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I posted this in Tech, but I figured it would be better off in here. (Mods, you can delete the thread in Tech if you want)
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)
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.
I guess there was an untested theory that air was reentering the hood at the rear due to a high pressure zone near the cowl 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 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.
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.
***UPDATE***
I managed to nurse the LS6 back into a usable state and managed to do some testing in between practice runs at Daytona's Trans Am race.
These groups of tests focused primarily on the front wheel wells of the car. I apologize for not taking any pictures of the test setup or creating any charts to map out the acquired data.
I would like to state beforehand that I do not currently have any form of pressurized brake ducting running behind the spindles. As one could imagine, ducting would introduce some amount of pressurized air into the wheel wells. This is something to test at a later date.
ΔP between the inner wheel wells and the engine bay consistently mapped in the -0.01 psi to -0.03 psi region, regardless of probe placement and average test speed. At first I did a baseline at a GPS verified 50 mph using a resolution of 3 inches starting at the front of the wheel well moving back. There was no significant ΔP across the wheel well fore and aft. I then tested along the Z-plane (facing the wheel), starting at the upper strut mount and moving towards the outer fender liner. Same results.
Interesting point: the slightly negative recorded data points indicate that under hood pressures are higher than those in the wheel well. I am unable to generate a CFD map of this area, but I would suspect that, despite there not being a significant ΔP, there would be a measurable amount of airflow. Despite this, ventilating the wheel wells into the hood would prove to be ineffective at reducing wheel well pressures as there are none.
Next is to map ΔP on the hood after the vents and yarn test how the air re-enters the vapor barrier.
-CFD
Last edited by Converted_Germ; 12-17-2015 at 11:05 PM.
GREAT POST !!! It is interesting that your mods were done using actual data from testing. Too many times mods are done by the seat of the pants. Good results, let us know what the summer differentials in temperatures are too. Your comprehensive photos and instructions will be of great help to the next person performing this mod
I've thought about looking for a cheap hood to do this to for track days. When I looked further, I found the hoods are $150ish and the vents are like $200. So $350 is a lot of money to not be sure if this has a positive effect on temps. You have confirmed it works, I appreciate it!
I'm running 18x10.5 square with 315/30/18 BfGoodrich Rivals.
I find myself to not really care about the appearance of the car, as you can see by the pewter on black combo. Trackspec offered me spacers for the center rear portion of the vent, but meh...
315s all the way around? Does it rub on the front out in the rear? Are you lowered or at stock height?
Can you possibly take some more pictures of the profile of the tires compared to the side of the car (preferably with the wheels straight)? I'm really curious to see how far they stick out past the fenders. You don't have to post them in this thread, a pm would do.
I've thought about looking for a cheap hood to do this to for track days. When I looked further, I found the hoods are $150ish and the vents are like $200. So $350 is a lot of money to not be sure if this has a positive effect on temps. You have confirmed it works, I appreciate it!
Dan
I've been wanting to do this exact mod on my C5. Where do you find hoods for $150?
Are the smaller side vents still just venting air from under the hood or do you also cut the wheel wells? I've been thinking about doing these myself, just worried about the side effects of it on a street car that often is driven or parked outside in the rain.
Did you center the vent on 19" or start from there up?
I'm not sure what you are asking. The measurements started at the front edge of the hood and ran along the center line to the weatherstripping under the windshield (full length of hood).
The data showed that the highest pressure zone under the hood of a stock rearward tilt radiator'd car is @ 19 inches from the front edge of the hood. I then placed the center of the large vent @ 19 inches from the front of the hood, as its the most effective positioning to evacuate under hood pressures.
Last edited by Converted_Germ; 11-23-2014 at 02:53 PM.
Are the smaller side vents still just venting air from under the hood or do you also cut the wheel wells? I've been thinking about doing these myself, just worried about the side effects of it on a street car that often is driven or parked outside in the rain.
For now, the smaller vents are limited to ventilating under hood pressures and radiant heat from the exhaust manifolds (a surprising amount of exhaust heat is evacuated through them at a standstill).
You could probably reheat food by placing it on top of the vents.
I plan on doing more ΔP testing on the wheel wells so see if they necessitate pressure ventilation. This was the original plan for the smaller vents on the hood.
FYI, this car is my non-garaged daily driver in Florida. GM did one thing right with these cars, the weatherproofing of the harness connectors and other assorted electrical bits. I have yet to have an issue with water intrusion.
Just be smart and do not vent the hood past the weatherstripping around the battery compartment (battery, fuse box, PCM, harness grommet).
Last edited by Converted_Germ; 11-23-2014 at 03:05 PM.
How does venting the hood reduce top speed...I would have thought the opposite. Is it due to more down force? Would it also kill quarter mile times?
Well, without doing more testing, I can only guess that in stock form, having a consistently higher pressure under the hood than outside, lift would be generated. This lift reduces the effective weight of the car and, in return, reduces rolling resistance and increasing top speed. Added wind resistance from trapped air would need to be included in the calculation, so as with everything I'm sure there is a happy medium. The vented hood sounds do the inverse of this.
As for 1/4 times, I'm sure it would effect it. The severity of said impact could be negligent, depending upon the car. My car is probably not effected very much, as it's only a 117.5 car. But I'm sure a 150 car will have a more noticeable reaction.
Last edited by Converted_Germ; 11-23-2014 at 07:34 PM.
For now, the smaller vents are limited to ventilating under hood pressures and radiant heat from the exhaust manifolds (a surprising amount of exhaust heat is evacuated through them at a standstill).
How did you determine where to place the smaller vents?
How did you determine where to place the smaller vents?
I placed the passenger's side vent in between the battery compartment and coolant reservoir, directly over where I would vent the wheel wells. Then it was a matter of squaring off the driver's side vent with the same measurements.
I forgot exactly what the measurements were, but I can take some if you would find them useful.
For now, the smaller vents are limited to ventilating under hood pressures and radiant heat from the exhaust manifolds (a surprising amount of exhaust heat is evacuated through them at a standstill).
You could probably reheat food by placing it on top of the vents.
I plan on doing more ΔP testing on the wheel wells so see if they necessitate pressure ventilation. This was the original plan for the smaller vents on the hood.
FYI, this car is my non-garaged daily driver in Florida. GM did one thing right with these cars, the weatherproofing of the harness connectors and other assorted electrical bits. I have yet to have an issue with water intrusion.
Just be smart and do not vent the hood past the weatherstripping around the battery compartment (battery, fuse box, PCM, harness grommet).
Awesome, thanks for the info. I ask about the wheel wells because in NASA TT I am allowed to vent the hood without penalty but if I vent the wheel wells I have to add ballast or reduce hp.
Do you have a stock positioned radiator or has it been tilted forward?
Awesome, thanks for the info. I ask about the wheel wells because in NASA TT I am allowed to vent the hood without penalty but if I vent the wheel wells I have to add ballast or reduce hp.
Do you have a stock positioned radiator or has it been tilted forward?
My research has shown that on the Corvette C5 the wheel well area in the front of the car is a low pressure area and not a high pressure area.
I would like to see what Germ's research determines.
11-22-2014, 12:51 PM
ΔP Testing on Stock C5 Hood - Trackspec Vents
