31 replies to this topic

[IronmanG]

Hi everyone I’m currently building my 1293 the planned spec is as follows.
1293 bore
AC Dodd rs road cam
MED duplex timing gear
Ported polished big valve mg head 35.6mm by 29mm might have more done to it havent looked in detail yet.
Single 44 carb
Lightened flywheel or weight taken out of the clutch/flywheel
Maniflow exhaust system
Aldon yellow dizzy or recurved to suit spec
Balancing of engine and clutch
I plan on fitting larger rockers but this may have to wait and be something I add in the future
The car has 13” wheels which I understand affects acceleration
Aiming for around 80-90bhp


Now I’m not sure what to do with the gearbox i plan on getting an oil pickup for it and stronger diff pin as well as having it taken apart and all worn items replaced by my local machine shop. Now I’m unsure what to do with the gear ratio/final drive I’m unsure what is in the car at the moment will find that out tomorrow when I visit the machine shop but I can only assume it’s a 3.44. So I don’t know what to do I’m thinking about changing it to a 3.1 so that I can have a higher top speed I’m aware the acceleration will suffer but I’m sure it won’t be slow off the line with a lightened flywheel and the rest of the spec. I’ve looked on guesswork’s and seen that with the 3.1 I will have a higher top speed and be able to cruise at higher speeds with a lower rpm which I like the sound of as there’s nothing worse than being at 70 with the engine screaming at 4K like my 998 does now I’d much rather be able to go 70 at 3.5k and be able to pull right up to 90. I saw another post on here with someone asking a similair question but he was putting a 276 cam in his and most reccomended not to as he was wouldnt be on the cam most of the time so I wondered what a 3.1 would mean for my spec any disadvantages or advantages. I’ve also never driven a 1275 with a 3.44 so what I’m saying could be completely wrong which is why I’m seeking advice as I’m out of depth here. I’m also open to other suggestions other than a 3.44 and 3.1 but won’t be able to buy anything too expensive as I’m only 19 and this is my first fairly quick engine build thank you for your time and I look forward to your responses.

My engine spec is similar and I am using the RS+ cam.
AC recommended 3.2 to me for that cam
I currently have 3.4 and if the box ever needs splitting I will probably go 3.1

[Luke Denver]

Road use on 10's or 12's - 3.44 all day, every day.

It’s on 13s mate still the 3.44?

[JXC Mini GT]

My 1293 GT has 12" wheels with a 3.44:1 final drive, 60 MPH is just over 3000 RPM

[Spider]

 

Road use on 10's or 12's - 3.44 all day, every day.

It’s on 13s mate still the 3.44?

 

 

Sorry my initial answer here was so brief.

Choosing a final drive ratio isn't really straight forward. Camshaft choice dictates the Gearset that is best used with that Cam and then you need a Final Drive Ratio that's then the compromise between getting the car moving and the highest cruising speed you want, while keeping in sight suburban speeds.

There's often I notice a fixation only with highway cruising speed and this often leads to a final drive ratio that leaves the car somewhat lifeless getting off the line and forever changing gears in suburban streets.

I've mentioned this elsewhere before, starting from the camshaft choice, you need to work out the gear ratios as best you can such that as a lower gear is just getting past it's peak power in the engines rev range and you then shift up, it puts you on the peak torque in the engine's rev range, though usually 1st gear is chosen a bit shorter. You can see here that the wilder the cam, the narrower the power band which then means the closer the ratios needed. This then also makes 1st gear taller. Combine that with a tall final drive and then 1st gear isn't too far off where 2nd gear was in a standard car, rather tall to be getting away from a standing start.

You'll also want to take in to account max cruising speed. I only did some calculations for Mokes the other week, these needing between 26 and 32 HP (there's a fair variation here on these cars) at the wheels to cruise at 60 mph. Keep in mind, this is part throttle steady-ish state cursing, not wide open throttle. You really need to look then at the engines power curve and see where you'll be making the power needed for stead state cruising speeds (taking in to account geartrain losses). This state ideally the engine needs to be running around 18 - 22" of manifold vacuum, any lower than that and you are burning more fuel than you should and also labouring the engine, leading to premature wear.

Or,,,,,

You can just try a few gear sets and final drive ratios to see what you like and what works for you.

[Cooperman]

My 1293 GT has 12" wheels with a 3.44:1 final drive, 60 MPH is just over 3000 RPM

The 3.44:1 gives 16.5 mph/1000 rpm in 4th gear on 10" and 12" wheels, so 3000 rpm is 50 mph.

 

Actually the wheel size makes very little difference due to the lower aspect ratios of the 12" and 13" tyres. It is ever only a low percentage, like about 4% maximum difference.

 

With regards to diff ratio, my preference is the 3.44:1.That is a good all-round compromise.

[ac427]

My 1293 GT has 12" wheels with a 3.44:1 final drive, 60 MPH is just over 3000 RPM

The 3.44:1 gives 16.5 mph/1000 rpm in 4th gear on 10" and 12" wheels, so 3000 rpm is 50 mph.
 
Actually the wheel size makes very little difference due to the lower aspect ratios of the 12" and 13" tyres. It is ever only a low percentage, like about 4% maximum difference.
 
With regards to diff ratio, my preference is the 3.44:1.That is a good all-round compromise.

I there a table showing the different final drive ratios and their speed in mph per 1000revs?

[nicklouse]

 

 

My 1293 GT has 12" wheels with a 3.44:1 final drive, 60 MPH is just over 3000 RPM

The 3.44:1 gives 16.5 mph/1000 rpm in 4th gear on 10" and 12" wheels, so 3000 rpm is 50 mph.
 
