Thread: smallblock cam selection

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Hey Tech. Can you tell what RPM's a cam should be able to turn buy the Duration and center lobe? Kurt

Originally Posted by vara4

Hey Tech. Can you tell what RPM's a cam should be able to turn buy the Duration and center lobe? Kurt

You can get an idea from the duration, but the lobe center can be all over the place, 104-114. The rpm numbers I've shown here are indicative of the power range of the camshaft, not necessarily the min and max r's of the grind.

This is from a chart I have on flat tappet hydraulic cams for a SBC....the number following the LC is the intake closing point @ 0.050". For those of you playing along at home, Lobe Center, Lobe Separation Angle and Lobe Displacement Angle all mean the same thing. These are not to be confused with Intake Centerline, the point of maximum lift of the intake lobe.... or Exhaust Centerline, the point of maximum lift of the exhaust lobe. If the intake centerline is 106 degrees AFTER top dead center and the exhaust centerline is 114 degrees BEFORE top dead center, then adding 106 and 114 together and dividing by 2 will reveal a Lobe Center of 110 degrees. A lobe center of 114 will give a dead smooth idle and high manifold vacuum. A lobe center of 104 will give a choppy, machine gun idle and low manifold vacuum. The last figures are the ideal static compression ratio to run with that particular cam timing. I didn't have all of them, but it shouldn't be difficult to fill in the blanks.
184/194...104/16.....500/4000
184/194...112/19...1000/3200
192/204...112/23... 800/4600...7.75-8.75
194/204...104/21.....800/4400
204/204...110/27...1000/4600...8.0-9.5
204/214...110/32...1200/4800
206/218...112/30...1200/5200...8.0-9.5
210/210...110/30...1400/5000...8.5-10.0
210/216...114/34...1600/5400
212/218...114/35...1600/5400...8.5-10.0
214/224...112/34...2000/4800
216/216...110/33...1600/5400...8.75-10.0
216/228...112/35...1800-5600
218/218...106/31...1800/5400...8.75-10.0
218/230...112/36...1800/5800...8.75-10.75
222/222...110/36...2000/5800...9.5-10.75
224/230...110/37...2200/6400...9.5-10.75
226/226...106/34...2400/6200...9.5-11.0
228/228...106/36...2800/6400...9.5-11.5
228/228...112/41...2800/6200...9.5-11.0
228/232...107/37...2800/6000...9.0-10.5
228/234...106/36...2800/6400...10.0-11.5
230/230...106/37...3000/6400...10.0-11.5
232/232...107/39...3000/6000...9.0-10.5
232/236...106/38...3000/6400...10.0-11.0
236/236...107/41...3200/6200...10.0-11.0
236/242...112/40...3200/6600...10.0-11.0
236/244...110/43...3000/6800...10.25-12.0
238/238...106/41...3200/6400...10.5-12.0
238/244...106/41...3200/6800...10.5-12.0
244/252...106/44...3400/7000...10.5-12.0
246/246...106/45...3400/7000...10.5-12.0

As the intake valve closing point numbers get larger, the intake valve is closing farther away from bottom dead center while the piston is headed up in the bore on the compression stroke. Compression cannot begin until the intake valve closes and as the intake valve closes later and later with a bigger and bigger cam, more and more of the fuel/air mixture that has just been drawn in through the intake valve, gets pushed back out the still-open intake valve by the piston and up the intake tract to disrupt the signal at the venturi. Result: rump-rump. As the cam gets wilder, it takes a higher and higher rpm point for the motor to "get up on the cam" and smooth out to begin making power.

This also explains why you have to coordinate the static compression ratio of the motor with your cam choice. The amount of mixture left in the cylinder after the intake valve closes has to coincide with the squeeze you're gonna put on the mixture. Close the valve too late (long cam) with a low static compression ratio (the most common rookie mistake) and you have a lizard due to insufficient cylinder pressure. Close the valve too early (short cam) with a high static compression ratio and you have a motor that will detonate on pump gas due to excessive cylinder pressure.