Graph showing HP over the full run.

 

Those are nice graphs. And now we have two different numbers to deal with. Which is correct? Dyno or G-tech?! =-] I'm putting my money on the g-tech cuz I know mazda wouldn't lie to us about something as important as this. :p

By the way, how much did that g-tech cost? Can you try it out on another vehicle to see if its accurate? Thanks for all the work that you've put into this with your OWN vehicle. =-]

-John

 

Firstly, I am quite sure the G-tech comp unit is very very accurate. Secondly, my 146 Hp rx7 once reached 140 mph so the argument of x amount of hp needed for y amount of speed is nonsense. Having said this there is a distinct possibility that a certain batch of rx8s might have the HP problem. Damn Mazda, make a statement for heaven's sake!

 

I know I probably should spend more time in the corner wearing the stupid hat, but the two graphs posted by Speed Racer, are very interesting:

Look at the max hp in his first and third graphs (page 19 and 21): The first tops out at 157hp, but the second (ignoring the noise on gear change) tops at about 175hp (after 10.5s).

To estimate the hp, Gtech takes the measured acceleration multiplied by weight and multiplies it with the speed. The speed is calculated as the time integral of the previously measured acceleration. What if the jerkiness of the acceleration caused a systematic error in the speed estimate? That would explain the 10% difference in the two hp numbers. It would also mean that the 1/4 mile time is underestimated because the GTech believes more distance covered than in reality, and the 0-60mph time is underestimated because the GTech believes the speed is higher than in reality.

I don't doubt that the momentary acceleration reading from a GTech is very accurate, but time integration over a jerky acceleration could build up a significant error.

Then again I could be wrong

/Elak

 

You guys are too scientific... I need to go back to school or just hang out with a different crowd...:D

-John

 

Without knowing anything about a GTech, surely if he can plot RPM then the unit must have a tach input therefore it's correcting speed on the fly.

BTW speed racer -> on your gtech dyno why is the selected area purely decelleration between 1st and second? Can you select the part that it uses to calculate the curves? Because I would have selected second if I were you.

-pete

 

That wouldn't work very well as soon as you have a wheel spin, which I assume is common in these situations. It also requires the GTech to know gearing and wheel radius.

/Elak

 

Obviously ( at least to me) it is the case that until somebody pulls out their engine and sticks it on an engine dyno, it is all simply SPECULATION

However, one thing to consider:
On the European sites, such as Mazda UK and Mazda Germany, the power claimed is 231PS ( that's 227.8 HP)

So, why would ones sold here in Canada make 250HP?

Don't believe me?

Look here:
http://www.rx8.de/de/advance_sale/default.asp


Or here:
http://www.mazdarx8.co.uk/upclose/specs/default.asp

 

Its good to have your knowledge on this board again Church!

 

In Europe the cars have different (more strict) emissions requirements. In fact originally the cars were to have 240PS, but a few months ago Mazda decided to aim for the next generation of emissions standards (over a year away) instead of the current one and so ended up with a lower than expected power of 231PS. Read some of the threads in the European Forum for more info.

 

Simple, different emissions standards.

 

It is common and it's in his graphs. You're wrong about the Gtech needing to know the wheel diameter and radius though. I mucked around with this with my own accel. but basically, you can use the accel. to estimate speed, and check that against the known rpm. Once you have 2 or more points in the same gear, you can calculate the line of best fit and get the gearing pretty much spot on. It works out to be pretty accurate. From then on you get speed from the RPM and gear. You only need the accel, to keep estimating what gear you are in and for instantaneous readings.

...that's enough sidetracking!

-pete

 

Errr... looking at that HP vs Time graph... if you exclude the noise at the gearchanges (probably due to clutch dumping), surely the HP looks like about 180 at best??

What are you guys seeing?

 

For about 25HP??
That's over 10%!

I am dubious..

 

I think you are wrong. Read here:
http://www.gtechpro.com/gtechprocomp_howitworks.html

The rpm is used to estimate torque.

