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I've been collecting specific HP numbers for years: how much power from what engine size and type.

There are lots of general patterns. Two-valve street engines, for example, typically run 50-80hp/liter. Four-valve street engines usually about 60-90hp/liter. There are a few standouts, like the Honda S2000 (120hp/liter), but most engines fall into the usual range.

Typically, you can always get more power out of an engine of a given size by spinning it faster (and getting it to breathe up there). Race engines are often over 100hp/liter, with two-valve engines being in the 100-120hp/liter range, and four-valve engines up in the 150-200hp/liter range. Again, there are standouts like the typical F1 engine making 900hp out of 3.0L (300hp/liter), but they spin them up above 19K rpm to get there.

This is all NA, btw. Boost changes the picture so much that there's no general pattern. The F1 turbo era had 1200hp out of 1.5 liters at 5 bar of boost, or 800hp/liter. 100hp/liter street engines are the norm when boosted, and 150hp/liter isn't at all uncommon.

In the most recent issue of Racecar Engineering, some numbers stood out. There's a new non-turbo class for rally cars called Super 2000, which is heavily restricted. 2.0L, 8500rpm, 11mm valve lift. This really seems restrictive, considering the S2000 revs higher than that, and it's a street engine. Yet, they're getting 270hp, or 135hp/liter from them, and the power peaks are often as "low" as 7000rpm. Considering rally engines have to have decent drivability and flexibility, this is fairly remarkable. btw, this class may end up supplanting the WRC class in a few years, so it's pretty relevant for rally fans. The good news is a great many manufacturers are building cars for this class, including Suzuki and FIAT, neither of whom compete in WRC now.

And yes, the stock 914 engines are pretty low on the scale: 47-48hp/liter. Even Jake's engines aren't doing much better than 65hp/liter for the long-lived street versions. Most of this is down to the bad head design, and 30 years of R&D by private individuals has managed to raise it up to the point where a decent design just starts off. An early 70s 911 engine, for example, would make 70-80hp/liter, still using two valves and air-cooling.

The kings of NA specific power are two-strokes. A full race high-dollar two-stroke engine can make 400hp/liter (200hp from a 500cc four). 350-380 is more typical for "production" racing engines, like those found in the RS125. Two-strokes are extremely efficient for their size and weight, but have been nagged by poor fuel economy and emissions. This is changing, thanks to scooters (of all things). 50cc scooters fit a legal restriction in the European market, where they're very popular (esp in Italy and Spain). To meet emissions regs, direct fuel injection is now being employed for these engines. There was an far-thinking company in Australia called Orbital that patented a clean injection system for two-strokes. A clean running two-stroke street engine of approx. 1 liter could easily make 200hp yet weigh only 100lbs. Unfortunately, Orbital was unsuccessful in really getting the car makers interested, mostly because even with the direct injection system and a catalyst, it was hard to pass the ULEV and SULEV standards which will become mandatory before 2010.