Gear ratios

"RamblinRover Luxury-Yacht" (ramblininexile)
03/13/2014 at 16:28 • Filed to: None

Does 800RPM = 6.5MPH in first sound a little fast to anyone, or about right? I ask because that's the lowest permitted by a BW Super T-10 wide ratio + 2.88 diff, presuming 26" overall wheel size. If that seems slow, there is a version with a 20% taller first...

DISCUSSION (20)

crowmolly > RamblinRover Luxury-Yacht
03/13/2014 at 16:33

2.88 diff? Isn't that first gear in the wide ratio ST10? What's your rear axle ratio? I'd say it would be a great first gear with a 3.73 - 3.23 rear.

dogisbadob > RamblinRover Luxury-Yacht
03/13/2014 at 16:46

Not really

Can't almost any do like 10 mph at idle? My car can probably do 10 mph in D with no foot on the gas or brake (automatic transmission, idle is 750 rpm)

RamblinRover Luxury-Yacht > crowmolly
03/13/2014 at 16:47

The diff is a Jag PowerLock, and yes - 2.88. The widest ratio ST-10 that they made is a 3.42 - which makes the first/diff balance exactly backwards from what you'd expect. Extrapolating the constant output torque #, it should be good for 250 ft-lb or so, rather than the ~300 IIRC for a 2.64 ST-10 (most common factory). I should be up against those kinds of numbers, but not violently past 'em.

The brand new ST-10s are available down to 2.88 - which would be ~275. Didn't come that way in any GMs originally that I'm aware of.

RamblinRover Luxury-Yacht > dogisbadob
03/13/2014 at 16:50

Well, tire burning would be amusing, and for the record it's a good bit taller than my Benz (auto).

davedave1111 > RamblinRover Luxury-Yacht
03/13/2014 at 16:59

It sounds a little fast, but not massively so. That's 8mph per 1000rpm, so at 5krpm you'd be doing 40mph. Depends on your engine. With a VTEC (yo) it's definitely wrong, with a big low-revving diesel it would be spot on, and there's some middle-ground in between where it might be an acceptable compromise.

crowmolly > RamblinRover Luxury-Yacht
03/13/2014 at 17:00

Oh, that's right, the iron ones got the 3.42 firsts.

I think any of them could hold well over 300 lb-ft.

And I think I'm at 6.5 mph @ 850 RPM. 4.11 rear, 2.66 first, 28" tire. Seems ok to me.

RamblinRover Luxury-Yacht > davedave1111
03/13/2014 at 17:02

Rover V8 4.2. Diesel-designed crank, will be decently high compression, and modified for higher revs. Still, in a sub 2500lb car, so fairly over-engined.

RamblinRover Luxury-Yacht > crowmolly
03/13/2014 at 17:05

Not just the iron ones! The PowerBrute iron ones yes, but I believe aluminum case ones as well mid '79 on. The tq figure is abstracted from Richmond's number for the ones they still sell - probably similar for an original BW. They rate 'em based on output torque, but it's a fact that I probably can't make use of max torque in first without spinning tires, so likely a non-issue - I'm not going super-fat.

Demon-Xanth knows how to operate a street. > RamblinRover Luxury-Yacht
03/13/2014 at 17:06

No, actually that's not too far off from the 5MPH in my Dakota and probably 7 in the Corvair.

RamblinRover Luxury-Yacht > Demon-Xanth knows how to operate a street.
03/13/2014 at 17:07

"About right" in other words. The next most available gear ratio used would be over 8 mph - pushing it.

davedave1111 > RamblinRover Luxury-Yacht
03/13/2014 at 17:09

I'd say it's a bit quick, but then you're not really going to need first with that set up anyway. I don't have any direct experience with this, so I don't really know what you want to do - you're not going to want to stay in first all the way to 50mph, but if you go the other way and make the gear shorter you'll be hard pressed not to spin the wheels every time you use first.

crowmolly > RamblinRover Luxury-Yacht
03/13/2014 at 17:10

Huh. I thought 81 Z's and such got the 3.42 in an iron case. It's been a while (if you can't tell, LOL).

GM rated them higher back in the day, and I think that was with a 24 hour torture test.

Are you buying a trans or trying to figure out what you've already got?

RamblinRover Luxury-Yacht > davedave1111
03/13/2014 at 17:11

I'm presented with a quandary - make car impossible to drive slowly without clutch slip under any circumstances, or convert all tires fitted into vapor. I *think* the proposed combo is the best balance, but I'm not sure - and it's impossible to go shorter without swapping diff sets.

Demon-Xanth knows how to operate a street. > RamblinRover Luxury-Yacht
03/13/2014 at 17:12

Corvair is a 24" tall tire, 3.5:1 1st, 3.27:1 rear.

Dakota is 29" tall tire, 4.01:1 1st, 3.55:1 rear.

...doing the math, both are pretty much equal to 5MPH.

RamblinRover Luxury-Yacht > crowmolly
03/13/2014 at 17:13

Buying. ~81 would have been the intro of the iron case, probably, but it's too damn heavy for my project. 3.42 aluminum case is probably my goal at this point, they made them in parallel with the iron.

RamblinRover Luxury-Yacht > Demon-Xanth knows how to operate a street.
03/13/2014 at 17:16

Somewhere around 26" tall tire, 3.42 first, 2.88(!) rear. You can see which element is, as the Brits would say, playing silly buggers. Not that I mind - makes for a high top speed, economy, and grumbley sound at highway speed without a big performance hit because stupidly light car.

crowmolly > RamblinRover Luxury-Yacht
03/13/2014 at 17:16

Why not a world class T5? Kind of a middle ground, plus you get an OD gear and an internal rail shifter.

RamblinRover Luxury-Yacht > crowmolly
03/13/2014 at 17:18

Four-speed: more brutish and primitive, and side-levers are easier for me to convert my column shift to. Yes, I know that part's daft, it's not the point. 10lb/hp = I need no extra gears.

crowmolly > RamblinRover Luxury-Yacht
03/13/2014 at 17:35

You are going to column-shift your 4-spd?

That's bad ass!

RamblinRover Luxury-Yacht > crowmolly
03/13/2014 at 17:43

I need to make/modify an extra detent plate and modify the rod slide for an extra 1/2" or so. Then, another lever and link rod - bam, four on the tree. Permits me to keep a clean (er) floorpan and use a bench of some kind - otherwise a floor shift with fixed position like a T-5 would end up pretty much right next to the front edge of the bench.