vertical scale is in thousandths of an inch, so 50 thou is the lower line... I have both metric and imperial versions to keep everyone happy
While I'm here I'll give comment on the whole "measured at 50 thou standard" that has crept into the cam terminology for our engines.
It's a totally meaningless spec.
it's is the industry standard in a market of pushrod V8 engines, where it actually means something. In a pushrod actuated valve train, there are MULTIPLE points of lash, and that method of valve actuation requires a slower ramp to gradually close up all these points of working clearances. i.e. cam lobe to lifter, lifter to pushrod, pushrod to rocker, clearance and play in the rocker assembly, clearance between the rocker and the valve stem...most of these are not just "pushing" they are also "pivoting" at the same time.
plus it give some meaning when trying to compare lobes designed for a regular hydraulic tappet, a solid tappet and a roller tappet... all of which have wildly differing ramp profiles.
In our engines the lash is only at one point (ok well two if you count the underside of the bucket to the valve stem, but clearance here is effectively zero as the bucket and valve spin independently and mate to each other) everything is in a straight line, and in service the lash closes, this method also suits much more rapid valve opening profiles.
The industry standard in a market that services high revving overhead cam engines (like Italy where our engines come from) is to specify the duration at recommended lash clearance, and generally lift at TDC, and a recommendation for individual I or E camshaft lobe centrelines or in the sohc's case a recommendation for the inlet centreline (as the LSA is fixed and the exhaust will "follow" the intakes "lead", just like in an OHV V8)
what matters to us, well there 's a few important points but in summary...
1. unfortunately simply knowing the stated open/close timing values of a cam lobe isn't really enough information to make any sort of evaluation...unless the maker also provides us with a few more clues... telling us what the overall duration at 50 thou is will add nothing.
2. we want to know where IVO and IVC points are (and to some degree where EVO and EVC are .. and I mean exactly (IVO = Inlet valve opening... IVC = closing ... EVO = exhaust valve opening, EVC = closing) now the specs that Pittatore / alquati and other Italian makers of cams quote is running duration at lash... when you see 40/80 at 0.4mm lash... that's where the IVO and IVC are if timed at that lash (and usually at "split" overlap unless a lobe centreline is specified) so when they say 40BTDC is IVO, that's exactly the point where the valve begins to lift off it's seat.
3. cam choice shouldn't be based on anecdotal "evidence" that someone fitted this cam to their engine and got XXX Hp, there are far too many variables to be able to simplify the internal combustion engine to that level... differences in head flow, static compression ratio and actual VE of the engine changes everything.
4. Cam choice should be based on what sort of revs you plan to be turning, and what sort of air flow you think the head is capable of.
5. If you study the motion of the piston in a four stroke internal combustion engine, the speed of the piston as it travels up and down the bore is not linear... it comes to a full stop and reverses at TDC and BDC, yes, but the speed of the piston as it travels in one direction up or down is far from constant... the reason is kinematics...which according to wickepedia is "the features or properties of motion in an object."
Anyway I am not going to go thru the math here... too much information for nearly everyone... but I can give you a rundown on what happens in the sohc engine, and then try and explain how this relates to cam choice. Piston speed is a function of the rod ratio, which is the relationship between the engines stroke and the engines conrod length.
In a 1500 the rod ratio is 2.007::1 (128.25mm conrod / 63.9mm stroke length) this puts the point of maximum piston speed at 77 degrees after TDC, and if you can get your 1500 to spin at 8000rpm, the piston at this point is travelling 27.5865 metres per second piston speed (instantaneous), the AVERAGE speed of the piston s travel at 8000rpm is just under 17.09 metres per sec.
so lets say "air demand" is driven by piston velocity and piston area...from around 70 ATDC to about 80ATDC the piston is imparting a heap of "demand" due it's high speed... in a 1500 at 8000 rpm the pistons speed is above 27m/sec from 67ATDC thru to 88ATDC... all that demand is being drawn thru the partially open valve, so IMO we want to look at what sort of air demand we have at this point around 77 degrees ATDC. One of the things that happens with a longer duration is usually more lift at this point, same with a cam with more total lift... at 77 ATDC more lift is more flow at the point of peak draw.
