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Mini-stroker Cam Geometry and Piston Motion Data
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Floating VW
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PostPosted: Tue Aug 30, 2016 7:30 pm    Post subject: Mini-stroker Cam Geometry and Piston Motion Data Reply with quote

I had some time to kill this summer, so I decided to clock my camshaft, and while I was at it, I calculated the instantaneous distances and velocities of my pistons too. A word of warning: I’m pretty sure there’s only me and about three other guys sporting a stock camshaft on this forum, so the following data may or may not be of much interest to you, but I did make a couple of really sexy charts to look at, so at least there’s that. Also, keep in mind that the values for instantaneous piston distances and velocities only apply to engines with a 76mm (2.992”) crank and 137mm (5.394”) rod (rod ratio of 1.803).

These are the formulas I used to calculate the piston distances and velocities (in case you have a rod ratio other than 1.803 and would like to calculate your own piston data). Don’t forget that you have to multiply the derivatives (x’ and x”) by your value for Crankshaft Angular Velocity (w) to get velocity and acceleration in relation to time, instead of radians (sorry if that’s obvious to you, but I’m half retarded and it took me a little while to figure that out).

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My camshaft is an aftermarket stock cam from CB Performance, which I installed “straight up”. When I was shopping for a camshaft to put in my engine, what I was looking for was something that would be very easy on the valvetrain, give great fuel efficiency and drive-ability, and not let the dynamic compression ratio drop below 7.5:1 from a static ratio of 8:1. The best solution, I decided, was to keep it bone stock. As a general rule, I don’t usually trust a manufacturer’s advertisements about the products they are trying to sell me, so I’m a little embarrassed to admit that I didn’t clock the cam before the final engine assembly. However, CB Performance is a reputable vendor in my opinion, and I had so many other things to do, so after checking the cam for straightness and clearance and finding half-lift for valvetrain geometry, I let the rest slide just this once. Here is a chart I made comparing the advertised specs with the actual measured specs:

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As you can tell, the advertised specs do, in fact, differ slightly from the actual specs, but probably not enough to cause any trouble. If anything, the actual specs seem to fit my design parameters even better, such as the earlier intake closing angle and the shorter duration at 0.050”. I’d also like to point out that, due to the difficult nature of finding the exact opening and closing points of the intake and exhaust valves using the equipment available to me, the values I obtained for these particular angles may not be 100% correct, but should be within a degree or so of the true value. I imagine this is why most cam grinders don’t start measuring cam duration until the lobes reach at least a few thousandths of an inch lift – the so-called advertised duration. Since this isn’t a standardized measurement between camshaft manufacturers (some use 0.003” lift as a starting point, some use 0.006” lift, and some use something else entirely), I didn’t bother measuring advertised duration, or rather, I attempted to measure it using 0.000” lift as a starting point, which is something no one really does.

Looking at the numbers, you can see that the ramp from 0” to 0.050” lift is a long, slow, gentle 65 degrees of crank rotation for the intake lobe, and 79 degrees for the exhaust. After that, the ramp from 0.050” up to half-lift is a much faster, much more aggressive 28 degrees for both the intake and the exhaust. The intake lobe then spends a modest but healthy 152 degrees (156 degrees for the exhaust) hovering from half-lift up to full lift and back down to half-lift again. The ramp from half-lift back down to 0.050” is the same fast 28 degrees for the exhaust, but is 30 degrees for the intake, which is a bit of an anomaly since I assume the back side of the lobe should mirror the front at this point, but I checked and rechecked all these measurements as if it were the only thing I had to do in life, and 30 degrees is what I came up with. Now that I’m thinking about it, the duration at 0.050” for the exhaust was also 2 degrees more than the intake (212 degrees as opposed to 210 degrees). I just wonder if those extra 2 degrees were actually designed into the lobes, or if it was just a little slop left over from the manufacturer. The split duration might actually have been on purpose, but the other one, I don’t know. From 0.050” down to fully closed then, is a really long and slow 125 degrees of crank rotation to ever-so-gently set that intake valve back down on the seat, and 109 degrees for the exhaust. This is a lot slower and more gradual than I imagined it would be, and that doesn’t bother me one bit. All told, the absolute total duration for both the intake and exhaust is a whopping 400 degrees, +/- 1 degree or so. This also means that both valves spend about 320 degrees worth of crank rotation fully closed, which is the time they have to transfer their heat into the cylinder heads via the valve seats. And for those of you who prefer to look at tidy little charts instead of long-winded sentences, this is for you:

Image may have been reduced in size. Click image to view fullscreen.


