cam profiles?

[Big J 283]

hi guys, just wondering if anybody can briefly explain how cams are measured? ive been reading about 268 cams, 264 cams etc but would like to know what these mean and what the difference is between the different shafts. always eager to learn!!

[magentaham 0]

There is no real short explanation. The 268, 264 numbers are degrees of crankshaft travel the valve is off it's seat. Most cam grinders have pretty well established the valve has to be off the seat about .050 inch to establish airflow so the actual open duration is more like 224, 218. The lift number, .410, .425, .456 is how far open the toe of the cam opens the valve. You also have a centerline figure. Somewhere between .110 and .117 this effects what is known as overlap. The smaller the number, more, larger, less. More overlap causes the engine to lope at an idle and raises cain with your emisions. VR's can't take alot of cam, about .270 duration and .450 lift and that would be pushing the limits of the ECM,and valve springs.

[Big J 283]

thanks for that, ill have to read it a few times and itl slowly make sense. i see the logic in it all tho with the duration the valve is open in relation to crank rotation.

[magentaham 0]

Jay, I'm not a good teacher. Try this, Crank turns 360 degrees per 1 rpm. It has to turn twice per cylinder fire.Intake(180 deg.)compression (180 deg.) power (180 deg.) exhaust (180 deg.) Total 720 deg. Lawn mower engines have a cam that opens at top dead center and closes at bottom dead center. Don't make alot of power but enough to do the job. Engine builders found that if the valve starts to open befor TDC when the piston starts down on intake the valve is open far enough that the piston would actually suck more air,fuel in, making more power, and if the valve actually closed after bottom dead center it also inhanced cyl fill because the piston was stopping to change direction and the inlet flow was not disrupted. So instead of cams with 180 deg. duration you get one alittle fatter, 240, 260, 268, ect.These numbers are actually correct as far as valve open time in crank deg. but as I said befor the valve has to be open a little to establish flow. However this is not so in the case of a turbo or supercharger. There is positive pressure in the intake manifold so the second the valve leaves the seat the cyl starts to fill.Lots more power!!! This is a short overview of whats going on and by no means all thats happening.

[Big J 283]

ottobomb, thats great, i can easily understand that one so thanks for explainin it. im doin a college course in mechanics, and dads a mechanic my trade so thankfully im not a complete dead head. its the performance side of engines and upgrades im enjoyin learnin about.

[Kelster 0]

Ottobomb, I think you have indirectly answered a question I have been trying to find the answer to, wondered if you or anyone could clarify.

The explanation regarding a fatter cam helping to suck air in and superchargers / turbos not needing this. Does this explain why I read somewhere on this forum that 248 cams are about all you would need with a Supercharger ? If so what would be the difference if a Supercharged engine had 268 Cams or higher ?

[magentaham 0]

ok, Fat cams, most have a grind profile based on natural aspration, ie, no stuffer.(turbo or charger) In order to achieve a longer duration the valve must open farther and farther away from top dead center. Also the exhaust opens earler and longer. just to pull some figures out of the air... Intake opens 20 deg befor TDC, closes 15 deg. after BDC(bottom dead center) Exhaust opens 20 deg. befor

BDC, and closes 15 deg. after TDC. So as the piston comes up on exhaust stroke almost to TDC, you have BOTH valves open at the same time(35 deg.) At idle the spent exhaust gets pushed into the oncomming charge of air and fuel causing the engine to run lopy. However at a given higher rpm(say for 6500 rpm) the overlap causes a vacume effect in the cyl that actually helps suck in the new air fuel. This is why with no stuffer Header, Intake and cams are your only performance angle.(with a good remap of your chip) As I have stated befor 270 deg. is what I would consider the limit with the stock computer. To much overlap and the ECM can't compensate, emissions go off planet. With a stuffer you really need a cam profile that can make the positive pressure in the intake work for the motor. Not a lot of overlap, (wastes fuel)(mind you we're talking street driven car) This is not to say 268's won't work with a stuffer but from what I've read in this forum nobody is turning 7000 to 8000 rpm. We have a few crazys here in the states building sand dragsters using vr6 power. Different valves, springs, no injectors...weber carbs, autronic ignition turning 8500rpm. You think they sound good on the street you should hear them at the hillclimbs. If you want a gofast vr6 dont waste your money on intakes fat cams or even an exhaust header...buy the stuffer, turbo or supercharger, it doesn't care if the head has long and short ports (cris at C2 is in the process of testing this with his short ram manifold). It doesn't care if you have cams.(you can change the pully on the drive of the supercharger and have more or less pressure to compensate for all of the vr6's imperfections. My personal pref is the supercharger. Utah in the summer is like 95 to 105deg. F most the time. Turbo's generate alot of heat. My corrado is already to hot. Would be kinda like putting an oven in a microwave. Anyway if you have read all this...your thinking, the old fart is rambeling. 248's have a nice idle (not a lot of overlap) infact if anybody has measured the stock vw vr6 cam they would probably tell you that a 248deg. cam is as close to stock as you can get so don't spend your money on them. The other thing to think about is lobe centerline. This has alot to do with overlap. VW, in the corrado manuel gives you the specs on opening and closing of the valves.....but no way to adjust the cams say after a head mill. Milling the head causes the cam timming to retard. Changes everything. Enough of that. I've already said enough to catch some heat from some of the guys here.