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Nitro Funny Car Motor’s “What Makes Them Tick”

By Ken Sitko

Tools are already set up on both sides of the pit spot, as the nitro funny car is wheeled back from a brilliant 5.69 run. The crew scampers from the tow vehicle, each grabbing a corner of the Plymouth Arrow shell, lift the body off the chassis and place it on the rack.  The car hasn’t really stopped moving as one guy grabs the wheel, and steers it into the center of the pit spot.

The front and back of the chassis are jacked up, then secured with solid jack stands; safety always!  Fluids are drained as the left and right-side guys pull the plug wires and spin the nuts off the valve covers.  At the same time, another crew member has already loosened the blower belt, disconnected the fuel lines, and is loosening the manifold nuts.  The blower and injector stays on the manifold, it would just be a waste of time to separate them.  The blower assembly is quickly lifted onto a table at the front of the car, and the fasteners on the inside head studs are loosened as the diver drops the oil pan from the block.  Impact wrenches remove the 13 head nuts on each side of the motor, and the billet aluminum heads, still smoking, are pulled off and left on each side of the car, draining hot oil into the drain pans in preparation for cleaning a little bit later.  15 minutes has elapsed since the chutes came out at the end of the last run, still no time to waste.  Gloves are an absolute must on the hands of the bottom end guys, the rods and crank are hotter than anyone can imagine.  The pistons are quickly pushed out and placed in order on the piston rack; the crew chief wastes no time in looking at each one, which ones are damaged, what type of damage, which ones are happy, or maybe TOO happy, which ones are scorched beyond recognition.  The wheels are already turning in the mind of the crew chief, does the fuel distribution or compression must be changed on certain cylinders, can the whole motor take a little more blower, a little more compression, or are we over-center already and have to back off a bit so the fuse is not so short.  A happy motor is a fast motor, but you must find that fine edge, a point where the aluminum doesn’t melt and the rings don’t get tight, but they are damn close to both.

A wheels up/fire in the pipes launch

There is no “magic” combination for a nitro funny car.  There are many variations of bore, stroke, camshaft, compression, system fuel pressure, cylinder head type, port size, chamber size, blower manufacturer, blower overdrive, nitro percentage, gear ratios, clutch type, finger weight and pivot radius, ignition timing, pump manufacturer and fuel curve, brand of tire, tire pressure, and wheelie bar height. Did I get all of them?  Probably not.  The only real constants for a nostalgia nitro funny car are that the fuel pump cannot flow more than 21.00 gallons per minute at 8000 rpm, and the ignition must be points style, and thus can only produce a maximum of 9 or 10 amps, once it is all juiced up.  The pump is not as constant as you might think, although the final flow numbers cannot exceed 21 gpm, the pressures and volumes produced at different rpms leading on the way to the finish line can vary greatly depending on the pump manufacturer.

The supercharger, although mandated to be a 671 GMC style, standard helix angle, has even more differences.  The Littlefield blower manufactured now (the LB series) makes more than enough boost to run big numbers, but it depends on how the rest of the combination is configured.  The SSI blower typically puts out a LOT of boost early, obviously improving performance, but this comes with high parts attrition if correct compensation is not made to compression, timing and nitro percentage.  Same deal with the billet PSI blower, while it looks like the other two, it is often spun at a slower overdrive to regulate the boost somewhat.  The lower blower speed comes with an additional advantage; it takes less power to drive it, so net horsepower is increased.  One thing everyone has to live with is the fuel pump size; barely enough to run a nitro motor, especially since the parts available now are WAY better than stuff we used in the 70’s.  The whole combination is built around the pump limitation.  If the crew chief wants to run high compression, then he likely is conservative on the blower and timing.  Big timing, high compression, lots of nitro, and a stout blower will likely end up as a pile of scrap aluminum.  A balance must be achieved that proves to run fast, but will also make the next run with minimal repair.

