By Larry Carley
The sport compact car market is hotter than ever. The market has moved beyond the simple bolt-ons such as wheels, tires, brake rotors and spoilers. It’s not unusual today to see sport compact engines producing upward of 400 horsepower on the street. With so much power under the hood, the challenge is getting the horsepower to the tires and maintaining reasonable traction and handling.
The weak link in the drivetrain is always the clutch. Stock clutches are designed to handle stock torque loads, not two or three times the power that a highly modified engine may be capable of producing. Consequently, if the stock clutch is not replaced with a stronger aftermarket performance clutch, it won’t last long. The stock clutch may not even have enough grip to prevent slippage when the car is accelerating hard, especially in higher gears.
Performance clutches use a stiffer spring to increase clamping force. A stiffer diaphragm clutch will increase pedal effort slightly, but usually not enough to be objectionable for everyday driving unless it’s a really high-pressure racing clutch. Some aftermarket clutches use centrifugal weights to increase clamping force at high rpm, which means the clutch springs do not have to be as stiff. The best advice for choosing a clutch is to ask your customer if the vehicle will be a daily driver, a weekend racer or an all-out racer. Then choose a clutch that best matches his driving needs.
The same advice applies to choosing the type of clutch disc and friction material for the clutch. Street clutches typically have full-faced non-asbestos organic facings with marcels between the facings, and springs in the center hub to cushion engagement and dampen shock loads. Racing clutches, by comparison, typically have 4, 6 or 8 friction pucks mounted on a rigid disc.
For street-performance applications, the friction pucks may be a ceramic/metallic or carbon/kevlar material. The disc may also have springs in the hub for torque dampening, but there are no marcels between the facings in this style of clutch. Consequently, engagement is harsher and the driver may notice some chatter (which requires riding the clutch a bit more when starting out). One way around this is to use a clutch disc with conventional facings on one side, and pucks on the other. The conventional facings provide smooth engagement, while the pucks concentrate the loading to handle higher torque loads.
For serious performance applications, sintered iron friction pucks are often used on the clutch disc, and the disc has a solid hub with no springs. This type of clutch is great for the track but can be very harsh for everyday driving and would probably not be recommended for a car that spends 90% of its time on the street.
Reducing the weight of the flywheel reduces its “moment of inertia” (momentum) so the engine can change speed faster. On a road course, a lightened flywheel allows the car to decelerate faster and brake later going into turns, and accelerate faster out of the turns. But on the street, lightening the flywheel too much can be detrimental to driveability.
The flywheel stores engine torque as inertia. The heavier the flywheel, the more energy it stores. When the clutch engages, the inertia is transferred through the drivetrain to get the vehicle moving. This reduces strain on the engine and smooths out the application of torque. If the flywheel has been lightened, but is too light, the engine may bog down when the clutch is engaged, and may even stall unless it’s revved up. That’s why most experts say the flywheel should not be lightened more than about 5 lbs. on a typical sport compact street car.
A flywheel can be lightened by machining metal off the back of the flywheel or off the rim. Weight removed further out near the edge of the flywheel will have more of an effect reducing inertia than weight removed toward the center of the flywheel. The other way to lighten the flywheel is to replace the stock flywheel with an aftermarket lightweight steel or aluminum flywheel. Aluminum flywheels may weigh only 7 or 8 lbs. (which is much less than the stock flywheel), and usually have a steel facing that can be replaced if the plate becomes worn.
Note: If a stock flywheel is lightened, it will also have to be rebalanced. New flywheels are factory zero-balanced, but may have to be rebalanced to a particular engine depending on how the engine is balanced. Some stroker cranks, for example, may require external balancing with the flywheel to achieve proper overall engine balance.
For drag racing, a heavy flywheel is actually better than a lighter flywheel. Here, you want a lot of momentum off the line, and that requires a lot of stored energy in the flywheel. The engine is going to hit maximum rpm fairly quickly anyway, and stay within a narrower rpm range as the driver bangs though the gears.
