help please dont understand what is being asked
 

there is a online calculator for headers size but couldnt understand one question.what will I write there:



Piston Stroke (inches) -ok
Piston Bore (inches) -ok
Exhaust Valve Opening Closing (ATDC) -? dont understand what is asked here?
Peak Power RPM -ok



thnx

i'm guessing but, ATDC = after top dead center.

They are looking for the number in degrees after top dead center of piston travel that the exhaust valves open and the duration they stay open. This should be listed on the spec sheets for your cams if you have them.

ATDC
 

it is a cam spec usually found on a cam card. There are actually 4 of them that are used to figure out duration and overlap. Charlie

I should write a number there only one number which one do I have to write?
here is the link:
click here

Tim here is the cam card for 139731 one of the most popular BBC Marine Roller Cams. hope this helps.



Crane Camshaft Specification Card
Part Number: 139731
Grind Number: HR-288-2S-12 IG
Engine Ident: 1967-UP CHEVROLET V-8 396 THRU 454 CU.IN.
WITH CAST IRON DISTRIBUTOR DRIVE GEAR.


VALVE SETTING: INTAKE .000 EXHAUST .000 ----> HOT

LIFT: INTAKE @CAM 345 @VALVE 587 ROCKER ARM RATIO
EXHAUST @CAM 359 @VALVE 610 1.70
ALL LIFTS ARE BASED ON ZERO LASH AND THEORETICAL ROCKER ARM RATIOS

CAM TIMING OPENS CLOSES ADV DURATION
@ .004 INTAKE 33.0 BTDC 75.0 ABDC 288 °
TAPPET LIFT EXHAUST 81.0 BBDC 35.0 ATDC 296 °

SPRING REQUIREMENTS
TRIPLE DUAL OUTER INNER
PART NUMBER 99896

LOADS:
CLOSED 132 LBS @ 1.906 OR 1 29/32
OPEN 405 LBS @ 1.326
RECOMMENDED RPM
RANGE WITH MATCHING COMPONENTS
MINIMUM RPM 2500
MAXIMUM RPM 6000
VALVE FLOAT 6500

CAM TIMING OPENS CLOSES MAX LIFT DURATION
@ .050 INTAKE 6.0 BTDC 40.0 ABDC 107 °ATDC 226 °
TAPPET LIFT EXHAUST 54.0 BBDC 0.0 ATDC 117 °BTDC 234 °

REMARKS: