The way this kit works is that unless you’re building it with one of a couple of fairly standard transmission and engine combinations, some brackets and mounts need to be custom fabricated and potentially a few pieces of the chassis need to be adjusted to make the slightly unusual drivetrain fit. It’s not a big deal, buy it does require the drivetrain to be at the manufacturer pretty early in their process so they can fit it. You can’t have them just send you the kit and then you start to figure out how to fit the engine – they have to do it custom. And you guessed correctly, I’m obviously not going to be doing a standard build.
Intially, I had some really crazy ideas, like doing a semi-automatic gearbox such as a dual clutch. That involves a lot of electronics that are encrypted and VIN-coded to work only in the original car, so it was dead end. Then I thought about doing a sequential transmission, which is what race cars use, but ruled that out for a few reasons. While the clutch-less upshift through pulling on a lever is cool, you still have to downshift and go neutral to first with a clutch – defeats the purpose a little bit. Moreover, sequentials wear out quicker than standard H-pattern transmissions and I have little interest in that kind of maintenance. Lastly, sequentials are pretty noisy, even without straight cut gears which are unbearably noisy in a street car.
I therefore had the choice of a few H-pattern boxes like a Ricardo unit from the recent Ford GT-40’s, a Mendeola unit, 1990’s Porsche Turbo G50/50, some Audi ones or a new unit called the Griffin being developed for these and similar kit cars. I ruled all these out for various reasons like cost and risk and instead went a different direction largely dictated by the type of engine I want to run, which is a recent-model Porsche flat six out of 911.
Irrespective of engine manufacturer, many builders use Porsche transmissions like the G50/50 or sometimes newer models. Especially the older ones are reasonable easy and cheap to get from salvage (usually crashed) cars and the Porsche Turbo versions are known to handle powerful engines well. Porsche transmissions also work well because the Porsche 911 is one of the few high-performance cars built in reasonable quantity where the engine doesn’t sit in the front, so supply and knowhow is decent. The SLC is mid-engined, so you have to invert the transmission, which many of the Porsche units are capable of (think about why they need to be inverted…).
In my case, since I wanted to go with a naturally aspirated (NA) engine from a very recent Porsche Carrera, I started poking around what transmission models would work mated to this specific type of an engine. I did a lot of research and put together a spreadsheet of the gearing of a most recent Porsche transmissions and after sifting through the numbers, this is what I learned:
- Transmissions are geared to get the most performance out of the specific engine they’re mated to, taking into account turbo vs. NA, max revs and torque band. Gearing between different eras of Porsche GT3, GT2 and Turbo is fairly consistent, but they’re geared very different between each other.
- Turbo engines are always geared shorter than NA, because they won’t rev as high and therefore need to get to speed with lower RPMs. They also will have more torque so they can get there quicker. The slight downside is you have shorter gears and thus need to shift more and this would be exacerbated if you don’t mate a Turbo engine to a Turbo-specific transmission.
- You need to match revs on upshifts to the engine’s power band (and manufacturers of course do this on their cars). Upon upshift, a properly geared transmission will be at a new lower RPM that’s right at the beginning of the engine’s power band. This is another reason why it’s bad to mix and match Turbo and non-Turbo engines and transmissions, because you could be wasting part of your power band if you’re hitting to high in the revs on upshift or your engine won’t pull very well if the gears are too long and you’re forced to start a too low of an RPM in the next gear up.
Beyond gearing, there are a ton of other factors to consider such as whether the transmission can hold the power that you plan to put through it, cooling, ease of inversion, condition of components if salvage and of course price. That’s a whole different story.
Without going into details about that, I landed on a G96/96 transmission out of 2004 Porsche GT3. For a naturally aspirated car, the GT3 transmissions are the clear winners in terms of gearing. An added benefit is the built-in mechanical oil cooler which avoids another failure-prone (and hard to diagnose) electronic circuit with a pump and external cooler needing to plumbed and installed. And this transmission can handle plenty of power. In stock form, the motor puts out 385 HP, but the GT3 is street-capable track day car, so the transmission was build accordingly.
I bought the unit sight unseen and had it shipped to GBox in Boulder, CO, where it was slightly refurbished and set up to run upside down. Overall, the unit was in really great shape though and exactly as advertised by the seller. Stan at GBox was tremendously helpful in this entire process and I can only recommend for you to work with him on getting your transmission set up. Very approachable, knowledgeable and willing to listen to your crazy ideas. I also got some great info on what I should be paying, which helped a ton.
Where’s the transmission now? On it’s way to RCR. Transmission: done. Woohoo!