Long Duration Camshafts & How they work

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Long Duration Camshafts & How they work

Postby pepa on Mon Apr 19, 2010 4:27 pm

These articles are courtesy of Panic-Mechanic(VW Club)

To cam or not to cam.
Terms you need to know:
TDC - top dead centre - when the piston is at the top of it stroke
BDC - bottom dead centre - Piston is at the bottom of the crank stroke.
BTDC - Before TDC
ABDC - After BDC
We often get asked that question in the club and it's never a simple answer. But let's first look at what it is and what makes it work. Hopefully most people will know what a camshaft is. It's that funny looking shaft that has odd lobes all around the thing. The purpose of a camshaft is to open the valves in the engine which basically makes an engine breathe. Camshafts turn at half crankshaft speed. End of the day all an engine is, is an air pump. So what that comes down to is that the better you can fill the capacity of the motor the more power it will make. Now let's look at cams. They will make the valve(s) stay open for certain duration of the time that the crank is turning. Quick explanation - 4-stroke engine is Intake - compression - power - exhaust cycles. You want the intake valve open during the intake cycle - both closed during the compression and power cycles and the exhaust valve open during the exhaust cycle. Each of those cycles will last 180 degrees of crankshaft rotation. So if you take TDC being 0 degrees then the intake cycle really last till 180 degrees or BDC. Then the compression cycle starts. That will last till TDC - spark happens and now you have the power stroke - that lasts till BDC then the exhaust cycle starts which lasts till TDC and all over again. Now these cycles are not as clear cut as exactly 180 degrees as there are areas of basically dead air movement at around TDC and BDC. Through the years engineers have studied the airflow inside a motor and found that you can open the intake valve before the exhaust valve is closed and use some of the scavenging effect of the exhaust air rushing out to start the intake cycle off. This is called valve overlap. So now already the intake cycle starts at BTDC. Then they found that because air is rushing into the intake port it carries momentum and it doesn't just stop immediately when the piston is turning at BDC - so you can close the intake valve ABDC and fill the cylinder better. So now already you can have intake cycle of say 5 degrees BTDC + 180 + 40 degrees ABDC to give you 225 degree duration. Same then happens with the exhaust. You can open it before the power stroke is finished, as combustion is complete - and you can close it after TDC to help the intake cycle. Standard cams run around that duration for reasons of economy - compromise of driveability and power. Now you get a cam with longer duration - which increases the area ABDC that the intake stays open. What happens now is that at Lower RPM some of the incoming air (mixture) is pushed out the open intake valve due to the fact that the piston is rising into the compression stroke and the valve is open. Effectively you have dropped the compression ratio. Why it works is that at higher RPM the speed at which the air is coming into to cylinder is so high that you do not get the reverse airflow - thus filling the cylinder to basically more than it's usual capacity. This is the reason why you get bad idle with long duration cams - there is a lot of valve overlap and the cylinder is being filled only partially - so you are running a low compression motor with bad combustion. The longer the cam duration the higher up in the RPM range the point is reached where airflow speed beats reversal. That is the reason why you lose bottom end power and the power moves up in the rev range. It now also means you basically have to rev the motor higher to reach power peak. If the duration is long enough you might reach a point where the power is made after where the normal rev limiter is so then you have to either move the limiter or use a shorter duration cam. All in all it's a compromise and any modification you make falls in the area of personal choice. Some people cannot handle the lack of drivability and the Fuel consumption that a 288 cam will give you and other can't live without the power it gives at higher rpm and don't care about the other losses. So when it comes to cam duration it's a case of seeing what you want and like or dislike and live with it.
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