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-   -   What rear end gear ratio to choose? (http://www.hotrodders.com/forum/what-rear-end-gear-ratio-choose-113211.html)

Rodney Mackey 04-06-2007 04:30 PM

What rear end gear ratio to choose?
 
Hi, I own a '68 Malibu with a modified 396. With the move to a big block making horsepower somewhere north of 400, I need to address the next weak link, the rear end. I have a 12 bolt that I intend to rebuild and am in a quandry as to the best ratio to choose. Really, this is a daily driver..........5 days a week to work, 5 miles one way. And though this is way more car than is appropriate for commuting.......it's a ton 'o fun. I will probably never race the vehicle, but I do like being quick off the line. The current recommendation from my friends is a 3:73 posi, which should give me a lot better launch than the 3.08 10 bolt I'm currently running. I really don't intend to use this car for the highway, it's for the city. One thing I'm struggling with is the 2400 stall converter that was installed with the 396..........although I'm starting to get the hang of it, the way the torque converter slips in city driving is irritating. I've been told the 3.73 will improve this circumstance. Thoughts? Thanks.

steve t 04-06-2007 05:04 PM

355 or 373, would do good on street

427v8 04-06-2007 05:21 PM

a 2400 stall isn't much, you sure thats right?
I know people running 5000 stalls...

I would run a higher stall and a higher rear end.
3000 stall, 3.20-3.50 rears because no matter what you say now, you will want to drive it at 70 for more than a few miles!

Rodney Mackey 04-06-2007 06:05 PM

re:What rear end gear ratio to choose?
 
OK, this is my first experience with a torque converter of this type (as opposed to a "factory standard", which I understand is in the 900 to 1200 range) so I'm pretty ignorant here, but my simple understanding is that the way the torque converter works is that the converter doesn't lock up or fully engage the engine with the transmission until it reaches the designated stall rpm (in this case 2400 RPM). So it seems like a 5000 RPM stall speed converter would create more opportunity for slippage at a low RPM or more of a tendency to have to run the RPMs up to simply move the car? My understanding was that a 5000 stall speed converter was more appropriate for a drag car rather than a street car. In other words, rather than having to really rev the car up in order to hook up the drive train, I want to apply torque sooner rather than later and I think a 5000 converter is going in the wrong direction..............does that sound correct? But you're correct about making sure that my choice of a rear end ratio works for the highway. I need to figure out what my RPMs will be a 70 or 75. I really appreciate the input!

red_rider52 04-06-2007 10:53 PM

What kind of transmission are you using? Need to know to suggest gear ratio.

Try out this table out just type in your info and it will do all the math for you.

http://www.merkurtech.com/merkurtech...LCULATOR.xlsry

427v8 04-06-2007 11:35 PM

Rodney,
Yea, 5000 is a little much of a converter.

BUT a 3000 stall is a pretty good for a fast street car.
You gotta remember a stall converter isn't a switch, it doesn't just suddenly lock up at 5000 ( or whatever) rpm.

And at part throttle it transfers power to the trans just fine.

302 Z28 04-07-2007 06:43 AM

If you choose a converter with too high a stall speed you will find yourself fighting the engine at stop lights. 2500rpm would be more streetable IMO.

Vince

curtis73 04-08-2007 03:15 AM

Wait... we're talking about a 400+ hp BBC? That doesn't require that much of a cam. I'm making almost 400 from a 454 with a 218/224 cam and I run a stock converter in a 4500-lb station wagon.

I think a stall that is slightly above stock should be fine in most cars for a BBC.

Anyway, Rodney... if your main purpose is street and you will see any highway use, pick a gear ratio you can live with. If you post the transmission you'll use and cam specs we can guess on torque peak which means you can predict best cruise RPM on the highway. We can also use torque peak to determine the stall speed and a ratio that will make the best of your launch and acceleration. From there its up to you to pick the ratio that will fit your driving style. For instance if you say you're picking a powerglide with its really high 1.76 first gear, that's a lot different than using a 700r4 with its 3.06 first gear.

