Cylinder heads need to be robust. They have to withstand huge pressures and very high temperatures, while retaining their shape and form to seal the cylinder block via the head gasket. They’re key to controlling air flow in and out of the cylinders and fuel deployment.
The cylinder head also holds the injectors and valves – and contains more moving parts than any other part of the engine.
Although largely unnoticed, the cylinder head plays a key role in your engine. It’s a solid item that sits at the top of your unit and covers its workings. It’s sealed in place with the head gasket.
The inside of the cylinder head contains a number of passages – known as ports or tracks – and the air mix travels along these to the inlet valves. Other tracks inside the cylinder head are the route by which the exhaust gases travel when leaving the main engine block.
Extreme temperatures
In all engines cooled by water – this applies to all Perkins engines – the cylinder head also contains ducts and passages.
The cylinder head houses the largest number of moving parts
These allow the coolant, comprising water and antifreeze, to flow and transfer excess heat away from the main body of the engine to the radiator, preventing the risk of overheating and damage. Not surprisingly, the cylinder head is subjected to extremes of temperature, which it has to resist.
What lies beneath
Beneath the cylinder head lie more moving parts than anywhere else in the engine. These mainly comprise the valves and fuel injectors, calibrated precisely to meet the needs of your engine.
Because they’re fixed in position and sealed with the head gasket, cylinder heads don’t suffer the same wear and tear as moving parts of the engine. They need to be replaced relatively rarely.
Choosing the “right” cylinder heads for an engine build can make all the difference in an engine that delivers and one that falls short of its potential. Horsepower, torque and throttle response all depend on how well the cylinder heads, camshaft and induction system work together. Choose the right combination and you’ll build a winner. Choose the wrong combination and you’ll end up with a mismatch that never achieves what you set out to achieve.
Choosing a set of aftermarket performance cylinder heads is not as simple as it sounds because there are so many different heads from which to choose. Comparing heads from various suppliers isn’t exactly a straight-forward process because numbers can be very misleading.
One thing is certain. There is way too much emphasis on peak airflow numbers. “The first thing a customer usually asks when they call about a set of heads is how much air do they flow? Or what’s the port volume? None of these numbers will tell you how much horsepower a set of heads will actually make because the power output depends on the combination of parts in the motor and the application,” said one head supplier.
Don’t Overdo It!
The one thing every head supplier cautions against is putting too much head on an engine that can’t handle it. Just like too much carburetion or too much cam duration, the heads, cam and induction system (and exhaust system too) all have to work together to produce the best possible results.
The common assumption (which is often a misconception) is that a set of heads that has the highest airflow numbers will always make the most power. They will under the right circumstances, if the engine can take full advantage of the flow potential of the heads. But often the tendency is to go with heads that are too large for a given application, which ends up hurting performance more than it helps.
For example, a street performance 350 Chevy with a hydraulic cam will never pull as much air as many race heads can deliver. Race heads are designed with larger port volumes and bigger valves so they can handle more airflow for large displacement stroker engines and/or high revving drag or circle track engines with long duration, high-lift cams. Bolt a set of race heads on an otherwise stock 350 in a street car and you’ll end up with a dog that has no throttle response or low to mid-range torque. The heads need to be sized to the application.
On the other hand, if you’re building a large displacement stroker motor, you’ll need heads with larger ports to handle the increased air volume the engine requires. A set of heads with 185cc ports would deliver the best all-round performance for a 350 CID engine, while a set of heads with 200cc to 220cc ports would work better on a 383 to 400 stroker. The larger the displacement of the engine, the larger the port volume the heads need to deliver the airflow. Ditto for a high revving race engine with a lot of cam lift and duration.
Airflow Numbers
Airflow is measured in cubic feet per minute (CFM) on an airflow bench. Typically, airflow is measured in 0.10˝ valve opening increments from about 0.200˝ of lift to peak valve lift. For stock heads and most street heads, peak valve lift is going to be 0.500˝ to maybe 0.650˝ with high-lift rockers, so measuring airflow beyond peak valve lift for these kind of applications is meaningless. Yet the peak airflow numbers advertised for some heads are at much larger valve lifts (up to one inch). If you’re building a ProStock drag motor that’s going to rev over 11,000 RPM with over one inch valve lift, then peak airflow numbers at one inch of valve lift is important. Otherwise it doesn’t matter.
Most head suppliers say the airflow numbers that count the most for street performance are those from about 0.200˝ to 0.500˝ of valve lift. A head that delivers the most airflow within this range will often outperform a head that has higher peak airflow numbers at 0.500˝ of valve lift and above. Why? Because peak airflow only happens at one point in the valve lift cycle (when the valve is fully open) whereas airflow that occurs when the valve is half open happens twice every valve cycle. Consequently, if a head flows well at part valve lift, it will do a better job of maintaining high air velocity for more power, torque and throttle response.
What about a set of heads that have great peak airflow numbers when the valves are at maximum lift but maybe not so great numbers at part valve lift? It depends on the application. In a high revving drag engine, low and mid-range torque are less important so there’s more focus on peak airflow numbers for maximum high RPM power. In a circle track engine, the driver needs good throttle response off the corners so peak airflow at maximum valve lift is less important than good airflow at part valve lift.
Efficiency is another factor that plays into the cylinder head selection process. Comparing port volumes and airflow numbers can tell you a lot about air velocity and efficiency. If a set of heads with 220cc ports delivers the same airflow numbers as a set of heads with larger 230cc or 240cc ports, the heads with the smaller 220cc ports are more efficient and will usually outperform the larger heads because they are better able to maintain maximum air velocity.
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