51 replies to this topic

[Tamworthbay]

Worth mentioning at this point that with ANY problem, always rule out the easy/ cheap potential problems first. No point in dropping the box only to find out it was a dirty float bowl!

Tiger - how did you do the swap in situ? Getting the diff out? And the drive pinion? As I am sure you are aware you should always swap pairs as it will most likely fail shortly afterwards otherwise.

[stevelane]

 

 

Ok, so here's the problem, you lost me at 'confirm the final drive ratio'. I wouldn't even know how lol

 
Do you have a rev counter?

Yes I do
 

 
Excellent.
 
If you've got a sat nav go out, and see how many revs the engine is doing at 60mph on the sat nav.
 
Then come back and post it and we'll work it out.
 
Also, what speed the speedo is reading when the sat nav says 60mph

Ok I've been out for a run. By the speedo I was at 3000 revs. My sat nav wasn't charged up so I used an iphone dash app which was slightly different, 3200 at 60mph

[Cooperman]

The problem is that with very high geared - low powered cars the drag builds up and prevents the engine getting to the required power to continue to accelerate the car in top gear.

At 3000 rpm the engine is not putting out enough power &/or torque to continue to accelerate in top above around 3000 rpm.

Peak power is probably at about 5500 rpm, so it needs to be taken beyond that in 3rd gear before changing into top.

For maximum speed it will need to be taken to about 6000 rpm in 3rd, then  changed up into top which will put it right in the best torque band and it may well then accelerate to around 78 to 80 mph, which is what is should do flat out.

The problem with the City E is the high gearing gives a wide ratio between gears. 

Try using a lot more revs through the gears and see what it will pull to. In fact in 3rd it should do about 70 mph or even slightly more at peak power.

[stevelane]

The problem is that with very high geared - low powered cars the drag builds up and prevents the engine getting to the required power to continue to accelerate the car in top gear.

At 3000 rpm the engine is not putting out enough power &/or torque to continue to accelerate in top above around 3000 rpm.

Peak power is probably at about 5500 rpm, so it needs to be taken beyond that in 3rd gear before changing into top.

For maximum speed it will need to be taken to about 6000 rpm in 3rd, then  changed up into top which will put it right in the best torque band and it may well then accelerate to around 78 to 80 mph, which is what is should do flat out.

The problem with the City E is the high gearing gives a wide ratio between gears. 

Try using a lot more revs through the gears and see what it will pull to. In fact in 3rd it should do about 70 mph or even slightly more at peak power.

 

In which case i'm wondering whether I am just holding back due to the noise - at 3000 revs the car seemed to be very loud so the thought of 6000 revs seems impossible but maybe I am just too used to more modern cars and having not driven this for a couple of years have forgotten how noisy it could be

[cal844]

Our 1989 998 mayfair pulls well, all the way to 70 possibly 75 in third, but at 70+ it was at 4k+ rpm

[Gremlin]

With the gearing and tyres I've got, due to the speed calculator I'm doing 60 mph at 4,500 in 3rd, and 60 mph at 3,000 in 4th, if it would do it I would would be doing 138 mph at 6,500

[Cooperman]

I just checked to see where peak power is with a standard 998 and it seems as though it is c.40 bhp @ 5000 rpm.

The theoretical maximum speed will be at the point where the power required to match the drag equals 40 bhp. Then, whichever gear matches the revs at that speed to 5000 will be the gear at which max. speed is achieved. It will be the case that if there is no gear which matches the ideal rpm, then the maximum will be slightly lower than theoretical.

The old 850 Mini had almost ideal gearing. Its top speed was about 75 mph, which it reached at 5200 rpm in top, which is almost exactly where peak power came in. That was with a 3.76:1 FDR.

With a higher FDR the car will cruise at lower revs for a given speed, but may struggle to get to the theoretical maximum in top gear as the revs will be too low to achieve the necessary power. Indeed, it will probably be faster as a maximum in third gear, say at 5500 rpm.

Unless you have the 'drag curve' for a Mini it won't be possible to calculate the theoretical max. MOH or the gearing needed to achieve this in top.

[Cooperman]

With the gearing and tyres I've got, due to the speed calculator I'm doing 60 mph at 4,500 in 3rd, and 60 mph at 3,000 in 4th, if it would do it I would would be doing 138 mph at 6,500

If it needs about 34 bhp to do 70 mph in 4th, to do 138 mph, i.e. double the speed, would need 4 times the power, that is about 140 bhp at 6500 rpm, to overcome the drag rise which is the square of the speed increase as a multiplier. So 'speed x 2 = power x 4'. However, it would be no good producing this power at higher revs unless the gearing were dropped to suit the point of maximum power.

