Tag Archives: lightweight OD transmission

Sorry for not posting

Because I don’t have anything to write about. Mostly what I have been doing is looking for an overdrive transmission that doesn’t weigh anything and has a torque capacity to handle large displacement LS engines and can be picked up for next to nothing at a junkyard. The difficulty is light weight and high torque capacity are pretty much mutually exclusive, or are hideously expensive. There is a T5 variant that weighs 80 pounds and has 550 ft-pounds capacity, but runs north from $4k when you can find it, which is slightly more often than “never”.

There’s a kit that makes a T5 handle 600 ft-pounds, but you have to have the T5 first, which I don’t. The good thing is the replacement gears in the kit are only a pound or so heavier than the stock gears so your finished transmission is 76 to 78 pounds plus a quart of ATF.

What I’m fighting against is the power to weight to cost champ is the LS-LT GM family of engines from the junkyard have more torque than the cheap transmissions can handle. The cheap champ OD transmission is the T5 at $500 and up for rebuildable cores locally, but only has a 300 ft-pound capacity and the 4.8 LS has 300-330 ft-pound output depending on tune which is the lowest of the engine families. If I wasn’t trying to build as light as possible because the classes I’m competing in have essentially no minimum weight, or a minimum weight so low that I will never get that low with the car I’m trying to build, I could go with the heavier transmissions. But I’m building light and cheap because F=m*a and reducing “m” increases “a” proportionally, and I’m a genetic tightwad. Or maybe a cultural tightwad because I don’t see any signs of it in my kids. But I’m building to a 900 pound with driver minimum weight for the class with a published minimum weight, or about 700 pounds empty, and the other classes have no minimum weight but do require equipment that kinda raise the weight the car has to be built to, like horn, headlights and fenders. And a pickup bed in one class. So we are looking at about 1500 pounds empty± in race trim (3 gallon gas tank).

And this is about where I’m putting this to bed, I’m running out of things to say.

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Kept awake again last night

I spent most of last night staring at my eyelids or looking at my phone. What happened was I stumbled on a website selling upgraded T5 transmissions that could be shifted without using a clutch or lifting throttle.

Now they didn’t give a torque rating on the upgraded clutchless transmission, just “stronger” than the factory 300 foot-pound rating, and reading about the fragility of the transmission proves that’s a hard limit. Since the engines I’m looking at are much higher rated for output than that the stock T5 is never going to work, but this company sells parts and complete transmissions that claim to greatly increase that throughput.

Sales page for the T5

Of course I had to see how this 75 pound transmission would work for my application including figuring out cruise RPM and how fast the car would go with the “40MPH in 1st” final drive gearing in 2nd (about 70). My mind wasn’t working right when I tried to figure “40 MPH in 1st” to “RPM at 70 MPH cruise in 5th”. I mean I know it’s just comparing ratios, except there are several ratios and I couldn’t get the equation to work out. There is the ratio between RPM and speed, the ratio between the street and race tires, and the ratio between 40 race and 70 MPH cruise. When I’m awake, that’s not a problem, but last night my brain was sleepy and did not work to let me calculate that it would cruise at 1850 RPM at 70 MPH in 5th and not being able to figure that out wouldn’t let me sleep.

But as I was looking for upgrade parts from other manufacturers I found this that has been track-tested at 550 ft-pds without breaking on slicks with a significantly heavier vehicle than the Sprint-T.
G-Force upgraded T5

But anywho I was looking for manual transmissions and stumbled on this: Strong 5-speed transmission. The super low 1st gear option only supports 500 foot-pounds but also only weighs 99 pounds (well all of them weigh the same, but the one with the lowest 1st only takes up to 500 foot-pounds without breaking). And that low ratio 1st gear turns 1860 RPM at 70 MPH in 5th with the same final drive that turns 6000 in 1st with the same final drive. Also tops out in 2nd at 66 MPH with the race tires. Obviously strong and light are not cheap at approx $2600 plus freight and taxes.