Why does my engine stop at 4500rpm in 5th...?

[tvradict]

I know this has something to do with Drag Coefficient or some pish like that, but why?

I sometimes drive quite fast (well, as fast as my car will allow, which is really quite slow) revving the engine quite high along the way. Holding 1st to 5000, 2nd to 5000, third I've been as high a 6000, 4th seems to stop at 5500rpm, more the power loses it's effectiveness rather than not being there and I have never had the engine about 4600rpm in 5th. Despite nearly 10 months of trying, I just can't get it above that. Why? This is usually around 110/115 depending on passengers and the vast quantity of sh*t that gathers in the boot. But strangely, I've had 115 4 up with a bootful going on holiday, which I can't quite understand

Can someone 'in the know' explain this one?

[MEMSDesign]

Air resistance (and friction) increases as you go faster. You engine can't provide enough power to overcome it. The 4500rpm, all the forces are balanced, and your engine can't push any harder.

You get variations mainly due to the effects of things like temperature and humidity on engine performance, and wind.

[Johnny Freon]

At higher speeds the weight doesn't limit progress nearly as much as the drag, which is why you didn't notice much difference with a full car.

J

[ultimaandy]

I think the two above posts have answered your question fully.

However as a side note, the strain on your engine flat out in top gear as you descibe will be enormous, if you are going to do this regularly keep it in tip top condition.

I used to do the same as teenager and did a few engine because of it (I also got caught), don't do it anymore because I would go to jail if I got caught (because of the speed).

[lotusguy]

MEMSDesign said: Air resistance (and friction) increases as you go faster. You engine can't provide enough power to overcome it. The 4500rpm, all the forces are balanced, and your engine can't push any harder.

You get variations mainly due to the effects of things like temperature and humidity on engine performance, and wind.

Hi,

Yes, you are spot on in all your observations with the exception of friction. Fluid Dynamics are waaay too complex a subject to try and summarize here, but leave it to say that air resistance increases exponentially to speed. Also, depending upon the car, tires, road condition etc. drag can vary as well - laminar flow vs turbulent flow.

There is always a limit for any design. The old Shelby Cobras (maybe AC Cobras to the Queen's Subjects), would go like spit 'til they reached 155-160mph. Then they hit the proverbial 'Brick Wall' with the literal feeling that someone had applied the brake, well below the car's 7,200 RedLine.

When the enclosed 'Daytona' Coupe body was installed, the car, with same engine/drivetrain topped out at 183mph!

An interesting experiment to confirm that aerodynamics, or more specifically, lack of them are the culprit here, try the same run, same road with the windows up and then down. A significant difference should exist between the two. Happy Motoring...Jim '85TE



>> Edited by lotusguy on Thursday 9th January 07:24

[skid]

Hi Guys,

One thing in addition which is probably the most significant is that the manufacturer geared the vehicle so high to give good mileage at 56mph in top and to satisfy promotional and brochure needs.

I'm assuming the vehicle is not brand new and at a guess a Ford or GM family car as its fairly common practice.

When Toyota originally introduced the MR2 way back the actively promoted the 'drivers car' side by highlighting it was geared correctly and redlined in top.

Yes various resistances effect all cars, it's how the manufacturers approach them.

Regards

Mark

[deltaf]

Just a thought matey, have you checked to see that youre getting FULL throttle?
Came across this kinda fault many times.
Get someone to plant their tootsie on the throttle, and have a look-see if its wide open. If not adjust da little bugga!