after changed the cat back , the air drag!!!!!
 

so here is my question
after i changed the cat back for mxp rs ti tip
there is a huge empty space between axe and rear bumper
i went for a track day the and the highest speed i got is around 145 km/h
and follow the by a high speed corner speed around 110-95 km/h
all that speed was on the stock toyota 86 stock tires exp the cat back!

now i have already order the new brake pad and rotor
new rpf1 rim and re71 tires

but after that track day , my reverse light wires was lay on my exhaust
luckily i found it out before the wires is burn

so this lead me to consider the air drag at rear bumper

i am going to buy a rear diffuses for help me cover it, but i think it will make the car under steering since there is more down force at back

so at this point i need a front lip for sure......
it was just tooo much for fix that air drag

any suggestion for create down force at front with out a big front lip?
or how to fix that air drag at rear?

here is the pics

145kph isn't going to ruffle your hair let alone slow your slow car down further or upset the delicate aero balance of your road car. Leave it as it is and just drive it mate.

IIRC from non-lip options hood vents should also reduce lift at front.
And yeah, there are several diffuser or cover options that may cover axleback exhaust to guide air around with less drag. With some exhausts some of them may need some cutting though for clearance.

But imho you overblown rear exhaust/bumper parachuting/drag issue a bit and confuse with downforce a bit. Drag rather will slightly hurt your long term fuel economy & acceleration .. even if it's by some 1-2kmh, is it an issue to care for unless you drive competitively for results, not for fun? Ordered stickier tires very probably will allow higher cornering and exit speeds, allowing higher speeds at end of straights anyway (though not sure if RE71R is best choice for HPDE, imho it's easy to overheat it in longer sessions, and it may wear down fast).

How do hood vents reduce lift, and how do they compare vs hood scoops (aero and cooling-wise?)

Sent from my iDroid

Check velox hood vents thread. IIRC there was some data both on temps & lift reduction.

Most of the air that goes into the engine bay vents under the car.

A diffuser by itself, on a stock, non-flat bottomed car, is probably not going to generate a whole lot of downforce, but on the other hand it won't be nothing. What the diffuser is actually doing is expanding and slowing down the air that is coming under the car as it exits the back. This aids velocity in front of the diffuser, where the downforce is actually created (high velocity = low pressure, low velocity = high pressure). But if you have a lumpy floor and transmission tunnel and diff and subframe and suspension links all there, well, flow is not going to be as smooth or as fast as it could be. That velox part that extends forward is a good start but there's still a lot of lumpy things and open space there.


At the front end, air goes into the front grille, through the radiator, then what?

The only place it can go is under the car and out the wheel wells, which creates lift and drag. Giving that air an exit on the top of the car, such as the fenders and hood, will generally reduce that front end lift.

Similar thing apples to a scoop- you are pulling air in at the top and increasing the pressure in the engine bay. That means more air going out the bottom and sides and more restriction to flow through the radiator.

Regarding aftermarket hoods with premade scoops/vents, worth checking where they are located (front/mid/or rear part of hood, closer to windows) and compare with CFD modelling Velox did for twins aero model. Depending on location there is different pressure above hood, which will help or not air evacuation from below hood. One can move separately bought vents and install in most optimal places. Not much options with aftermarket hoods with integrated vents, where only option is to buy different hood.

Well.. what was your speed before on the straight? With no comparison it's hard to tell if you were actually slower.

Honestly, I wouldn't worry too much about aero until everything else on your car is already sorted out.

A lot of replacement exhausts do indeed turn the rear bumper into a parachute.

- Andrew

[ame]http://www.youtube.com/watch?v=quDLzxmJl5I[/ame]

here is the imfo i got !!!!
very nice one
i wont worry about that now
[ame]http://www.youtube.com/watch?v=Fq8hkPobpes&t=744s[/ame]

this one is talking about roof but give me alot info too