Using only cam for triggering

[wulfgang]

I know there are negative comments in the wiki with regard to using only cam angle sensors for triggering (because crank triggering is considered more precise). However, I also know that many OEMs use CAS for normal ignition triggering, and on many of the OBDII engines, the crank sensor is only there (often on the flywheel) to diagnose errors with the CAS. The engine I am using, Audi AKB V8 (from 2001) has two cam sensors, one on each bank, and only a VR sensor on the flywheel (obviously there are no missing teeth on the flywheel).

Setting up VEMS to use the cam synch with crank triggering currently looks to be complex, as judged by the forum comments and minimal documentation in the wiki. So what's the problem with just using cam triggering alone, with no crank trigger? The lack of precision can't be that big a deal... if it were, the OEMs would all have crank triggers.

[EDIT] Actually, I think I can answer this now. The flex plate has a cage-looking thing bolted to it which is essentially a 60-2 setup. However, I wasn't planning to use the flex-plate, so it looks like I'll be adding a wheel to the crank pulley, making this post moot.

[rob@vems.co.uk]

You can use a non-missing tooth crank trigger and single cam pulse if you want, it just takes some cranking experiments to check the setup of the cam trigger.

[wulfgang]

Hmmm... the engine was from an automatic with that flex plate/tone ring setup. But I'll be using the transmission from a Porsche Boxster, which has the tone ring on the engine side of the flywheel. So everything is there for both setups, but it looks like they probably won't line up, which means that there will be no place to mount the VR sensor unless I find a good spot to drill a hole in the bell housing.

[wulfgang]

How does variable cam timing work with cam sync. As I understand it, there is a *very* small window for cam sync on a V8. What happens when the variable cam timing causes that sync point to move around a bit?

[rob@vems.co.uk]

How far off the trigger tooth is the cam sync tooth?  The ecu doesn't care much about the position of the cam sync - it just starts the count again when it reads a cam signal.

[mattias]

How does variable cam timing work with cam sync. As I understand it, there is a *very* small window for cam sync on a V8. What happens when the variable cam timing causes that sync point to move around a bit?

When the primary trigger is of "missing tooth" type the cam sync timing does not matter, it should just always appear before or after the missing teeth.

[wulfgang]

When the primary trigger is of "missing tooth" type the cam sync timing does not matter, it should just always appear before or after the missing teeth.

Does this mean that the firmware has been changed? Previous posts, the wiki, and the VEMS triggering pdf all say that the cam sync must come after the last previous ignition event that is before the next crank trigger, but before that next crank trigger.

So you're saying that the cam sync can float around within almost a full crank revolution so long as it does not race with the missing teeth on the crank trigger? That makes perfect sense.

[rob@vems.co.uk]

Not quite what I am saying - how mcuh does the trigger actually float around?

[wulfgang]

Not quite what I am saying - how mcuh does the trigger actually float around?

From what I can find, variable cam control authority is 22 degrees for the 40-valve V8.

I'm curious though. The h[0] or h[2] or whatever tables have 8 entries, right? So regardless of whether or not a cam sync comes around, the sequential should continue to work fine once it has been properly sync'd on a V8, shouldn't it? So maybe there's a way to pick up the sync right at startup when the cam timing is stable and then just hide that signal after a few seconds before the cam timing starts to vary.

Here's the reference about the cam timing that I found. It says 2003 year model, but I think the 2001 engine was essentially identical:

http://www.audiworld.com/news/02/a8launch/content4.shtml

[mattias]

Previous posts, the wiki, and the VEMS triggering pdf all say that the cam sync must come after the last previous ignition event that is before the next crank trigger, but before that next crank trigger.

That is only true for a simple primary trigger, where each tooth represents a single cylinder event.

So you're saying that the cam sync can float around within almost a full crank revolution so long as it does not race with the missing teeth on the crank trigger? That makes perfect sense.

Exactly.