter - which feeds lots of lovely cold air directly onto the trumpets at speed. I know this works, as I've also got an under bonnet temperature readout on the dash (sensor next to the air filter) which reads significantly lower with the scoop on than it does off . Entire set up came to less than £25, providing you're happy to utilise some resoures imaginatively!!!

Can post pics if anyone is interested and I can figure out  how to.

Mike

Grand idea, and as a bonus it makes use of some of the otherwise unwanted air underneath the car too

[Jenko]

An interesting discussion, and some good points...............

Stu, I'm not suggesting the filter is restricting airflow, although it will to some extent....but if you imagine holding a sponge in front of a hair dryer, the air felt on the other side of the sponge will certinly be slowed, and I would imagine that would be the case with a foam air filter....so, if you are travelling forward at 30mph, the apparent wind in the filter would be considerably less than out side, hence I dont think there would be much of a ram air effect...............at the same tim, I don't think the air is rushing over the tops of the trumpets......

Also, my car has a ducted nose cone, so the rear of radiator is actually covered.......Although, I thought when fitting a cold air induction kit to tin tops, you normally put the filter behind the rad as the air is actually cooler (again, could be wrong here).....

Back to the hot air coming out of the bonnet cut......I'm thinking the huge hole under car (i.e. the entire engine bay that is not covered), will off far more suction of hot air removal than a small hole in the bonnet, and thus cooler air would be sucked in through the hole (open to be shot down in flames on that one)............

[mike150]

Best way of removing the hot air from under the bonnet wouldbe via slats / louvres on the tub side or bonnet top - which suck the hot air out at speed (similar to a self bailer in a dinghy / yacht) and in theory would allow cooler air to be sucked through the radiator or up through the bottom of the engine bay. Filter then wouldn't then have to suck in hot air and my feet wouldn't have to cook after 20 mins in the car!!

For the ducted nose cone, may be a couple of ali pipe runs from from the front of the car (or between the wishbone mounting points) to the filter would help?

[stu999]

But how fast is the air travelling out of a hair dryer?  

I know what you are saying, and yes, I agree it will go *some way* to slowing down airspeed. Now Mr Saddo here (*waves*) has previously conducted a very unscientific experiment on the way back home from Demon Tweeks when I purchased a foam filter for my car last year. I thrust the filter out of the car window with my hand inside (SWMBO driving) to see if I could judge how much air does get pushed through - and it is a surprising amount

The 'huge hole' under the car is the route the hot air from the engine/rad is forced to take - but often the area under the car is a 'higher pressure' area - there is a lot of wind trying to force its way under the car at speed. Drilling holes in the bonnet is likely to allow air to vent into a lower pressure area instead -which it will want to do more readily...

[Tigger]

The 'huge hole' under the car is the route the hot air from the engine/rad is forced to take - but often the area under the car is a 'higher pressure' area - there is a lot of wind trying to force its way under the car at speed. Drilling holes in the bonnet is likely to allow air to vent into a lower pressure area instead -which it will want to do more readily...

An interesting discussion indeed and can be argued many ways without concrete proof of which we will probably never obtain.

Stu,

I would have thought that the pressure just around the carbs/throttle bodies would have been a lower pressure than the areas either side i.e. bottom of car and outside of car.

I would come to this conclusion thus.

Air travelling under the car would be the same pressure/speed as air travelling over the bonnet.

Any air in the engine bay would be slower as it has to circumvent engine,rad and various other parts on it's route  past the car.

Surely this would make it lower pressure hence any air over the bonnet and under the car would find it's way into the engine bay area not the other way around.

My head is starting to hurt now working this one out

Tigger

[ljsanders]

Isn't dumping the bonnet and buying a V8 one just as good?

and its a damn sight easier.

Lets loads of cold air in whilst driving along, although admitedly its not directly into the intake, surely its nearly just as good?

My Zetec has a V8 bonnet and it looks the dogs as well.

[Chasmon]

I think I've made my mind up to have a cut in my bonnet if only to allow the inlet to be close to the optimium length.

