MrTomT

Down here in the Australian summer, my 928S can run pretty hot in the midday sun. I've been wondering about the benefits of extraction vents towards the rear of the bonnet (or hood) to enable more airflow around the engine. Has anyone tried this modification and had much success?

I've had a pretty good search around rennlist, and found a lot about forced air induction and intake scoops and vents at the front of the hood, but I'm not interested in that. From what I can tell the air flows quite well through the front of the car to the intake tubes, so don't want to unbalance that system, just want to find a way of clearing more air out of the engine bay to aid in cooling?

Thanks in advance,

Tom

RKD in OKC

The rear of the hood is a high pressure area. Air will go in, not out. The result would be less air going over the top of the car and more air exiting under the car. If you look at Mark White's 200 mph car, The White Car, he covers the intakes on the front of the car and takes all his air in at the back of the hood. A lot of muscle cars have reverse hood scoops for the same reason.

belgiumbarry

time ago i was playing with that idea to take high pressure air for the carbs... but quit that because i didn't found a scoop that matches the 928 lines and was giving me enough clearence above the carbs... it was tooooo ricy

it started with a sketch .....


ofcourse paper is forgiving.....

jorj7

I put cooling vents in the back of my hood. I made sure that they were forward of
the back part of the wheel well around point 9 in the graph below, so it's still a low
pressure area:

MrTomT

That's the sort of thing I was thinking of George. What sort of results did you get?
RKD I don't think air would go in. If you look on the above graph the highest pressure area is at the lower air intake in the nose. The next peak at 13 is the angle change from the windscreen. 10 is about as far back as you could go and is almost zero on the graph, so air would have to flow out from there surely. I'm no expert though ; )

WallyP

The pressure graph is drawn upside down! High pressure goes down on the graph, low pressure goes up. Lowest pressure on the car is at the top of the windshield (windscreen), while the base of the windscreen is a high pressure area.

jorj7

Tom,

It seems to help keep the engine compartment cooler. I also wanted to have a place for
the air to escape after going through the radiator that wasn't under the car. I was hoping
to keep as much air out from under the car as possible.

I've just installed soon air temp sensors on the intake, one of which is behind the radiator.
On a 53 degree (F) day, the air going into the engine bay behind the radiator was already
143 degrees. That was with the water temp gauge sitting a little over 170 degrees. I'm
sure the air is hotter by the time it gets to the back of the engine, but I don't have a temp
sensor there, so I can't confirm it yet. I do have one on the intercooler outlet, and the air
there was around 93 degrees, 40 above outside air temp and 50 below the air that passed
through the radiator. Also, the temp at the supercharge seems to be 20 degrees
higher then the air passing through the radiator. All this was without pushing the car very
hard, so I expect the temps to be a lot higher when I'm in 100 degree weather and doing
170+ mph. I hope I'll find out soon.

hacker-pschorr

From a different thread with the same topic:

https://rennlist.com/forums/3487465-post17.html