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.

Technology is, but when a person says that a Small Displacement DOHC motor is more "technologically advanced" than a large displacement pushrod OHV motor they are actually showing a lack of understanding of the technology in question.



A Pushrod OHV motor can be and today often is just as technologically advanced a Variable valve timing DOHC small displacement turbo motor. Why do I say that?



1.) OHV, DOHC, Turbos, Superchargers etc all are technologies that have been around since the 1920s.



2.) Manufacturers choose a configuration based on design and purpose.



3.) Technological advancement is measured on total execution.



Case in point, Z06 vs. BMW M5. Both have V8s that make at or around 400 hp. BMW uses a DOHC 4.9 Liter V8. The Chevy uses a 5.7 Liter pushrod V8. At first glance one would think the Bimmer motor to be more “technologically advanced”, but is that so?

• The Chevy motor makes more Horse Power and Torque.
  
  
• The Chevy motor gets better Fuel Economy (19/28 vs. 13/21) (Largely a function of weight, but also because the LS6 is more efficient)
  
  
• The Chevy motor has better emissions (ULEV vs. the BMW LEV)
  
  
• The Chevy motor is dimensionally smaller
  
  
• The Chevy motor is cheaper (you can buy one at the parts counter for $6K)
  
  
• The Chevy motor is lighter (Check the Shipping weight)
  
  
• The Chevy motor has fewer moving parts.
  
  
• The Chevy motor contributes to a better Cg. (Shorter deck height from a tighter OHV package lowers the Cg)
  
  
• The Chevy motor does a better job using less expensive parts
  
  

So What car do I think is more technologically advanced, M5 or Z06? M5 of course (I mean seriously you get 80% of the performance and it’s a 4 door!).



What Engine is more technologically advanced? The Chevy motor. It does it’s job better because it: creates more power, more efficiently while doing so with better fuel economy and emissions. It does all this using fewer parts, in a tighter package using less expensive materials at an overall lower overall costs.



That is the definition of “High Tech”



-Adam

So What car do I think is more technologically advanced, M5 or Z06? M5 of course (I mean seriously you get 80% of the performance and it’s a 4 door!).

What Engine is more technologically advanced? The Chevy motor. It does it’s job better because it: creates more power, more efficiently while doing so with better fuel economy and emissions. It does all this using fewer parts, in a tighter package using less expensive materials at an overall lower overall costs.

That is the definition of “High Tech”

However I would like to comment on your last set of quotes, and I appologize if they are taken briefly out of context:


I believe one factor that has not been mentioned is the "exotic" factor. Whether you agree or not, much of the appeal of a 4-V DOHC engine is twofold:

1). The ability to breathe substantially better at the same time using higher RPMs, and

2). The sound of the exhaust, valvetrain, etc, that makes that V8 "shreak" or "scream", rather than "burble" or "roar" like our small blocks do.

Now, I agree they have been around about the same so "advanced technology" is not the issue, but "Potential Energy" is key.

In other words, the better breathing DOHC 5-liter of the M5 has 7500 rpm and 500 rwhp potential, without hardly breaking a sweat. That is an advantage IMO regardless of the known drawbacks. Because hp is just a number calculated from tq & rpms, has nothing to do with weight.

Dinan has massaged the current 5L M5 to 500 bhp, just with shorties, different length intake runners, and some A/F tuning. (Not even a cam!) But the fact remains this motor in modified-to-modified form could reach up there.

Now when you consider it has less rotating mass, and revs freer than an ls1 for example, you now have a consumer engine that is closer to an F1 engine (or crotch-rocket engine for that matter) than you would otherwise.

That's where the "exotic" factor plays a role, in the known potential of the engine, not what it may be putting down in one version... So, with less weight/displacement it makes more hp while revving easier and breathing better.

Maybe it does have something to do with expensive exotic materials effecting the weight of the valvetrain after all...

Sure, this is apples to oranges but FWIW DOHC engines own the high RPM segment...just look at the CobraR a 5.4L DOHC Modular Ford engine that dynoed higher than 02+ Z06's do FROM THE FACTORY (~380 rwhp to be exact). With some bolt-ons only they can develope well well over 450 RWHP, and with SHM cams over 500 rwhp and still drivable, because they have the best of both worlds at their disposal: RPM AND Displacement.

Quote:
Originally Posted by Dragula
I see to many people concerned with HP per Liter, when I think a better stat is HP per weight of engine.

This is one of GM's factors for sticking with the pushrod engine.

This leads me to this question. What is the weight of a crate ls1/ls6 engine?

In response to the original question, the weight of an engine is rather irrelevant. The weight of the engine is determined by its construction material. For example you could have a cast iron 5.7 with cast iron heads and intake, and an aluminum 5.7 with alum heads and intake with the exact same same internals and very similar hp output, but the cast iron version would weigh almost twice as much.