-4 HP is just an estimate from seeing other cars dyno'd. By going from 18 to 20 your doing two things that are going to decrase HP at the wheel.

1) Increase in MoI (moment of inertia). The further you move the center of mass from the axl, the more inertia (resistance to change) the mass has. Since torque is the force to turn the wheel, and it has not changed, but you have made it's job harder, you create a net decrease in the amount of work the motor is able to do (HP).

2) 20" wheel has 3.14 x 2" or 6.3 (x the width) more inches of of material (ie weight or mass) that has to be turned, further increasing the MoI. Perhaps this has a bigger impact on MoI than increasing the radius. If you do not take measures to reduce weight (carbon fibre or magnesum) and do something like use chromed wheels (added weight) it has even more impact. Keep in mind that your tire will also be larger/heavier as a result in increased diameter compounding the MoI impacts of larger wheels.

Bigger brake rotors will have a similar impact. However, better brakes does more to help you go fast than it hurts, the same can not be said for bigger wheels. The "purpose" of larger diameter wheels is to make room for bigger brakes, therein lays the performance justification. However, 18" is more than adequate for very potent brakes.

All I was saying in my post is that as much as I love the car, and understand it isn't about HP numbers, I don't want it degraded any further. If I want to go fast, I have 3 motorcycles, two of which will blow the doors off ANY car on the road (I don't see any Enzos or FI cars ON the road). You won't see me doing anything to slow down the 8 any further though.

 

The RX8 is not a slow car, it performs very nicely stock; what are you guys talking about? With a few mods, like my car she is fast and when I get done with her I will put her against the FD as well... I had a 1993 I bought new back in the day and the stock 0-60 times on that car were 5.5... When they first came out there were not many parts for them either, and the RX8 is a better car in many ways... Give it some time and you will start to see some very fast RX8s...

 

c'mon tigre don't take it personal, beside you know we belong on a fd, the 8 it's just a car to go buy grocery you know that, relax, dog, we'll be back on track on a real sports car......


Evil....

 

Well said. You didn't even have to throw in a equation.

I also wanted to use this opportunity to talk more about the weight of the brakes.

True brakes have a moment of inertia, multiplied by the angular velocity, but they never are in contact with the ground. Only the tire is in contact with the ground. The wheel will carry more of the interial moment, so you can approximate the wheel and tire combination as one.

 

lol i hear ya evil
i need boost and mazda will not take it lightly if i take apart there new SPORTS CAR. i still have stuff from the fd,i have 3 boost controllers, one im hooking up to my brothers tII this week.and (white boy) is working on my beater car
i might make that a street car but i have no real love for piston cars.
p.s remember what we talked about i found some info on silvia
i think its in california so im shoot the owner a email and see if he's intrested in selling it back to me.go get your blue fd
dont make me get some old pixs.

 

I was watching Sports Car Revolution on SPEED and they were modding an Acura NSX, testing each mod along the way. They swapped out the stockers for 18" wheels and large brake discs and lost -9 HP. -9 HP!!! You're right about the brakes having minimal impact on the MoI, probably less than 5%. Of course you aren't moving the entire mass (and increasing the mass at the same time) out, you are just adding a small amount to a very centralized location. I just wanted to point out that increasing the MoI to the drive shaft has performance concequences. Discs of equal mass have a small MoI than a ring (wheel) which is why the brake impact is small. -4 HP for 20"s is probably conservative. People like Cortc spend hundreds of $$ to gain 9 HP through exhausts, intakes, chips and pully systems (money well spent in my eye). From the kits I've seen you could gain about 50 HP for about $3k (Not including spraying or soon to be available FI kits). A lot of people think weight is the enemy, but it is actually inertia (which is directly correlated to weight or mass).

Basically 20" dubz counter the benefits of the carbon fibre drive shaft.

 

You also don't want to have to low of an inertial moment. Otherwise, you'd have too much acceleration; you'll just slip. The frictional force is also what makes your car wheels turn. It looks weird when you write out the equations for dynamic equilibrium.

I think the optimal wheel size for the RX-8 is 17 x 9. It would provide nominal acceleration, and it will reduce the weight at the corners. Mass will always be a culprit; I believe inertia is a by-product, at least when a particle is in motion.

 

actually i didnt feel a power loss from my stock wheels to the 20 hre's. jus my .02

 

Your Butt Dyno isn't sesitive enough.

Besides, HP is rarely "felt". Our cars are lacking torque. HP, as it relates to torque is about efficenienct use of rotational force and how it relates to top speed. The only way you'd notice a difference is if you took scientific top speed measurements in each gear on a controlled run (consistant wind, grade, etc...).