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Supercharger Tuning Through Cam Selection and Cam Timing
Camshaft tuning is an essential part of supercharger tuning. Camshafts orchestrate the valve opening and closing events in the engine and decide whether what comes out of our motor is beautiful high power music, or a mess of dysphonics.
The use of the proper supercharger optimized cam shaft can go a long way towards supercharger tuning and give considerable power gains for the money invested.
To understand camshaft timing and camshaft selection we have to understand first:
Relativity: Changing when the valves open or close (intake or exhaust) changes the the valve timing with respect to:
- The piston position inside the cylinder. Depending on where the pistons is in the stroke, and where we are in the combustion cycle, then opening the valves will exploit the pressure difference between the cylinder and the intake and exhaust manifolds.For example it would make sense that the ideal time to open the intake valve is when there is peak vacuum inside the cylinder so that when the valve opens, the maximum amount of fresh air can be ingested. Similarly, it makes sense not to open the exhaust valve until peak cylinder pressures have been achieved inside the combustion chamber and the combustion is complete and all the power is extracted.
- The high and low pressure pulses created by the design and runner lengths of the intake and exhaust manifolds.It would make sense to open the intake valve just as the reflected pressure waves in the intake manifold reach the intake valve as a high pressure portion of the wave, thus opening the valve at this high pressure point gives a ‘ram air’ effect through volumetric efficiency resonance tuning increasing air ingestion which increases power.Similarly on the exhaust side, it makes sense to open the exhaust valve, just as the reflected low pressure (vacuum) portion of the exhaust wave (reflected back from the collector) reaches the back of the exhaust valve. At this point in time there is both peak pressure inside the cylinder, and vacuum in the exhaust which creates a higher pressure differencial and a faster evacuating exhaust gas.
- With respect to the ignition timing event, for example a shorter duration or advanced exhaust cam, opens the exhaust valve sooner with respect to when the mixture was originally ignited, this means that although by advancing the exhaust cam we may have matched our header design and opened the valve with the lowest possible exhaust back pressure for best efficiency, at the same time, we have reduced the amount of time that the mixture is combusted and possibly opened the valve before reaching our peak cylinder pressures and thrown away some horsepower.
- The intake valves with respect to the exhaust valves: and this is usually described in terms of lobe separation angles (the offset in degrees between the center of the exhaust cam and between the center of the intake cam), or in terms of how many degrees of overlap (the number of degrees that both intake and exhaust valves are open at the same time).
Since the combustion inside the cylinder occurs at a much higher pressure than atmospheric pressure, and since exhaust valves are usually smaller than intake valves (for this same high pressure reason) then exhaust gas velocity is much higher than intake gas velocity. So, in some engines it is beneficial to open the intake valve earlier than usual during the last part of the exhaust stroke, this is called overlap. During overlap – at the very end of the exhaust stroke – the amount of pressure left in the cylinder is low so it is possible to breathe in new air under atmospheric pressure, at the same time, the high velocity of the exhaust gasses exiting help draw in even more fresh air from the intake side in an effect much like ‘syphoning’ where the fluid (in our case air) flows as a continuous stream drawing in new intake air after the old exhaust gas leaves.
The other part of phenomenon that relates to timing intake valves with respect to exhaust valves is the duration of time where both valves are absolutely closed, which is your power stroke. This is the part of the combustion cycle where the mixture can be compressed and combusted. If either (or both) intake or exhaust valves are open you will not be able to neither compress nor combust the mixture, and the absolute duration of time (in degrees of rotation) that your mixture is combusted and allowed to reach peak cylinder pressures is affected by camshaft selection and cam timing. One thing to note is that the valve angle has a lot to do with exhaust scavenging, obviously you will get maximum scavanging if the exhaust and intake valves had ‘line of sight’ i.e. if the valves were separated by an angle of 180*. If so, the exhaust air can directly pull in new air. Conversely, you would have the least possible scavenging if you had valves that were at a narrow angle (zero degrees at the extreme) between each other, so that the air would essentially have to make a U turn to come in through the intake and get pulled out the exhaust.
So different motors respond differently to overlap depending on the exhaust back pressure and the valve angle.
Cam duration is the number of degrees of the entire 360* rotation that the intake or exhaust valve is open. The longer the duration, the more air you can get into the motor, the more overlap you have (which helps more with higher rpm power performance), the shorter your power stroke is (which reduces your combustion duration and your peak cylinder pressures reducing low rpm fuel efficiencly and clean idle….etc
Increased duration (with it’s increased overlap and scavenging) also gives the opportunity for exhaust gasses to get to the intake, or intake gasses to leak to the exhaust, and so are more sensitive to proper timing events otherwise we can get some negative effects from being ‘overcammed’
Lift is how far or how deep the valve opens into the cylinder. The more lift you have, the less the valve is a restriction to incoming air because it is farther away from the direct path of entering or exiting air. Adding lift in general adds power to all rpms, depending on how well the camshaft (and valve train) can accelerate the valve to a higher lift number in a short duration. It’s like a ramp, the shorter the duration and higher the lift, the steeper the ramp. So what happens here is that if your valve train isn’t light enough and well controlled (Through proper valve springs or hydraulic lifting and damping) to operate that rapidly then lift will give you improved performance at lower rpms (where there is a lot of time to move the valve to peak lift) but reduced performance at higher rpms, where there are more rounds per minute and so less time per round, and thus less time to go up the steep ramp and push the valve out to full extension.
