10 replies to this topic

[iDemonix]

Reacquired my old mini from someone that bought it from me as an MOT'd driving project ~12y ago and never moved it off their drive. It's a 998cc auto, needs a full recondition, and I'm resuming my plans of doing my first engine swap + rebuild. For me a mini needs an A series, so I'm keeping my eye out for a carby model A+ 1275 manual, I'll be rebuilding it, paying someone local for skimming/boring, megajolt kit, and eventually get it rolling roaded naturally aspirated. The aim is a fast road carb 1275, and I live right by Donington so it'd be rude not to do one or two days in it.

 

Everything sounds straightforward so far, but eventually I will want more power. I usually ride sports bikes, and whilst it'll never be on par with that, I don't want to be stuck behind someone doing 40mph on an A road and unsure whether I've got the power to pass, I want to feel a bit of being pushed in to the seat when I floor it. Numbers wise, I like to focus on bhp/ton, so if we assume the car is 700kg then 85bhp, which I think would be achievable with the above setup, gets me 120bhp per ton which is on par with a Ford Fiesta 1.6 Zetec S 2008, still fun in the classic mini, but could be more exciting...

 

This leads me obviously to forced induction. I've always wanted to turbocharge a car to experience the whooosh of the turbo coming on, and I love the noise, but I'm doing my first engine swap + rebuild, so adding a turbo on top is a lot to learn all at once, and I'm workin to a budget. It'd be ideal if I could rebuild the engine and run it N/A for a year or two, and then add on forced induction later. All the reading I've done suggests that my best option in this case would be a supercharger instead of a turbo, as I wouldn't have to change the pistons/cam for the turbo later on - I can build an N/A now and bolt a supercharger on later without having to get the engine in bits again and replace 1-2y old parts.

 

If I could get 140bhp, that puts it at ~200bhp/ton, which is on par with a 2020 Audi A4 S4 3.0 V6 Turbo, which is a bit more respectable. 

 

I'm aware of certain upgrades around oil pick up, centre strap, x pin etc, but I read a lot of conflicting info around whether or not the standard gearbox would be good for ~140bhp or more. Part of me wants straight cut gears, but I ran a Tyga Maggot exhaust on a VFR400 for a while and I can still feel my ears ringing, I'd hate to rebuild and get it going, then realise I can't stand it after a month.

 

I'd like to run N/A for a bit first, so is the supercharger later the best route (vmaxscart most likely)? Or just suck it up and save for another set of pistons + camshaft and turbo it in future?

 

 

Yet to start work on the car and it has holes in the floor and needs tons of work before I get to the engine, so I have time to learn and plan. Any advice etc welcome!

 

 

 

[lsto]

My last mini was a 1275 cooper, it had a stage 1 kit but as it was it could easily keep up with modern traffic. As long as your car is properly set up there is no reason to be worried if it can perform.
As for the forced induction side of things, I currently run a 998 turbo, it's very mild, other than a cam and slightly modified head, for the CR, the rest of the internals are standard as is the box. It happily keeps up with traffic and although it's not blistering performance it does push you a bit into your seat when it spools up, it sounds good and I never worry about overtakes. As it stands I'm only running around 7psi without any charge cooling so I have room to improve on that in the future. With regards to noise I do about 40 motorway miles every day and although my exhaust and air filter sounds good blasting round the streets it does become very tedious on the long cruises.
As for running turbo or SC they both have their quirks, the main problem you will have if you want to go from NA to forced induction will be your compression ratio, wether you go down the route of simply adding a decompression plate or new head will be something else to think about, as the lower that is, the more power you can safely run.
Things I would advise to consider is what do you want out of the car? Mine is a daily so as I said I kept it mild so it's still fun to drive in traffic and reliable. The more aggressive it is, the less fun it will be to live with on a daily basis, unless you are after just a weekend warrior.

[Steve220]

To expand on what Isto has written, there's a few things to take into consideration if you're intending to go forced induction later on.

