please educate me in gear ratios
Guest
Posts: n/a
please educate me in gear ratios
i just started driving a m/t when i bought my used '00 RS last month. i'm trying to learn more about parts and mods, and one thing has always eluded me. i don't understand the effects of of close gear ratios, could somebody explain to to me please?
Guest
Posts: n/a
Close gear ratios = small steps or gaps from one to the next, compared to a wide-ratio box with large gaps. If an engine is tuned "peaky", with a relatively narrow powerband, a wide-ratio transmission can cause the engine revs to drop below the powerband when upshifting. With the engine developing less power at those lower RPMs, acceleration tends to bog down until the engine once again reaches up into the powerband.
A close-ratio box allows the engine's revs to stay up in the powerband at each upshift, translating to stronger, more consistent acceleration.
The trade-off is that on a close-ratio box, either first gear has to be considerably taller than on a wide-ratio one (making strong launches more difficult), or else top gear on the close-ratio box has to be shorter than on the wide-ratio one, meaning that even in top gear you're buzzing along the freeway at higher RPMs than you might like for good miles-per-gallon and relaxed, quiet highway cruising.
One solution (though expensive) is to swap in a gearbox with another gear. All other things being equal, a six-speed box automatically has closer ratios than a 5-speed, because the entire range from 1st to top is divided into 6 steps instead of 5.
A close-ratio box allows the engine's revs to stay up in the powerband at each upshift, translating to stronger, more consistent acceleration.
The trade-off is that on a close-ratio box, either first gear has to be considerably taller than on a wide-ratio one (making strong launches more difficult), or else top gear on the close-ratio box has to be shorter than on the wide-ratio one, meaning that even in top gear you're buzzing along the freeway at higher RPMs than you might like for good miles-per-gallon and relaxed, quiet highway cruising.
One solution (though expensive) is to swap in a gearbox with another gear. All other things being equal, a six-speed box automatically has closer ratios than a 5-speed, because the entire range from 1st to top is divided into 6 steps instead of 5.
Registered User
iTrader: (4)
Joined: May 2003
Posts: 2,341
From: your friendly neighborhood hairpin
Car Info: '03 PSM Sedan
close ratio gearboxes also make downshifting easier too. on a wide ratio gearbox, you have to blip the throttle more to match revs when downshifting. its easier to accurately match revs when there isn't such a large differance between gears.
even when mated to an engine with a wide powerband, close ration gearboxes allow you to really optimize your gear selection for corner exits. often times with a wide ratio box, you'll encounter a corner that is too slow for say 3rd gear, but you can go through it quicker than 2rd really allows. close ratio boxes reduce the occourance of this, espescially close ratio boxes with an extra gear.
even when mated to an engine with a wide powerband, close ration gearboxes allow you to really optimize your gear selection for corner exits. often times with a wide ratio box, you'll encounter a corner that is too slow for say 3rd gear, but you can go through it quicker than 2rd really allows. close ratio boxes reduce the occourance of this, espescially close ratio boxes with an extra gear.
Guest
Posts: n/a
ok, i have a question then. currently, the gear ratio for my tranny is as follows:
(1) 3.454
(2) 1.947
(3) 1.366
(4) 0.972
(5) 0.738
i've been looking at other gearsets, and the following thread:
Speed in gears comparison thread
and i've come to the conclusion that generally:
lower gear ratio = better acceleration/increase in top speed of a gear
higher gear ratio = lower acceleration/decrease in top speed of a gear
are those statements true?
if so, does that mean that the following gear ratios will improve my car's response in the lower gears?
STi Gearset:
(1) 3.166
(2) 1.882
(3) 1.296
(4) 0.972
(5) 0.738
sorry guys, i'm really new to this, and i'm still learning...but then again, i guess its better that i ask and learn, than get stuff done without understanding how it works
(1) 3.454
(2) 1.947
(3) 1.366
(4) 0.972
(5) 0.738
i've been looking at other gearsets, and the following thread:
Speed in gears comparison thread
and i've come to the conclusion that generally:
lower gear ratio = better acceleration/increase in top speed of a gear
higher gear ratio = lower acceleration/decrease in top speed of a gear
are those statements true?
if so, does that mean that the following gear ratios will improve my car's response in the lower gears?
