Hot Heads For Cool Cars: Powerful Secrets Revealed
By Larry Carley
Adapted from Larry Carley's article in ImportCar
As you are probably aware, there is a growing demand for high-performance cylinder head work and other engine modifications on a variety of sport compact cars. The most popular nameplates are imports such as Honda, Acura and Mitsubishi, but coming on fast are Nissan, Toyota, Mazda, Subaru and Volkswagen - plus domestic models such as the Ford Focus, Dodge Neon and even the Chevy Cavalier.
Of course, nobody is abandoning the small-block Chevy or any of the other V8s that have traditionally been the mainstay of the performance aftermarket. But in a highly competitive market, there's always pressure to grow your customer base and to find new opportunities. One such opportunity is doing performance work on sport compact car engines.
For those of you in school across this country, you already know that the market for this kind of work is not just a West Coast phenomenon. It's nationwide and growing every day.
Who's Looking for Performance?
Research shows that the typical prospect for performance work is an 18- to 28-year-old male who owns a four- to 10-year-old sport compact car. He's already spent several thousand dollars on trick accessories such as flashy wheels, ultra low profile tires, cross-drilled brake rotors, coil over struts, spoilers and wings.
Now he's ready for some serious power upgrades. He may have already done some bolt-ons such as a bigger throttle body, cold air intake system, exhaust headers and megaphone muffler. Maybe he's even added a bottle of nitrous oxide. Bolt-ons are great, but if the guy wants a killer street motor or an all-out race car, he's going to have to put some significant money in the engine itself.
Most of these people are computer savvy, but are lacking in mechanical skills - and certainly don't have the equipment or expertise to do head modifications or other internal engine work such as boring, balancing, porting and polishing, valve guide and seat installations, etc. That's why they will be coming to you, seeking the services of an experienced race shop.
It's not unusual for some of these tricked-up sport compact car engines to produce 300 hp in naturally aspirated form, and well over 450 hp with a turbocharger or supercharger. That's a lot of power from engines that typically produce around 160 hp in stock form. The secret to getting these kind of numbers is maximizing air flow and compression. Reworking the cylinder heads to flow more air, which typically involves porting the heads, installing oversized valves, massaging the bowls and combustion chambers, and increasing valve lift and duration are all traditional hot-rodding tricks that work just as well on sport compact engines as they do on traditional V8s.
To find out how some shops are reworking these little motors, we interviewed a number of shops on both coasts. Some are specialists who do only certain nameplates, while others do anything that comes in the door.
Making Magic from Mitsubishis
Jerry Jackson of Jackson Auto Machine in Hanover, MD (www.jacksonautomachine.com), runs a shop that does all makes and models of engines, but has developed a niche for himself by focusing on Mitsubishi.
"We got into building performance import engines about six or seven years before it became so popular. Now everybody wants to be involved with imports. But you have to be willing to make the investment in time and equipment to do the work. A lot of people don't really understand these smaller engines and underestimate their potential. They say a four-cylinder can't make serious power. But with the right modifications, these little engines are capable of producing up to six or seven horsepower per cubic inch. Those are numbers Chevrolet engine builders would have killed for 20 years ago."
Jackson says a modified street/strip four-cylinder Mitsubishi will generally put out 350-450 hp at the wheels. He also said one of his all-out Mitsubishi race motors cranked out 760 hp on a dyno - without nitrous.
"We currently offer a CNC ported cylinder head for the Mitsubishi 2.0L 4G63. We start with a factory casting and take it from there. The port locations in the stock head can vary quite a bit so we CNC machine the ports to optimize air flow. Our Stage 1 version of this head sells for $2,475 and is capable of producing more than 550 hp. This same head design ran 8-second times on the drag strip over three years ago. It includes our own high-performance springs, titanium retainers and stainless steel valves.
"We use stock-sized intake valves, but can go larger if needed. We also use stock valve seats, but use our own custom-made bronze valve guides. The cams we use are custom ground to our specifications, and are always changing as we make new improvements. And every head is flow-tested to make sure there is less than half a percent variation cylinder-to-cylinder.
