GTLM - GT3 - GTD Do you know the difference?

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GTLM - GT3 - GTD Do you know the difference?

Unread postby sickwititHD » Tue Jan 31, 2017 10:03 am

Found this article by Ben Wedge and wanted to post to see who all new the difference between the three main GT classes racing all around the world. In an effort to build a new STA Endurance series I'm hoping we can tweek all the GT cars towards the GT3 class. this class is usually a little slower than GTLM and just a little faster than GTD.

By Ben Wedge

Okay, we all know that GT racing is awesome. Whether the series is Pirelli World Challenge, the old ALMS or Grand Am GT, IMSA GTD, IMSA GTLM, FIA GT3 or the Blancpain Endurance series, the racing has been world class. The interesting thing is many of the same cars compete in all these series but under difference specifications. Cars such as the Ferrari 458, SRT Viper, BMW Z4 and Porsche 911 are all cars that compete in Grand Am Daytona (GTD), GT LeMans (GTLM) or FIA GT3 specifications.

A few general things before we get into the details. GT class rule books are kind of weird. They tend to be very open and use the phrase “must be homologated” and “may be adjusted”…a lot. The GTLM and GTD rule books are approximately 25 pages long. In the GTLM rule book, French is included in 25 pages. The GT3 rule book is approx. 50 pages and that also includes two languages. To put this in perspective, the 2013 LeMans Prototype regulations were 55 pages (included French and English) and the Formula SAE collegiate design competition, that rule book, 163 PAGES! In general sanctioning bodies say “here are these general guidelines, now build something that goes about this fast. We will tweak it later.” The tweaking is the BoP that everyone loves. Usually the rules themselves help limit how fast a car can go simply by limiting modifications that can be done to the engine and suspension.

Speed

The first thing that is obvious from the time sheets is the speed. For example, at Sebring, the GTLM class Ferrari was approximately 4.5 sec quicker than the GTD spec version. And at Silverstone, the GTLM version is about 2 seconds quicker than the GT3 spec car. These gaps are similar in all the cars. GTLM is the fastest, GT3 is the next quickest and GTD being the slowest.

Downforce

Downforce is one way to that speed differential is created. Let look at the rear wings of the Ferrari 458 cars. GTD runs a spec wing produced by Crawford performance. This wing has a constant profile with a certain width no matter what the car in the class. The GT3 wing is mounted much higher but has a non-constant profile. The same shape is also seen in the GTLM car. This shape increases the efficiency of the outer portions of the wing which is in cleaner air than the parts of the wing behind the driver’s area. The location, size, angle, camber and chord of a wing are all things that can greatly affect the performance of the car. (stay tuned for a future post on wing design)

wings

On the front, downforce gets a little different. For this example we will look at the BMW Z4. This is a perfect example because both the GTD and GTLM versions of the car are based off of the GT3 spec car. The GT3 spec car, which was developed first, has large fender flares and multiple dive planes. The GTD and GTLM versions of the car share the same fender flares as the GT3 car but lack the dive planes of the GT3 model.

BMW

Other rules for each series can affect the cars performance. For example, in 2013 the Audi R8 that ran in Grand Am started out as GT3 spec car. In Grand Am the car was required to run without windows, this caused the airflow to the rear wing area to be much different than originally intended and greatly reduced the efficiency of the wing.

Engines

Engines are where we see some differences. The Ferrari 458 and BMW Z4 all use the same engine in all three cars with some minor differences. All three specs of Ferrari use a 4.5L V8 but tuned to fit each series. A similar story for the BMW Z4, all three specs use a 4.4L V8 producing similar power numbers. Performance differences will occur due to different restrictors required by each series as well as different fuels. The Aston Martin Vantage, however, is a little different. The GT3 and GTD spec cars both use a 6L V-12 engine where the GTLM variant uses a 4.5L V8. The Porsche is again similar, with the GTD spec using a 3.8L motor while the GT3 and GTLM spec using a 4L motor.

Driver Aids

Driver aids such as traction control (TC) and anti-lock braking systems (ABS) allow a driver to push the car closer to its limit with less risk of spinning or locking up a tire. GT3 allows for both of these systems to be attached to a car. GTLM does not allow ABS but does allow for traction control (engine only) to be used. GTD doesn’t allow either system to be used.

Tires

One of the largest differences in performance is the tires. Tires are the only part of the car that makes contact with the road and also has the largest effect on performance. The GTD class runs a spec continental tire and GTLM spec cars are an open tire, Michelin and Dunlop being the most popular. GT3 spec cars run on Pirellis in the Blancepain Endurance Series. While I don’t know the differences in construction between all the different types of tires, it is safe to say that whatever the differences are they greatly affect the cars performance.

Behind the Wheel

Mike Hedlund, DragonSpeed Ferrari driver in Pirelli World Challenge, was kind enough to answer a few questions about what it’s like to drive the cars. Mike has experience behind the wheel of a GTD spec Ferrari 458 as well as a GT3 (Pirelli World Challenge) spec Ferrari 458. Mikes first comment was on the how you drive the cars.

“The GTLM and GT3 cars have a lot more downforce so you tend to roll a lot more speed through the entry and middle part of the corner compared to a GTD car. GTD cars tend to square off more corners in order to get rotated early and back to the power since they can’t roll the same speed through the corner. The horsepower and weight is very close between them all. Driver aids don’t help much in terms of lap time, just makes them easier to drive close to the limit.”
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