Advanced valve timing.....

General discussion about our beloved Tercel 4WD cars
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takza
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Post by takza »

Have heard of this recently. Seems the typical advance used is around 4 degrees. Advantage is more torque at lower RPMs.

Since the cam gear on the Tercel has 40 teeth, each tooth = 9 degrees.

Been a while since I've had the cam gear off, but looks like is it held in position using a pin instead of a key?

A new hole or keyway could be drilled or cut on the other side of the gear as long as it was offset in the right direction vs any gear tooth...in fact several holes or keys could be cut...allowing several 1 or 2 degree adjustments from OEM....2-4-6....or 1-2-3-4-5-6 degrees advanced.

Question is: Is the Tercel already set up with this type of valve timing considering the higher torque level it has relative to HP?


This page outlines a method to find out:

<a href='http://www.iskycams.com/degreeing.html' target='_blank'>http://www.iskycams.com/degreeing.html</a>


"CHECKING VALVE OVERLAP WITHOUT DEGREE WHEEL OR DIAL INDICATOR

When installing a camshaft, or when an occasion arises where it is necessary to make a check on valve timing and no appropriate instruments are available, the recommended Isky procedure is as follows:

Insert the camshaft and mesh the timing gears on the stock marks. Do not as yet install the timing gear cover.

Adjust the valve lash of the intake and exhaust valves of the No. 1 cylinder.

Using a long wrench or lever, turn the engine over in the normal running direction. Use enough leverage to get an even, steady movement instead of a jerky motion. Rotate until the intake and exhaust valves of No. 1 cylinder are in the overlap position (both valves opened slightly). Stop exactly on T.D.C., which is marked on the harmonic damper.

Now loosen and back off the rocker arm adjusting screws until the intake and exhaust valves are just barely closed. Lock the adjustment screws so that the intake and exhaust valves are at exactly zero clearance.

Now turn the engine over exactly one revolution of the crankshaft to T.D.C. on the harmonic damper. You are now at T.D.C. on the compression or firing stroke.

Take Notice! Now there is a large space between the rockers and valve stem tips. The space indicates the actual amount the valves were open at T.D.C. of the overlap period (less valve lash, of course).

We will measure this gap space by probing with common feeler gauges of various thicknesses combined until we determine the gap space. After computing the gap, record the figures for both intake and exhaust in your notebook.

If the amount of gap on intake and exhaust is exactly the same, you have a perfect split overlap."


"Advanced Cam Position: If your intake happens to come out with .100 gap, and the exhaust with say .080 gap, your cam is in an advanced position. In this position, the cam will produce more low-speed power or torque. However, there might be a slight loss of power at high RPM."


<a href='http://www.huntracing.net/tech/valve_timing.htm' target='_blank'>http://www.huntracing.net/tech/valve_timing.htm</a>

"The single most important valve timing event when considering an engine’s desired operating range is the intake closing event. The reason for this is that there is only a given amount of time to complete any process within an engine at a given speed; as engine speed either increases or decreases, there is a corresponding increase or decrease in time available for properly filling the cylinder. Therefore, for increasing engine speed, the intake valve must be closed at a later time due to the decrease in time given for completion of cylinder fill. However, closing the intake valve at a later point in the cycle will hurt low end performance to some degree because of the fact that there is not enough charge velocity at low engine speeds to maintain the high inertia needed to completely fill the cylinder. The result is a certain amount of reversion of the charge back into the intake manifold at low engine speeds, which results in lower signal across the carburetor and poor idle and throttle response characteristics. Conversely, if an earlier intake closing point is selected, the engine will have good idle characteristics and tremendous low speed throttle response and torque. The trade-off in this scenario is that as engine speeds rise, the valve will close before cylinder fill is complete, resulting in a decrease in power at these elevated engine speeds. "

Other info:

"I have advanced the cam on two 2.2 engines, one in a 87 Daytona (turbo), and one in a Shadow (NA). The results were the same on both. More low end torque, and better gas mileage.

[Webmaster note: these results are reported by many other people as well, but none provided us with details on how to do it!]

The advance was accomplished by moving the cam gear one belt tooth clockwise, being sure not to slip the belt on the crank gear.

To advance the cam timing the cam gear must be moved forward clockwise from its normal alignment position while all other gears remain in their normal alignment position. Moving the cam gear one tooth clockwise advances the cam timing 9 (nine) degrees."

"Altering cam timing is one way to enhance engine performance, but it's often a trial-and-error process. In general, advancing cam timing moves the power curve lower, and retarding it moves it higher. That's more effective when enhancing a stock cam than aftermarket cams, since the aftermarket guys usually factor in a little timing advance beforehand.

The best reason to alter cam timing is to correct timing errors caused by manufacturing tolerances in the parts involved. It isn't uncommon to check the cam timing with new replacement parts (especially no-name and el cheapo specials) and find errors of 5-8 degrees. If you ever wondered why two identical engines perform at opposite ends, this is one of the reasons.

If you do need to alter cam timing, it can be done several ways, depending on your engine. One way is with offset camshaft-to-cam gear keyways, which can be bought in 2- to 8-degree increments, or with offset dowel pins, which advance or retard cam timing."

"Assuming constant lobe centers and cam duration:

- Advancing the cam slightly improved low rpm torque,
- slightly reduced peak power
- imperceptible change in emissions, idle & fuel consumption

****+5 degree cam timing results are a 3 to 5 change in torque from 2000 RPM to 4000 RPM

- Retarding the cam = opposite of the above"
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