Does anyone know how to calculate how many RPMs the WRX will be turning at 70 MPH in fifth gear? The gear ratios and final drive ratio are reported as:
1st 3.454:1
2nd 1.947:1
3rd 1.366:1
4th 0.972:1
5th 0.738:1
fdr 3.9:1
Assume you're riding on the stock 205/55R16 rolling rubber. As an aside, I'm also wondering how much boost you'd be running at this RPM.

Approximatly 3333 rpms.

where did you get those gear ratios?

Doing the math I get 2716 rpm at 70 miles per
hour which seems about right as my GT does just about 3000 at that speed and the final gearing is a bit lower in the RX and the 5th gear is also a bit lower. Which ought to put 3000 rpm at around 78. In 4th gear, 70 would
be 3578 rpm and 6800 would give you 133mph so that will be just about perfect for forays to the top speed.

The boost at 70 mph in top gear depends on where the throttle is.

Your figure is too high (based on passenger seat observation) so I calculated it myself and I get 2810 RPM in 5th.

The formula is:
engine_rpm =
(road_speed * axle_ratio * gear_ratio * tire_revs_per_mile) / 60.

Tire revs per mile is 837 from the <A HREF="http://www.bridgestonetire.com/products/RE92tec.htm">Bridgestone site</A>.

At ~2,800-3,000 I'd expect max boost (13.5 or 14.2 PSI) at full throttle (based on boost/torque curves I have seen). Obviously, that doesn't happen instantly. At 70MPH cruise of course you would probably have manifold vacuum.

Glenn

[This message has been edited by Glenn Wallace (edited February 03, 2001).]

Ah, I was just using my approximate calculations for the tire diameter. With the exact figures from the bridgestone site I get 2721.1590821886275 rpms.

What formula did you use?

I just used an online rpm calculator. http://www.quiknet.com/~ke6vut/rpmcalc.html

This is a sharp crowd! As for the gear ratios and final drive ratio, I pulled those from the recent Sport Compact Car article (March '01). About 2750-2800 seems about right. As for the boost, seems even if you stomp on it in 5th at 70, it would take a bit for the boost to spool up so it wouldn't exactly be a kick in the pants. Thanks everyone! 37 days and counting.

Can I ask where you got the axle ratio? I either missed it or forgot.

That will be better than the RS. That kicks ass. That means better gas mileage.

As I was first with the right answer, and did NOT use any online tire calculator, I'll pass on the simple way to the answer.

The tire is a 205/55-16. That means the sidewall is 55% of 205mm. Add twice that (in inches) to the 16" diameter of the wheel and you have the diameter of the tire. Then multiply by pi for distance traveled per tire revolution. Divide that by 12 to get feet. Divide 5280 by the distance the tire does in one revolution. This is about 809.6 revolutions. Multiply by 3.9 and .738 to get engine revolutions at 60 mph. Multiply by 70/60 to get the answer for 70.

Math is fun!

I thought this is a very interesting topic.

I created a spreadsheet that calculates RPM in each gear based on MPH or MPH in each gear based on RPM.
http://www.oakos.com/wrx/gears.htm

That's really nice work. My question is about the accuracy. Not in your calculations, but when put to the test. According to the spreadsheet, at redline (7k RPM), the car will do 38.5 in 1st, 68.3 in 2nd, 97.3 in 3rd, etc. If this will stand up to a real-world test, that would be pretty sweet (by my standards, anyway).

These formulas work very well. But it depends a little on the inflation pressure, how worn the tires are and the speedometer and tachometer accuracy. I've checked my speedometer against a GPS and it's very close, but I have not really checked the tachometer against a digital one at high speeds.

On my 98 GT with 4.11 final ratio and 2nd gear as a 2.11 that would give 6500 rpm at 55.5 MPH and that's pretty close to what I see at redline. So the formulas work fine.

anyone notice the big drop in rpm when shifting from 1st to 2nd?

assuming the gear ratios printed from SCC are correct, my 'fuzzy' math comes up w/ a drop from 7K rpm (assuming shifting @redline) to just under 4K rpm, 1st to 2nd shift. What's up w/ that?

