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BMW Camshafts Explained

When you are dead set on improving the horsepower and torque output from your BMW, your first thought is often to throw parts and a tune at the engine and hope for massive gains. On Naturally Aspirated engines, the biggest improvements all come down to three principles: add volume, increase rpm, and improve engine timing for increased output. The methodology behind these changes is simple since more airflow at a higher velocity and cooler temperature allows for more fuel to be added, the engine can produce a bigger explosion that translates to more horsepower and torque. The restrictions of performance parts like a better flowing intake, a higher volume plenum, sport-tuned exhaust, and engine tuning are that they are at the mercy of the camshaft or camshafts. In this installment, we will explore how camshafts work, what it takes to increase the performance of your engine with different camshafts, and how it all ties together with timing and your Air/Fuel Ratio (AFR) for optimal results in your BMW.

First, let's take a closer look at the two basic layouts you will find in BMW engines. All modern passenger car BMW engines are either a Single Overhead Camshaft engine (SOHC) or a Dual Overhead Camshaft engine (DOHC). These designations refer to the number of camshafts per cylinder head. Engines with only one intake valve and one exhaust valve per cylinder, like the M20B25, have only one camshaft in their cylinder head. These engines are perfectly capable of benefiting from a higher performance camshaft but are limited in their fine-tuning for something called angle separation, which we will cover later. The DOHC engines have two camshafts per cylinder head, like in the S52B32, where there are more valves that require more cams then there is space for them on a single camshaft.

Your BMW will either be a SOHC or DOHC engine, but the principles of how a camshaft operates and their function in the engine remain consistent. Your camshaft is a spinning shaft that is driven by an external camshaft gear or sprocket by the timing belt or chain. The spinning shaft has egg-shaped lobes, called cams, that are clocked at specific angles. These cams rotate and use their pointed end to push the rocker arm up, which in turn pushes the valve to an open position to allow for the induction of air and fuel or the expulsion of exhaust gasses. In order to get a picture of how it works inside your engine, imagine slowing down the rotation of your engine to bullet-time.
As your rotating assembly spins, your pistons are driven up and down by their connecting rods for what is called their stroke. When your piston is at top dead center or TDC, the firing cycle is initiated. The piston begins to descend to the bottom of its stroke by way of the rotating assembly, which is timed with the camshaft to begin opening the intake valve. As the piston travels down, the valve is opened fully and begins to close as the piston returns to the top of the cylinder. The valve is fully closed when the piston reaches the top once more for the spark, combustion occurs, and the piston is driven down. As the piston reaches the bottom of its stroke, the exhaust cam is rotating to open the exhaust valve. The exhaust valve opens fully as the piston returns to TDC and expels the exhaust in time to close the valve and start all over again. This, of course, is a basic representation of how the firing sequence happens, however, there is no perfect world where each part of the sequence happens exclusively alone. In reality, an optimal balance between performance and comfort is achieved by allowing some overlap in the valves open positions.

The timing of your engine is an important part of camshaft maths. Without jumping into the nitty gritty equations, the basics are fairly straightforward. We know that the firing sequence is initiated when the piston reaches TDC, however, the spark plug actually needs to fire BEFORE this happens to allow the air/fuel mixture to ignite. Engine timing refers to where the spark is introduced in degrees BEFORE the piston reaches TDC. Ignition timing can be advanced (earlier in the stroke for a shorter stroke and larger volume of combustibles) or retarded (later in the stroke for greater reliability but less power) for different engine performance characteristics. Timing changes can provide maximum gains when coupled with a performance camshaft that allows the valves to be opened both longer and farther into the cylinder. The longer the valves are open, the more air and fuel can be introduced, and the bigger the explosion when advancing the timing.

So what makes a performance cam improve your engine torque and horsepower output? The fundamentals for camshaft optimization can be summarized by the three H's of performance: High duration, High lift, and High RPM. Since horsepower is just a figure of Torque x RPM divided by 5252, increasing the torque and RPM capabilities of your engine will greatly improve top end horsepower output. The whole idea with more aggressive cams is to do just that. If your cam lobes are both a longer duration and a higher lift, your valves will be open longer and extend farther into the cylinder for the best performance and optimal air/fuel delivery. While the differences on the low end will be minimal, the torque curve will be moved farther down the RPM range with greater output, and you will experience a smoother curve. That torque curve will continue for a greater duration with performance cams. In some cases, you can experience a loss of torque and horsepower at low RPM, but there's always a trade-off.

