Redlinez

iTrader: (5)

OK, this isn't shown in most cam specs unless you see the cam card or cam doctor report I guess. I would think this is a fairly important spec that I don't often see discussed. Could someone tell me what the difference is in one with say 9 degrees versus 5-6? I would imagine the higher degree of overlap also affects the fuel coming out of the exhaust. I know with my cam, it affected PTV clearance and we couldn't machine the heads. What is ideal for a street car with heads, if there is such a thing? I know there are valve event timing, lobe type, ramp speeds, and other things that come into play, just looking at this from a more generalized point.

TXZ0603

iTrader: (1)

I cant explain it but i know i have 20.5 deg of overlap with mine

5_02ls1

iTrader: (20)

@050 int dur+exh duration divide by 2 subtract lsa twice= overlap

the more positive you have the more its gonna lope the more negative the smoother it will idle and be easier to tune....the more positive usually means that it will create more cylinder pressure.....hope that helps theres a few good threads on here in the stickies..lots of reading but educational

LS1C5Vette

iTrader: (11)

I have 12 on mine

wht01ws6ta

iTrader: (45)

20.5 degrees of overlap Daily drive with that much? what's the specs on your cam? 245/245 on a 112 would get you 21 degrees of overlap

wht01ws6ta

iTrader: (45)

Not to hijack but my uneducated question is this: at 0 degrees overlap the intake and exhaust valves are not open at the same time correct? Or, is it wiht an overlap of 0* that they are just not open at .050 but they are both still open at the same time at a lower lift like .006? Once they are both closed and not open at the same time and there are no degrees of overlap, it shouldn't matter how much negative the overlap is, right?

ShevrolayZ28

iTrader: (4)

0 degrees means the intake and exhaust valves are never open at the same time. 0.050" durations will not give the correct overlap, you need to use advertised durations.

Negative overlap is undefined. There's no such thing.

Here's a link with some good examples.

http://members.uia.net/pkelley2/Overlap.html

HEMIETR

iTrader: (2)

For comparison sake. overlap at .050 is fine. With that being said I have 25*

wht01ws6ta

iTrader: (45)

That's some good info, thanks. It seems every time I see overlap shown on here it is measured at .050 though but I guess that's not completely accurate.

I guess that's where some good tuning skills come in handy.

It seems the main thing with measuring the duration at .050 versus the advertised duration are the opening and closing ramp rates.

ls2 bait

iTrader: (11)

i have 23* of overlap and my car is a gas sucking pig that bucks like a pissed bull in the low rpms.

wht01ws6ta

iTrader: (45)

Mine is 9 degrees overlap figured at .050

I just checked the cam card from COMP and did not see any advertised duration, only duration at .006 and .050and it listed the valve events at .050 from which I figured up the overlap (INT opens at 8* BTDC and EXH closes at 1* ATDC so 8* + 1*= 9* overlap which is the same overlap as just figuring the overlap from the durations at .050

So, I have no idea what my real overlap is.

Patrick G

iTrader: (14)

For a daily driven street 346 ci LS1, you can have perfectly acceptable street manners with almost no issues up to 7 degrees of overlap at .050". You will continue to gain power with more overlap, but once you pass 15 degrees of overlap, the gains begin to diminish quickly. With 25 degrees of overlap, you will need a very skilled tuner to make this cam enjoyable with an EFI setup.

Here are some popular cams and their overlaps.

TR 224/224 on a 112LSA = 0 degrees at .050"
228/230 F-13 on a 112LSA = 7 degrees
G5X3 on a 112 LSA = 14 degrees
T-Rex = 25 degrees of overlap at .050"

DrkPhx

iTrader: (15)

My previous cam had 21 degrees and the new one has 24 degrees of overlap. The new one required alot more timing in the cruising range to reduce the bucking,

HEMIETR

iTrader: (2)

It is not a good time for me to take up the art of tuning. But, I have it running pretty well. I'm not a cam expert but this thing is fairly smooth for the amount of overlap. I can idle through a parking lot in first gear the majority of the time. More tuning to come...

Beaflag VonRathburg

iTrader: (176)

How does increasing cubic inches affect this? Lets say a 408. Since with the larger bore you typically have larger valves. The amount of overlap you need reduces as there's a greater area for flow. You can end up diluting the intake charge with too much overlap. Semi educated guess.

Redlinez

iTrader: (5)

Thanks PatG, that's what I wanted to know in a nutshell.

Patrick G

iTrader: (14)

Increasing cubic inches without increasing valve area makes the engine "undervalved" and it will want more overlap. Increasing valve size area while increasing displacement keeps overlap from having to increase as much, but as a general rule, you want to add 8 degrees of duration (and overlap) for every 50 cubic inches of displacement increase. A 408 is 62 cubic inches larger than a 346 so it will want around 10 degrees more duration to have the same street manners and power range as the 346 (62/50 = 1.24 x 8 = 9.92 duration increase).

ls2 bait

iTrader: (11)

while your being technical what is the advantage to cams with split durations? one you just specd for me had an 8* split i was just kinda curious about that.

Beaflag VonRathburg

iTrader: (176)

Thanks, that's pretty much what I had in mind. I'm going to be sending you an odd cam request then.

Patrick G

iTrader: (14)

Tommy,

The amount of split between the intake and exhaust is mostly a function of getting the valve events where you want them. We'll want the intake valve to open at a certain point and close at a certain point to work best with our rpm range and the tuned resonance of the head/intake manifold combo. The exhaust valve will want a certain opening point depending on its flow, the exhaust header size, the rpm range, and whether there's a power adder involved.

In your case, you are running a smallish exhaust port with a smallish exhaust header (1 3/4") on a large engine (416) with a 200-250 shot of nitrous. The larger split was my approach to extending your rpm range without having to run too big of an intake lobe. It's all about balance.