air filters

[chrisj]

Whilst at a rolling road session I was advised by a fellow westfield chap to cut a hole in the bonnet and fit a new air filter to draw cool air in from outside.  I currently run 2 x 40 dellortos and have k & N filters fitted.  The car is a SEI wide bodied and at present the filters fit under the bonnet.  Can anyone recommend a filter assembly, do I need trumpets from the carb as present I dont not have any.  Also does anybody know the easyest way to cut the hole???  HELP

[stu999]

Hi Chris. K & N air filters come in varying heights, not to much information to go on, but I should imagine that you have quite shallow ones fitted. Its merely a case of purchasing some more with a greater depth.

If you remove the lids of the air filters, you should find the trumpets inside. If not, buying some should be your first priority. If you have to purchase some, you will notice that they are also sold in an assortment of lengths. When choosing the size that you require, theory says that short ones make more top end power, longer ones hike the bottom end up. In practise, this is not always the case, but either way your engine will make a lot more power than without!

Another thing to bear in mind, if sticking to K&N's do not buy trumpets that come close to the lid of the air filter, you want at least 40mm from the lid to the trumpet mouth.

As for cutting holes for clearance, careful measurement with the air filters fitted (using masking tape on the body to mark either end of the filter protrusion) and the height of the filters from the upper chassis member. Remove filters, stick bonnet on, and transfer measurements to the bonnet. You now have a base line and the length to work from. For the rest of the shape, you can use the filter lids to draw around. Remember, the resulting holes will have to be slightly bigger than the filters to allow for movement from the engine.

One last thing, I dont really know whether it is actually worth the effort! Yes, cold air is definately better than hot for your engine, but in 'real-life' when you are driving, under the bonnet there is a slight positive pressure that *may* be slight turbocharging for free! Also, with the air filter hole, hot air is going to be forced past the filters from the positive pressure under the bonnet. And with the way Mother Nature works, if air is rushed past a hole (your trumpets sticking throught the bonnet) it will actually try and pull air from the hole. Ideally, an air scoop which isolates the filters completely from the engine bay with a foward facing intake outside of the bonnet is the way to go. Most of this is theoretical and I have no figures to prove either way and now I am going to hide under a table.....      

[Liam]

I have 45's on a standard Pinto with 165mm trumpets and Ramair foam filters with an internal wire skeleton to keep the shape (Rally Design) that just push on. It develops 100.3 at the back wheels so just the road/rally cam to fit and that'll do for the minute.

I had to cut an oblong hole and used the trusty Dremmel. All masked up as suggested - use a cutting disc for the staights (it's not as frightening as it sounds - they are remarkably controlable if you have something to rest your arm on)and a sanding barrel to finish the corners - dead easy - wear a mask, close all the windows and take the washing in!!!!

Liam

[Blatman]

I'm with Stu on the "possibly not worth the effort" comment.

My green car has no hole for the filter, and makes well over 180bhp. It's sister car (GeeFin's carbon car) made almost exactly the same bhp on Dave Walkers rollers. The only difference is that his car has twin 45's. Internally, I think the engines are identical, or as near as dammit, so Stu may well be right, wrt effort expended for bhp gained.........