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Old 02-16-2010, 10:51 PM
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The first thing you need to know to spec a camshaft is the exact static compression ratio of the motor. You need to know this so that you can choose the proper intake valve closing point to trap just the right amount of fuel air mixture to make good power without detonation on the available fuel you have to run. Just looking through my Crane catalog, I'll give you some for-instances....

Powermax H248-2, power range 800-4600 rpm's, advertised duration 248/260, 0.050" duration 192/204, lobe separation angle 112, intake valve closes at 23 degrees after bottom dead center. This cam is recommended for use in a motor with 7.75-8.75 static compression ratio. If you average the SCR in this case, you can see that a motor with 8.25:1 SCR needs to close the intake at around 23 degrees ABDC.

If you closed the intake valve earlier than that, you might trap too much mixture and introduce detonation. If you closed the intake valve later than that, some of the fuel air mixture could be blown out the (still open) intake valve by the piston coming up the cylinder on the compression stroke. When the piston blows mixture out through the open intake valve, it travels back up the intake tract and disrupts the signal at the carburetor venturi. The venturi sees mixture going both ways and it doesn't know whether to sh** or go blind, so there isn't a good homogenization of fuel/air being blended.

Now, let's move up on the cam selection. The H260-2 Powermax has an operating range of 1200-5000, advertised duration of 260/272, 0.050" duration of 204/216, lobe separation angle of 112 degrees, intake valve closes at 29 degrees ABDC. This cam is recommended for use in a motor with 8.0-9.5 static compression ratio. If you average the SCR in this case, you can see that a motor with 8.75:1 SCR needs to close the intake valve at around 29 degrees ABDC.

For the last example, let's use the 284H12 Energizer. Operating range 2800-6200, advertised duration 284/284, 0.050" duration 228/228, lobe separation angle 112 degrees, intake valve closes at 41 degrees ABDC. This cam is recommended for use in a motor with 9.5-11.0 static compression ratio. If you average the SCR in this case, you can see that a motor with 10.25:1 SCR needs to close the intake valve at around 41 degrees ABDC.

Now, this little bit of knowledge isn't going to allow you to choose a camshaft for your motor. There is so much more to it than this, but we have to start learning somewhere.

Go to the various cam grinder's websites and read everything you can. Come back here and ask questions. Read some more. That's the way we aquire knowledge.

Start by reading this page on Performance Camshafts by Dimitri Elgin. He's the main force behind Elgin Camshafts and is, in my opinion, a very smart man....
http://www.elgincams.com/campaper.html

One of my pet peeves is the newbie that has the 8.0:1 motor and wants a lumpy camshaft. It should be clear to you even with the little bit I have outlined here for you that this AIN'T GONNA HAPPEN. A lumpy camshaft, as a general rule, will have quite a lot of duration. A lot of duration closes the intake valve later, so you need more SCR to compensate and keep the balance between SCR and intake valve closing point. Bottom line: If you want a lumpy cam, coordinate the components in the motor to reach the proper SCR to match the cam.
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