Buger

Cartest normally uses a hp/torque curve profile for cars which is modified to match the peak hp/torque numbers that are entered. Since the renesis hp/torque curves are very different than a normal piston engine, I modified the hp/torque curves to match the renesis. (see attachment)

Below are some of the parameters that were used:

hp:........... 250 @ 8500
torque:.... 159 @ 7500
redline:.... 9000 rpm
wheels: ... 225/45R18

Diff - 4.1
1st - 4.10
2nd - 2.61
3rd - 1.84
4th - 1.39
5th - 1.06
6th - 0.75

curb weight:........ 2900 lbs
driver weight:..... 160 lbs
drag coeff:.......... .29 (estimated using value from Mazda Millenia)
grnd clearance:... 4.9" (estimated using value from Mazda Millenia)
launch rpm: ........ 2000 (optimum est by cartest using all data)

Below are the some of the results:

0-60: ........... 5.74 seconds
1/4: ............. 14.36 @ 99.08 MPH
Top Speed: .. 160.69 in 131.37 seconds
Mileage: ....... 20.4/32.1 mpg (combined 24.4)

Below are some results with a 3000 lb curb weight:

0-60: ........... 5.89 seconds
1/4: ............. 14.49 @ 98.19 MPH
Top Speed: .. 160.39 in 134.70 seconds

Below are some results with a 2800 lb curb weight:

0-60: ........... 5.6 seconds
1/4: ............. 14.23 @ 99.97 MPH
Top Speed: .. 160.97 in 127.44 seconds

It will be interesting to see what the final weight and gearing specs end up at. This should give us all some idea of what may be possible.

Brian

sheylen

Thanks Buger for the great info! Do you know it the 0-62 (0-100km/h) will be very different?

wakeech

holy effin' eff... 0-60 mph in 5.6s?? again, holy...

i know that this approximation (using approximated and guesstimated numbers) is only an approximation (using approximated and guesstimated numbers), but still man, that's pretty damn fast for a sports sedan...

boowana

Thanks.:D

Much better.:p

I can see I will have to go on a major diet Oh well, it'll be worth it. Now I have a real reason to drop a few.

RedRotaryRocket

Buger,

Nice numbers. It sounds like your version of cartest is much more sophistocated than mine...mine must be some old demo version or something...it's dos based and I downloaded it for free almost two years ago! Needless to say, mine doesn't have a help menu and only accepts one weight, entitled "test weight". Since you've got the fancy-schmancy version, we'll let you generate the numbers from now on :D

BryanH

Does CarTest2000 account for drivetrain configuration and losses?

RedRotaryRocket

That's a big 10-4 good buddy.

Or....

Yes.

Buger

Hi Sheylen,

It shouldn't make much of a difference. Add about .02 seconds at that speed to the 0 - 60 time.

I got the new version from www.cartest2000.com. I think the same guy has been tweaking it a bit. I can run some other numbers if anybody wants to but it would be good to buy a version to reward the guy for all of his work.

Yes, it appears to estimate drivetrain losses pretty closely but you can modify the losses if you have more specific information. See the attached screenshot of some of the parameters.

Brian

sheylen

Thanks again

Buger

Here is the attachment with some of the drivetrain loss parameters (it was too big earlier):

Buger

The performance of the 1999 Mustang gt coupe and the estimated performance of the rx-8 are pretty close. It would be interesting to look at how two closely matched performance cars with vastly different approaches compare. The Mustang has far greater torque but the Rx-8 will have a much higher redline.

Attached are a couple of curves comparing the 1999 Mustang gt coupe and the rx-8 acceleration in first gear. The graph on the left illustrates how the renesis can pull over a much wider rpm range (almost 60% longer) than the Mustang v8.

In the graph on the right, you can see how the torque curve of the example rx-8 was shortened to approx the same speed range of the Mustang by shorter gearing. Although the Mustang engine has a huge advantage in torque(302 - 159), it is also approx 300 pounds heavier in this example (3205 vs 2900). The Mustang still pulls harder (by approx .1g) at its torque peak but we can see that the sample rx-8 pulls harder at the low and high ends of its curve.

