Timing

[Nismo_Freak]

Alright, my head is fighting with itself again and I can't understand this concept at the moment. Can someone explain it all out in detail for me?

Here's the scenario... you have an RB26 .. you advance the intake cam 4 degrees and retard the exhaust cam 2 degrees. Assume base timing is at 10 deg. total advance. Now the cam angle sensor is optical on the 26. My question is... what is the resulting ignition timing change? Is it 4 degrees ... 6 deg. total advance resulting timing? How will the ECU react to this... I'm all twisted up, as I've read several different posts/ideas on it.

[DAEDALUS]

I *think* the CAS is keyed to the cam sprocket, and not the camshaft. Also, I assume you mean that you're rotating the sprockets relative to the camshafts to effect your advance/retard. *IF* both of these are true, then I believe the ignition timing will not be affected. The chain controls the relative angles of all sprockets (crank and cams), so they will maintain their timing to one another as long as you don't remove the chain, regardless of how you adjust the camshafts relative to them. Then assuming, again, that the CAS obeys the sprocket, the spark will still occur 10 deg. BTDC. The also assumes the "eye" is fixed with regard to the block.

I think that's an RB26 in your sig...saw one a couple weeks ago, but it wasn't near as pimp as yours.

[Nismo_Freak]

Well ... you can't count on the sprocket being 100% on the spot when it's advanced either. I guess I'll just have to monkey with it and see what happens. Reason for the question is that it was brought up on the AEM tuning forum, and they battled back and forth over it and I can't make anything out of the babble they posted. I figured someone like Q45 Tech or one of the Nissan designers might know.

[Q45tech]

Don't worry about what you do with the intake cam.......just correct for exhaust cam offset as you are really trying to set the advance so that the burn is 70-80% completed at 16-17 deg ATDC.

The CAS is 2 degrees different from the old position [but 2 cam degrees is 4 crank degree]..........sprocket ratio. Remember you are trying to set the CAS with the crank position.

[DAEDALUS]

Well ... you can't count on the sprocket being 100% on the spot when it's advanced either.

I'm confused then. If you keep the chain in the same place (marks all lined up) the sprockets will always keep their relation to the crank, won't they? The crank (#1 piston) is what everything else is timed against.

[Q45tech]

If you keep the chain and CAS the same, yes except for chain stretch.......each cam sprocket is 10 degrees, each crank sprocket is 20 degrees.

[Nismo_Freak]

Don't worry about what you do with the intake cam.......just correct for exhaust cam offset as you are really trying to set the advance so that the burn is 70-80% completed at 16-17 deg ATDC.

The CAS is 2 degrees different from the old position [but 2 cam degrees is 4 crank degree]..........sprocket ratio. Remember you are trying to set the CAS with the crank position.

You mentiond advance on the exhaust side... now generally most engines respond to the retarding of the exhaust side cam. In fact, it's where most power gains are had on a turbo motor such as the 2JZGTE.

Are you refering to setting up the exhaust cam to correspond with ignition timing so that it isn't opening too early/late? Or are you talking about the actual timing of the camshaft itself.

Also, since the CAS is optical on the SR and RB series motors the CAS won't detect the change in timing as far as I can think. However the KA's distributer system will since it's driven by the "screws" on the exhaust cam. Therefor the KA will change it's timing if the distributer is not removed. Am I correct in those statements?

[Q45tech]

When I say advance I am speaking of ignition advance. What you do via variable sprockets has nothing to do with ignition advance in respect to the piston position.

You set the timing with a timing light looking at the crank pulley -------------piston position!

As to cam positioning retarding the exhaust cam [relative to oem position] increases the overlap thus reducing low rpm [charge dilution] and increasing high rpm [earlier blow down].

Think in terms of when the intake closes not when it opens and the opposite for the exhaust cam.....when it opens is the most critical.

You didn't provide duration and lift for the cams you are using, the turbo takes care of the filling but an extra 10 degrees of exhaust duration or higher lift is the way you get the gasses out.

What rpm are you trying to optimize peak for dyno or real world acceleration?

[Nismo_Freak]

Just theoretical, nothing applied as of yet... I fully understand about how to tune the valve timing in terms of piston position.

More or less I can do monkey tuning by just adjusting and seeing what happens to the powerband on the dyno, but I like to fully understand how and what exactly is changing inside the engine in terms of signals and overall flow. With this understanding I can reduce my dyno time, increase my ability to find power, and decrease the likelyhood of any mechanical malfunction.

BTW, thanks for answering my questions.

[s13sr20chris]

now thats customer service...

...except it was free...

...nico so rocks