81 replies to this topic

[daenesh]

A classic Mini can be safely revved to 6000 rpm or just over. If the engine produces maximum power at, say, 5800 rpm, those are the revs at which maximum speed should be achieved. Thus it needs to be geared to achieve this.

If geared too high, say a 998 with a 2.9:1 FDR, it may actually be faster flat out in 3rd rather than in top.

Changing from, say, 12" wheels to 10" won't make any significant difference because the rolling radius of the 12" with the lower profile tyres will give approximately the same rolling radius, certainly no more than about 3% difference.#

The higher the revs at which max power is produced, the lower the FDR needs to be.

There are so many variables in this maximum speed business that it is not easy to explain them all, but the key thing is gearing. The 998 Cooper gave its maximum speed from 55 bhp which was produced at around 5800 rpm and with a 3.76:1 FDR that gave about 88 mph at those revs.

The Cooper 'S' 1275 with 75 bhp had a 3.44:1 FDR and gave about 96 mph at the same sort of revs, i.e. 5800. So the additional 20 bhp only gave an additional 8 mph due to the drag increasing as the square of the speed. So double the speed = 4 x the drag. There is no answer to this and those who claim silly maximum speeds don't understand that their cars are governed by the laws of physics.

So to achieve 100 mph requires approximately 90 bhp and as the engine will be modified it will probably give this power at just over 6000 rpm, so it must be geared down to do this.. 

These figures ignores the torque curve which is needed to enable the engine to continue to 'pull' against the drag curve after changing up.

Another thing which improves max speed by way of drag reduction is to remove the huge wing mirrors and just fit a small rectangular A-post mounted mirror on the driver's side only. Removing any wheel-arch extensions and fitting narrower wheels & tyres will add a couple of mph at the top end as well. The huge 'Sportpack' arches do create a lot of additional drag - look at the cross-sectional area they present to the airflow.

As I said, it's complicated  .

 

i respect you but i would say you're wrong.. with my fairly modded 998 i did a GPS 160kmh which equates to 100mph.. i wasn't able to screenshot it but i will do it and post it here just so you'll know im not bulls**ting.. :)

Edited by daenesh, 05 May 2015 - 02:38 AM.

[Ethel]

That's actually a very good explanation from Cooperman. From reading the forum it's clear not everyone grasps the relationship between speed, wind resistance, power and gearing.

 

I don't think anyone doubts 100mph+ speeds are possible in any car - all you'd need is a tall enough cliff. It's whether or not you include gravity and tail winds, which aren't part of the car so don't help comparisons.

 

Though I do think it's an aspect of tuning that's not often mentioned. If your efforts reduce how quickly the power tails off after you've passed the peak it can increase the top speed where you're making up the deficit for an assist off a hill or tail wind. Especially true for non-bouncing points, valve springs and longer duration cams.

[Cooperman]

 

A classic Mini can be safely revved to 6000 rpm or just over. If the engine produces maximum power at, say, 5800 rpm, those are the revs at which maximum speed should be achieved. Thus it needs to be geared to achieve this.

If geared too high, say a 998 with a 2.9:1 FDR, it may actually be faster flat out in 3rd rather than in top.

Changing from, say, 12" wheels to 10" won't make any significant difference because the rolling radius of the 12" with the lower profile tyres will give approximately the same rolling radius, certainly no more than about 3% difference.#

The higher the revs at which max power is produced, the lower the FDR needs to be.

There are so many variables in this maximum speed business that it is not easy to explain them all, but the key thing is gearing. The 998 Cooper gave its maximum speed from 55 bhp which was produced at around 5800 rpm and with a 3.76:1 FDR that gave about 88 mph at those revs.

The Cooper 'S' 1275 with 75 bhp had a 3.44:1 FDR and gave about 96 mph at the same sort of revs, i.e. 5800. So the additional 20 bhp only gave an additional 8 mph due to the drag increasing as the square of the speed. So double the speed = 4 x the drag. There is no answer to this and those who claim silly maximum speeds don't understand that their cars are governed by the laws of physics.

So to achieve 100 mph requires approximately 90 bhp and as the engine will be modified it will probably give this power at just over 6000 rpm, so it must be geared down to do this.. 

These figures ignores the torque curve which is needed to enable the engine to continue to 'pull' against the drag curve after changing up.

Another thing which improves max speed by way of drag reduction is to remove the huge wing mirrors and just fit a small rectangular A-post mounted mirror on the driver's side only. Removing any wheel-arch extensions and fitting narrower wheels & tyres will add a couple of mph at the top end as well. The huge 'Sportpack' arches do create a lot of additional drag - look at the cross-sectional area they present to the airflow.

As I said, it's complicated  .

