What exactly causes a cam to be lopey, and if you can give me an example of a cam that does and doesnt, and lets say the motor is a 350.

 

It is a contamination of the air/fuel mix in the combustion chamber. That's right! All it means is the engine is not able to run properly at that rpm.

This is caused by an overlap of duration between the intake valve and exhaust valve--in other words both valves are open at the same time, allowing the exhaust to reverse direction and re-enter the combustion chamber.

This bad fuel/air mix problem goes away as the engine rpms accelerate due to the acceleration of the combustion process. That is why it shouldn't lope at 2000 rpm, unless that cam has massive lobes!

 

is there anyway to tell, by looking at the cam specs, how much it overlaps,

ok for instance what about a comp 268 high energy, 218 duration @ .050, and .454 lift, and 110 lsa

and what about comp 262XE 218/ 224 duration @ .050, .462/.469 lift, and 110 lsa

350 engine ported 305 heads, and bout 9.5:1 compression.

thanks for any info

Kevin

 

Yes, the LSA (or Lobe Separation Angle) is generally the key to determining whether or not a cam will provide a decent idle for an engine. LSA is a function of the degrees of duration the intake and exhaust valves have ground into them. Obviously, then, an LSA of 110 degrees is LESS than a cam with 114 degrees. The higher the degree of separation, the smoother the idle will be because there is less time in the cycle that both valves are open. As a general rule of thumb, and acknowledging that larger engines can handle both valves being open longer, a 305 that is fuel injected should have at least 112 degrees of LSA because fuel injected engines are more reliant upon a strong vacuum signal (which is a function of how long the valves are open, thus allowing vacuum to decrease). A 350 could just as easily work fine with 110 degrees of LSA. Carbed engines are somewhat less dependent on LSA because they are not reliant upon sensors that read the vacuum signal. So a carbed 305 would not have trouble with a 110 degree LSA.

I hope that makes sense

 

thanks alot, that does make alot of sense.

 

Yup. Basically, to be computer friendly, the more duration you have the more LSA you want. So for a medium to large cam you might want 114* LSA.

But if you had a tiny cam with specs like 196/206 @ .050" then it'll still work even tho it has 109* LSA.

As the lobes get larger (more duration) the further apart you need to spread them so they don't overlap.

...make sense? :lala: