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#113 Dual VVT Tunning - A question about Overlap

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Hi,

I'm building a 1998 BMW M3 3.2l Dual Vanos, in fact is already built, its mods is:

- 284 schrick cams intake and exhaust

- 0.5mm oversized valves,

- custom pistons with 12,3 compression ratio

- Fuel E100

It was tuned on it's oem Siemens ECU and now I'm removing MAF and all intake sistem to use a fiber carbon Plenum. And for this reason I decided to use a MegaSquirt MS3 Pro Ultimate.

It's all built and now I will start to tune the engine.

My question is about overlap at high RPM. All I've read says at higher rpm more overlap will increase power. But in this webinar (at 21min05sec) 113 | Dual VVT Tuning Strategies - MoTeC M1 the Intake VVT map shows that the Intake cam advance follows the VE curve. At higher RPMs the Intake Cam is almost in the same position of Idle.

I also checked my OEM map and it follows the same pattern.

So, at higher rpm overlap will increase power? If not, why not?

Thanks in advance!

the optimal cam timing will be very dependent on your specific cams as well as the rest of the engine combination. That being said, the results of the FA20 tuning are reasonably typical. At high rpm the air speed in the inlet is higher and hence it has more inertia. If you use a lot of overlap under these conditions you can end up with some of the inlet charge transferring straight to the exhaust. There's a lot of variables here including exhaust back pressure so it's hard to make a sweeping generalisation.

Generally you'll find that the intake cam will want to see maximum advance at 2500-3500 rpm or thereabouts and as the rpm climbs it will retard back. The exhaust cam's relaxed position will be maximum advance and this follows the opposite pattern to the intake, retarding through the midrange before advancing as the rpm increases. It's also important to understand that you do need a 3D table for cam timing as the cam timing will want to be much closer to their zero points at low load.