A range of tech tips

Posted on November 2nd, 2003 in Opinion by Julian Edgar

With the use of remote filter airboxes (and intercoolers in all turbo cars), there’s plenty of plumbing through which air has to flow before it can become part of the combustion process. Then of course after the burn has happened, the exhaust – again, a long piece of bent pipe – has to be negotiated. The flow through these bends therefore becomes an important part of performance. Sometimes in automotive modification we tend to think that we are inventing something new, but there are plenty of other industries where the flow performance of long pieces of bent tube is critical to the performance outcome.

Click for larger image I recently picked up a book first published in 1960 – the Handbook of Air Conditioning System Design, written by engineers of the Carrier Air Conditioning Company. It’s an extremely comprehensive manual of refrigerative air conditioning design, but in addition it has extensive chapters on facilitating the movement of air through pipes – the latter a fundamental of air-conditioning systems. The pressure drop experienced is even more critical in these applications, where pipe runs are much longer than you’ll find in any engine bay, and the pressures being dealt with are so much less. So these guys looked long and hard at the restrictions posed by bends, elbows, 180-degree hairpins – and quantified the results.

So let’s cut to the chase – what are some of the useful figures? Let’s take a 3-inch tube: that’s pretty common these days on both high performance exhausts and intakes. A tight 90-degree bend (where ‘tight’ means a radius about equal to the diameter, ie in this case 3 inches) poses the same restriction as 7.5 feet (2.3 metres) of straight pipe!

A long radius 90-degree bend (a bend radius of 4.5 inches, or 11.4cm) has a flow restriction equal to about 5 feet (about 1.5 metres) of straight pipe.

A 45-degree bend? Well, one with a radius of bend the same as its 3-inch diameter has an equivalent flow restriction of 4 feet, or about 1.2 metres. A 180-degree bend with a 1:1 radius/diameter? It’s got the same flow restriction as 12 feet of straight pipe – that’s an incredible 3.7 metres!

Get the picture? Those bends – even when they are relatively open – drop flow to a major degree. If the mass of air contained within the pipework isn’t critical (eg as it is for intercooler plumbing, where throttle response loss needs to be minimised) it makes a helluva lot of sense to go much longer rather than put in tight bends.

And we’ve all seen those intercooler plumbing constructions where someone hasn’t bothered using a bend at all – instead they’ve welded a 90-degree mitred join into the plumbing. The Carrier book suggests that in 3-inch tube, such a join has the equivalent flow restriction of nearly 4.6 metres of straight plumbing….

And what about other pipe diameters? Some other data from the book is reproduced here:

Losses in equivalent feet of straight pipe:


Pipe Size 90-degree standard 90-degree long radius 45-degree standard 180-degree standard
2 inch 5.0 3.3 2.6 8.2
6.0 4.1 3.2 10
3 7.5 5.0 4.0 12
9.0 5.9 4.7 15
4 10 6.7 5.2 17
5 13 8.2 6.5 21

‘Standard’ = radius/diameter ratio of 1. ‘Long Radius’ = radius/diameter ratio of 1.5.


Pipe Size 90-degree 60-degree 45-degree 30-degree
2 inch 10 4.5 2.3 1.3
12 5.2 2.8 1.7
3 15 6.4 3.2 2.0
18 7.3 4.0 2.4
4 21 8.5 4.5 2.7
5 25 11 6.0 3.2

Quite a while ago – in fact in June 2001 – I mentioned how the clothes prewash cleaner Preen worked very well at getting brake dust off alloy wheels. At 81.5 per cent liquid hydrocarbons, it is very effective and much cheaper than most alloy wheel cleaners. However, what I didn’t tell you at the time is that right when you’re cleaning your wheels it’s also possible to get a very good feel for the camber settings you’re running on your rubber.


Click for larger image

It works like this. Step 1 is to clean your tyres and wheels while the car is standing on light-coloured concrete. Spray the Preen on…

Click for larger image …then wash it off rapidly without using too much water. You just want enough to get off the chemical and the brake dust, not flood the place.

Click for larger image The water and its accompanying dirt will form a puddle around the base of the tyre.

Click for larger image Give it time to dry and then when you move the car away, hey presto! you’ll see this..

Click for larger image …or maybe this.

Click for larger image Have a look at the two tyre footprints and you’ll soon see that they show a different shape. This was one of the front tyres, where I had the car set up with a little more negative camber (ie the tyre leaning in at the top) than for the back wheels. You can clearly see how the inside of the tyre is being pushed harder against the pavement than the outside – in non-cornering situations the inside of the tyre is taking a greater part of the load.

Click for larger image One of the rear tyre footprints shows that there’s still negative camber being used, but less than that the front.

Now I am not going to suggest that these dirt footprints are going to put an electronic wheel alignment machine to shame, but they can be startlingly clear in showing the change in the shape of tyre contact patch caused by camber variations. Plus when you can do it for nothing at the same time as you’re cleaning your wheels, then…

Click for larger image Another popular ‘tips’ type of story that we ran a while ago was “Ballistic Bellmouths” in January 2002 (if you don’t know what advantage a bellmouth provides – read the article). The story covered how to produce a bellmouth from the stainless steel eggcups available then (as they still are) from Woolworths stores in Australia. The resulting bellmouth had an opening of about 60mm with a taper down into the tube.

But what if you want different-sized bellmouths?

Click for larger image I’ve been keeping my eyes open since and have found an excellent source. They are cookware tins and plastic trays designed to produce jellies and cakes which look a bit like large donuts – the finished foodstuff will have a big hole in the middle. The jelly containers (maybe microwave cakes as well?) are plastic, while the oven-cook cake tins are available in either coated steel or aluminium constructions.

Click for larger image When they’re turned over, the bellmouth formed through the middle can be clearly seen.

Click for larger image Then it’s just a case of using some hand tools (sharp knife, file, sandpaper) to remove all the surplus plastic around the bit that’s wanted. This particular bellmouth is about 90mm in diameter across the outer edge and tapers down to a 52mm tube size. On a small car it would make an ideal engine intake bellmouth or brake duct bellmouth.

Click for larger image If you want bigger, this monster should suffice. It’s a cake tin and the central hole (when that little flange has been filed away) is a whopping 100mm (~4 inches)! Depending on where you trim the edge of the bellmouth, the overall diameter will be about 180mm. As an engine airbox intake or brake duct this is a bloody good thing – all for a new price of about ten Australian bucks from your local cookware supplier…

One Response to 'A range of tech tips'

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  1. Mal said,

    on October 30th, 2007 at 7:20 am

    Bends cause friction loss & turbulence. Tighter bends cause more turbulence than shallower bends.
    I don’t have time to try this experiment but air conditioning ducts that have tight bends contain turning vains to reduce turbulence. If you were to take a tight radius bend and cut 2 longitudinal slots opposite each other in the bend along the centre axis (the slots will look like a right angle with a radiused corner) and inserted a piece of sheet metal (some tin can) throught the slots and silicone it in place and then push/suck air through it you may find there will be less pressure drop.
    As I can’t attach a pic to this blog I can e-mail a pic to describe what I’m taking about.

  2. Julian Edgar said,

    on October 30th, 2007 at 7:35 am

    Yes, as we covered at http://www.autospeed.com/cms/article.html?&A=0491