Lift is good, but usually people don’t try to radically increase lift on their aftermarket cams because of a few considerations:
- Make sure that at this new lift, that there is still enough clearance between the valve (at full extension) and the cylinder (at top dead center) to prevent any catastrophic mechanical failure.
- Upgrade to lighter valve train, with stiffer springs or dual valve springs to have more control over the valve with this steeper cam profile.
- It does add power but it doesn’t shift the power curve up or down as radically as changing cam duration does, and so in most aftermarket applications we really want a cam to give us peak power at a certain rpm range and so we care much more about the best duration (and some added lift).
I know this is a somewhat complex topic, but I need to make sure we’re speaking the same language before we go into how this relates to superchargers. Before you decide which camshaft to use (or how to adjust the timing on your stock cams) you have to look at one very important thing:
Your exhaust system and exhaust back pressure:
If you have a stock log type exhaust manifold, with a close coupled cat, with a dual cat exhaust system, small exhaust tubing, and a couple of restrictive mufflers on your car then it is possible at peak power to have up to 10psi of back pressure.
If this is the case, my first recommendation would be to upgrade to a high flow, low pressure exhaust system because of the potential power gains; however, I do know that some of our readers have cars that they are setting up for their parents or for dual use where their partner or the laws in their location …etc are really strict when it comes to any added exhaust noise or any aftermarket exhaust. In this case, where exhaust upgrades are not an option, then you must select your camshafts, and tune your cam timing to where you have ABSOLUTELY the minimum possible amount of overlap. If you have significant overlap, then the more you rise above about 4500 rpms the more your supercharger will suffer and the more power you will waste. If the supercharger is geared to 7psi of boost for example, then during overlap, the cylinder sees 7psi of boost on the intake side, and 10psi of back pressure on the exhaust side, the net result is that air will flow from the high pressure zone (the exhaust) to the lower pressure zone (the intake) and so your cylinder will start to fill with exhaust gases. As the rotation continues, the exhaust valve will close and overlap will end, and the intake valve will stay open for the remainder of the intake stroke (for the rest of the duration of your intake cam), and the rest of the cylinder will fill with fresh air.
What happens here is that we get a cylinder that filled for 30* of overlap with exhaust air, and then filled for another 210* (of the original 240* of duration for a typical street cam) with fresh air. The result is a cylinder that is only 85% filled with fresh air or an engine that is literally 15% smaller in displacement! On the other hand, if our supercharger is geared for 18psi for example, then during overlap we will have 18psi on the intake side and our exhaust back pressure of 10psi on the exhaust side, the net result of this overlap is that our supercharger is effectively only producing 8psi worth of differential pressure between the intake and the cylinder and so we are only going to get a power boost of 8psi during overlap. So, during those 30* of overlap the supercharger is only effectively producing 8psi of boost, and after that once the exhaust valve closes, the supercharger will be able to go back to operating at full boost for the other 210*. The net result is something like 16psi of boost so 2psi (or about 12%) of our power was wasted.
Supercharger tuning through cam selection and cam timing
Because of the negative effects of overlap on a supercharger car’s performance, and especially in the case of high exhaust back pressure as is the case with most factory supercharged cars, we find that the optimal cam duration for the intake cam is typically 30-40* of duration less than a normally aspirated camshaft for the same peak power RPM. The decision to reduce the intake cam duration rather than split the duration reduction between the intake and exhaust cams, is that the intake cam will flow air under pressurized conditions (due to the addition of the supercharger and the increase in intake manifold pressure) and so at a reduced intake cam duration the engine will still be able to get it’s full share of intake air. At the same time, the high rpm efficiency improvement from the reduction of overlap will also boost power production with a more conservative cam. Finally, if we would like to get more flow from the intake cam, there is still the option of using a higher lift camshaft (with a steeper profile due to the decreased duration) with supporting valve train modifications to make sure valve float doesn’t occur at higher rpms.
Intake cam timing:
The cam timing for the intake cam would ideally be retarded which would move the intake cam opening event farther away from the exhaust valve closing event so as to reduce or eliminate overlap, and as a side effect the power stroke duration will increase by retarding the intake cam which can also compensate for the lost power from the duration reduction.
The exhaust cam duration and lift for a supercharged version of the motor should be similar to a nitrous camshaft, in the sense that the exhaust cams on nitrous specific builds have:
1- Very healthy cam duration & very healthy cam lift to allow a severely elevated amount of exhaust gases to be able to efficiently exit the motor when the nitrous is activated and the horsepower (and thus the exhaust gasses) have both doubled in quantity.
