Building a 7age.

[fantapants]

penis, i dont understand how a 20v head on a 7a block with vvt and no adjustment to timing can work perfectly? In fact i dont understand how any of the 7a builds can be perfect timing with no modification? At the end of the day, correct me if im wrong, a belt is a belt is a belt. If its the right length for tension, its going to have the same interaction between crank and cams as any other belt?

And if you use an exhaust adjustable wheel, you are only adjusting the exhaust cam???

and ffs please put some effort into making your posts legible? i get a headache every time i have to decipher something you have posted.

[ben_ray_91]

sooo the way i understand it, if you slap on a 20v head and use a 16v belt the timing will be out by a few degrees? this can be fixed with adjustable cam gears yes?? but the cam gears will be offset at standard timing adjustment?

Getting Bazda to recut the keyway is the best and cheaper option if you are using stock cams and duration?

[Matt]

i'm keen to know the answer to this as i'm literally a week or so away from getting my 7a block machined...

Lets not turn this into a pissing competition either.

To summarise the whole reason for the recut keyway...

7a block is 10mm higher than 4a
You use a 4a Crank Cam belt sproket (Bit the belt goes around )
4a Bottom timing belt cover

The crank timing lines up with the markings with the 4a crank gear... and is considered to be correct.

Put on a 16v or 20v head, and your cam gears are out by a couple of degrees? Why is this the case due to the height? The extra height doesn't add any gears and it should spin at the same rate regardless right providing it starts in the same spot?

[fantapants]

tbh matt i got no idea. im like you, weeks away from putting this pos together, but i did all my researching and buying about a year ago.

i went with the bazda recut keyway. Plan is a 16v belt, and i have a ae101 oilpump, so no hydraulic tensioner)

What you have said makes sense, but thats probably because im mildly retired?

[70XIN]

Cam timing is completely dependent on the spacing of the teeth (not length of the belt), and the overall height between gears.



I have attached a photo of a stock 4AGE, just to explain things a bit easier. This is WAY easier to imagine on a smaller scale, as the 7A problem has nothing to do with being multiple teeth off - it's the problem that it's multiple teeth and a few mm off.

Imagine that the deck height (or easier yet, the distance between the cam gears and the lower sprocket) is only 1mm higher, and that the crank sprocket is stuck in the perfect TDC position and cannot move no matter what. Because the timing belt between the exhaust cam and the lower sprocket ALWAYS has to be "X" number of teeth apart, and always remain tensioned, you quickly see that you HAVE to rotate the cam gear just the tiniest bit to the right to line up the teeth perfectly. This change also effects the intake cam gear, which also always wants to be exactly "X" teeth away from the exhaust cam, so it wants to rotate a little to the right to line up as well.

Your timing is now fux0red - this is similar to why a 7A is screwed up - if you try and put it together with stock everything, you'll notice your timing belt teeth are almost exactly half a tooth out - literally the worst possible timing.

I hope that makes sense - i'm pretty herp derp at explaining things.


**This is also why decking your head/block, and running different thickness head gaskets has interesting effects on cam timing**

[fantapants]

and thats why i didnt try and confuse any one any more, knowing seamus would eventually be in here to explain things WAY betterererer than i could

Very clear explanation mate, thanks

[dying3k]

also the reason, people who go out and deck blocks and shave heads to the shit house and run adjustable cam gears get decent power out of a basically standard 4a

[fantapants]

what?

[Seventy]

Bringing this back up again, so the fwd 7a flywheel will be fine to use with an jdm 4a starter? thanks

[70XIN]

Correct