Why don't engines like the F20C and 2ZZ-GE just open and close the valves more quickly to begin with? I'm asking because there's an upgrade for the engine in my car to lower the crossover point from 6500rpm to 5500rpm, which people seem to be very complimentary of. It's Lotus approved, and I'm thinking of having it done, but I do have to wonder why engines don't just work like that from the word go.

Ok I understand fuel economy is one reason, but why not just have a switch or something to have it on all the time if you want?

 

If you mean why not have VTEC from 0 rpm upwards, I believe it's because you would actually get less or no extra power. The reason for the change in valve timing is something to do with the extra air present at higher rpms.

Basically the variable timing takes advantge of the different dynamics at different rpms.

 

 

 

Seems to me it should be quite easy for the manufacturer to pick the optimum changeover point based on the two cam profiles used, and so there's nothing to be gained by moving it unless you've also made other substantial changes to the engine.

 

Not necessarily true. it depends what you're trying to achieve with the engine. Most of us want maximum power/acceleration. others want longevity or torque. For any given set of criteria, there is probably an optimum point, but people criteria will differ.

 

I would think that's more to do with the choice of cam profiles. Once you've decided that, then there's a clear point where one starts working better than the other and that's your changeover point. If the valve timings are working less well, you're probably losing out on efficiency as well as power, so it's not just about power vs efficiency.

But I don't know anything about engines.

In FTO land, the change over point was 5,700 rpm. There were always a few people that would waste money on controllers thinking it would allow them to MIVEC from 3000rpm and have much more low range torque, but in reality it just made the car slower. I'm assuming VTEC land (and Toyota VVTi land) is much the same.

 

But on the subject, I have always wondered why the Japanese go for a switching system with two profiles rather than a constantly adjusting system like the Rover K-rap series engines in the Elise 111s and MGF. The latter ought to be better in theory.

 

VTEC et al is a solution to an engineering compromise. Designers are always working to compromise one set of goals to achieve another. Power vs smooth, fast vs efficient, etc, etc. Vtec allow the use of a low-lift profile to achieve a smooth running engine with reasonable economy at low-rpm and a high-lift profile for high rpm power production. I see little point of fannying around lowering the point of change-over since in both cases the driver should be selecting the optimal gear for his situation. In other words if you want to go fast; change down and drop her into vtec. At a guess Honda did their sums when they picked 6000rpm as an appropriate switching point.

I'm amazed that nobody has made a production engine with valves controlled electronically by some sort of soleniod. This would allow continous adjustment of the lift and duration of not only the inlet and exhaust valves but each valve individually (if there was ever an application for such detailed level of control). Where currently we have machined cam profiles you'd be able to get your profile remapped in the way we can set boost targets, ignition advance and so on when mapping turbo cars currently. You'd also do away with a cam-belt/chain which can cause the death of many an engine. I'd like to know what the limitation is that has prevented this from being done - I can only assume its one of reliability. (i.e. if the ecu or any solenoid ####s up even once for a split second it'll screw the engine!).

With the traditional belt and camshaft system once its set up its locked into the cycle of the engine so that the only way it can fail is if something physically snaps/sheers.

 

I would say that's exactly why, the slightest electronic failure, even a short circuit would result in bent vales/ possible engine failure.

When you consider the expected life-span of an engine it just wouldn't be reliable enough.