The Three Variables

Getting a high-horsepower car down a drag strip begins with understanding three interconnected variables: engine power, clutch setup, and traction. Michael Marriott, crew chief for the Spinozzi Racing Pro Stock team, explains the relationship.

"Any time you change one or three, it will show you in the middle. If you put more power in there and there is still enough traction you will see more clutch slippage. Or if you have the same amount of power and you increase the amount of traction the clutch will also slip more. The clutch will be an indication of how the track is." - Michael Marriott

An adjustable clutch is uncommon in Group Three competition but standard in Group Two and nearly universal in Group One. In a nine-second Super Sedan, the clutch typically has its total load built in with no external adjustment, making the initial clutch selection critical. The clutch must generate enough force to deliver 1:1 drive at a given rpm in each gear.

"One of the things is that the smaller the clutch, the smaller the window the clutch operates in. It makes the clutch less forgiving. It comes down a lot to the clutch surface area - a bigger clutch will end up with more surface area and is able to dissipate heat quicker." - Michael Marriott

The Adjustable Variables

Moving up the clutch hierarchy, fine adjustability becomes critical. The first variable is base pressure, controlled by an allen key that adjusts the spring install height. Base pressure is measured in turns.

"Generally speaking there will be six small springs and depending on the style of engine that base pressure will vary from 150 pounds to 1,000 pounds." - Michael Marriott

The second adjustable variable is centrifugal force, controlled by weighted levers that engage as rpm increases. Adding or removing weight changes the application rate at which the clutch engages.

Clutch size itself is not adjustable between rounds but is determined by engine horsepower. A Pro Stock car runs a six- to seven-inch clutch. A Top Doorslammer uses a 10.5- or 11-inch unit with significantly more surface area and heat capacity.

"You can go one step further in something like the Doorslammers where they run a lock-up clutch, so in first gear they might use their six primary levers and six levers as their secondary levers, activated by the driver. A Pro Stock car reaches a 1:1 point on average around 8,500 rpm where it locks up." - Michael Marriott

Applying the Setup

Track surface quality determines how aggressively the clutch can be loaded on launch. A better start line surface permits more base pressure - the initial force the tyre sees when the clutch is released.

"The better the surface the more base pressure you can run. If the surface or the start line isn't as good you have to reduce the base pressure you run. You also reduce the application rate the clutch comes in at." - Michael Marriott

Clutch tuning is mathematical. Marriott works from calculated load rates versus rpm for every clutch he manages. Each gear shift determines where the clutch rpm drops, and precision matters.

"Every time you shift gears with a Pro Stock car it determines where the clutch rpm drops to on the shift. If the driver is more than 200-300 rpm out on the shift it has a very big effect on the clutch. If the driver shifts too early you won't have the centrifugal force and so the clutch may then slip too much. If the driver shifts too late you have too much force and it may make it too aggressive on the change and the car will shake." - Michael Marriott

The Advantage of Consistent Power

Pro Stock engines produce relatively consistent power, which simplifies clutch tuning. Turbocharged and supercharged applications introduce a complication: boost is easily adjusted, and changes to power output require corresponding changes to clutch settings.

"On the turbo cars you need to increase the power gradually and bring the clutch program with it otherwise it makes it difficult to know how much clutch you have in. The tuners have that adjustability at their fingertips so they tend to make bigger changes quickly. The Pro Stock approach is more methodical. Pro Stock is about repeatability and the clutch is a very repeatable part." - Michael Marriott

Between Rounds

Every Spinozzi Racing crew includes a dedicated clutch specialist. After each pass, the car goes on stands. The driveshaft is removed, the gearbox comes out, the bellhousing is pulled, and the clutch is inspected. Heat exposure is assessed by examining the condition of the plates and floaters.

All clutch discs are reground. Based on track conditions and anticipated changes, base pressure and centrifugal weight are adjusted. The clutch is reassembled to a known stand height so every other variable remains consistent.

