Size of hood vent

[i_am_socket]

It also depends on the vehicle.

[aerohead]

Originally Posted by NeilBlanchard

Hi,

Velocity means that air is being deflected, or pushed up and over the front of the hood, and this is where drag is most easily created. The air velocity is almost nil at the base of the windshield (I have seen maple seeds just sit there) and so while it may not be generating lift, I don't think that there is much pressure there.

So, the diagram makes sense if you think about velocity (changing direction), and this also means pressure against the surface of the vehicle. Low velocity is closer to still/stagnant air, and therefore is lower pressure against the surface of the car (because it is closer to the way it was before the car pushed it out of the way).

The vectors are showing lift, I guess. They only shade the parts that have a positive Y value. I guess it's the X values that I have a problem with -- why are the vectors below the neutral point (on the front bumper) going the right (positive X) and the vectors above the neutral point go to the left? Is this area of the car pulling the car forward?

I'll draw a picture later that is more helpful, to me at least.

The velocity will be imparted to stagnant air as it is displaced by the moving vehicle,going from rest,and atmospheric pressure,to some local velocity determined by the time/displacement function,and lower static pressure.---------------- Drag will be imparted by viscous shearing forces of the air of the boundary layer,induced from positive or negative lift generated by the body of the car, along with attached- vortices ( all from pressure differentials ),and primarily from form-drag ( profile-drag ),associated with separated flow and it's attendant turbulent wake,with low base pressure acting against the back of the vehicle.--------------- The drag is not created by the impact of the vehicle against the air,but rather the pressure differential "across" the vehicle.

[holypaulie]

Originally Posted by IkoIko

holypaulie, I like the hood mod. Did holes of that size effect the hood's strength? Does it flex under normal use?

I've played around with the a Magnehelic gauge and the aerodynamic flow profile makes sense to me. I found lower pressure 6 inches back from the front edge of the hood, and higher pressure up by the cowl.

I'm going to next check along the side of the front fenders and further off the center line of the hood as suggested by aerohead.

Thanks again for all the input.

My hood is still strong because I didn't remove the main vertical hood support which is locatated between vents. I cut very small portion of front horizontal support. No flexing at all. Everything was well planned out. I'm glad that I could share.

[aerohead]

Originally Posted by NeilBlanchard

Hi,

I've attempted to show the air flow lines, overlaid on the "velocity" image (if that is what it is showing):


And here it is with just the flow lines:


As you can see the greatest velocity occurs where the air is being pushed aside at the greatest angle, and this is what gives it the greatest velocity. I think that the nose pushes the air up above the main part of the hood, and then the upper part of the windshield again pushes it up; creating another higher velocity zone.

Neil,I think a smoke-rake image of the car would better show how the body deforms the airflow.I think the car is a Porsche 944,and I've seen imagery from Porsche,and it's a little different.------------------------------- To illustrate "lift" associated from velocity,I've gleaned the following: In the windtunnel development of Sunraycer,too much air went under the car,causing negative lift under the very long,cantilevered tail.The "lift" would pull the tail down,causing the nose to lift.Fairings behind the wheels would catch the air and yaw the car sideways,and then the car would liftoff,flying.This was at 20-mph(32 km/h).For the World Solar Challenge,the nose was re-configured and wheel fairings deleted,trading increased drag for better stability.-------------------------- A 1969 Z-28 Camaro was tested and found to generate 500-pounds(227-kg) of lift at the nose at 128-mph(206 km/h).Addition of a chin spoiler cut lift to 275-lbs (125kg).-------------------------- A half-million pound (227,272 kg) Boeing 747 leaves the ground at 165-mph (266-km/h).------------------------------- Racing Beat's 3,000-lb (1,363 kg)Mazda RX-7 LSR car "flew" at 235-mph ( 379 km/h),landing on it's roof at Bonneville.-------------------------- In 1989,Art Arfon's "Green Monster #27" ( which looks like the fuselage of a 1951 Grumman F-9F-2 Panther ) took off at 300-mph (484 km/h),did a complete flip,landed on it's side after 300 feet in the air,grinding to a halt after a half-mile(0.8 km) on Bonneville's salt.----------------------------------- Lastly,during the Monte Carlo Grand Prix,when the Formula-1 cars go through the tunnel at Monaco,they experience oversteer,do to the ceiling of the tunnel,20-feet overhead.It's been found that the rear wing can suffer lift losses from any disturbance within up to about 30-feet.Using inverted logic,one can deduce that an F-1 car disturbs the air up to 30-feet away.---------------------------- If windtunnels were much larger than they are,we could better appreciate how much our cars really affect the air around them due to "lift" and the implications with respect to mpg.

