MazdaMike

i was at a fast food place talking with a few friends, one of em said i should drop a 6speed tranny into my car. i started talking about how lard it would be and pretty much the only 6 speed with rev i could get would run me way too much if it DID exist. anyway some guy started talking and the convo went a bit like this

guy: oh yea u can put a 6speed into a rx7
me: oh heh hi, yea r u sure cause i heard its pretty much impossible
guy: oh yea all it is, is a second overdrive
me: huh, what do u mean second overdrive. im not sure what u mean
guys *** kissing friend: oh heh then u dont know cars
guy: [insert gay snicker here]
me: ok well then im just saying i dont know what u mean
guy: oh it brings ur ratio from 1.4 to 1.1 or something
mejust standing there waiting for him to finish)

here is where it getsgood

guy:u know u can just drop another rotor in
me: well in know there is a 3 rotor, the 20b but thats 40k i dont have. i saw one down in florida once actually.
guy: no there is a kit where u can take off the rear of the engine and bolt on the rotor and housing, takes like 20 min.
me: i just started laughing ordered my food and made sure to rev my engine as i left.

yea so is there any truth to that he was saying about 2nd overdrive im not sure what he was trying to say. if someone could explain to me what overdrive is (i mean i know basicaly how it affects a auto, either on or off, but i got a manual i didnt even knnow they made them on mani's)

im really agrivated now at that stupid "u dont know about cars" comment made me feel like an ***. anyway yea id appreciate any feedback

-Mike

rynberg

I believe the only 6-speed you can drop in is a dog-box -- no synchromesh. (HKS or Greddy, I can't remember). They cost over $5k.

The Tremec 6-speeds used in the viper and F-bodies and C5s have two overdrives.

Overdrive simply means a ratio less than 1. 5th gear in almost every 5-speed made is an overdrive. (last gen M3s had 5th as 1:1)

and yes the guy was a dipshit. typical person that knows nothing about FDs trying to tell you how it is. I have gotten that several times, even from FC owners.

MazdaMike

so what are our gear ratios?

rynberg

1st: 3.48
2nd: 2.02
3rd: 1.39
4th: 1.00
5th: 0.72
reverse: 3.29

final drive ratio: 4.1

Van Sema

iTrader: (1)

go to rx7.com and look in the catalog, then in the transmission section, then in the gears ... it shows the gear ratios for each gear and the speed in each gear at 7500 rpms.

MazdaMike

cool what does the final drive ratio and how does it equate to each gear, heh sorry m drilling u all for info but im loving it
-Mike

jimlab

"Overdrive" refers to any gear with a ratio less than 1:1. (input shaft rotations vs. output shaft rotations)

The Mazda OEM 5-speed's 5th gear is an overdrive gear, with a ratio of 0.719:1. In other words, the input shaft is turning slower than the output shaft, allowing the engine to run at very low rpm while still traveling at high speed. For each rotation of the output shaft, the input shaft only makes 0.72 of a rotation.

At 70 mph, for example, you'd be running at approximately 5,400 rpm (stock tires) in 3rd gear, which is 1.391:1. In 4th gear, which is 1:1, engine speed would drop to about 3,880 rpm. In 5th gear, which is the overdrive gear, engine speed drops to 2,790 rpm at the same speed, which doesn't work the engine as hard and nets better gas mileage.

In a 6-speed car, like a Camaro, there are two overdrive gears. 4th gear remains 1:1, but 5th gear is ('94-up Camaro and Firebird) 0.74:1, and 6th gear is a very deep overdrive with a ratio of 0.5:1. That means very good gas mileage, despite having a 346 CID engine, and only turning a leisurely 1,880 rpm while cruising at 70 mph.

So to answer your question, yes, most production 6-speeds have a second overdrive gear, with both 5th and 6th gear ratios being less than 1:1.

A performance close-ratio 6-speed like the McLaren F1's, on the other hand, would only have one overdrive gear, since 5th gear is 1.16:1 (over 1:1), and 6th gear is 0.93:1. Count the number of gears with ratios less than 1:1, and you'll know the number of overdrive gears a transmission has.

Van Sema

iTrader: (1)

haha ... You're my hero Jimlab

MazdaMike

thanks man heh hearing that form u is like getting advice on my philosophy paper from Plato
-Mike

jimlab

Final drive is calculated by multiplying the transmission gear ratios by the gear ratio of the differential. For the RX-7 5-speed, final drive is as follows:

1st gear - 3.483:1 x 4.10:1 = 14.28:1
2nd gear - 2.015:1 x 4.10:1 = 8.26:1
3rd gear - 1.391:1 x 4.10:1 = 5.70:1
4th gear - 1:1 x 4.10:1 = 4.10:1
5th gear - 0.719:1 x 4.10:1 = 2.95:1

Final drive ratios are the number of input shaft (transmission) rotations compared to the number of wheel (and ring gear) rotations. So in 5th gear, for each 2.95 rotations of the transmission input shaft, the tires will make one full rotation.

The real importance of final drive is in acceleration. There is a trade off between "pushing power" (maximum torque at the axles) and vehicle top speed. The higher the final drive ratio, the more torque is applied to the axles, and the faster the car accelerates. This is because we reach the maximum power output of our engine very quickly, because engine rpm climbs very quickly in relation to vehicle speed. Unfortunately, the limit to this acceleration is the redline of the engine, so we eventually have to shift.

In the next gear up, we're not applying as much torque to the axles (lower final drive ratio), and engine rpm isn't climbing as quickly, so we're not accelerating as quickly, but we are increasing our top speed past what we could have managed if we had only one gear. We lose "pushing power" as final drive decreases, but it becomes somewhat less important because the mass of the vehicle is already in motion, and we're gaining a higher top speed in exchange.

As you'll have noticed, your car accelerates much more quickly in 1st gear than it does in 5th, but in 1st gear, you'd be limited to about 42 mph, while in 5th gear, you could attain speeds in excess of 190 mph, assuming you had enough power. So unfortunately we can't accelerate as hard in, say, 4th gear, as we can in 2nd (or even 3rd), but we can't reach 140 mph in 2nd gear either.

CYM TKT

iTrader: (4)

lol this one guy at mcdonalds was telling me how mazda was buying up ALL the first gens they could for 5k each because they are going to "re-release" them in 2005 with 3rotors or some crap like that... I was trying so hard not to laugh.. then he went to say how the front of my FD looked like a corvette and the back looked like a firebird! I was about to smack him! but instead I told him he had it the otherway around since the FD was out way before the C5.. didnt even wanna mention the firebird comment he made.. ughhhhh