CONTROLLING WHEEL SPIN
Engine modifications that produce more horsepower and torque won’t make a car accelerate any faster if the tires lose their grip and go up in smoke. The open differentials that are found in most front-wheel-drive transaxles need serious help when it comes to maintaining traction. If one wheel starts to break loose and spin, the side gears in the differential route all the power to the slipping wheel and none to the one that still has traction. The result is a lot of tire smoke and noise off the line, but not much acceleration.
Maintaining traction at higher speeds can also be a challenge if the engine has been modified with more turbo boost or a nitrous oxide system. When the power kicks in, the torque tends to overpower the drive wheel with the shortest halfshaft, or the inside wheel if the vehicle is turning into a corner. One wheel goes up in smoke and the other is just along for the ride.
The best all-round performance drivetrain configuration is all-wheel drive (AWD), but it’s available only on a limited number of vehicles (Mitsubishi EVO, Subaru WRX, older Mitsubishi Eclipse, etc.). So one of the handicaps of front-wheel-drive cars is that they have traction problems when they accelerate.
The cure here is to replace the stock open differential with an aftermarket limited slip differential. Torque-sensing differentials such as TorSen and Quaife use cross-mounted worm gears to shift torque from one side to the other if one wheel starts to lose traction. These are the best alternative for street cars because the torque transfer isn’t as abrupt as a locking differential that uses clutch packs. Torque-sensing differentials also tend to be longer lived on the street because they don’t have clutch pucks that wear out.
Another problem that plagues many modified FWD cars is torque steer. Under hard acceleration, the wheel with the shortest halfshaft tends to toe-in more than the side with the longest halfshaft. The short shaft is usually the one on the left (driver’s) side, so the car usually pulls toward the right. Vehicle manufacturers have tried to minimize torque steer by going to drivetrains with equal length halfshafts on both sides. They can also counteract torque steer somewhat by using stiffer bushings on the short side. Even so, some FWD cars still pull under hard acceleration even with stock engines. Modifying the engine must multiply the problem and make it worse.
Installing stiffer polyurethane control arm bushings can help reduce suspension compliance that causes torque steer. The plastic bushings replace the stock rubber bushings, and the result is less torque steer and better handling.
RIDE HANDLING MODIFICATIONS
Lowering the suspension looks cool, improves aerodynamics by rerouting air around the body rather than under it and lowers the vehicle’s center of gravity. But a vehicle that is closer to the ground doesn’t necessarily experience less body roll when cornering. So if your customer is really serious about handling performance, a variety of suspension modifications may be needed to achieve the kind of chassis control needed to make a vehicle competitive.
Lowering can be accomplished in a variety of ways including replacing the stock springs with shorter, stiffer aftermarket springs or coil-overs, or by replacing the stock front spindles and rear control arms with lowered parts.
The nice thing about installing lowered spindles and control arms is that no other modifications are required. It doesn’t affect wheel alignment or ride quality, so if a customer wants only a “lowered look” without sacrificing kidney comfort, this is a good alternative to shorter, stiffer springs or coil-overs.
Installing shorter springs or adjustable coil-overs can change the geometry of the suspension, so the wheels have to be realigned after the modifications have been made. On some cars, there are no factory camber adjustments or only limited adjustments, so an aftermarket camber kit must be installed.
Lowering ride height usually produces negative camber causing the tops of the wheels to lean in. Negative camber can help traction and handling, but it can also cause the tires to wear unevenly. There are various types of kits from which to choose that will provide up to 1.5 degrees of camber correction. On Hondas, the kit may use offset bushings to replace the stock bushings on the front upper control arms and the rear upper arms.
With coil-overs, ride height and spring stiffness can both be changed by turning the adjusters on the struts. This provides a lot of flexibility and allows the suspension to be tuned for better weight distribution and handling. Changing the load that each wheel supports can shift weight front-to-rear and side-to-side to achieve understeer, oversteer or neutral handling (in theory on some vehicles).
Although it’s hard to see, there is actually quite a bit of chassis flex in a unibody when the vehicle is cornering hard. The strut “stops” tend to deflect in and out, which can be felt as camber/caster changes in alignment. Movement in the lower strut mounts at the rear of the vehicle can have a similar effect on rear alignment.