The nitro motor nestled in the funny car chassis

A nitro motor and drive train is a complex arrangement of highly technical parts. The “brain” of all motors is the camshaft, and again, the choice of camshafts, and the parts that complement it, is critical.  Camshaft centerlines for nitro engines vary from 108 to 112 degrees.  The center line, or valve overlap, controls the timing between the opening and closing of the intake and exhaust valves.  Small numerical center lines tend to close the exhaust valve sooner, increasing the cylinder pressure dramatically as opposed to the static compression ratio.  this can make a lot of power, but sometimes at great cost.  The cam always comes with a recommended install position, for example 36 degrees before the piston is at top dead center, but is sometimes advanced another 2 to 4 degrees if more power is required at the launch.

The Intake ports in the cylinder heads are huge compared to a street car

The pistons are made of aluminum, and are usually coated with a tough Teflon because of the high temperatures that it must endure.  The pistons are generally referred to as fuses, as they are consumables because they typically don’t survive for more than a quarter mile.  Because nitro motors need only a very low compression ratio (between 6 and 7 to 1), the pistons generally have flat tops, or maybe a very slight dome.  Wholesale changes to the compression ratio can be made by installing a different thickness of head gasket.  In good air (sea level) a thicker gasket is usually needed.  Thin copper gaskets are installed for high altitude tracks to make as much horsepower as possible.  Piston damage is due mostly to detonation, so it is common for the ring grooves to collapse, which pinches the ring, thus preventing the ring from doing its job.  If the motor is lean, which seems to be the most popular way to make power, temperatures in the cylinder can reach well over 1300 degrees, which can melt a hole in the center or down the side of the piston.  Sometimes burning pistons is a good trade off for performance, unless the damage is so severe that the end of the connecting rod is burned off as well, at which point the whole engine has to be replaced.

Detonation has ripped the ring lands right out of this piston

There are many variations of cylinder heads with different combustion chamber and port sizes, and corresponding flow characteristics.  The intake valves are usually 2.4 inches diameter, and are made of titanium to allow the motor to safely operate in the 9000 plus rpm range.  The exhaust valves are a little smaller, 2 inches or less, but must be made of Inconel to withstand the high exhaust temperatures.  Even though these are made from this super, heat resistant alloy, the exhaust valves must be checked often, as they tend to stretch, and will eventually fail, sometimes leading to the impressive body launching blower explosions that you see on T.V.  The valve springs are way tougher than you would see in your regular street car, the spring pressure at rest (valve closed) is about 450 lbs), and when open, almost reaches 1000 lbs.  These high pressures are needed to ensure that the valves return to the seat on time, to avoid allowing the flame from getting back into the intake manifold, which would cause the devastating blower explosion.  Obviously, a smooth, efficient valve train also promotes maximum performance, as fuel and air is gated into the cylinder, and exhaust gases out with maximum efficiency.

Piston rings are very important, not just to seal the fuel and gases that fire above the piston, but also to keep oil from entering the combustion area.  Tight gaps are typically the norm in a Pro Stock type application to make as much horsepower as possible, but with a nitro motor, this is not possible.  Relatively large ring gaps must be used, as there is such a huge swing in temperature that the ends of the rings can butt, resulting in massive damage to the piston/ring/sleeve and even connecting rod, as well as a huge drop in horsepower. A ring gap of .040 or larger is common. The oil rings are very important in a nitro motor, as they must keep the oil from going into the combustion chamber.  For this reason, oil rings should be changed after no more than 3 runs.

The supercharger used on today’s nitro car still resembles the GMC blowers used on old diesel trucks, but instead of a couple of unsealed rotors banging against each other in a cast case, these are now high precision air pumps, with sealing strips placed at the exact right location for peak performance and longevity.  The advent of the billet aluminum cases allows even tighter tolerances and size stability, which pumps clean, cool air into the motor.  These blowers are so efficient that most don’t ever get run at the 18.9 percent maximum allowable overdrive, they can produce the required boost at less than half that speed.

