948cc cruising speeds

[MoggyMatilda]

My car struggles to get to 50mph & seems quite high revving at that. Hardly normal that is it? I’d of thought it would do 50mph effortlessly

What diff comes in a pickup? As I can get my hands on one but maybe it’d be no different.

[nutmegct]

If you're talking about a 948 saloon, I'd think 50 mph is about 15-20 mph lower than top speed. I believe Morris rated the 948 at 75 mph.

What's the rpm when you hit 50? Today's cars are much quieter than those of 60 years ago, so the engine sound you hear may be normal for the 1960s, and you still have another 10 to 15 mph to go.

Just a thought.
Tom M.

[MoggyMatilda]

I don’t know the exact Rev’s as I’ve no rev counter, but yeah a car from that era is going to be a lot nosier than a modern car. It just feels like I’m ringing it’s neck to get to 50mph.

[liammonty]

Cruising at 60 mph should be no problem in a 948 with a standard (4.55:1) diff. Very roughly, the gearing should give around 15 mph per 1000 rpm, so at 50 you’d be around the 3500 rpm mark. It’s possible you’ve got a diff from an 803cc car fitted I suppose (5.375:1) , if the revs are higher than that, but I would think that’s unlikely.

Regarding getting to 50 mph in the first place, you’re right, the car shouldn’t struggle with that at all, so it sounds as though the engine could be down on power for some reason. Is it all serviced and set up well?

[MoggyMatilda]

Is there any handy way of finding out what diff I have other than taking it out?

There’s a larger cylinder head on it, everything else is stock (far as I’m aware) . H2 carb & stock exhaust.

I’ve been tinkering at it myself but I might just get the mechanic to give it a once over.

[nutmegct]

Could you give more details on "larger cylinder head"? Sounds like some modifications have been made.

Carburetion and ignition are correct?

Edit: this might be helpful:

https://shiftsst.com/blog/post/how-to-d ... ratio.html

[MoggyMatilda]

12g202 head
Fully reconditioned
Unleaded seats
Double springs
Skimmed 40
Compression raised 9.8 - 10.1


My dad set the ignition as he worked on mini’s back in the day, now to say how accurate it is would be another question lol

The carburettor was giving me trouble running rich all the time. I put the standard needle from a 1098 in which helped he car run nicer but doesn’t have much poke. I’m going to try a 1 1/2’’ carburettor next because I don’t think the 1 1/4” carb is up to it

[myoldjalopy]

My car has a 948cc engine and cruises comfortably at 60mph, and acceleration, whilst hardly record-breaking, is eager, smooth and constant. Any number of things could cause poor performance but, as 'liammonty' hints, a car 'out of fettle' will not perform as it should. Wrong points and plugs gaps and valve clearances will definately limit potential performance.

[ManyMinors]

As others have said, there is something wrong with your set-up if it is struggling to reach and maintain 50mph. A well serviced 948 in anything like good condition is a very driveable little car and a delight on the road. Not fast of course but should indeed cruise happily at 50 - 60mph. I find the gearing is ideally suited to the power output but a good 948 will manage with a 4.22 differential if cruising is your main aim - particularly as most of us now have radial ply tyres which alter (lower) the overall gearing slightly when compared with the original crossply tyres.
Have you had a compression test carried out? Checked the valve clearances? That would be a good start but wouldn't explain the high revs at 50mph.
The differential ratio (or at least the teeth count) is stamped into the outside of the casing. It isn't always easy to see though.
A 948 engine doesn't require a larger carburettor. Spend your time getting it up to standard first

[MoggyMatilda]

As others have said, there is something wrong with your set-up if it is struggling to reach and maintain 50mph. A well serviced 948 in anything like good condition is a very driveable little car and a delight on the road. Not fast of course but should indeed cruise happily at 50 - 60mph. I find the gearing is ideally suited to the power output but a good 948 will manage with a 4.22 differential if cruising is your main aim - particularly as most of us now have radial ply tyres which alter (lower) the overall gearing slightly when compared with the original crossply tyres.
Have you had a compression test carried out? Checked the valve clearances? That would be a good start but wouldn't explain the high revs at 50mph.
The differential ratio (or at least the teeth count) is stamped into the outside of the casing. It isn't always easy to see though.
A 948 engine doesn't require a larger carburettor. Spend your time getting it up to standard first

Yes I think you’re right, as I’ve only noticed these issues from the cylinder head was upgraded, which I thought would be a good thing! I’m tempted to put the original head back on & keep it as standard as possible.

[paul 300358]

Is there any handy way of finding out what diff I have other than taking it out?

Put the car in neutral on axle stands, mark a rear tyre and the prop shaft, rotate the rear wheel one full turn and count the turns of the prop shaft. One full turn of a wheel should result in the prop shaft rotating 4.2 times

[MoggyMatilda]

So I jacked the car up. Prop only turned twice & a quarter for one rotation of the wheel. That can’t be right!?

[nutmegct]

Read the link I put in a previous post. That'll tell you the ratio.
Tom M.

[MoggyMatilda]

Ah yes, I’ve read the link.

I turned the wheel Two full turns, the diff turned 4 & abit so I take it to be a 4.2 diff.

Must be something else odd or else I just think the car is revving a lot.

I’ve drove my sisters traveller before & it doesn’t have the same issue cruising at 55 but then it’s a 1098 & maybe had a different diff.

[ManyMinors]

The standard diff is 4.55:1 on a 948cc Minor 1000 and 4.22:1 on a standard 1098cc Minor 1000.
Of course, it is perfectly possible that the diff could have been changed at some time on your car. Or your sister's come to that

No point changing the whole cylinder head just for fun unless there is something wrong with it. Fitting the head from a 1098 car will make little real difference in practical terms but there should be no downside to it. As I said before though....have you done a compression test and checked the valve clearances? Do them both before changing the cylinder head

[nutmegct]

+1 !

[MoggyMatilda]

Haven’t done a compression test but I will. One thing I will say is the car starts incredibly easy, like first turn. which makes me rule out a compression problem. But I may be wrong.

As for the valves, should I set them at 12 thou or 15 thou?

[philthehill]

The correct clearance is 0.012" but I have always found the 'A' Series runs better with 0.015" clearances though a bit noisy.
If the hammer tip of your rocker is worn hollow you will need to compensate for the wear. The SPQR valve clearance adjuster is ideal for getting the right clearance with worn rockers.

SPQR.jpg (25.51 KiB) Viewed 2750 times

Forget the fancy prices being asked for the tool - I recently paid £17 for nearly new complete tool (pictured above) with the SPQR 1/2" UNF socket. Any 7/16" UNF 1/2" drive socket will suffice.

[MoggyMatilda]

The valves were tight, now have them at 15 thou.

What would be the consequences of tight valve clearances? The engine having to work harder?

Apologies for all the questions, amateur mechanic here so I’m learning as I go!

[Monty-4]

Thinking it through - tight valves could perhaps result in higher valve lift which would be better, but then perhaps them not closing completely after thermal expansion, in a really extreme case, but that strikes me as unlikely? Also an amateur.