Differential Change

[donalchanner]

Hi, I have a pickup, which used to be a van, which has a reconditioned engine. It runs quickly up to 55mph but feels as if there is plenty more power available.I had an ex telephone engineers van years ago which would do 70.mph. I am thinking of changing the differential which I assume is 4.55 for a 4.22. How would this increase the top speed? How would it change the speedo reading? Might it be worth thinking of a 3.9?. I am not thinking of carrying any significant weight, the wheels are 15in.and it has disc brakes. Any advice would be welcome. Donal[frame][/frame]

[MarkyB]

See what differential you have fitted first.
The number should be stamped on top of it somewhere.

GPO vans had a restrictor plate with about a one inch hole in, make sure it isn't that.

[bmcecosse]

15" wheels ?? They look standard in that picture? A change of final drive ratio won't change the top speed - unless of course it has a 5.3 fitted at the moment. when it will indeed run in to valve bounce at lowish speed........ As above - find out what you have - then decide. A 3.7 ratio works well if the 1098 engine is upgraded. Otherwise -a 3.9 should be fine. Just don't expect it to change the top speed!

[wickermonkey]

Sorry to hijack your thread but im having the same sort of issue, I've got a 57 2 door with a 1275 marina engine, hif44 carb plus minispeed manifold, type 9 (5 speed)gearbox with what i was told was a 4.22 diff and standars rear axle, oh and 15" mini lights. I always seem to need a gear higher than i should be using and running out of revs at 55-60.

[bmcecosse]

And is the speedo reading correctly? If so - must be wrong ratio final drive - certainly not a 4.22....

[plastic_orange]

A 1098 minor will go well 'off the clock' with standard 4.22 gearing if in good order. You should still see 80+ on the speedo with a 948 and 4.55 diff.
If you have a tweaked engine, then yes, upping the gearing will allow you to pull a higher speed.

Pete

[donalchanner]

Hi again. The diff is 4.55 and the wheels are 15in diameter without the tyres and there is no restrictor. Has any one some useful advice about what increase in top speed might be achieved if the revs are the same? Or what effect the change would have on the speedo reading. Donal

[bmcecosse]

So - the wheels are not the ones shown in that picture? What size tyres are fitted? Changing the final drive ratio will not necessarily change the top speed - since with the 4.55 you should have been able to reach 80 +mph easily. Changing to a lower ratio final drive will certainly drop the revs. So going to a 3.9 say would drop the revs in top gear by approx 14 %. The top speed may stay the same if the engine has enough power, or may be slightly lower if the engine can't make the power required at the lower revs.... And obviously to keep the speedo correct - the tpm figure must be changed by the same %. I can tell you that a 1248 tpm speedo (ex Mini) is absolutely correct with 155X14 tyres and a 3.7 final drive - with a Minor 4 speed gearbox of course...

[donalchanner]

Hi, The wheels shown in the picture measure 15in in diameter. I have checked again. I dont understand why the pickup will not go above 50mph. There is no restriction on the pedal - it is not even fully depressed at 50. What else can I try? Donal

[bmcecosse]

These are 14" wheels - what size tyres are fitted? Read off the sidewalls....
As for not going above 50 - well - assuming the speedo is correct (what have you checked against?) - then it could be serious clutch slip or engine WAY down on power - or of course brakes dragging hard - are the hubs hot after a run? Are you measuring the speed in 4th gear? Does the engine seem to be revving hard? You say a reconditioned engine - but you don't say what capacity? If it is an 803 engine - well - 50 may be it's limit!!! Assuming none of these problems - check if the throttle is going full open when you press the pedal to the floor. And a compression check will tell us the general condition of the engine. Also - what colour are the spark plugs after a good run - and what needle is in the carb?? Another possibility is fuel supply restriction - so you could try pumping a pint of petrol through the pump into a container and tell us how long that takes.... Sorry - there are no easy answers !

[donalchanner]

Hi, You were right about the wheels although they measure 15in accross the tyre is 14in. Very confusing. It is because the engine is not revving high at 50 and the clutch is not slipping or any of the other things you mention and the acceleration is fine that I thought of changing from the standard 4.55 of the pickup or van to the 4.22 of the saloon. I have known another pickup which has the same performance. Donal

[bmcecosse]

Well -if the engine is not revving high at 50 - you don't need to change to a lower (4.22) final drive ! You don't say the engine capacity?

[MarkyB]

"There is no restriction on the pedal - it is not even fully depressed at 50."
The restrictor is in the throat of the carb on the plate that holds the throttle outer.
How fast does it go with the pedal to the floor?
Not sure what the performance of the low compression engines is like, anyone care to comment?

[donalchanner]

HI, I know where the restrictor would be as I took one off my first minor a telephone engineers van in 1969. The pickup gets up briskly to 50 and then feels as if I should be changing up a gear. My point about the pedal was that it is not lack of travel which is causing the problem. Since I had the same situation in a pickup previously I wondered if anyone else had found a way of increasing the top speed. I dont have a rev counter and the only way I have of judging the accuracy of the speedo is by all the other cars overtaking me. Donal

[bmcecosse]

You say the engine is not revving hard at 50 mph - and then you say you need to change up a gear at 50 mph???? We need to know the engine capacity - and you need to check the speedo accuracy. Compare with the flashing signs as you enter a villaage!. This is not a pick-up problem......it is an engine power problem!

[Bazzalucas]

He said, "FEELS as if it should change up a gear" at 50mph. I understood that to mean that his sense is that the engine has more power to offer, but that something else is responsible for the low top speed.

[bmcecosse]

Well -I change up a gear when the engine is revving high - then into the next gear to get lower revs at the same speed.... A Minor should do well over 50 mph in third gear!! The problem is likely low engine power or just possibly dragging brakes! Provided of course the final drive really IS a 4.55 at the moment - has this actually been checked?