Lotus Elan

How many rpm at 60 mph

PostPost by: Lotus 50 » Fri Jan 01, 2021 2:03 am

I think my tach reads too high. How many rpm is 60? 70?
Thanks
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PostPost by: 1owner69Elan » Fri Jan 01, 2021 2:39 am

Need to know what differential ratio, tire size. Assuming 4th gear (1.0).
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PostPost by: mbell » Fri Jan 01, 2021 2:50 am

I ended up buying:
https://www.amazon.com/Maintenance-Tach ... 07PMJH7QV/

For a reference while adjusting my spyidia converted tacho. Simply wrap the lead around the coil wire and you have a reference tacho.

The tacho tend to be of by different amounts at different rev so this can let you map it out across the range.
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PostPost by: stugilmour » Fri Jan 01, 2021 4:00 am

Assuming

Stock 165/80R13 tires
Fourth gear 1:1
Final drive 3.78

Approximately 3,200 RPM at 60 mph

With the optional 3.54 Final Drive

Approximately 3,000 RPM at 60 mph

This tool is a very easy spreadsheet to get engine speed in the other gears, change tire size, etc. I just picked the engine RPM off the graph, so plus minus a bit.

http://westfield-world.com/gearboxshifts.html

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PostPost by: h20hamelan » Fri Jan 01, 2021 4:17 am

mbell wrote:I ended up buying:
https://www.amazon.com/Maintenance-Tach ... 07PMJH7QV/

For a reference while adjusting my spyidia converted tacho. Simply wrap the lead around the coil wire and you have a reference tacho.

The tacho tend to be of by different amounts at different rev so this can let you map it out across the range.


Has the battery runout (died) yet? Does it log data? Or are you simply useing it for a specified time and gathering results?
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PostPost by: Andy8421 » Fri Jan 01, 2021 8:21 am

mbell wrote:I ended up buying:
https://www.amazon.com/Maintenance-Tach ... 07PMJH7QV/

For a reference while adjusting my spyidia converted tacho. Simply wrap the lead around the coil wire and you have a reference tacho.

The tacho tend to be of by different amounts at different rev so this can let you map it out across the range.


Thanks for this. I have just ordered one from Amazon in the UK. I have an old Fluke digital frequency meter that I have used in the past, but it is mains powered and a major pain to use balanced on the engine. For those who would like one of their own:

https://www.amazon.co.uk/gp/product/B07XYLXXQP/ref=ppx_yo_dt_b_asin_title_o00_s00?ie=UTF8&psc=1
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PostPost by: Andy8421 » Fri Jan 01, 2021 8:47 am

Lotus 50 wrote:I think my tach reads too high. How many rpm is 60? 70?
Thanks

The Smiths tachometers are a 'best guess' device, and the guess gets less accurate as time goes by and the components begin to fail. The same is true of the speedo, so unless you are using a GPS app for speed, the chances are you are comparing two wrong numbers.

As pointed out above, even a properly calibrated Smiths tach is only correct at the calibration point, it will not be accurate for the entire rev range. To add to the fun, the moving parts in the tach have mass, and cornering forces will move the needle around by a few hundred RPM, depending on where the needle is pointing to start with and how hard you corner.

Modern tachs have given up on all this analogue needle waving business and use a micro controller with a stepper motor drive to the needle, which gets the display spot on. You can buy a period looking stepper tach if accuracy matters (in racing for example) but they don't match the original Smiths tach found in the Elan.

If you want to have a go at calibrating or rebuilding the tach, it is very straight forward with some very basic skills. There are two different types of Smiths tach, but the many 'how to' guides on the web explain this and how to go about working on them. I came across this guy on Youtube who does a good job of explaining how the tach works and how to repair / calibrate. There are less 'techy' versions available, but I liked this guy's approach, and he seemed to be using a proper camera setup for the video which made it very clear.

https://www.youtube.com/watch?v=w5auLGtzTqY

Good luck.
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PostPost by: gjz30075 » Fri Jan 01, 2021 9:52 am

Lotus 50 wrote:I think my tach reads too high. How many rpm is 60? 70?
Thanks


How certain are you that the speedometer is correct?
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PostPost by: mbell » Fri Jan 01, 2021 1:54 pm

h20hamelan wrote:Has the battery runout (died) yet?


Nope, think it's been in the car 6-9 months. Battery should last a few years and can be replaced in the one I got.

h20hamelan wrote:Does it log data?


No. It's just displays engine rpm and tracks engine hours.
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PostPost by: Lotus 50 » Fri Jan 01, 2021 6:04 pm

I assume 3.77 gears. If I believe the tach it would be 4 something. The speedometer is perfect at 62 mph per cell phone GPS. 186/70-13 Sprint Classic tires. Perhaps wider then they need to be but they look skinny by today’s standards

Thanks
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PostPost by: 1owner69Elan » Fri Jan 01, 2021 9:35 pm

Lotus 50 wrote:I assume 3.77 gears. If I believe the tach it would be 4 something. The speedometer is perfect at 62 mph per cell phone GPS. 186/70-13 Sprint Classic tires. Perhaps wider then they need to be but they look skinny by today’s standards

Thanks


I assume you meant 185/70 not 186/70 (typo)

With your specifications, looks like ~3200 at 60mph and ~3750 @ 70mph:

Revs at 60 and 70.jpg and
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PostPost by: Lotus 50 » Fri Jan 01, 2021 10:46 pm

Thank you
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PostPost by: NedK » Thu Jan 21, 2021 4:58 pm

Here's another YouTube video of a tacho conversion.
This guy has been working on his Europa for quite a while, and seems to be doing an excellent job.

[url]https://youtu.be/oX-3TUubU_Q
[/url]
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