"We are also monitoring the weather because that is having an effect on how much power we are making." - Michael Marriott

The principle is direct: if power changes, the clutch must change with it.

Heat Management and the Burnout

Excessive heat warps clutch surfaces, reduces contact area, and destroys consistency. Managing heat begins with the burnout.

"Every time you come back from a pass in a Pro Stock car the clutch plates are refaced and that keeps the surface of the clutch consistent from run to run. We try to be as consistent as we can in the burnout so wear and heat is consistent." - Michael Marriott

If a car is stopped after a burnout and forced to redo it - as happened to Spinozzi Racing at Willowbank during a sun break - the additional heat changes clutch settings. The clutch slips more, which generates more heat, compounding the problem.

"The burnout sets the clutch, it sets the tyre temperature, it controls the engine temperature, so it is a massive part of the run." - Michael Marriott

A Pro Stock car requires substantial water in the burnout box because the clutch cannot be dragged down the way a Top Doorslammer's rolling burnout permits. Marriott describes the Pro Stock car as the most difficult car to drive in drag racing.

Floater Disc Technology

Floater discs separate clutch discs within the assembly. A three-disc clutch consists of the flywheel, a floater disc, a clutch disc, a second floater disc, and the hat. Floaters are typically steel and can be treated with a titanium coating to improve heat resistance.

"After X amount of passes you have to turn around and go and get those floaters re-coated. You don't grind it as much as a non-coated floater, else you remove the coating." - Michael Marriott

For high-heat applications such as turbocharged cars, Marriott prefers non-coated floaters, grinding the plates after each pass to maintain a consistent surface.

Clutch Technology in Pro Stock

Despite the growth of automatic transmissions in Top Doorslammer, the manual clutch remains the standard in Pro Stock.

"At this stage the manual would be the better mousetrap. I still see that the clutch, with the amount of variables we have in gear ratios, ends up being a better package." - Michael Marriott

The primary choice is between a six-inch triple-disc clutch and a twin-disc seven-inch clutch. The triple-disc has less centrifugal mass and can be easier to drive at rpm. The larger twin-disc is more forgiving.

This season, Spinozzi Racing worked with Ulf Leanders to develop the Leanders clutch for the Australian Pro Stock application. The result is the smallest-diameter clutch in Pro Stock racing, offering precise adjustability.

"I think that the Leanders clutch is a high-quality piece and being at a development stage it took a little bit to get the tuning window to where we needed it to be, but now we are there. The car is the fastest 60-footing Australian Pro Stock car in the history of the sport." - Michael Marriott

With Australian Pro Stock cars now running in the 6.8-second range at over 200 mph, the engineering reference point has shifted toward NHRA-level setups.

"What we look at now is much closer to what NHRA Pro Stock was using a few years ago, or what they run at Mile High. Those teams run completely different clutches for that one event." - Michael Marriott

The Broader System

While the power-clutch-traction framework simplifies setup, the actual system includes additional variables. Gear ratios work in concert with clutch settings - both are determined by track surface quality. On a better surface, both can be more aggressive.

"In a Top Doorslammer or Top Alcohol car the track conditions are the primary situation and quite often as the track gets better they need to become more aggressive with a shorter gear ratio so that they don't underpower the track. Ultimately torque drives the car - it's the amount of torque you can apply through the rear wheels to the track." - Michael Marriott

Tyres introduce another variable that changes with every pass. Tyre height is measured each run because the tyre is the final element in the drive ratio, after gearbox ratios and the differential.

"When the tyre is brand new you need to be less aggressive. We will use a figure of five runs on most cars before the tyre starts to come in. Each tyre then has a certain amount of life, and what you find is that you measure a tyre brand new, but by the time it has done 200 mph the tyre will then grow. You don't really know what your final tyre height will be until you run the car." - Michael Marriott

Every element in the system is connected. Changing one variable without adjusting the others introduces inconsistency. In Pro Stock, where the margin between winning and losing is measured in thousandths of a second, the clutch is where engineering precision meets the track surface.