Tall story!

After my experiences last summer buzzing down the French peages, I have decided to fit a 3.54 diff to reduce the revs a bit. Doing a bit of calculating with the revs/Diff ratios/Gbox ratios I find that there is a potential top speed of 160mph at 6500rpm redline in an Elan or +2 with a 5 speed gearbox and a 3.54 diff running standard profile wheels and tyres. You don’t want to know what 7500rpm gives… The superior aerodynamics of a +2 may win here.

Anyone got even close!!

Jeremy

:open_mouth: Really! :open_mouth:

My hood (slightly tired) used to pop up at the front when over 110 mph in temps higher than 25*c.

You’ll need the benefits of aerodynamics for all the extra weight of a +2 of course :smiling_imp:

BARNEY

And maybe the long range fuel tank that was dropped into an Elan plus 2 as a joke at the car show at the NEC!!!

Plus someone lunatic enough to try it :laughing:

Andy

The Europa front spoiler also fits the +2 perfectly !

With my (original) 5 speed sprint, (with the normal diff), the speedo needle would go all the way round to the ignition light on a pretty long downhill bit of the autobahn - but I have no idea whatsoever what speed that was! (Speedos are notoriously inaccurate and often lose their original adjustment). On the flat the top speed was approx. the same in 4th (in the red a bit) as in 5th. Pity I didn’t have a GPS then - not sure I feel like doing that any more now that the car is much older (and me too)!
It could be, that without more power, you’ll be faster in 4th than in 5th (on the flat).
Gordon

Recreational use of Excel lead me to this musing. I attach the spreadsheet for interest. I don’t guarantee the figures, and welcome comments/improvements. Two sick days off work got me bored!

Jeremy
Gearing and Road speed ratios.xls (25 KB)

Interesting comments

The early Elans had a speedometer with numerical marks every 20 mph to 140 mph. Later speedometers had different numerical marks, up through 130 mph. However, both speedometers had the same sweep, so the 130 mph speedometer had a tick mark that would correspond to 140 mph.

Getting the speedometer needle into the ignition light implies a reading above 140 mph. Of course, as others have said, this calls into question the speedometer calibration. I have read of other +2s getting above 130 mph.

You could also look at the tachometer reading, since that often times is more accurate.

Knowing the tire size, differential ratio, gearbox ratios, and engine RPMs should allow car speeds to be calculated using simple math. However, life is almost never that simple.

If the tire were perfectly rigid, the calculations would be accurate. However, a real tire deforms at the contact patch. This results in the actual tire rev/mile figure being greater than the calculated figure, or, conversely, the actual speed attained to be less than calculated.

The heavier the weight on the tire, the more the deformation and the higher the rev/mile figure, conversely, the lighter the weight the lower the rev/mile figure.

A higher inflation pressure results in a lower rev/mile figure (higher speed), while a lower inflation pressure results in a higher rev/mile figure.

As the tire wears, the rev/mile figure gradually increases. For a 10 mm tread depth, this results in a 165-13 tire going from 887 rev/mile to 919 rev/mile, a well over 3% change.

The tire manufacturers generally publish figures for the rev/mile of their tires, for each size at a default weight, rim width, and inflation figure.

However, different manufacturers have different rev/mile figures, even for the same tire size.

Some years ago, I made a spreadsheet with tire sizes and different reported rev/mile values. Averaging all the differently reported values resulted in a reported rev/mile figure 1.03 times greater than that calculated from the nominal tire dimensions. Discussing this with another expert wanting to use the data revealed an industry recommendation to use 1.031, pretty close.

To complicate matters, some manufacturers report basically garbage, so you have to know about what the figure should be! So, this gets close, and adjustments can be made for weight, inflation pressure, rim width, and tread wear.

A previous post, lotuselan.net/forums/viewtop … c&start=28 (msd1107 post of Tue Jan 22, 2008 12:53 pm) has a downloadable spreadsheet that, for a user input tire and differential ratio generated a two dimensional table of user input max/min/increment RPM and user input gearbox ratios.

There are six combinations of these four factors. Another post,
lotuselan.net/forums/viewtopic.php?t=15596, had four of the other combinations. There were sheets with varieties of tire sizes and differential ratios to allow the user to easily see the differences that different tire sizes or differential ratios had.

With all this theoretical speed data, you might be curious as to the power requirements to achieve any speed. The second post has a sheet (nominally set up for a baby Elan but modifiable for the +2) that calculated horsepower required for any speed, or alternatively, for a given horsepower, estimated the speed that could be achieved.

And finally, if you know the power curve of your engine, there is another sheet that, with some effort, can give you a reasonable idea as to the actual top speed that could be achieved with any engine and gearing combination.

There are somewhat reasonably priced measurement systems available now that measure various aspects of vehicle performance. But borrow your neighbors GPS portable navigation system. Most have a speed readout to the nearest MPH, but that should be sufficient.

All this is a little too blue sky, but curious types can now waste their time in a new way!

David
1968 36/7988

Sean,
have you fitted a spoiler and did it make any noticeable difference apart from cosmetic?
Mike