A Squared
Well-known member
- Joined
- Nov 26, 2001
- Posts
- 3,006
The reason the auto engine is putting out more HP per cubic inch is because of higher RPM *and* higher compression ratio. When you make relevant comparisons, the differences aren't that great.
I don't think that you're going to find too many normally aspirated production auto engines producing 140 bhp from 90 Cubic inches as has been stated. Let's take a look at a real auto engine, instead of some of the fictional ones that have been mentioned.
VW Golf engine. 2.0 liter, 121 Cubic inch displacement. Normally aspirated.
Compression ratio, 10:1
There's your first clue. a 10:1 compression ratio is substantially higher than the more 6.5:1 or 7:1 which is common in aircraft engines. Higher compression ratio results in higher brake specific fuel consumption (more horsepower per gallon of gas) and greater power per cubic inch.
Now here's the rest of the story: the engine puts out 115 hp at 5200 Rpm. That's fast. That's in the neighborhood of twice as fast as the rated RPM of an aircraft engine.
So we have 50% higher compression ratio, and 90% higher crankshaft speed. This is hardly an apples to apples comparison. Let's make a more relevant comparison. At 2600 RPM the Golf engine develops 60 hp. Ok that's a little closer. that's 0.496 bhp/cubic inch
Compare that to an Lycoming O-235-C engine. 233 cubic inch displacement, 6.5:1 compression ratio produces 115 bhp at 2800 rpm. That's 0.493 bhp/cubic inch. That's less than a one half of a percent difference in power per cubic inch. Not much of a difference at all really. Not bad for a lower compression engine.
What if you upped the compression in the airplane engine?Lets compare the Golf engine to the O-235-F That engine has the same 233 CID, but it has a 9.7:1 compression ratio. It develops 125 BHP at 2800 RPM, or 0.536 BHP/cubic inch. That's *more* power per cubic inch than the Golf engine, about 8% more, of course the airplane engine is turning faster than the Golf engine, in fact it's turning 8% faster.
Anyone think that it's a coincidence that the O-235-F develops 8% more power per cube while turning 8% faster? Anyone? Bueller?
Here's another real life engine:
The 144 cubic inch, fuel injected engine in my Toyota Pickup truck develops 73 BHP at 2800 RPM. That's 0.507 Bhp/Cubic inch, *less* than the airplane engine at the same RPM.
Yeah the numbers seem wildly different when you don't look to carefully, when you actually sit down and make legitimate comparisons, there really isn't that much difference in terms of power output.
I don't think that you're going to find too many normally aspirated production auto engines producing 140 bhp from 90 Cubic inches as has been stated. Let's take a look at a real auto engine, instead of some of the fictional ones that have been mentioned.
VW Golf engine. 2.0 liter, 121 Cubic inch displacement. Normally aspirated.
Compression ratio, 10:1
There's your first clue. a 10:1 compression ratio is substantially higher than the more 6.5:1 or 7:1 which is common in aircraft engines. Higher compression ratio results in higher brake specific fuel consumption (more horsepower per gallon of gas) and greater power per cubic inch.
Now here's the rest of the story: the engine puts out 115 hp at 5200 Rpm. That's fast. That's in the neighborhood of twice as fast as the rated RPM of an aircraft engine.
So we have 50% higher compression ratio, and 90% higher crankshaft speed. This is hardly an apples to apples comparison. Let's make a more relevant comparison. At 2600 RPM the Golf engine develops 60 hp. Ok that's a little closer. that's 0.496 bhp/cubic inch
Compare that to an Lycoming O-235-C engine. 233 cubic inch displacement, 6.5:1 compression ratio produces 115 bhp at 2800 rpm. That's 0.493 bhp/cubic inch. That's less than a one half of a percent difference in power per cubic inch. Not much of a difference at all really. Not bad for a lower compression engine.
What if you upped the compression in the airplane engine?Lets compare the Golf engine to the O-235-F That engine has the same 233 CID, but it has a 9.7:1 compression ratio. It develops 125 BHP at 2800 RPM, or 0.536 BHP/cubic inch. That's *more* power per cubic inch than the Golf engine, about 8% more, of course the airplane engine is turning faster than the Golf engine, in fact it's turning 8% faster.
Anyone think that it's a coincidence that the O-235-F develops 8% more power per cube while turning 8% faster? Anyone? Bueller?
Here's another real life engine:
The 144 cubic inch, fuel injected engine in my Toyota Pickup truck develops 73 BHP at 2800 RPM. That's 0.507 Bhp/Cubic inch, *less* than the airplane engine at the same RPM.
Yeah the numbers seem wildly different when you don't look to carefully, when you actually sit down and make legitimate comparisons, there really isn't that much difference in terms of power output.
