(21-01-2017, 10:32 AM)welshpug Wrote: that's not high tbh, I'm sure Darren revved one of his old engines further, fairly commonplace for engines with more valves to rev higher, though the old M51 BMW inline 6 goes to 4800 and that's just a 12 valve.
its usually more a function of bore vs stroke plus rod lengths.
Airflow through the engine is a tiny portion of the story... Valves do help move more air at high RPM, but you'll tend to always limited by the injection system rather simply by airflow - which can usually be upgraded more than enough by fitting decent turbos.
Usual limit is simply the time you have to start the injection, get the quantity of fuel you need in and finish the injection, ensuring it's all in before the compression is fading away - there's a limit to how much timing you can give it... There's also a fixed amount of time that it takes from the point of fuel going in until the first sign of combustion (ignition delay) which has to be factored in...
At 1400uS duration (yes, 0.0014 seconds...) at 4000rpm - it'll take 33 crank degrees to get the fuel in... Move that to 5000rpm and that's now taking 42 crank degrees... At 5800rpm you're up to 49 crank degrees... That's just to get the fuel in, ignoring the fact that 50 degrees ago the first of the fuel went in and is now starting to combust - the last of the fuel still has some time to go before it even begins to combust! To make that power up high you need insanely short injections and quite a bit of timing, even to ensure that the fuel is in early enough to compensate for the ignition delay and ensure the fuel is combusting at the right time.
Joy of commonrail is that you can have those huge injectors that inject a huge amount of fuel in a very short time at high pressure... However even at lower pressure - you can get an acceptable idle and midrange and ensure you're getting good spray patterns for decent combustion, but crank the pressure and you've got craploads of fuel going in a short period. Just need some bigger nozzles!
