Overdrive Euphoria

Shift From Old School to New Age With a Simple Swap

Randy Fish Apr 11, 2005 0 Comment(s)

In the past, automatic transmission swaps were normally done so the tranny would handle the extra power of a new (or bigger) engine combination. Rarely, if ever, were gear ratios or the benefits of overdrive a consideration for the general street enthusiast. Now, with a bunch of choices out there, overdrive transmissions are quickly becoming known for producing many more benefits, aside from just fuel mileage, and we're about to show you why.

Enter Randy Manning, an outside sales representative for Edelbrock. His daily driver, a '70 El Camino also happens to be his favorite toy. What's more, this long-time enthusiast of speed has installed plenty of trick upgrades on this proven performer. Check out our Tech Specs box to see what powers his hauler. Then, we'll tell you why he thought this swap was a great idea.

After hearing so much about the benefits of GM's newer transmissions, Randy felt that swapping out the trusty TH400 would offer a better level of driveability, more favorable gear ratio splits, and maybe even an increase in fuel economy. He chose a Hughes Performance 4L60 (not to be confused with the 4L60E), which is an updated version of the popular 700R4. What we're focusing on here is more of the "meat and potatoes," as opposed to a step-by-step install. We figure most of you are already familiar with the basic "how-to" wrench-turning parts of this swap.



In crunching the numbers above, the 4L60 delivers at both ends. The 3.00:1 low-gear ratio launches harder, but when coupled with its .67 overdrive represents a 33-percent reduction from 1:1 for highway cruising. To break it down further, when you consider that the El Camino has a 3.73:1 rear gear, this 4L60 delivers the low-gear performance of a TH350 with a 4.44:1 rear, or, a TH400 with a 4.51:1 rearend ratio. In overdrive mode, the 4L60 performs like a TH350 (TH400 or Powerglide) with a 2.50:1 ring-and-pinion!



The drop in cruise rpm represents more than improved fuel economy (from 13 to almost 18 mpg); it also reduces strain and friction on engine internals. Anybody who has cruised a considerable distance with a performance car equipped with aftermarket exhaust and a relatively low rearend gear knows what the interior noise levels are--hell, most times you can't carry on a conversation, much less hear the stereo. Swap in a 4L60, and you might find your comfort zone has gone up several notches. Another great trick from Hughes is their 10-inch performance lockup converter. It provides more stall down low, but it locks up on the road for another 100-200 rpm, as opposed to the TH400's larger, "street performance" converter.

* 700R4 and 4L60 use the same yoke as the TH350 (smaller than the TH400).
* Driveshaft needs to be shortened (measure from the back of the tailshaft, or flat transmission rear seal surface, to the flat of your pinion flange (at, or as close to ride height as possible).
* Transmission cooler lines fit with minor realignment.
* Speedometer cable requires no modification. Hooks right up!
* Transmission case is slightly larger on 4L60 (no clearance problems on this El Camino--could be tight in other applications).
* Transmission crossmember needs to be moved forward from TH400 location (rearward from TH350 or Powerglide). This El Camino had multiple crossmember mounting holes from the factory, already there for different combinations. Even so, we did have to drill and tap two new holes.
* If using a single-pattern flexplate, be sure it matches the bolt pattern on your converter before installing the transmission. Dual-pattern flexplates should not be a problem, though you may consider using one that's SFI approved.
* Proper adjustment of the TV (throttle valve) cable is the most important part of this swap. The TV cable performs the functions of both transmission kickdown and line pressure. Follow the factory instructions to the letter, or call the Hughes Tech Line if you're in doubt. You can fry the transmission in no time at all, if this adjustment isn't done properly. The TV cable resembles those used on a TH350, but the 4L60 cable does a lot more. We can't over emphasize the importance of this step.
* Another key item is properly setting the vacuum modulator. Here again, Hughes supplies easy-to-follow instructions.
* Wiring for the lockup converter is simple: one 12-volt, "key-on" wire to the front, driver's side terminal of the factory plug.
* Additional needs:
* Three, 1-1/4 X 5/16-inch Grade-8 bolts and locknuts for the torque converter
* A 4L60/700R4 dipstick and tube
* For aftermarket or Factory shifters, you'll need two metric pan bolts (longer than those supplied with the transmission).

The Drag Strip Results chart chronicles three runs from late last year with the old TH400 transmission--using both street and BFG Comp T/A Drag Radial tires, and slightly different driving styles. Please note; the best 60-foot time from that outing was 2.02 seconds with a quarter-mile clocking of 13.053 at 106.89 mph. Not too shabby. With his new Hughes Performance 4L60 transmission, Manning ran a 1.89 60-foot time, with a best of 12.82 at 106.90 mph. During his track testing at the end of March, Manning made what most racers would consider "normal" adjustments--tire pressure, length of burnout, and launch rpm.

In conclusion, the net yields include a 60-foot improvement of .34 and a quarter-mile betterment of .225 seconds. Couple that with two days of general driving, which included in-town and freeway (stop and go and just plain go), as well as two demonstration, full-throttle blasts for some clients. At the end of that unofficial fuel test, the consumption came out to 17.8 mpg! Just imagine what constant freeway speeds would yield. We're guessing well over 20 mpg.Seeing as how most performance-minded folks are of the curious nature, we estimate a swap like this would fall between the $1,500 to $3,000 range, depending on sources, converter selection, accessories, etc. However, please be aware of the fact that your intentions are always best served by dealing with a reputable "high-performance minded" company, for the maximum results and benefits.


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After removing the driveshaft, exhaust system and related components, the trusty TH400 was relegated to "has been" status. This isn't the kind of project we'd recommend doing on your back. A sturdy lift and transmission jack both make life much easier.

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The old TH400 is shown in the foreground, right next to the new Hughes 4L60 and our assorted components from Energy Suspension and B&M.

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We put a straight edge at the back of each tailshaft to demonstrate the slight difference in overall length. Without an extra plug in the shop, we left the yoke in the TH400 so it wouldn't bleed all over the floor.

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In this photo, the straight edge shows the difference in the location of the transmission mount. When bolting things together, we used a urethane mount from Energy Suspension.

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Here's one of those common sense steps we're happy to pass along. Before any automatic transmission swap, it's a good idea to flush the cooler and its hard lines. This neat, pressurized can of flush was also supplied by Hughes.

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Always double check your converter fitment before trying to bolt up the trans and converter as a unit. As we mentioned in the text, this check is really important if you're using a single-pattern flexplate. Dual-pattern flexplates should not be a problem, though you may consider using one that's SFI approved, especially if you plan a trip to the drag strip anytime soon.

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Once our flexplate and other clearance issues were double checked, the new 4L60 was jacked into position.

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