I understand how duration and lift work, in theory at least. But overlap is not intrinsicly obvious.

Thanks,

 

Basicly overlap is how long in degrees both valves (intake & exhaust) are open at the same time

 

I understand that, but how and why does that benefit performance?

 

 

http://www.newcovenant.com/speedcraf...camshaft/5.htm
http://www.holley.com/HiOctn/TechSer...IECCTech5.html

 

In my understanding, When you have both exhaust and intake valves open, "at the right time of course" the exhaust pulse creates a vacume "high pressure high heat likes to travel to a place of low pressure and low heat, but still expanding, the exhaust opens, and it jets out, creating a low pressure/high vacume point directly behind it, this helps pull out the remaining gasses. Then the intake valve opens, and the pressure differences help pull in a fresh intake charge. Now I don't know how long this can occure before it becomes ineffective and hinder performance. As at some point the exhaust valve needs to close and the down stroke needs to finish pulling in the intake charge.

but then again I could just be talking out my *ss. But thats how I *think it works* I am sure someone will chime in and better explain the whole reason.

 

As already posted, overlap is the period in which both the intake and exhaust valves are open. Advertised overlap figures aren't always easy to compare because the don't take into account the amount of lift during overlap. Two cams may have identical overlap specifiations but if one has more agressive ramps, increasing lift faster within the cycle, they may behave differently. Overlap is still probably the best way to "guestimate" a cams idle characteristics though. The disadvantage of more overlap is that it can allow more reversion, i.e., exhaust gasses mixing with the fresh air/fuel charge. This also slows down the intake velocity which can result in the fuel/air not staying mixed as well. The end result is that the engine doesn't run as well as lower rpm. That's why big cammed engines lope at idle. The benefit of more overlap is that, for a given intake/exhaust duration, it means more time with the valves closed during the rest of the cycle, which increases torque and hp at higher rpm when flow velocities are high enough to prevent reversion. Like most any aspect of cam timing, overlap is a compromise and the goal is to make the best compromise for the intended use.

 

Overlap is necessary on high speed engines because gas - exhaust or inlet mixture - does not start and stop instantly. Inertia is involved, so it's advantageous to open the inlet valve prior to TDC and close the exhaust vavlve after TDC. This improves VE at WOT high revs, but it's a double edged sword.

At low revs and manifold vacuum the exhaust pressure is higher than the inlet pressure, so exhaust gas migrates into the inlet port during overalp at idle and part throttle. This is why "big cam" engines have a lumpy idle and poor low end torque.

Overlap is also very useful if headers are installed to create a rarifaction wave that is reflected back from the end of the primary pipe and reaches the cylinder during the overlap period. This creates lower pressure at the exhaust port than the inlet port at WOT, so the exhaust wave dynamics help initiate the inlet flow and "scavenge" the exhaust gas.

Racing engines with headers need lots of overlap for best high rev specific output, but street engines with manifolds don't need much, and too much will hurt performance. Many aftermarket cams IMO have too much overlap for street engines, particularly with the OEM exhaust manifolds, and this hurts idle quality and low end torque without paying much back at the top end.

Modern high performance emission controlled engines have much less overlap than vintage HP engines, but the modern versions have signficantly higher output.

Duke

 

Hey John - One more thing: If you want to determine how much overlap a cam has, it's listed as LSA or lobe seperation angle (or lobe centerline angle). You have 2 cams with the same lift & duration, but one cam has a lobe seperation of 108 deg, and the other has a seperation of 112 deg, the 108 has more overlap & will make more power, but idle quality will suffer. The 112 deg cam will have less overall power & overlap - but will idle smoother. Most cam companies use 1 or 2 LSA's they use on most of their cams (90 % of comp cams have 110 LSA)
Just becuase a cam has a wide LSA doesn't mean it won't make power. The Chevrolet factory LT-1 & the BB LS-6 mech cams were built with 116 deg lobe centers with lots of duration.

Cams can get confusing in a hurry
:crazy:

 

That's what I was looking for.

Thanks,
JR