GD engine with auto vs. standard cam installed

[raygreenwood]

[SGKent]

There is so much misunderstanding on cams in this site.

Car owners want free power. Meaning, someone wants to choose an item, or a bolt on accessory that makes them power. It is and isn't that easy.

To make more power the car engine needs more air. Specifically it needs Oxygen to make more power. The more oxygen that passes thru it the more power it has. The simplest fix is move to a spot below sea level like Death Valley, or the area around El Centro CA. Move to a cold dry climate. The air is denser in both places.

So what do people do otherwise - they add turbos, they add super chargers, they change cams, they go to larger displacement engines, they add nitrous oxide to the engine etc.. Any of those things gives an engine more oxygen.

What you don't have going for you in your 1978 FI bay. First you have an air snorkel slightly larger than a quarter for all the air to flow into the engine. If you look at the back there are holes drilled in the housing. My guess is VW figured they had to find a cheap way to get more air into the air cleaner - answer, drill holes. The air cleaner entry is less than optimal. Next you have a weirdly shaped plenum once the air get thru the funky AFM and throttle body. Air has to do all sorts of contorsions to get thru that plenum. Then it runs down these lanky inlet runners, thru the cast ports and into the cylinder. The air that makes it burns with the addition of fuel to it, and it exits out these funky ports into a poorly constructed exhaust system. The F-pipes are anything but streamlined. Then comes a 180 degree turn thru a loop VW added to help give the exiting air longer to burn off the HC, CO, then thru a restrictive heater box, and out into a funky muffler. A very long path exists for the exhaust.

What this means is that your engine has the practicality of a tractor engine. It was designed to operate at a low RPM. If it were designed to operate at a higher RPM the intakes would be shorter and tuned to the higher RPM, and the exhaust route would be larger in diameter, less restrictive, and tuned to a higher RPM. You would not have heat provided by an exhaust system, and if you did it would be designed completely different.

Now, as the engine goes faster and faster the air needs to begin its journey in and out of the engine sooner and sooner. So the camshaft lobe duration gets lengthened, advanced, and has more overlap. That is great at higher RPMs but it sucks at lower RPMs. Modern cars have multiple camshaft lobes that are designed to operate at different RPMs. We have a single cam lobe because we are a 50 year old technology. But there is this guy from the 1950's named Eddie Iskenderian (Isky Racing Cams) who proved that you can harness the exhaust and intake pressure waves to go faster, just by timing when the valves open and close. It's just like pushing a kid on a swing. If the column of air in the intake system is moving towards the valve, you open the valve at that time and shut it before the wave heads back out into the intake from whence it came. That pushes more air into the cylinder which makes for more power. Then on the exhaust side you open the exhaust valve as the exhaust pressure wave is moving away so it pulls (scavenges) more of the exhaust gas, which makes more room for the incoming charge. If you do it well enough it even creates a little extra vacuum that helps pull more air into the engine.

So to do these things you have to choose a cam that will operate at the RPM you plan to run the engine. Choose wisely. If you choose a cam designed to operate at too high an RPM you will be pushing against the kid on the swing as he/she arrives and in effect stopping them, lowering performance. I have no gripes with the WebCam 142 on a 2L engine. I highly recommend it. Probably it is even a little too much cam but there isn't a cam that works between a stock GD bus cam, and a stock GA European Porsche cam. The 73 and the 86 are designed for higher RPM than the 142 is so they work against themselves at a lower RPM. Will you notice it, probably not but I would in a bus. If I was building a rally car to drive fast on country roads, I'd run higher compression than a bus has, a longer duration cam, dual carbs, a nice header system, and no heat. I'd probably find a 5 or 6 speed that fit it and a complete IRS suspension. Rims would be light, all brakes would be sprung and up on the trans etc.. But gee, this is a 50 year old VW bus so I have no interest in tying up my money in something like that.

It's a free country and people should buy the cam they think is best for them. But that said I've written all that you need to know in choosing a cam duration so unless you have lots of time and a dyno at your disposal, and a track for time trials, you won't be able to find the perfect cam. I won't name names but there are people who say X cam tested for blah blah blah, but when you have been around a long time you run into people who know who gets what where, and what it is. You'll be happy with the 142 on a 2L bus. But, I doubt if you would be happy with it on a rally or street racing 912e or 914-4, or especially a dune buggy you wail on. The vacuum signature at idle and very low RPM will fall with the 142 compared to the stock cam so you may need to richen the AFM a notch. If you have a 1700, or 1800 it will be geared lower than a 2L so with the carbs and less restrictive exhaust than the FI 2L you may want the 73.
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