glen2002

I am about to do so motor upgrades and as part of that am considering pressing some old school hood vents into the hood to aid cooling. Have searched plenty here and also outside and am unsure of the best position. Many cars seem to have them starting just before the struts and finishing near the firewall (which would also mean they work when the car is stationary) and this would agree with the diagram below. Is that the consensus this is the best spot? Just want to get all the information before punching holes in my bonnet

V2Rocket

About on the struts is good towards the firewall. The lowest pressure zone over the hood is where the vents should go.

Dash01

Per your graph above, about point 11 toward the leading edge of the hood, point 30 on the roof not being convenient for your purposes.

Note that point 11 is pretty near where the vent is is Cayman, etc. since that's where the wind is going fastest and so making for the lowest pressure. Higher the air speed = lower the pressure, so vent as close to there as practicable.

Could be, places on the side of the car, perhaps just aft of the front wheel wells, would also work well. I've considered that to vent turbo and exhaust manifold heat.

HICKS

Why do you think you need hood vents?

968 GUY

Check out the windshield cowl on the 968 turbo RS. it is at the lowest pressure point on the hood, just at the windshield.

rlets

This was function over form. I placed them as far forward as possible to be in the low pressure zone over the front half of the hood. There's an oil cooler in place of the intercooler and a big all aluminum radiator in the stock location. Cooling is much improved.

Rich

rlets

Actually no. That is a high pressure point. The lowest pressure point is #12 in the above diagram, toward the front of the hood.

Rich

Adker

A couple of points about that chart that I ran into when looking at it. You need to count the dots on the car and then find that number in the graph, the car and the graph are not lined up. The positive pressures are below Zero, backwards from Cartesian coordinates we normally look at. Most vents on the middle part of the hood are sucking air out so the louvers should point toward the windshield. The car is a 924.

Some nice stamped hood vents.
https://rennlist.com/forums/10768621-post18.html

glen2002

Adker, big thanks for pointing out the car and graph aren't lined up, hadn't realised that! The stamped vents you link to are exactly what I was thinking, in that position.

I realised the car was a 924, wasn't sure if there was any information on 951.

V2,Dash, 968 guy thanks for the confirmation I am heading in the right direction.

Dash I had an old BMW 2002 Turbo which I put holes from the engine bay into the wheel well, worked pretty well at extracting heat though I am not sure what it did for brake temps! I was always worried it compromised the shell strength, so was going for hood vents this time. I also like how hood vents will expel rising hot air when stopped, thereby helping components like hoses etc.

Riets really interesting setup, I assume you have a V8 in there or if not where is the intercooler?

Hicks, don't think I actually need vents, more while I am there. My hood is just starting to show signs of needing a respray, despite being parked in an underground garage the last 15 years (I know guards red is a bad colour for the sun). While I am going to spray it would be nice to help underhood temperatures, mainly in the interests of longevity of components. It also means peace of mind if I take the car inland in summer where 100 F temps are commonplace and warms days hotter.

Adker

I have never seen a 951 diagram but I am sure the front of the car will be close to the same as a 924 but the back points between 40-50 will be different due to the spoiler and the rear diffuser.

MAGK944

If I'm not mistaken the 951 engine under tray forces cold air into the engine bay. So regardless of the placement of the vents in the hood, if there is a vent the hot air will find the path of least resistance to exit anyway.

glen2002

Mike & Gina, it looks like that when you take the under tray off, but I realise I really have no idea about aero - made even clearer after reading Patrick's (333pg333) thread which had his aero results with all his extras. You may be right, my thought is if I place the vents at the lowest external pressure point, that will provide the maximum pressure differential to underhood pressure and exhaust more air - hopefully with positive results.

Arominus

https://rennlist.com/forums/944-turb...an-needed.html

MAGK944

Whether the under tray has an effect or not, physics dictates there is only one way that hot air is going and that's up towards the hood where it will eventually find it's way out through the gap by the windshield. If there is a vent anywhere in the hood it will provide another escape path.

My thinking fwiw is that it would be better to assist the airflow inside the engine bay to help push that hot air towards the windshield gap, maybe add some NACA ducts drawing air in from the front of the hood. These were not used for that purpose on this car but the placement would probably work in pushing hot air out. Interesting topic.

V2Rocket

it wont get out through the cowl, that is a very high pressure zone. hence cowl-intakes for muscle cars etc.

over the middle of the hood span is the best place for evacuation, lowest pressure spot.
as far as the car in the picture above, that looks more like an aerodynamic aid to keep air from hitting the wipers and to smooth it over the windshield corner.