GIGAPUNK

iTrader: (2)

Presuming an optimal street exhaust system with good headers and cats, what is an optimal overlap? I've heard that it gets really hard to pass a smog test with much more than 6 degrees. I would presume that is about the point where gas milage tends to plummet also. Any one agree or disagree with these approximations?

Given the restrictions that even good mufflers and high flow cats present, would you even be leaving any/much hi-rpm torque on the table by not going to a higher overlap?







JUST FOR REFERENCE: I'm thinking about street strip hydraulic cam for a (.020" over) L76, that will be reving to 7200rpm:

3.622” stroke:

RPM FPM
6600 3984
6800 4104
7000 4225
7200 4346
7454 4500

presuming an L92 head flows at .05 @ 311/206 = 66.24% I/E

[372/2 x7200]/1728= 775
775/8=96.875 cfm
96.875/311= 0.3115
0.3115 x 720 = 224.28 (round up to 225 for kicks)

I/E = 66.24%
80% (presumed ideal)-66.24%=13.76
13.76/2 = 6.88 I/E split (round up to 7 for kicks)

225/232
457/4=114.25

114.25
-110 LSA
=4.25
X2
=8.5 overlap

114.25
-111 LSA
=3.25
X2
=6.5 overlap

114.25
-112 LSA
=2.25
X2
=4.5 overlap

Patrick G

iTrader: (14)

-6 degrees of overlap seems to be the magic number between pass and no pass, not positive 6 degrees...that's too dirty for the sniffer.

Beast96Z

iTrader: (21)

Were did you get this math from? Unless you know something I don't, you can't choose duration that way. While a 225 cam may be a good "mild" cam in a 372, it won't turn 7200 rpm's efficiently. You have a ton of other factors to go through. While you did calculate overlap correctlly, you listed it as split. Really confused on the math thing.

GIGAPUNK

iTrader: (2)

thanx, i'll edit that.

The 225 is just a ruff calc based off of raw cfm that I once saw CStraub use.

What do you use to determine duration? Do you agree with the +7 on the exhaust.

Beast96Z

iTrader: (21)

Lots of things factor into choosing an exaust lobe. I would think you should look into the exaust timing instead of giving a set x amount of split. The .050" duration should be a by product of the correct VE's.

GIGAPUNK

iTrader: (2)

Was I calculating positive overlap degrees in the three examples above?

How do you determine VEs on an engine that's never run?
And wouldn't the cam change the VE as well?

thehammer69

iTrader: (2)

If you pick up software like Dynosim Advance with pro tools...it will list the estimated VE per rpm point in the Pro Data tables.

Hammer

GIGAPUNK

iTrader: (2)

So would you then calculate your duration off of the VE at your maximum RPM? And what would that calculation look like?

Beast96Z

iTrader: (21)

If your looking for a math problem to give you a camshaft it's not going to happen. Peak rpm is needed in deciding on a cam, but if you choose performance way up in the rpm's, your going to give up on the low end. Diffrent lobes have diffrent durations at givin lifts, so cylinder head flow/intake flow/induction/exaust/gearing/and so forth all play into which lobe may work best in your application. Cams can also be geared toward you driving habits, such as road racing, street, and drag. Even in my cam selection, I still plan to have to swap after my initial tuning. We choose what we thought was close and we'll tweak from there.

jimmypop13

iTrader: (17)

I passed the sniffer with a TR230 cam, I wanna say it has 8 degrees of overlap at .050 . Before I went to the station, I screwed in the throttle body set screw so the idle was around 1100rpm, I put on cats after my longtubes, poured RXP in the gas tank (was told it helps with emissions big time) and ran the car hard on the way to the station to make sure the cats were extra hot. I passed first try!
-Chris

GIGAPUNK

iTrader: (2)

I originally wasn't very concerned with passing the sniffer. I was hoping to gain a little insight into the play between exhaust back pressure, and the overlap where you start to have diminishing returns.

Formulated

iTrader: (16)

Any 224 cam with 114 LSA seems to be the benchmark street cam that easily passes smog tests. Others say that a 224 cam on a 112 LSA will pass with a carfull tune. The difference between these is the 114 LSA has -4deg overlap (at 0.050), and the 112 LSA has 0deg overlap. I have to get through the California smog test myself, so I've been using these two as benchmarks for what is acceptable. Selecting a cam on overlap is very restricting, though, because I would like something with enough duration to make some power, but I end up with way too early EVO, or way too late IVC. I've tentatively come up with 228 intake and 224 exhaust on a 113 LSA. I'm not sure what this would run like with the reverse split, but it's typical of the weirdness that results with trying to make a well behaved street cam.