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RPM at a true 60 MPH

PostPosted: Mon May 10, 2021 4:07 am
by tdskip
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)

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 6:55 am
by 1owner69Elan
Can you supply the following?

Tire size
Diff ratio

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 8:04 am
by Andy8421
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.

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 1:08 pm
by rgh0
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

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 2:26 pm
by tdskip
Thank you for the responses.

Stock S4 differential so I assume stock ratio back there.

Tires are 155/80/13 on all four corners.

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 3:44 pm
by Andy8421
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.

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 6:35 pm
by tdskip
Just broke down and spent the required $12.99.

:D

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 9:30 pm
by 1owner69Elan
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.

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 9:34 pm
by tdskip
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.

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 9:52 pm
by mbell
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

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 10:24 pm
by tdskip
I have, running a Pertronix which is likely the leading suspect.

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 10:31 pm
by mbell
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.

Re: RPM at a true 60 RPM

PostPosted: Mon May 10, 2021 10:49 pm
by tdskip
Very helpful – thank you for the suggestion.

Re: RPM at a true 60 RPM

PostPosted: Fri May 14, 2021 12:19 am
by m750rider
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.

Re: RPM at a true 60 RPM

PostPosted: Fri May 14, 2021 7:22 am
by lotusfan
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!!