Buger

The example rx-8 has the following gearing:

Diff - 4.1
1st - 4.10 * 4.1 = 16.81
2nd - 2.61 * 4.1 = 10.701
3rd - 1.84 * 4.1 = 7.544
4th - 1.39 * 4.1 = 5.699
5th - 1.06 * 4.1 = 4.346
6th - 0.75 * 4.1 = 3.075

The 99 Mustang GT has the following gearing:

Diff - 4.1
1st - 3.37 * 3.27 = 11.02
2nd - 1.99 * 3.27 = 6.507
3rd - 1.33 * 3.27 = 4.349
4th - 1.00 * 3.27 = 3.27
5th - 0.67 * 3.27 = 2.191

With the above gearing, the renesis will turn almost 17 times before the rear wheels turn once in first gear. The mustang engine will turn a little over 11 times before its rear wheels turn once in first gear. The difference in gearing and weight mitigate the peak torque advantage that the mustang has. With the above gearing, simulated world (not real world) testing shows that the rx-8 actually beats the mustang in 0-60, 1/4 mile and top speed.

Two more graphs compare the drive power of the 2 cars thru the gears. You can see how the higher drag coefficient affects the top speed of the mustang.

fritts

If you would could you try adding 50 hp to the top end. Just to show us what kind of times the mazdaspeed version of the rx8 would produce. I would think the 50 hp would come in late in the powerband.

Thanks
Ryan Fritts

MikeW

What kind of swept area are you using for the top speed calculation? If the RX-7 had about 1.9 square meter frontal area, how does the RX-8 compare.

What was the coefficient of drag of the RX-7? 0.35?

Also since the Miata 6 speed has a 1:1 5th gear, so should the RX-8. That benefits top speed because the power doesn't flow through a gear mesh in the transmission.

BlueAdept

0.35 is very high these days... I'd be surprised if a modern design was above 0.30... infact I'd be surprised if the RX-7 was that high!

RedRotaryRocket

A quick web search seems to indicate that the FD RX-7 had a drag coefficient of 0.30.....

Buger

Hi Ryan,

I am at work now but will try using the single turbo torque profile with the extra 50 hp and the added weight tomorrow morning at home.

Hi Mike,

As there has been no real info on the frontal area of the rx-8, I let cartest estimate it from the given parameters. It is an optional field and the help menu says that "CarTest will estimate it if it is not known more precisely". I could probably try some values to geta ballpark figure of what cartest is using though.

The drag coefficient of the 3rd gen rx-7 was .29. I estimated .29 for the rx-8 as well since the Mazda Millenia also had the same coefficient. Surprisingly, even the 2003 Toyota Corolla has a low drag coefficient of .296. It may be more conservative to estimate the rx-8 drag as .30 but no more than that. A convertible version of the rx-8 (if it is ever made) might have a drag of something close to .35 but the rx-8 coming out next year should have .29 or .30.

It is a good idea to try a 1:1 ratio in 5th gear as it is much more common on 6 speeds than the gearing #s I used. There are several cars out there without a 1:1 ratio gear but you make a good point that having a 1:1 5th gear could marginally reduce frictional losses for that gear.

I think the best ratio for top speed was somewhere around 1.04 or 1.05 to 1 because the drag losses curve meets the 5th gear torque curve close to the highest peak. A 1:1 5th would be more fuel efficient though and would probably be more in line of what Mazda is planning. I will try a 1:1 5th tomorrow morning and move 3rd and 4th to space them evenly with the new 5th gear.

Brian

Buger

A google search on the web comes up with conflicting info for the Cd of the 3rd gen. Most places generally quote .29 or .30.

I remember reading that the base had a .29 Cd while the r1 had a .31 because of the added body items.

I also remember reading that the 2nd gen rx-7 had a similar Cd of .29 to .31 depending on the model.

I fully expect the rx-8 to come in at a similar range of .29 - .30 for the base model. The Cd doesn't make too much of an impact at lower speeds but is the major factor in limiting top speeds of cars.

The top speed of the rx-8 is definitely not a factor for me as I would not like risking a ticket at anything over 100 anyway.

Brian

MikeW

Most Japanese/German (soon to be american) companies like to use Cw instead of Cd. Well Honda doesn't.

The Insight is 0.25 Cd, Toyota claims that the Lexus LS430 is also 0.25 (my ***!!!) I would believe 0.25 Cw if the car was lowered to its bump stop (air suspension) and the wheels weren't rolling in a wind tunnel.

