For either two or four valve per cylinder heads the canted valve arraignment where the valve stems stick out to the sides at about 30 to 45 degrees tends to flow best. This canted valve arraignment is fairly universal on four valve per cylinder heads, and is only sometimes used on two valve per cylinder heads. There are three reasons that the canted valve arraignment flows better, and one of these is simply that the intake air does not have to turn as sharply as it enters the engine. The other advantages of canted valves have to do with maximum attainable valve sizes and shrouding of the valves by the cylinder bore.
Two Valves per Cylinder
Canted Valve Advantages
Why Are the Intakes Bigger Than the Exhausts?
Valve Angle Trade Offs
Canted Valve Small Engines
Generally two valve per cylinder engines need a whole lot of valve lift, the obvious reason for this is that the single intake valve has to be a whole lot bigger than the size of each of the two intake valves on a four valve per cylinder engine. The reality though is that two valve per cylinder engines in fact require disproportionately more lift to flow as well as they possibly can. A four inch bore four valve per cylinder engine would usually have about 1.6 inch diameter intake valves and a four inch bore two valve per cylinder engine would usually have about two inch diameter intake valves. Obviously the two inch intake valves are going to require at least 25% more valve lift than the 1.6 inch intake valves, this is just a simple proportionality. The reality though is that the two valve engine will benefit from even more than 25% more valve lift than the four valve engine.
The reason that a two valve engine benefits from a whole lot more valve lift is because of the different ratios of valve head area versus valve head circumference on a two valve engine versus a four valve engine. The two inch diameter intake valve on the four inch bore two valve engine has a head area of 3.1 square inches and a circumference of 6.3 inches. The two 1.6 inch diameter intake valves on the four inch bore four valve per cylinder engine have a total head area of 4.0 square inches and a total circumference of 10.0 inches. That is 28% more head area but a whopping 68% longer total circumference. The basic consequence of this is that four valve per cylinder engines flow a whole lot better with smaller amounts of valve lift. A two valve per cylinder engine has the potential to flow almost 80% as well as a four valve per cylinder engine, but in order to do this the valves must lift extremely high so that it is the valve head area that dictates the flow.
Because valves lift gradually it turns out that even with quite large and aggressive camshafts the total circumference ends up being very significant for how well an engine flows at high engine speed. Four valve per cylinder engines always flow a whole lot better than two valve per cylinder engines, and getting a two valve per cylinder engine to work as well as it can requires a very large and aggressive camshaft with lots of lift, lots of duration and the fastest opening and closing of the valves that can be attained.
With a really big camshaft that pushes the valves up far away from the seats and holds them there for lots of degrees of crankshaft rotation it is mostly just the size of the valves that is important. From this perspective canted valve engines are a bit better just because there is a more room for the valves. With the valves canted 40 degrees there is room for 30 percent larger head diameter valves than for a parallel valve engine. Those 30% larger valves have a For most traditional pushrod engines though the camshaft was not nearly big enough or aggressive enough to get the two valve per cylinder engine to flow anywhere near as well as it could. With these milder camshafts the circumference of the valves is of critical importance, and any shrouding of the area that the valve opens into is a large hindrance to performance.
With a really big camshaft that pushes the valves up far away from the seats and holds them there for lots of degrees of crankshaft rotation it is mostly just the size of the valves that is important. From this perspective canted valve engines are better just because there is a more room for the valves. With the valves canted 40 degrees there is room for 30 percent larger head diameter valves than for a parallel valve engine. Those 30% larger valves have nearly a 70% larger head area For most traditional pushrod engines though the camshaft was not nearly big enough or aggressive enough to get the two valve per cylinder engine to flow anywhere near as well as it could. With these milder camshafts the circumference of the valves is of critical importance, and any shrouding of the area that the valve opens into is a large hindrance to performance.
A significant problem with parallel valve engines is that when the largest possible size valves are used the valves end up very close to the edge of the cylinder bore, and no matter how far the valves open the valve heads don’t get any farther away from the cylinder bores. Having the valves open close to tangent to the cylinder bore effectively reduces the circumference of the valve by nearly one quarter. With the canted valves there is not only more room for larger valves, but even if the valves go all the way out to the edge of the cylinder the valves do open up away from that cylinder wall alleviating the problem of the effective circumference being reduced.
