Maxing a Car. What Happens?

[Huntsman]

I'm sure some of the petrol-heads here have taken whatever they drive until it wouldn't go any faster. Today I realized I have a question about that.

Say you have a car that with no load, will not exceed 8,000rpm. First of all, why? I'm guessing either you, 1) run out of ability to pump air/fuel through, 2) run out of advance, or 3), float the valves.

Ok, so if you now take that car and put it on the road, and say you figure that at 8k rpm with your gearing and diff you should have a theoretical top speed of, oh, 180. But most cars never get to a 'theoretical' top speed, because wind resistance is increasing at the square (IIRC) of velocity and mucho more power is required. So what happens?

I am presuming that the engine climbs to whatever RPM is the peak power output and just sits there, right? It sounds logical but I wonder. It must be something to be doing like 150 in top gear, pedal to the floor, and see the tach frozen well below redline.


In the other case, the the engine does have the power, I suppose it winds up being limited by whatever limits the RPMs in the no-load condition -- but since max RPM is always(?) higher than the peak hp RPM, that should never occur, right?

 

[Serg]

In any somewhat modern car the chip cuts gas at the limit. it can still pass (briefly) but it will not pass the limit but not by much.

 

[Reuters]

Originally Posted by Serg

In any somewhat modern car the chip cuts gas at the limit. it can still pass (briefly) but it will not pass the limit but not by much.

+1.
There is a rev cut, impossible to hold in red for long.

 

[Huntsman]

Yeah, ok, that's certainly true but that's a very artificial constraint and doesn't tell you anything about how an engine works under those condition. And chips can be removed when they exist. My car doesn't even have a chip (not that I want to max it), so it should be able to outrun any normal MB on the road, which is an inaccurate comparison.

Regardless, the red is red for some reason, and I'm sure an SL65 is not running at redline when the speed limiter cuts it out at 155. So the questions still stand.

 

[dtmt]

The engine would not likely get stuck at exactly the peak hp rpm. While theoretically it would be possible to arrange the gearing as such, the odds are that it won't work out exactly that way in real life.

If you make a graph of how much power your car is putting down to the wheels according to rpm, and then overlay that with a graph of the amount of power it will take to overcome wind resistance at the corresponding speed in top gear*, then you answer will be a the point at which the second line crosses the first.


(*I assumed for this example that the car would be able to reach its highest possible speed in the highest gear, although for many cars the highest speed will actually be hit in the second highest gear)

 

[johnapril]

Originally Posted by Huntsman

Regardless, the red is red for some reason, and I'm sure an SL65 is not running at redline when the speed limiter cuts it out at 155. So the questions still stand.

The red represents hemoglobin smeared down a road.

 

[Journeyman]

Huntsman,

I don't know if my answer will help, but this is my experience with my 1990 1.6l MX-5 (Miata).

I've only hit the rev limiter a couple of times in my car when on a public road, and both of those times was when I was racing through the gears trying to get the drop on a Lotus 7. On both occasions, I hit 8500 revs in both first and second gears. Clearly, as I was accelerating as fast as I could but was on a public road, I didn't hit the car's top speed at that time.

The other occasions that I've hit the rev limiter was on the race track.
At those times, I hit the car's top speed (a bit over 110 miles per hour, if I remember correctly and if my mathematics is correct for the conversation factor) going down the main straight in fifth gear, and I was again bouncing off the rev limiter as I had the accelerator pressed flat to the floor.
Therefore, at least in my car, it would seem possible to achieve maximum speed and have the engine hit the rev limiter.
Thankfully, none of those experiences seems to have done my car the least bit of harm.

Regards,
J.H.

 

[yachtie]

Absent an electronic speed control, the car will sit at the speed where output equals drag ( which will be much slower than the theoretical top speed, or where the valves start floating). BTDT.

 

[briancl]

Originally Posted by Huntsman

Say you have a car that with no load, will not exceed 8,000rpm. First of all, why? I'm guessing either you, 1) run out of ability to pump air/fuel through, 2) run out of advance, or 3), float the valves.
Oiling is why cars have redlines. The manufacturer sets the redline at a safe point at which the engine can remain lubricated. If you want to raise your redline, you have to upgrade the oiling system. **edit** this is a gross oversimplication, but a good rule of thumb. high rpms = better oiling system required.

Ok, so if you now take that car and put it on the road, and say you figure that at 8k rpm with your gearing and diff you should have a theoretical top speed of, oh, 180. But most cars never get to a 'theoretical' top speed, because wind resistance is increasing at the square (IIRC) of velocity and mucho more power is required. So what happens? I am presuming that the engine climbs to whatever RPM is the peak power output and just sits there, right? It sounds logical but I wonder. It must be something to be doing like 150 in top gear, pedal to the floor, and see the tach frozen well below redline.

Right. Electronic speed limiters aside, the tach will remain "frozen" where resistance and output are equalized. Manufacturers do not set gearing to max out with top speed. They set gearing to meet certain acceleration and fuel economy targets. 99% of the time, the top gear (or top 2 or 3 gears) are purely "overdrive" gears in that they are designed to improve fuel economy, and the ability for a car to redline a top gear is really never intended.

In the other case, the the engine does have the power, I suppose it winds up being limited by whatever limits the RPMs in the no-load condition -- but since max RPM is always(?) higher than the peak hp RPM, that should never occur, right?

Yes. The manufacturer set redline limits the speed in this case which means that the lack of lubrication beyond the redline is limiting the top speed. Also, max RPM is not always higher than the peak HP rpm. There are plenty of cases where higher RPM would equal higher HP, but other factors limit the max RPM, so the real world max HP is lower. Of course, having peak HP at such a high RPM may not always be desirable, depending on the application of the engine.

