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Torque Converter and Rearend Ratio

Old March 13th, 2019, 01:46 PM
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Oldscarnut
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Torque Converter and Rearend Ratio

As I finally get the engine parts in a single pile, and when the machine shop has done their balancing act, it is time to start the engine build. So many have warned me about the low ratio rear ends in the 80's G Body, and a TH 250 hiding where a TH350 should be. I think I've resolved that this is a TH350 BOP because of the Buick 3.8 V6 (NOT CHEVY 3.8!!!) The Car must have been a desert SW car because there is virtually no rust, but every single rubber part is dried and cracking, so I have replaced the door gaskets and window rubber, the body mounts etc. The dash looks like a rpg hit it and I will someday deal with that, but for now the engine goes in and bolts to a TH350 which as a converter set for low rpm, low output boat anchors How to I figure out which ratio I have in the 10 bolt non LS rearend? I thought to get an accurate speed check and then check the tach at 60 to see what RPMs are happening. My Olds 350 is being built to the W-31 specs with a W-30 cam for automatic, Qjet RPM Edelbrock, factory cast iron headers, dual exhaust, and the pistons are 10.25:1 EGGE. I am assuming the V6 Torque converter must go, and I am being told by a lot of people about higher stall speed, and lower gearing. I want a cruiser not a drag racer, so I have been warned by my Olds racer friends to watch out on the stall speeds much above idle for street use. I DO NOT EVER WANT IT TO FEEL MUSHY! I'm also told that with higher rearend ratios, the higher stall can cause slipping and overheating. With all that in mind, to me the ideal setup for this is 3.08:1, maybe one step higher. I would want the stall to be almost instant, certainly not a launch! Can anyone shed some light on their experiences with matching torque converter to rear end ratio for cruising efficiency vs off the line? I do not want to spend needless money for performance I do not need. I don't want to burn up my tranny because I made the wrong matches either. My engine will be close to the W-31 power curve, with HEI blueprinted distributor, MSD Capacitive Discharge, big valves and higher rate valve springs than stock 350's, and higher compression. I'd like to hear advice, and not criticism of how I chose to build my engine on a very tight budget.
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Old March 13th, 2019, 03:39 PM
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If you buying a new convertor, contact the company you are buying from. They should have a tect person, that you tell them what you have and what you want the car to do, and they can help you.
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Old March 13th, 2019, 03:50 PM
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Honestly if you plan on 3.08 gears, I would not consider much above a stock converter otherwise it will be slipping a lot.
With a lot of gearing, you can get away with more stall.

Keep in mind then, you are not building much of a performance combination, but will end up with a nice street cruiser with decent mileage.

You can easily check your gearing by marking the driveshaft & rolling the car until the tires move 1 revolution (or jack it up & spin both tires 1 turn) & count the number of driveshaft revolutions.
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Old March 13th, 2019, 05:12 PM
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Oldscarnut
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Thank you Lonnie. I will try that the next time the wheels are off the ground...so I am guessing 1 tire rev to 3 shaft rev means 3.00:1. So my 3.08 is one and a smidge; 3.42 :1 is almost 1.5 revs etc? I like the driver/cruiser idea. I'm not a kid any more...or any less either, but I don't see me racing too many unless their big mouths overreach their brain. The engine will have enough poop to get out of its own way, just don't want to burn up a tranny figuring it all out when I have the time to fix it now. One question that comes to mind is; in BOP trans, is the stock converter the same with V6 and V8, so that the stock converter working in my TH350 now with the Buick 3.8 will work or not?
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Old March 13th, 2019, 05:13 PM
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Thank you! You can never go wrong asking the right person the question and then listening to the response!
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Old March 13th, 2019, 05:48 PM
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My car has a Hughes Performance 2500 RPM converter and I have had both a 3.23 rearend and a 2.56 rearend under the car and the converter worked great with both rears. Driving around town at light throttle it feels a lot like the factory converter, with no apparent slipping when taking off from a stop - it just goes like normal.

I did some RPM measurements when the 2.56 rear was installed and found the converter slipped only about 250-300 RPM while steady throttle cruising, both above the stall speed and well below the stall speed, like the car running at 45-50 MPH and the engine turning 1700-1800 RPM. Many people complain about excessive converter slippage and the resulting heat generation but the Hughes converter doesn't have that issue.

Bottom line is if you get a quality converter it won't slip much until you go WOT.
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Old March 13th, 2019, 06:55 PM
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I will check it out! That is good information. I have always said you cannot go wrong with factory feel, settings, adjustments because they were able to choose from an infinite supply of parts and they picked the best for performance/fuel mileage, feel and sound, at least until 1972!
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Old March 13th, 2019, 08:49 PM
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Im running ~400hp BBO with an 11" TCI Breakaway converter, (2400 to 2600 RPM flash stall), a TCI TH400 Street Fighter manual/automatic valve body trans, Denny's aluminum driveshaft and 308:1s held together with ARP hardware. Love it! Makes great squirrely 11s all day. Great launches, decent highway manners. Feels stock...mostly. Yes, a high 3 or low 4 series would be even more fun, But then I'd need an OD trans. This car is street driven with 1-2 annual trips to the track.

Look at the Jegs link below. They have a chart which lists the recommended applications by HP, cam, rear gear, engine characters, and block size.
Get the TCI clear synthetic trans fluid.

FYI for those who know me, I tout a numbers matching car. Fear not the # matching trans will go back in this summer after I go through it. It ran well with the Breakaway converter as well. I have to detune it. Keeps self-destructing. It's internally "dual fed" with a custom valve body, Red frictions and band/Kolene steels, HD Drum & Planetaries with extremely high line pressures. What a nasty shifting beast. It idles into 2nd and leaves a scratch. Hummm could that be what out-of-phased the OEM two-piece d.shaft...oops.

https://www.jegs.com/i/TCI/890/24110...276+4294828826

Click on the "overview" tab to see chart
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Old March 15th, 2019, 05:00 AM
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What year of a TH350 is it?
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