White_Knuckles

So to not hi-jack the poster asking about swapping an overdrive trans in his '66, I'm thinking a TH350 will be dandy fine. I have a functional switch-pitch, Jetaway. Or in other words, a "Junkaway" as Joe P. defines them.

So with a 350 swap, what rear-end gearing would work best for performance? I spotted, in the other thread, an overdrive trans may run too low, (RPM) lugging at highway speed with stock gearing? I'm thinking a step up to a peppy ratio may be fine as I never take long highway drives? Having a 1st gear, a mild shift-kit and gearing that makes for low-end grunt would make her scoot off the line and be fun! But 70 mph can't be 3800 revs either.

I have no idea what the stock gearing is or if it was optimized for a two-speed? Anybody have experience going from a ST300 to a TH350? I'm not sure what to expect?

Note: I'm set on a 350 so please no 4-speed, lock-up or overdrive suggestions. Cool, but too much for my goal. This is not a "driver", it's a Mad Max escape vehicle simply for scaring women and children. Mileage is not a concern.

oldcutlass

I don't know what gearing is in your car, mine has 3:08. I went with a turbo 400 because of an engine swap, it was available and functions fine over the ST300.

The T350 is the simpler swap. Your transmission cross member stays where it is. The drive shaft will not need to be modified, your shifter linkage should be fine. You will need the a passing gear cable and a different torque convertor with a stall speed based on your engine mods and gearing choice.

tmaleck

The final drive for the two and three speeds are the same, 1:1. The big advantage is an extra gear in the middle. I like numerical rear gears in the 3:42 to 3:75 range for street cars. Gives you a little better pull without putting you over 3500 rpm at most highway speeds. I also did a spreadsheet to calculate rpm vs. gear ratio vs tire diameter. I can email it to you if you like. With a 26 inch tire, 3:73 rear gear and a 1:1 transmission gear, I get an engine rpm of 3400 at 70 mph.

Tim

joe_padavano

Actually, no. The big advantage is the lower first gear. Second gear in the three speed ATs is close to first in the Junk-a-way. (1.48/1.52:1 vs. 1.76:1)

Of course, the 200-4R has the lower first gear AND the O.D.

Sharky146

I'm also thinking of swapping my jet-away. I think I'm going to go with the 200-4R. I had an 87 Monte Carlo SS with that tranny and really liked the punchy first gear and overdrive. There's a guy selling one locally for $350, but they're north of $1000 remanufactured. I'm not totally sold yet though. The TH350 is cheaper and just as easy to swap....so I hear.

oldcutlass

I think on any, the torque convertor will make or break the drivability of the trans.

don71

"This is not a "driver", it's a Mad Max escape vehicle simply for scaring women and children. Mileage is not a concern."

First, I think this should be in your sig line

I don't know what your rear ratio would be, short of crawling under there and finding the code or simply pulling the diff cover to make sure.

I have no doubt you would like the 3 speed th350 regardless of diff ratio. Good plan to put a smile on your face.

White_Knuckles

Okay, good we're getting the streetable rear ratios up here.Sounds like my existing rear-end is a mystery without getting numbers off it.

Good stuff. I thought a lower first gear may have some tire shredding fun involved!

Okay, let's talk about that. Stall to me means slip before lock-up. A high-stall converter, let's say 3500, is not a good choice for a driver unless power braking or line-lock is desired to launch your rocket? So, do you have a recommendation for a stall speed?

Hmmm, may have to add that? You see, the car is heavy gold metal-flake and looks 70's "for sure man". It has dual Flow Master 40's pokin' out from a 70's Olds 350 with #6 heads and a particularly nasty cam. It shakes windows and has a lovely blat, blat, blat exhaust note. I get strange looks from people all the time. Yes, I'd say it's your basic scary car.

I like your numbers matching line. I'll state I do have a numbers matching car. My front plate matches my back plate!

oldcutlass

I would call a few of the torque converter companies and tell them what your engine specs, gear ratio, trans, and see what they suggest. I'm guessing a little over stock 15-1800 RPM.

White_Knuckles

^^ Yikes! I just checked out your monster engine. Wheeee! I'm thinking the same range but still don't really understand why more slip is better for normal driving? Okay, not normal but no real staging or line-lock need.

oldcutlass

It raises your flash point up into a better power range for your relatively stock engine. You'll be surprised at how much better your car will launch. It's basically one step over stock.

joe_padavano

For normal driving, you really want the LOWEST stall speed that won't bog the motor. High stall speeds are for dedicated drag cars. Don't fall prey to the misconception that what's best on the track is also what's best on the street. Drag cars operate under a very narrow set of conditions. Street cars require the maximum performance flexibility possible.

oldcutlass

That's why he should call the torque converter company and give them all the spec's. They will recommend a torque converter that's suited to his setup and driving habits.

White_Knuckles

Ah, both your comments make sense. There are several variables here. Problem is with a Franken-car, the specs are mysteries. It has a Olds 350 that produces ??? HP. It's final gearing is ??:??. So, my questions are looking for known good results with what one can guess from similar model cars. Getting the right parts the first time would be nice.

Installing a TH350 1st will reveal any strange new conditions. The torque converter should not be too crazy as I will certainly discuss this with the trans rebuilder. The gearing is the "hope I get it right" part.

Interesting info. you guys are posting. Good stuff for an informed process.

oldcutlass, could you describe "flash point" in little more detail please? Not sure I quite grab that?

oldcutlass

The stall speed is, for example 1500 rpm, that's where under normal circumstances the torque converter locks up. Up until that point it will slip a bit to allow the rpms to come up a bit. The flash point is based on torque, so a 1500 rpm stall speed will actually flash to a stall of say 1800 plus or minus under full throttle conditions. This helps keep your launch up in the power band of the engine.

Since you have a lopey cam of unknown origin, I would go a little over stock.

White_Knuckles

Right, the slippage allows revs to ramp up before lock. I did understand for racing applications, they go up in RPM stall speeds to unleash the optimal power band to the wheels on launch. If it didn't slip, the motor would strain being held back under braking. I didn't know street cars can benefit from a slightly higher stall. Interesting.

So, from a dead stop idling, you release the brake and pull away rapidly with a slightly higher than stock converter. Would you see an improvement or would you need to hold the brake, run the engine up to 1500 and then release the brake to get a pay-off? That's what I'm puzzled over.

oldcutlass

Under normal driving you won't notice much of a difference. You would just drive it like normal. Now if you only have 2 speeds like my wife (stop and go go go) you will notice some difference in the engine rpm as it will ramp up higher to a higher rpm when push on the big fun pedal.

White_Knuckles

That's it, no braking required. Someone told me at sometime (you know the ambiguous statement) that, " higher stalls are only useful for line revving before launch". Maybe not so much? I think I've learned a lot of bad information over the years. Doh!

Thanks