Date: Tue, 10 Dec 1996 17:35:50 -0800 (PST) From: Mike West Subject: Pistons 'n Cranks 'n numbers Well we have to talk , about the piston in the engine and such. We've discussed what happens in the combustion chamber and the pressures and temperatures attained. We've managed to keep the head on for the moment. The head if you recall is about 4"(100mm) thick. I'll talk about the stock 1600 with 69mm crank, 6.9:1 compression. The head on the piston is only about 3/8"(10mm) thick. If the piston doesn't move it's going to blow the top out. We have a pressures in the area of 340 psi peak in there on a 3.37 dia.(85.5mm) surface of the head. The piston top has an area of 8.9sq.in.(57.4 sq.cm.)which slammed with the pressure is 340X8.9 or 3026 lb of force. You metric people will notice I've abandoned you here. I can't translate metric pressures without blowing all my concentration. For you Americans it would now go into KPa , kilopascals for the metric. At some point, even before the peak pressure the piston will start to move. This will depend on a lot of variables. Friction, load on the crank from the transmission, mass of all the moving components and so on. I can't deal with them all in this little bite of time so I only mention them. It has been suggested that the piston is like a bullet going out a gun barrel. To a point it is, but there are some major differences. In a big old 3"-50 cannon the projectile will keep picking up speed until it leaves the gun barrel. In theory anyway. Let's take a look at the "crank connection" on the piston. Some of you will recall the relative "flat spot" at the top of the stroke if you ever looked for TDC. There is so little movement of the piston for the travel of the crank that it seems non-existent. The crank is, like the piston, approaching it's very top. It is moving sideways more than up and down until it gets sideways enough for up and down to become the major motion again. If you haven't had some physics this will seem totally alien to you. Draw a little circle and trace it with your finger, you'll see it. Not to worry, as the car salesman said, "trust me". At the very top and at the very bottom the forces required by the piston to move the crank are the greatest. At 45 degrees rotation the side forces equal the down forces. It should be noted that these "up and downs" I'm talking about are really in and out from the center since the pistons lay parallel to the ground. So I'm mentally impaired! You'll have to cope with it. Where this is going is that at 90 degrees crank or halfway down the sleeve, the piston reaches it's greatest speed and then begins to slow down again. The piston has "per se" its own flat spot down there in the middle. You see what I mean? I can only hope. :-) This is really a boon for both the heads and the bearings as the momentum of the piston itself is not "slammed" into the ends. Still at the top end there is the greatest force required to move the crank and that's the whole 3000 lb on those bearings on the rod ends. Hey, while digging around and salvaging from our last storm I found this book I picked up a few years back called "Auto Math" by John Lawlor. It's like a Petersons or what, $15. Some of you might find it interesting. I'm pulling data out of it right now. Lots simpler than dads old books. Lets talk about piston speeds and the limits. The limits are imposed by two things, the rod/crank and bearing material and guess what? The "flame front travel". Frankly to go faster than the flame front would be a "down hill event". The limits on piston speed seem to be, for stock rods and cast crank, 2500 feet per minute. Above that, things start to break. On a VW with a 69mm stroke that would be 5514 rpm. Looking at a 78mm stroke that would be at 4885 rpm. Not to be disheartened, racing sproggies, I got you covered. Graham Bell who has some fame as a racer says with modern metallurgy, cast crank and rods, 3500 fpm. Forged crank, heavy rods and main caps, 3800 to 4000 fpm. I do not think "modern metallurgy" covers the stock VW parts so if you want big stroke and fast speeds you should go "after-market". Notice how I avoided all the problems with friction and mass and gas efficiency and drag and . . . . can I dance or what? :-) There is an interesting equation that reads: hp = (rpmXtorque)/5252. What makes it interesting is that at 5252 rpm the numbers cancel out and the horsepower and torque are the same. In theory of course. Well, I said I was retarded. Maybe a little bent too. :-) west