6. Opening the inlet valve earlier will generally give us more lift at the point of peak air flow demand...BUT... opening the inlet valve earlier and earlier (before TDC) is only really effective at much higher RPM (think VTEC) and low speed power is poor with this sort of valve timing...remember that at lower engine revs, flow past the inlet valve before the piston passes TDC is only going to be initiated by a pressure inside the cylinder (at the moment the inlet valves lifts off it's seat) which is less than atmospheric, the only way this can happen is if the exhaust gasses have been scavenged effectively... so if exhaust flow / scavenge is poor for whatever reason (port/valve/seat throat/header/system) then this will affect how well the engine behaves / performs as the inlet valve is opened earlier. At higher RPM the air flow speed improves, as the inertia imparted to the air column allows it to keep moving forwards even when the valve is closed (effectively "piling up" behind the valve head for a millisecond) so at high RPM the flow actually becomes "continuous" due to the inertial effects, so then the initial low lift flow and VE improves and power benefits, some people might describe this as the engine "coming on to the cam" .... and can feel, in extreme cases, like the power band of a two stroke motor cycle!
So in summary (for now as I'll add to this for the "ultimte sohc" thread) we want to calculate AIR demand at this 77 ATDC point and size the port/throat and most other parts of the inlet based on some number we get, an engine is after all... a glorified air pump!
Then based on the air DEMAND at around 77 atdc, based on the cam lift at around 77atdc, we work out the valve curtain area , the throat area, the port average cross section.. . we have an idea of the engines air flow cycling value (but the VE is never 100% of course, and at 8000 rpm a 1500 is cycling 190.5CFM at 90% VE) and know that the relationship between valve curtain area, port throat area , average cross sectional area of the port and the port air speed all have mathematical significance to one another, we can use all this to do a little math and figure if the cam is suitable for what we want to do.
But bottom line is, if flow at this lift, and this moment when the piston is moving fastest is really good... then the column of air gathers a lot of speed, even when the piston slows down and approaches BDC the column of air in the port still has loads of inertia and despite the piston reversing and heading back up the bore after BDC the column of air will flow virtually uninterrupted until the valve curtain area again becomes too small for the high air speed, the both port air speed and port flow drops rapidly after this point where the reducing valve curtain area becomes a restriction... so lift at BDC is also an important value for a cam, and I like to plot the lift values for each degree of lobe operation, and like to work out the valve curtain area / degree / lift curves and see where things intersect... that's all coming... it's all part of the reason why I'm going thru this exercise with the accuracy needed.
pittatore 77 grind / Alquati A16 has about 9mm of lift at 77 degrees, full lift 110 atdc of 10.35, inlet valve is closing quite rapidly at BDC but still about 5.75mm off its seat....
the actual IVC closing point is very important too (mathematically) as it allows us to calculate the (simple) dynamic compression ratio of the engine in question. Dynamic compression calculation is very much like the static CR calculation, this is a SIMPLE dynamic CR calc, other factors come into play to work out the actual running dynamic CR (which is difficult but not impossible to calculate/measure, but definitely a bit beyond the home tuner, but it's one of the things you need to understand to be able to make an informed decision about the best parts for the job.
A basic DYNAMIC compression ratio calculation looks like this:
Volume at inlet valve closing point is (remaining swept volume + minus deck volume - dome )+ flycut volume + ring land clearance volume + gasket volume + chamber volume.
Volume at TDC is (volume in gasket + minus deck - dome) + flycut volume + ring land clearance volume + chamber volume.
The stroke measurement for the dynamic CR depends on the actual valve closing point.
A recent Fiat engine build (a 1603 SOHC for an X19) made 109HP at the wheels and just under 120lb/ft at the flywheel, on premium pump fuel (no additives or boosters) 1603 cc, 11.4:1 static CR, 42/82 10.6 cam, 7.79 dynamic CR (in the build planning stage I was aiming for 7.8:1 simple dynamic CR) - zero pinging or knock or signs of detonation - close attention to details like SQ clearances. Very well developed head ports and valves / valve seats that flow in excess of 150cfm @10mm lift 28" of H2O depression put the VE close to 90% at the greater part of the rev range.
Can you tell me these sorts of numbers for any of the cams from the usual suspects? No because the makers / sellers don't have most of this available, and will probably look at you like you've got two heads when you ask, but to make an informed decision on cam choice this is all info we need... I've been thru all this before (the measuring part) but not to the accuracy I'm doing it to now, and I do these sort of calculations for EVERY engine that I build.
SteveC
