R. Bruno

I am in the preliminary stages of considering a crate motor replacement and am researching all this stuff - not the least of which is the fun smog stuff here in California. What is the explanation for cam lift and duration and how does it affect the way a motor performs? Thanks in advance :cheers:

ORVette

ALL THE FOLLOWING IS TO THE BEST OF MY KNOWLEDGE, CHECK IT OUT WITH A FEW OTHER MEMBERS!

Cam lift is measured a couple ways, the height of the cam lobe from the base circle(?), but more commonly, the amount of lift produced by the cam lobe as measured at the valve. That is to say, many small block cams are given lift figures that represent the amount of lift at the valve that the cam provides with a rocker arm ratio of 1.5:1. This translates into the cam lobe measurement times 1.5. Obviously, the same cam with a 1.6:1 rocker ratio arm would produce a lift figure .1 taller than the advertised lift. Duration is listed as "advertised", or more commonly as lift measured at .050 valve lift. Either way, it represents the amount of time, in cam degrees, that the valve is open. Since "advertised" duration is measured at varying lift figures(usually somewhere around .003-.006(?)) , the .050 figure is a better determining factor in how a given cam will perform. It's important to remember that even cams with identical lift and .050 duration figures can perform quite differently based on lobe design. Some offer more duration at a given lift figure than others based solely on lobe design. I bought a Summit cam(#1102) with 204/214 duration @ .050, and .420/.442 lift figures because it was identical in that respect to the Edelbrock Performer SBC cam. As I later found out, the two cams actually had very different "advertised" duration figures and therefore performed differently(how much I don't know). I always think it's a good idea to discuss cam selection with a few manufacturers, and more importantly, with several Forum members who can relate real world experiences. I think it's probably accurate to say that generally, the greater the lift and duration, with a given set of heads, will provide added top end HP at the expense of low end TQ. Again, this is a generalization, and cannot accurately represent what someone will find to be true on the street. I think the best thing to do is post very detailed information on your engine, especially head specifacations, and find out what other people think based on thier personal experience.

Side note: get a big block, and run a big lumpy cam. If you start with a big torque motor, you can afford to give up a little low end in the persuit of that NHRA chop! :cheers:
I know this simple minded observation does you no good if you live in an area where you need to take emission control into consideration. Good Luck!

Mystery Machine

I would attempt an explanation but I would only be regurgitating what others have told me. Try this site:

http://auto.howstuffworks.com/camshaft.htm

Alwyn678

:iagree: That is a great analysis :cheers:

Ganey

Advertised duration became meaningless a long time ago. Since some want larger nos., then larger advertised duration helped sales & some manu. advertised a much larger no. while actually selling less cam since they want the customer to be happy & realize that larger nos. is not always the way to go, i.e. you want to match the setup.
Harvey Crane of Crane Cams seeing all this proposed & promoted measuring at .050 as a method of comparison probably starting in the 60s.
The cam can be thought of as the engine's mechanical "program". The duration is the amout of time the valve is open & lift is how much.
As the cam gets larger typically both duration & lift increase.
As the cam increases, the rpm range is shifted up. A stock cam typically has an rpm range of idle to 5000/5500. A performance cam can be 3000 up.

:cool:

R. Bruno

Thanks for the replies. They are very helpful.

Jvette73

Cam lift is how high off the seat the valve is lifted. Higher lift=more TQ

Duration is how long the valve is off the seat. Longer duration increases the rpm level where maximum TQ is achieved.