Zmans69Malibu 04-08-2007 12:29 PM

Gear ratio
 
I have a 69 Malibu and I run a 3.73/1 behind a 700R4 transmission. that gives me 1800 RPM at 60 MPH in OD and I still get the acceleration and torque in 1st-3rd. The 700R4 will handle 500HP easily especially if it's not a race car, just a hotrod

Rodney Mackey 04-09-2007 01:36 PM

re: What rear end gear ratio to choose?
 
To red-rider52:

OK, it looks like I'm going to a slightly beefed up TH400. Based on the calculator you suggested (by the way thanks!), with a 3.73 it looks like 67 MPH at 3000 RPM. A little high, perhaps, but workable as street car? A 3.55 gets me to 70 MPH at the same RPM.

To 427v8:

So a 2400 converter is pretty conservative? Will going to a 3.73 dampen the slipping effect?

To 303/Z28:

By fighting the engine, do you mean having to "punch" the engine to get an reasonable rate of acceleration?

To curtis73:

Yes sir, it's a street car. TH400. Cam is a Lunati with a .54 lift in and .55 lift ex. The guy who built the engine indicated that this engine's torque curve is at a higher RPM, which (if I understood him correctly) is the reason he selected a 2400 converter. Sound correct?

To Zmans69Malibu:

Yeah, an overdrive would be cool! Solve all the problems! Are you running a standard torque converter?

Zmans69Malibu 04-09-2007 04:30 PM

rear end gears
 
Not stock but not a high stall, 2000rpm or a little less. Built to 69 LT1 specs with edelbrock performer heads and intake.

427v8 04-09-2007 05:26 PM

I'm running a 2400 stall on my vette.
it's got a 450hp 500ft/lb 434 a TH400 and a 3.70 rear end. I've driven it >200 miles on the highway, went racing and came back.

I Get a little bog on the track, no bog on the street, just tire smoke :welcome:

I'm torn on more converter or solving the bog with a shot of NO2

More converter would definitly make it faster...

red_rider52 04-09-2007 09:15 PM

I think if your not going to be going to the track at all you would be happier with 3.55 even in the city and if you do happen to go on the highway you will be able to drive a little faster.

It is really up to you in the end but I think 3000rpm at 67mph is a little much. and if you don't have a lock up converter you will have a little bit of slip witch might put you around 3100-3200rpm at 67mph.

Ozz1967 04-10-2007 12:45 AM

With a BBC you are going to have GOBS of torque. I don't know how much exactly but running a 2500 stall with 3.73 gears you are going to get off the line in a HURRY!!!! I mean smoking!

What he means by fighting it at the light (as I figure) is that you'll be having to stomp on the brake to keep the car from going anywhere. It'll WANT to go forward.

For true street with a TH400, especially since this is going to be basically a daily driver...I'd personally go with a lower gear, say 3.32 or 3.42. You'll be much happier around town and It'll give you good highway with that 1:1 final gear. Not to mention with the torque you'll be shoving to the pavement with the 454 you won't burn all the rubber off your tires just easing it into first gear. With a 3.73 and up you won't be able to keep tires on the thing.

spinn 04-10-2007 10:02 PM

maybe i just like cars that drive right
 
3.73-4.10 ( no OD) are terrible on the street. take your daily driver out. put it in second, and drive at 60 mph. is that cool ?? cruising at 3500 rpm is no fun. garranteed police attention in my town.

3000-5000 stalls are rediculous on the street. no fun part throttle either.

its always some hillybilly hotrod. i like a balanced car that drives right.

------------

a race car is a race car. they sound cool, and are fast. they dont belong on the street. you cant cruise at 4500 like a boat to buddies house. you just cant.

a muscle car has incredible on demand power at any time. mastering the open roadways at all speeds. they fall short on the race track. sometimes you wont even get into 4th, i think thats a good thing.


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