2- As little or no overlap if possible, as any overlap would mean that nitrous would be sprayed from the intake side and out the exhaust, which is wasteful of our limited supply of nitrous. Similarly the more overlap we have, the harder the supercharger will have to work because of what we explained earlier about either exhaust reversion into the intake, or the supercharger pressurizing the exhaust.
Exhaust cam timing:
Advancing the exhaust cam both opens and closes the exhaust valves sooner. Opening the exhaust valve sooner slightly reduces the power stroke, but at the same time it reduces overlap and makes better use of our supercharger. Typically an an advanced exhaust cam combined with retarded intake cam will provide the best results on a supercharged car, especially with a restrictive exhaust.
If we had a high flow exhaust system installed, then it may not be beneficial to advance the exhaust cam, a high flow exaust system that is optimized for our engine’s power requirements can clear the combustion chamber of all it’s gasses very efficiently. Having a high duration exhaust cam, a low back pressure exhaust system and a no overlap what so ever camshaft means that we are giving the exhaust gas plenty of time to exit they cylinder, the intake valve still hasn’t opened (because the we have decide to retard it, or use a conservative cam with less duration) and so the supercharger is not pushing any new fresh air in yet, now the cylinder is void and so some of the exhaust gas can revert back into the cylinder, then the exhaust valve will close, and then the intake valve will open only to find the cylinder already partially filled with exhaust gases.
This isn’t a problem with a restrictive exhaust because a restrictive exhaust will take some time to clear the cylinder at a lower velocity, however with a higher flow exhaust system we must be careful not to dial out ALL of the overlap in the cam timing, or to over-cam the exhaust cam (using too much duration).
So exhaust cam timing can be advanced or retarded, depending on the exhaust modifications and the intake cam selection and thus must be dyno-tuned.
It’s important to note that with all of these changes in cam selection , overlap, power stroke duration, and cam timing, that the power stroke duration is effected and if it is effectively shortened then we may need to retune the car’s timing advance on the dyno (for increased advance) to regain losses in duration of the power stroke (again this against popular thinking of never to advance timing on forced induction cars, if we have a shortened power stroke, or an application with significant overlap then it may be necessary to do so).
So we see here that the end result here a lop-sided camshaft with a conservative duration, high lift cam on the intake side, and a normal duration, high lift cam on the exhaust with minimal lobe separation angle and minimal (but not necessarily no) overlap.
The exception to the rule:
Sometimes people take a car that starts off with a 9000 rpm redline, has an 11.5:1 compression ratio, and a 280* duration camshaft, and an aggressive naturally aspirated-esque timing curve and decide to supercharge it for more power. One suck example is kleemann’s kompressor for the SLK55 AMG (which already makes 400 hp in normally aspirated form from an 11:1 compression ratio motor). In this type of application, if you use a more conservative cam, and dial out all the overlap, and increase the power stroke, in combination with an already high 11:1 compression ratio and a healthy amount of boost pressure (7psi or above) you will end up with a motor that produces extremely high peak cylinder pressures and those intense pressures and heat may easily start off a chain reaction of pre-ignition and detonation and you will find that no matter how much you retard the timing that the setup will end up both powerless and still not that safe.
In this case, I would consider RPM and compression my primary power adder, and my supercharger as my secondary power adder (that is unless I decided to change that and went ahead and lowered the compression ratio of the motor). In this case it is OK to sacrifice some supercharger high rpm efficiency for preventing high-load & low-rpm detonation. Furthermore, to overcome the overlap inherent in this kind of high rpm normally aspirated power-plant it would be very advisable to use a centrifugal supercharger that is capable of producing more boost and flow with increased rpm rather than a roots type charger that will easily run out of boost and flow capacity (CFM) when facing an aggressive camshaft ‘leaking’ boost away.
Here is a great example of how cam tuning can affect supercharged power:
The car is a 1.8 liter Honda motor equipped with:
- Supercharger optimized big primaries and short runners Kamakazi header
- A greddy 2.5? SP2 catback exhaust system.
- An LHT ported “S” supercharger inlet tube
- An LHT ported intake manifold ( Non intercooled)
- A Carbon fibre intake
- A Jackson racing eaton M62 supercharger geared for 7.5-8 psi.
The black line is the baseline run with all of these modifications before tuning with peak power coming in at: 223 wheel hp @ 7600 rpms.
The blue line is the power achieved after a full tine (camshaft timing redone for reduced overlap, ignition timing re-optimized, and air fuel ratio optimized for peak power), with peak power coming in at 248 hp @ 8400 rpms.
You can see on by the dyno results that by reducing overlap and properly tuning the car the power peak not only increased by 25 horsepower, but more importantly shifted up by 1000 RPM’s due to increased supercharger high rpm efficiency from reduced overlap.
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#Supercharger #Tuning #Cam #Selection #Cam #Timing