Supercharger

Gear _________ Speed at Redline

________ 247 hp RX-8 ___ 228 hp Boxster

1st _______ 39 mph ________ 43 mph

2nd _______ 65 mph ________ 71 mph

3rd _______ 89 mph ________105 mph

4th _______124 mph ________137 mph


The Boxster can keep up with the RX-8 despite having taller gearing and a lower power-to-weight ratio.

Conclusion: The Renesis probably produces less than 247 hp.

Buger

Hi Supercharger,

The results are interesting. The crazy gearing and better traction of AWD explain the Subaru Legact 2.5XT fast times but I would have thought that the Porsche Boxster would be geared a little lower. Knowing nothing else, it makes sense to assume that a higher hp car with the same shift points will be a little faster.

I went to the link with the R&T Boxster stats and was surprised to see that the Boxster has some crazy rear wheels/tires. 265/35zr-18's are not a common tire choice to say the least. The width of those tires and the rear weight bias of the Boxster do a good job of adding more traction off the line.

When you consider the margin for error (results taken from different magazines and conditions), it is a bit presumptuous to come to a conclusion based on a single comparison.

I would have thought that the first comparison you would have made would be between the G35C and the (preproduction) RX-8. Both are more similar in price and purpose and Road&Track already did the work of putting their test results side by side. They even graphed their acceleration curves together (April 2003). The power to weight ratios on the 2 cars are very close and Road&Track said that the (preproduction) RX-8 was a little faster than the G35C in 0-60 and the 1/4 mile.

Magazine comparisons with the preproduction RX-8s aside, we will have to wait for Mazda's response on the dyno testing results that have production RX-8s missing some torque (and thus horsepower) above 6250 rpms.

Does this one comparison mean that the G35C probably produces less than 280hp? Margin for error and different testing conditions are 2 major reasons why magazine racing does not always yield reliable results. Go and drive the cars Supercharger! You know you want to!

Brian

FamilyGuy

If you'll forgive me for quoting myself, I think this explains why the Boxter does better than the RX-8. It has more torque to play with earlier. To put it into other terms, the Boxter has 173 of its horsepower available at 4750 RPM. If the RX8 has 160 torque at 4750 RPM - and it probably has less - than it would only have 144 horsepower. Even if the RX8's peak horsepower is higher than the Boxter's, for a big chunk of the RPM range the Boxter has the edge.

For a further example, the Forester has one of the lowest horsepower ratings in your little comparison - but it has one of the highest torque ratings.

Buger

Hi FamilyGuy,

You are forgiven but I thought you would understand that engine torque without gearing and tire size information is meaningless by now. Think wheel torque and tire size NOT engine torque!

If you remember, gearing was discussed in your thread about your V6 Impala in February. In one of my posts in your thread (https://www.rx8club.com/showthread.p...9615#post29615), it was pointed out that the RX-8 is geared about twice as much as the Impala in first gear (16.71 vs 8.35). The end result is that the 159 ft-lb RX-8 has almost much more wheel torque than the 205 ft-lb Impala.

Your comparison which you quoted yourself on might be valid if the drivewheels for the RX-8 and the Boxster were bolted onto the output shaft of their engines. Unfortunately, things are not that simple and both cars have gears.

As an exercise to further understand that things are not that simple, we can use released figures and some simple math. Note however that Mazda released figures seem to differ from production results at high rpms right now:

Boxster:
engine torque ................... : 192 ft-lbs
1st gear .......................... : 3.5:1
final drive ......................... : 3.56:1
total gearing ..................... : 3.56 * 3.5 = 12.46 (in first gear)
approx peak wheel torque ... : 12.46 * 192 * .8 = 1914 ft-lbs (in 1st gear)
approx weight .................. : 3000 lbs

RX-8
engine torque .................. : 159 ft-lbs
1st gear ......................... : 3.76:1
final drive ....................... : 4.444:1
total gearing ................... : 3.76 * 4.444 = 16.71 (in first gear)
approx peak wheel torque ... : 16.71 * 159 * .8 = 2125 ft-lbs (in 1st gear)
approx weight .................. : 3000 lbs

You can see that the 159 ft-lb RX-8 has more peak wheel torque in 1st gear the 192 ft-lb Boxster. The wider rubber of the Boxster gives it a little more traction off the line however.