STi Gearset:
(1) 3.166
(2) 1.882
(3) 1.296
(4) 0.972
(5) 0.738
sorry guys, i'm really new to this, and i'm still learning...but then again, i guess its better that i ask and learn, than get stuff done without understanding how it works
Registered User
iTrader: (4)
Joined: May 2003
Posts: 2,341
From: your friendly neighborhood hairpin
Car Info: '03 PSM Sedan
and i've come to the conclusion that generally:
lower gear ratio = better acceleration/increase in top speed of a gear
higher gear ratio = lower acceleration/decrease in top speed of a gear
lower gear ratio = better acceleration/increase in top speed of a gear
higher gear ratio = lower acceleration/decrease in top speed of a gear
the STi gearset you're looking at has a slightly taller 1st, 2nd and 3rd gears so your can won't accelerate as quickly in first through third. fourth and fifth are the same as you quoted for stock.
Guest
Posts: n/a
ok, well, i plan to do auto-x in the near future, and i'm not that into top speed anyways, the most i'd probably drive (even on the highway) is probably 90 mph or less (i'm not a speed demon)
since i plan to do auto-x (and also because most of my driving is not really highway driving), what kind of gearset should i be looking for to get better acceleration in lower gears?
also, you said the sti gears for 1st through 3rd are taller...does that mean that a lower number means a taller(larger) gear and that a higher number means a smaller gear?
once again, i'm sorry for being such a newbie, just trying to learn...and by the way, thanks for all the help...
since i plan to do auto-x (and also because most of my driving is not really highway driving), what kind of gearset should i be looking for to get better acceleration in lower gears?
also, you said the sti gears for 1st through 3rd are taller...does that mean that a lower number means a taller(larger) gear and that a higher number means a smaller gear?
once again, i'm sorry for being such a newbie, just trying to learn...and by the way, thanks for all the help...
Guest
Posts: n/a
"Taller" (also called higher) ratio means a gear with a smaller NUMERICAL reduction ratio (example: 0.738 is taller/higher than 0.780) and will attain higher speeds at any given RPM, but with weaker acceleration because the engine has less leverage.
The converse is also true: A "shorter" (also called lower ) gear is one with a larger numerical reduction ratio (3.45 would be shorter than 3.10) and accelerate more strongly but will "max out" at redline at lower speeds.
Same goes for final drive ratios. The Forester XT's 4.44 final drive is "shorter" than the 4.11 in regular Foresters, which is shorter than the 3.9 in the WRX. However, you also have to consider differences in tire diameter. Larger-diameter tires (such as the Forester's) drive a car to higher speeds per RPM than smaller ones (same effect as a "taller" gear), but smaller-diameter tires accelerate more strongly.
The converse is also true: A "shorter" (also called lower ) gear is one with a larger numerical reduction ratio (3.45 would be shorter than 3.10) and accelerate more strongly but will "max out" at redline at lower speeds.
Same goes for final drive ratios. The Forester XT's 4.44 final drive is "shorter" than the 4.11 in regular Foresters, which is shorter than the 3.9 in the WRX. However, you also have to consider differences in tire diameter. Larger-diameter tires (such as the Forester's) drive a car to higher speeds per RPM than smaller ones (same effect as a "taller" gear), but smaller-diameter tires accelerate more strongly.
Registered User
iTrader: (4)
Joined: May 2003
Posts: 2,341
From: your friendly neighborhood hairpin
Car Info: '03 PSM Sedan
for autocross, you won't be using many gears. picking a 2nd/3rd combo for auto-x use is going to be pretty impractical unless you're a big $$$ sponsored driver. that said, a gearset with shorter gears would be great for auto-x, and aggressive street driving. if you're not going to spend a lot of time cruising on the highway, and have the money to throw on a stronger gearset with better ratios, i think it'd be very worthwhile.
Thread
Thread Starter
Forum
Replies
Last Post