my car's top speed.
 

I just got home from a GREAT night-drive in my recently purchased 1990 GXL (all stock, 60k mi. on rebuilt motor). I learned two things:
1) I need brighter headlights.
2) My car's top speed is 120mph.

I was in 5th gear with the pedal to the floor. Speedo was reading a hair under 120 and tach was reading 5000 RPM. Being the novice that I am, I'd always assumed that any car's top speed was acheived at redline in high gear. Apparently it's not. Based on my assumption, I expected my car's RPM and MPH to continue increasing until the wheels fell off, the car flipped over, or I wet myself. Obviously, none of this stuff happened. Of course the car continued to slowly gain speed... but for all intents and purposes my top speed was 120.

So what factors are preventing my car from acheiving higher and higher RPM, and a higher top speed? Why doesn't the motor just continue to rev and push the car faster? This is all new to me, and I'm interested to learn what's going on. I'd appreciate any light that can be shed.

btw, 120 on good roads = pretty friggin' :cool:

You need to take into account the gearing of the car, (tranny & rear end) as well as resistance. There comes a point in which hp and resistance (wind, driveline, etc.) meet, this is your top speed. if you were to change your gearing you could increase your top speed, but probably not by much. In 5th gear at 120 you are turning 5k. you just aren't producing enough hp at 5k to continue accelerating. I remember reading somewhere one time that wind resistance is tremendous at that speed. If i can find the article i will post it for you. I'm no expert but I hope this helps.

Also, I used to live in Gainesville. Look up evot23, he works at Shands, he is a cool guy to hang out with and be envious of..!

It probably would have gone a little fast in 4th, and not 5th. CJ

yup, 5th is gas saver gear for FCs, try it again in 4th you may go higher. but like rgordon said at some point the power of your car cannot overcome the resistance from air. So if you want to go faster either make more power (higher RPM like I said in 4th) or make the car more aerodynamic.

interesting.... faster in a lower gear (spoken a la homer). Thanks for the replies. Concerning the aerodynamics: is it a matter of keeping downward pressure on the rear wheels, or is it a matter of slicing through the air altogether and avoiding resistance?

To go faster, you need less resistance.

To get better grip, you need downforce (which adds resistance).

Imagine you're pushing a fully-laden shopping trolly around. Pretty hard to push isn't it? Now imagine that another person helped you push it (doubling the power available) you can now go faster.

The same goes for engines, you either make it lighter to make it easier to "push", or you get more power to push it.


Aerodynamics: Look at F1 cars, do they have big flat front-ends like a truck? To lessen the resistance, (which makes it easier to "push" )you need to have smooth airflow, where the car slices through the air, creating as little disturbance as possible. If you had a F1 car drive past you, you'd feel a little breeze. If you had a truck go past you at the same speed, the air being pushed out of the way would be like a gale!!



In basics, the reason why your car stopped accelerating at 5K rpm, is because it didn't have any more power to move it any faster. If you ported it and added headers, then it'd rev higher, and your top-end speed would be higher

nicly explained chairchild. to make more power without mods just go down into 4th so you would have higher RPMs and a little more power than the lower RPMs in 5th.

sweet. looks like I need more guts under the hood :)

in the meantime, I haven't tried topping out in 4th gear yet, since I need to make the car last at least until I've saved up and "Oh #@$!" fund for it. lol. Anyway, thanks again for the input.

-marshall

it's pretty simple, acceleration=excess horsepower, at some point drag=available HP and you stop accelerating.