I see the advantage to be not just the cold air (jury still out) but the fact that the filter won't be pushed up against the bonnet. I remember reading (thought it was on the Jenvey website but can't find it now) that the filter being close to a bulk head or bonnet etc can lose some serious power. Fomr memory I beleive it was backed up with examples... But can't find the article now

I'm aware that K&N style filters attempt to use the harmonics to increase power but thinking about htat stuff gives me a headache.

[Lurksalot]

Well I think that particular can of worms has been opened

In my case ,I also have 100mm ducting from the inside of the front rhs wishbones to the underside of the air filter.

I don't think I have the time and patience chasing marginal performance in this manner

Thanks for all the input on the subject.

I shall have to find a tuned 2ltr Zetec and drop it in  

[busapower2001]

Ram air pressurisation only becomes signicant >>150mph local air speeds (not road speeds). Forget it basically, unless your talking perhaps drag racing bikes/cars or definitely aircraft or missles.

Filters cause pressure drops when air is forced through them. The pressure drop generated increases as a square of the flow rate (velocity) through them. The pressure drop vs flow rate curve is a characteristic of the filter material (foam, paper etc), thickness (thicker implies higher pressure drops) and it surface area.

The smaller the filter surface area the higher the pressure drop caused by the filter (for a given flow rate).

So by having your bonnect cover a large proportion of your filter your stopping the air from being able to flow through where your bonnect touches your filter => (implies) your available filter surface area is actually less => the filter generates a higher pressure drop for the same previous flow rate => the flow rate would drop until the pressure drop across the filter (and induction system) is equal to the suction generated by the engine  => less power (since less air flow also => less fuel).

What can be done to counteract the situation? Don't let air filters touch (or come close to) other objects such as bonnets. A larger surface area (of the same material) filter will generate lower pressure drops and allow more air to flow. Short of a very poor filter design you aren't really going to gain much BHP by doing a filter mod (as Jenko's says his filter mod resulted in much better throttle responsiveness due to bigger surface areas), however I could see that by having a lot of the surface area of your filter blocked (touching the bonnet) you could well be loosing lots of BHP since your trying to ask your engine to suck through a straw rather than say a large diameter waste pipe! Side note ... have you tried sucking up water through one of those purification water filter straws that you can use at the edge of a lake or river? Did you find it made your lungs collapse when you tried.    Try the same through a similar sized mcdonalds straw with some tap water. Ah easy!

On the cold air front. I believe that engine power varies roughly linearly with intake air temperature (density). So lets say we are drawing in air from the engine bay at 80C and our ambient air temp is 20C. If we had a ducting system  (as mike150 has. It needs to be large diameter ducting though else the ducting pressure drops start to become signifant! 60mm sounds great!) or we expose our filters directly to the ambient air then what might we gain? Well if we could achieve 20C at the filter then that represents an air density increase from 1kg/m^3 to 1.2kg/m^3 (20%) and as engine power is essentially proportional to charge density so a similar power gain should result!

Some final thoughts... Exposing your filter to the air through a cutout in the bonnet is fine but make sure you seal the filter to the bonnet gap to stop hot air being drawn out from the engine bay and into the filter (doh). Essentially, seal the airflow path your after.

The same principal applies to rads and fans and fan shrouds. One mod I need to make is to make a bulkhead out of some plastic sheet and to place it around the gaps between the sides of my rad and the nose cone. This will then mean that 1.) air has to pass through the radiator core and cannot then recirculate back around the sides (fan on stationary vehicle) and through the rad core again (hot air trying to cooling hot water is not good!). 2.) It will also ensure that more air flow passes through the radiator since currently a lot approaches it and goes "OH NO here is a radiator that has a massive pressure drop! There is no way I want to go through that as it is too much like hard work! Hang on a minute there is a massive gap all around the sides of the radiator. Yippee I'll take this easy route and not flow through the radiator core!", and this means you miss out on valuable cooling air! (sorry for giving air a personality!)