It would make very little difference in terms of EPA equipment now. Not anymore. What more restricts motors is mufflers and octane limit of gasoline. They will make more power under biofuels than petro-based fuel. Catalytic converters restrict mega-power motors some, not much for regular cars. I think you're living in the past. Inefficiency not only causes emissions, but also less fuel economy and less power. In the past, the 70s thru mid 2000, EPA standards did severely affect power and gas mileage. Direct cylinder sequential injection and computer management systems have almost complete negated those loses.
As much as I do not like anything that limits power on a car, most cities do have terrible air quality and that is mostly due to vehicles.
Even I was surprised that the Ford Fusion comes with a mere 1.5 liter motor (less than 100 cubic inches) and still is a peppy car that will go 100 mph. Modern cars are quite amazing actually. They now can go 12.5-1 compression on a mere 91 octane unleaded, which was unheard of in the past. The Merc we have has 8.5 -1 compression, but then puts out a massive 22 psi boost from the turbos - and can run on pump premium unleaded making 800 hp, 1000 ft/pounds torque and still get 20 mpg in a 4600 pound car. And that technology is now almost a decade old.
I was surprised how little the quiet mufflers and cat-converters limit power on our ubber powerful motor car as we explored powering-up options, though going cat-less and o2 sensor delete would allow then 110 octane leaded racing gas. (At 8 per gallon). The extra octanes would allow taking up the boost on the turbos. However, none of this would improve daily driving fuel economy.
There is 100 octane unleaded racing fuel (about $6 a gallon). While 100 octane unleaded could be offered at stations, few people would be willing to pay for it. Most people just want a car to get them where they are going.
What would more increase fuel economy would be to eliminate safety factors to increasingly lighten cars. But that does increase danger factors. Only 1 subcompact even reaches an 'acceptable" rating. Most are dismal in crashes.
What is most inhibiting development and running up vehicle prices is primary safety regulations and crash standards, not EPA regulations. Modern computer managed motors are so efficient they put out very little emissions. The other inhibitor is products-liability. Safety features go beyond structural integrity and air bags to now also controlling the brakes, suspension, and motor. This drives the price up, but also makes for much faster cars in the corner.
The rate at improving technology is incredible. I had no idea how far it's gone until really exploring it. However, it is not consumer adjustable and makes a person totally dependent upon the experts/shop to modify and for most mechanical repairs.
That is frustrating. I got rid of a Jaguar XKR (supercharged XKE) because it was literally impossible to change the rear axle ratio. No one even among Jaguar enthusiasts had any way to do it either, and Jaguar refused to release the codes. If the rear axle ratio changed, the rotation of the front tires will tell the computer something is wrong and throw the car into limp mode (2nd gear and reverse only). Many tried and none succeeded at defeating that. So I got rid of it.
But modern car are superior they way a calculator is superior to an old mechanical adding machine - though the adding machine might be more reliable.
I never said what they achieve today is not in may ways amazing. I said it could be better.
And, no, I'm not living in the past talking about EPA equipment.
We no longer have 100 octane gas on the street because that would allow much higher compression ratios. Those higher ratios give more power, because they burn more efficiently. Unfortunately, when you increase efficiency in an internal combustion gasoline engine, you also increase CO2 production. Why, simple, Water and CO2 are the only things that would be produced in a perfect burn of gasoline. EPA restricts CO2 emissions, worse, it does so by using a ratio measurement. You cannot increase efficiency of the motor without increasing CO2, especially when measured as a ratio instead of by volume. I am also referring to Fuel Air mixture, timing restrictions and other idiocy put in place to meet requirements by an even stupider EPA.
Weight, but only a little, gearing and aerodynamics give some gains. But ignoring the EPA and tuning for maximum efficiency of the motor offsets that on older vehicles. The best option would be no EPA on newer vehicles. The next best option, and the one that is legal, is to install that new technology in vehicles which are not required to meet EPA standards.
Non-Emissions and early emissions, pre-cat vehicles give the best options for tunning. Just keep in mind that you actually have to modify the older block, not transplant a new engine to stay legal.
The next best option is the Post-Cat, pre-OBD I vehicles. Modern cats have very little restrictions, however, since there were EPA measurements, your options on tunning are limited. These cars also came at the time when manufactures switched from really heavy to lighter cars.
Next would be the OBD-I vehicles. Many have good aerodynamics and lighter weight. However, any gains due to aerodynamics and other factors will be offset meeting the tighter EPA restrictions. These are also the models that saw the move from 3-speed to 4-speed automatics and the introduction of the 6-speed manual transmissions, greatly improving gearing.
There are all kinds of mods out there for changing a lot of factors involved. Including weight reduction options, better transmissions and even some aerodynamic mods for older vehicles that reduce drag. If Direct Injection for older engines were to become available, it would greatly improve even what could be achieved today.
One of the best designs, if you are looking to improve economy on a chevy small block is to put on the TPI intake. It is by far the best intake ever designed for low to medium range torque. Couldn't move air well enough for high-end performance, but great for a daily driver.
You also keep mentioning horse-power (HP). That is nothing but a number for them to hook you on a sale. Those numbers you are talking about are at such high rpms that they are never or only rarely ever achieved. HP really doesn't mean crap outside of racing. If your motor builds 300 hp at 6500 rpm but only makes 65 ft/lb torque at 2000 rpm, you are in far worse shape for everyday driving than if your car only built 175 HP at 5500 rpm but makes 125 ft/lb torque at 2000 rpm.
You want to pick a car for economy, ignore HP totally, it doesn't really take affect until peak torque is reached, usually around 1000 rpm lower than peak HP. Instead, look at the torque curve, then test drive the car and see which is vehicle provides the best torque at the normal operating range at highway speeds.
