I was having overheating problems with my car (used and abused). A friend of mine gave me a spare hood, so I said why not. I did some testing with a manometer and this is what I found:


The data shows pressure readings prior to hood venting. I plan on conducting another string of tests for comparison, but it takes a long time due to the 3 inch resolution of the test itself (lots of stopping and re-taping)

The data shows an average ΔP over a 30-60 second time frame @ GPS verified 50 mph (would like to get 120+ data, but need to find directions to Mexico.)

In theory, ΔP increases exponentially as the velocity of the vehicle increases. Again, more testing would be required to answer that question



So a little explanation: the negative(-) is the amount of pressure difference between the inside and outside of the hood along the center line. Negative being that there is higher pressure inside the hood than outside.

As you can see, the highest pressure zone is ~19 inches from the front of the hood, which is where I placed the center of the large vent.

OK. Here is what the vents look like. I ordered them from Trackspec Motorsports. These are the full GT2 vents (largest version) without the spacers they offer:

Work from the middle out towards the corners when riveting:


I radius'ed the corners to prevent splitting with a 1/4" drill bit:



I used Cleco clamps to hold and align everything. VERY HELPFUL TOOLS TO HAVE!!!










Finished product:








Findings:
- Oil temps in traffic dropped from averaging around 235-245 to 215-225 with decently hot Florida summer weather.
- Coolant temps typically stay within a couple degrees of thermostat (187-192). HUGE difference. Better temperature comparisons will come when ambient temperatures climb.
- The front of the car is noticeably more stable at 140+.
- Top speed has been decreased from ~176 mph to ~168-170 mph.

 

Nice work. I'd like to see pictures of your test setup.

I did find it interesting you found the pressure was always negative. There have been lots of "theories" posted saying the pressure turns positive towards the back of the hood.

 

Multiple CFD tests have shown that there is a major high pressure zone at the at the base of the windshield, a couple of which were done by GM themselves. This may be why there is a piece of weather stripping running along the back side of the hood that would prevent air from reentering the hood. The data shows that just before the weather stripping there was an increase in outside pressure, with little to no increase in under hood pressures, which would indicate that air is not reentering the hood. Keep in mind, this is ΔP testing. As outside pressures build when moving towards the base of the windshield, you want to see the pressure differentials level out and go positive (+). This is what the acquired data started to show.

Unfortunately I only have one good photo of how it was setup. I mapped out the compound curves of the hood with masking tape, along with the primary areas of interest (center line and just over the fenders).

This ended up being a waste of time, as the pressures were similar laterally. The only variable that altered the pressure was moving the manometer probes fore and aft on the hood.

I did the test at a resolution of 3 inches @ 50 mph.

 

What I was meaning is that you found the engine compartment pressure was always higher. There have been claims here before that a vent too far back would actually push air into the engine compartment which would then end-up under the car.

I understand that the cowl is supposed to be a high pressure area. Still, there used to be a racer here who ran a C5 and he did claim another C5 with the hood seal missing vented steam and oil smoke from the cowl and over the windshield when the engine blew up while on the track.

How did you ensure the air flow over the hood wasn't affecting your result?