Originally Posted by birkey
i was told that an oversquare motor that was meant to run at high rpm would be better off with a 2 valve head as opposed to a 3 or 4. i never really delt with many DOHC motors especially for a somewhat high-performance build, and didnt quite understand this. He said that the way the air flowed over the valves that the piston speed would be reduced. Any experienced cylinder head people out there, looking for some input from actual builders or very knowledgable people.
Piston speed is not dependent upon how the valves are operated. Piston speed as a ratio of RPM is lower with a short stroke crankshaft (over square). It is higher with a long stroke crankshaft (under square); the bore in either case being adjusted from bigger than the stoke to smaller to maintain a given total displacement. In any and all cases, the piston speed changes based upon the crankshaft's position in degrees of rotation. The piston speed is also sensitive to the length of the connecting rod from the throw to the pin centers. The crankshaft and rod open and close a set of lines that go from straight through the pin, rod and crank throw when the crank is at the top and bottom of its stroke to these same parts opening a set of triangles starting from the top an Acute that arrives at a Right triangle when the crank is half way down its stroke. From there it becomes an Obtuse triangle as it recloses toward a straight line at the bottom of the stroke. This sequence is reversed when the piston is rising from the bottom. There are fairly complex mathematics for computing the piston's position and velocity based upon degrees of crankshaft rotation and the length of stroke and rod centers. Generally in reading the piston speed is given as an average in feet per second, but it varies wildly from that in the specific locations between. This same math becomes the basis of calculating the performance difference between long and short rods and is a major part of the calculation of the Dynamic Compression Ratio (DCR) which is used to adjust the compression ratio for the point where the intake valve closes in crankshaft degrees which makes the stroke appear shorter to the engine than it physically measures.
Generally a short stroke, large bore engine will rev higher, often much higher than an equivalently sized long stroke engine. The problem of higher revs is finding the time to fill the cylinders. Since time becomes so critical, the valve movement must become more precise to use all the duration (which is really a time function) available as quickly as possible. Along with that is to use as much port and valve area and volume as possible in that period of time. To do that efficiently and effectively is a difficult problem for the cam in block, push rod operated valve train. The masses of the lifter, push rod, rocker along with their tendency to deflect and bend under the acceleration rates and forces make it difficult to support an ultra high speed engine. Although the sprint and cup cars have pushed that upper end a long way in my life time. There was a time not so long ago that 6000 RPM NASCAR engines were at the edge of push rod valve train technology. Today that's moving beyond 10,000 RPM.
However historically, the simpler approach to breathing at high RPMs has been with overhead cams and multiple smaller sized valves. The Single Over Head Cam (SOHC) can operate valves in a row with direct action of the lobe running a cam follower that acts directly on the valve stem. This reduces the masses and part deflections of a cam in block but makes a taller in line engine and a bit wider with a more complicated cam drive system in a Vee engine. The SOHC can be used to operate multiple valves of a Hemi or Pent chambered head but now a rocker arm has to be introduced which increases masses and deflections again, not as bad as with push rods but it does become a high rev limiting factor and is a fairly expensive solution without quite enough benefit.
The DOHC design allows the direct valve actuation of a cam follower acting between the valve and lobe for an engine with multiple valves in a Hemi or Pent shaped combustion chamber. For extreme RPMs this buys the precision and amounts of lift needed in very short time periods to support running at those RPMs and having some power to show for it. The price to be paid beyond the fact these tend to be pricey designs is width, this isn't too bad with an inline, but it sure makes Vee engines very wide.
Motorcycle engines are more akin to Formula 1 car engines in that they need to be small and light, they don't need a lot of torque as the vehicle is also quite light but they need a lot of top end horsepower for speed and acceleration. Overhead cam engines in this application are highly effective when geared properly.
For full sized cars and trucks a lot of torque is needed to get the substantial weight moving, and while some driving may be moderately fast most isn't so high power at high revs isn't needed but space and cost is usually a premium so a cam in block operating the valves with pushrods and rocker arms is the less costly more space efficient solution. One could make a good argument that the Ford modular engine with multiple valves and overhead cams is largely wasted engineering that serves no purpose between home and grocery store. The Europeans went that route to try and extract adequate power from small engines that were taxed based upon their displacement. So the way they went to minimize the tax impact was to make small high revving engines. I rather think the Japanese and other Asian imports are impacted by that as well. While all the imports now suffer from overhead cams of some sort mostly as an advertising claim that makes it look like they do unusually great engineering compared to Detroit's offerings.
When it comes to the latest generation of engines from Detroit I'm impressed with everybody's offerings for vastly different reasons:
- The GM LS is an amazing clean sheet engineering effort. The cam in block is very cost, space and weight efficient and is a proper level of engineering consistent with its end use so the consumer isn't paying for rocket science they have no way of using. The bottom end is excellently done, for once with aluminum; the engineers showed an understanding of the material instead of treating like cast iron which when done in the past had proven to be a failure for everybody so many times. GM showed us what can be done when you understand the technologies involved, the market target, and have an unlimited budget.
- The folks at Ford with the modular's history of 2 valve, 3 valve, 4 valve and back again show just how schizophrenia Ford can be. The overhead cams are a waste, in my opinion, as are the multiple valves as it makes an expensive, wide motor that's hard to fit in smaller chassis with more design level Rocket Science than is needed by the application. Don't get me wrong it's a nice motor but getting it to where it is today took a lot of pain then finally my old buddy Alan Mullay to get their engineering act together. Ford showed us what happens with management that didn't function either technically or fiscally.
- Chrysler; what can you say? They've been running on grandma's make do budget for so long that if nothing else they have learned to make a sows ear into a silk purse. The gestation of the Magnum from the herky-jerky LA block is a sight to behold. Again a cam in block application aimed right at its market place, a space and cost efficient design. Chrysler showed us how you can take an almost good, but never quite right design and with a moderate investment correct the deficiencies to make it into the outstanding product it should have been.
Sorry for the length but this stuff doesn't lend itself to "sound bite" answers and this barely scratches the surface.