yellow8

Saturday morning at 8.00 am,outside temp was 77 degrees F.
redlined 1,2,3,4 but 5th will only rev to 8,250 good for 133mph.
shift into 6 gear,rpm dropped to 6,750 at 130mph.My car already has 2,200 miles

brothervoodoo

Well, I hope you are driving somewhere safe at that speed... Maybe it's still running rich and hasn't totally loosened up yet. I'm not going to look at these dyno's or user facts until we get a bunch of people say over 5-6k miles on the car.

My car had a little over 700 miles and I got it up to 136mph. And it seemed that the road was not totally level, I was going up "a perceived" slight incline. Whatever that is plenty fast for me.

I'm more curious as to the horsepower issue than the top speed. Via documented stats this car is electronically limited to 148mph, it's not the engine, if anything, it's the ECU..

dcfc3s

Might also be a speed limiter of some kind. That would be a first - no RX-7 had a speed limiter.

Dale

Magnesium

I also got up to 135 and still picking up speed in 5th. I was on a slight decline, so maybe that is helping out a bit.

Superfan

136mph in 5th

BRealistic

The only time manufacturers put speed limiters on their vehicle is when the vehicle can (gear limited) go faster than the OEM tires allow. What are the standard tires speed rating?

brothervoodoo

I have read this multiple times about the 8 being "governed, electronically limited" to 148mph.

But ofcourse, I can't find a friggin source to back up my statement!

Anyway, maybe I was dreaming but if true, I'm sure somebody could offer to quote a source.

Thanks,
BV

DisneyDestroyer

I've seen somewhere from Mazda for maximum speeds for each gear. Here, it stated maximum speeds for gears 1-5 at 9xxx RPM, but for gear 6 the max RPM was only around 6500 - which would put it somewhere around 140 MPH.

ProtoConVert

Is it true that stock odometers typically overstate speed? If so, the issue might be more serious

Buger

The above results seem to match with the dyno results that seem to be missing torque over 6000.

I suspect people will see an extra 10-15 mph at the top end when the mystery is solved.

Brian

brothervoodoo

Check out Buger's excellent graph here:
http://www.rx8forum.com/showthread.p...&pagenumber=11

MrWigggles

Due to the amount of drag on the car due to wind resistance at higher speeds it takes 8 times as much power to double your speed.

Granted that the RX-8 is down 10% of its horsepower at the upper RPMs, don't expect a car that is doing 136 MPH now to do more than 140 MPH with 10% more power.

I don't know what the coefficient of drag on the RX-8 is but it seems that after 60MPH the car even in the Road and Track / Car and Driver tests had poor performance after 100 MPH.

I think odd-shaped pairly-closed large front air inlets of the car are hindering the aerodynamics.

-Mr. Wigggles

Supercharger

Car&Driver test data

_________________ Top Speed ____ Power

Dodge SRT-4 _______153 mph _____ 215 hp

BMW 330i PP _______ 152 mph _____235 hp

Chrysler Crossfire ___ 151 mph _____215 hp

Saab 9-3 Vector ____ 148 mph _____210 hp

Mazda RX-8 ________ 144 mph _____247 hp

Acura RSX Type-S ___142 mph _____200 hp


The data suggests that the RX-8 tested by Car&Driver has less than 247 hp.

MrWigggles

Supercharger,

The coefficient of drag (cd) and the car's horsepower determine top-speed. Those are the two main factors.

I think the cd is higher than other vehicles out there for the reasons I menitoned earlier.

-Mr. Wigggles

P00Man

doest top speed have to do with torque (i guesse and thus hp...)

ok, maybe im an idiot...this post is pointless i guesse since torque and hp are directly related

i was just thinking cause of this thing this MB guy told me regarding some AMG model..anyway...
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rx7aggie

depends on hp, not torque. torque is a measurement of work, hp is a measurement of power, or work per unit of time, different things, different measurements, different units.

a car's hp is simply the torque multiplied by the rotational speed, and converted using a certain number, depending on the units you want.

power (hp) = torque (ft-lbs) * engine speed (RPM) / 5258

or something to that effect.

the hp required to keep a car at a top speed is simply the drag forces acting on the vehicle multiplied by the top speed, converted to hp.

