Hot Rod Forum banner
1 - 1 of 1 Posts

· Registered
16,302 Posts
This would be a Throttle Body Injected (TBI) engine unless somebody molested it. The computer on these engines does not have a whole lot of learn capability, it is there mostly to compensate for wear. If the engine is still in decent shape that is it holds good compression or passes a leak down test then its breathing should be in the range of the computer program.

The computer is looking for three principle data points these being manifold vacuum which for these engines is called Manifold Absolute Pressure and is tracked by a MAP sensor mounted on a bracket on the right side of the engine viewed as you would be seated behind the wheel. The others are throttle position which is the Throttle Position Sensor (TPS) mounted on the right side of the TBI in line with the throttle shaft. There is also the Idle Air Controller (IAC) on this side so don’t confuse them. Last big data input is RPM. The way this works in programmed in the computer is PROM chip (Programmed Read Only Memory) for all possible fuel and ignition timing values expected in normal operation. These data sets reside as a package assigned to an address. The address is a calculation derived from an integration of the MAP to TPI to RPM at specific steps so this thing does not allow much variation of the integrated address.

Making cam changes makes changes in the integration of the MAP, TPS and RPM readings to where the wrong or a invalid address for fuel and ignition, resulting in poor engine running, but there is built in help if the cam isn’t too big.

Built in help is a functional O2 sensor. This is key to the engine wear compensation of the learn program. If the calculated data address content is close enough for the O2 sensor to correct it will do so and after so many repeat events it will modify the data in that address making a permanent change. You might at first sense this as a hiccup but will eventually smooth out.

For a big cam about as far as the computer can be pushed is to a timing that looks like 256/268 degrees measured at .006 inch lift as the lobe’s ramp starts to move the lifter. Measured at the .050 lift point the duration would be about 195/202 degrees. Lift at the valve the top would be like .417 inch intake and .439 inch exhaust. More cam than this simply goes beyond the computers learn function and forces a custom program be made which isn’t cheap and is often problematic resulting in a lot of vehicle down time till the chip program gets dialed in. The sighted cam timing is about 30 degrees more than the cam you have in the truck so it is a considerable increase the lift is a little better as well. Your engine has the Swirl Port heads, these are more sensitive to increased duration than increased lift simply because the ports of these heads don’t flow big numbers so holding the valve open longer has more value than opening it further.

This computer program problem also extends to major displacement changes. The usual .030 to .040 overbore is not upsetting but a 383 kit sure is.

Headers and duals work with this cam through the computer as well. Long primary tubes with a couple feet on the end devoted to collector does much to improve mid RPM torque these work with this combination of cam also. Yes they also improve top end high RPM power but this engine isn’t going there so if you can take advantage of what they do for mid range torque.

1 - 1 of 1 Posts