 

i respect you but i would say you're wrong.. with my fairly modded 998 i did a GPS 160kmh which equates to 100mph.. i wasn't able to screenshot it but i will do it and post it here just so you'll know im not bulls**ting.. :)

 

In that case you have either:

 

Around 88 to 90 bhp with the gearing so that it will pull through the drag curve to the revs for max power in top.

A tail wind of around 10 to 15 mph when doing the measuring.

A road with a downhill sl.ope

An optimistic GPS reading (not impossible).

 

If one of those, or a combination of those was not present, then you have defeated the laws of aerodynamics.

[nicklouse]

A classic Mini can be safely revved to 6000 rpm or just over. If the engine produces maximum power at, say, 5800 rpm, those are the revs at which maximum speed should be achieved. Thus it needs to be geared to achieve this.
If geared too high, say a 998 with a 2.9:1 FDR, it may actually be faster flat out in 3rd rather than in top.
Changing from, say, 12" wheels to 10" won't make any significant difference because the rolling radius of the 12" with the lower profile tyres will give approximately the same rolling radius, certainly no more than about 3% difference.#
The higher the revs at which max power is produced, the lower the FDR needs to be.
There are so many variables in this maximum speed business that it is not easy to explain them all, but the key thing is gearing. The 998 Cooper gave its maximum speed from 55 bhp which was produced at around 5800 rpm and with a 3.76:1 FDR that gave about 88 mph at those revs.
The Cooper 'S' 1275 with 75 bhp had a 3.44:1 FDR and gave about 96 mph at the same sort of revs, i.e. 5800. So the additional 20 bhp only gave an additional 8 mph due to the drag increasing as the square of the speed. So double the speed = 4 x the drag. There is no answer to this and those who claim silly maximum speeds don't understand that their cars are governed by the laws of physics.
So to achieve 100 mph requires approximately 90 bhp and as the engine will be modified it will probably give this power at just over 6000 rpm, so it must be geared down to do this.. 
These figures ignores the torque curve which is needed to enable the engine to continue to 'pull' against the drag curve after changing up.
Another thing which improves max speed by way of drag reduction is to remove the huge wing mirrors and just fit a small rectangular A-post mounted mirror on the driver's side only. Removing any wheel-arch extensions and fitting narrower wheels & tyres will add a couple of mph at the top end as well. The huge 'Sportpack' arches do create a lot of additional drag - look at the cross-sectional area they present to the airflow.
As I said, it's complicated  .

 
i respect you but i would say you're wrong.. with my fairly modded 998 i did a GPS 160kmh which equates to 100mph.. i wasn't able to screenshot it but i will do it and post it here just so you'll know im not bulls**ting.. :)

In that case you have either:
 
Around 88 to 90 bhp with the gearing so that it will pull through the drag curve to the revs for max power in top.
A tail wind of around 10 to 15 mph when doing the measuring.
A road with a downhill sl.ope
An optimistic GPS reading (not impossible).
 
If one of those, or a combination of those was not present, then you have defeated the laws of aerodynamics.

Might be deseamed. That has quite an effect. As do many other things that I won't go into.

[Cooperman]

De-seaming reduces frontal area by around 40 sq. ins, which is quite a lot.

Then there are the huge wing mirrors fitted to later cars which measure about 6" x 4", so that's 48 sq. ins.

If Sport-Pack arches are fitted, the profile drag is increased by a further 30 sq. ins per side = 60 sq. ins. 2" wide arch extensions give around 20 sq. ins. per side.

Wider wheels also increase the drag and typically a 13" x 7" wheel with a 175 section tyre as compared to the original 3.5" x 10" wheel with a 145 section tyre will account for an additional 20 sq. ins. per side = 40 sq. ins.

Adding that lot up gives a figure of around 178 sq. ins of frontal area increase over the original Mini/Cooper/Cooper 'S'.

If under-trays were fitted below the rear sub-frame to each side of a centre exhaust that would improve things a bit more.

So simply going back to the original frontal profile with a later car will increase the maximum speed by reducing the drag. I know a wing mirror is needed for a later car, but the law requires just one on the driver's side and the small 4" x 2" A-post rectangular mirror works fine.. or even a 'bullet-shaped' period '60's mirror can be good. Streamlining does work.

All this still won't give 100 mph on anything less than around 88 bhp with compatible gearing as the figures are all based on the early cars which did not have wheel arch extensions or big wing mirrors.

[CMXCVIII]

All this talk of wind resistance and frontal area is very good but it's old fashioned! 

 

This is the internet age ... don't forget the power of red paint!?

[daenesh]

All this talk of wind resistance and frontal area is very good but it's old fashioned! 

 

This is the internet age ... don't forget the power of red paint!?

 

i like where you're getting at.. hahaha