But all the other redline shifts for 2nd to 3rd, 3rd to 4th, etc. drops the rpm to rougly 5k rpm. Much better. What's Subaru thinking? Hopefully SCC got the ratios for 1st or 2nd wrong.

assuming the gear ratios printed from SCC are correct, my 'fuzzy' math comes up w/ a drop from 7K rpm (assuming shifting @redline) to just under 4K rpm, 1st to 2nd shift. What's up w/ that?


Well that would mean it's a long 1st gear, right?


Also...where's peak torque? 4K RPM is closer to 3K RPM than 5K is...better on the auto-x when you rarely get out of 2nd. http://www.i-club.com/ubb-files/smilies/wink.gif

--KC

>Well that would mean it's a long 1st gear, right?>

well, uh, no. More like 1st is too short, or 2nd is really tall. The answer is 'B'.

My point is why go through the trouble of making gears 2-5 close ratio, but not 1-2?

A tall 2nd gear is ok for those who like weaving through cones, but what about guys like me who race Solo I?? I'd downshift into 2nd maybe once per lap, and only if the tires are starting to wear off. ditto for those who prefer drag racing.

The gear ratios listed here are correct.

The 1-2 change is pretty nasty, worse than a 2.5RS for sure (http://home.kscable.com/lockes/colin/imp_gears.htm) . Besides the long 1-2 shift though, the gear ratios aren't bad at all.

I was a little surprised also when I saw the gap between 1st and 2nd. It could have been worse, we could have 3 overdrive gears like the Stealth/3000GT 6 speed boxes.
The good thing about the WRX is that there are plenty of other options like new gearsets or JDM boxes (STi 6 speed http://www.i-club.com/ubb-files/smilies/smile.gif). I know the Talon/Eclipse cars have major gearbox issues and there aren't many (any?) options.

mmm... sequintial 6speed.

ive seen a VW cabroilet with a VR6,AWD and sequintial 6speed converion. Now I want to do the same (well, just the 6speed http://www.i-club.com/ubb-files/smilies/smile.gif )

peace

Why stop there? If your gonna go exotic maybe you can get your hands on a joystick controlled semi-automatic box from a WRC car. http://www.i-club.com/ubb-files/smilies/cool.gif

The gear ratios seem very similar (if not identical) to those on the cars in the UK. From my own experience (I've owned two in the past 4 years), 5th gear is pretty much useless for acceleration at anything below 85mph. If you could get 7,000rpm in 5th, then the speed would be something like 185mph! That is a tall gear, and is designed for quiet/economical cruising at around the 70mph mark.

For real acceleration around the 70mph mark, then you need to drop to 4th, or even 3rd gear.

Well, that is one thing that will take me alot of getting used to. I knew I would miss my 6-speed, I have a 97 Z28 and with a .50 6th gear and 3.42 gears at 70 I am about 1,700 rpm and it actually pulls quite well in 6th, 85 mph shows me about 2,100 rpm....oh well, I'm ready for a change http://www.i-club.com/ubb-files/smilies/biggrin.gif.

Whats with 4th and 5th being overdriven? Has anyone really run the WRX hard enough to shift into 4th from redline in 3rd? Does it have the power to still pull this? I just got rid of my celica GT-S (6 speed) and 5th and 6th were overdrives... I hated that because the celica was gutless in both of these gears.

-Nathan

I've shifted from red line in third to fourth. To be honest, It actually felt like it was pulling harder in fouth (which is very odd). I thought it was cool though.

Perhaps I was just gitty though.

-Ed

At 73 mph my tech reads 3000 rpm

I guess i need to remember that while the celica is 500lbs lighter, it has 100FT/lbs less torque and about 40 less HP.

-Nathan