Turbine AR, trim, cams and airflow- what does it all mean?

[Sydneykid]

Hi Warspeed, the R34 GTT RB25DET seems to have more adjustment built into the VVT. Is it possible the camshafts are physically the same, but the timing quoted is due to the different VVT positioning?

[Warpspeed]

Hi Sydneykid,

I thought exactly the same thing after typing that last post, there is something very odd about it.

I am thinking that those figures are probably the fully advanced position of the VVT which would then make more sense. Unfortunately those are the only figures I have available and they come without any further clarification.

What are your own general ideas on turbo valve timing ?

[Steve]

Very interesting to see the difference from R33 to R34.

Any ideas why there is so little overlap on the R33 timing? Is this for emissions control, which would not be of as much concern with the R34 due to its extra VVT range? ie retarding at idle/low rpm?

Really appreciate all the info warpspeed, thanks heaps:)

[Warpspeed]

You are right. All modern engines that have to pass idle emissions cannot have more than a very few degrees of overlap, and most run zero overlap these days.

The way VVT gets over this is to retard the inlet cam at idle, so you get zero overlap and clean idle emissions. At some speed just above idle (1500 RPM?) you advance the inlet cam fully.

Because the engine is now under load the inlet manifold vacuum is far lower, and it will stand some overlap without messing up the emissions. The earlier inlet valve closing can significantly increase the low end torque.

At higher RPM (>4,000) you again retard the inlet cam. The later inlet closing will help cylinder filling at the top end.

I have a sneaking suspicion that the R34 figures for the VVT Neo engine give the fully advanced inlet cam timing figure. this might not actually be unreasonable because the engine will spend most of its life between 1,500 RPM and 4,000 RPM anyway. But I cannot say that is the case for sure.

It is important to realize that VVT is put there for emissions, not for performance. If you throw away the VVT and fit a decent inlet cam it will kill the VVT everywhere in the RPM range. Of course if you go totally nuts with the duration it might also just as easily end up worse than stock everywhere.

So the tip is go a bit more lift and duration on the inlet cam and dump the VVT. You can go a bit further on the exhaust side without stuffing things up. If you do it right it will be noticeably better everywhere.

For people still not convinced, they could easily have fitted VVT to the GTR if it had increased performance in any way. Think about it.