camshaft overlap

[chevytech777]

after reading the turbo cam thread i have been trying to figure out about overlap - what do you want to be running on n/a engine - i have heard close to zero - what do you think the difference would be changing from a -26 overlap (if i did the math right) to a cam with -1 degree of overlap

[banker099]

this is a good read....hopefully it will answer your question

cam overlap

my cam overlap is 54 degrees i believe

[ATX ENGINE MAN]

Higher over lap = more high rpm scavenging of the exhaust gas "train" leavin the cylinder. That scavenging helps create a higher pressure drop in the cylinder, "basically". That means more intake charge. Up to a point. So if you miss calculate your particular engine combo according to it's particular air path and volume needs by running too much overlap, you basically leave unburnt gasses in the combustion chamber that take the place of your you nice and stoich incoming charge. And ya loose power. And keep in mind lobe center lines and duration both effect overlap as well as lobe separation.


Call each big manufacture, get a cam recommendation from them. Then call again the next day, you'll get two different answers.

[chevytech777]

i have read the article - does not explain anything about negative overlap - what does that create/used for

[banker099]

yeah i have never understood the negative overlap....someone learn me

[chevytech777]

i am so freakin confused

[Jermoslin]

this is a good read....hopefully it will answer your question

cam overlap

my cam overlap is 54 degrees i believe

So your LSA on your cam is 84? NOt 112,114...

[banker099]

So your LSA on your cam is 84? NOt 112,114...

now thats confusing....

LSA is not overlap

the only reason people think that LSA=overlap is because LSA has a direct effect on overlap...the lower the LSA the higher the overlap will be...assuming all other cam specs are the same....does that make any sense?

[ATX ENGINE MAN]

Negative overlap is generally a smaller duration cam shaft with a wider lobe separation, which I'm sure we've figured out by comparing the two cams examples. It's used in factory applications that are more focused on lower rpm and part throttle efficiency... Or in boosted application with the same constraints.
Most forced induction cams can be smaller on the duration and wider on the lobe separation, which gives you less overlap, due to the fact the engine isn't relying on the scavenging from cam timing and pressure from the good ol atmosphere to fill it.
You can obviously take advantage of scavenging effects on a boosted motor to bump up the efficiency at higher rpm but on a street motor why? Just turn up the boost. Inevitably the best results come from stabbin cam after cam in a particular combo on the dyno to get the perfect one.