Data recorders on race cars were not even thought of in the 70’s, and now almost every nitro car has one.  Parameters such as fuel flow, exhaust temperatures, wheel speed, and clutch slippage are now easily gathered with a glance at the computer screen, and can be corrected for the next run if any values vary from the norm.  In one way, it is really a shame that computers are allowed in a nostalgia class; the smart guys had a distinct advantage with the “seat of the pants” tuning as opposed to those that can just read a computer graph now.  It does come in handy for a lot of things though.  Nitro flowing through the injector at idle is generally around 2 gallons per minute in a nostalgia car, and as he is staging, the driver flips the lever to divert ALL the fuel the pump must offer, which is about 3 gallons per minute.  This can be viewed live on the drivers dash board, and is a nice feature as it makes sure you have enough fuel at the hit to prevent premature piston damage.  You can tell when the driver flips to the High Side as he rolls into the final stage light, the rpm drops a bit and the motor sounds much throatier.

While we are talking about staging, the clutch is something that the average fan doesn’t know much about.  Most clutches in nitro cars are not the type you are used to, they are actually a centrifugal clutch, which, when set properly, allows the driver to let the pedal all the way out without the car moving.  There are 6 stall springs that keep the disks apart, the distance between the clutch discs and the pressure exerted by the springs are adjusted in the pits to regulate how much the clutch will pull the motor down when it is engaged, and how easily the car will move ahead at idle.  The hand brake is used to stop the car while the discs are lightly rubbing against each other, but as soon as the throttle is hit, and the rpm goes up, no amount of brake is going to keep the 3500-horsepower car from launching.  The purpose of this type of clutch is to put some load on the motor before the car leaves (nitro likes load), and to let the motor flare up before the launch so the fuel pump feeds enough fuel into the motor.  The dis-advantage is that it takes an instant for the clutch to engage, costing the driver about 6 hundredths of a second compared to a standard pedal clutch. Therefore, you are starting to see more pedal clutches in the class. The reaction time advantage is huge, but you must be able to keep the motor happy as well.  If you hear the rpm come up to 4000 rpm or more as the car is staging, then you know there is a pedal clutch in the car.  The extra rpm is necessary to feed enough fuel into the motor for the launch.

The transmission of choice is typically a Lenco 2 speed, which has a set of 3 planetary gears running in a drum which operate in low gear.  This unit is attached to a set of fiber clutches, like in an automatic transmission, which are forced against each other when the driver shifts high gear, basically making the planetary unit and clutch pack rotate as a single assembly.  The low gear ratio can be anywhere from 25% under to 44% under, depending on the preference of the crew chief.  High gear is always 1 to 1.  The low gear ratio is one of the ways that the wheel speed can be controlled, too little wheel speed will cause the tire to wrinkle, and the car will shake violently, slowing the run down, breaking parts off the car, and even damaging parts in the engine.  Too much wheel speed will mostly end up in the car smoking the tires, or spinning them so it fishtails down the track.  Wheel speed is one of the critical elements of the nitro funny car tune-up, it is looked at every run, and has to be considered when the track changes temperature or the power level in the motor is increased or decreased.  On some cars, the wheelie bar is used to control wheel speed as well, if the WS is too low, the wheelie bar can be lowered a quarter inch to keep the car UP on the tire.  Wheel speed is changed when more counterweight is put on the clutch, or more power is fed to the motor, so there are a lot of variables involved.

Ignition timing varies greatly between combinations.  A lot of tuners choose to fire the spark plug at more than 70 degrees before top dead center, as nitro is a very slow burning, although powerful fuel.  This helps the car to leave the starting line hard, and gets it up on the tire (there is that wheel speed thing again), but has the dis-advantage of being hard on the motor, and doesn’t allow it to rpm freely.  In essence, it is fighting against itself, running on individual explosions in each cylinder as opposed to a smooth-running Swiss watch. The Swiss watch approach likely needs different gearing than the cars with big timing, but as said earlier, there are a thousand ways to get a nitro car to the finish line, everything just must work together perfectly.  Another thing you will notice on a car with lots of timing is that it doesn’t run big mile per hour at the top end, is has either used up pistons by then, or it is still fighting itself, basically acting like a rev limiter.

These are the major components in a nitro funny car, and how they work together.  As you can see, there are many possible combinations, some of which are just ludicrous, and won’t work at all, but most are workable if other parts of the combination are able to work in unison. Every motor and every car also has its own unique personality, so something that works for one guy won’t necessarily work for anyone else.  I hope you have gained some insight into how complex and difficult these cars can be, and how much an achievement it is to force the car to make a “perfect” run.

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