Cheers for your input Chip... So is being able to rev further what gives bike engines the ability to make such big bhp for the size of the engine?
What is it exactly that allows a vtec to rev so high and freely in comparison to other engines? Ive read a few of your posts now where you mention that the F4R engine is more suited to boost because its not suitable for high revs from an engineering point of view?
The size of an engine pretty much dictates the torque it will make.
Most petrol 4 stroke car engines will make between 60-80lbft a litre, that really doesnt change much.
Only way to get much more torque than that is forced induction, even the absolute best N/A petrol engines wont get much over 100lbft per litre, even in things like formula 1, it simply cant be done.
So now that we know our 2 litre N/A petrol engine is going to make something vaguely in the region of 140lbft do we also know what power it will make?
Thats what it varies far more, power is a combination of rpm and torque, torque is basically a measure of how much work the engine can do in one cycle and the rpm is how many times it does that work a minute.
So if we can keep that torque output high at big rpm then we can end up with a lot more power.
140lbft at 5252prm = 140bhp
140lbft at 8Krpm = 213bhp
Honda car engines are specifically designed to rev well so that they can make good power, motorbike engines even more so.
From a bottom end point of view the key things to getting an engine to rev well are:
Square bore/stroke ratio or better, as if you have a silly long stroke and a small bore like an F4R does to achieve its 2 litres (its over 10% longer stroke than the honda engine in this thread) then it means that the cylinder being a smaller diameter will restrict the amount of room in the head for large valves and decent flow around those valves, this limits the engines ability to breathe at high rpm.
A long stroke means that you need to either make the engine VERY tall (not good when needing to fit under a bonnet) or you have to limit the length of the rod relative to the throw of the crank, again this is an area where the F4R is massively compromised, a piss poor rod angle from a low rod ratio combined with a long stroke means that the piston speeds in a high revving F4R are actually HIGHER than in a formula 1 car at 19Krpm, what this means is that the burn cant effectively push onto the piston top hard enough cause the piston is rushing away nearly as quick as the burn can get there, so torque at high rpm naturally falls as a result.
Then when you look at the top end, my honda 1800 engine has solid lifters from the factory and can happily rev to 9K+ on the standard top end, the F4R though as hydraulic lifters and weak rockers that start meaning you are taking a risk the moment you go much past 8Krpm as well, so not only does the F4R struggle to make decent power at high rpm, it also suffers from reliability problems if you try for long.
Some engines are simply better than others, and although the F4R is extremely well specced from the factory in terms of the cams etc compared to a lot of engines and hence makes good power as standard, the moment you start trying to tune it you realise how fundamentally rubbish the underpinnings are especially in terms of the geometry which you simpy cant do enough to correct, that size block is only really suitable for a 1600 engine, that would then have a good rod ratio and plenty of valve area, but the head would still stop it revving so no point even trying that one sadly (williams head is better though, you could make a decent 1600 out of a williams)