So is there a point that you can fit a camshaft that will outperform your engine? Yes. You can have a camshaft with too aggressive an angle of separation, which will cause too much valve overlap and will hurt your performance. Valve overlap occurs near the end of the exhaust stroke and it is defined as the amount of time the exhaust valve takes to close after the intake valve has begun to open. Valve overlap allows the incoming air/fuel charge to help expel the remaining exhaust gasses from the cylinder. Too much overlap will cause the engine's efficiency to drop as a result of too much crossflow between the exhaust and intake valves. This will happen if the angle between the clocked intake and exhaust cams is too acute. That angle is measured by imagining the 'egg' shape of one cam pointed straight up. The cam with which it is paired for that cylinder will be offset or pointed a slightly different direction. If you drew a line back to the camshaft to meet the line of the first cam down the centerline, the two lines intersect and form an angle that is referred to as your lobe angle of separation. A street cam will have, for example, a 110-degree angle for minimal valve overlap. An aggressive angle will be 108 or 105 in a high-performance race engine in this example and will have much more valve overlap. Were that angle to be any narrower, the valves would be open for too long simultaneously and combustible materials will be lost with the exhaust gas so performance will suffer. The notorious 'big cam' sound is actually more of a result of that valve overlap than it is anything else. The more aggressively narrow the angle of separation, the more 'lobey' the engine will sound and the rougher it will idle. However, this narrow-angle separation does allow for the greatest increase in engine RPM as it allows for the most air to be introduced at the top end. Normally, your engine power drops as it is not able to deliver enough air to meet the fuel demand at high RPM. With the intake valve open farther and longer, more air is delivered and engine RPM can be increased without dropping horsepower as quickly.

BMW Schrick Performance Camshafts

Here at Turner Motorsport, we stand behind the capable and dedicated performance engine building folks at Schrick. Their camshafts and engine parts can be found in F1 winning engines all the way back to the 1970's and that technology persists today in their performance camshafts for road vehicles. The manufacturing process of their camshafts is consistent with their highest performance F1 camshafts all the way to their OEM replacements. They apply the same processes, technology, design tools, and even use the same engineers when producing camshafts for the general public.

Schrick Performance Camshafts are designed for your BMW to achieve the desired duration, valve drop (lift), and to generate higher RPM that is within the capabilities of each engine for which they are developed. For optimal performance in your BMW without a custom set of cams (which requires a mind-boggling amount of math for us non-engineers), Schrick's off the shelf performance camshafts and OEM replacements are your best choice.



BMW VANOS Sytems

We also encourage those with the confidence and experience under their belts to perform their cylinder head service, VANOS service, and camshaft replacement at home. So what is your VANOS? The modern BMW VANOS engines use either a single or dual unit which contains a bearing, race, and oil pressure operated valve of sorts. Oil pressure engages the unit and adjusts the engine's timing by slightly advancing it at high RPM for the best power output when the VANOS unit is engaged. A VANOS unit allows the engine to run a milder engine timing under normal driving conditions, but an aggressive one when the throttle is wide open or the engine is running at high RPM. VANOS failure occurs when the seals fail, which causes oil delivery to be reduced, and the bearing and race to wear. This will cause play in the assembly and the unit will either fail open, or in mild timing, or closed aggressive timing. This will cause you to either experience massive torque losses, fuel economy loss, and/or the notorious 'marbles in a tin can' sound from your engine.

Speaking of cylinder head service and VANOS service, you will need to consider these aspects when upgrading camshafts or are considering maintenance service on the cylinder head. In order to increase engine RPM, cam size and angles, horsepower, and torque, you will want to make sure your valve springs are the proper rating and that your valvetrain is properly timed. Once you have replaced your camshaft or shafts and interchanged your valve springs for performance versions capable of providing the desired rate of return to the valve seat, you can adjust your valve timing, or the gap between the cam lobe and lifter, to specification. When you increase the size of the cam lobe, your gapping, or valve timing, will need to be adjusted within the proper distance specification. A feeler gauge to individually measure these gaps is the only way to perform this service.

For the VANOS, we encourage you to fully service and replace the internal components when performing these modifications for two reasons. Primarily, that an older VANOS will be prone to wearing more quickly and failing sooner under harder operating conditions. Secondarily, in order to take full advantage of your BMW's variable valve timing for peak performance, your VANOS needs to be functioning properly. In older BMWs, the single or dual VANOS units which have not been serviced will surely need to be refreshed either way.

Our VANOS repair kit, or fully refurbished/new units, easily solve this issue and contain an anti-rattle spacer to protect against a future failure. The Schwaben BMW camshaft locking tool gives you the confidence to safely perform these services without risking damage to valves or the camshaft/cams themselves. This tool is exactly like the ones used by BMW to perform these services and puts the ability to do it yourself at exceptional cost savings right in your own garage. Our VANOS repair kit, paired with the Schwaben Camshaft Locking Tool, allowing you to perform this service easily at home while you are upgrading your engine's camshaft.


Takeaways

With your camshaft upgrade, newly refreshed VANOS, and proper timing, you can achieve massive improvements in Torque and Horsepower output from your BMW. Finding everything you need to do this at home is easy with everything in one place here at Turner Motorsport.


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