mark kibort

ahem, I live this every weekend I race! just put on the 305s up front last race and it is amazing how much stompiing power you have under braking, and the cornering is better, enough to loosen up the rear, which i was able to add more rear wing to help fix. 285s on the rear vs 305s is a big difference in rear grip too. if you are racing me with 275 285 same tires vs my 305 /305 or even 275 /305 set up, you will be at a disadvantage.

Ive watched the vets, vipers play with 275 to 305 and then 335 rear tires and the bmws go from 225 to 245 and then 275 tires, as did the S2000 with the blower. the larger tire just works better!

mk

Louie928

When you add vents to direct unwanted air from a high pressure zone to a lower pressure zone, you should also provide some ductwork, inside the body outside of the airstream, to accelerate the vented air and direct it in the same direction as the slipstream air. That is, do not simply cut a hole in a low pressure zone to vent the higher pressure air. That air will exit normal to the direction of the airstream and greatly increase the drag. Have the air exit at a small angle, less than 30 deg, to the surface and near the same velocity as the slipstream. Use yarn tufts to check out the flow direction and turbulence at the exit.

mark kibort

you guys are missing the point of what causes more downforce.

vacuum does nothing, its pressure that does the work. there is a high pressure zone at the nose. primarily, under the nose we are concerned with where that air goes. under the car as it is bad and adds to the air molecules traveling under the car which is near ambient. around the car is good, as it keeps that air mass flow from going under the car. the inlet air can be diverted into the car, to add to the mass flow going over the car. this helps reduce the amont of negative pressue above the car. in particular right at the roofline. The real benifit is diverting the air that normally would be going under the car, adding to that air cushion, to divert it above the car at the hood area. now there is more mass flow traveling over the entire car, increasing the pressure above it in all areas, becuase the speed is the same, and the Mass flow has gone up.

Using hoses to find the superlow pressure zones on the roof doesnt work, as you are limited by volume. what is the volume of air that can be routed out of my vent vs a FEW 4" diameter hoses???? No you get it!! . go to any engineeing hand book and look up the losses for 3-4" tubing 6ft long. you have pressure drops along the way, for what? you are trying to increase air flow from the engine compartment to above the car. what kind of pressure drops can we expect from a couple of 4" tubes? it probalby would be a wash with the greater pressure differential. BUT, you have bypassed the windshield and roofline, so you have lost added downforce there , by its abilty to increase the pressure over the car. news flash: if it made sense to do it, dont you think the sports car would be venting air through the cockpit to the roof? Now, its not to say it cant work but what might be better, might be additionally, to the hood vent, these vent hoses from the engine , bottom of the car, to the roof.

These kind of ideas are what are proven with pressure sensors and wind tunnels.

mk

mark kibort

an analogy. you have a wing.(our cars are wings basicaly). if you cut holes in the front or bottom of the wing and routed it to the rear of the wing exiting up, what would happen? you would still have lift. if you vented the nose of the wing to just after the nose of the wing, you would reduce more lift as more air would be traveling over the wing now, (vs prior) so, you would be raisng the pressure along the top of the entire wing. venting to the rear, wouldnt have the same effect . why do airplanes have to be so concerned with icing? just a thin layer of turblulence over the wing reduces the differntial pressure vs the air running under the wing. result, lift is reduced (ideal for what we want to do), but drag is increased as well. We want to reduce the lift, as well as keep the flow as laminar as possible. letting the air vent from the hood area does this best and most efficiently, with the least amount of drag.

its been awhile since Ive looked at this stuff, but thats the general idea.

mk

dprantl

Because if you are serious about aero (i.e. racing) you will want to have a front splitter go as low as possible. Otherwise you would have air going under the car doing all sorts of nasty things that you don't want. Unless of course you have a full underbody tray that has been designed in a wind tunnel.

Dan
'91 928GT S/C 475hp/460lb.ft

mark kibort

but you were talking about frontal area going down. I know about the benefits of lowering the car, but its mainly due to the air that gets routed around the car, so actually the lower splitter will do this without lowering the car.
mk

ew928

Guess it's back to dive planes and hood fences (see pics of BMW) to get some downforce on the front to balance out any diffusers in the back. Removal of the battery box and fuel tank relocation on race cars will open up lots of room behind the rear axle to install your favorite flavor of diffuser.



Hood fences are the 2 black aero fences on left/right side beside the hood.

.

mark kibort

I use some of the air underneath the nose to cool my oil via the oil pan!

mk

jorj7

Just put a hole in the sunroof, box it in on the inside with the hose going to it, and
put vents or slots on top to the outside.

Slots or vents in the hood would be simpler...

RKD in OKC

ah ha ha ha...Now THAT's some aero.

Lizard928

RKD,
are you offering to let us try those mods on your car? I have some space particle board we could use

RKD in OKC

I was thinking MDF.

ew928

MDF wilts when wet.
Lexan works much better.
Use the extra stuff left after cutting pieces for rear hatch and windows.

Daniel Dudley

Radiator flaps. Electric motor, thermostat controlled. Had them on my 89, and I think they stopped using them after a year or two.

dprantl

Deleted on '91+ models.

Dan
'91 928GT S/C 475hp/460lb.ft