 

- Compression Ratio (CR) - Due to the nature of the 5 port having leaner outer cylinders, it is more susceptible to knock (detonation. If you see pre-detonation, don't take their advice, there's no such thing). The way to overcome that in old school terms is lower the CR into the 8's (usually around 8.5 is a sweet spot, lower will effect off boost and higher you'd be limited to the amount of igntion timing you could have before knock).

 

- Cam choice - You don't necessarily have to chose between a NA or FI cam. Ones such as the Swiftune SW5 of Kent 266 work great for both. I ran 150bhp/150lbft on the SW5 and 14psi of boost in my MPi turbo for years!

 

- Gearbox - This often leads to heated discussion, so i will tell you what i've learnt over the 7 years of turbo mini ownership. The standard gearset and diff can be fragile at low rpm torque (usually with a supercharger), they've been known to strip teeth clean off gears. As a minimum go for a cross pin diff and something like a 3.1 final drive (FD). I ran the standard gear set at 150bhp for 2 years without an issue, but i was sympathetic in low gears. If you want to take it up a notch, then the Minispares Evo helical gearset is very strong! I'm running c190bhp / 19psi now on this set up and give the car very little mechanical sympathy.

 

- Pistons - If you're going for a rebore then consider a set of MED cast pistons with the 10cc dish, you can then do the maths for CR based on your cylinder head pocket. These are very strong pistons for cast, and will serve you well!

 

- Cylinder head - As a minimum, i would consider changing your valves. The standard size are 'ok' but they will be restrictive with boost, they could also melt on boosted applications. Upgrade to a set of stainless steel and double valve springs.

 

- Fuelling - To do a forced induction set up, you'll need a high flow pump. Most just put an MPi tank in and run the standard pump to a carb regulator in the engine bay. But you can run an external pump and put it somewhere like your rear subframe. If you go VMAX supercharger, it'll come with the carb, but if you go turbo, you'll need a modified HIF44 carb so it can pressurise the bowl and provide the correct fuel flow.

 

- SC or Turbo - **this is Steve's opinion based on fact, and what i've been witness to, over the years** - The VMAX kit is a cheap way to get power. HOWEVER. Many have faced fuelling and running issues based on the carb being so far away from the inlet valves. When you put your foot down quick, there is a significant delay getting the fuel into the cylinder. Imagine putting your foot down and the engine gulping in all that air, yet the enrichment fuel is still a way behind; you get a serious lean event and can make the car bog. The charge temperatures are also in the region of 60-90 degrees C, this will limit performance significantly, and often most VMAX kitted cars only get around 120-130bhp (rolling road dependant). Fuel also runs over the supercharger vanes, which in the PTFE lined versions, can cause wear quite quickly. They also run HOT, so you'd need to consider radiator size and potentially an extra fan. There's a reason you see so many kits go up for sale second hand on Facebook.

I've been around turbo minis a fair few years now, and can honestly say it is by far the best option. Off boost, the cars drive great and delivers good MPG when cruising. And depending on your turbo size, you get a progressive increase in power up the rev range. You also have the option to adjust the boost depending on how far you want to go - plus BOV noises!

 

This may seem like an exhaustive list, but you need to be honest with your budget. There are 3 things to consider with a mini engine:

 

- Power

- Reliability

- Cheap

 

You can only pick 2 and sacrifice the 3rd.

 

Edited by Steve220, 09 April 2025 - 05:15 PM.

[iDemonix]

Can't thank you enough for the write up Steve, that's why I'm glad forums like this one are still around!

 

Your engine bay looks incredible, I'll give the build thread a read soon. I'd be happy if I could achieve 140bhp or more when boosted, 190bhp sounds mental (and 19 psi sounds scary). Must put it around 270bhp/ton which is fast Jag/Merc/Audi territory - I assume you get through front tyres a lot faster than rear! 

 

My preference has always been turbo over super, but I was erring towards super due to thinking the car would be rough if I set it up for a future turbo and ran it N/A - whereas the setup for N/A and supercharging can be the same-ish.

