seal cowl induction?

**PaulL** · 05-18-2012, 01:36 PM

We all know the Viper hood has about a 2" cowl induction opening in the back. After doing a lot of reading, the higher air pressure caused by that can affect the cooling of the radiator. Would it be beneficial to attempt to seal this opening? If so, what could be used (i.e., maybe some kind of large seather stripping)?

Discuss.

**BigTyla** · 05-18-2012, 03:43 PM

Don't know where you read that, but that couldn't be more wrong. At speed, there will be LOWER static pressure across the hood, creating a vacuum effect that will pull the air from the engine bay out through the duct.

**PaulL** · 05-18-2012, 06:00 PM

Originally Posted by BigTyla

Don't know where you read that, but that couldn't be more wrong. At speed, there will be LOWER static pressure across the hood, creating a vacuum effect that will pull the air from the engine bay out through the duct.

Originally Posted by Permanent grin

I agree. I have a Seibon hood and 2 problems we have identified are 1. I can fit my hand between the cowl and weather strip by the wind shield and 2. the rear vent is an intake, so at speed causes the pressure in the engine bay to rise. This means that the fans can't pull any air through the radiator. Even in neutral the temperature doesn't start to fall off until my speed drops to say 40 mph!

The first problem may not sound like a problem until you realise that the air by the cowl reverse flows back into the engine bay rather than going up and over the car. You can test this quite simply by taping a few short lengths of wool to the back edge of your hood and watch to see if they get sucked towards the engine or up the screen whilst driving.

post #14
http://www.3sgto.org/f13/overheating...rs-6158-2.html

That's not the only time I have ever read about this either. I have read that the bottom of the windshield contains the most pressure, and the easiest place for the pressure to dissipate is through the rear vent. I've never tested this myself, but I have read a lot of articles about this.

**PaulL** · 05-18-2012, 06:27 PM

More from other people on other forums about cowl induction

I hooked up a home made manometer using water and clear tubing to measure pressure delta between engine compartment and the cowl. Indeed above about 35, the cowl was in an area of higher pressure than the engine. With the AC on this speed increased to about 45 due to the pressure from the fans. Never was the pressure significantly higher. Not more than about an inch of water at 70.

That little experiment always made me wonder why the majority of folks adding padding to the seal between hood and cowl noted a reduction in vent temp. Like GTX said some is better than none and it's likely the air there is indeed cooler....

The padding helps keep engine heat and smells out of the HVAC intake when the car is slow. The padding returns the rubber cowl seal to original shape and function sealing the cowl away from the engine compartment. (Not all parts on Miatas age gracefully?) I consider this a good thing and a civilizing influence on my driving experience. A good firewall/hood seal also keeps the cowl higher pressure air from 'backwinding' the radiator at speed. The last thing a radiator needs in some other high pressure air coming into the engine compartment to lower the radiator's pressure differential before and behind it.
Jacking the rear of the hood to create a large permanent gap does just the same thing at speed. Terrible for cooling going fast. Great for engine ventilation when stopped or very low speed.

**BigTyla** · 05-18-2012, 06:52 PM

If they are doing as they say then it isn't a cowl at all or is a poorly-designed one. If the cowl were too large or at too large an angle of attack I can see this occuring. It would also occur if pressures under the hood were low, which could be caused by poor ducting and/or missing splashguards.

EDIT: I will say that I could definitely see this being an issue if the cowl opening were too close to the windshield. That is a significant area of fluid stagnation.

**anyonebutme** · 05-18-2012, 09:51 PM

Originally Posted by BigTyla

If they are doing as they say then it isn't a cowl at all or is a poorly-designed one. If the cowl were too large or at too large an angle of attack I can see this occuring. It would also occur if pressures under the hood were low, which could be caused by poor ducting and/or missing splashguards.

EDIT: I will say that I could definitely see this being an issue if the cowl opening were too close to the windshield. That is a significant area of fluid stagnation.

"Cowl's" by design are intakes. It would have to be about 8 inches forward of the hood trailing edge at least to become an extractor.

For the visual crowd:

**BigTyla** · 05-19-2012, 02:26 PM

Semantics I guess. I've always thought of cowls as extractors and being significantly upstream from the windshield. I've never known of a cowl to be referred to as an intake device. Those are usually called ducts in my realm of engineering.

**PaulL** · 05-19-2012, 04:49 PM

Regardless of semantics, the viper hood is raise someting like an inch or two in the back. By your terminology, it appearantly serves as a duct being at the bottom of the winshield. Is it better to seal it off?

**anyonebutme** · 05-19-2012, 05:08 PM

Originally Posted by BigTyla

Semantics I guess. I've always thought of cowls as extractors and being significantly upstream from the windshield. I've never known of a cowl to be referred to as an intake device. Those are usually called ducts in my realm of engineering.

"Cowl" is not the duct itself, it's the location proper. Any duct device is the cowl is an intake due to the static pressure developed in the region.

Best examples I can give is the intake system of a NASCAR type cup car. Or any 60-70s domestic fitted with the hot new thing of the time: Cowl Induction.

Anyways, it's a very basic principal that's still used in vehicles today, mostly to force air into the HVAC system without the use of the fan for the fresh air input for the mixing box.