With the biggest camshafts canted valve engines are an advantage because bigger valves will fit and the intake air does not have to turn as sharply. With milder camshafts the bigger canted valves are still a huge advantage, but the canted valves opening away from the cylinder wall becomes very important as well. What it comes down to is that canted valve engines always flow better, but the advantage to canted valves is even more dramatic when a mild camshaft is used.
In most engines the intake valves are significantly larger than the exhaust valves. This tends to seem backwards because there is so much larger a volume of hot exhaust flowing out of the engine than there is intake air flowing in. The explanation for this is that making power on a normally aspirated engine is all about getting more air into the cylinders. Any vacuum in the cylinders on the intake stroke reduces the power potential of the engine. The exhaust on the other hand is forced out under pressure as the piston moves up. As the exhaust valves close near top dead center most of the exhaust has already been forced out and the pressure in the cylinder easily drops off to nearly nothing.
A big reason that the intake valves are so critical on a normally aspirated engine is the fact that the valves open and close gradually. As the piston begins to move down there is a delay in the movement of intake air beginning. This delay causes the bulk of the flow of intake air to take place late in the intake stroke as the piston slows towards bottom dead center. With the intake valves already beginning to close as the bulk of the intake air flows in any extra size possible on the intake valves tends to be a huge advantage on normally aspirated engines. The slower the intake valves close the bigger they need to be. Somewhat ironically bigger intake valves are heavier and tend to require even more slowly opening and closing camshaft lobes. An aggressive fast opening and fast closing roller camshaft with stiff valve springs is the usual solution to this conundrum on a two valve per cylinder engine. Four valves per cylinder alleviates the intake flow problems for two different reasons. The two smaller intake valves are lighter and can run on faster opening camshaft lobes up to higher engine speeds with less spring pressure. The much larger circumference of the two intake valves also means that flow is better late in the intake event as the intake valves are already closing.
Carbureted and port injection engines have to get the fuel through the intake valves as well as the intake air, and this also tends to make bigger intake valves important. The much larger intake valves than exhaust valves found on most engines are inevitable for high performance normally aspirated engines, but mild camshafts and carburetion tend to accentuate this trend.
On a forced induction engine this plays out differently. With the intake air being forced in under pressure there is less need for oversized intake valves, while the larger amount of fuel and air burned tends to require larger exhaust valves. Forced induction engines would tend to work well with equal sized intake and exhaust valves, and very high output forced induction engines might actually be able to attain higher efficiency under a full load with larger exhaust valves than intake valves.
The same advantages of canted valves apply to a four valve per cylinder engine, but they play out somewhat differently. Canting the intake valves away from the exhaust valves on a four valve per cylinder engine reduces shrouding of the valves by the cylinder bores, but only up to a certain moderate valve angle. Canting the four valves more than this actually causes worse valve shrouding problems. Canting the valves on a four valve per cylinder engine does allow larger valves to be used, but not for exactly the same reason as on a two valve per cylinder engine. Just about as big of valves will actually fit on a parallel valve four valve per cylinder engine, but they can be made to flow much better by canting the intake and exhaust valves away from each other. Four valve per cylinder engines tend to work best with the valves canted at smaller angles than two valve per cylinder engines, but four valve per cylinder engines are sometimes built with quite radically canted valves. Even though less advantage is to be had by canted the valves on a four valve per cylinder engine the huge advantages of four valves per cylinder still means that they can do better than the best canted valve two valve per cylinder engines.
The more the valves are canted away from each other the better a two valve per cylinder engine can flow at high engine speeds. The limit to how far the valves can be canted away from each other though has to do with the shape of the piston and the compression ratio of the engine. With the valves canted by 40 degrees there is little difficulty getting the compression ratio up to 10 or 11:1 using domed pistons. A slight upward dome to the crown of pistons increases their strength under a heavy pressure load and actually allows them to be lighter than flattop pistons. If the valves are canted farther away from each other though then an 11:1 compression ratio requires not just domed pistons, but pistons that have a wedge shape to them. This wedge shape is not a particularly good shape for the piston crown, and it tends to make the pistons heavier. More radically oversquare canted valve engines have an even worse problem with requiring extremely tall wedge shaped pistons, and higher compression ratios of course also accentuate this problem.
