A DEE O

iTrader: (2)

Do we see any, most, or all overlap when car is idling, or is it continuous throughout the entire rpm band. Is this directly related to the LSA on the cam. All replies are are appreciated..Angelo

LS1astro

I have no clue what your talking about but Overlap is the same as LSA just a diffrent way of reading it.

TimZ28

Overlap is NOT the same as LSA. In fact, LSA by itself tells you nothing about overlap. For two otherwise identical camshafts the cam with a lower LSA will have more overlap area. Overlap is desired to take advantage of exhaust scavenging effects. Two other factors effect scavenging will be RPM and exhaust tuning. A cam with a high amount over overlap area will not perform well in a relatively low RPM range. You will see alot of your intake charge flying out your exhaust. You also would not want a low overlap cam if you are intending to use a high RPM operating range.

Lobe profiles also directly impact the overlap area. For the same LSA the cam with the more aggressive profiles will have more overlap even if the advertised duration #s are less.

Going with a lower LSA may not increase your overall peak... you may only see increases outside your intended operating range. It's all about finding the right timing events for your particular combination.

Tim

A DEE O

iTrader: (2)

Thank's for the great info. The reason for asking, is that I am getting ready to install my D1SC Procharger, and I was told that higher LSA,s would be better for FI because of less overlap, and it pushes your power into the higher portion of our rpm range. Just as you explained, I am concerened that most of my boost will be wasted? Should I really do a cam swap, again. I have the TR 224/224/561/561 112 LSA. How much boost will actually be wasted, While both intake and exhaust are open at the same time? And does overlap occur through the enire rpm band. Thanks.....Angelo

TimZ28

Angelo,

A cam with less overlap is desirable in a FI application. Reducing overlap can be accomplished with reducing duration and/or LSA. You may want to go to the FI section and see which camshafts people have been using with their FI setups.

Tim

Colonel

"Reducing overlap can be accomplished with reducing duration and/or LSA."

I think you meant to say by reducing duration or increasing LSA. <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

TimZ28

Yeah what Colonel said! <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

A DEE O

iTrader: (2)

Thanks Chris, that was the answer I was waiting for. I will give this a shot with the TR224 and see how much boost I can make with a 3.6in pulley. <img border="0" title="" alt="[Smile]" src="gr_stretch.gif" />

JimmyKash

for all you techies out there:

valve overlap = (intake dur @ .050 + exhaust dur @ .050/2) - (LSA * 2)

for example my mild turbo flat tappet:

232/232 114lsa

(232+232/2) - (114*2)
232-228 = 4* of overlap

CHRISPY

For boost levels under 15#'s and considering your low duration numbers I wouldnt bother swapping cams unless you changed duration specs.

With the blower and around 8-10#s boost for stock displacement a 226/234 114LSA cam with 4 degrees advance works really well.
cheers,
Chris

92 B4C 396 LT-1

overlap=period in crankshaft degrees when both valves are open. Intake opening degrees BTDC + exhaust closing degrees ATDC = overlap.

Crane HR 296 @ .004 lift

int. open 37* BTDC + ex. close 39* ATDC =

76* overlap

JimmyKash

ok..ok.. i should stated that was for calculating overlap @ .050

better now?