What does a cam's "overlap" actually mean?

[berms]

Does a higher overlap give a cam that nice "lope" sound we all love?

[2006FXDCI]

I am frequently wrong but I think the cam lobe separation angle is what gives you that nice lope , overlap is the amount of degrees that both valves are open simultaneously . I think 

[RevFastEddy]

Does a higher overlap give a cam that nice "lope" sound we all love?

Not really that much, it is the early exhaust opening and long duration...   Comp cams has some good examples they advertise as "lopy"

[panic]

It's the period around TDC (360° away from ignition) when both valves are partially open. How much? From the cam data card:
intake opens: XX BTDC
Exhaust closes: YY ATDC
Overlap = XX + YY degrees

Yes, that rilly kewl irregular low speed misfiring, coughing, gasping, nearly stalling sound that all the kids at the magazine rack love is due to residual exhaust gas contaminating the fresh charge. LSA is only a meaningful comparator among cams with similar event durations.
But that means big power? No, it means less power until the engine speeds up.
Early exhaust opening makes the idle loud (the gas is still expanding when the valve opens for that crackling noise), and also reduces power (by cutting down on the duty cycle). If you guess right, this loss is more than made up for at higher RPM by more complete evacuation of exhaust products.
As with most cams chosen for high RPM power (or chosen by mistake) you get the "compromise" part immediately and constantly - it's useless at low speed. If you guessed right, you get the benefit any time you wind the piss out of it.

The chances of a novice builder inventing a successful cam (i.e., not off-the-shelf) is about the same as predicting where the next meteorite will land.

[2006FXDCI]

What panic said !

[berms]

It's the period around TDC (360° away from ignition) when both valves are partially open. How much? From the cam data card:
intake opens: XX BTDC
Exhaust closes: YY ATDC
Overlap = XX + YY degrees

Yes, that rilly kewl irregular low speed misfiring, coughing, gasping, nearly stalling sound that all the kids at the magazine rack love is due to residual exhaust gas contaminating the fresh charge. LSA is only a meaningful comparator among cams with similar event durations.
But that means big power? No, it means less power until the engine speeds up.
Early exhaust opening makes the idle loud (the gas is still expanding when the valve opens for that crackling noise), and also reduces power (by cutting down on the duty cycle). If you guess right, this loss is more than made up for at higher RPM by more complete evacuation of exhaust products.
As with most cams chosen for high RPM power (or chosen by mistake) you get the "compromise" part immediately and constantly - it's useless at low speed. If you guessed right, you get the benefit any time you wind the piss out of it.

The chances of a novice builder inventing a successful cam (i.e., not off-the-shelf) is about the same as predicting where the next meteorite will land.

Thanks panic, this cam business sure is complicated. I was always one of those "kids" who loved the slow lopey idle of my buddy's carbed evo and 20 years later I guess I never grew out of it. Damn these complex fuel injected machines!

[barny7655]

back in the 60s we use to grind the bottom of the cam lobes  to give them more lift ,adjustable tappets , we use to call them silent ground cams ,couldnt hear them but they did work , that was in the mini coppers, any ways what you can do with a cam is usable, but on here we havent got a cam that can vary degrees , like cars  so we can have another 1000 revs on inlet  valve , now exaust as well , it will come  in HD , so a cam will do several things with the ecm ,higher reves still producing power , cheers Barny

[Phu Cat]

Barney, You've got me completely confused about this:  "back in the 60s we use to grind the bottom of the cam lobes  to give them more lift"   

How is it you grind something to give it more lift?  I don't get it.  You grind the bottom of the lobe, 180 degrees from the what would be the highest lift point on the lobe?

PC

[panic]

Barney's right.
It's also called "undercutting the base circle".
The lobe height isn't the distance the tappet moves, it's the difference between the base circle (the "round" part of the lobe) and the top of the lobe.
If you remove .030" (example only) from the base circle, the lobe height doesn't change but the difference between the 2 is now +.030" = more lift.
It also (invariably) changes the opening and closing points, usually making them farther apart = more duration.

[barny7655]

I couldnt of said it better my self  panic ,they did work well in there day ,varible cam timming will come to HD to give us extra usable power ,may be on both cams,at the moment 4000revs is what a cam produces power , so when it starts and finishes is up to the cam grinder, cheers Barny

[PoorUB]

Thanks panic, this cam business sure is complicated. I was always one of those "kids" who loved the slow lopey idle of my buddy's carbed evo and 20 years later I guess I never grew out of it. Damn these complex fuel injected machines!

