One of the doctrines of NASCAR racing is to have rules that more or less equalize competition between the vehicle manufacturers. Sometimes the rules work. Sometimes they need to be re-evaluated and adjusted. Such is the case with NASCAR’s recent decision to allow Ford to run an all-new cylinder head for the 2004 racing season.
Winning races comes down to five factors: horsepower, aerodynamics, handling, driving skill - and luck. As far as horsepower is concerned, NASCAR’s rules limit engine displacement, compression, carburetion, and the basic configuration and dimensions of the cylinder heads so no single vehicle manufacturer theoretically has a significant edge over the others. In Ford’s case, they’ve been running the same basic engine design and cylinder head since 1990. General Motors, on the other hand, got NASCAR approval for its second generation SB2.2 cylinder head in 1998 for Cup Series races.
In 2003, GM seemed to have a significant advantage over its rivals thanks to its engines and aerodynamics. In the final point standings, GM teams held seven of the top 10 positions, Dodge had two and Ford only one.
Greg Specht, manager of North American Racing Operations, Ford Racing Technology, said he was relieved that NASCAR finally approved a new head for Ford because it should help Ford win more races in 2004. “It will allow us to have better performance in the low rpm ranges, and I think that’s where we’ve struggled. It’s going to be a better feel for the driver now coming out of the corner and getting on the throttle. There will be plenty of power there to help them get back out of the turn and get going down the straightaway,” said Specht.
Ford Has a Better Idea - Maybe
The cylinder head that Ford has been using in NASCAR races is essentially a modified 351 Cleveland head. The new head is also for the existing Ford block but has bigger valves to improve airflow. The old head will still be allowed because supplies of the new one will be limited this season.
Doug Yates of Robert Yates Racing said the new head will have port height comparable to GM and Dodge (a 4” intake height). The challenge now, he said, is to fully develop the head and manifold to maximize power. The intake manifold for the former head won’t work with the new head.
Performance engine builders who are interested in running Ford Motorsport SVO Yates heads on a 302 or 351 V8 can choose several different castings. The C3, C3L and C35 castings have 67 cc combustion chambers, shaft mounted rocker arms and are typically used with titanium 2.10” intake and 1.60” exhaust valve.
The heads also have larger water passages between the combustion chambers and use AGS series spark plugs. The CH3 “high port” heads are similar to the C3 castings but have 40 cc combustion chambers, raised intake runners with a straight intake bolt pattern and use longer pushrods. The SC1 castings are the new design with raised intake and exhaust runners, thicker metal around the ports to allow additional porting, and can accept larger 2.180” intake and 1.625” exhaust valves. The SC1 head requires an
M-9424-W352 intake manifold.
GM Heads
Randy Dorton of Hendrick Motor Sports says Chevrolet teams are running GM’s SB2.2 heads that have intake ports angled toward the carburetor to improve airflow (called a “mirror port” design). Intake valves are tilted (splayed) 4° toward the center of the cylinder and tilted sideways 11° while exhaust valves are angled at 8°. Intake valve centerlines are 0.250” closer to the intake flange and 0.080” toward the cylinder bore centerline than the previous SB2 generation head. The spark plugs are also 0.125” closer to the bore center for better flame propagation. Spring pads are also larger for use of 1.625” valve springs.
The SB2.2 is a purpose-built cylinder head for NASCAR Cup racing, and is available to anyone through GM Service Parts (P/N 12480129). The required intake manifold for the SB2.2 head is P/N 12480048, with a separate valley cover P/N 12370840.
Dorton said NASCAR’s rules limit the modifications that can be done to heads. Rules fix the spacing between valves to 1.935”, so if a larger intake valve is used the size of the exhaust valve must be reduced to maintain the same center-to-center spacing. He said most teams are using titanium 2.15” intakes (5.65” length) and 1.60” exhaust valves.
NASCAR provides a template to set the roof height of the port in the cylinder head. Port height must be within 0.050” of the specification, which limits how much the ports can be reworked. The rules also do not allow any changes in port centerlines or moving the original location of the spark plugs. Compression is limited to a maximum of 12 to 1.
Dorton said most NASCAR engines these days are capable of revving 9,000 to 9,500 rpm, which is really hard on valve springs. Because of this, spring durability is extremely important. Valve springs are replaced after every race. Valve openings are typically in the 0.800” lift, so most teams run some type of lightweight rocker arms such as Jesel to reduce the stress on the springs and valvetrain.
Other GM heads used by NASCAR teams include 23° “Pontiac” heads, which have siamese center D-shaped exhaust ports, 62 cc combustion chambers, 3/4” reach 14 mm spark plug holes. For 9:1 compression NASCAR engines, GM has 18° heads (P/N 10134363). Aftermarket performance heads by Dart, Brodix and others are not allowed under NASCAR rules.
Dodge
Bob Fisher of Ernie Elliott Inc. said Dodge’s small block cylinder head is more like a big block head in that it has staggered valves with the intakes canted for better breathing. Stock Dodge heads, by comparison, are “wedge” heads with all of the valves in a row and the same plane (18°).
The Dodge cylinder head is a special NASCAR casting available through Mopar Performance or Evernham Performance Parts. The head number is P-5007428. It is designed for use with engine block P-5007427.
NASCAR rules specify a valve spacing of 1.916”, which is a little wider than Ford (1.900”) but narrow than Chevy (1.935”).
Fisher said most teams use beryllium exhaust valve seats because of their ability to handle high temperatures and cool the valves. Titanium valves are the norm, with double or triple springs.
“One of the most important components in any NASCAR cylinder head is the rocker arms. We’re never happy with what we have and are always trying to develop something better. We’re always trying to take more weight out of the valvetrain. Many teams use aluminum rocker arms but we also use a special steel matrix rocker arm that is very expensive. But the steel rocker arms are actually lighter than aluminum and a lot more durable,” Fisher said.
Power Secrets Revealed
Not! None of the engine builders we interviewed for this article would reveal their closely guarded secrets for squeezing maximum power from their respective cylinder heads. But all said that hundreds of man-hours go into race prepping a set of NASCAR cylinder heads.
Much of the development work is done on a flow bench. Once the optimum runner profile has been determined for an engine, it is electronically mapped and reproduced by CNC machining the head castings. Dorton said heads that used to cost $20,000 to $25,000 can now be done for about $10,000 - which helps keep costs down. It also speeds up the development of new port profiles.
Other head modifications that are commonly done include narrowing valve guide bosses, shortening the amount of guide that protrudes into the port, and unshrouding the valves to improve airflow.
Additional gains in airflow can also be found by carefully matching the intake and exhaust ports in the cylinder head with the intake manifold runners. Sharp edges obviously interfere with airflow, so the area where the head and intake manifold join is carefully aligned and blended to improve airflow.
Other performance tricks that are usually done include equalizing 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.
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