Query about gearing...

[Ian Williams]

There is something seriously amiss with a lightweight special that is not capable of more then 45mph I don't believe the gearing is going to be it. It is very hard to answer the questions without seeing the car in the flesh and knowing what has been done to it, which Duncan appears not to know either, first stop would be ignition timing and mixture. You mentioned the exhaust system what is on the car, is it seriously restrictive which if so may be the problem? The manifold is, as on all sevens, dreadful but that should not stop it from achieving a far better top speed than quoted.

[Spex]

Just a thought.
Is it possible that your rear brakes are dragging when you sit in it?
Peter

[Chris KC]

Long time since I've checked the numbers but I recall possibly incorrectly that the rolling radius of a typical 17" tyre is not a lot different from a 3.50 x 19.

[Malcolm Parker]

For a lightweight road going special a 5.25 axle ratio with 19 x 3.50 tyres is a well tried and tested gearing.  My Ulsteroid has that ratio and has pulled almost 5000rpm  in top, that with a full windscreen.  The Pytchley saloon is similarly geared and although lacking a rev counter will cruise happily at 50mph.

[andrew34ruby]

There is something seriously amiss with a lightweight special that is not capable of more then 45mph I don't believe the gearing is going to be it. It is very hard to answer the questions without seeing the car in the flesh and knowing what has been done to it, which Duncan appears not to know either, first stop would be ignition timing and mixture. You mentioned the exhaust system what is on the car, is it seriously restrictive which if so may be the problem? The manifold is, as on all sevens, dreadful but that should not stop it from achieving a far better top speed than quoted.

Leightweight special? The weight makes no different at all to the eventual top speed on a level road.
Top speed depends on the engine power at the revs concerned, and the wind resistance.

Remember the 45mph is an accurate figure, not a speedo reading which would likely be 50mph.

The mk2 Ruby was given in reports from 1936 as a stopwatch top speed of 53. A saloon will be less air resistance due to a much smoother shape so in perfect condition a special might be a top speed of 49. The 45 is less, but not a lot less.

The exhaust on the special is a standard front pipe, but with a kink at one point, then a small silencer of unknown construction, and a tailpipe about a foot long.

What top speed would you expect Ian?

[Bob Culver]

The brakes on when laden idea interesting. Should be easy to check. Is the silencer back to front and does it matter? If it is not new, has a baffle come loose? Does it have a standard cam? Is there any scope for it being timed wrong? Does the carb have a buttrfly choke or a separate starter circuit as many Solex?

[Colin Reed]

HI Duncan,
I think it would be a good idea to temporarily fit a rev counter and you will know exactly what revs you are getting in each gear.
rev its nuts off not a good way to know.

Colin
NZ

[Renaud]

Duncan, I agree with Colin that yoy'll need revs to understand what's what.

[Chris KC]

Just re-read the original post. Could it possibly be that the engine is simply not run in yet?
At 450 miles I would be pottering down quiet lanes on a mere whiff of throttle.

[Howard Wright]

Hi Duncan

I’ve been meaning to try a couple of apps on my iPhone but haven’t got round to it yet.  One uses the engine noise to record revs, the other the camera to “strobe” onto a marker.  Probably neither will work as you are going along but at least you can get a handle on the engine noise at certain revs AND depending on what rev counter you end up with provide a calibration. My Cheap Indian rev counter is obviously way out so I hope to use the phone app to calibrate and fit a new dial.

Cheers

Howard

PS Bill Williams book provides some great tables in the back which correlate engine revs to speed for various CW/p ratios and tyre/wheel sizes.