I understand when designing camshafts the idea is to try and keep IVO=EVC to promote good carry after peak. I also understand that IVC is a good indication of where the power will peak. Finally, I understand that the lower the EVO the more midrange power will be produced. However, in an ideal world you want the EVO to open "later" (i.e. numerically lower degrees) to ensure all of the Power Stroke is being used but opening too late will not allow proper exhaust scavenging which could "pollute" the incoming air/fuel misxture on the next Intake Stroke. EVO and IVC are all dictated by ICL & LSA. Overlap affects idle quality/emissions but also the more overlap the more the power. My question is as follows: If the above is true (and please correct me if I am wrong) when does lowering EVO hurt more than help? The reason I am asking is because I have been looking at reverse splits for a good torquey street "Truck" cam. The reason being is because they seem to offer the abilty to run a lower LSA/ICL (with a small duration) which will promote good low end-midrange power/torque (i.e. low EVO) and if I understand overlap correctly help high rpm power as well. For some reason I think my understanding is flawed and I am looking to get put back on track.

 

Good ideas. I didn't like this part. Just because the exhaust lobe is smaller, doesn't give reason to run a tighter LSA to make back some of that overlap. exhaust lobe was shrunk to make the most average power within your power range.

 

Sport Side, thanks for the one-and-only response. I guess what I am having a hard time trying to figure out is for example (4) small reverse split cams (i.e. truck cams):

212/208 109-1
214/210 108-1
216/212 107-1
218/214 106-1

All (4) of these cams have IVC @ 36 ABDC and EVO @ 32 BBDC. They also all have IVO=EVC. The only differece besides duration is overlap. Naturally as the duration increases so does overlap. I guess what I am having trouble visualizing is they should all peak around the same RPM and they should all make midrange torque around the same RPM (if I understand IVC=peak power rpm and EVO=mid-range torque). So what would the difference be as duration increased?

 

More duration will push the powerband upwards. Like in your example where duration increases and the LS is tightened, peak power is met at relatively the same RPM, but power at this point will increase due to the added overlap. This of course assumes the motor needed overlap. Just remember that one given LSA will make the most power within your RPM range... whether it is wide/narrow/inbetween.

 

Again, thanks. So basically what you are saying is, if, in my example above, I increased duration (and decreased LSA/ICL) I should make more torque (power is just a factor of torque) IFF the engine can support the added duration/overlap? I think I should also asume because I will make more torque at peak I will make less torque below that peak? So there would be some give-and-take.

To be honest, I think in my application (non-race) I would like to keep most of my power to around 2000 - 4500 but I do not want it to fall off. If you look at that power band it brings me back to the late 1980's-early 1990's L98 TPI 350 engine from Camaro/Firebird/Corvette. People claimed it was "Chevy's best truck engine never put into a truck". I know we are speaking of a whole different animal but the torque is what I am talking about. However, due to its insufficient intake volume, and realitively poor flowing heads it fell on its face after around 4500 whereas I think our truck intake/heads will not have that problem.

 

Not necessarily. On a lot of these motors, too wide of a lobe seperation is used and power from 2000 up can be increased be simply tightening the seperation angle. But yes. Tighten the seperation too much, and the powerband is pushed upwards due to the added overlap.