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Building and Preparing a Factory Five Challenge Car for Competition

The Factory Five Challenge Series was envisioned to give racers a place to run where fair competition and equal cars brings the fun back into racing. The goals from the beginning have been to give every racer an equal chance at being competitive while maintaining low operating costs and minimal in between race preparation. While the series cars must be built to strict specification set by the National Auto Sport Association (NASA) there are some variables that are left to the builder/racer that can affect overall performance and it is maximizing these areas that we will try and help share our experiences with. It should be noted that rule enforcement and interpretation is completely done by NASA and rules are subject to change from year to year. None of these suggestions or methods should be thought of as being legal just because they are documented here, any grey areas or questionable interpretations should be clarified with a NASA official before entering the car into competition. This information is not a cold hard set of facts, rather what has proven to be one successful way to build a competitive car. In reality there is not one right way, only what makes you and your car the fastest it can be.


There is an old saying in the home built aircraft industry; if you are considering adding something to your airplane, hold it out in front of you and drop it. If it falls down to the ground, itís too heavy and should be used for something else. This same basic thought process is required when building a competitive car. For every piece that is not specifically called out, thought should be given over how to do it lighter. Even if you donít think that making the minimum weight will be a problem, your car will benefit from being under the minimum weight and adding ballast where it will do the least harm. Most of the way the car is built is already specified for you but what is left is certainly enough to make a significant difference.

One of the most important places to be weight conscious is with the unsprung weight, or weight that is not attached solidly to the chassis but moves with the suspension. Wheel choice is open down to a minimum of twenty pounds, this is one of the more critical areas as the wheels are not only unsprung, but they have to be rotated as well. (as well as stopped from rotating under braking) Get the wheels as close to the minimum as possible. Also open in the suspension department are the springs, front and rear. There may not be as much variance in springs as in wheels but every little bit adds up. You can use an eight inch spring instead of a ten for the weight savings, however make sure whatever springs you use there is enough travel that they do not bottom out and limit suspension movement.

The wiring harness is another area where attention to detail can make a significant improvement. Getting rid of all the wires, plugs, modules, sockets, and anything else you donít need can cut the harness weight by more than half. Finding a battery that is very close to the weight limit is important. The battery we have found to work the best is the Odyssey brand PC925 which is just over the limit at 25.5 lbs and has a very short overall height to help keep the center of gravity lower. Radiator size can also make a big difference; not only in its own weight but in that the capacity of coolant it carries can add up significantly as well.
A standard 3 core radiator can be more than 12 pounds heavier than a single core aluminum when full. Removing the cooling fan to save weight can also help but if you take it off youíll need to make certain that you remember when idling in the pits or during a red flag.

Transmissions and clutch containment devices also have a very wide range, using a blanket instead of a steel bellhousing can save as much as twenty pounds but make sure if you use one that it is made for a flywheel and clutch and not just to cover an automatic transmission. The Tremec 3550/TKO transmissions are also quite a bit heavier than the standard Mustang T-5, plus they tend to rob slightly more power. Other items allowable to change that can have an affect include, the seat, steering wheel, accessory pulleys, gauges, shifters, pedals, master cylinders, oil pans, and overflow tanks among others.

The starter from the later Mustangs, 91-93 is lighter than the older ones. It does require slightly different wiring to make it work as outlined in the manual.

Fasteners are also open and while it isnít cost effective to replace them all, trimming any areas where there are excess threads that can be cut off or replacing bolts with a similar but shorter ones will help add up over the course of the whole build. (Make sure you still leave a few good threads showing past the locking part of any lock nut) Any additions that are needed or desired for street driving, testing, or getting licensed should be made easier to remove. A passenger seat and full harness set not only adds weight but also drag if it sticks up above the body. An oil cooler also adds significant weight and often can reduce power, only run one if you feel it is absolutely necessary. If you need a horn for the street make it easy to remove.

On the engine side there may be extra brackets or emissions equipment that can be removed if not done already (The EGR spacer must stay in place but the rest of the emissions stuff can go). Another allowable change is the clutch and pressure plate. This is another area where the weight reduction is more valuable because it must be rotated. The clutch we have found to work best is from ďSpecĒ brand clutches, we use the stage one with the lightweight option which you must specify. Ford Racing also sells a lightweight heavy-duty clutch that is lighter than the regular heavy-duty clutch. (It is about the same weight as the stock Mustang unit but with more clamping power and better feel.)

Last but not least and quite possibly one of the biggest variables between cars on the starting grid is the body prep and paint work. The amount and type of fillers and paints used can vary the weight of the body by as much as 50-75 pounds. Having a body shop that understands what is important in a race car body vs. a street or show car body can make all the difference. If you are using your car as strictly a race car then it is best, and generally much less expensive, to skip the 2 and 3 stage paints and use single stage basic colors and to minimize the amount of body prep and filler added to seam areas and edges. Both Body filler and paint can add up pounds quickly.