"With our springs and retainers, the Mitsubishi heads can safely handle 8,500 to 9,000 rpm depending on the cams. The redline for the factory cams is about 7,000 to 7,200 rpm."
Jackson said he doesn't have heads sitting on the shelf ready to go, but custom builds each one to suit the customer's needs and application. He also offers a Stage 2 and Stage 3 version of his Mitsubishi 4G63 head. Stage 3 is an all-out racing head that is fully hand-finished and polished after it has been CNC machined. The Stage 3 head goes for about $3,000.
To seal the heads, Jackson uses multi-layer steel (MLS) head gaskets which he says work well for engines up to about 450 hp. For the all-out race motors, he prefers to use copper head gaskets with O-rings and receiver grooves.
Paulus Lee of AEBS Racing in San Diego, CA (www.aebsracing.com), specializes in sport compact engines almost exclusively, with Honda being his number one most popular engine, followed by Nissan and Subaru. Lee says his AEBS Racing Heads are custom-made to order, with all porting and polishing being done by hand. Each cylinder head is reworked for the type of driving or racing the customer wants to do.
Lee says three things affect how much horsepower a head will make: Combustion efficiency, volumetric efficiency and thermal efficiency. Sometimes you have to trade one for another, but the goal is to optimize each so the engine will make the most power possible.
"One thing we always try to do is to maintain or increase air velocity. The cubic feet per minute (cfm) capacity of a head depends on air velocity times the area of the ports. You want to keep the momentum of the incoming air high so that it will fill the cylinders more efficiently and make more power. We also like to produce swirl in the combustion chambers because this improves combustion."
Lee says he reworks the combustion chambers on the Honda heads to create a "cloverleaf" shape that increases swirl. It's the same design concept that is used in many Indy Racing League motors. The cloverleaf works best in naturally aspirated engines, but is not needed in turbocharged engines, he says, because the air is already turbulent as it exits the turbo. For turbo motors, an unshrouded combustion chamber works best.
"Our typical customer will spend $750 up to $1,500 or more on head work. The cost depends on how much welding is required and how much work is done to the ports."
Lee says one of the keys to making power gains in the head is the angle and radius of the valve seat. The right contour can add as much as 25 cfm to the flow capacity of the head. Lee says he's spent more than 100 hours perfecting the optimum valve seat contour and keeps it as a closely guarded secret.
In some applications, he installs tungsten carbide seats and prefers bronze valve guides for most. He uses stainless steel valves because titanium valves are too expensive for most of his customers and aren't really necessary in these engines. For camshafts, he uses Toda cams from Japan. He says the Toda cams may not have the hottest grinds, but they are bullet-proof reliable, unlike some other brands of cams he has tried in the past. For valves and springs, he uses REV products.
So how much power do some of Lee's Honda engines make? He says a typical naturally aspirated 2.0L Honda B street engine with a 86 mm bore and stock 89 mm crank can produce 280 hp at the wheels (or about 329 hp at the crankshaft). He's also built a turbocharged 2.2L Prelude that puts out more than 1,000 hp at the wheels running 40 lbs. of boost pressure.
Lee says the demand for import performance work has been very strong, and should continue to expand in the years ahead. "We are currently building a 600 hp supercharged Nissan 3.5L V6 and are using it to develop future products. This engine should be very popular in the future because it is used in so many Nissan models. We have already developed a cylinder sleeve for this engine that allows us to increase the bore size from 95.5 mm up to 102 mm."
Lee said another engine to watch is the new QR25 four cylinder that Nissan has used in 2002 and newer Altimas. It is a very strong engine with a high flow head that should easily lend itself to many performance upgrades.
Fast, Furious and Focused
Dennis Hilliard of Focus Central in Tehachapi, CA (www.focus-central.com), said his company is focused on the Focus, Ford's entry in the sport compact car segment. "It's a natural offshoot of our Mustang business," he said.