Now if you want to compare the peak wheel torque of the Porsche to the RX-8 at the same speed...

Boxster in 1st gear:
wheels/tires .................................. : 265/35r18
Speed @ redline .............................. : 43.5 mph @ 7200 rpm
Speed @ peak torque ...................... : 29mph @ 4750 rpm
approx wheel torque @ 4750 rpm ..... : 12.46 * 192 * .8 = 1914 ft-lbs

RX-8 in 1st gear:
wheels/tires .................................. : 225/45r18
Speed @ redline ............................. : 41.6 mph @ 9000 rpm
eng rpm @ 29mph ........................... : 6250 rpm
eng torque @ 6250 rpm ................... : approx 157 ft-lbs (according to Mazda)
approx wheel torque @ 6250 rpm ...... : 16.71 * 157 * .8 = 2099 ft-lbs

Given the gearing and wheel/tire size of the Boxster 7200 rpm calculates to 43.5 mph. The 1st gear speed at peak torque (4750 rpm) is approx 29 mph.

You wanted to compare the HP of the Boxster (@ it's trq peak) to the RX-8's HP at the same speed. Notice that the RX-8's 1st gear rpms are 6250 @ 29 mph (not 4750).

Boxster @ 29mph has approx 173.6 HP (4750*192/5252)
Boxster 1st gear wheel torque @ 29 mph (assuming 20% dt losses) is 1914 ft-lbs

RX-8 @ 29mph has approx 186.8 HP (6250*159/5252)
RX-8 1st gear wheel torque @ 29 mph (assuming 20% dt losses) is 2099 ft-lbs

Note that this comparison is in no way trying to denigrate the Boxster (which is another great car). It is only meant to demonstrate that engine torque by itself is meaningless.

Once again, a couple of things to remember:
1. HP is a better indicator of wheel torque.
2. HP/weight is a pretty good metric if cars are geared to shift at the same speeds.

Lastly, please note that the Subaru 2.5XT gets it's wheel torque by tremendously low (high ratio) gearing. It was geared for more torque but less range in each gear. The entire reason I wanted somebody to find out the speed @ redline for it was to demonstrate this. You can see that 1st gear only has a range up to 32 mph and 2nd only goes up to 54 mph. The tradeoff to gearing for more wheel torque is less of a range in gear.

This was much longer than I meant it to be. I think I'm done for awhile.

Brian

Buger

Short version:

Absolutely wrong. The Boxster does not have more torque to play with earlier. It is a fallacy that the Boxster has a big chunk of it's RPM range at higher HP than the RX-8. Gearing affects the RPMS that the cars should be compared at.

The Forester has one of the lowest horsepower ratings in Superchargers comparison but it has AWD and it has been geared for more torque and less range per gear. Peak engine torque by itself is practically meaningless.

Brian

FamilyGuy

Buger,

Thanks for clearing me up on that. My mistake. I was trying to make things simpler than they are. Allow me to summarize the situation then, as best as I understand it - and I'm welcoming correction here, this is how I learn (I may get lots of things wrong, but I think I've come a long way since first reading this forum):

When comparing two cars' performance in straight line acceleration, you have the following factors:
peak horsepower
RPM point for peak horsepower
peak torque
RPM point for peak torque
drivetrain losses to horsepower and torque (usually 20%)
final drive ratio
transmission drive ratios
combination of where the transmission 'tops out' in each gear.
tall gearing reduces effective torque to the wheels but
offsets that by getting a larger mph range out of each gear,
short gearing does the opposite
AWD, FWD, or RWD and the presence or absence of
a limited slip differential (LSD, no not the drug)
vehicle weight
tire radius
(and of course) driver skill

Did I miss anything?
Here's two questions.
1. All other things being equal, if you put tires with a 5% larger radius on a car, does that mean you get 100/105 = 95.24% of your previous effective torque to the ground? Is that totally wrong, or is there another formula you can use?
2. How much does the weight of your tires factor into this? Heavier tires would require more energy to accelerate in rotation, yes?

