What's the Real Impact of Dual-Clutch Transmissions on Lap Times?

People always say a dual-clutch transmission is quicker on track. But is the actual amount of speed gained worth less involvement?

Matt Tierney

It’s an annual habit of mine to read the Web chatter after we release the PCOTY results in an effort to see what we can do better in the year that follows. There are a million opinions on the Internet but in my experience there are usually one or two topics that really stand out. This year it was something along the lines of “How much faster would the 911 GT3 have been if it had been equipped with the PDK transmission?”

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There are three possible answers to the question. The first is… Who cares? The manual gearbox is the best way to get the car. The truth of the matter, however, is that many drivers prefer not to shift nowadays, and that’s just the way it is. The second answer is We won’t know until we test two otherwise identical cars under identical conditions. The nice people at EVO tried that a few years ago with 911 Carreras and they found out that . But the two Porsches in question weren’t actually identical, and the writer admits that his driving was adversely affected by thinking about using the PDK because it didn’t feel natural to him. We can't really take that test result as gospel.

Which leaves us with this third answer: Let’s look at some facts and try to figure it out. We can start by ruling out one bit of bad math that the Internet likes to use:

(number of shifts per lap) x (0.2 seconds) = laptime saved

Richard Pardon
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That equation comes from the generally accepted fact that PDK (or DCT, or whatever trademark you want to use) can complete a shift almost immediately, while a skilled driver needs about 0.2 seconds for a no-sympathy upshift. The flaw is that you don’t actually lose 0.2 seconds with every shift. You only lose 0.2 seconds’ worth of acceleration. It ain’t like the car stops moving. So forget that nonsense right away.

To get a better idea of what PDK does for a laptime, we start by trying to figure out how much acceleration we lose during that 0.2-second pause. I spent last night looking through VBOX trace data from PCOTY and I have concluded that each shift I took during testing cost, on the average, about 1.5 mph in acceleration over an average distance of just under an eighth of a mile. At NCM West, I made between seven and nine shifts per lap depending on the car over a total straight-line distance of about 1.3 miles. If you keep boiling the math down, you wind up with a theoretical difference at NCM West of about 0.9 seconds.

Honestly speaking, that sounds like a pretty big gap for a track that small, but it fails to take in account one way in which a PDK can be an even bigger advantage: upshifting in fast corners. The nearly uninterrupted drive provided by a dual-clutch transmission can go a long way towards keeping a car from getting upset in a very fast turn. Most modern tracks don’t have increasing-radius fourth-gear corners, but the Nurburgring sure as hell does. I’m also thinking of Laguna Seca’s Turn Nine, where a manual shift can upset the balance of a car but a PDK just powers through. So if you were on a track like that, you’d see an even bigger advantage with a dual-clutch transmission.

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Against these obvious advantages, we have to consider the drawbacks. The additional hardware needed by PDK can weigh as much as sixty pounds, which hampers acceleration, braking, and cornering. The energy needed to operate the transmission has to come from somewhere, whether that somewhere is an engine-operated hydraulic pump or an electric manipulator. And there’s always the chance that the gear ratios in the PDK won’t work as well at a given track as the manual ratios do. I think that might actually be the case at NCM with the six-speed manual in the GT3 against its seven-speed PDK sibling.

Richard Pardon

Would a PDK-equipped 911 GT3 have beaten the Camaro ZL1 1LE around NCM? I’m going to go out on a limb and say Absolutely. But that’s not an apples-to-apples comparison, because the Camaro was also equipped with a manual transmission. More importantly, both cars are appealing to the enthusiast driver in large part because they are available with a clutch pedal. Trust me–if Lamborghini sold a Performante with a chrome-gated six-speed manual, it would be more desirable even if it was ten seconds slower around some wacky racetrack in Germany. If you don’t believe me then call your local exotic-car dealer and ask him what he’d be willing to pay, sight unseen, for a stick-shift Ferrari 599.

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In the end, it all boils down to this: Lap times aren’t the whole story when it comes to street cars. If you’re 100 percent serious about setting the fastest possible time, you won’t waste money or effort messing around with any of the cars on our PCOTY list. You’ll head straight to the nice people at Radical, or Stohr, or Lola, or Crawford, or any of the other shops which specialize in true race cars.

Consider the following: In 1983, a fellow in a Van Diemen Formula Ford set a 7:39 lap of the Nordschliefe. You could duplicate his car for about twenty grand nowadays. If you want to spend more than that, Radical will sell you an SR8 that breaks the seven-minute mark with time to spare. And you’re still not up to the price that your local Porsche dealer probably wants for an early-build 911 GT3 manual.

Nobody but a buffoon would buy a road-going performance car with the sole purpose of setting lap times. You do it because you want to enjoy yourself behind the wheel. And for many, shifting your own gears is a nearly non-negotiable part of that. So consider that when you talk about the laptime benefits of PDK. In driving, as in many other things, there’s considerable satisfaction to be had with doing it yourself. A good time is better than a fast time–at least sometimes.

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