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HiFlite · 10-24-2015, 10:44 PM

Well, you picked a challenging vehicle to improve! Plus, racing and efficiency pull mods in the opposite direction. Here's the fuelly history of my 2006 RX-8:

Batmobile (Mazda RX-8) | Fuelly

Before putting a big wing on the back and getting onto a track, I could regularly do ~22 mpg of mostly highway driving and ~24mpg with the AccessPort "economy" tune loaded. Pushing as hard as I could on mountain roads, the worst I could manage was 13.5 mpg and on a track, 7.9 mpg! With every other car I've had, beating the EPA estimates was easy - not this one, however.

For reference, here are a list of my current mods:
RX8Club.com - HiFlite999's vBGarage

Also, there's a guy on the RX-8 forum who has done a lot of work, both with N/A and turbocharging with efficiency as a priority. Here's one of his threads - an interesting story:
Harlan's Impossible turbo build. - RX8Club.com

In terms of aero - detailed extrapolations from the world of 400 mph, 2000 hp, 1940's airplanes is of limited use. The Reynold's Number is very different from cars as are the thermo considerations in dealing with 100 gph's worth of heat. Also, manufacturers and racers have discovered that what may work well at 20,000 feet, does not work so well 6 inches from the ground. Additionally, what may work with an 80 mph wind blown against a parked car, works differently when the air is moving only relative to the car (hence moving road wind tunnels).

Careful venting of cooling air may help slightly and may help cooling significantly, but is unlikely to make much of a difference in the overall drag. Why? First, the car is too short, or, saying the same thing really, the maximum height of the car is reached too far toward the rear. A piece of evidence supporting this view is Mazda's claim that its "breadbasket" optional "spoiler" reduced Cd from .31 to .30 which is a pretty big difference for a small piece of plastic. The "spoiler" is not producing downforce but extending the airflow out a bit towards the teardrop ideal. Second, the stock tires are already 225 wide and best cornering requires at least 245 or 265. That's a bunch of drag. Wheel covers might help, but will cause the brakes to overheat quickly if the car is being pushed. Even narrow tires still leave the huge wheel wells designed to fit large ones. Third, the suspension is great, but very complicated, making a clean underside difficult.

All that being said, IMO, there are a few mods that have a chance of helping with Cd. First, lower the car, which is desirable on the track anyway. Don't go too far, about 3 cm will still maintain good suspension geometry. Second, do something about the stock rear bumper cover. As is, it's a great scoop for air that's flowing under the car. Extend it forward to mesh with the underbody. However, since that would also cover the stock muffler, some thought will have to go into muffler location and cover materials. As is, I've seen modded RX-8s set the rear bumper cover on fire from exhaust heat.

Eliminate any pathways for incoming air to go around the radiator and oil coolers. Use the best possible radiator (which would have to be custom made for your Series II car):
MAZMART - Serving The Mazda Community Since 1980. REMEDY RX8 Radiator - 04-08 MT
And water pump, and every other cooling component. Then slowly close off the radiator opening until it runs okay with minimum airflow.

I found this, plumbed into the heater core circuit, to be useful in controlling temperatures:
ACS BASIC MT by RX-8 Performance | RX8Performance.com

Remember, a single excursion of water temps to ~235 degrees is enough to blow the coolant seals and ruin the engine.

I'm also pretty convinced that a front splitter will improve airflow around the car and through the cooling inlets.

10-24-2015, 10:44 PM	#5 (permalink)
HiFlite EcoModding Lurker Join Date: Dec 2014 Location: Michigan Posts: 34 Thanks: 1 Thanked 26 Times in 17 Posts	Well, you picked a challenging vehicle to improve! Plus, racing and efficiency pull mods in the opposite direction. Here's the fuelly history of my 2006 RX-8: Batmobile (Mazda RX-8) \| Fuelly Before putting a big wing on the back and getting onto a track, I could regularly do ~22 mpg of mostly highway driving and ~24mpg with the AccessPort "economy" tune loaded. Pushing as hard as I could on mountain roads, the worst I could manage was 13.5 mpg and on a track, 7.9 mpg! With every other car I've had, beating the EPA estimates was easy - not this one, however. For reference, here are a list of my current mods: RX8Club.com - HiFlite999's vBGarage Also, there's a guy on the RX-8 forum who has done a lot of work, both with N/A and turbocharging with efficiency as a priority. Here's one of his threads - an interesting story: Harlan's Impossible turbo build. - RX8Club.com In terms of aero - detailed extrapolations from the world of 400 mph, 2000 hp, 1940's airplanes is of limited use. The Reynold's Number is very different from cars as are the thermo considerations in dealing with 100 gph's worth of heat. Also, manufacturers and racers have discovered that what may work well at 20,000 feet, does not work so well 6 inches from the ground. Additionally, what may work with an 80 mph wind blown against a parked car, works differently when the air is moving only relative to the car (hence moving road wind tunnels). Careful venting of cooling air may help slightly and may help cooling significantly, but is unlikely to make much of a difference in the overall drag. Why? First, the car is too short, or, saying the same thing really, the maximum height of the car is reached too far toward the rear. A piece of evidence supporting this view is Mazda's claim that its "breadbasket" optional "spoiler" reduced Cd from .31 to .30 which is a pretty big difference for a small piece of plastic. The "spoiler" is not producing downforce but extending the airflow out a bit towards the teardrop ideal. Second, the stock tires are already 225 wide and best cornering requires at least 245 or 265. That's a bunch of drag. Wheel covers might help, but will cause the brakes to overheat quickly if the car is being pushed. Even narrow tires still leave the huge wheel wells designed to fit large ones. Third, the suspension is great, but very complicated, making a clean underside difficult. All that being said, IMO, there are a few mods that have a chance of helping with Cd. First, lower the car, which is desirable on the track anyway. Don't go too far, about 3 cm will still maintain good suspension geometry. Second, do something about the stock rear bumper cover. As is, it's a great scoop for air that's flowing under the car. Extend it forward to mesh with the underbody. However, since that would also cover the stock muffler, some thought will have to go into muffler location and cover materials. As is, I've seen modded RX-8s set the rear bumper cover on fire from exhaust heat. Eliminate any pathways for incoming air to go around the radiator and oil coolers. Use the best possible radiator (which would have to be custom made for your Series II car): MAZMART - Serving The Mazda Community Since 1980. REMEDY RX8 Radiator - 04-08 MT And water pump, and every other cooling component. Then slowly close off the radiator opening until it runs okay with minimum airflow. I found this, plumbed into the heater core circuit, to be useful in controlling temperatures: ACS BASIC MT by RX-8 Performance \| RX8Performance.com Remember, a single excursion of water temps to ~235 degrees is enough to blow the coolant seals and ruin the engine. I'm also pretty convinced that a front splitter will improve airflow around the car and through the cooling inlets.