cold air intake for weber carbs

greku · #1 (**permalink**) 02-07-2008, 11:53 AM

Hi there, lately i've been discussing with my alfa mechanic about fitting K&N's on my IDF's what he told me was, it would have a gr8 sound but it will reduce performance because it will suck only hot air from the engine compartment. what i want to know is, why? and how is cold air better in carbs? cheers thanks beforehand

77SudTi · #2 (**permalink**) 02-07-2008, 01:40 PM

Cold air is more dense, this means that the colder the air the more air molecules per 'suck' stroke of a piston you are sucking into the cylinder. The more air you have the more complete the combustion of the fuel is and you feel this as increased power. With hot air, the molecules are furthur apart, so you get less molecules of air into the cylinder per 'suck' stroke of the piston.

Interestingly enough, pilots call cold air, 'Heavy air' and due to the density of the air being greater than hot air, they handle the aircraft slightly differently, as in cold air there is more lift when compared to hot air, just as you get more bang in your engine with cold air.

greku · #3 (**permalink**) 02-07-2008, 02:09 PM

cheers thanks, what i was thinking of doing is remove the airfilter box off my 33, and with each elbow i fit a pipe that is directed to the grille where cold air can be sucked, and on the front i will place a normal cone filter this will give me the cold air, and a gr8 sound, basically like the one on the injection cars. what do you think? open for suggestions and critisism! hehe

Greek · #4 (**permalink**) 02-07-2008, 02:34 PM

That ws my thought also but you have to put some kind of filter otherwise they will suck a lot of dust and other thinks into the carbs...

greku · #5 (**permalink**) 02-07-2008, 02:44 PM

Quote:

Originally Posted by Greek

That ws my thought also but you have to put some kind of filter otherwise they will suck a lot of dust and other thinks into the carbs...

evilgidget · #6 (**permalink**) 02-08-2008, 02:02 AM

This cold air intake is a modification to the original airbox. Basicly, I've cut the front off the airbox & added a revised fibreglass intake.

Filtration is kept the same (above average for this size engine), yet the engine now draws cold air from between the grille & bonnet, and breaths better.

Oh yeah, it sounds pretty meaty too

I'm not going to build any more, as it was an absolute b@st@rd to form into shape to work around everything. Alternator, thermostat, emissions equipment & coolant lines all come DAMNED close to interfering, but with persistence, it works well, despite looking a bit rough

evilgidget · #7 (**permalink**) 02-08-2008, 02:03 AM

Oh yeah, it does look better than that now, These were pics from initially building it. It's smoother & all black now, hole on the airbox is larger due to different alternator being used etc.

greku · #8 (**permalink**) 02-08-2008, 08:23 AM

Yes, it's also a good idea, but i just don't want to cut in it's original airbox, neways, today i was telling my mechanic about it and told me the only drawback would be that on idle and when i'm not moving the carbs wil be chocked because of the longness of the pipe, when moving it would not be such a problem because the air is coming against the car and wil help itself in. also there is another pipe coming into the airbox. it comes from i think is the exhaust pipe, is it putting air in? or getting air out? if it is putting air in i am supposing it is hot air, why is that?

damonb · #9 (**permalink**) 02-08-2008, 02:42 PM

That is the hot air intake. There is a thermostatically controlled flap inside the airbox which directs warm air from around the exhaust manifold when it's cold and switches to the main cold air intake once the intake air reaches about 30-40°C. It's well explained in the Haynes manual. There are 2 main reasons for this feature - one is to aid the running of the engine when warming from cold, and in very cold conditions (near or below freezing) it prevents the carbs from icing up. If you live in a warm climate where temps never get to 0°C or lower it isn't really required.

Note that the thermostat part often fails, leaving you with hot air only. This is not good.

The std twin carb airbox with it's huge filter and nice curved aluminium "trumpets" on the top of the carbs is a very good design and hard to improve on if you have a standard engine. You would need a very greedy engine to require more air than what it can supply.

dave33 · #10 (**permalink**) 02-08-2008, 03:30 PM

One thought,

If you are not likely to need the warm air to stop your car from icing up, you could remove the thermostatic flap from inside the warm air intake, and redirect the warm air duct forwards to catch more cold air into the air box.

lenus · #11 (**permalink**) 02-08-2008, 04:25 PM

Pointless, but, the other day I was fiddling around with airboxes on my sprint and also the little bends for the top of the carbies, as I have quite a few laying around, I noticed that the bends that come off a 1.7 carb engine are much MUCH larger then the ones that come off the 1.5 engines.

It looks like they have just been bored out to match the size of the carbies, so maybe one way to get a little more airflow down the guts of a 1.5 engine would be to get the flutes off a 1.7 for their larger diameter? or even boring out the 1.5 ones...

Also, if you are afraid of cutting into the original airbox I have some laying around that aren't in use you can practice on if you want one! .... If you are in Australia.

lenus.

greku · #12 (**permalink**) 02-09-2008, 08:07 AM

Quote:

Originally Posted by lenus

Also, if you are afraid of cutting into the original airbox I have some laying around that aren't in use you can practice on if you want one! .... If you are in Australia.

lenus.

cameron · #13 (**permalink**) 02-20-2008, 04:57 AM

im thinking of cutting my box but dont want to stuff it so ill wait for one of you guys to do it and see how it turns out. dose any one know the k&n filter number for a 1987 33 boxer 1.5 as the local car shops cant find one.

Alfam · #14 (**permalink**) 02-22-2008, 02:33 PM

Quote:

Originally Posted by cameron

im thinking of cutting my box but dont want to stuff it so ill wait for one of you guys to do it and see how it turns out. dose any one know the k&n filter number for a 1987 33 boxer 1.5 as the local car shops cant find one.

77SudTi · #15 (**permalink**) 02-22-2008, 03:19 PM

Does anyone in Australia know where I can get lengths of that flexible hose that people use for cold air intakes and brake ducts? Never seen it anywhere, except maybe in a part store for a rediculous price!
When I was in Germany, you could buy this really great clothes dryer ducting by the metre off a reel, I should have bought some! So flexible, light and never wanted to collapse.