ChrisW

The quoted top speed is achieved in 5th gear, 6th is geared too high. I suspect 5th is geared too low for maximum top speed and you just run out of revs. You really need a ratio somewhere between 5th and 6th to get the best top speed.

I think this is why the quoted top speed is disappointing compared with other cars with similar power (the 8's drag coefficient is actually quite decent, 0.31). This reason behind this strange gearing is to help with fuel economy.

The top speeds people are seeing in the 130's are definitely too low though, and seem to confirm the dyno results.

eccles

The whole subject of gearing and hypothetical top speed was discussed last year in this thread. Buger ran the projected numbers through CarTest, and came up with the set of cascades below. While they don't match up with the top speed reported in this thread, they do demonstrate the difference in available torque at the wheels in each gear. Top speed is where the "losses" due to wind resistance, friction, etc, match the available torque.

(The cascades also indicate that for maximum acceleration you should take it to the redline in every gear. This is often not the case for other vehicles - see the lower set of cascades for a Mustang, in which the lines actually intersect indicating that the next gear has more torque available before you hit redline in the lower gear.)

Supercharger

To get a better idea of how a car performs, let’s examine acceleration and top speed.


Car&Driver test data

__________________ 0 – 1/4 mile ___ Top Speed __ Power ___ Weight ___ P/W

Dodge SRT-4 ____ 14.1s @ 102mph __ 153 mph ___215 hp ___ 2920 lb ___ 0.074

BMW Z4 3.0i ____ 14.0s @ 99mph ___ 152 mph ___225 hp ___ 3090 lb ___ 0.073

BMW 330i PP ____ 14.3s @ 97mph ___ 152 mph ___235 hp ___ 3370 lb ___ 0.070

Porsche Boxster __ 14.6s @ 98mph ___148 mph ___228 hp ___ 3024 lb ___ 0.075

Mazda RX-8 ______14.5s @ 96mph ___144 mph ___ 247 hp ___ 2940 lb ___0.084

Chrysler Crossfire __14.8s @ 96mph ___151 mph ___215 hp ___ 3111 lb ___0.069


All the above cars have manual transmission.

Some cars are geared for acceleration while others are geared for top speed.
It seems that Car&Driver tested a RX-8 with less than 247 hp.

1stRX8

Probably a headwind more than a power problem. Car & Driver seems too visible for Mazda to send an underperforming car.

MrWigggles

Guys,

I don't think the RX-8 is all that aerodynamic and that is its "problem" at high speed. Some of the air intakes on the front are partially blocked or completely blocked. That can only hurt the coefficient of drag.

-Mr. Wigggles

Supercharger

For those who live close to these facilities, you may want to test your RX-8’s aerodynamics.

Full-Scale Wind Tunnel Testing

NASA Langley (Virginia)
http://wte.larc.nasa.gov/facilities/...d=1&id=2&fac=1

NASA Ames (California)
http://windtunnels.arc.nasa.gov/

Elak

I've removed this post since the calculations in it were wrong and did nothing but add to the confusion.

Sorry 'bout that.

/Elak

Skyline Maniac

My last car, bont stock Millenia S 4AT sustained 145mph+ speed on a desert highway. (top speed over 150mph) The car had 210hp 210lb-ft torque and runs on 16" wheels. The 90-94 designed Millenia had the CD value of 0.29. The car was very quiet at high speed and did feel like it was slicing through wind. (except the tail end design, which caused slight lift and noise at high speed) In the RX-8, the drag on highway and wind noise was much more noteable than the Millenia. Did Mazda really designed this car in a wind tunnel or did they compromise some aerodynamic design for styling?




Here is a diagram detailing the relationship between speed and power given the same aerodynamic. Improving the CD figure will lower the slope of the curve. In the case of low top speed of the RX-8, is it:

1) Driver issue (Wrong gear? Inability to plant the right foot?)
2) Engine power issue (215hp?)
3) Environmental issue (100% humidity, 120F?)
4) Aerodynamic (too much drag at that 130mph?)
5) Port Opening issue?
6) Break in period ECU doesn't allow top speed runs?
7) Check engine light?

Cast your votes

P00Man

OOPS...i posted a dup. of this in the discussion board...

anyway, had the 8 at 145 tonight, dsc completely off

oh, i heard the buzzer too, i still think it needs to be a TAD louder
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