If after you have taken advantage and maximized every area you can and you still canít make minimum weight then it may be time to take advantage of the overbore rules. The 5.0 engines in the Challenge Cars run somewhere in the neighborhood of 240 hp at the crankshaft, or about .8 horsepower per cubic inch. Overboring the engine by .030 gives an additional 4 cubic inches, and theoretically about 3 horsepower. What this means is if you take the additional 30 pounds you must run for an overbore and gain 3 horsepower you will end up with essentially the same power to weight ratio as the lighter cars.


When it comes to the engine and drivetrain there are only a limited number of modifications that are legal however making sure that your engine is in a good state of tune and running the way it is supposed to is critical. Keeping a clean fuel and air filter, fresh spark plugs and wires, new cap and rotor, along with making sure there are no exhaust or intake leaks can go a long way.

Optimum timing on the engine may vary slightly from car to car however we have found 14 degrees (set the base timing with the spout removed) to work best. This may require higher than standard octane fuel depending on weather and air quality so be careful running anything less than premium.

Accessory drive pulleys and shorty headers are also open and what we have found works best is the March brand race underdrive pulley set in aluminum, (to keep the weight down) part number 1035. This kit with the 3" crank pulley is no longer available from March. You can still get the alternator and water pump pulleys but not the crank pulley. We have found another source for the 3" crank pulley from ASP. The part number is ASP880100 but check their web site and call to order. For headers their isnít as clear an answer however the 1-1/2 inch diameter sets seem to make more torque than their 1-5/8 counterparts. The stock headers have proven to be quite good and many people use them. The headers we have settled on are JBA unequal length 1-1/2Ē part #1620-2.

The clocked position of the mass air meter in the intake tube can have a significant affect on your air fuel ratio. The best way to optimize this is on a dyno, but what we have found is that most engines run slightly richer than optimum and running the meter with the sensor in the 6 oíclock position or pointing straight down works best on most engines. Be careful to keep the plastic sensor and plug protected from the header as it will melt and produce some difficult to diagnose problems.

Some other tips to maximizing the stock set up include:

If you need to replace your injectors buy a matched set. What this means is that someone has taken the time to measure the flow of many stock injectors and grouped them according to flow characteristics. We havenít seen a perceptible increase on the dyno over a stock set and if yours are functioning properly this is probably a waste of money but if you need new ones it is probably worth the slight increase in cost.

Make sure that your air intake tube is not kinked or pinched in any way and the filter is as far from the header as you can get it without changing the tube.

Use the lightest clutch and wheels possible as described above, both can make a noticeable difference on a rear wheel dyno.

Ground the Engine computer directly to the battery. The two grounds at pins 40 and 60 are the reference grounds for all sensors and work best when connected directly to the negative terminal of the battery. For a more in depth explanation see the 2007 Ford Racing catalog.

Use synthetic fluids in the engine, transmission and rear axle.

Run a windage tray in the oil pan.

Do not run a high volume or high pressure oil pump, they rob power and are not needed on such a low rpm engine. It is much more important to engine life make sure that the pickup can never suck in any air.

Try to shift at or below 5000 rpm. There may be some circumstances where holding out a gear makes sense but due to the torque curve of the stock engine, revving it any higher before shifting will most likely slow you down.

Remove the PCV (emissions) system and put a breather or catch tank on the valve cover. This allows the engine to ingest more clean air instead of hot crankcase vapors.

Set the throttle position sensor as aggressive as feels comfortable. Adjusting the sensor is done by loosening the two screws on top of the throttle body and turning the sensor while measuring the voltage coming from its green wire. (Key must be on and even better if the engine is running) The closer to .99 volts you get the quicker the throttle response will be. If you want instant response you can go just above 1.00 volts but the car may be jumpy to drive slowly.

Running a cooler thermostat does not usually make the most power.

Bolting a new or freshened up set of heads on a good condition used short block can make good power. When changing the heads make sure and double check the amount milled off of any used set and match it to a gasket set that will put your compression just under the maximum.

Replacing the remote oil filter set up with a 90 degree adapter from Ford Racing wonít give any added power but does reduce the number of places for having a leak and cleans up the engine bay.


The majority of our race cars have been built with a single tandem master cylinder and this set up is in reference to that. As with the engine there are a very limited amount of legal changes you can make to the braking system, however make sure that what you do have is functioning at its best. The two main choices to make when setting up your brakes are what rear system to use and what brake pad to use. The two most popular rear brakes systems are from 1987-88 Thunderbird Turbo Coupes and from 1994-2000 Mustangs (non SVT models). Neither one of these choices has proven to be clearly superior to the other but due to the caliper volume the set-ups differ slightly between them. The newer Mustang calipers have a smaller diameter rear piston and give a good pedal feel with a 15/16Ē bore master cylinder, whereas the T-bird calipers need a little more volume and work best with a 1Ē bore. All of these master cylinder combinations work best with either the 4 cylinder mustang brake pedal or the stock pedal modified for leverage as described in the manual.