[Novaz_nz]

For those wondering about lifting the rear of the bonnet/hood with spacers - I tried this on a 3rd Gen RX7. The heat generated in the engine bay is massive, so any extra cooling is helpful.
I was fortunate enough to see where the heat was spilling out in the mornings when the bonnet was iced up.

The raised edges cleared first along the whole length of the bonnet, the de-iced strip being perhaps 2" wide from the edge. The next area to de-ice was directly above the turbos, followed by a criss-cross pattern that marked the underbonnet reinforcing. The *last* area to thaw was the very rear edge of the bonnet. It seemed that very little hot air was expelled from here.

I thought about raising the bonnet in a similar fashion on the Laurel to smooth the transition to the windscreen, however it looks like the turbulence generated by the raised edges might counteract this effort.

[hypermiler01]

If it were mine, I would use no hood vent at all, and go with holes in the inner fenders to vent to the wheel wells. The wheel wells have low pressure air so they will suck the high pressure air right out of the engine compartment very well.

Just cut the holes there down near the axle so that there isn't a large amount of tire spray flying back into the engine bay.

Also use a grill block to keep out more air than you need for cooling.

[Daveedo]

Ever seen hot rods with the "scoop" pointed towards the windshield and ending right near the base of the windshield? They do this as a "ram air" setup due to the forces shown in the aerodynamic pic on p.1. At the base of the windshield when you are traveling at a decent speed air is trying to force its way down and into the back of the hood. Even firebirds came with this stock in the late 70's and 80's.

As for venting the hood by raising it. I believe this works well at slower speeds. Seen it help with cooling problems on 4x4s that saw trail use and slower speeds.

A person I knew had overheating problems in arizona while one a trip. He stopped at some podunk place and asked "cooter" if he could put louvers in his hood. They did it for a decent price and it fixed his cooling problem. It was a mid '80s toyota pickup that was hauling a huge load. The louvers were formed into the entire middle of the hood but not directly in the center line where the main support is. They didn't go within back 1/4 of the back of the hood (towards the windshield). Basically air would come through the radiator then up and over the motor and out the louvers.
There are louvers on a number of vehicles that comes that way from the factory. FJ40 landcruisers have some by the way on the back corners of the hoods but that seems to be to there to help more with slow operating speeds with high loads. seen them on other cars but can't quite recall which ones...grand am?

[QuickLTD]

If I remember correctly, the forward facing scoop located midway back and just to the right of center on the 84-86 Mustang SVO actually flowed the opposite it was supposed to. It moved air from the engine compartment through the intercooler and out the scoop while the vehicle was moving instead of pulling in cooler outside air for the intercooler. I Googled it but found nothing to back up my memory.

[IkoIko]

A quick update, and end, to this thread.
I decided to not cut my hood. There are many reasons but one of the biggest is the expected impact on economy was pretty small.
Since I've joined ecomodder I've been faithfully tracking my mpg. The biggest effect on mileage, without a doubt, is time of day. If I leave home for work during "rush hour" and wind up stuck idling in traffic my mileage for that tank is crap. I have a flexible work schedule, so the biggest improvement to my mileage is to travel during off peak hours.
I've decided the garden edging on the bottom of the bumper was a good improvement http://ecomodder.com/forum/showthrea...mods-5835.html.
My next project is a 1985 Toyota MR2. If I'm stuck in traffic it should be more efficient to feed a 1.6L engine. But mainly more fun to drive...

[eagle]

I thought I shouldnt open a new thread that is why I am resurrecting this one

I/C Water Spray Test on Subaru GRB

0:55 onwards

subaru OEM intake scoop seems to reverse its flow (!) to the outside

wasnt subaru aware of that? how can that be?