Why American V8's Make Low Power for Their Size December 03 2015, 14 Comments
Why do large-displacement American V8s generally produce less horsepower per liter of displacement compared to their European and Japanese counterparts? A common answer is that American engines are tuned to favor low-end torque instead of top-end horsepower, but that answer is vague and only half true. Large displacement American V8s are typically pushrod engines, and this engine design has characteristics that make it more difficult to produce high horsepower per liter. There is a sliver of truth that American engines are tuned for low-end torque, but that is not entirely accurate. Once set on using a pushrod design, it's not as if the engineers at General Motors could easily have chose for the engine to have high-end horsepower instead of low-end torque. The engineers have some ability to shift the peak of the torque curve up and down the rpm range, but the general vicinity where torque peaks is largely already determined by the engine architecture.
As we talk about the reasons why American V8s make relatively low horsepower per liter, we'll compare the LS3 out of a 2008 Corvette with a S65 out of a 2008 BMW M3, since this pair provides a good comparison. Both were built around the same time, and both were designed to be put in performance cars. The LS3 was one of the best American V8s at that time, and the S65 was one of the best German V8s at the time. Both engines produce similar power figures, but use different approaches to get there.
1. American pushrod engines generally have lower redlines than overhead cam engines. Horsepower is proportional to torque multiplied by rpm, so if you can rev your engine higher and still maintain torque, you can produce more power. The limiting factor to how fast an engine can rev is usually the valvetrain. The valves are held in the closed position with springs, until they get a bump from the cams, causing them to open. When the cam lobe leaves, the spring acts to close the valve. At high enough engine speeds, the spring can't close the valve fast enough, and the valvetrain momentarily loses contact with the cam. At this point, the motion of the valve is no longer fully controlled. This is called valve float. Since the pistons and valves operate in the same space in most modern engines, they have to be in sync and move out of each others' way at the right times. Therefore, valve float can damage an engine by causing the valves and pistons to collide.
A pushrod engine usually has a heavier valvetrain than an overhead cam engine, since the camshaft is in the center of the block and the cam pushes on a rod that pushes on a rocker that pushes on the valve. That is a lot of stuff to move, and that limits max rpm. Compare this to an overhead cam engine where the cam sits right above the valve stem with only a lightweight rocker arm separating them.
Can pushrod engines be made to rev higher? Trick LS engines can be built to rev upwards of 7,500 RPM, but they require the use of parts like titanium pushrods, solid lifters and stiffer valve springs. This adds more cost and maintenance than the average consumer will tolerate, so most manufacturers don't do this.
2. Pushrod engines struggle to flow at high rpm. The pushrod engine architecture makes it difficult to have more than 2 valves per cylinder. By comparison, overhead cam engines often have 4 or even 5 valves per cylinder. More valves generally means better flow through the engine, especially at high rpm where large amounts of fuel, air and exhaust are trying to flow in and out of the engine. High torque at high rpm is where big horsepower numbers are made, and high flow is necessary for high torque at high rpm.
For the LS3, even if the engine was modified to rev faster, it would unlikely be able to produce any more power because the engine would struggle to flow at higher rpm. Horsepower peaks at 5900 rpm, and drops off as the revs climb. Compare this to the S65, where peak power is made at 8300 rpm. The S65 still manages to flow effectively at high rpm.
3. American engines generally have lower compression ratios than their foreign counterparts. Increasing compression ratio is awesome, because it increases both horsepower and efficiency, as you may remember from our fuel octane article. What keeps American engines from increasing their compression ratios?
Many factors are at play, but a major factor is combustion chamber geometry. As the spark plug ignites the fuel air mixture, a flame front moves through the cylinder and a pressure wave moves ahead of it. This pressure wave can produce localized instances of autoignition in the cylinder. Without launching into a dissertation, there is a delay between when the unburnt air/fuel mixture reaches autoignition temperature and when it actually auto-ignites. If the flame front can burn the fuel before it auto-ignites, it prevents autoignition. A combustion chamber geometry that allows the flame front to reach all corners of the combustion chamber more quickly is more knock resistant, and allows for higher compression ratios. This means that symmetrical combustion chambers are more ideal. 4-valve designs generally allow for better geometry compared to 2-valve designs.
As a sidenote, direct injection can lower combustion chamber temperatures and allow for higher compression ratios, but neither the LS3 nor the S65 have it.
4. Pushrod V8s generally do not have variable valve timing. Variable valve timing is able to vary the timing, duration and/or lift of valves at different rpm. What does that mean in English? For each engine speed, there is an optimum timing for when the valves should open (timing), how long they should stay open (duration) and how far the valves should open (lift). That optimum amount of each of these varies with engine speed. At low rpm, you typically make more torque with later valve openings, moderate duration, and low lift. At high rpm, you typically make more torque with earlier valve openings, longer duration and high lift. For engines without variable valve timing, the valve timing, duration and lift are set and do not change, so the engineers had to choose valve characteristics that are a compromise between low-rpm and high-rpm performance. The engine misses out on some high-rpm ponies because of this. An engine with variable valve timing can optimize valve events at both low and high rpms. This is especially helpful at high rpm, because high torque at high rpm makes big horsepower numbers.
So if variable valve timing is so great, why don't American pushrod engines incorporate variable valve timing? The pushrod architecture makes it harder to implement variable valve timing. There is less room for a variable valve timing mechanism, and the intake and exhaust cams are all on one cam, making independent exhaust and intake valve adjustments difficult.
Pushrod engines have their own advantages. It may sound like we've been bashing on pushrod engines for this whole article, but American engine manufacturers have their own reasons for choosing this architecture. (a) Pushrod engines are generally lighter and more compact than equivalent overhead cam engines (b) Pushrod engines are simpler than overhead cam engines, which drives down cost and increases reliability.
Some American V8s do have fancy bells and whistles. The Ford Coyote 5.0 has overhead valves and variable valve timing, and a specific output of 87 hp/liter compared to the LS3's 70 hp/liter. Some of the latest GM V8s have cylinder deactivation as well.
So what if my engine has a big displacement? The downside to having a big engine is all the weight that comes with it, but if a pushrod design can negate the size and weight downsides, then who cares? It then becomes a matter of preference of which design philosophy you prefer. BMW makes their engine only as large as necessary and then squeezes out nearly all the potential of that displacement. GM, on the other hand starts with a big engine and only optimizes for power as much as it needs to. At the end of the day, both engines make similar power, but how they get there is very different.