Torque and Hp bands (power curve) on DOHC 2 and 4 valves per cylinder

[Tikked Again]

Hello....
Here's a question how much does the torque band (also the hp) differ on similar size and stroke engines that are DOHC but either have 2 valves per cylinder or 4.

I ask this because I am planning a rebuild for my 86 mk2 5mge supra engine this spring. Basically getting stronger everything in the engine, upping the compression to 11/1 (most likely won't change the stroke) and not changing the cams (yet). And since I would like to remain NA. I was also considering building up a 7MGE engine which would bolt right in, which is why I'm wondering if the 7mge would have a better power band (other than the fact it is a slightly larger engine). Thoughts....pains....general discontent?

(NOTE: 5MGE = 2.8 liter, 12 valve, Inline 6
7MGE = 3 liter, 24 valve, Inline 6

[vvtlikick]

Alot of people seem to think that multi-valve (3,4,5-valve) engines can't develop as much low-rpm power and torque as non-multi-valve engines (2-valve), but I don't think so.

I think that multi-valve engines are generally tuned more towards mid to high-rpm use than 2-valve engines, but if you designed it for low rpm use, I'm sure a multi-valve engine could put out as much power/torque at low-rpm as any 2-valve engine (if not more).

As an example of a low-rpm multi-valve engine, I think Chrysler is going to soon be using a multi-valve pushrod diesel in its' new pickups (3-valve?). Since it's a diesel, you know it's tuned for low-rpm use.

As for the 5MGE vs. 7MGE question, I don't know enough about the two engines to give specific advice. Intuitively, I think the newer, larger, multi-valve 7MGE would be a better choice, but you already own a 5MGE, and I don't know how much a 7MGE would cost.

Additionally, you might also want to consider aftermarket availability, i.e. which engine has more/cheaper parts?

[Chui]

For a given bore and stroke a 2-valve engine generally makes more power at lower crankshaft speeds for one reason:

Lack of combustion efficiency at those speeds.

Why would this be the case? Due to the larger area of the valves which reduce the velocity of the incoming air. This reduces the bulk flow of the inlet air [reduction in turbulence either thru tumble or swirl or any combination] thus we have a corresponding reduction in combustion efficiency, volumetric efficiency and torque.

Can one design a 4 valve head to produce more torque than an optimized 2 valve design? Probably. But at what cost? If one were to use mild cam lift and overlap profiles, smaller valve diameters, lots of squish in the combustion chambers and a slightly smaller bore:stroke ratio then "yes" one could. Look at Toyota's engine in the Lexus LX470 and full-sized pickup truck as an example. The cost is poor higher rpm performance, however.

As with everything, there are trade-offs.