VVT, what are the pro's/con's?

[aragorn]

Can anyone tell me what the VVT system actually does?

At a very basic level i'd have thaught it would alter the timing to give good low end torque and a fast spool, then once it reaches a set RPM, switches the cam over so as to provide a good top end?

Is that what happens? Is the effect visible on the dyno between similarly tuned cars with/without VVT?

 

[Sean_S3]

On our cars i dont think variable valve timing does much for performance , i think its more for emissions but im not 100% sure on this.

Cheers.

 

[mk1chopper]

Its not like the Vtech system or VVTI found on the Honda's and Toyota that have a second cam profile, it alters the timing via a tensioner on the timing chain I think. Its probably for emissions and possibly a minor improvement on fuel consumption.

 

[mk1chopper]

from VWVortex.com - 1.8t FAQ - Please Read

VCT or Variable Cam Timing. Synonymous with Variable Valve Timing
The AWW, AWP, and AWV engine codes have VVT, while the AWD and APH do not.
The VCT in later 1.8T's is simply for emissions purposes. It has only two states - normal and massive overlap. The overlap is used to draw exhaust gasses back into the cylinders at startup.

 

[aragorn]

A lot of VVT systems only have one profile. The difference comes from advancing or retarding the cam timing.

Take your cam and advance it, and you may find it makes more power right up top but with a big hole at low RPM with a really lumpy idle. Similarly ****** it and the idle might smooth out but now you've limited the top end.

Obviously with fixed timing you need to pick the best compromise, but with VVT you can have the timing set for excellent bottom end performance until the revs increase, then adjust the timing to get the best of both worlds.

 

[<tuffty/>]

VVT on 20v's is for emissions use.... there is no performance advantage in having it as std (AFAIK) but..... it has been used on custom maps to improve spool time on larger turbo's (as seen on the monster 750 odd bhp lime green Mk2 Golf) and Custom Code do a 'VVT Attack' version of their software but I haven't seen any results of what effect that has as yet...

<tuffty/>

 

[aragorn]

Ah cheers, posted at the same time.

Does look like its a pretty pointless implementation then!

Although the description doesnt make any sense. It says it uses massive overlap to improve emissions, but massive overlap usually results a lumpy idle and tonnes of unburnt fuel emptying down the exhaust and very poor emissions...

 

[mk1chopper]

The Vtec system on Honda's should really be called variable valve lift as that what occurs, the cam profile changes to allow longer cam opening times at high rpm.