Q&A / Tech Articles

Ask Away! with Jeff Smith: Choosing a Transmission for Big Block-Powered Cruiser

This is a GM4L80E electronically controlled, four-speed automatic overdrive. It is really just a TH400 with a built-in overdrive. It will require an aftermarket stand-alone trans controller, but this trans is very durable and is more than capable of being used behind the big-block you plan to run. (Image/Jeff Smith)

I am building a ‘72 Caprice as a tribute patrol cruiser. I am pulling out the original 400 SBC and replacing it with a big-block 454 just so the look is correct for a patrol car of that era. I have bought ‘70 heads, a roller cam, and a ‘70 aluminum factory intake. I am also going to put in a built 700-R4 and change the rear to 3.73 posi. Does all this sound good? — B.H.

Jeff Smith: Let’s start with what appears to be the combination that is way out of sorts with the rest of what you want to do.

Police cruisers are just that — highway cruisers; not drag strip heroes.

So the addition of the 3.73:1 rear gear seems out of place. The 700-R4 four-speed automatic (which later evolved into the 4L60E electronically controlled) uses a 3.06:1 first gear ratio.

If we multiply 3.06 x 3.73 = 11.4:1, this is the overall first gear ratio.

If we divide that by the first gear ratio of a TH400 automatic transmission (which is 2.48:1), we come up with an equivalent rear gear ratio of 4.60:1.

So your combination in first gear would act the same as a TH400 with a 4.60:1 rear gear. Let’s use that as a basis for comparison.

This is a very steep gear for a highway cruiser.

Plus if we take the 700-R4’s overdrive of 0.70:1 and multiply that times 3.73, we get 2.61:1 which isn’t a bad highway gear.

While the highway cruise rpm will probably be acceptable with a tall-enough rear tire, you may not like the way the transmission operates around town at part throttle. With this deep of a first gear, it will shift into second gear almost immediately. That doesn’t sound bad, but if you’ve ever driven a car with deep gears and an automatic it often becomes annoying.

The 700-R4 was designed originally to be used behind the tiny 305 V8 and even smaller 2.8L V6 engines in the early ‘80s. Plenty of companies are now building much stronger versions of this transmission, but you will need to invest in some pretty serious components to upgrade the 700-R4 to handle the torque of that big block.

Sonnax Industries makes an entire lineup of heavy-duty components for the 700-4 to upgrade the trans, so if you are committed to this program, then I suggest you look into the Sonnax lineup. They offer a Performance Pack, a SmartShell upgrade, and other components that will improve the life of this transmission. They have a complete catalog on the 4L60E trans and many of the later parts will interchange with the 700-R4.

Frankly, if you have not yet purchased a transmission, my suggestion would be to go with a later 4L60E electronic transmission for several reasons.

Yes, the electronic transmissions require a separate stand-alone controller and these add to the overall cost. There are literally 10 different controllers to choose from and among the best is the one from HGM that is called the Compu-Shift Sport model that uses Bluetooth to allow you to put input and monitoring with your smart phone. Another excellent controller is the TCI TCU 2.0.

A major reason the 4L60E transmission is a better choice is that you can dial in a specific shift firmness and speed points for part-throttle and full throttle up-shifts with a few keystrokes. That is difficult and time-consuming with the 700-R4 because it requires changes to the governor with springs and weights. Frankly, that’s a pain.

Yes, the 4L60E will be more expensive to buy and upgrade with the cost of the controller.

The 700-R4 also requires a very critical throttle valve (TV) cable adjustment to ensure the transmission line pressure is correct.

We know two friends who have killed their 700-R4 transmissions in performance cars due to incorrect TV cable adjustments. The critical nature of this adjustment cannot be overstated.

A 20-minute test drive with an incorrect TV cable adjustment can kill a transmission. That sounds unrealistic or overblown but it is not.

Unless you become or know someone who is a 700-R4 TV cable expert, you might want to consider the 4L60E as both easier and ultimately less expensive.

We’ll throw another idea at you if it isn’t too late. You might consider investing in a 4L80E transmission instead of the 700-R4 or 4L60E.

Yes, the same stand-alone electronic trans controller will be necessary, but this transmission is essentially a TH400 with a built-in overdrive. Unlike the 700-R4, the 4L80E is a true three-speed with a 2.48:1 first gear ratio. The 4L80E’s overdrive is only 25 percent (0.75:1) so it won’t lower the cruise rpm quite as much as the 700-R4 will.

However, the 4L80E is a durable transmission that was designed to handle big-block torque from the very beginning. Better yet, add new clutches and a few other simple upgrades again from Sonnax and bolt it together and it will likely last forever.

If you still want to run that 3.73:1 rear gear, let’s look at how this would play out.

The overall first gear ratio with the 4L80E will be far more streetable so that won’t be a problem because you have 2.48 x 3.73 = 9.25:1 which is taller than the 700’s 11.4:1.

With a 0.75:1 overdrive, this reduces the effective rear gear ratio from 3.73:1 to 2.82:1. If you combine this with a 28-inch tall rear tire, which should be easy with a big car, this will put your 70 mph cruise rpm to 2,370 rpm assuming a lockup converter.

This would be a very nice cruise engine speed but with a mild enough cam, you could go lower and improve fuel mileage a little but that’s probably not a major concern. The only other consideration is the torque converter.

Considering this mild situation, you might go with an RV-style converter which will only be slightly more stall speed than stock but regardless of the stall speed you choose, it should be a lockup-style converter. This is important because with a non-lockup converter, the low rpm at highway speeds will create excessive heat and even with a good cooler, the trans temperature will likely be excessive.

High heat is what kills automatic transmissions so make sure to use a lockup converter.

Hope this helps.

Tags: , , , ,

One Comment

  1. Pingback: Ask Away! with Jeff Smith: Choosing a Transmission for Big Block-Powered Cruiser

Leave a Reply

Your email address will not be published. Required fields are marked *

*

This site uses Akismet to reduce spam. Learn how your comment data is processed.