Cw differs from Cd in that Cw is what the drag from blowing air at a car. Cd is what results from hitting stationary air (Real Driving)


I would hope that the RX-8 has low front/rear lift rather than a remarkably low Cd

BlueAdept

How do you test Cd then if stationary in a wind tunnel doesn't cut it?

Buger

Trying different values, it appears that cartest estimated the frontal area to be 23.821 sq ft. This seems a bit high to me as I found a website that had the Mazda 929 at 20.76 while being both wider and taller than the rx-8. Whatever the case, 20.44 sq ft (1.9 sq m) seems to be reasonable number so I will use that number for the rx-8 from now on. A quick check on cartest shows that the change in frontal area increased top speed by 5 mph and marginally decreased the 0-60 and 1/4 mile times.

While researching some of the variables of drag, I found some good info on Cd and frontal area. An excerpt from the site (http://www.thirdgen.org/newdesign/te...deffects.shtml) is below:

Coefficient of drag is the most widely used number in relating the Aerodynamic efficiency of a car, and is very misleading. It is just a measure of how much out of 100% (like a barn door) a vehicle lets air flow around it. Typically sports cars have coefficients of between .28 and .45 (Viper roadster uggh!). The better (lower) the number is, the easier it is for air to pass around a car. It can be misleading because I recently read an ad in MT,and it claimed that the '97 Infiniti Q45 has the best CD of any production car (I think it was .27). You don't realize that the car has 10" more height, and a few inches more width than a third generation f-body (length isn't a factor). This means that it has more sq feet of air to push out of the way.

A good example of frontal area is the Mazda Miata. The car has a greater CD than an f-body (camaro?), but because its frontal area is tiny (I think it is 16.5 sq ft), it is more aerodynamically efficient.I use an equation to compare cars with different CD and Frontal Area figure, the number generated is merely a factor to be compared 1:1.

CD x Frontal Area(sq ft) = factor #
It works well for direct comparisons, because doing it the long way would take 10 minutes to compute (true engineering method).

Trying different values in the optional field tire circumference, it appears that cartest estimated the value to be 82.44 inches while a 225/45R18 should have a tire circumference of 81.59 inches. Real world circumference will be a little smaller because of the weight of the car so I just used a value of 81 inches. Note that this has the effect of additionally gearing the car an additional 1.75%.

The changed parameters for the new test are below:

frontal area: ................ 20.44 sq ft (instead of cartest est 23.821)
wheel/tire circumference: 81 inches (instead of cartest est 82.44)

Diff - 4.1
1st - 4.10 * 4.1 = 16.81
2nd - 2.65 * 4.1 = 10.865
3rd - 1.83 * 4.1 = 7.503
4th - 1.33 * 4.1 = 5.453
5th - 1.00 * 4.1 = 4.1
6th - 0.75 * 4.1 = 3.075

curb weight:........ 2900 lbs
launch rpm: ........ 1800 (optimum est by cartest using new data)

Below are the some of the results:

0-60: ............ 5.64 seconds
1/4: ............. 14.32 @ 99.32 MPH
0-160: ........... 57.28 seconds
Top Speed: ....... 168.49 in 144.82 seconds

Brian

fritts

Brian,
Is that estimated 0-60 time for the 300 hp mazdaspeed version?

Buger

That was for the na version with revised gearing, front area and wheel parameters.

I just did a quick run with 100 pounds added weight and using the standard torque curve with the turbo profile and the below hp/torque figures:

hp: .......... 300@8500 rpm
torque: ... 220@6500 rpm

The rest of the parameters are the same as for my previous post. Below are the some of the results:

0-60: ............ 4.77 seconds
1/4: ............. 13.43 @ 107.49 MPH
Top Speed: ....... 178.47 in 171.67 seconds

I really wanted to do some research to estimate what numbers would be reasonable to input but I didn't have the time. The above numbers seem a bit unreal don't they?

Brian

MikeW

Those top speed seem very unrealistic. I would hope that the RX-8 (regular) would do 160, and a 300 hp version should do 170 (hopefully).


A wind tunnel can be used for estimating the aero drag, and so can a computer model. Empirical testing determines how much high the real Cd is.

Anyone remember 'roll down' testing in Motor Trend many many years ago.

Hercules

Not like it matters anyway, you'll never get up to that speed, even on a track, unless it's an oval :D

I have gotten up to 150 in an E36 M3 on a straightaway, but otherwise I stay around the 80-85 limit on the highways.