The two graphs show an 18hp discrepancy on the same car. Why? We all seem to agree that the accelerometer has only a small momentanious error - however integrate that error over time and it could explain the hp difference. With this explanation the "fast" GTech run becomes consistent with both other runs and the dynos.

However much I want it to be otherwise, I believe the 0-60mph and 1/4mile times are wrong in this case.

/Elak

 

well guys,as long as no rain on sunday i will have time slips and will have a better idea if there is any missing ponys.i have a red gt that has 1200 miles and has been great thus far.ill post slips if anyone is intrested.going to go to the last street performance/import event for the season and run test and tune and maybe race but the classes are 4 6 & 8 cyls,dont know if i can run or not.

 

How do you propose it does that without knowing the gear ratios?

-pete

 

P(hp) * 5252 / RPM(1/s) = TQ(lbft)

/Elak

 

Chill out a little bit with the nay saying. I've just given you guys proof that the car is capable of meeting the claimed performance specs and the only thing that you are interested in is discrediting my info. What's wrong with this picture?

There is an 18 HP discrepancy between the two runs because the second run was dramatically slower. If you read the Gtech page that you linked to then you should understand that the HP figures are calculated from the measured G forces (acceleration) / time = speed, speed * acceleration * vehicle weight = HP. Looking at this general formula you can see that time is inversely proportional to HP (i.e. if the time increases the HP decreases). For that reason alone, the Gtech is not an accurate tool for measuring HP or torque for that matter which is determined from the HP and current RPM as measured by the voltage through the cigarete lighter. The HP and torque numbers will not match those of either a chassis or engine dyno but the general shape of the curves should match them when done properly (i.e. smooth shift at low RPMs into preferred gear and hold until redline).

As for the recorded times to 0-60 and 1/4 mile, they should be accurate to 0.01 seconds (manufacturer's spec). Real world accuracy should be about 0.1 seconds. So I don't think you should be so quick to discredit the times. Would it really be the end of the world if it turned out that there was no major Mazda conspiracy surrounding the RX-8's performance? More likely it would just be an end to this run away thread.

P.S. I apologize for taking this out on you , it is nothing personal. I just feel that all of the speculation is getting a little out of hand and its time for all of us to have a reality check.

 

Elak, d your formula doesnt work. You can't substitute in RWHP and FW RPM and get FW torque!

Unless you're suggesting that it's only calculation RWTQ and then I ask, how does it get the WHEEL RPM?

-pete

 

When you setup up the Gtech it requires you to calibrate its tachometer with your car's tach. This is done by reving the car to two known RPMs and recording the values on the Gtech. From that point on it can measure the current RPMs. It is pretty slick setup but even with that I wouldn't trust the calculated torque values because the HP is suspect (see previous post for explanation).

 

Thanks, I'm just trying to putting this whole debacle into perspective by adding some real world performance numbers (0-60, 1/4 mile). I think the Gtech is an acceptable tool for that and it is definitely better than a "butt dyno" but it is really not a very good tool to measure engine performance. At this point I'd really like to see one of the rotary tuners put their engine on an engine dyno as that is the only way that we are going to know for sure if there really is a problem with the engine's output. Until then we should all be out on the road enjoying our RX-8s.

I picked up my Gtech Pro Competition directly from the manufacturer for $250. Its a little pricey but it has given me a little piece of mind, in that I now know that my car is not a total dud. :P

 

Do a Google on horsepower and torque and you'll find the information.

/Elak

 

No worries, I am sure many with me appreciate the effort you (and your car) made.

I wish I was wrong for the second time today, but I fear I'm not.

/Elak

 

Just a thought... if the car is capable of the claimed performance (0-60 times and top speeds), why do we care what the hp is?! We paid for the 0-60 times and the top speed, we knew what it will do, why do we care how much hp it really produces?!

Just my pennys into this bucket of speculation.. :p

I can't wait to get my car, put 3 friends in it and scare the crap out of them as I carve some corners .. muhahahahhaha

-John