Supercharger

FamilyGuy,

Car&Driver did an 8000rpm clutch drop standing start to get the 0-to-60 time of 5.9s for the RX-8. The RX-8’s engine rev never drops to 4750rpm in that run.

0-to-60 and 0-to-1/4 mile are not low-end torque tests. 5-to-60 is.


______________ 5-60mph

228 hp Boxster ___ 6.7 s

247 hp RX-8 _____ 7.5 s

FamilyGuy

You're right, I was wrong.

Supercharger

Car&Driver test data

______________ 0-100mph ___ Power ___ Weight ____ P/W

Volvo S60R _______13.9 s ____ 300 hp ___ 3717 lb ___0.081

Dodge SRT-4 _____ 14.0 s ____ 215 hp ___ 2920 lb ___0.074

Subaru Forester
2.5XT ___________ 15.0 s ____ 210 hp ___ 3290 lb ___0.064

Honda Accord
EX Coupe ________ 15.2 s _____240 hp ___3300 lb ___ 0.073

BMW 330i PP _____ 15.3 s _____235 hp ___3370 lb ___ 0.070

Mazda RX-8 ______ 15.8 s _____238 hp ___2940 lb ___ 0.081


All the above cars have manual transmission.

Supercharger

Road&Track test data

________________ 0-100mph ___ Power ___ Weight ____ P/W

Honda S2000 _______14.2 s ____ 240 hp ___ 2835 lb ___0.085

Porsche Boxster S ___14.4 s ____ 258 hp ___ 3080 lb ___0.084

Infiniti G35 Coupe ___ 14.9 s ____ 280 hp ___ 3435 lb ___0.082

Porsche Boxster _____15.7 s ____ 228 hp ___ 3000 lb ___0.076

Mazda RX-8 ________ 15.9 s ____ 238 hp ___ 3000 lb ___0.079

BMW 330i __________ 15.9 s ____ 225 hp ___ 3340 lb ___0.067


All the above cars have manual transmission, NA engine and RWD.

A pre-production RX-8 probably has less than 230 hp.

Supercharger

AUTOCAR (U.K.) test data

______________ 0-100mph ___ Power ___ Weight ____ P/W

Honda S2000 ____ 14.3 s _____ 237 hp ___2780 lb ___0.085

Alfa Romeo
156 GTA Sedan ___15.1 s _____ 250 hp ___3110 lb ___0.080

VW Golf R32 _____ 15.6 s _____ 237 hp ___3260 lb ___0.073


All the above cars have manual transmission and NA engine.

kostas*

Or maybe porche has more than 228Hp

rabinabo

FamilyGuy, don't forget that the width of the powerband is also important. Just imagine the extreme example of an engine that only makes considerable horsepower in a span of 100rpm. Basically that peak is useless because it would be impossible to maintain that rpm as you accelerate (unless you have tons of gears).

Supercharger

Car&Driver test data

__________________ 0-100mph ___ Power ___ Weight ___ P/W


Acura TL (manual) _____14.6 s _____270 hp ___3521 lb ___ 0.077

Mazda RX-8 (manual) __ 15.8 s _____238 hp ___2940 lb ___ 0.081


TL report: January issue Car&Driver (p.120)

Hanzo

You forgot to mention something very important, boxter is a mid engine car.

Hanzo

It is basically the same engine as the STI without the bigger turbo and intercooler. Same short block.

Hanzo

You have to look at the torque not just horse power.

Hanzo

Don't forget the Boxter has a mid engine layout therefore less dt loss, I would say more like 10%.

Also from standing still your formula doesn't apply since you can rev the engine up at neutral and drop the clutch at any rpm you desire. It only applies going from 1 to second gear and so on.

Hanzo

Weather condition, temperature, altitude, tyres (stickyness of the tyres), and engine layout (where the engine is located).