05 May Building a Reliable and Powerful Street Engine II
To the camshaft..and beyond.
The next item in the lower end of the engine to consider is the camshaft. The California Look crowd has always held very strong views in the choice of camshafts for their street/strip machines. Often, these street sedans will run a full-blooded race camshaft, and do so quite successfully. It is quite common to find the serious contenders erring on the wild side of the camshaft spectrum. 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. Both of these cams exceed 270 degrees of actual duration (measured at .050″ cam lift), and lift the valve over .550″. In comparison, the popular Engle 110 “street” cam measures 247 degrees actual duration and lifts the valves about .430″. Yet, you are unlikely to find the Engle 110 in a very high-output engine. 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.
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. 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. 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.
Secondly, these very fast street cars utilize an intake system that provides a separate throttle plate and intake port for each cylinder. 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. If these wild camshafts were used with an intake system that shared a throttle plate for all four cylinders, the engine would run very, very poorly. A very rough idle would result, as well as a very “peaky” powerband, only making good power and torque at high rpm, if at all. By isolating the pulsations and reversions within each intake runner, the intake system can more dampen the effects of increased valve timing and overlap.
Another reason these radical cam grinds work well in so many all-out street cars is due to their transaxle gearing. Most of these cars are primarily used for drag strip action, and not surprisingly, the cars are geared super closely. The reason for this is to keep the engine “on the cam” in between shifts. If stock gear ratios were used by these very fast street cars, their engines would climb the rpm range very slowly in the second gear to third gear shift and in the third to fourth shift again, resulting in slower acceleration and decreased performance. With close gear ratios, these engines fall in the rpm very slightly between shifts, keeping the rpm’s well within the cam’s “sweet spot.” Typically, the wilder the cam, the closer the gear ratios will have to be. This is something to not consider lightly, as close ratio gears will decrease your street car’s ability to be a “daily driver” (increased highway cruise rpm’s will result).
Hopefully, one can see how the successful choice of a camshaft will require evaluating several factors. As mentioned before, the larger an engine’s displacement is, the more cam it can use. And knowing that added displacement can be gained by different means (by increased bore-diameter, increased stroke-length or a combination of both), different camshafts should be chosen, even for engines of similar displacement. For example, a 1915cc engine can be built using 94mm pistons and the stock 69mm crank. Assuming we are using stock-length connecting rods, this engine will have the same piston speed as a stock VW engine. For this engine, I would think twice before dropping a wild, long-duration cam in. However, if a 1900cc engine were built by using a 74mm crank and 90.5mm pistons, because of the increased stroke-length, this engine would run better with a fairly hot cam. Of course, add 94mm pistons to this combination, and you’d have a better running engine yet!
Other details to consider when deciding on the cam:
Cylinder heads
Typically, an aggressive camshaft requires extensively modified cylinder heads. It makes little sense to use a high-rpm cam if your choice of cylinder heads will limit the engine’s powerband.
Intake system
Again, a hot camshaft will not work well with a restrictive intake system. For the engine we are planning here, dual 2-barrel carburetors using a 44mm bore would be a good choice, and 40mm would probably be on the small side (restrictive).
Exhaust system
Like the heads and intake, you must marry the exhaust to work with the camshaft in harmony. A small-diameter header will restrict power-output and cause engine heating problems as well.
Compression ratio
Long overlap camshafts run better with increased compression ratio. Why? The increased valve-timing cause the engine to “bleed-off” cylinder pressure at lower rpm’s, resulting in a soggy and flat powerband. By increasing the compression ratio (within reason), some of the effects of the increased valve-timing can be nullified, while also enhancing the cam’s ability to improve cylinder filling at higher rpm’s. A word of warning here though: short duration cams and increased compression ratios don’t agree as well. A conservative camshaft needs a conservative compression ratio as well. More about compression ratios later.
Vehicle weight and transaxle gears
Generally speaking, the lighter a car’s curb weight is, the hotter the cam’s specs can be. If you plan on hauling heavier loads of luggage and people in your Volkswagen, choose a conservative camshaft (along with the heads, intake and exhaust). If your car is stock weight or even lighter, and is equipped with close ratio gears, a long duration cam can be utilized to drastically increase power output and acceleration. Using your car on the highway for cruising? Forget about the racer’s cams and the close gears. Stick with a stock geared transaxle and a fairly conservative cam.
So let’s assume we are building this 2276cc engine for a near stock-weight VW sedan with close ratio transaxle, and plan to use it frequently around town and for the occasional drag race at the strip. We will be using modified, large valve heads and larger carburetors and exhaust systems. I would suggest a camshaft grind with specification as follows: actual duration measured at .050″ lift around 265 to 268 degrees and cam lift at lobe around .380″ (assuming 1.4 ratio rocker arms will be used, if stock 1.1 ratio rockers are to be used, choose a cam with about .415″ lift at the lobe). A cam with much more actual duration will cause the engine to be a bit on the wild side, idling rougher and always begging to be wound up to make power. Of course, these wilder cams can be selected, just be prepared to spend more time tuning and shifting gears.