 

[A Y]

I think Brian's laid it out pretty well. Also power required to overcome wind resistance is proportional to the cube of speed. So for speeds at which air resistance dominates (which is not very fast), going 10 percent faster requires 1.1^3 = 1.33 or 33 percent more power.

In the Miata example, it sounds like gearing and not engine power was the limiting factor, which is not surprising as the Miata has pretty short gearing.

--Andre

 

[Huntsman]

Originally Posted by briancl

Oiling is why cars have redlines. The manufacturer sets the redline at a safe point at which the engine can remain lubricated. If you want to raise your redline, you have to upgrade the oiling system. **edit** this is a gross oversimplication, but a good rule of thumb. high rpms = better oiling system required.
That's what I needed to know. I didn't realize this.

Originally Posted by briancl

Right. Electronic speed limiters aside, the tach will remain "frozen" where resistance and output are equalized. Manufacturers do not set gearing to max out with top speed. They set gearing to meet certain acceleration and fuel economy targets. 99% of the time, the top gear (or top 2 or 3 gears) are purely "overdrive" gears in that they are designed to improve fuel economy, and the ability for a car to redline a top gear is really never intended.
Thanks. It most be a pretty eerie thing to see. Yeah, I realize about the OD gearing, I just forget as I've never driven a car setup like that -- four fwd gears max, but usually three. I forget about these 5,6,7 speeders that we have nowadays.

Originally Posted by briancl

Yes. The manufacturer set redline limits the speed in this case which means that the lack of lubrication beyond the redline is limiting the top speed. Also, max RPM is not always higher than the peak HP rpm. There are plenty of cases where higher RPM would equal higher HP, but other factors limit the max RPM, so the real world max HP is lower. Of course, having peak HP at such a high RPM may not always be desirable, depending on the application of the engine.
In other words, most cars do tend to drop off in power before running out of speed, just for practical purposes of getting the curve efficient for normal driving.

Originally Posted by Andre Yew

I think Brian's laid it out pretty well. Also power required to overcome wind resistance is proportional to the cube of speed. So for speeds at which air resistance dominates (which is not very fast), going 10 percent faster requires 1.1^3 = 1.33 or 33 percent more power. In the Miata example, it sounds like gearing and not engine power was the limiting factor, which is not surprising as the Miata has pretty short gearing. --Andre
Cubed -- yeah as I typed square it just didn't feel right. I've forgotten my Aero eng classes. Powerful stuff, air.

Originally Posted by johnapril

The red represents hemoglobin smeared down a road.
Death always awaits the foolish, and sometimes, the unlucky. ~ Huntsman

 

[A Y]

Originally Posted by Huntsman

I forget about these 5,6,7 speeders that we have nowadays.

7 forward gears is so 2006. 8 is what's trendy now.

In other words, most cars do tend to drop off in power before running out of speed, just for practical purposes of getting the curve efficient for normal driving.

I think this depends on the torque curve and gearing as well. The 6-cylinder used in the Nissan 350Z is a very drivable engine, but it makes the most power right at redline. Some other cars, like the Honda S2000, make max power at redline, but many people don't like its drivability in normal situations. Things that can affect your torque include all the fancy variable valve controls many engines now have in addition to traditional measures like engine capacity.

The V10 in the new M5 can actually make more power at 10K RPM, but BMW limited its rev range so the engine would last longer. In addition to lubrication issues, the crankshaft resonates at certain frequencies. Those resonances are probably damped just for revs in the allowed range of the engine. Going above that is an invitation for trouble.

More revs generate more heat, and for very tightly packaged cars like BMWs (which run hotter than most any other production car), that could lead to other issues outside of the engine.

The engine computer may also not be able to keep its control loop going for higher engine speeds. Spark plugs and fuel-injectors may also poop out at higher speeds, and if they run, the spark plugs will certainly be generating more RF energy, which may screw up some other system in the car.

Cubed -- yeah as I typed square it just didn't feel right. I've forgotten my Aero eng classes. Powerful stuff, air.

Actually, I think you had it right in your original sentence because you were talking about air resistance instead of power to overcome that resistance.

--Andre

 

[j]

On my bike with the stock (tall) gearing, the top speed is actually only achievable in 5th gear, not 6th, because in 5th the engine is able to push nearer to the "redline" (Desmo engines don't have one, but they have a limiter in the computer around 9k) while in 6th it is in a lower part of the power curve of the engine. Most owners who tweak the bike start by putting a 14 tooth front sprocket in place of the stock 15, to lower the gearing across the board and make all the gears usable throughout their range.

It's said that the tall gearing is to allow the bike to pass strict Euro3 noise and emissions regulations. Also included is a fuel map that is very lean at idle and doesn't approach 14.1:1 AFR until somewhere in the 4k+ RPM range. The bike pops a lot on low RPM deceleration because of this.

 

[dl20]

I tried to max out my Z4 on a looong vacant stretch in PA a few times. Last run was around 155 and my speed was still slowly rising with the RPMS at 7k in 6th gear. Didnt have the balls to keep goin but I think 165-70 would be possible.

dl

 

[ms244]

Keep in mind too, that as you go faster the air moving on top of your car goes faster then the air on the bottom.

Faster air = less pressure.

It might lead to some handling problems down the road.

But yea, it comes down to power needed to overcome resistance (rolling, air, etc) vs. power available at the wheels.