Like I mentioned before, on another forum I'm on the guy made hood spacers. Drove the car before and took temp tests, then drove it again with the spacers after letting the car sit. Drove it the same way he did the first time and the temps were a lot cooler. I can go find the thread and post the pictures he has with all the temps on it if someone wants to see it. Another guy also did the same thing and got cooler temps.

Okay it didn't take as long as I thought it would to find it. Now I'm not sure if you have to be registered or not on the site, but if you are I can copy and paste http://myspecv.com/f/t162345.html

 

You're not getting what I'm saying. I've already said that this will work well if the hood/windshield area is designed to not have an area of local flow separation and high pressure. I just consulted a few of my racecar aerodynamics books, and verified that some of the prototype racecars use this method because they have a flat underbody. The cockpit glass is shaped like a bubble, and the flow from bodywork (there isn't really a hood obviously) to windshield is very carefully matched, with a slight upsweep on the edge of the bodywork where the cooling flow leaves and flows over the cockpit. Elsewhere in the book it mentioned how bad this was for passenger cars because of the inverted flow area at the change in angle between bonnet and windscreen (I love British books). They had some SAE cooling outlet recommendations for passenger cars, none of which included anything at the base of the windshield. The non-normal ones that I noted were the hood vent type, which is what most racecars use, and the inlet type, like the top mount intercooler on most turbo Subarus.. They noted that the flow MUST exit into a low pressure area (which is NOT the base of the windshield because of the inverted flow) and that the engine compartment doesn't have much pressure in it because most of the pressure is dissipated when going through the radiator. The best cooling flow exit is out of the bottom of the engine bay and the wheelwells, both of which are low pressure areas. It seems that given the pressure under the car, and the pressure at the base of the windshield, air will probably enter at the back edge of the hood, and exit under the car, which would help in cooling if the pressure difference was big enough. Also, at low speeds everything changes because of different boundary conditions related to air velocity and flow attachment.

So at the recommendation of some very highly regarded experts, I'm going stick to my previous statements. If the whole area at the back of the hood, including the windshield, is designed to have cooling flow exit there, it should work great..

 

There is an area of flow seperation and inversion at the back of the car, just like at the base of the windshield. You can put a skirt all the way around the car so there is zero flow under the car, and this will still happen.

There are some very solid reasons that production based racecars don't do this when they need more cooling, and go with something like a vented hood or a "reverse cowl" instead.

 

So what you are saying to me is that if you're traveling 55 mph in an average commuter car (i.e. a civic), there's a stream of air going over the top of the hood at 55 mph and an equal stream of air going through the engine compartment under the hood also at 55 mph?

 

I had to read that like five times cause I'm tired and my eyes kept skipping lines but it sounds like somewhere you or I got lost because your talking about air passing through the radiator area and coming out the back of the hood and I'm referring to about a 1/4-1/2 inch gap between my hood and grill
I have because I removed the weather stripping which lets air flow through much more free. If someone has not done this(as i previously stated) spacing the hood is not as effective.

 

Ok, so I'm not impressed at all. That guy, in all his wonderful unscientific...ness, measured a max difference of 10.5 degrees on the headers. that's a fraction of a percent of the header temp, and the max difference in temp of 13 degrees on the intake manifold. Those numbers are so small that just driving normally could give a larger variation in temp. One thing that this guy failed to recognize is that sitting still, heat is getting vented out of the engine bay too. So at every stop light, and while he's getting out of the car and getting ready to lift the hood, heat is escaping.
The other interesting thing is that those infrared temp probes that he used don't have an accurate reading off of reflective materials because of their emissivity. Shiny stainless headers are not going to give an accurate or repeatable reading, sorry. Regardless, doing one test with not the right instruments in a completely random way and with no way to control the conditions, it doesn't say anything. If he had put thermocouples on what he wanted to measure (no IR emissivity problems) so it could be logged real-time, and done repeated tests on different days along the same stretch of highway at the same constant speed, then he would have something that actually might have some merit. Without something like that, there are way too many variables to even guess at, and the temp differences are small enough that accuracy really comes into play, not to mention repeatability.

It occurred to me that just driving through his neighborhood at normal neighborhood speeds would be enough to change the temps significantly, regardless of how the hood spacers worked at speed.

I'm just saying that his results don't really show anything, and if they actually do, it's not enough to justify raising the back edge of your hood by over an inch (holy shit).
Letting air out or letting air in, it's most likely doing something with the hood raised that high, that's for sure. At that ridiculous height, I wouldn't be surprised if air is actually flowing out of the sides of the hood along the fenders, which could make a lot of sense. Could be flowing in the sides too, hard to tell on that area of the car.
At that height it might also be creating less separation at the base of the windshield, so pressure would be less than before. So yeah, I guess it could work if you want to raise the hood that high, which is insane. Racecars definitely wouldn't do anything like that for drag reasons, and the rules would never allow something like that on the track for obvious safety reasons.

Holy shit though, do you really want to raise your hood that high??????

 

If the weather stripping in the front of the hood and rear of hood are gone and there is space for the air to travel then air will travel at a high rate of speed possibly like 55 mph but it doesn't have to be going the exact speed. It needs to have enough pressure to keep the air from on top of the hood from pushing air back into/ keeping air in the hood. This is getting so far off the topic though. This is about whether hood spacers work or not. Which they have been proven to.

 

The velocity will be very, very different because of the temp, and because the underside of the hood isn't smooth (and there isn't a smooth cover over the engine like a second hood). That's beside the point though.

Go for it then. I don't see enough proof anywhere to make it worthwhile, and I couldn't justify the looks for the marginal (and even debatable) performance, . I'm not convinced obviously.


This is a good debate, the kind that makes you really step back and think about how stuff works..

 

You don't honestly think that he wanted to do this for the performance/cooling gain do you?The only reason he wants to shim it in the first place is because he has seen it done,and wants to be like the rest of the cool kids with the new trend.Maybe he will shim it atleast 2 inches,the damn thing will fly off going down the interstate and we will all get yet another good laugh out of this thread.

 

Yeah.

If I need more cooling, I'm more likely to get a better radiator and then do a little duct work to up the efficiency. Lowering underhood temps isn't going to net much; engines don't lose a lot of heat through engine block convection, most is still put into the coolant water. If slightly lower underhood temps make your engine run cooler, you have a problem somewhere with your cooling system.