Why do similar engines have wildly different specs?


Why does a Porsche Flat 6 with 256 cu in able to push 500 horsepower, while the AMC 258 Flat 6 in my Jeep barely hitting 100 horse power? The Porsche flat 6 feels right at home up to 8000 rpm, while the AMC 258 is screaming by 4000. Why can do engines with very similar blocks be so different performance wise?

I am talking about naturally aspirated of course.

In: 3

Because all you are looking at is displacement.

There are a lot of factors that effect the power output of a motor. Displacement is only one of them. Another important factor is compression and stroke length. There is also the machining to reduce internal resistances, valve placement/design, porting, exhaust design, intake design, and ECU mapping.

Maximizing the performance of all of these takes a lot of time and effort.

There are so many things going on in an engine that all I can really do is list examples.

Horsepower is limited in large part by RPM, but also by airflow and compression ratio. Something like a turbo can improve the horsepower a ton by cramming more air into the same displacement.

Compression ratio is limited by intake temperatures, as well as the engine’s structural strength and the fuel.

RPM is held back by the weight of the parts as well as their strength. The use of alloys and computer-generated structures can raise the RPM ceiling quite a bit, but so can reducing the compression ratio and changing the stroke width.

Airflow is very complicated and depends on the shape of the parts.

Other things come into play, too, like ignition timing and fuel/air balancing.

They’re designed for two different jobs. One is designed to haul ass and the other is designed to pull stumps.

Bore and stroke size matter and that determines how fast an engine will accelerate or how much torque it has. Different compression ratios also can make a huge difference.

Another important factor is the cam shaft profile. The cam profile can be mild to help stimulate low end torque, and good fuel economy (for the size of the vehicle), along with low emissions, but it doesn’t do much in the top end, and they’re all tapped out at around 5,000 rpm. You also have your mild to wild cams that move the power band up, allowing the engine to wind up quicker, but they’ll have a lot less torque down low. Those vehicles depend on lightweight flywheels (manual transmissions) or a smaller torque converter (automatic), along with numerically higher differential gearing. With a mild cam, you can move the rpm range up to 6,000-8,000+rpm before you’re tapped out. A full race cam with the right bottom end can get your rpm’s up to and beyond 10,000 rpm.

Intake, exhaust, fuel management, engine timing, and spark timing also make a difference, but those all make a minor difference.