I've been reading into cam overlap some over the past few days and have a few questions. I understand that on boosted applications, overlap is bad because the pressurized intake charge is going right out your exhaust. But my question, why is this ever good? What about cam overlap is good on N/A applications?

Thanks,
Alex

 

Scavenging.

 

The exiting exhaust gases help to pull in the intake air. What he^ said.

 

this article was posted in this forum a couple of months ago, it'll tell you everything that you'll want to know about valve overlap...
http://compcams.com/Community/Articl...?ID=1726205736

 

Not a bad article, but I'm sure someone is going to call the first paragraph,
3 column on the first page:

"overlap is ground into the camshaft and cannot be changed"

You can alter overlap using higher ratio rockers, and pre-load/lash adjustment.

 

That was a good article, thanks for the help guys. Does anyone know the formula to determine the amount of overlap your cam has?

 

If you use pianoprodigy's DCR calculator spreadsheet...It will give you the valve events and tell you overlap for a given cam.

Hammer

 

How do roller rockers change overlap. Duration determines valve events, and thus overlap. Going to a higher ratio roller rocker would give you more lift, but it wouldn't change the duration. So, how would it effect overlap?

 

A higher ratio rocker would bring the .050 lift point sooner in the curve. Since the duration will be larger at this earlier point on the ramp, the effective overlap will increase at this point.

Hammer

 

The best way to understand this is to graph a set of points on a X-Y
graph and then connect the dots.

Make up a cam lobe with a set of lift points.

Make the X axis degrees of duration (between IVO and IVC) and make the
Y axis lift in increments of 0.050".

Draw one curve at 1.5 ratio lift and another at 1.7 ratio

Do the same for the exhaust valve (EVO and EVC)

Your graph will look very similar to the example in the link (Figure A).

If you compare the shaded areas of overlap, you should see that the
region has more overlap.

If not, try using a higher ratio that 1.7 (IE: 1.9)

 

Alex,

A Centrifigal Blower setup and a NA setup run on the same principals....

The difference between the intake port pressure and the cylinder pressure, and the difference between the exhaust port pressure and the cylinder pressure.

If the intake port pressure is higher than the cylinder port pressure, air/fuel will travel from the intake port to the cylinder. If during overlap the exhaust port pressure is lower than the cylinder pressure it will then lower the cylinder pressure more to create a larger difference from intake port to cylinder.

A blower just raises the intake port pressure more, and doesn't increase the exhaust port pressure.

The correct overlap for any motor is based on the same things that the duration is based on, RPM range, cubes, power etc... Overlap can help and hurt you.

Bret

 

So there will be more lift at an earlier point. So the duration is also effectively raised as well as lift when going to higher ration roller rockers, correct?

 

hold on...I think it just sunk in. The valve events don't change. The valves are just at a higher lift point during overlap with higher ratio roller rockers. So, speaking in terms of the shaded overlap region of the graph in the link(fig A), the overlap would increase about the y axis, not the x axis ( in the verticle direction, not horizontal), right?

 

You nailed it.

 

No, the valve events do change slightly. The will change at all the points we normally look at .006, .050, and .200. This may be less pronounced at.006 since the increase in ratio isn't going to move that much. The change at .200 may be a bit more however.

Think about this. If you take you .050 reading with 1.7 rocker, and you swap it to a 1.85 rocker. The 1.7 duration point for .050 reading will now be at .054. Another way to look at this is your .050 value with an 1.85 rocker is only .045 with a 1.7 rocker.

While the gains are not gigantic, with a swap to a larger ratio, you pick up about 2 degrees of duration on most of the cams we're runninggoing from say a 1.7 to a 1.85, along with an increase in total lift of about 8-9%.

.600 lift with a 1.7 rocker is 0.3529 lobe lift
0.35294 lobe lift with a 1.85 is 0.6529 lift
.600/.6529 = 0.918
.918 = 91.8% of the lift, so about a 8.2% difference in lift.

 

Oooohhh...splittin' hairs....

Anything above 0.0" lift is multiplied...but I guess you can't find a cam card that
has absolute valve timing at zero lift. In effect, the true seat to seat events
wont change.

 

Simply put, cam overlap CAN NOT be changed, but valve overlap CAN be changed.

 

I don't know why they even bothered stating that? It's a little pet
peeve of mine. Who really cares what's happening at the cam lobe
anyway? We're not using 1:1 ratios.

You can change pretty much anything such as lift, effective duration, overlap, etc.
at the valve.

The only parameter that never changes is the lobe center angle.

 

Right all that matters is valve events.

 

Really?

The ratio of the rocker arm can actually cause the valve to open sooner (with respect to the cam lobe)? Hmmm.... think about that one again.

I will agree that different rocker arms with different ratios would allow a valve to open more (or less) at a given point on the cam lobe, but given a zero clearance (i.e. - "lash") Hyd cam, the 0.050 inch lift is mearured on the cam, not at the valve, and therefore the rocker arm doesn't even play into the equation.

Now, for the solid lifter crowd, I will conceed that rocker arm clearances (i.e - "valve lash") can indeed impact when a valve begins to move, but this is not the intended / primary function of valve lash; the clearance is there to allow for the thermal expansion of the materials involved, not to modify when the valves start to open.

Don't misunderstand, I am not disagreeing that opening up the valve lash will change (actually retard) the valve "events" - rather I am stating that is NOT the primary purpose of valve lash. As for rocker arm ratios, different ratios change the area under the curves (as Twerker stated) but at zero lash (the standard for Hyd cams), rocker arm ratios will not impact the relative time of the valve "events" because 'lift' is measured off the cam circle not relative to the valve and the combustion chamber.

So, J-rod gets partial credit, as his answer is correct for solid lifter cams but not hyd cams.

I hope that clears things up.

MIKE

Oh, and for Adrenaline_Z - I am NOT a mechanic, I'm an Engineer. Which means that when I tighten three fasteners, I start one, use a torque wrench to get it to the correct value, then I wonder why the other two bolts won't line up! Doh! (Or, as my step-father, a master mechanic says: "Stop thinking like an engineer!") I'm only in step 1 of my 12 step program so far; "Hi, I'm Mike, and I'm an Engineer..."