Lotus Elan

RPM at a true 60 MPH

PostPost by: tdskip » Mon May 10, 2021 4:07 am

Hope everyone had a good weekend. I am hoping to get some feedback on what you were 60 mile an hour RPM is to see if my tack is as optimistic as I think it may well be.

Ideally hearing from an S4 owner running standard height modern tires would be great

Thank you

(And man, this thing is a hoot)
Last edited by tdskip on Fri May 14, 2021 4:51 pm, edited 1 time in total.
SoCal
1969 Elan DHC
tdskip
Third Gear
Third Gear
 
Posts: 437
Joined: 13 Sep 2012
Location: SoCal

PostPost by: 1owner69Elan » Mon May 10, 2021 6:55 am

Can you supply the following?

Tire size
Diff ratio
'69 Elan S4 SE
Street 181 BHP
Original owner
1owner69Elan
Fourth Gear
Fourth Gear
 
Posts: 965
Joined: 16 Jun 2015
Location: Napa, California

PostPost by: Andy8421 » Mon May 10, 2021 8:04 am

For the princely sum of £7.39 plus 99p postage, you can buy a digital tach unit that just needs a pickup wire wrapped around an HT lead to work.

Just search on eBay for:

Digital Tach Hour Meter Tachometer RPM Counter For Snowmobile Motorbike Skis#

There are equivalent models available on the US eBay site, just search using the above text.

It has the advantage that when you have checked the accuracy of your tach, you can fit it to your lawnmower.
Andy8421
Fourth Gear
Fourth Gear
 
Posts: 585
Joined: 27 Mar 2011
Location: Surrey, UK

PostPost by: rgh0 » Mon May 10, 2021 1:08 pm

Yes I need that as despite my Elan Tach being reasonable accurate for 1960's electronics my lawn mower keeps stalling in high torque long grass situations and I need to monitor it better :lol:

cheers
Rohan
User avatar
rgh0
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 7995
Joined: 22 Sep 2003
Location: Melbourne, Australia

PostPost by: tdskip » Mon May 10, 2021 2:26 pm

Thank you for the responses.

Stock S4 differential so I assume stock ratio back there.

Tires are 155/80/13 on all four corners.
SoCal
1969 Elan DHC
tdskip
Third Gear
Third Gear
 
Posts: 437
Joined: 13 Sep 2012
Location: SoCal

PostPost by: Andy8421 » Mon May 10, 2021 3:44 pm

My slightly flippant reply was actually meant in good faith. Checking your tach against a speedo of unknown accuracy with tyres of unknown rolling diameter is never going to be a precise business.

To improve things, you could put a chalk mark on the tyre and measure how far (say) 10 revolutions gets you letting the car roll forwards. You could also use a GPS speed app on your phone for speed.

Depends how accurate you want to be. In a past life I did measure the rolling diameter of a number of tyres of supposedly the same size from different manufacturers and there was significant differences (up to 10%).

For the price of one of those digital tachs (I bought one myself) you can at least have some faith in your measurements.
Andy8421
Fourth Gear
Fourth Gear
 
Posts: 585
Joined: 27 Mar 2011
Location: Surrey, UK

PostPost by: tdskip » Mon May 10, 2021 6:35 pm

Just broke down and spent the required $12.99.

:D
SoCal
1969 Elan DHC
tdskip
Third Gear
Third Gear
 
Posts: 437
Joined: 13 Sep 2012
Location: SoCal

PostPost by: 1owner69Elan » Mon May 10, 2021 9:30 pm

The diff ratio on a US S4 should be 3.77.

If that is what you have then, with the tire size specified, you should be at 3340 rpm at 60 mph, assuming both your tach and speedo are accurate.

This is ignoring higher order effects such as tire inflation, tire wear, tire heat, load, ... Also, the presumed 80 profile might not be that accurate either.

If one uses the more precise diff ratio of 3.7777777... (34/9) instead of 3.770, for example, the rpms at 60 would be 3345 instead of 3340.
'69 Elan S4 SE
Street 181 BHP
Original owner
1owner69Elan
Fourth Gear
Fourth Gear
 
Posts: 965
Joined: 16 Jun 2015
Location: Napa, California

PostPost by: tdskip » Mon May 10, 2021 9:34 pm

Thank you, that’s very helpful. I expect the supplemental digital tack that I ordered today to confirm this tomorrow. It does raise the question of why my current unit is reading anywhere between 1000 to 1500 RPM high.

Appreciate the details and response.
SoCal
1969 Elan DHC
tdskip
Third Gear
Third Gear
 
Posts: 437
Joined: 13 Sep 2012
Location: SoCal

PostPost by: mbell » Mon May 10, 2021 9:52 pm

What you ignition setup? Have you gone electronic?

Two likely hoods if missing reading:
1) Electronic ignition causing more current pulse being picked up by tacho
2) Capacitors in tacho are failing due to age causing miss readings
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
mbell
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2079
Joined: 07 Jun 2013
Location: Austin, TX (UK Ex-pat)

PostPost by: tdskip » Mon May 10, 2021 10:24 pm

I have, running a Pertronix which is likely the leading suspect.
SoCal
1969 Elan DHC
tdskip
Third Gear
Third Gear
 
Posts: 437
Joined: 13 Sep 2012
Location: SoCal

PostPost by: mbell » Mon May 10, 2021 10:31 pm

Yup, try powering straight from start solenoid instead of the coil to test. If that sorts it a simple fused relay, using coil to switch it and solenoid as power source should sort it out.
'73 +2 130/5 RHD, now on the road and very slowly rolling though a "restoration"
mbell
Coveted Fifth Gear
Coveted Fifth Gear
 
Posts: 2079
Joined: 07 Jun 2013
Location: Austin, TX (UK Ex-pat)

PostPost by: tdskip » Mon May 10, 2021 10:49 pm

Very helpful – thank you for the suggestion.
SoCal
1969 Elan DHC
tdskip
Third Gear
Third Gear
 
Posts: 437
Joined: 13 Sep 2012
Location: SoCal

PostPost by: m750rider » Fri May 14, 2021 12:19 am

1969 S4 SE with 155HR13 XAS tires, standard diff

60 mph 3300 rpm =/- no markers between 3 and 4 so estimating.

At 3000 rpm, speed is 53 mph

Looking at the two gauges while driving in 4th gear, speedometer and tach, the needles should be in line with each other on their respective gauge - same position on each gauge.

That's the simple way to tell if your tach is accurate.
m750rider
Second Gear
Second Gear
 
Posts: 149
Joined: 10 Jan 2010
Location: Asheville area, NC

PostPost by: lotusfan » Fri May 14, 2021 7:22 am

I don’t mean to be picky Mr tdskip but could you correct the thread title to read 60 MPH please. My OCD can’t stand it anymore!!
Mike
72 Sprint DHC
User avatar
lotusfan
Third Gear
Third Gear
 
Posts: 344
Joined: 15 Sep 2003
Location: S Bucks, UK
Next

Total Online:

Users browsing this forum: No registered users and 7 guests