PROJECT TORANA SS REBUILD
A classic-style build like our giveaway LX Torana hatch calls for a classic transmission, and thatís precisely what a Turbo- Hydramatic 350 has become.
Of course, itís 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 isnít 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 thereís 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 the one pictured here for us. As you can see, it also had some problems. When the sump came off, Gilbert, whoís been rebuilding transmissions for over 20 years, said heíd never seen a valvebody with this
much rust on it.
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 didnít 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 hadnít 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.
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 isnít keen on them.
Rather, he feels itís 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 arenít 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 higherperformance 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 doesnít do much for the speed of engagement.
Making things happen more ier To Find 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 itís 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.
BBE Auto can build a trans to suit your specific application.
Contact them on (03) 9528 2071