An easy bolt-on product for stiffening up the suspension is to install a brace between the tops of the upper front struts, and a tie bar between the lower rear struts. A strut brace can help minimize understeer due to chassis flex, and is a “must” for serious handling improvements especially if larger or wider tires and wheels are installed on the car. The braces may be T-6 aluminum or steel tubing. Aluminum bars are often anodized in different colors to dress up the engine compartment.
Replacing the stock shocks or struts with aftermarket gas-charged performance shocks or struts is also a good way to improve handling agility and ride control. Gas-charging reduces cavitation and fluid foaming when the shock is working hard to reduce shock fade. High-pressure monotube shocks and struts are a popular choice here, but gas-charged twin-tube shocks and struts are also available.
Stiffer valving and/or a larger piston inside a shock reduces body roll and tire bounce to enhance handling and traction. But shocks that are too stiff can be very harsh on the street. Consequently, shock manufacturers offer adjustable shocks with a variety of settings. Turning the adjuster changes the internal valving to suit changing driving conditions. The shock can be set “soft” for everyday driving, and tuned to a stiffer setting on the track.
On newer Hondas, replacing the stock front struts is a challenge because the steering arms are attached directly to the struts. This requires a similar type of replacement strut or modifying the original strut by cutting it open and installing an aftermarket strut cartridge.
To install the cartridge, you first remove the OEM strut and disassemble the spring with a spring compressor. Then you drill a hole in the bottom of the strut to vent the gas charge and oil. Using a pipe cutter, you cut off the top 40 mm of the old strut, pull out the stock guts and install the new cartridge.
The last link in upgrading handling performance are the sway bars. The front and rear sway bars (if equipped) connect the right and left sides of the suspension together. When the body starts to lean going into a turn, the movement of the body twists the bar. The stiffness of the bar resists this twisting to help keep the body flat. The stiffer the bar, the better the car corners and can handle sudden lane changes without losing control.
The reaction time of the bar depends on the links and bushings that connect it with the control arms. Polyurethane bushings reduce compliance and the reaction time of the sway bars.
Replacing the stock sway bars with larger, stiffer bars improves handling but, as a rule, both bars should be replaced as a matched set to maintain the proper balance. If a stiffer rear bar is installed, but the stock front bar is left in place, it may have an adverse effect on handling and cause oversteer. Likewise, installing a big, stiff front sway bar on a vehicle with no rear sway bar or a relatively mild rear bar may induce understeer.
WHEELS AND TIRES
Big 19-inch wheels look cool on a small car, but larger wheels are often much heavier with more inertia than the stock wheels they replace. Wheel weight is detrimental to performance because it takes more torque to accelerate and brake, and it increases unsprung weight, which adds to ride harshness. So if a customer wants bigger wheels, have him buy the lightest ones he can afford. Wheels with a more open design will be lightest.
Tires also make a huge difference on traction and handling. Soft rubber compounds with large smooth expanses of rubber across the tread and minimal grooving and edge siping are great for dry traction, but may not be the best choice for wet-weather driving. These types of tires tend to be rather slippery, even if they have directional tread patterns. A better choice is a performance street tire with an “A” wet traction rating and compounding that provides reasonable wear characteristics.
Low-profile tires with high aspect ratios are also popular with oversized wheels. Shorter, stiffer sidewalls reduce tire deflection when cornering for improved handling agility, but the trade-off can be a much harsher ride and increased risk of rim damage if the vehicle encounters a pothole.
The inflation pressure inside the tires also has a significant impact on handling and weight distribution. Higher pressure reduces rolling resistance and friction may cause the tread to bulge, lifting the edges away from the pavement. High pressures also increase ride harshness. Lower pressures flatten the tread and reduce harshness, but too little pressure may cause the tire to run hot and fail. Low pressure also increases the risk of rim damage with ultra low-profile tires.
Adjusting the pressure in individual tires can also shift weight front-to-rear and side-to-side to tune suspension handling. Only a couple pounds difference can make a noticeable difference in how the car handles.
Aftermarket tire pressure monitoring systems (TPMS) are a nice addition here, not only to protect a customer’s investment in expensive wheels and tires, but also for driving safety. These systems warn the driver if tire pressure drops 25% or more under the normal inflation pressure.