The "lope" on the Evos and Shovels was probably more from the slow idle RPM and poor idle fuel mixture than the "hot" cam. Most of the time these engines would mis-fire regularly and add to the lope. Guys would idle these engines 500 - 750 RPM where a Twin Cam idles at 1000 RPM. Then add fuel injection and the idle smooths right out. I messed around with my '05 Ultra with Andrews 26 cams and Power Commander. I could richen up the idle mixture until it almost loped like the Evo or Shovel did, but the exhaust would burn your eyes plus the excess fuel was probably washing the oil off the cylinder walls. Probably not a good idea!

[panic]

One of the things that overlap does is establish a "window" where both valves are significantly open (depends on who you believe, some say .100" etc.) pretty close to TDC.
This allows a strong tuned negative wave coming back from the exhaust to cause a strong vacuum in the chamber when the intake "pull" (piston coming down ATDC) hasn't started yet.
This both starts the intake stroke earlier, and adds to the total resulting in a significant increase in VE - at and near the tuned exhaust length. Just my opinion: you get some benefit from this at a range of RPM (not a single point) when the wave returns, because both valves may be open for 80°.

If the overlap period is too short or small (or the pulse is too weak), not enough vacuum gets in.
If the overlap is too tall or big, exhaust gas doesn't completely "flush" out of the chamber until higher RPM, which makes that unpleasant negative spike in the dyno torque curve.

David Vizard says that overlap "triangles" (the shape as shown on a cam diagram) should not be compared by duration, but by the square of duration so that the practical effect of 75° of overlap isn't 1.5 × that of 50° of overlap (75 ÷ 50), but 2.25 × it (75^2 ÷ 50^2).
This is why small changes in LSA have such great effect on the cam's "manners", even a few degrees tighter makes it a bitch in traffic.

[Admiral Akbar]

This both starts the intake stroke earlier,

How do you start the intake stroke earlier?  Does't the intake stroke always start at TDC?



Max

[aswracing]

This both starts the intake stroke earlier,

How do you start the intake stroke earlier?  Does't the intake stroke always start at TDC?



Max

Pretty sure he meant to say intake "cycle" ...

[RideFree]

How do you start the intake stroke earlier?  Does't the intake stroke always start at TDC?

The effective intake stroke can start sooner than TDC with valve overlap when the amount of gas being pushed by the piston as it approaches TDC becomes less than the amount of gas moving out the exhaust valve. Remember the objects (including gases) in motion rule and momentum. Exhaust flow can actually assist the intake flow by creating a lower pressure area above the cylinder for the intake to start flowing to, BTDC. You could take a cam with no overlap and a cam that is the same in all aspects except does have overlap and you will get more good stuff in the cylinder while leaving behind less bad stuff at RPM. It also idles really wicked because the effect needs rpm or higher exhaust flows to really start working well.

Take the SE 204 with 30 degrees of overlap and that is mild. The SE 260 has 52 degrees of overlap. So for the 204's 30 degrees of piston travel around the intake stroke's TDC the exhaust flow is helping accelerate the intake gases at RPM. The intake opens 22 degrees BTDC, then the intake stroke's TDC happens, and the exhaust closes 8 degrees ATDC. She runs real smooth above about 2200 rpm but wicked rough below that.

SE 204
Intake: Opens 22˚ BTDC/Closes 34˚ ABDC
Exhaust: Opens 52˚ BBDC/Closes 08˚ ATDC

I think I said that all right. At least that is my understanding.

[Admiral Akbar]

The effective intake stroke can start sooner than TDC

So if I add up all the strokes, I can get a 4 1/2 stroke?   

Max

[JBarrettB]

The effective intake stroke can start sooner than TDC

So if I add up all the strokes, I can get a 4 1/2 stroke?   

Max

This is getting rich...

JB

[gordonr]

The effective intake stroke can start sooner than TDC

So if I add up all the strokes, I can get a 4 1/2 stroke?    :scratch

Not 4.5 but 5th!

[RideFree]

Kindly explain your 4 1/2 strokes. I like rich.

