Ask Away! with Jeff Smith: Transmission Recommendations for a ’64 Chevelle

I have a 1964 Chevelle convertible. Several years ago I put a 700-R4 in it, which has never shifted correctly. It slips, doesn’t work right with the carburetor, etc. I just want to switch it out. What would you recommend as a good replacement—a TH400 / TH350, GM 4L80E, or what? — D.E.

Jeff Smith: I put a 4L60E in a ’64 El Camino with a small-block and I really like the conversion.

While it is very similar to the 700-R4 in terms of operation and gear ratios, the big difference is the electronic command of the 4L60E. It controls the line pressure that was previously dictated by the TV (throttle valve) cable.

If you are considering the switch to a 4L60E, you will need a stand-alone, aftermarket trans controller. I’ve tried several of them and really like the HGM Electronics version sold under the Compu-Shift brand. It’s more expensive than other controllers but I like the way it works. Other options that also work well include the MSD Atomic transmission control module, Performance Automatic’s controller, as well as B&M’s Shift Plus 2. Even Chevrolet Performance offers a stand-alone controller that uses a laptop to tune it. These controllers range in price from $800 to $1,200.

While the TH350 and 400 transmissions have both proven themselves to be durable performance automatics, neither offers an overdrive gear, which is almost a requirement these days. I will assume that you do lots of cruising and highway driving with your car, and that places an absolute necessity on an overdrive transmission.

The 4L80E is also a great choice but may require some floor pan surgery to fit under a ’64. Plus, it will also need an electronic controller, a custom crossmember, and a new driveshaft. Since you already have the 700-R4 in your car, the 4L60E will bolt right in.

The advantage of the electronic control on the 4L60E is that it is easy to adjust the line pressure with a couple of keystrokes on the hand-held controller or a laptop. This also offers the freedom of fine adjustments on the shift points—which is near impossible with the 700-R4.

The issue with the 700R4 has always been setting the TV cable correctly. I recently installed a Sniper electronic fuel injection system on a friend’s big-block El Camino and we struggled to set the TV cable on that application and frankly, we’re still not completely happy with it after multiple adjustments.

The trick is to set the TV cable so that it is piano-wire tight at wide-open-throttle (WOT) and yet will offer part-throttle up-shifts at the appropriate points. If you have driven your 700R4 for a couple hundred miles with an improperly adjusted TV cable, it’s likely that the transmission may have been damaged. You indicated that it seems to slip on up-shift. This is a clue that the part-throttle line pressure is inadequate to fully engage the clutches on the shift. This will quickly burn up the clutches and eventually the trans will fail.

Most often, this is traced to the high-gear clutches.

It might be worth the effort to have a reputable trans shop rebuild your 700R4, but that will only be worthwhile if the TV cable is adjusted properly. If you have a local shop do the work, then it’s possible they will have the expertise to set the TV cable up properly. If they only do rebuilds but not installations—this can be an issue. If you install the trans yourself but don’t get the TV cable adjusted correctly—you will be right back where you are now.

We don’t have the space to really get into all the details necessary to run through a proper setup of the TV cable. The best way to do it is to use a pressure gauge hooked to the line pressure port on the transmission. This is a great tuning tool.

For example, when the TV cable is mounted on the carburetor, the slightest movement of the throttle should produce a line pressure increase in the transmission. If it does not, then there is a problem with the throttle cable, the connection to the trans, or potentially a problem in the transmission.

As you can see, there’s a lot to consider here and why a short answer won’t get into all the details required to address this properly.

There are a couple of places that offer a decent fully adjustable TV cable mount. One is the FAST (Fuel Air Spark Technology) version. This FAST kit is designed for use with the FAST EZ-EFI but offers enough adjustability that it may also work with a typical Holley carburetor. Bowler Transmissions also has a kit that appears to be well thought out.

You mentioned the 4L80E as a possible option.

This is an overdrive version of the classic TH400. It uses the same first three gear ratio set as the standard TH400 (2.48:1, 1.48:1, 1.00:1) then adds an overdrive ratio for fourth gear of 0.75, which is 25 percent. This transmission also requires a stand-alone controller and in most cases will use the same controller as those listed for the 4L60E.

The disadvantage of the 4L80E is that it is larger, longer, and heavier than a TH400. On the flip side, it is incredibly durable. But unless you’re running a high output small-block or a big-block, its size and weight make it less advantageous.

If you prefer to not go with an electronic overdrive transmission, there are still alternatives.

One would be to go back to a three-speed TH350 transmission and then adapt a Gear Vendors overdrive to the back of that trans.

The Gear Vendors is a small overdrive unit that replaces the stock extension housing on the back of the trans. The Gear Vendors offers a 22-percent overdrive, or the equivalent of converting a 3.55:1 rear gear ratio to 2.77:1. Plus, you can gear split your transmission if necessary. This can convert your TH350 into an effective 6-speed automatic by overdriving each forward gear.

Here’s how that would work:

The final drive ratio on any car is the transmission gear ratio times the rear gear ratio. For a 3.55 rear gear with a TH 350 with a 2.52:1, first gear = 8.95:1. Then if you hit the Gear Vendors, it reduces that ratio by 22 percent, making the effective overall gear ratio 6.98:1.

Then, you shift into second gear on the TH350 and remove the overdrive. This makes “third” gear 5.40:1. Then hit the overdrive, and it drops to 4.20:1. Then, shift into “fifth” gear which is really high gear in the TH350 (1:1 x 3.55 = 3.55:1). Then, engage the overdrive and that becomes an effective 2.77:1 ratio.

Splitting gears helps acceleration because the engine drops much fewer rpm between gears.

Generally, this will help acceleration.

We tested this gear split combination on our QUARTER Pro drag strip simulation program and it gave us an improvement from 12.30 at 111.1 mph to 12.03 at 114.8 mph with a 3.55:1 rear gear Chevelle powered by a 450-hp small block.

In the real world, the improvement would not quite be that good because we didn’t add the equivalent weight increase (roughly 40 pounds) that we would see with the Gear Vendors overdrive unit.

But this would only slow the car by only 0.04-second or so. The gain would still be roughly a quarter of a second improvement.

We’ve given you a bunch of different ideas on ways you can go. None of these are inexpensive plans unless you go back to the TH350 trans.

But there are plenty of options, and you simply have to decide which one is right for you.