Ok, so about 10cm. A bit of a crude measurement from my "hacked to pieces" manifold gives me a runner length of about 45cm. So the total distance from the start of a runner to the valve is around 55cm (not very exact, but it's a starting point).
From what I've found on the internet, there is a strong relation between runner length and diameter and the rpm peak torque is being made at. You can do some simple guestimates.
One way to calculate is:
To get peak torque at 10.000rpm, you need runner length of 17.8 cm. For every 1000rpm you want to lower peak torque add 4.3cm runner length.
So in this case that would mean that we subtract around 8.5 times 4.3cm, which would mean peak torque at around 1500rpm. Or, if this method of calculation doesn't include the 10cm in the head, it is still around 3000rpm.
Another I found is:
N * L = 84000
N == Engine RPM at peak torque
L == Runner length in inches fro opening to valve
So for us L would be around 21.5", so that would mean peak torque at 3900rpm. Still quite low.
And a last one I found about plenum size (so the volume of the space between the throttlebody and the intake of the runners) is that if you want peak torque to fall between 5000 and 6000 rpm, the volume of the plenum should be about 50-60% of the displacement of the engine (for a 4 cylinder engine).
I haven't done any measurements on the plenum size, but I would be surprised
if it is 1,1L.
So I'm guessing, that shortening the runners (removing the 180 degree bend
for example), and thereby increasing the plenum size should move peak torque up. The longer the engine can produce torque at higher RPM, the more horsepower etc etc. This might explain the standard more "lazy" feel of the engine at higher rpm..
Would be very interesting in people's opinion. Not really an expert on these kinds of things, but it is pretty interesting
Mark 😃