[smithbrl]

Barney's right.
It's also called "undercutting the base circle".
The lobe height isn't the distance the tappet moves, it's the difference between the base circle (the "round" part of the lobe) and the top of the lobe.
If you remove .030" (example only) from the base circle, the lobe height doesn't change but the difference between the 2 is now +.030" = more lift.
It also (invariably) changes the opening and closing points, usually making them farther apart = more duration.

OK, I know enough about engine theory, etc. to get myself in trouble. I'm having trouble understanding this increased lift you speak of.

Is the valve not closed when the cam is on the base circle?  How can undercutting allow the valve to close any more than it is already? It seems to me that the undercut would only introduce .030 "slack" in the system. The lifter/pushrod would have to move .030 before engaging the tappet and the lift would be then unchanged.

I hope I'm not embarrassing myself here.

[aswracing]

Barney's right.
It's also called "undercutting the base circle".
The lobe height isn't the distance the tappet moves, it's the difference between the base circle (the "round" part of the lobe) and the top of the lobe.
If you remove .030" (example only) from the base circle, the lobe height doesn't change but the difference between the 2 is now +.030" = more lift.
It also (invariably) changes the opening and closing points, usually making them farther apart = more duration.

OK, I know enough about engine theory, etc. to get myself in trouble. I'm having trouble understanding this increased lift you speak of.

Is the valve not closed when the cam is on the base circle?  How can undercutting allow the valve to close any more than it is already? It seems to me that the undercut would only introduce .030 "slack" in the system. The lifter/pushrod would have to move .030 before engaging the tappet and the lift would be the unchanged.

I hope I'm not embarrassing myself here.

It's not a dumb question at all.

Hydraulic lifters self adjust, within a certain range. Put in a smaller base circle cam and now the pushrod is too short, you're correct, but the lifter automatically lengthens to take up the slack.

In the ideal world, though, you lengthen the pushrod by the amount you reduced the base circle radius, so that you keep the lifter's plunger approximately centered in it's travel.

[smithbrl]

 
[/quote]It's not a dumb question at all.

Hydraulic lifters self adjust, within a certain range. Put in a smaller base circle cam and now the pushrod is too short, you're correct, but the lifter automatically lengthens to take up the slack.

In the ideal world, though, you lengthen the pushrod by the amount you reduced the base circle radius, so that you keep the lifter's plunger approximately centered in it's travel.[/quote]

OK, now I understand completely. Pretty cool trick. Thanks for the clarification.

[KE5RBD]

Does a higher overlap give a cam that nice "lope" sound we all love?

The overlap is a result of the cam timing events.  It is achieved by adding when the intake opens before top dead center and exhaust closes After top dead center.  The amount of overlap does change due to the cams timing curves.  The stock cams up to 2014 had a -8 degrees overlap.  The 255 had 13 degrees of overlap.  The 254E had 5 degrees overlap. The 2014 cams have 13 degrees. None of these will lope much.  The SE204 has 30 degrees overlap which will lope.  A cam like the Andrews 48 has 28 degrees and a TW 555 has 40 degrees.  Any drop in cam with 40+ will benefit greatly from a compression boost.  The 2014 Air cooled have 9.8 compression and the Twin Cooled have 10.0:1.  Pre 14 103's had 9.6. 

[aswracing]

Any drop in cam with 40+ will benefit greatly from a compression boost.

You're not one of those people who believes overlap bleeds compression, are you?

[KE5RBD]

Any drop in cam with 40+ will benefit greatly from a compression boost.

You're not one of those people who believes overlap bleeds compression, are you?

I use compression releases to do that.  What my above statement is intended to say is cams with over 40 degrees of overlap will not be efficient at low rpms.  When tuning these type cams you have to reduce the Volumetric efficiency at low rpm areas about 25%.  With 30 degrees about 15%.  These numbers are very general as the exhaust type makes a lot of difference.  By the time the rpms are in the cruise area the cams are nearing stock on efficiency.  Adding compression is one way to improve the performance at low rpms thru high rpms. As far as bleeding compression the SE255 cams with 13 degrees overlap need compression releases in 103 or 110 for some reason.  If you put a Cam like the TW555 with 40 degrees overlap you don't need compression releases even with 10.5:1 compression. You have less cylinder filling with the larger overlap and they are less efficient at cranking rpms.    A static compression test with SE255 will give you 210#'s or there abouts.  With the TW555 about 185 to 190 everything being equal.  If you are calling that compression bleeding I am guilty.