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Old 09-28-2015, 09:49 PM   #21 (permalink)
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Quote:
Originally Posted by vibrating_cake View Post
Isn't the opposite true about the sealing at the windscreen side of the hood? Taking that off would try venting bonnet heat into a high pressure zone, which would effectively act as an auxiliary air intake into the bonnet that bypasses the radiator?
Hot air has high pressure too. A temperature difference of 55deg C (100 deg F) gives about 18% increase in pressure. As per the picture, you would think that its easy for hot air to flow down. Rather, the air stagnates inside the engine bay, which can cause a drop in differential pressure across the rad. I would say that air leaks out to the back rather than being drawn out. I do not think that ecomodders would drive fast enough to have high aerodynamic pressure to force the hot air back in. One would need to travel beyond 90km/h which isn't very eco anyways.

See also http://www.autospeed.com/cms/A_113157/article.html Unfortunately that article did not say what pressure that sensor is supposed to trigger, but you get the general idea.


Last edited by bobdbilder; 09-28-2015 at 10:31 PM.. Reason: Added reference to Autospeed and Pressure difference due to temperature.
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Old 09-29-2015, 07:56 AM   #22 (permalink)
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i don't want to steal the thread, although the OP hasn't updated on this thread since starting it?
It makes sense with the hot air creating pressure in its self, never thought of that!
I've got a feeling from looking, the air vents mostly out the wheel arches, and also through the exhaust tunnel (which would be a transmission tunnel in a RWD car) so i'm not sure how it'd effect the general flow down there, although it's not exactly well thought out in the engine bay of these little cars

I've sealed the trunk opening (not the top but the sides) thanks for that one, it made a big difference to road noise (not sure about mpg, i drive around town most of the time so it's not going to effect me really)

I've done a lot of mods to reduce the noise in this car, but MPG wise, nope!

I'm still interested in seeing this active grill block and any other mods to come!
Any updates?
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Old 09-29-2015, 08:03 AM   #23 (permalink)
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It would be interesting to see the effect of removing the engine bay seal to the aero of the front of the car. It will definitely help with some engine cooling. Third gen camaros and firebirds had poor airflow over the radiator, and removing that seal was a popular fix to get them running cooler. I had a friend that had an 87 trans am. He had to instal an air dam of sorts to force enough air up and over the radiator to keep it from overheating.
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Old 09-29-2015, 02:04 PM   #24 (permalink)
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Also consider my favourite car of all time; a Lotus Elise, you would not think Hethel would want to spoil the aeros on a track car. Although the inclined radiators are vented roughly a further distance before the windshield. But the Evora has it much closer. I suspect it does help with some modifying the aeros of the front windshield.


Last edited by bobdbilder; 09-29-2015 at 02:07 PM.. Reason: Added a picture of an Evora
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Old 09-29-2015, 04:41 PM   #25 (permalink)
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It probably has a lot to do with the shape of the car before the vents and windshield.
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Old 09-29-2015, 05:20 PM   #26 (permalink)
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sealing the hood

Quote:
Originally Posted by bobdbilder View Post
For hot weather application, perhaps you should look into removing some sealing close to the windshield. This seems to seal the engine when the hood is closed. Removing this allow hot air to escape your engine bay and move right over your windshield. I think this sealing it is to keep the heat in during winter. Since I live in the tropics I removed mine. It helps to increase air flow across the radiator as the engine bay is not entirely closed. If the engine bay is closed, there is a drop in differential pressure across the rad. When there is a drop in differential pressure, flow across the rad also reduces. Be sure to put the sealing back on in winter.
The ventilation intake is at the base of the windshield.If you remove the weather stripping you'll allow hot air from the engine bay access to the cabin.
It defeats the fresh air.
The carmaker took into account the delta-P across the heat exchanger during development of the car,based upon the 'production' architecture.The seal was engineered into the system.
If you're racing,you might be obsessed with underhood heat,as it affects charge density and horsepower,but for a daily driver or ecomodding,you'd be thinking about tuned extractors which would require a full-scale wind tunnel for proper development.
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Old 10-02-2015, 01:30 PM   #27 (permalink)
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Yes agreed. Where it is hot, I seldom open the fresh air vents for the AC. There is a workaround if one still have this need. You can see where the fresh air intake is, remove the seal except those that are right adjacent to the intake hole. The recommendation is for cases where a car is found to be overheating. As Iamnotahippee mentioned, it is a known quick fix.

I was hoping that the discussion has opened up the idea that hot air at the base of the winshield might modify airflow over it. There might be a remote chance of reducing Cd. This might be useful for Pickups and old Saabs where the angle of the windshield is more upright than your normal sedan. If this is proven, then that energy waste wouldnt be such a waste after all.
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Old 10-02-2015, 06:17 PM   #28 (permalink)
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If all the heat expelled through given opening is assumed to be recovered at parity (1:1) in mechanical effort in the airflow and compared to the energy needed drive the airflow (for the whole vehicle, or just the windshield area), then that will put you in the rule-of-thumb ballpark.

It will never be 1:1.
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Old 10-03-2015, 03:22 AM   #29 (permalink)
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I don't know if the usdm crz has the under engine tray, but that has a funky design to create a low pressure zone just under the exhaust tunnel, which leads me to think that's where the hot air is designed to go... I'll get a photo after I get mine back from mot time.
Also the crz has a well raked windscreen and its got a huge amount of curvature to it!
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Old 10-03-2015, 02:15 PM   #30 (permalink)
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modify airflow

Quote:
Originally Posted by bobdbilder View Post
Yes agreed. Where it is hot, I seldom open the fresh air vents for the AC. There is a workaround if one still have this need. You can see where the fresh air intake is, remove the seal except those that are right adjacent to the intake hole. The recommendation is for cases where a car is found to be overheating. As Iamnotahippee mentioned, it is a known quick fix.

I was hoping that the discussion has opened up the idea that hot air at the base of the winshield might modify airflow over it. There might be a remote chance of reducing Cd. This might be useful for Pickups and old Saabs where the angle of the windshield is more upright than your normal sedan. If this is proven, then that energy waste wouldnt be such a waste after all.
Actually,Professor Kamm got a patent for doing this.
The air from the engine bay was ducted to the base of the windshield,where it helped re-energize the flow heading up the windscreen and delay separation downstream.You can see what may be this extractor vent on Kamm's personal car (lowest drag of all K-Cars)

Today,the convention is to create a small burble at the cowl which is isolated from the outer flow,leaving the rest of the air to blast past this region,with just a tiny drag penalty.
Professor Morelli dedicated much wind tunnel time at Pinifarina to create tuned extractor ducts from the CNR research car's engine bay.


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