Bonnet louvres

Great response, really interesting too.
I have been looking at the idea of getting some louvres stamped into my bonnet as cooling can be marginal so I will do some more research.
Interesting that you talk about how they are only effective where there is a pressure differential, and how they can reduce lift. I have reading about their use in Simon McBeaths book and starting to appreciate how these things work together.
I am currently thinking that a couple of NACA Ducts, positioned in a high pressure area of the bonnet, to provide greater air under the bonnet would help provide ambient air for the carbs and force fresh, cooler air to the exhaust manifold. This, combined with some louvres to exit more air, as you say, over the car rather than underneath it, seems to make sense.
I love this aerodynamic enginnering stuff and am hungry to read and learn more - it's not exactly straightforward is it!

I don't think a lot of cars as standard had overheating issues - only suffering as a result of poor or incorrect maintenance or abuse.

If you think about it, it would be pretty silly to release a car that even in fine condition couldn't operate properly.

Dolomite Sprint (and likewise the Stag) cooling system can be fragile if not looked after properly - but a well sorted one will deal with even heavy traffic without much issue.

Don't forget that most of this stuff is at least 25 years old now....hence the prevalence of problems.

Aero can seem very very complicated and hard to understand, especially if you attack it from theory from books. I struggled when I first started learning at uni, but was lucky enough to get some time in the MIRA full size wind tunnel as part of the course, and once the practical element was there it almost seemed easy.

I like autospeed.com for its aero info - here is the 1st part in their practical aero testing series, well worth a read autospeed.com/A_108656/cms/article.html

and something about locating vents - autospeed.com/cms/A_2159/article.html

Just search Aero or Aerodynamics on there and you'll keep your brain interested for hours and hours

alolymipic - regarding vents to get cold air into the carbs (and cool the exhaust) plus other vents to get hot air out and maybe give some aero benifit - this is totally possible, but requires a lot of thought, and even more testing to make sure it's actually working.

If it were me, and I wasn't planning on spending a year choosing where to put the vents, I'd build a cold air intake for the carbs, plummed into a high pressure area (base of the screen is good - see very good article here autospeed.com/cms/A_110680/article.html) that way you know you're getting good flow of cold air to the intake.

Regarding exhaust header temps. I'd simply wrap the exhaust (if its steel or cast - its a bad idea on stainless as the higher temp often cracks open the welds) and thus keep the under bonnet temps down that way.

As for aero stuff - well if you think you have high pressure behind the rad, the best place to release it is from the back of the front wings - this is normally a low pressure area so it will draw the air out, here's a nice pic



That gives you some idea of pressure levels on the body (red is high, dark blue low) and has flow speed lines too for more interest!

If you want to reduce lift - well venting it out the side will help, out the top is better, but much harder to choose where to vent it from position wise! I'd not worry about it as you need to do some proper testing to get it right. But if you fancy being practical, go for it ;D

Hope that makes some sense and isn't too boring either

Well if hot running is due to under bonnet temps from exhaust etc, rather than inefficent cooling from the rad, then in traffic, yes.

While this is true when sationary, things get more complex moving. True it might work, on that porsche at that point on the bonnet I'd have thought it'd generally draw air out from the engine bay when moving - but the question is the benefit worth of the effort of louvres or what many would consider the disadvantage (in terms of looks and water ingress) of holes.

In my own opinion, if the engine is running too hot due to exhaust temps, then the exhaust needs redesigning or wrapping/shielding, cutting holes to let the heat out is a bit of a bodge.

Think it's important to pin down why something is running too hot, then work on how to best improve things to counter the hot temp.

Several things to check for cooling problems, I always make sure the rad is sealed to the front panel, so all the available air is pushed thru the rad, and not sneaking out the edges, I use self adhesive foam tape to seal this, which is available in different widths, thickness's and density, same goes for fans, they must be either ducted or fitted as close to the rad as possible, even using foam again to ensure a good seal, have the fan stat set as close to the rad thermostat as possible