And here is the chart I made of the instantaneous piston distances and velocities from 0 to 180 degrees, at 3000 and 5000 RPM. You'll probably have to click on the image to be able to read it:

Image may have been reduced in size. Click image to view fullscreen.


You’ll notice that maximum piston velocity occurs at approximately 75.498 degrees ATDC. And at 5000 RPM, the piston’s maximum velocity is 4065.32 feet per minute, which translates to about 46.2 MPH. To put that into perspective, this means the piston accelerates from a dead stop, to 46.2 MPH, and back to a dead stop again in the space of about three inches. And it does this in six thousandths of a second, 10,000 times a minute. And you thought your life was rough!

If you put these charts together and compare the data, a much clearer image of what is going on inside your engine begins to appear. For example, if you look at when the 0.050” opening and closing points of the intake lobe occur, and compare that to the position of the piston at those same angles, you can easily and precisely calculate your dynamic compression ratio. Something else I noticed was how everything seems to happen within a couple degrees of 30: The ignition event occurs at 30 degrees BTDC. The intake 0.050” closing angle occurs at 30 degrees ABDC. The exhaust closing half-lift is at 32 degrees. The intake opening half-lift is at 28 degrees. Both the intake and exhaust ramps from 0.050” to half-lift and back are 28 degrees (except for the intake down ramp, which is 30 degrees). The intake hits full lift 28.502 degrees after maximum piston velocity occurs. The exhaust 0.050” opening is at 36 degrees BBDC, which is a little outside the range, but not much.

Of course, the two events that really caught my attention were the intake closing angle and the intake full-lift (centerline) angle. I believe one of the reasons that a stock cam works really well in a low-RPM, mini-stroker is because of this type of synergy. Because air is springy, it doesn’t immediately begin to fill the combustion chamber when the piston starts going down. And for the same reason, it doesn’t immediately stop filling the chamber when the piston starts going back up. The trick, then, is to open and close the intake valve a little bit after the piston event, and preferably the correct amount after. In my case, this occurs at about 30 degrees after for both the max piston velocity event, and the BDC event. I imagine peak torque for my engine probably occurs close to the RPM that best matches the 30 degree delay in “springiness” of the intake charge. But if I had chosen a cam that, say, hit full lift at 25 degrees after max piston velocity, but had an ICA of 42 degrees, it’s possible that I might lose some performance due to the lack of synergy. I doubt such a cam actually exists, however, as I imagine most cam grinders are well aware of this fact.

And one more thing I noticed while I was clocking my cam (here is where I get to brag a little): One of my goals when building my engine was to reduce friction between the moving parts as much as possible – DFL coatings, straight-cut gears, low tension rings and springs, anything I could do. In this, I succeeded beyond all imagination, as evidenced by the fact that when I was attempting to measure the half-lift angles, the partial weight of the valve spring pushing back on the cam lobe was enough to turn the whole engine over, cam, crank, pistons and all! I had to be extremely careful not to even touch the car when I went around to check the dial indicator, or else the crank would spin backward a few degrees and I would have go back and start all over again. Good job, me!

Alright, I hope I didn’t bore the pants off you, and if anybody disagrees with any of the formulas I used or the results I obtained, please feel free to give me a proper bashing.
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mark tucker
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PostPosted: Wed Aug 31, 2016 9:04 am    Post subject: Re: Mini-stroker Cam Geometry and Piston Motion Data Reply with quote

reminded me of that old Styx song.....
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