CarTalk.com Blogs Car Info Our Show Deals Mechanics Files Vehicle Donation

Maximum Horsepower

I was just curious, if anyone knows the answer; what is the maximum horsepower that can be extracted from a 2.0 liter 4 cylinder naturally aspirated gasoline engine? Say redline is around 7K RPM.

Check with Honda in street cars, 2.0, 240 horse power. Honda 2000. More I’m sure can be produce (Farrari) but at the sacrifice of reliability or very high cost. Those are the real limiting factors IMO. There is really no race to the top in hp if it isn’t accompanied by gobs of torque. That’s why doing it with turbos is better and cheaper.

If you have lots of money to spend, and you weren’t concerned with emissions or longevity, you can go well beyond 240 HP.

The areas to increase the horsepower are still the same as they have been for years - just a little more complicated. These include modifications to the camshaft, pistons, intake manifold, exhaust, fuel delivery system, engine sensors, ignition timing curves, and I’m sure I’m missing several.

If you have lots of money to spend, and you weren't concerned with emissions or longevity, you can go well beyond 240 HP.

A bit too lazy to do the math that would be required, but probably not in a 2L limited to 7kRPM. Kill the rev limit and you can get some very impressive numbers.

Probably 500 HP for a few races. All depends upon how much money you want to spend on modifications and how long you want it to last between rebuilds.

500 hp from a naturally-aspirated 2L limited to 7k rpm?

I believe that Honda developed 240 hp at 9k rpm.

500 sounds like a reasonable number… maybe even more if you have 20K or more to spend on the mods and don’t care about daily driveability, noise, or long term longevity.

Or you could buy a V8 with this much HP and enjoy the much more effortless power, better sound, and at least 200K trouble-free miles, plus the flexibility to go much higher in HP if you wanted. At that level of HP, you’ll likely get the same miserable MPG out of either too. But to each their own.

Once you realize the 2.0 can only suck in so much air, the next reasonable progression is to add a turbo. So much easier to get power from that route. None can easily enlarge the cylinder head ports to flow a LOT more air. It is physically impossible in almost all cases.

I’d be amazed if one could get 500 hp out of a non-turbo/supercharged 2.0 l with a 7000 rpm rev limit. HP = torque X RPM, you need to rev to get crazy hp.

2.0 liter 4 cylinder naturally aspirated gasoline engine and a redline around 7K RPM. The answer is not very much. The redline at 7K is for a V8, an inline 4 can rev way beyond 7K.

In a racing version even a naturally aspirated inline 4 could run up to 15K and would likely be running 11-12K in normal racing. With those kinds of rev’s you could get into 300-400 hp.

Normally aspirated at 7K you are talking 200 to 250 max. Now, put a blower or turbo on it and you’ll get about 300hp out of 7K and 500hp at 10-11K.

With or without N2O?

To find out, figure the total volume that will be drawn in at 7K, calculate the total amount of gas that’ll be in the volume based on a theoretically perfect ratio, and figure the total energy contect of that gas. That gives you the true theoretical maximum power, as it assumes that all energy is converted to torque (none wasted in heat and expelled through the exhaust) and none wasted on friction.

But there’s a huge gap between that and the maximum HP that can be attained in real life. And it’s loaded with variables. Such as thermodynamic efficiencies in the flow of gasses, the weights of the reciprocating masses, and a hole list of others. Simply allowing an impractical idle speed opens up opportunities for more HP.

I’m not certain the question is answerable with only those constraints. All one would need is a new development in piston and rod material, a new way to fill and purge the cyllinder, or a new development in fuel technology to boost HP. Or even a way to perfectly balance crankshaft operation without the flywheel and harmonic damper.

The maximum that’s been gotten out of a naturally aspirated engine of 2L is probably about 300, but that doesn’t mean that’s the true maximum possible.

It seem 100HP per liter can be extracted with reasonable reliablity. This figure keeps going up as engine refinements and better materials are developed. The upper limits are much higher, but at the expense of reliability engine life and cost.

My 1948 Chevy “stovebolt 6” developed only 85 HP from 235 cubic inches or 3.76 liters.

@Docnick
And that was a hot engine. Wow! Soon, as Ford and others are proving, until electric power in the drive motor is the norm, small turbos in daily driving is preferred…torque is where it’s at. Getting lots of hp out of a small motor is vanity for the rich…or power hand tool and ouboard motor makers.
Now, if this conversation included two strokes…

Stroke and connecting rod length seem to be the most limiting factors in RPMs.

Years ago, Toyota had an engine called the Formula Atlantic, it was racing only. It started with a 1.6 liter 4AGE engine and with about $30,000 worth of parts, plus precision machining and meticulous assembly, it would produce 320 HP at well over 7k rpm though.

I’d add “reciprocating masses”. A longer rod length weighs more as well as changing the angles at which it pushes the crank. A heavier piston is another drain on HP and on RPM ability.

All other things equal, the lighter the reciprocating masses are is the more HP ends up in the crankshaft and the better able one is to get higher RPMs and a better torque curve.

Y’know, the more I think about it is hte more I realize how much of an impact valvetrains have on RPMs. Some design company had a rottating ballvalve system developed to replace traditional valves, and it worked. I wonder whatever happen to that.

Wasn’t Mercedes fooling with that a few years ago? I remember it was supposed to go in a C-class, but I don’t think it ever did.

Latest I’ve heard is electronically activated valves. But it must be difficult to come up with something that’s better all-around than the ancient cam/valve setup…

It’s an ingenius setup, but those springs and rockering rockers take energy to operate. It would be an asset if someone could find a better way.