[Gremlin]

With the gearing and tyres I've got, due to the speed calculator I'm doing 60 mph at 4,500 in 3rd, and 60 mph at 3,000 in 4th, if it would do it I would would be doing 138 mph at 6,500

If it needs about 34 bhp to do 70 mph in 4th, to do 138 mph, i.e. double the speed, would need 4 times the power, that is about 140 bhp at 6500 rpm, to overcome the drag rise which is the square of the speed increase as a multiplier. So 'speed x 2 = power x 4'. However, it would be no good producing this power at higher revs unless the gearing were dropped to suit the point of maximum power.

That's something I hadn't thought of, if your gearing is such that you can't get into the power band your stumped, I had 56 BHP from my 998 to which I then put a piper 255 cam in so not sure what sort of power I'm getting but it goes well from 2700+, It cruises nicely at 60 (3000rpm) and will overtake from 60 pretty well

[Tamworthbay]

It is a problem on the city E (as anyone who followed me on the A5 last year will testify). The time taken to go from third to fourth on a steep hill is enough for the speed to drop sufficiently that you can't pull fourth. Very different even with a stage one kit fitted.

[T.Harper]

Hi - I think this is still an issue of engine tuning. City Es do have very tall gearing but once they get there they can easily hold 70-80 on the flat in still conditions - standard without stage 1.

I would recommend a good service (including new ignition consumables such as ht leads as breakdown of leads and plugs can produce similar issues)) and then a tune before more significant things are considered.

[Cooperman]

Interestingly, the old 998 Cooper had 55 bhp with a 3.76:1 FDR. This gave the same gearing as an 850, so at the max speed of around 90 mph around 6000 rpm was being pulled, which is exactly where maximum power happened. So the formula works.

Also the increase of max speed from 72 of the 850 to 88 of the 998 Cooper, with the same gearing, is about right according to the formula. The increase in factor of speed from 72 to 90 is 1.25. So the square of the increase is 1.25 squared = 1.5625. Multiply 1.5625 by 34 bhp = 53.2 bhp, which is virtually the stated power of the 998 Cooper.

However, the drag did not stay at a constant figure across the entire Mini range. In fact, the later cars with their wheel arch extensions, larger wing mirrors (early cars had no wing mirrors), wider wheels & tyres put a significant amount onto the power needed to overcome the drag at any given speed (increased the fuel used as well) and this reduced the maximum speed of the later cars. It is definite that the later 1275 Minis with Sportpack arches, huge wheels, etc, with around 65 bhp were slower at the very top end than the earlier 1963 1071 Cooper 'S' which had 3.76:1 FDR, 3.5" wheels and no arches wing or mirrors and the same 65 bhp.

The fastest production Mini was the 1964 - 67 Mk.1 Cooper 'S' 1275 which would max out at around 97 mph at 6000 rpm on a 3.44:1 FDR. Without the drag-creating arches, wheels & mirrors it was very quick indeed. It had 75 bhp at about 5900 rpm and stacks of torque as well.

Drag curves, power/torque cures and gearing is a complicated subject and even major manufacturers can get it wrong. However, the higher gearing and consequent lower rpm at normal cruising, to the detriment of max speed, was seen by manufacturers in the late 70's and the '80's as being a good idea to reduce engine wear and consequent warranty claims. If you want better top speed and faster acceleration, just drop the overall gear ratio by lowering the FDR. That will let it rev to max power in all the gears, including top.

[Carlos W]

In answer to the OPs question, it is worth making sure the brakes aren't sticking on, the engine is well serviced, and set up.

It may be worth a stage 1 kit and a rolling road session.

Its also worth checking the exhaust for leaks as this will make the car louder and make you less inclined to rev it

[Ethel]

Isn't the power/speed ratio cubic? You aircraft wizards deal in thrust which is force not power. The car equivalent is the torque  on the driving wheel side of the gearbox. 

 

We're about shoving air out of the way:

 

Double the speed and you travel twice as far so you also double the mass of air you encounter, and have to move in the same time, but you also have to accelerate it to the speed of the car, which has also doubled. For good ole F= MA that wheel torque (F) now has to equal a double helping of air mass times a double helping of acceleration to get it out the way. 

 

Scribbled this while Cooperman was posting at 12.08 - dunno if I'm right but I do know my head hurts   

 

I shouldn't think aloud when its late, double times double is double squared  to get from 1F to 2F requires 2M times 2A, or the square of the increase in F  (just like Cooperman says).

[The Matt]

Effel...In the equations where you consider the pocket of air you're pushing your car through, you multiply air mass x CD x area (or volume depending how CD has been degined) x V^2 (velocity is squared).

So the effect of speed is masseev. Changing speed from 10m/s to 20m/s won't double the force, it'll quadruple it....weeee. Lotsa ponies required to drag yer cart along at 100mph.

That aside, a well tuned City E (in standard nick with no mods) should be able to sit at 70. Mine certainly did. There may be some tuning issues to address before considering.

Disclaimer: I wrote the above whilst sat on the loo, most of it may be codswallop.