fictionrx

some one told me that my fb would be quick if i put 14/40 gears or something like that does any one know what they are talking about. If so how.

peejay

Firstly, Mazda never made a 14/40 gearset. All Mazda gearsets are "hunting" gears, meaning each gear tooth on one gear will eventually see each gear tooth on the other. "Nonhunting" gears (both even, or both sharing a common denominator) don't do this, say for example you have a 3 tooth and a 9 tooth gear, a given tooth on the 3 tooth gear will only see three different teeth on the 9 tooth gear. The upshot is, if you have "nonhunting" gears you have to be extremely careful to mark which teeth mesh where, since the gear teeth can and will have distinct wear patterns, whereas it doesn't matter for "hunting" gears because each tooth will see each other tooth and they'll all be uniform.

Now that you know that little tidbit...

Anyway. What your friend might have meant was a 14 tooth pinion gear and a 40 tooth ring gear. That's a 2.86:1 ratio, and waaaaaaaay taller than stock. Just fine if you like using 2nd gear on the highway. What your friend probably said was 4.10, which is either barely shorter than stock (3.91 for 12A cars) or pretty much the same as stock (for GSL-SE) that it's not worth the bother.

Directfreak

iTrader: (2)

14/40 sounds like 4.44 gears.

Brianhsval

No top end with those, but you sure will get 0 to about 70 quick.

peejay

No top end?

Top speed in a stock RX-7 is about 118-120mph.

This is achieved in 4th gear.

You could go to 4.78 gearing (aka Sportage gears) which leaves your 5th gear where 4th gear is stock, and your top end *will* not change.

Now say you have a modified engine. Say you're playing it smart and safe and keeping it below 8500 because you like engine longevity.

Your RPM-limited top speed becomes, with the FB 5th gear, 4.78 rear, and stock tire size, 149mph. And you'd *need* a nicely modified engine, too, since pulling that kind of speed would require about 200-220hp just to GET to that speed, let alone have any decent acceleration up to that point.

Basically gearing doesn't matter until you get some seriously insanely deep gearing.

Brianhsval

Not true guy....But I am not going to get in a pissing match with you. I have done this one before. 425 CAT, 3:70 rear, 10 speed tranny, top out 2100 rpm at 74 on tall rubber. Go to a 4:11 rear and top out at 66 mph at the same rpm. WTF? Comon peejay I know you know better this. I have alot of respect for your work but you are off on this one. Top end is 120 on my stock SE. Canged rear end and now is 128, not achived in 4th gear but in 5th. It is possible to get more out of a lower greared rear but you will need to go with a fuel mangement system to get the it up to a higher RPM to achive this. The higher gear will let you travel.....Oh for the hell of it 100mph at 5000 rpm. The lower gears, say a 4:44 will go like 60 at 5000 rpm. You know better. By the way this was just an example.

Feds

Brian, you are right and wrong at the same time.

A 4.11 gear is 11 % "shorter" than a 3.7 gear (4.11/3.70 = 1.11). This means that an equal input speed will be output 11% slower on a 4.11 gear.

66 MPH is 11% slower than 74 MPH (66/74 = 0.89), so you are right.

4.78 is 22% "shorter" than 3.91, so your theoretical 100 MPH is 82, and not 60.

Asuming a top-gear redline speed of 150 MPH in the stock car, a switch from stock to 4.78 will reduce that to 122. Still eminantly useable on the street and the track (ie, you will never ever aproach that with a stock motor on a track, and you will need a tonne of miles to see that on the street).

So your Diesel experience is correct, but so is PJ

Brianhsval

The 100 mph and 60 like I said was an example,at any rate the lower the gears the slower you go at the same RPM that was my point in the first place. Therefore to go with the 4:44 rear isn't logical unless you are just going to drag race it just isn't economical.

Thank you for confirming what I already knew. I am not trying to be a smart ***, but I have made a living off of gearing my trucks over the last 30 yrs., and contrary to somes belief a diesel rig is not very different from gearing a car.

Feds

You are right, but the point of lower (numerically higher) gearing is that you rarely will need to go 150 mph, so the reduction in top speed is an easy trade off for the increased acceleration.

Also, since most of these are not daily drivers, the reduction in fuel economy is not a big deal.

REVHED

I don't think you understood what peejay was saying. Sometimes the theoretical top speed of a certain gearing is different to the actual top speed. There is no point having tall gearing if you don't have the power needed to get the rpm high enough in 5th gear. The stock 12A Rx-7 which achieves top speed in 4th is a perfect example of this.

peejay

What I am saying is, even with the 4.78 gears, unless you have an engine putting out over 200hp @ 8500, your top speed WILL NOT BE REDLINE LIMITED.

Note that 8500 is the "true" redline for a stock internal engine, in the sense that exceeding that speed puts your engine in grave danger. 7000rpm ain't it. (For best engine logevity you shouldn't hold the engine over 6200 for any protracted length of time, though, due to apex seal chatter and due to water pump cavitation)

I don't doubt your truck experiences. However, we're not dealing with vehicles that are already redline-limited. They are air resistance limited.

Brianhsval

My mistake I appolagize for the misunderstanding of what you were saying. I re-read and yes you are right. Again I am sorry for the misunderstanding.