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beatr911 · 11-17-2009, 02:23 PM

Jack,
Because of the cab rearward characteristic, the more or less fixed value is the low pressure in the rear of the car. As you've said, some tapering can occur but not really that much. The bright side is that there isn't alot of area to start with so potentail still exists.

Instead of trying to make it more teardrop shaped, I would think that the focus should be on smoothing. Try to ensure that each air molecule encounters only one high pressure point along its path to the low pressure area in the rear.

The rear fenders present a large second high pressure point to air that has already encountered the nose and or the front fenders. Iether fair in the rear fenders to the front, and/or narrow the rear track with wheel offset or axle width. This may mean installing half height "doors" to allow entry/exit. Fish are made with wider mid-sections, so bowing out the middle might help here also. Steeply sloped front glass would reduce the peak pressure for air coming over the nose allowing more gradual spill-off to the sides and top.

I also had this response to the MEN thread on a grille block but was unable to post:

I've read somewhere that for nascar cooling systems that are fully ducted in and out, the rule of thumb is about 4 square inches per 25 horsepower produced. For my 112 hp Ford Ranger the 20 square inches of grill that is not blocked off is plenty - provided I'm moving. The radiator fan, clutch and shroud have long since been removed.

Most grille opening tuning is done by how often the electric fan comes on with FWD vehicles. I'd suggest removing the belt driven fan if you have one and install an electric fan or closely monitor the gauge. Then make the grille opening size by the above rule should be about 5.5 square inches if I have your horsepower correct at 35 hp. Best to duct or seal to the radiator to reduce leakage around the radiator, and center the opening to the radiator.

For air exit, there should be plenty of space around the driveshaft tunnel to exit out the low pressure in the rear, if you install a full length undertray. You’d end up with air pressure in the front and suction in the rear making the system quite efficient. Monitor oil temps because the sump can do a fair bit of oil cooling. If additional exit area is needed for more cooling, the front wheels are in a low pressure area enabling more cooling air evacuation.

Test and adjust as needed for your expected driving conditions. Additionally an adjustable louver or door would allow a small advantage in cooling system aerodynamic losses when not running full throttle or in heavy traffic.

Looking forward to seeing you in the spring for our MPG Rally in Lacey, WA!

11-17-2009, 02:23 PM	#18 (permalink)
beatr911 Master EcoModder Join Date: Feb 2008 Location: West Coast, USA Posts: 516 B2300 - '96 Mazda B2300 SE Focus - '05 Ford Focus ST The red car - '00 Honda Insight Thanks: 6 Thanked 77 Times in 56 Posts	Jack, Because of the cab rearward characteristic, the more or less fixed value is the low pressure in the rear of the car. As you've said, some tapering can occur but not really that much. The bright side is that there isn't alot of area to start with so potentail still exists. Instead of trying to make it more teardrop shaped, I would think that the focus should be on smoothing. Try to ensure that each air molecule encounters only one high pressure point along its path to the low pressure area in the rear. The rear fenders present a large second high pressure point to air that has already encountered the nose and or the front fenders. Iether fair in the rear fenders to the front, and/or narrow the rear track with wheel offset or axle width. This may mean installing half height "doors" to allow entry/exit. Fish are made with wider mid-sections, so bowing out the middle might help here also. Steeply sloped front glass would reduce the peak pressure for air coming over the nose allowing more gradual spill-off to the sides and top. I also had this response to the MEN thread on a grille block but was unable to post: I've read somewhere that for nascar cooling systems that are fully ducted in and out, the rule of thumb is about 4 square inches per 25 horsepower produced. For my 112 hp Ford Ranger the 20 square inches of grill that is not blocked off is plenty - provided I'm moving. The radiator fan, clutch and shroud have long since been removed. Most grille opening tuning is done by how often the electric fan comes on with FWD vehicles. I'd suggest removing the belt driven fan if you have one and install an electric fan or closely monitor the gauge. Then make the grille opening size by the above rule should be about 5.5 square inches if I have your horsepower correct at 35 hp. Best to duct or seal to the radiator to reduce leakage around the radiator, and center the opening to the radiator. For air exit, there should be plenty of space around the driveshaft tunnel to exit out the low pressure in the rear, if you install a full length undertray. You’d end up with air pressure in the front and suction in the rear making the system quite efficient. Monitor oil temps because the sump can do a fair bit of oil cooling. If additional exit area is needed for more cooling, the front wheels are in a low pressure area enabling more cooling air evacuation. Test and adjust as needed for your expected driving conditions. Additionally an adjustable louver or door would allow a small advantage in cooling system aerodynamic losses when not running full throttle or in heavy traffic. Looking forward to seeing you in the spring for our MPG Rally in Lacey, WA!