Clutch Cars Need Data! Clutch Cars Need Data!   Ford Mustang Chassis, Roll bars, Cages, Suspensions, shocks, brakes, wheelie bars

Posted on Tuesday, July 08, 2014 Categories: Bobby Fazio's Blog

Clutch Cars Need Data! 

When we decided to bring our Super Stock Mustang out of its 18+ year retirement in 2009, we made the foolish yet cool decision to leave it as a manual transmission car in the heavily populated transbrake/air-shifted world of NHRA Super Stock.  Let's face it, dumping a clutch at 8500 rpms and hitting second gear before you reach the 60 foot marker is way more exhilarating than releasing a thumb button and holding two hands on the wheel for 1320 feet.  However, our decision clearly meant lots of work, experimenting, and consulting of other stickshift racers.  We became friends with an Arizona racer, Kip Martin, who was racing a similar Mustang in Arizona.  As a multi-time national record holder he was a little more seasoned than we were.  We were both using the same McLeod Soft-Lok 10.5 clutch, aluminum flywheel and Jerico DR4 transmission so Kip asked how much the clutch was slipping in high gear.  Huh?  How the heck was I supposed to know that?  That is when I bit the bullet and purchased an Autometer playback tach with drive-shaft rpm. 


Tuning a clutch to be just on the verge of slipping in high gear is where the car will make the most HP and will also make shifting without the use of the clutch easier on parts as well as your right arm.  Too much clutch can hit too hard at the launch, be very difficult to shift a clutch-assisted transmission, and slow the car down as the clutch actually pulls the engine down on a low HP and torque motor, slowing the ET.  However the only way to tune the clutch to perfection requires analyzing recorded runs on a laptop. 


Here are two downloaded runs where the first is a run where the clutch was slipping excessively and the second is just on the verge of slipping which is where we want to be.  Notice on the bad graph, the top blue line (engine RPM) takes a while to meet the bottom green line (driveshaft RPM) once I shifted into a 1:1 high gear.  This is too much slip which slows the ET and decreases clutch lifespan.  The second graph takes less time for the two lines to meet and shows an engine to drive ratio of 1.01:1 so the clutch is slipping very little for roughly two tenths of a second until full 1:1 engine to drive ratio kicks in.  This translates into a quick and consistent ET, easier shifting, increased clutch life. 

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