Just what the ideal valve angle would be on a canted valve engine depends on the fuel that is to be run. A fuel that requires a higher compression ratio means that the pistons would become heavy with large amounts of angle between the valves. A fuel that can run with a minimum of spark lead down at a 10 or 11:1 compression ratio though can use a larger angle between the valves which means larger valves can be used go get better flow at higher engine speeds. Interestingly though the lower compression ratio never ends up being any real advantage because higher compression ratio engines inherently flow better by virtue of more of the hot exhaust gas being forced out before the intake stroke begins. One final note is that very powerful exhaust gas scavenging tends to remove the flow advantage of higher compression ratio engines. Still though, in practice, normally aspirated OHV engines always flow better over a wide range of engine speeds with a higher compression ratio. Trade off versus trade off versus trade off. Another bit of information of the subject is the fact that the highest flame front travel speed fuels which work best for getting any gasoline engine to run better over a wider range of engine speeds and loads tend also be the lower pressure fuels that use lower compression ratios. Another point for more heavily canted valves. The flip side of this though is that it is the higher pressure and lower flame front travel speed fuels that tend to be available in the largest quantities at lower prices.
A special case where extremely large valve stem angles would be a big advantage regardless of what fuel was to be run is for a very small radically undersquare engine with a very low displacement per cylinder. The main reason that a smaller displacement per cylinder is desirable is to keep the cylinder count up at six or eight for good transmission efficiency while also keeping the total engine displacement low enough to support light loads in applications requiring small amounts of power. This very small engine tends to have a quite short stroke and is not run with extremely high piston speeds. The point with the very small and radically undersquare engine is not to get the most power at the highest piston speeds possible, but rather the goal is the highest possible efficiency at a reasonable engine speed. For a gasoline engine this reasonable engine speed is 4,000 to 8,000RPM and mostly above 6,000RPM where gasoline engines run best. Getting the efficiency to be high at 8,000RPM requires a good flowing engine so that pumping losses do not get out of hand. And if the goal is a small displacement per cylinder then the bore size is going to be small making it of paramount importance to get the largest valves possible to fit. Since this smallest gasoline engine has a short stroke, probably about one and a half to two inches, the piston speed is not all that high even up to 9,000 or 10,000RPM. This reasonably slow piston speed means that the weight of the pistons is not of such huge concern as is the case with a longer stroke engine that needs also to attain 6,000 to 8,000RPM to run well. It is not that heavier pistons are desirable, not at all. Any engine benefits from lighter pistons for good light load performance at higher engine speeds. Rather it is just a case where getting this very small engine to flow as well as possible would be worth the slightly heavier wedge shaped pistons required for a high compression ratio on an engine with extremely canted valves. An under square engine also pushes more intake air up into the small head, meaning that higher compression ratios can be obtained on a canted valve engine just with slighlty domed pistons which are not heavier.
The same rational applies for very small diesel engines as well. Diesel engines generally operate at much slower speeds, but getting a diesel engine to be as small as possible certainly does run into the same problems of a radically undersquare configuration with tiny valves that have trouble flowing well. The smallest efficient diesel engine has a much longer three to four inch stroke and operates at about 1,500 to 4,000RPM but getting the displacement per cylinder down does require very small bore diameters that leaves little room for valves. The much higher compression ratios of 16 or even 20:1 that diesel engines use tend to make radically canted valves even less appealing but being able to get the bore size down is of great importantce for a small engine. Again if the goal is the smallest possible efficient power output per cylinder then the piston speeds tend to be on the low side and a bit heavier wedge shaped pistons can be tolerated. Since a very small diesel engine might use just a two inch bore on a four inch stroke a whole lot more intake air is pushed up into the head. This means that quite high 16 or 18:1 compression ratios could be attained on a canted valve engine with flattop or slighlty domed pistons.
Going up to a 45 degree valve angle means that 41% larger diameter valves could be used compared to a parallel valve engine. These 141% diameter valves would have a head area nearly two times larger, a very substantial boost to high speed performance for a small radically undersquare engine. Of course this 50 degree valve angle would not be used on a four valve per cylinder engine, but a radically undersquare 45 degree two valve engine might be made to work quite well with rapidly opening valves on a roller camshaft. The circumference of these big 141% valves is still not quite up to what can be obtained with four valves per cylinder, but the difference is no longer so dramatic.