Why can a Honda Civic make 100 horsepower per liter but a Ford Mustang can't?

Comparing small four cylinders to big v-eights is comparing apples to oranges. By there very nature small four cylinders have a better power efficiency because they have less rotational mass. The problem is you can't reliably make 400+ horsepower on a four cylinder without expensive turbochargers otherwise everybody would do it. Also the question of torque and power curves comes into play. American manufactures have tended to use camshaft profiles that result in flatter torque curves with more power down low, this comes at the expense of high rpm power which is where if tuned with the correct cam an engine makes the most power. hondas go more for the high rpm power but if youve driven one you know theres no low rpm torque to speak of.

You are mixing up can't and don't. Ford certainly has the engineering know how to make more HP - but at what cost? A Civic Si makes 200 HP (almost), but there are a lot of cars with equal or less HP that will blow it away - a VW 1.8 turbo, for example. depending on year they were rated at either 150 HP or 180, but either (in a Golf/GTI) was quicker than the 200 HP Si. Why? More torque.

Ford is going for a broad powerband - strong down low (which the Si is not) and strong up high, and good on gas and reliable.

A V6 Mustang will both blow the doors off a Civic Si and gets 2 MPG better on the highway - those high strung Civics are not that efficient, just 21/29 (V6 Mustang is 19/31).

don't get too caught up on the spec sheet, the performance on the road is what matters.

They could, (a little more aggressive cam) but they want it to last longer and get better mpg.

The new Corvette is putting out 720 HP with a 427, That's just over 100hp/liter.

You need to compare torque not HP. HP sells cars, torque wins races.

Honda's use the same iron and the same technology as every other car company, they don't have magic motors.

I outran a Honda civic with my old 1991 Olds Custom Crusier (5000 lb station wagon), it had a 305 v8 with 180hp. I had 700 lbs of concrete (9 80lb bags) in the back of my wagon at the time.

100 Horsepower

It's a matter of design trade-offs among power, torque, and efficiency. Ford has been emphasizing flatter torque curves for more than a decade, and while that means more useful performance it means reducing the power peak. American drivers tend to prefer flatter torque over power that can rarely be accessed (how much time do you spend at peak power rpm, anyway?)

Are you trying to ask why Ford didn't engineer the mustang to make 100hp/liter in stock trim? They both can exceed 100hp/liter. Problem is you are reading peak numbers and not reading where they are actually engineered to make power.

A modern Mustang can. Stop being an American basher.