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Understanding cam durations with single or dual VVT

Understanding cam durations with single or dual VVT

Understanding cam durations with single or dual VVT

Hi everyone,

I am interested in VVT control and I have some questions about the choices done by manufacturers about cam durations.

First, basics I have:
IVC is the most influent cam event in engine behavior. This is why valve phasing first appeared on intake cam.
EVC and IVO then, and moreover their relative opening with the balance between scavenging and intake mixture going to exhaust. Idle and high vacuum/low rpm situation is very sensitive to EVC/IVO overlap, more than IVC.
Late EVO is slightly better for efficiency at low rpm to benefit from full gases expansion, but that is pretty much it.

Particular facts read in different places, with no strong proof, that may be discussed:
- on stock, non VVT engines exhaust duration is often longer to compensate for a restrictive exhaust. When performance headers are fitted, similar intake/exhaust duration is better
- independent throttle bodies make huge intake durations possible with no big adverse effect such as poor idle, and it is clear when seeing cam specs
- In general, retarding all events together suits to higher rpms (only doable VVT on single cam V8s...)

Then, facts about the 2JZ NA engine:
Toyota 2JZ-GE (NA) non VVTi has the following specs (advertised duration, not 0.050'' / 1 mm):
in 3;50: 233° / ex 53;3: 236°
Toyota 2JZ-GE VVTi has :
in -12;65 to 48;5: 233° / ex 40;6: 226°
(open;close angle)

(other fact: turbo version 2JZ-GTE VVTi keeps the 236 exhaust cam, but I will not ask why, or not today :) let's stick to the common statement that a big exhaust duration helps low rpm turbo spool)

Here is a 2JZ single VVT map :

Seems pretty clear to me that at full load, IVC is retarded to follow RPM. This later intake closing comes with a smaller valve overlap that is not ideal at high RPM but less important than IVC?
Partial load : 60° advance of IVC allows partial cylinder filling without throttle pumping losses, and same advance IVO makes reversion: EGR without EGR valve (not a balancing act between sooner IVO and IVC, both are beneficial for emissions).

And here is the heart of my questions: why reducing the exhaust duration when having VVT on intake? Preventing fresh gases passing through exhaust valves when VVT advance is big? But we see that the difference is mainly taken on the opening side...

Other fact, BMW M5x straight six engine
M50B25 (no vanos, probably no catalytic converter) in 24;36: 240º / in 35;13: 228º
M50B25tu (single vanos) in 24;44: 228º / ex 35;13: 228º
M52B28 (single vanos) in 1;49: 230º / ex 39;9: 228º
M52B28tu (dual vanos) in -6;54: 228° / ex 47;17: 244°
S50B30US (single vanos) in 252° / ex 244°

I did not check angles of vanos and could not find any vanos map with load and RPM. I do not know if it is an extreme or center position of the phasing.

S50B30US has single vanos and no ITBs, and follows the same apparent "smaller exhaust" rule (S50B30 and B32 engines have ITBs, let's not look at those. S50B32 is dual vanos, S50B30 is single)

Other questions: dual vanos have a much larger exhaust duration, why is that?
How does exhaust phasing vary? Always following intake to a specified overlap (at partial load to have the desired amount of reversion/no passing through, and optimal at full load?)

We see that generally speaking (two manufacturers might be a bit small to take conclusions...):
- when adding single VVT from no VVT, general tendency is to slightly reduce some cam durations
- on single VVT, exhaust duration is often lower that intake. (Why? Performance is ensured by VVT and adverse effects of long durations are removed? It is necessary to avoid a big overlap when VVT is advanced?)
- when going from single to dual VVT, now exhaust duration is bigger than intake!

I cannot truly explain these two last statements. Any idea?

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