ryanrlamothe
Moderator
Moderator
| Posts: 221 |   |
|
Re:Mustang Vs RS - 2006/11/22 12:27
Yes, that post was something you see a lot of...comparing one factor of performance and claiming which car is "faster".
Unfortunately, dyno's tell you nothing of the absolute performance of a car, just the engine torque per RPM.
So, without further ado, here is an equation for you:
Horsepower of a Rotating Mass
Horsepower is never measured directly, it is dependent on time, and is calculated by measuring the instantaneous torque.
So when considering the absolute performance of a car there are a number of things to consider:
1) Maximum RPM
2) Maximum Torque
3) At what RPM the torque is made
4) How fast the torque is made
5) Gear ratios in the transmission
6) Final drive ratio
7) Number of gears
8) Drivetrain (AWD/FWD/RWD)
9) Suspension
10) Rim size
11) Rim weight
12) Wheel weight
13) Engine weight
14) Weight distribution
15) Weight of the car overall
16) Aerodynamics of the car
17) Center of gravity
18) More...
Now, almost none of these performance factors have to do with driving in a perfectly straight line, obviously. And some of these factors are dependent on other equally important factors such as compression ratio, fuel grade, flywheel weight, camshaft profile, operating temperature, oil viscosity, bore, stroke, etc. So, to limit this discussion to driving an NA engine in a perfectly straight line, we will only focus on a few of the items in the list.
Comparing a 2.5L four cylinder NA engine to a 5.0L eight cylinder NA engine with identical bore, stroke, cylinder head design, compression ratio, RPM range, gasoline grade, elevation, temperature, humidity, 3rd gear ratio (since most dyno's use 3rd gear for testing), etc. should yield 1/2 the output for the 2.5L four cylinder engine as the 5.0L eight cylinder engine.
So, you ask, why can I take a 200bhp 1.8L four cylinder Lotus Exige to Virginia International Raceway bone stock and hand even modified Mustang's their asses over and over on the track? Because, we are talking about driving in perfectly straight lines. Again, the dyno curves tell you nothing about real absolute performance.
Back to the straight line discussion. The comparison of the two engines above, the 2.5L and 5.0L, assumed that everything was equal between the two engines except for physical cylinder displacement and number of cylinders.
So, a few of the factors to consider then when looking to make your 2.5L NA engine compete with a 5.0L NA engine:
1) Gear ratios
2) Final drive ratio
3) Flywheel weight
4) Torque output
5) Maximum RPM
6) Compression ratio
7) Camshaft profile
8) Bore
9) Stroke
I think that about covers it. Obviously, that is somewhat simplified, but hits the major points.
Now, gear ratios are very important when considering a particular type of performance. For example, take a look at some commonly used Subaru gear ratios at this link from Rallispec:
http://www.rallispec.com/Subaru%20Gear%20Chart.htm
You can see that changing from the stock gear ratios to RA gear ratios will increase your acceleration immensely. Also notice that along with the change in gear ratios there can also be a corresponding change in final drive ratio, which is both the center and rear differentials in the Subaru's. Do you want to see a 0-60 time in the high 3 seconds? Try RA gears with a 4.44 final drive ratio. The stock gear ratios on the Subaru are meant for fuel economy and highway driving, neither of which are great for racing.
The next thing you will want to consider is your engine's output. Can you make a huge power 2.5L NA engine? Sure. Formula One 2.4L engines run at 15,000-20,000+ RPMs and produce almost 1000bhp. So you can too. What is important is that you match your engine build to your drivetrain gearing. If you want long, track racing gears then you will want a different build than if you want short, Rally sprint gears.
I have to get going. More on this discussion later...
|
Forums
