26 replies to this topic

[driftz]

What is needed to do to an mg metro 1275 to allow it to happily rev lets say 7000rpm for sustained periods of time. For my gearing that would be a good engine speed however i'm hesitant to rev it much past 6000. 

[Dusky]

Balanced Bottom end , lighten what you van.
Improved valve springs and or a lighter valve train

[ACDodd]

High revving = short life.

Ac

[Northernpower]

What is needed to do to an mg metro 1275 to allow it to happily rev lets say 7000rpm for sustained periods of time. For my gearing that would be a good engine speed however i'm hesitant to rev it much past 6000. 

Just out of interest what's the fascination with 7,000 revs? Why don't you just change the gearing if you're unhappy with it?

[carbon]

What is needed to do to an mg metro 1275 to allow it to happily rev lets say 7000rpm for sustained periods of time. For my gearing that would be a good engine speed however i'm hesitant to rev it much past 6000. 

Depends on what type of driving you have in mind.

 

If you're using 1st/2nd gear and 7000rpm for long periods of time on tarmac with sticky tyres I would be more worried about the gearbox and diff holding together.

[Cooperman]

Gear the car to suit the engine, particularly the cam.

If, for example, you are running a Kent 286 which has peak power at 6400 rpm, you need to be able to rev safely to 6800 to eep it fully on the cam during competition(it is a competition cam after all).

If you are running a cam which has peak power at, for example, 5700 rpm (like an MG Metro of Kent 266 cam), you want to be at maximum nominal speed at 5700 rpm with the capabilty to rev to around 6200 rpm through the gears, so you gear it for that. In fact, with a 266 the ideal final drive ratio is either 3.2:1 or, even better, 3.44:1. A 3.4:1 will give around 16.5 mph per 1000 rpm in top, so 5700 would be just over 90 and a well-built engine with a 266 or MG Metro cam will give around 80 bhp which would give a max speed of around that figure.

My Cooper 'S' rally car has a 3.9:1 FDR and gives about 14 mph/1000 rpm in top, so 7000 rpm is just under 10 mph. But it has a 286 cam and over 100 bhp and with a proper 'S' engine (EN40B crank, forged pistons, etc) it will rev to well over 7000 without blowing up.

As 'AC...' says, high revs = high engine wear rate.

[driftz]

Thanks for the info. My car has an MG metro engine and a cooper s remote change gearbox so the gearing is quite short. It accelerates well, but for top speed it would be nice to get a few more mph before over revving.  As it stands now it tops out just a little under 100mph. I hesitate to change the final drive and loose that acceleration, maybe I just have unrealistic expectations for an old car. 

 

I am planning to pull the engine/gearbox this winter and give it a re fresh, figured while I had it apart if there was some particular mods to do to the engine to allow it to rev a bit more I would do them. 

 

With a rebuilt mg metro 1275 a+ with a stock bottom end and maybe some stiffer valve springs and 1.5 rockers, would you say 6500 is a good target for a rev limit? 

Edited by driftz, 04 July 2017 - 04:12 PM.

[Cooperman]

What is your FDR? That determines whether the overall gearing is correct for the cam being used. It sounds like you have an MG Metro cam which, as I said earlier, at 5700 rpm so a 3.44:1 FDR is about ideal. It's not about target revs, it's about gearing to suit the engine characteristics. With an MG Metro cam you don't need more than around 6000 rpm. The gearing is meant to be 'short' as it was designed that way. It gives good acceleration with acceptable cruising speed.

You don't need higher rate valve springs and you will gain very little with 1.5:1 rockers.

The first thing to do to make it go slightly better is to gas-flow the head and put the compression ratio at around 10.3:1 with good induction and exhaust systems.

The Mini is an old classic car and it won't go like a modern car. With a 3.44:1 FDR you can cruise at 70 mph all day long at just over 4000 rpm. That is the gearing which the original Cooper 'S' 1275 had as standard.

Just enjoy the retro-driving experience for what it is.

[Northernpower]

You say it currently tops out in the region of 100mph yet you want a higher rev limit but dont want to change your current final drive. I don't understand why you want to optimise the engine revs to give you a potentially higher top speed in a classic mini. That's not what they're about.

You may want to consider building an engine with great torque to give great pulling power which is more useful in today's driving conditions. Consider how much of your time will be spent using really useful acceleration out of bends and roundabouts compared to how much of your time (and enjoyment) will be spent driving over 100mph?

Of course you can have both but then you're into substantial spend, and with high revs, frequent rebuilds.

[Cooperman]

There are more questions which stem from this thread:

 

What speed do you want to cruise at? (If you want to cruise at modern car speeds, then buy a modern car).

How important is acceleration compared to high cruising speed?

How often are you prepared to re-build the engine?

Why do you want to use very high revs with a fairly mild cam - you gain nothing by so doing?

Why drive a classic car if you want to drive quickly on main roads? Classic cars are just that; classic in terms of performance i.e. slow.

[nicklouse]

change your FD.

[cal844]

Our minis(2 998s and a 1275) will cruise at 70 at approx 3500 rpm(I say approx as the 1275 and my estate don't have rev counters

[driftz]

Fair enough. I like everything about the current setup/gearing, and for most city use it is great, I was more so thinking that if the engine was happily capable of say 7 grand then I would feel less terrible about revving it out to 6000 on the long straits. It seems from everyone that a high revving a series just isn't the way to go and it would be best to change the final drive and build the engine to have a strong torque within the lower rev range. My old smiths tach bounces around like crazy when it gets up in the rpm, I should get something more modern that will give me an accurate measurement (maybe that will calm my nerves about over revving it). 

[nicklouse]

Fair enough. I like everything about the current setup/gearing, and for most city use it is great, I was more so thinking that if the engine was happily capable of say 7 grand then I would feel less terrible about revving it out to 6000 on the long straits. It seems from everyone that a high revving a series just isn't the way to go and it would be best to change the final drive and build the engine to have a strong torque within the lower rev range. My old smiths tach bounces around like crazy when it gets up in the rpm, I should get something more modern that will give me an accurate measurement (maybe that will calm my nerves about over revving it). 

but a 7k engine will be crap under 3k

 

change the final drive.

 

i have a 8.5k 100mph power unit and it would be impossible on the road and would need many rebuilds.

[Northernpower]

Fair enough. I like everything about the current setup/gearing, and for most city use it is great, I was more so thinking that if the engine was happily capable of say 7 grand then I would feel less terrible about revving it out to 6000 on the long straits. It seems from everyone that a high revving a series just isn't the way to go and it would be best to change the final drive and build the engine to have a strong torque within the lower rev range. My old smiths tach bounces around like crazy when it gets up in the rpm, I should get something more modern that will give me an accurate measurement (maybe that will calm my nerves about over revving it). 

I still don't understand, you say you're uncomfortable about revving it at 6,000 on long straights which you've said is 100mph, where are you driving for long periods at 100mph?