watch and learn....
this guy tears them down in about 20 minutes!
this guy tears them down in about 20 minutes!
Transmission Options
- Thread starter Rgpmalibu
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I got a carbed 6.0 gunna go into my 83 Grand Prix, thinking about going with a TH400. I don’t know the science behind it but I’ve seen a lot of people use that combo. Can someone tell me why? I read online they can take a beating. Someone else said I won’t be able to take it on long highway drives which kind of sucks.
TH400's are known to carry the workload of many w/o failure. They're not as efficient as other auto trans options but even in basic form they're solidly built.
The 4l80e is just the electronically controlled over-drive version that works just as hard & offers a better highway rpm driving range.
If you want an all around fun driver then an overdrive transmission is the way to go. If you're going with an LS, then put a 4L80E behind it, in my opinion. Go with 3.42 rear gears with a mild cam and a good converter and you have a very fun combination. Not a drag strip star, not a dog. Just a very fun driver.
Tire size (height), rear axle gear ratio, final drive ratio of your transmission and how you want to use the car are all factors in the equation.
If you want to be dominant at the drag strip, then you need a lower (numerically higher) rear axle ratio like 4.11. If you want something really terrific on fuel economy, then you want a high (numerically low) rear axle ratio like 2.29. For a driver, you normally want something in between.
3 speed transmissions (automatics) have a final drive ratio of 1:1. That's to say, for every one turn of the engine, the output shaft of the transmission turns once. This is also true of most 3 and 4 speed manual transmissions. With an overdrive transmission, you get more than one turn of the output shaft for each turn of the engine when you are in overdrive (highest gear). This lowers engine RPM when you are in high speed cruise. It also gives you the flexibility to run a lower (numerically higher) rear gear than you would with a 3 speed transmission.
Here is an RPM calculator I like to use:
Here is a tire height calculator I like to use:
Tire Height Calculator - Wallace Racing
Tire Height Calculator by Wallace Racing
www.wallaceracing.com
Here are a few examples for you:
TH400, 3.42 rear gear, 26" tire - A TH400 has a 2.48 1st gear ratio. Multiply 2.48 x 3.42 (rear axle ratio) and you have a final drive ratio in 1st gear (taking off) of 8.48:1. This is acceptable but will be suited to something with some horsepower and a little bit of stall speed to accommodate the relatively high 1st gear ratio in the TH400. Now let's look at highway cruise. At 80 MPH you will be turning 3536 RPM. This sucks with loud exhaust and just plain sucks over long distances.
4L80E, 3.42 rear gear, 26" tire - A 4L80E has the same 2.48 first gear ratio so you have the same 8.48:1 final drive ratio in 1st gear. A 4L80E has an overdrive ratio of 0.75:1. At 80 MPH you will be turning 2652 RPM. This is much much better if you are going to use the car on the highway.
With a 6.0 and a mild cam, you probably don't want to go steeper (numerically higher) with the rear end ratio than 3.42 for the street or it's going to just fry the rear tires when you stomp on it. You just won't be able to get traction. The only "negative" with a 4L80E is that the stock truck converter is very tight and will feel doggy. You really need to spend the coin on a good aftermarket converter with a stall speed to match your combination. If you do that, it is super fun.
Here are a few more examples for illustration:
TH350, 2.41 rear gear, 26" tire - This is a good example of a malaise era G Body. Gutless 305, three speed transmission and a very high (numerically low) rear axle ratio to make it get fuel economy and pass emissions requirements. 2.52 1st gear x 2.41 rear gear = 6.07:1 final drive ratio in 1st gear. Gutless turd that won't get off the line. At 80mph it will turn 2492 RPM and get good mileage doing it. At 55 it turns 1713 RPM. This is how the Big 3 initially evolved to get the cars to pass fuel and emissions standards in the late 70's.
200-4R, 3.73 rear gear, 26" tire - Here's your Monte SS or Olds 442 setup. The engine made around 180 HP. Give it a steep (numerically high) first gear and a steep rear gear to get it off the line and make it feel decent, and give it overdrive so it gets good highway mileage. 2.74 1st gear x 3.73 = 10.22:1 final drive ratio in 1st gear. Gonna feel good even with that suffocated smog dog under the hood. A 200-4R has a high overdrive at 0.67:1 (big RPM drop when it shifts from 3rd gear to overdrive). At 80 MPH it will turn 2584 RPM. At 55, it will lollygag along at 1776 RPM. A good package for the times with such low power engines.
TH400, 4.11 rear gear, 26" tire - 2.48 1st gear x 4.11 rear gear = 10.19:1. Gonna get out of the hole very well. Will likely shift into second before you get across the intersection. At 80 it turns 4249. Not gonna work on the highway. At 55 it turns 2921. This is a local in-town car and is really set up for drag racing.
It's all a compromise. Hopefully this helps you understand the math behind determining what setup to run.
Hey man thanks for that info, car has a Posi rear end 3.42 gears. Not sure on the tire heigh I’m going with, car sitting on some cragars right now. But I do want a fun car that I can daily and drive on the highway. I was leaning towards the 400, might go with the 4l80.
The trucks with 4L80e have 32" diameter tires and do ~2200 RPM at 80MPH. Stock tires on g-body are 26" (I have 235/45R17 = 26" OD). Engine will be screaming at 80MPH (3535 RPM) with the 3-speed; 27" tires on the g-body only knocks RPM down to 3405 at 80MPH. I heard only the 2004R will do a 3-4 shift at WOT.
If you go with a 4L80 then you need a trans controller of some type. The TH400 doesn't need anything electronic. If it's going to be daily driven, and you decide to go with a 4L80, then it lends to the question of WHY use a carb.
To answer your question directly though, the TH 400 in mild built form can handle 1000hp all day everyday and it's cheaper to build than 4L80. A built TH400 can be had for $1500-2000 in this area and a freshened (shift kit and new stock clutches) can be done for about $700-800 on your core. They are beasts, the same goes for a 4L80, but more money. A stock 400 or 4L80 will handle (laugh at) 500hp.
What year is your 6.0? You have be careful of the early ones with the different crank - need the proper flexplate spacer/adapter.
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One can always do a TH400 w/a Gear Vendors OD unit to eliminate the need for the controller. Yields about the same rpm compared to a 4l80e when OD is engaged (the 4l80e can lock the converter to drop rpms a little more). A built 4l80e w/a GOOD converter prob runs about the same price tag as the Gear Vendors unit. Either way won't be cheap but lower highway cruising rpms make longer drives much nicer.
Dad's Galaxie has a GearVendor's behind the Top loader. It makes the car. The GearVendors has a .78 overdrive ratio compared to the .75 of the 4L80E, an imperceptible difference. The GearVendors unit is a little girthy. It was very tight in Dad's car. We had to massage the tunnel.
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