There are custom 2.5 L S14's running around on the racetracks of this country which put out anywhere from 250 to 300+ horsepower. The basic recipe for such power is relatively well known - basically Evo III everything plus a little more. However actually achieving such impressive numbers is another matter entirely. In the end it comes down to details. But the risk is that in developing the top end power for the track, you may well loose driveability at lower RPM's. For some this is not an issue. But I like to auto-x my M3 (as well as track it) and for this purpose it really helps to have a wide power band. Thus my thinking was to build an engine similar to these "monsters" but to back off a bit on a few of the specs in order to retain higher flow velocity, and therefore a little more torque, in the lower part of the powerband. I figured I would be happy if I came close to the lower HP range of the truly high performers. |
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With that plan I developed the following specifications for my custom 2.5 L S14: |
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| • Cylinder head ported by Don Fields w/ stock intake valves |
According to Don, the porting job is aggressive, but not over the top. The goal is to retain some flow velocity at lower RPM's. Thus the stock size intake valves are retained as opposed to the larger Evo III units. Stiffer Evo III valve springs are employed in order to handle the accelerations imposed on the valves by the higher lift cams. |
| • Throttle Bodies Bored to 48 mm and ported by Don Fields |
The BMW Motorsport Evo III throttle bodies are larger then the 2.3 L units. They have 48 mm butterflies while the stock TB's have 46 mm butterflies. That amounts to almost a 10% increase in cross-sectional area. What Don did is enlarge my stock throttle bodies to accommodate the larger 48 mm butterflies and then port out the rest of the body to match. The result is not quite as free flowing as the Evo III units but it's close. The larger throttle bodies are matched to the larger Evo III intake trumpets to keep things balanced. |
| • BMW Motorsport Evo III 2.5 L Crankshaft |
An 87 mm stroke vs. the stock 84 mm stroke gives an increase in displacement but also a big increase in torque due to the increased mechanical advantage of the piston on the crankshaft as it passes through 90° past TDC. |
| • JE Forged Pistons w/ 11.35:1 Compression Ratio |
The pistons were designed by Steve Nelson at Top End Performance in Van Nuys. I ordered them in a 95 mm bore which I had deemed acceptable based on sonic testing of the block. The JE pistons have extra deep valve pockets to prevent piston-to-valve interference from bonking the engine. Total Seal rings are employed. |
| • Schrick Camshafts - 284 intake & 276 exhaust |
These cams are definitely more aggressive than the Evo III cams. But the maximum lifts are lower than on the all out Schrick race cams (the 305 and the 320). This way the engine still idles ok and doesn't fall on it's face at low RPM's. |
| • BMW Motorsport Gp A "Rally" Exhaust Header |
This header has much bigger primaries and slightly bigger secondaries than the stock 2.3 L header. The Gp A "Rally" header has an excellent reputation for producing a well balanced powerband. A stock 2.3 L engine might not flow enough air to put this header to use. But a modified 2.5 L engine can really use the extra breathing. |
| • Evo III AFM w/ Evo III "Big Green" Injectors (TMS/Conforti Software) |
The recalibrated AFM and the larger injectors are a common upgrade when building a high flow 2.5 L S14. Plus they are designed to work with the special TMS/Conforti chip which I employed. |
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