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Old 07-08-2005, 04:04 PM
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How to choose correct camshaft

Disclaimer: While based on facts about racing, this page contains my views
and experience.

Every racer wants to get the best return for the money, and choosing the correct camshaft will produce more power for the money. It is simple to understand camshaft technology with the correct tools, and choose the correct camshaft the first time.To learn more about choosing the correct camshaft, read on...

First, you will need to know a few measurements about your engine to plug into the formulas. I will use an example engine to walk you through the process. The example engine will be a 355 cubic inch Chevrolet small block with the following specs:

BORE: 4.030

STROKE: 3.48



CAMSHAFT: 249-12-908-9
Bracket Race
Drag Race 300BR-6 Mechanical Roller Camshaft Only
Operating Range: 4200-7200 RPM
Duration Advertised: 300 Intake / 308 Exhaust
Duration @ .050'' Lift: 264 Intake / 270 Exhaust
Valve Lift w/1.5 Rockers: .630'' Intake / .630'' Exhaust
Valve Setting: .026'' Intake / .028'' Exhaust
Lobe Separation Angle: 106

Take your Duration @ .050'' Lift: 264 Intake / 270 Exhaust and plug them into the formula atOpel Tuner's source (click on Software in left menu, then click on Download it here under Multi-purpose Excel-sheet, then click on Camshaft in the excel sheet and plug the duration numbers in to find camshaft events).

Cam data to event timing
Intake duration : (264)
Exhaust duration: (270)
Lobe Separation: (106)
Cam advance (+) / retard (-) 0.0 [ Crankshaft]
Intake lobe centerline: (106)
Intake opens before TDC: (26) [ BTDC]
Intake closes after BDC: (58) [ ABDC]
Exhaust lobe centerline: (106) [ BTDC]
Exhaust opens before BDC61) [ BBDC]
Exhaust closes after TDC29) [ ATDC]
Overlap (55) [ Crankshaft]

Then take take your number that the Intake valve closes after ABDC (58) and plug into this into the dynamic compression ratio formula at Rb Racing .

RSR Advanced Compression Ratio Calculator
Bore in Inches : (4.030)
Stroke in Inches : (3.48)
Rod Length in Inches : (6)
Static Compression Ratio : (13.1:1)
Inlet Valve Closes ABDC : (58) (0 to 90 Degrees @ .050" Lift)
Boost Pressure in PSI : 0
Target Altitude : (1000 Feet)

When you press calculate Dynamic Compression ratio, a little blue box will pop-up with all your dimensions. The one that is important is the dynamic cranking pressure of 230.88 PSI. This is perfect cranking pressure for a naturally aspirated engine is 230.0 PSI. When you choose the correct camshaft, you engine will perform better. Play arouund with the numbers on your own engine and see your dynamic cranking compression. WARNING: If you are running your dynamic cranking compression over 235.0 PSI, you are in dangerous territory for engine failure.

Wendall Carmichael

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