Actually the wheel size makes very little difference due to the lower aspect ratios of the 12" and 13" tyres. It is ever only a low percentage, like about 4% maximum difference.
 
With regards to diff ratio, my preference is the 3.44:1.That is a good all-round compromise.

I there a table showing the different final drive ratios and their speed in mph per 1000revs?

 

You can plug your info on the guesswork page. 

[alex-95]

 

 

My 1293 GT has 12" wheels with a 3.44:1 final drive, 60 MPH is just over 3000 RPM

The 3.44:1 gives 16.5 mph/1000 rpm in 4th gear on 10" and 12" wheels, so 3000 rpm is 50 mph.
 
Actually the wheel size makes very little difference due to the lower aspect ratios of the 12" and 13" tyres. It is ever only a low percentage, like about 4% maximum difference.
 
With regards to diff ratio, my preference is the 3.44:1.That is a good all-round compromise.

I there a table showing the different final drive ratios and their speed in mph per 1000revs?

 

Pretty much this guessworks calculator. can change the wheels, diff, drop gears etc.

 

I've got a 3.2 diff in my mini, 1293 similar spec but with 10's. goes very well off the line and reduces the revs on motorway journeys. I was used to 998's with 2.9 diffs though 

[Gaz66]

Get a 4.1 on 10's but stay round town. Lol

[JXC Mini GT]

My 1293 GT has 12" wheels with a 3.44:1 final drive, 60 MPH is just over 3000 RPM

Apologies for my previous post, when I checked the Mini today on a run 60MPH is 3500 RPM which ties in with the Guessworks calculator, which is 17MPH per 1000 RPM,

[Cooperman]

Get a 4.1 on 10's but stay round town. Lol

 

I fitted a 4.1:1 in a 1071 Cooper 'S' rally car with a 649 cam, etc, and it went fantastically with up to 7500 rpm available.

My son had a 970 'S' rally car and that also had a 4.1 but with a 286 cam. That would rev to almost 8000 rpm, but not in top gear obviously.

[ac427]

Here is a table of the MPH per 1000 rev in top gear for 165/70/10 tyres

Attached Files

•  ScreenShot009.jpg   35.28K   7 downloads

Edited by ac427, 25 April 2022 - 02:47 PM.

[Luke Denver]

Road use on 10's or 12's - 3.44 all day, every day.

It’s on 13s mate still the 3.44?

Sorry my initial answer here was so brief.

Choosing a final drive ratio isn't really straight forward. Camshaft choice dictates the Gearset that is best used with that Cam and then you need a Final Drive Ratio that's then the compromise between getting the car moving and the highest cruising speed you want, while keeping in sight suburban speeds.

There's often I notice a fixation only with highway cruising speed and this often leads to a final drive ratio that leaves the car somewhat lifeless getting off the line and forever changing gears in suburban streets.

I've mentioned this elsewhere before, starting from the camshaft choice, you need to work out the gear ratios as best you can such that as a lower gear is just getting past it's peak power in the engines rev range and you then shift up, it puts you on the peak torque in the engine's rev range, though usually 1st gear is chosen a bit shorter. You can see here that the wilder the cam, the narrower the power band which then means the closer the ratios needed. This then also makes 1st gear taller. Combine that with a tall final drive and then 1st gear isn't too far off where 2nd gear was in a standard car, rather tall to be getting away from a standing start.

You'll also want to take in to account max cruising speed. I only did some calculations for Mokes the other week, these needing between 26 and 32 HP (there's a fair variation here on these cars) at the wheels to cruise at 60 mph. Keep in mind, this is part throttle steady-ish state cursing, not wide open throttle. You really need to look then at the engines power curve and see where you'll be making the power needed for stead state cruising speeds (taking in to account geartrain losses). This state ideally the engine needs to be running around 18 - 22" of manifold vacuum, any lower than that and you are burning more fuel than you should and also labouring the engine, leading to premature wear.

Or,,,,,

You can just try a few gear sets and final drive ratios to see what you like and what works for you.

I’m finding it pretty difficult to select what final drive to go with if I’m honest. My current engine and the only mini engine I’ve ever driven is a 998 not sure on its ratio but it tops out around 70-80 and to get it to that it needs to be at around 4100 revs 5k being the redline and I’ve never liked it being like that just doesn’t feel right so I’m building a 1293 so far it has an ac Dodd rs road cam duplex timing chain gearbox wise I’ve asked for a cross pin diff, central pickup, and for the final drive to be changed to a 3.1 so that I don’t encounter the same problem of topping out at 70 and having the engine screaming for its life. Reading some of the responses I’m now not so sure on my choice you seem pretty clued up would you be able to work out what sort of speed revs and all that I’d be getting from the 3.1 and from the 3.4 I’m aiming to have around 90bhp the head will be stage 3 and along with the changes I’ve listed a decent exhaust decent carb so I’m sure ill achieve this the car is on 13x7 wheels if you could help me in any way I’d appreciate it let me know if you need any more info thank you

[nicklouse]

Luke get to a Mercia Minis meeting and get taken out in some cars.

 

With that cam I would go 3.44 final drive some great driving roads in the area.

And learn to let the engine rev.

[T.Harper]

I would recommend having a play with the Guessworks calculator to see the difference.  At road speeds you are talking at +/- 7mph between your FD ratios, which is not really a big deal if it's a weekend warrior.

 

http://www.guess-wor.../Tech/ratio.htm

 

I personally have a 3.1 but wouldn't really be fussed if it were a 3.44 as the car that I built as an everyday is now a weekend warrior (as it took too long and life moved on!).