What to Expect When You Convert from a Classic Automatic

We often hear an overdrive transmission is the single most dramatic improvement our customers make to their cars. Not too many of them back it up with dynamometer results. Our customer, John P. sent us this email with his dyno results:

I bought a TKO600 kit for my 1973 Corvette from you last April. I got it running last weekend, and the difference between my TH400 and this TKO600 is astounding. I can hardly believe I ran that TH400 for all those years. It was nicely built with higher stall, shift kit, 34 tooth sprag & good clutches & steels, but it is still an absolute slush box compared to the performance of the TKO. This was worth every penny of the cost. And the kit was beautiful, everything was there and of excellent quality and perfect fit.

The thing that impressed me the most was the increase in power and response to my wheels. I had expected to feel some of course, but the amount I did feel was much greater than my expectations. I put it on a chassis dyno this morning and the numbers prove it: The TH400 used up fully 25% of engine power, this TKO600 uses only 12%. Half! I got an increase of 60 lb/ft of torque and 48 horsepower to the rear wheels! I did not expect that great a difference, but the numbers prove it. And I shall show you:

Attached are my dyno sheets. One for the engine on the engine dyno with full exhaust and air cleaner as on the car, one with the TH400 on the chassis dyno, and one with the TKO600 on the same chassis dyno.

I'll also send a photo of the engine on the dyno and one of my car. It is a bit of a street rat. The paint is original, with chips, stains and faded front bumper. So is the interior, with ripped driver seat and some loose trim. Radio, A/C, and other useless stuff has been removed. None of that helps it run better. But underneath there are many suspension, brake, drive train and other upgrades. The engine is a mildly built 383 with AFR heads, smallish roller cam and OEM Quadrajet. I run a 3.36 rear end.

Thanks for your help & the great kit. I love it. The 500 miles I have on it so far have been the most pleasurable and fun I have ever had driving this car. It is the best thing I have done for the car and really adds a crowning touch to all the other modifications I have made --and will keep making.


John P.
Loveland, CO

As you can see, John is really happy with the Tremec TKO600 5-Speed and his PerfectFit™ Kit. His results made us curious about why his horsepower and torque improved exponentially, so we asked the smartest guy we know, our in-house engineer, Dick McCord. Dick examined the dyno test results and took inventory of the drivetrain components in John’s car. This is what he found:

  1. The TKO has a lower rotating mass (inertia) than the larger, bulkier TH400. Energy is lost as the heavy gears rotate.
  2. The TH400 torque converter efficiency (even with a higher than stock stall speed) would affect the dyno results.
  3. The TKO was designed to be more efficient than older model transmissions; so many considerations were made to reduce parasitic power loss.
  4. The A41 automatic, LGT-700, and Magnum 6-Speed would have all had similar results for the same reasons as the TKO.


Dick also pointed out that when the car is in 5th gear with an overdrive ratio of 0.82 verses the TH400’s 3rd gear at 1:1, the reduced engine RPM at highway speeds will increase gas mileage and reduce cabin noise.


These are some impressive results. See the photos of John’s dyno results and car below.  

TH400 on the chassis dyno

TH400 Dyno Test

TKO600 on the chassis dyno

Tremec TKO 600 Dyno Test

TH400 on the engine dyno

TH400 Engine Dyno Test

The engine dyno setup

Engine Dyno

John's 1973 Corvette

1973 Corvette