Just thinking out loud...

Why are most VVT engines only variable on the inlet cam and not both the exhaust and inlet cams? Old man's Beemer for example has VANOS on the inlet cam only and that's the same as the VVT on the ST170. The M3's and M5's however run double VANOS so they run on both the inlet and exhaust cams. I'm assuming that's how the duratec Vi-VCT runs as well (only on the inlet cam). How comes it isn't introduced on the exhaust cam as well then on the ST170 for example? Also why isn't it used a lot on a turbo cars? The Evo 9 and 10 though has the MIVEC system I think.

Cheers!

 

cost? technology?

as youve already pointed out BMW have been doing it for donkeys years on their top end engines

 

MIVEC on the Evos was inlet only for the 9 and twin on the 10.

A lot of the reasoning for it is purely to satisfy the emissions laws and keep the green folk happy. Yet still allowing performance at the top end. Has the added bonus of improving drivability/powerbands/response as well.

However, think how much it costs to bolt a pulley to the end of a cam. Now think how much it costs to develop an adjustable centre to that pulley, create a failproof system to adjust it, control it, evolve it, map all possible combinations extensively etc. Ask yourself if you were a car manufacturer - would you pursue it?

 

Mainly emmissions as above

 

Ahh right fair enough. Was just wondering why we hardly ever see it on the exhaust cam and I thought it was purely a power modification as it seems most of the quick-ish stuff has it like the ST170, Racing Puma, various BMWs, the V6 Mitsubishi's like the FTOs. The Honda system is different as that actually changes the cam profiles.

 

The standard 1.7 Puma has VCT as well, not just the FRP. It gives a bit more power, but makes the engine more responsive / driveable rather than out and out power.

It's nowhere near as aggressive on Fords as the Vtec on a Honda as it runs from oil pressure advancing and retarding the cam timing, rather than a cam lifter on a separate cam lobe in the Vtecs.

http://www.youtube.com/watch?v=AcT_ZyY3F0k

 

you can also get a 1.6 focus with variable cams .115hp. all the new petrol fiestas have it as well i believe. 3.0 m3's had single vanos,3.2 evo m3's had it on both cams.

 

Many BMW engines are double vanos - nearly all of the current ones.

Inlet has the biggest advantages - add in the exhaust and its a smaller gain.

Many turbo cars use VVT - Skylines, 200SX, GTir,

Even Rover used VVT and called it VVC and it was inlet and exh

 

Iam I right saying that that BMW`s VANOS system does not infact advance the cams at revs but reteards the cams at lower revs? Iam sure thats what I read when IO rebuilt mine many years ago.

 

It will depend on many factors... but most VANOSs are infinatly variable ie constantly changing.

Generally speaking you right - low revs needs lower valve over lap so the inlet and exh are moved apart.

Higer revs need more overlap.