what happends if high valve lift happends at lower rpm?

[neo_geo]

As i know.. variabe valve mechanism works by having different profiles..one for lower speed and the other for higher rpm..

at higher rpm the engine needs more air so if the valves lift can be increased together with the duration the valves stays open.. then sufficient air/fuel mix can be supplied to the engine..

what happends if there is only one valve profile which is only highlift? are toda killer vtec cams one of these kinda cams? and like having the vtec controller opening at lower rpm..

how is it that the fuel is not burn completely when this happends? engine not running fast enough to want more? i thought once the fuel/air goes into the chamber the spark ignites and how the hell is it not burnt all?

And why the car when left idling sounds like its gonna stall ?

please help clear my mind. thankyou

 

[dawg]

for vtecs/mivecs, the role of the variable valve timing and left is to ensure that street cars have the benefits of both world (low cam profile for low end, daily drive convenience, high cam profile for more power at higher rpm powerband)...

for comparison, take a look at GTi and DC2 (both 1.8 engine), but the hp is different (140hp vs 180hp at crank -estimate figure-). At low end, both will have the same hp and torque figure, but at the top end after VTEC enabled, the integra will maintain to have more hp climb. Reason is that the GTi without variable is not taking in extra air, but the integra have engaged higher lift and longer valve duration to suck in more air. More air = more power (and the right fueling). A GTi with high cams, will have crappy low ends, but when it gets to high revs it should come up with about the same hp figures are the DC2 with vtec engaged. The only sacrifice the GTi has is bad idling, less responsive low end drivability, while the DC2 still have good low end response.

So if you want to make VTEC point engage sooner, you are looking at the GTi with high cams example. There's a reason why those brainees in Honda made VTEC open at certain stage of RPM is to maximize the powerband efficiently.

 

[Zeroed]

High valve lift + high duration during low rpms causes dynamic compression losses.

Static compression is the fixed compression of your engine, i.e.: 10:1 compression.
Dynamic compression is the actual compression of air-fuel volume during the ignition phase, and this varies according to valve lift, duration and engine speeds.

Because the valve overlaps are huge for high cams, if the engine's rpm is not high enough the piston pushes the air-fuel mixture out of the combustion chambers during the compression phase as the valves are still open. By the time it closes and the spark plug ignites, the dynamic compression would be too low to sustain healthy compression. Hence the engine is weak and stalling; and the exhaust fumes smell bad due to unburnt fuel.

Once the engine rpm gets into the correct powerband for the cams, dynamic compression then becomes healthy again. This happens as the valves closes too fast before the fuel mixture gets pushed out of the chambers. If you dont know already, valve overlaps are designed to make use of the exhaust scavenging effect for a determined powerband. High cams have overlaps that are designed for higher rpms; causing you to lose low rpm efficiency.

While on this topic, note that for street cars the more ideal cams are high lift and medium duration cams. There are some high duration but low lift cams out there that perform less than ideal for road cars.

A general rule of thumb is the higher your cams are (bigger overlaps); the more static compression you need to compensate for the losses in dynamic compression. Thats why some cars with high cams are able to run at 13:1 static compression safely; however if you put normal cams in these engines they will most likely blow up.