Posted: Sat Oct 14, 06 4:57 pm
I will have a go at this for you.
Duration is the amount of time (in degrees of CAM rotation) a valve is open. The cam revolves at half the crank speed because the piston has to come up and down twice in every 4 stroke cycle.
The longer the valves are open the more time fuel and air has to get in and exhaust has to get out.
Without going into why, the effect of this is to move the torque curve further up the rpm scale. This makes more horsepower because HP = (Torque x RPM) divided by 5252 so you can see that if rpm is more HP will be more.
However the trade-off is you now have less torque at lower rpms.
As I said, duration is measured in degrees. Most manufacturers measure duration from the point the cam lifts the tappet from 50thou up to the point it reaches 50thou again on the way back down because lifts below this point do not effect the engine much at all.
This is called "duration at 0.050" and is the industry standard as an indicator of the cams real size.
Forget "advertised" duration. This is the way Chrysler describes its cams but is not a very good indicator of how much effective duration you have.
Most hot street cams are between 200 and 240 degrees @ 50thou duration on the intake valve. The exhaust can be a bit more because chrysler engines seem to like a bit more duration on the exhaust.
If you go above 240 degrees on the intake the engine will start being more of a race engine and will have little torque at normal rpms. You would need a 3000+rpm stall torque converter and low axle gears etc.
You may also need more static compression ratio because with big duration cams you bleed off some cylinder pressure.
So to sum up this bit;
More duration = more HP but you loose low end torque, need more compression ratio, need lower axle gears so the car operates at higher rpm, and a higher stall converter.
Now lets look at lift.
More lift = more torque. Up to a point.
Your heads can flow only a limited amount no matter how far the valve opens. Once the valve has opened enough the head will flow no more even if the valve opens more.
BUT! Opening a valve past this point makes the average time the head is at peak flow increase. So the effect is that the head flows more which makes more torque.
Torque BTW is made directly by the amount of fuel that fully burns. More fuel burns = more torque = more HP.
Also the faster the vlave reaches that high flow point, the more air/fuel can flow.
So when looking at a cam see how much duration it has @50thou and look at how much lift it has. The more lift it has the better but don't go too mad on the lift or you have to start worrying about piston to valve clearence.
Duration is the amount of time (in degrees of CAM rotation) a valve is open. The cam revolves at half the crank speed because the piston has to come up and down twice in every 4 stroke cycle.
The longer the valves are open the more time fuel and air has to get in and exhaust has to get out.
Without going into why, the effect of this is to move the torque curve further up the rpm scale. This makes more horsepower because HP = (Torque x RPM) divided by 5252 so you can see that if rpm is more HP will be more.
However the trade-off is you now have less torque at lower rpms.
As I said, duration is measured in degrees. Most manufacturers measure duration from the point the cam lifts the tappet from 50thou up to the point it reaches 50thou again on the way back down because lifts below this point do not effect the engine much at all.
This is called "duration at 0.050" and is the industry standard as an indicator of the cams real size.
Forget "advertised" duration. This is the way Chrysler describes its cams but is not a very good indicator of how much effective duration you have.
Most hot street cams are between 200 and 240 degrees @ 50thou duration on the intake valve. The exhaust can be a bit more because chrysler engines seem to like a bit more duration on the exhaust.
If you go above 240 degrees on the intake the engine will start being more of a race engine and will have little torque at normal rpms. You would need a 3000+rpm stall torque converter and low axle gears etc.
You may also need more static compression ratio because with big duration cams you bleed off some cylinder pressure.
So to sum up this bit;
More duration = more HP but you loose low end torque, need more compression ratio, need lower axle gears so the car operates at higher rpm, and a higher stall converter.
Now lets look at lift.
More lift = more torque. Up to a point.
Your heads can flow only a limited amount no matter how far the valve opens. Once the valve has opened enough the head will flow no more even if the valve opens more.
BUT! Opening a valve past this point makes the average time the head is at peak flow increase. So the effect is that the head flows more which makes more torque.
Torque BTW is made directly by the amount of fuel that fully burns. More fuel burns = more torque = more HP.
Also the faster the vlave reaches that high flow point, the more air/fuel can flow.
So when looking at a cam see how much duration it has @50thou and look at how much lift it has. The more lift it has the better but don't go too mad on the lift or you have to start worrying about piston to valve clearence.
