![]() By providing each cylinder and intake valve with its own intake tract and throttle plate, reversions caused by increased cam duration are isolated to each cylinder and intake tract, allowing for smoother running. Secondly, these very fast street cars utilize an intake system that provides a separate throttle plate and intake port for each cylinder. The effect of longer duration has even less effect on low-end torque with cranks longer than 82mm! The reason is again, that the longer stroke cranks increase piston speed, offsetting the effects of increased valve timing. This increased displacement generates much, much more torque than a smaller engine (say a 1776, for example), and this torque makes up for the loss of bottom-end torque that is usually associated with radical valve timing. So, are the owners of the 12-13-second VW sedans throwing driveability out the window with their radical cam grinds and large cc engines? The answer is “no”, and there are reasons for this! First of all, most fast VW’s run engine displacements of 2000cc’s or larger. Not that it won’t produce good driveable power, it just will not allow a large cc engine (equipped with the right cylinder heads, intake system, exhaust system and gearing) to wind to rpm’s much over 5500 rpm. Yet, you are unlikely to find the Engle 110 in a very high-output engine. ![]() ![]() In comparison, the popular Engle 110 “street” cam measures 247 degrees actual duration and lifts the valves about. Both of these cams exceed 270 degrees of actual duration (measured at. ![]() A review of many 12 and 13 second VW’s will find cams such as the Engle FK-87 or the Web Cam 86C, both very radical grinds, that normally would run very poorly in a street sedan. It is quite common to find the serious contenders erring on the wild side of the camshaft spectrum. Often, these street sedans will run a full-blooded race camshaft, and do so quite successfully. The California Look crowd has always held very strong views in the choice of camshafts for their street/strip machines. Now in this one the intake is left open all the way to 72 degrees after BDC-ABDC so the piston has gone even higher up the bore before the gaping hole is close.The next item in the lower end of the engine to consider is the camshaft. (Incoming charge migh be whistling down the ports at anywhere from 200-320 mph so it has a lotta inertia, it WILL keep on coming in past BDC-that is why they leave the valve open, but how much depends on a number of factors but you can't ignore that.but that wasn't the question) The piston only goes up 180 degrees of crank rotation to move the piston from the bottom to the top.and 51.5 degrees of crank rotation before the hole is close means you don't have 11:1 There's a big hole that is open from BDC up to 51.5 degrees "I has 500cc cylinder volume + 7 cc gasket volume + 43cc head volume so 500+7+43= 550cc at BDC (Bottom Dead Center) and at the top at TDC theere's gasket + head volume of 50cc Ok everybody talks about "I haz 11:1 compression" and they refer to service book that says 10:1Īnd then maybe they go and measure cylinder volume by running the piston to BDC, the running it up and saying Still, it's a more useful approximation than static compression ratio. Note that all of this crap is, of course, an approximation, as are any estimates of how much compression something can take on whatever gas and so on. Translating that kind of thing into crank degrees is a little more annoying in terms of math, so here's a handy calculator I found in 20 seconds of googling: You can look up governming equations if you want to also it's basically just looking at piston motion. The long and short of it though is that as there's more duration, the valve is closed later in the compression stroke which means it doesn't get squeezed as much. If the cam were closed with the piston halfway up the cylinder, dynamic comrpession ratio would be half static compression ratio, so on and so forth. If the cam were closed at BDC, dynamic compression ratio (what this is called commonly) would be equal to static compression ratio. ![]() Ignoring inertia effects of the intake charge and stuff, the engine only starts building compression once the valve closes. ![]()
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