Editor’s Note: This is the second half of an article on how to interpret cam specs to select the right cam for your engine and vehicle needs. You can read the first half here: Comparing Camshafts: A History of Camshaft Specs and Choosing the Right Cam.
Most of us know the four strokes of an engine. When looking at camshaft timing, we actually start near the end of the power stroke.
This graphic gives us a better visual of what’s going on. (Click to enlarge)
Exhaust Opening (Also Known as Blow-Down)
At this point on the power stroke, the piston is more than halfway down the cylinder.
Now, there is little positive downward pressure on the piston to create horsepower, so we crack the exhaust valve open. You’ll see most street/strip cams start to open between 40-60 degrees (@.050″) before bottom dead center (BDC).
Opening a little earlier or a little later is a compromise. Earlier means positive pressure is no longer working to push the piston down. At the same time, any positive pressure on top of the piston at BDC is costing you power because the piston is pushing against crank rotation.
By BDC, most of the residual pressure has already left the cylinder and the piston starts to push out whatever is left.
Near the end of the exhaust stroke, the piston is nearing top dead center (TDC). On a street/strip cam, the intake valve is starting to open and the exhaust hasn’t quite closed. This is called Overlap and it’s critical to get right.
Why is overlap so important? As the piston is coming up the bore, residual pressure takes the path of least resistance. At low speed/low throttle opening, intake manifold vacuum is high and high exhaust backpressure can flow backward into the intake—which is called reversion. Let’s look more closely at what’s going on in each port.
Overlap (from the Intake Perspective)
When we open the intake before TDC, a few things are happening.
- The Good: Opening the intake valve sooner won’t delay cylinder filling on the intake stroke. It’s least inhibited when we’re at WOT and running an open exhaust.
- The Bad:
- Any residual exhaust pressure backflows into the high intake manifold due to high vacuum at part throttle.
- Any burnt mixture from the cylinder goes back in the intake runner. Not only will it not burn again, but it’s taking up the space where the fresh incoming mixture should be going.
These are seen as lower idle vacuum and the engine runs rougher. Carbureted engines suffer more than injected engines because they rely on this pressure differential to flow fuel.
Overlap (from the Exhaust Perspective)
When we close the exhaust after TDC, there are a few things happening.
- The Good: At WOT and higher rpm, a well-tuned intake runner and exhaust primary length (along with a free-flowing exhaust) will flush out burnt gases.
- The Bad:
- If you are running a standard exhaust, you may have up to 8 lbs. of backpressure at WOT. Exhaust (that’s already past the exhaust valve) can reverse direction into the intake. Luckily, a free-flow exhaust often has less than 1 lb. of backpressure.
- Fresh air/fuel may be pulled out through the exhaust without being burned as a complete waste. This is what causes engines with a lot of cam overlap to smell of raw fuel.
Overlap (General Observations)
Overlap pulls in fresh mixture at high rpm, wide-open throttle (WOT) with a free-flowing exhaust, but hurts efficiency and driveability at low-rpm part throttle.
Here are a few more generalities:
- Engines with big cams show bigger gains with headers and a free-flowing exhaust than stock cams.
- At WOT and under boost, a supercharged engine is more tolerant of overlap because the increased intake manifold pressure behind the intake valve inhibits backflow. The reason you often see wider lobe separation is because intake closing can be delayed due to higher residual intake stroke pressure. This is a prime example where the valve event calculator should be used.
- A turbocharger is less tolerant of overlap because backpressure may be as high as 3:1. In other words, you may have 30 lbs. of backpressure between the exhaust port and the turbo for every 10 lbs. of boost. Older turbos with antiquated turbine side design are particularly bad. Newer turbos are better in this respect. When we close the exhaust valve before we open the intake valve, we won’t have backflow. This is seen as smaller duration exhaust lobes and wider lobe separation.
- Nitrous adds more exhaust volume and the valve needs to open longer to get rid of it.
Taking Advantage of Intake Manifold Ram Tuning and Inertia
Let’s talk about the period right after overlap. The intake valve is open and the piston is moving down the cylinder. Peak piston speed happens roughly 73 degrees after TDC and peak inlet airspeed is slightly behind that.
Even when the piston passes BDC, the downward rush of air/fuel coming in is still stronger than the upward push of the piston. On a typical street/strip cam, we take advantage of this velocity by closing the intake valve 40 to 50 degrees after bottom dead center. This packs and traps in more mixture.
Fun Fact: The volume of air/fuel mixture trapped in the cylinder is always the same whether the engine is at idle or 9,000 rpm WOT. At part throttle, it’s less dense. At WOT, it’s more. Check out this article explaining volumetric efficiency.
Effects of Closing the Intake Valve Earlier or Later on the Powerband
At lower engine speed and part throttle, the density of the mixture is low.
- To build compression, we must close the intake valve earlier. This traps as much mixture as possible, begins to compress it earlier and builds torque lower in the power band.
At high engine speed and WOT, we can advantage of the intake manifold runner length and cylinder head velocity.
- With more velocity, we trap the air a little later—which raises the powerband. This is when we see horsepower going up, but the overall peak-torque going down. It also allows a high compression ratio engine to run on lower octane fuel.
The valves are closed for most of the compression stroke and power stroke—until the exhaust valve begins to open again.
Being a degree off here or there is of little consequence.
Remember the order of importance of the events. The intake closing event is the most important to the powerband so you could degree the cam into that. If idle or piston to valve clearance are the most critical, you would set it by intake valve opening.
Want to see how a camshaft is made? Check out this factory tour.