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Home // MR2, 4A-GE, 7A-GE Tech // Fixing 7A-GE Cam Timing

Fixing 7A-GE Cam Timing

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Camshaft timing is where a 7AGE gets tricky. Any 7AGE has inherently incorrect camshaft timing because the distance between the crankshaft and camshafts is increased by a distance that is not an even multiple of the tooth pitch of the timing belt.

When Toyota created the 7A-FE, they started with the basic 4A layout, but increased the stroke and made the 7A block 15.4mm taller than the 4A block.  The stock 7A block also puts the pistons 0.6mm below deck height at top dead center (TDC) because the 7A-FE head gasket is only 0.6mm thick.  To use a 4AGE head gasket, machine (deck) the 7A block by 0.6mm.  Deck height will now be 14.8mm taller than a 4A block.

The taller block moves the camshafts farther away from the crankshaft. With a 14.8mm net increase in deck height, the tangent points (where the timing belt effectively makes contact) of the crankshaft’s timing pulley and the exhaust camshaft pulley are farther apart by 14.5mm in a 7A-GE than they were in a 4A-GE.

Timing belt pitch is 3/8″ (9.525mm) on a 4AGE; each tooth on the timing belt is 9.525mm apart, center to center.  Divide 14.5 by 9.525 to get 1.52; the cam timing is “off” by 1.52 teeth.  We can move the cam pulley relative to the belt by one full tooth, leaving the camshaft pulley 0.52 teeth ahead of the position Toyota wanted it in, relative to the crankshaft.

This disparity at the camshaft is the reason a 7A-GE normally has very wrong camshaft timing.

There are 36 teeth on each cam pulley.  Each tooth represents ten degrees at the camshaft and twenty degrees at the crankshaft (valve events are always discussed in crankshaft degrees).  Multiply the 0.52-tooth figure times twenty crankshaft degrees per tooth gives a 10.4 degree advance on both camshafts.  You could also install either camshaft 9.6 crank degrees retarded by moving them one tooth back.  Either way, it’s wrong.

There’s another problem. With any 4A-GE head, the distributor is driven by one of the camshafts.  If your cams are mistimed by a given number of degrees, so is your ignition timing.  The bad camshaft timing of a typical 7A-GE is made worse by also messing up the ignition timing.

Correcting the valve timing on a 7AGE can be done one of two ways:

The first way is to compensate for it with adjustable cam gears.  This works, but your baseline (starting point) is still off.  In other words, since you have to move the cams by ten crank degrees just to get to “zero”, your starting position is not what it is marked at on the cam gear.  You’ll have to remember how much it’s offset by if you want to know where your cams really are in relation to the crank.  Another drawback is that you will use up about ten degrees (again, as measured at the crank) of your new cam gears’ adjustment range just getting the cams back to the correct starting point.

A better way to correct the cam timing is to re-index the crankshaft timing gear by having a machine shop broach a new keyway into the gear, so that when the gear is installed, the new keyway retards the gear by 10.4 degrees.  This operation would automatically put the cam timing back where it is supposed to be, even with stock cam gears.  If you’re using adjustable cam gears, this would put their zero point back where it belongs and let you use the full adjustment range.  Since ignition timing is directly dependent on camshaft timing, getting the cam timing set to its default position automatically makes it easier to set the ignition timing correctly.

Unfortunately, many people build cheap-and-dirty 7A-GEs by throwing their 4A-GE head on a worn-out 7A-FE bottom end.  They are usually disappointed with the results and typically dismiss the 7A-GE as a bad performer that will doesn’t like to rev.  Correcting the cam timing would restore a lot of the performance they are missing. A well-built 7AGE can produce a broad, usable torque curve with plenty of power up top.  It’s much like an equivalent 4AGE with more power everywhere.

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