Project Torana: Transmission
BBE Auto builds and tests a tough TH350 for our LX Torana hatch
Project Torana: Transmission
A classic-style build like our giveaway LX Torana hatch calls for a classic transmission, and that’s precisely what a Turbo-Hydramatic 350 has become. Of course, it’s not the only choice available; adapters or modifications make it possible to fit almost any transmission to a 308 Holden V8 these days.
The Trimatic, for example, is a good trans but it isn’t as robust as a Turbo 350. In standard form it will start to suffer beyond about 400hp, though with the right components that figure can be extended to as much as 600hp. For heavy-duty applications there’s the classic Powerglide, and these can be built to handle any power level, but for regular street use there are better choices than a two-speed transmission.
Although it has some clout, this hatch is a cruiser rather than a dedicated bruiser, so a Turbo 350 is perfect. John at BBE Auto agreed to help us get a strong, fully tested 350 into our giveaway Torry for the lucky winner to enjoy.
The original unit we were going to fit turned out to be beyond salvage, so BBE found this one for us. As you can see, it also had some problems. When the sump came off, Gilbert, who’s been rebuilding transmissions for over 20 years, said he’d never seen a valvebody with this much rust on it.
Removing the original TH350 sump revealed a world of pain. Fortunately, BBE Auto were always going to fit the replacement valvebody shown
Valvebodies can be refurbished by boring out valve bores and fitting oversized valves and other replacement parts. But pretty clearly, this transmission was way beyond that. In any event, the rust didn’t matter too much because it was always going to be converted to a full-manual trans, so an aftermarket BTE valvebody and separator plate were always part of the plan. The rust hadn’t penetrated too far and the internals cleaned up quite well. Most of the rust you can see on the parts in the dismantling shots was transferred during the valvebody removal.
This is a factory sprag. Getting these replaced in higher-powered applications is essential. The rusty material has been transferred in the dismantling process
The factory direct drum was also rusted, but it too was scheduled for replacement. The direct drum carries a sprag-clutch, and the factory unit is a particular weakness in the Turbo 350. The outer races on these sprags tend to break at around 350hp. There are aftermarket races available but John isn’t keen on them. Rather, he feels it’s best to replace the entire drum with an aftermarket performance version. In addition to being thicker and stronger, these drums come with a 36-element high-performance sprag that can hold as much as 1000hp.
Putting transmissions back together properly is as much about diagnosing worn components as it is about fitting new parts.
If the old parts aren’t diagnosed properly problems could be missed and may recur when the transmission is back in service.
John suggests that friction material for automatic transmissions is often misunderstood. Sure, it needs to be high-quality, but it needs to be matched to the application. Friction material designed for higher-performance applications is harder and may not work as effectively under less demanding conditions. Surprisingly, harder friction material may result in a slower, softer grab because of the lower temperatures and pressures. Softer friction compounds work much faster and can grab virtually instantly, but if the material is too soft it will wear very quickly. Originally, our idea was to fit Raybestos blues because of their reputation, but after John explained all this to us we followed his advice and fitted the Raybestos tan types shown.
Clutch engagement speed is vital for performance, but holding power within a clutch assembly is just as important. Obviously, increased holding power is achieved by increased pressure within a system. Standard pressure within a TH350 might be something in the order of 120psi. If an apply piston has an area of, say, four square inches, the clutch pack on which it acts will have a clamping pressure of 480lbf/in2. Line pressure in this transmission was upped to 180psi, so clamping pressure would also rise to 720lbf/in2. That will hold quite a bit of torque.
While increasing pressure in the main circuit will push the clutch plates together more tightly, it doesn’t do much for the speed of engagement. Making things happen more quickly calls for valvebody modifications. There are a number of aftermarket valvebodies available and these also should be matched to the correct application.
The final modification to our Turbo 350 was fitting a deep cast-alloy sump. This serves several purposes. First, it simply holds more transmission fluid, so operating stresses are dispersed through a greater amount of fluid. The increased fluid volume can also absorb more heat. Also, because the sump is finned, it has a greater surface area than the smooth factory unit pressed from steel sheet. Greater heat rejection through a larger surface area also helps keep operating temperatures lower.
One more feature of the thick cast-alloy sump is that it’s much more rigid than the pressed factory unit. This bridges the void in the underside of the transmission case with a strong structural member, which strengthens the case and resists torsional strain.
The Axiline transmission dynamometer tests elements of a transmission that would be difficult or impossible to assess after installation. The dyno runs the transmission through the gears under a variety of simulated operating conditions.
Simple noises are a case in point. The operator can stand right next to a running transmission and listen to it without engine noise while visually checking it from every angle. Hearing exactly when any noises in a trans are present is an invaluable diagnostic tool. A high-pitched noise may indicate a worn pump. A noise from the converter can be confirmed by listening to when it occurs. If, for instance, there’s no noise in Park or Neutral but there is in Reverse and Drive, it’s likely a turbine bearing fault. When gears are engaged, pressure increases and the two halves of the converter tend to separate, which loads the bearing and increases noise.
Pressures, shift points and speeds, down-shifts, converter lock-up characteristics, stall speeds and more can be tested, logged and recorded. Vacuum circuits on older-style transmissions like the 350 can also be tested, as can solenoids and modern electronic control systems.
Transmission dynamometers are an excellent idea for testing your transmission before it goes in the car. We were able to run the transmission through the gears and test all the line pressures before it went into our Torana hatch.
Pressure lines with quick-connect fittings are hooked up to the input and output feeds on the transmission.
Here are all of the new parts BBE Auto supplied and fitted for the Torana transmission. The cooler a transmission is while in service, the longer it will last, so BBE Auto also supplied a trans cooler.
Some of the components, like this clutch pack, are assembled into sub-groups and then fitted into the case. Clutch pack clearances are meant to be between about 0.020in-0.090in. Most rebuilders set them at about 0.008in-0.010in for each friction plate. The clearance is adjusted by fitting different-thickness intermediate steels, usually referred to at just steels.
This apparatus is used to set longitudinal freeplay. The central measuring rod is dropped until it makes contact with the thrust bearing and the gasket must be in place for this as shown. The measuring rod is then set in place above the direct drum in the transmission.
The marks around the edge of the case are from the water, but they're just that, marks. The surface is completely smooth and will seal perfectly after the new valvebody lower half is fitted. A number of gaskets are supplied with each rebuild kit. The holes in the gasket must match the holes in the separator plate.
Finally, the sump is bolted in place and the Torana transmission is ready to go on the dyno. There are many more steps involved in both dismantling and re-assembling a Turbo-Hydramatic 350, but we're simply showing you an overview rather than how to rebuild a transmission.
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