Unreal

Yep, and compression too. My motor is 9.7:1 which changes the low RPM drivability too.

Michael_D

Boost can be a wonderful thing. I drove a buddies manual hellcat the other day. I cannot lie, it was a downright pleasure that had me thinking seriously about selling my Z and buying one.....

Unreal

Next door neighbor traded his 2012 Centennial Edition vette in on a bright green hellcat. Loves it. Said it does everything better but turn, but still turns well enough to get arrested on the street. He has only 1 car, so the Hellcat really fits his needs a lot more than a vette.

NW94Z

Let's talk about overlap. Am I correct in calculating the overlap on the stock cam is +21.5 degrees? What are the pros and cons to overlap above and below 0. How does one consider overlap and it's effect on drivability and performance when modifying additional aspects of the car. Ported/ milled heads, headers, CAI, ported TB and intake.

I like the information this thread has developed and the relative civility by all contributors. Let's keep this going so those of us out there now and in the future will be better prepared to make a cam selection that isn't only concerned about max power at the expense of everything else. Not that there is anything wrong with that. This is a different approach to modding a car and often neglected on this forum I think.

MyLS1Hauls

Stock cam has negative overlap...actually around -20*. The more overlap a cam has, the more lope you will get, and the more drivability issues you will have. Overlap does help make more power though.

When the intake valve opens as the exhaust valve is closing (the more overlap, the more pronounced this is), the exiting exhaust gasses help to pull more air through the intake valve, helping fill the cylinders more, and "flush" any exhaust gasses out. At low RPM and low throttle opening, the opposite can happen, where exhaust reverts back into the intake, diluting the charge, resulting in rough running.

If you want a car that is streetable, keep the overlap down. Make no mistake, a cam with 20* overlap isn't going to be very friendly. I'd stay under 15*, and even that can be too much for some.

Unreal

Stay at 5 or even zero for good driveability IMO.

Michael_D

Overlap is a pretty complex topic. Not one that's easily jotted down in an internet forum post. There are some very general rules of thumb for how much to target per engine application, but they're very rough. Hell, I have a couple I put together over time, but they suck too. There's just way too many variables in an engine to put valve overlap into little boxes.

To know the amount of overlap a camshaft has, you need to know the seat to seat duration (advertised duration). And if you are going to compare camshaft profiles, you also need to know valve lift at several points of camshaft rotation, so you can drop that data into Excel and plot a bell curve. Then you will know what the "under curtain" overlap really is, as well as know what kind of profile the cam lobe has.... Good luck getting this information from a reseller, or even the cam grinder. I don't even see it on cam cards anymore. Or, you get a Cam Doctor report, but then again.....good luck.

What you see nowadays, is at fifty numbers. Rarely will you get valve event numbers, or advertised duration. The whole damn world talks "in fifty" anymore. And that doesn't really tell you shiit, other than give you one data point for comparing camshaft grinds - but it's only one data point of many that you really need.

Rambling aside.....

The stock LS7 camshaft valve events at .006" lift (I presume these are correct):
IVO BTDC: 13 deg
IVC ABDC: 79 deg
EVO BBDC: 83 deg
EVC ATDC: 29 deg

If in fact those are correct (got them off the internet, so they MUST be correct...lol), then the valve overlap in degree of duration, is 42 degrees. The "at fifty" degree of OL is negative 19.5 deg.

So, how much is enough, but not too much?? Depends. Here is what I've come up with as my VERY ROUGH, rule of thumb for cam selection (N/A builds only). The overlap duration numbers are ADVERTISED duration - NOT .050".

The shorter side of each group works for smaller CID while the longer side of each group is for larger CID.

10*-35*: trucks and RV’s that require low-end torque and where gas mileage is a concern.
30*-55*: cars and trucks where idle quality, low-rpm performance, good street manners and strong off-idle throttle response are desired.
50*-75*: high performance cars where performance is the primary goal with reasonable street manners.
70*-95*: roundy-round, open road race, and strip engines that consistently operate at high rpm’s
90*-115*: all-out drag-race engines.

NW94Z

The formula I've been using to calculate cam over lap is:

((Intake Duration + Exhaust Duration)/2)-(2xLSA)

Any obvious flaws with it?

I understand what your saying about advertised numbers at fifty degrees. This is only one of a multitude of data points and the overall shape of the cam lobe really tells the complete story which can be different than what one may expect. Still it is a common point of reference for comparing different cams and the best we can hope for.

In looking at your rough scale of overlap and behavior what would your values equal at .050?

The cams I'm looking at are close to zero plus or minus only a couple of points. Actually I calculate the BTR cam at +8 degrees. In order to take advantage of the better flowing heads and exhaust at least a little positive overlap is desired. Reliability and drivability remain my first two goals so an EPS lobe cam is already decided. I still need to make those two phone calls about the cam. Maybe something with a hair more duration to increase the overlap by a couple of points. Was I reading in this thread or a different one about the LS7 liking more separation. between the intake and exhaust.

Michael_D

Cam lobes are rarely symmetrical anymore. So the formula will get you in the ball park, but will not be exact. Also, make sure you account for advance that may be ground into the lobe, as most LS cams have this.

I stated this engine likes a wide split in duration, and it does. There is a reason why some of the grinds you see do not have much, like ten or less. Because the grinder wants their cam to make good power (so they can sell more of them), but they don't want to add too much exh duration, or the car will loose drivability. So, they make compromises. If they would have keep the split wider, like 15 or more, then power would be higher and flatter. Take a look at the camshafts GM offers for this engine. You'll see a trend.

All engines, even turbo rigs, must have some degree of overlap. That's what purges the combustion chamber of exh gasses. I do not have any good rules of thumb for .050" duration numbers. I never buy a camshaft that I don't have the valve events for, and if the grinder wants to keep them "secret", then he can keep his product too. The real secrets, are the shape of the lobe, not the valve events. Any idiot can spec a lobe and centerlines from a lobe catalogue. Then give it a cute name, call it secret, or my favorite "proprietary".....hilarious.

NW94Z

I'm getting closer again to a cam decision and have a couple more questions. With the exhaust duration and wide split you have on your cam how is the low rpm throttle response? Does it feel at all sluggish or delayed over stock? If so where does it catch up and surpass the stock cam in your opinion?

fueledpassion

That's almost possible. PM me because I'm in the same boat and I have a builder that is doing just that for me. We're aiming for 560/520 on nothing but Stage II heads/cam and CAI + Pfadt headers. The cam selection is critical when using Pfadt headers. Those headers don't care for huge overlap and large durations. They don't need that much which is why the tri-y is an excellent choice for street manners because u can hit ur goals with less cam.

Power under the curve is simply better for a street car. It will be faster even if u gave up a but of power up top.

Les-10

Drove a friend's 07Z yesterday with the 116 Torquer & found it to be much more to my liking (lack of surging/bucking, etc.) than my 110 Torquer. Didn't think there would be enough of a difference to warrant a switch but there definitely is. I do realize there will be a power deficit. I'm wanting to go with EPS lobes vs XER for less VT stress. I'm very interested also in the 224/244 .625/.625 on a 115 that Michael_D had suggested to the Op.
Thoughts?