9000 RPM! Hello everyone and welcome,
in this video we're going to be talking about how high revving engines happen.
How do you get an engine to rev to 9000 RPM? For example in this AP1 S2000,
but there are plenty of other vehicles out there that do it,
and so we are going to be talking about how does high revving engines,
how does that all work?
And so first of all why would you want an engine to rev high? And so this is all about power
so horsepower is a function of torque multiplied by RPM,
so if you can keep torque constant, or relatively constant, and you can increase RPM, then you can make more power.
So that's the reason, the logic, behind having a high revving engine
very simply put it means you make more power.
So what makes it possible?
Well the first thing I want to talk about is a little bit of geometry and average piston speeds
and so something you'll notice across all engines is piston speeds wont exceed you know a certain value
they don't get too much faster than about 60 mph in movement
and part of that has to do with flame propagation and how quickly combustion can occur
if you think about a piston moving downward at a certain speed
once it gets really fast it's going to start you know matching the speed
at which that flame front is travelling and at that point it's not producing that much useful work.
So there is an effective speed at which combustion will you know, occur and still be very efficient and still be very useful
and so if you do the math, looking at engines that are high revving,
you'll find that the piston speeds don't actually differ all that much from lower revving engines
they may be you know a bit faster, but ultimately they're not going to exceed a certain point.
So if you look at the math for this AP1 S2000, you'll see a certain speed somewhere around 56 mph.
Now with the second generation Honda S2000, they increased the stroke of the engine
to increase the displacement from 2.0L to 2.2L.
Well in increasing the stroke, they increased the average piston speed,
and so Honda stated the reason they decreased the redline, the fuel cutoff
for the AP2 S2000 down to 8200 RPM from 9000 RPM is because of piston speeds.
And so if you do the math for that, if you look at you know that increase in stroke length
that means a faster piston speed but decreasing the engine RPM means slower piston speed
so ultimately the piston speed isn't much different than the AP1 Honda S2000, it's a little bit slower
but they reduced the engine RPM because the stroke is longer.
So this has an oversquare engine which means that the bore is wider than the stroke,
the distance, the width of the cylinder versus how far that piston moves up down
versus the AP2 S2000 has an undersquare bore to stroke ratio,
where the bore is actually slightly narrower than the stroke, the distance that piston travels up and down.
Ultimately whether its undersquare or oversquare doesn't dictate whether or not it can be high revving
8200 RPM is still very high revving engine for the AP2 S2000, but overall it has a very short stroke,
and so that short stroke is what allows you to have those lower piston speeds.
Now aside from geometry and piston speeds, you want to make sure your reciprocating mass isn't very high.
So for example in this S2000 the pistons are forged aluminum,
aluminum because they're less weight that means you've got less reciprocating mass moving up and down.
Same with some engines will use hollow intake valves,
and the reason being is to reduce that reciprocating mass.
Likewise you know rotary engines, the Mazda RX-8 if you had the manual version it could rev to 9000 RPM
Well there is no reciprocating mass in that engine, it is all rotational and so because of that,
because you are not changing the direction of a mass and it's all spinning in one direction constantly
you can rev higher, it allows you a bit of freedom in how you rev up that engine.
And obviously the engines have to breathe. So if you can have a torque curve that is flat, great.
But as you start hitting those higher RPMs it is harder to get a lot of air into the engine
and so you know the trick that Honda has which everyone knows is VTEC
so once you get above 6000 RPM you'll hear that cam profile change
and what's that's doing is it's opening your intake valves more to allow you to bring in more air,
and there is all kinds of different systems out there that optimize this
every brand you know has their method of doing it, but ultimately a lot of different companies
will choose methods that allow for more air flow at higher RPMs
so you can make additional power, for Honda it's this step system, so it switches cam profiles
and so there is a little pin that will lock up and it will allow to switch over cam profiles
once you get to a certain specified RPM and you'll hear that, you'll have that audible note
where it changes from the lower cam profile to the higher lift cam profile which allows for more air
within the engine and allows you to still have usable torque even at higher RPM when you have less time to pull in air.
Now if an engine can't rev high is that a bad thing?
Absolutely not, so you know diesel engines tend to have very high torque,
long stroke, very low revving, with high compression ratios.
They tend to be heavier because they are built for these higher torques
the internal combustion higher heat, higher pressure in there
but they are much more efficient and they can still produce plenty of power, plenty of torque.
So you know, it's not necessarily that it's good or bad to have a high revving engine
it's just a different strategy to produce power so you can rev high
and have lower torque and not worry about it because you can compensate with gearing for torque
because you can rev so high, or you can have high torque engines that may not rev quite as high
but you know you don't want to have to worry about revving as high because you have all that torque down low.
It's just a different strategy, its not that one is necessarily better than the other
but it is cool from an audible standpoint revving up an engine to something like 9000 RPM
having those high revving notes, that come through the exhaust
it's a cool thing to experience.
So I think it's neat, I think it's cool that they're multiple strategies out there.
Thank you guys for watching if you do have any questions or comments feel free to leave those below.