There are many reasons for making a small displacement high hrosepower motor vs a large displacement high horsepower motor.

First off, efficiency. Hp/Liter is an argument reserved for Rice-Burners. Speaking of which my Rice Burner makes almost 200 hp /liter


Does that make my $8K sport bike more advanced than the BMW? I think not. The smaller the motor's moving parts the easier it becomes to make more power/Liter. By design it’s hard to make more hp/liter out of a M5 or LS V8 than it is to make it out of a M3 Inline 6.

The LS V8 series is more efficient than the BMW M5 V8 (I won't speak to the M3 since it's an Apples to Engagement Rings comparison...sorry, big event on the mind). But comparing the LS V8 to the M5, the LS is more efficient as it makes more power using less fuel with lower emissions. That's efficiency.

The reason GM does not use an OHC design on the Corvette is for the following reasons.

Packaging: OHC motors are much larger and heavier as the valve cover to deck height is much larger. This means an OHV motor can sit lower in the bay and takes up less room between the shock towers allowing the front suspension to sit closer in making for a narrower package. Additionally, less weight and lower deck height lowers the center of gravity of the car affording better handling.

Price: less of an issue on a BMW, more of an issue on a Chevrolet, an OHV design uses fewer moving parts that do not require near the level of precision of an OHC design. Fewer parts and less need for precision mean lower manufacturing cost.

Now the talk about power to displacement, anyone here would agree that the M3 motor is highly tuned and eeking another 100 hp out of the motor is about impossible without nitrous or a blower. Getting that same gain out of a LS V8 is very easy. There are stock displacement 5.7 Liter LS1/LS6 motors out there making 450 RWHP (530 crank HP) with modifications. (No, I doubt they pass emissions, but that's not the point). The point is by designing a large displacement motor with a lot of headroom you can use significantly cheaper and less durable parts. This reduces cost which allows you to put a high performance motor in a much cheaper car. Case in point the GTO and Camaro.

Technology. The idea that OHV is "less advanced" than "OHC" is not a valid statement. Both technologies are 80+ years old they are just simply different design executions given the compromises of packaging, cost and expected performance.

Why is the M3 an I6 and not a V8? Cost there as well. There is no V8 planned for the 3 series until the next generation. By launching the M3 as a Inline 6, 99% of the engineering work from a 330Ci to a M3 is already done. If they put a V8 in the M3, BMW would have to reengineer and test bell housings, accessory drives, charging and cooling requirements, etc etc that would get passed on to us, and we would be stuck with a $100K M3! When you also factor in the lower volume due to expense they would not be able to leverage the R&D cost over enough units and the business plan for the M3 fails.

If OHV is such a great solution, why doesn't BMW make an OHV large displacement motor? Answer, no heavy duty truck line. The Majority of the LS-series V8s that GM makes go into their light and heavy duty trucks so they are able to leverage that Corvette investment over many different models and further lower costs. The 4.4 Liter or 4.8 Liter X5 V8s would make sub-par heavy duty truck motors and BMW would have a hard time selling that to a general contractor. The Range Rover can afford the cost of a high-$$ lower output V8 as it's towing ability and ability to drive onto a construction site are not as much of a factor to a Range Rover buyer as they are to a Chevy Ford truck buyer.

Why would you want a small displacement OHC design over a OHV design? At the same displacement, a 4 valve per cylinder motor has the ability to make more power than a 2 valve per cylinder design. Displacement is the limiting factor here. GM designs’ it’s V8s to go in trucks, BMW designs their V8s to go into cars. As such, BMW is more concerned with the length of the motor more so than GM. It’s easy to fit a longer motor in a long nose Corvette, GTO or Firebird. It’s hard to do the same in a shorter nose 5 series. Because of that Bore-spacing becomes very important. IF you make the bore too large then you either have to dig into the cooling jackets or lengthen the motor to accommodate. Additionally you have to worry about crowding the combustion chamber creating combustion inefficiencies. The M3 inline motor suffers doubly from this since its cylinders are in a line. (That is a V-6 is half the length of an Inline 6, but much wider and therefore creates other packaging problems). So since BMW is more concerned about packaging their V8s into a shorter hood, they have to run a smaller displacement motor.

But I said above that OHV motors are more compact than OHC motors? Yes I did, at the same displacement a pushrod motor is smaller in height and top-width. A larger displacement is larger in length, and to a lesser degree width, assuming the bore centers have changed. Thus the M5 motor is much larger than the LS6 motor due to its huge plenum and large complicated heads.

Can’t you increase the stroke of a motor to compensate? Yes, but bore center placement dictates largest bore, and the stroke/bore ratio dictates power curve shape. If you get to long of a stroke you hit problems with thrust angle, wrist pin height etc, so you can only go so far before you need to look at other ways to increase bore to compensate.