The Ford Focus has not yet achieved the popularity of the Honda or some of the other imports because it is still a relatively new model (introduced in 2000) that has not reached the used car market in big numbers yet. Even so, it has been well-received and is developing a following as more aftermarket parts for the vehicle become available.
Hilliard says the Ford ZTEC 2.0L DOHC four-cylinder engine is capable of developing more than 500 hp with a turbocharger. Even without a turbo, a modified, naturally aspirated engine can pull a Focus through the quarter mile in 12.9 seconds at 106 mph.
"The stock head is really quite good and actually has higher flow numbers than a Honda head. There are no aftermarket head castings available for this engine yet, so we rework the stock Ford casting to increase flow 30 to 35 percent. It's good for 290 to 300 hp without a turbo. We also offer a Stage 3 racing head that usually sells for $1,295 to $1,395.
"Typically, we install 1 mm larger intake valves, upgrade the springs and retainers and replace the stock cast iron guides with bronze guides. We currently hand-port the heads, but are looking into CNC machining."
Hilliard said Ford introduced a 2.3L Duratec engine in 2004 that is available in the Ford Ranger and Escape, and Mazda 3 and Protege. He said this engine has a lot of performance potential and is a good candidate for aftermarket parts and modifications. The engine has an aluminum block and heads, making it 40 lbs. lighter than the 2.0L ZTEC engine. He also said that Cosworth in England is developing a CNC-ported head, pistons and other internal parts for this engine, which will be offered exclusively through Ford Racing - along with seven bolt-on performance items that are currently available from Focus Central.
Every engine builder has his or her own secrets for improving cylinder heads. The type of modifications that are made depend on the application, rpm range of the engine, cam lift and duration, fuel system and so on.
The key to developing a head's peak power potential is flow bench testing. A lot of trial-and-error time will reveal what changes work and what changes don't. Modifications that work typically include raising the intake ports, narrowing the valve guide boss, shortening the amount of guide that protrudes into the port, smoothing and blending the bowl area under the seats, and installing larger intake valves or valves with undercut stems just above the valve head and/or swirl polished heads. Removing metal in the combustion chamber to unshroud the valves usually helps, too. Polishing the chambers can minimize carbon buildup and the risk of detonation.
Additional gains in air flow can also be found by carefully matching the intake and exhaust ports in the cylinder with the manifolds. Sharp edges obviously interfere with air flow, so carefully blending the area where these parts come together is a must for good air flow.
Another performance trick that's usually done is to equalize the volume of the combustion chambers ("ccing" the head). This assures the same compression in each cylinder and evens the power output cylinder-to-cylinder.
Valve lift, duration and seat angles also play a big role in air flow. The combustion chambers in Japanese 4-valve heads are pretty cramped and the valves are canted toward each other so there's a limit on how much valve overlap can be safely run without valve interference problems.
For higher rpm, stiffer valve springs are a must, along with lightweight retainers. Titanium valves can also reduce valvetrain weight, but are fairly pricey for most street engines.
When installing performance valve springs, pay close attention to the installed height. This ensures the springs have the required pressure to keep the valves shut. Height is checked by measuring the distance between the spring seat in the head and the retainer on the valve stem. Most performance valve springs are closely matched, but if adjustments are needed it can be done by shimming the valves to equalize pressures.
Another performance trick that's usually done is to equalize the volume of the combustion chambers ("ccing" the head). Shims are made of hardened steel, come in various thicknesses and are usually serrated on one side to prevent rotation (the serrated side faces the head). The thicker the shim, the more it increases spring pressure. Don't overshim, though, because doing so may lead to coil bind with a high lift cam or rocker arms.
The spacing between the spring coils must also be checked with the valves at full lift to make sure the springs do not bind. High lift cams and/or rocker arms push the valves further open, so it's important to make sure there's still some room between the coils at maximum lift. This can be done by inserting a feeler gauge between the coils. A minimum clearance of .060" is usually recommended.