Brake pad material is important as well and there are many good race compounds available. What we have found to work best with the T-bird rear brakes is a more aggressive rear pad, usually by one step over the front, while the Mustang rears can use the same compound as the front. Most of our cars are running Carbotech pads in around a 10 compound. With pads this aggressive brake ducts, particularly in the front are needed and the pads and rotors should be checked often as they are working harder. Also using good fluid like Castrol SRF is more important.

Setting brake bias is equally important and if you are using the set up as described above what we have found is that even with a slightly more aggressive rear pad there is still a tendency for the front brakes to lock first. For this reason we generally mount an inline brake proportioning valve in the line to the front brakes and adjust until the fronts lock just before the rears. It is critical that the car is scaled before doing any brake bias testing.


The final piece in the puzzle and the one that will be most sensitive to individual driver preference is how the car is set up to handle the way you want it. Some of the basic things that can be done when building or improving the car will be of benefit to any set up, however when setting the balance of the car to your particular driving style the best measure of what is working will be the stop watch.

Giving yourself a good solid repeatable chassis that does all of the basic things well will mean that setting up the car to be fast will be much easier. Start by mounting everything on the chassis as low as possible. Many things are fixed according to the rules but the ones that arenít (for example a fire bottle) should be as close to the ground and usually toward the passenger side you can safely get them.

Make sure that the suspension components are well lubricated and move as freely as possible without the coil-overs in place. The front upper control arms can easily be bound up if the adjustments are made without loosening the clevis bolts on the ends. Be sure and tighten these bolts last as any lengthening or shortening of the arm can put a bind on the pivot otherwise. The front lower control arms should be able to move relatively easily as well and should drop under their own weight. If they do not then it is important to grease not only the sleeves but between the bushings and the frame tabs. The bolts holding the lower arms in place may need to be loosened up some in order to accomplish this but not so much that the arms have side to side play. The same is true for the rear lower control arms.

The following set up is only one example of a car that has been proven to work. This set up is what we have come up with over the last few years. It provides a good baseline and a fairly neutral car for most people around most tracks.

Spring rates: We are running 700lb front and vary from 400-500 in the rear with 500 being the more aggressive toward oversteer type set up.

Alignment settings: Almost all of our cars use the same basic settings of 7-8 degrees caster and 1/8 in. total toe out. We have run just under -2 degrees camber for the last few years but this should be experimented with as it has been mostly with a different tire than we now run.

Ride Height and track width: The car seems to work best at or just above the minimum ride height. Rear track width should also be maximized.

Steering: We have found that power steering enables us to do two things that make the car faster overall and offset the small weight and power penalty. The first thing the power assist gives is the ability to use a faster steering ratio and with the assist the ability to add steering correction much quicker. This has enabled us to drive harder closer to the edge and still make quick corrections (and saves when pushed too far) that keep the car on the limit. The second benefit of the power assist is it allows for much more caster to be dialed into the front end of the car which helps counter the suspensionís relatively high kingpin angle (SAI) while still allowing the car to be steered with low effort. Also the higher caster raises the steering arm on the spindle which in turn improve bump steer (with the tie rod mounted on the bottom of the arm) making the car more stable and easier to drive.

If you are using power steering put the car is twitchy and the steering effort is too light you can reduce the assist by trimming the spring right behind where the high pressure line connects to the pump. Start by trimming 1-1/2 coils and work up from there until the steering feel is where you want it.

Tire pressures: From recent testing with the Spec tire in a 255 width we have found that 25-29 psi. hot pressures work the best for us and any fine tuning of the suspension can usually be done within that range.

Rear Axle: If your rear axle was used and came from a mustang it should be checked for straightness in both toe and camber. A little bit of toe in is fine as is some negative camber as long as its below the maximum. Any toe out or positive camber in the rear needs to be fixed or the car will be near impossible to set up. Also make sure that your limited slip is in good shape, it will wear out from time to time and need to be rebuilt after every couple seasons or more frequently if the car sees double duty as an autocross or street car.


Ultimately what makes the car fast is a combination of set-up, power, braking and driver comfort. None of the set up information will be of any help if you are not comfortable in the car. It always pays off to spend the time to set the pedals, steering wheel, seat, and any other driver interface to where you (or your driver) is most comfortable with them. Small things like pedal height and spacing may just take a few minutes to get right and the results are well worth it. Some stick on convex mirrors work wonders for visibility, a 3-3/4 diameter one will fit perfect over the stock side mirrors. Once the car is comfortable then the real set up can begin.

Having help at the track helps but even without it, it is important is to keep track of what changes you make and how they affect the car, as well as lap times, tire pressures and conditions. Without the record of how the car has changed it becomes difficult to make any real progress or repeat past success.

See you at the track.

Page last updated: 03/18/2009

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