On the intake side, the closest point between the valve and piston typically occurs at 10 degrees after top dead center (ATDC). But the chief reason for the difference is the relationship between the piston and the valves. Why does the exhaust valve need more clearance than the intake valve? First, the components are hotter on the exhaust side and they expand more due to the intense heat. An engine that is maintained rigorously with fresh valve springs and regular inspections can run less valve clearance, but an extra safety margin is advisable for a sportsman engine that will run dozens (or hundreds) of races between rebuilds.
In a typical sportsman drag racing engine, I recommend.
COMPRESSION RATIO CALCULATOR BBC FULL
100-inch of valve clearance in a 632ci big-block costs a full point of compression ratio, and that has a major impact on performance. But you have to pay attention to valve clearance even in a large displacement engine. Those engines had relatively little swept volume compared to the big-inch engines we build today, so squeezing every bit of compression was a challenge. I learned about the relationship between valve clearance and compression ratio back when we were building 287-cubic-inch Modified engines in the ’70s, and it’s a lesson that has stayed with me for 40 years. To a large extent, valve clearance determines the compression ratio because the piston’s valve notches account for much of the combustion volume. The gains in efficiency produced by increasing compression can be documented on a dyno as the brake specific fuel consumption (BSFC) goes down. In an engine that requires more spark advance, some of the power of the expanding gases is wasted as negative work, pushing against the piston while it is still rising in its cylinder. That’s beneficial because more of the power pulse occurs after the piston has passed top dead center (TDC) and the expansion of gases in the cylinder is driving the piston downward. The more these gases expand, the more power they make.Ī high-compression engine typically produces best power with less spark advance than a low-compression engine. You can also think of compression ratio as the engine’s “expansion ratio” – as the combustion gases expand from a small volume to a larger volume, they produce usable power. Raising the compression ratio increases cylinder pressure, and cylinder pressure is what drives the pistons, turns the crankshaft, and ultimately makes the wheels go around. Why is compression desirable? In short, higher compression makes an engine more efficient. We often find that the owner or engine builder has left a lot of compression on the table.Įxcessive piston-to-valve clearance is the enemy of compression – and compression is your best friend in a naturally aspirated racing engine (but only up the point where it induces detonation). One of the rules at our shop is that we always check the valve clearance in any engine that we didn’t build before it’s torn down.
One of the most common mistakes I see in engine building is excessive valve clearance. “Excessive valve clearance is the enemy of compression – and compression is your best friend in a naturally aspirated racing engine.” By David Reher, Reher-Morrison Racing Engines
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