Thread: 350 track engine

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Originally Posted by nhrafan

i have a 350 bored .40 over with flat top pistons. the heads are aluminum 190cc runners and 64cc combustion chambers. 2.02 intake and 1.60 exhaust valves with .550 net lift on springs. what is the best cam for this set up to get maximum hp and torque for the track?

Like Glenn stated, we need more information to intelligently suggest a camshaft for your motor. One of the first things a tech guy from your favorite camshaft grinder would ask you is: What's the static compression ratio of the motor? You cannot intelligently choose a cam for any motor until you know the exact static compression ratio. There are 5 values that are needed to figure the SCR and you have provided us with only 2 of them. We also need:
1. Piston deck height. (the measurement from the crown of the piston to the block decks where the heads bolt on)
2. Piston crown configuration so that we can figure the cc's (either positive or negative) of the crown.
3. Head gasket bore and thickness so we can figure the cc's of displacement in the gasket.

The reason you need the static compression ratio of the motor in order to choose a cam for the motor is that the cam profile dictates the intake closing point of the cam lobe, the most important of all 4 events on the cam (intake open, intake close, exhaust open, exhaust close). From top dead center on the intake stroke, the piston comes down the bore with the intake valve open and atmospheric pressure pushing the mixture into the vacuum created by the descending piston. As the piston comes to bottom dead center and reverses itself, going back up the bore toward top dead center to compress the mixture that has just been pushed into the cylinder, the intake valve is still open a little and the inertia built up in the column of mixture going into the motor is still a force. It has mass and it has velocity and will continue to pack the cylinder until the intake valve is completely closed. Here's where the juggling act comes into being. If we close the intake valve too soon, excessive cylinder pressure may be created when the plug fires and the motor may detonate on the fuel that is being used. If we close the intake valve too late, the ascending piston will be able to push some of the mixture back out the intake valve and back up the intake tract, past the carburetor venturis again, where more fuel is added to the mixture. The venturis have no idea where the mixture is coming from or where it is headed, all they know is that as the mixture passes through the venturis, a low pressure area is created (even with the mixture going backwards through the venturis) and all they know to do is to add more fuel to the low pressure area. So, on the next intake gulp, a very rich mixture is again drawn into the cylinder and a condition is created where the motor does not want to continue to idle. That's why, with an over-cammed motor, you have to keep blipping the throttle to keep the motor running, otherwise it will die from an overly rich fuel/air mixture.

Anyway, I got a little off track with explaining that. When you close the intake valve too late, some of the intake mixture that has already been drawn into the motor is expelled back out the (still open) intake valve by the rising piston and that contributes to less explosive mixture in the cylinder, so power is down. The trick is to close the intake valve at exactly the right time based on the static compression ratio of the motor. If you don't know the SCR, then you cannot choose the right cam.

Please read through this turorial that I wrote and that has been improved by a few other fellows....
http://www.crankshaftcoalition.com/w..._compatibility

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