1970 El Camino 4L60 Transmission - Overdrive Euphoria

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.

After removing the driveshaft,... 

   

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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.

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.

Transmission Swap News And Notes
* 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).

How To Make It Hook
With an El Camino that just had an infusion of more low gear, it was obvious that producing favorable test results would have a great deal to do with traction. The biggest fear was having an empty pickup bed-as in no weight over the rear axle with a limited amount of options to make it stick on the starting line. We turned to BFGoodrich for a set of their 275/60R15s Comp T/A Drag Radials.

BFGoodrich has been manufacturing its Comp T/A Drag Radial for several years now, with proven results. Targeted specifically for the Street-Legal drag racing crowd, the tire met with rave reviews. With its minimal tread, few cross grooves, and softer compound, the Comp T/A Drag Radial is yet another tool to be used in your arsenal of speed, and it's available in sizes ranging from P295/65R15 to P315/35R17.

Referring to the Drag Strip Results chart, the improvement in 60-foot times are eye opening. Three-tenths is a bunch, when you realize that's a figure you can't count on your watch, and lately, an entire 16-car NHRA Pro Stock field has less of a spread from the first to the last qualifier. So, if you're looking to improve traction at the drag strip, opt for a new set of shoes!

The old TH400 is shown in...        read full caption 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.

We put a straight edge at...        read full caption 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.

In this photo, the straight...        read full caption 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.