 

My main worry is that I don't want to have to buy pistons/cam twice, and rebuild the gearbox/engine twice, so let's say I went with this for a rebuild:

 

• Rebore block + new pistons (+10cc)
• Get head (likely 12G940) ported + new seals/guides
• Roller rockers, either 1.3 or 1.5
• Upgraded valves + double springs
• SW5 cam
• Crank balanced
• HIF44 + megajolt
• X-Pin diff
• 3.1:1 FD
• Upgraded clutch
• Possibly could stretch to helical gears now, think I'll avoid straight cut for now

If I was able to get CR to mid 8s, would this be an ok driver naturally aspirated, as well as a strong enough base for a turbo build later on? The reason I'd binned off the turbo idea is that all the N/A builds I see are aiming for high CR so I assumed a mid 8s CR would be a bit weak to drive with for possibly a couple of years.

 

I'd love to do it all now, but I've realised the car is going to need most if not all the paint redoing, and it's one of several expensive projects (the main one of which is a full house reno!) so I have to tackle it in phases. Happy to spend money on things, but it'll just take me longer to do - hence why it'd be nice to get it painted and running with a rebuilt engine for a bit, so I can save up and collect parts ready for FI another year.

 

Thanks again!

Edited by iDemonix, 11 April 2025 - 01:00 AM.

[timmy850]

With 10cc pistons and a standard combustion chamber head you’ll have a decent NA compression ratio - somewhere around 9.5 to 10:1 . That’d be a good NA engine and get you going well

Then in time when you swap to the turbo setup - swap to a head with larger combustion chambers to suit a turbo compression ratio.

If you want 140hp and a turbo you’ll have to plan to spend lots more money and keep fixing things when you break them, no matter how well you plan ahead

[IronmanG]

To expand on what Isto has written, there's a few things to take into consideration if you're intending to go forced induction later on.

- Compression Ratio (CR) - Due to the nature of the 5 port having leaner outer cylinders, it is more susceptible to knock (detonation. If you see pre-detonation, don't take their advice, there's no such thing). The way to overcome that in old school terms is lower the CR into the 8's (usually around 8.5 is a sweet spot, lower will effect off boost and higher you'd be limited to the amount of igntion timing you could have before knock).

- Cam choice - You don't necessarily have to chose between a NA or FI cam. Ones such as the Swiftune SW5 of Kent 266 work great for both. I ran 150bhp/150lbft on the SW5 and 14psi of boost in my MPi turbo for years!

- Gearbox - This often leads to heated discussion, so i will tell you what i've learnt over the 7 years of turbo mini ownership. The standard gearset and diff can be fragile at low rpm torque (usually with a supercharger), they've been known to strip teeth clean off gears. As a minimum go for a cross pin diff and something like a 3.1 final drive (FD). I ran the standard gear set at 150bhp for 2 years without an issue, but i was sympathetic in low gears. If you want to take it up a notch, then the Minispares Evo helical gearset is very strong! I'm running c190bhp / 19psi now on this set up and give the car very little mechanical sympathy.

- Pistons - If you're going for a rebore then consider a set of MED cast pistons with the 10cc dish, you can then do the maths for CR based on your cylinder head pocket. These are very strong pistons for cast, and will serve you well!

- Cylinder head - As a minimum, i would consider changing your valves. The standard size are 'ok' but they will be restrictive with boost, they could also melt on boosted applications. Upgrade to a set of stainless steel and double valve springs.

- Fuelling - To do a forced induction set up, you'll need a high flow pump. Most just put an MPi tank in and run the standard pump to a carb regulator in the engine bay. But you can run an external pump and put it somewhere like your rear subframe. If you go VMAX supercharger, it'll come with the carb, but if you go turbo, you'll need a modified HIF44 carb so it can pressurise the bowl and provide the correct fuel flow.

- SC or Turbo - **this is Steve's opinion based on fact, and what i've been witness to, over the years** - The VMAX kit is a cheap way to get power. HOWEVER. Many have faced fuelling and running issues based on the carb being so far away from the inlet valves. When you put your foot down quick, there is a significant delay getting the fuel into the cylinder. Imagine putting your foot down and the engine gulping in all that air, yet the enrichment fuel is still a way behind; you get a serious lean event and can make the car bog. The charge temperatures are also in the region of 60-90 degrees C, this will limit performance significantly, and often most VMAX kitted cars only get around 120-130bhp (rolling road dependant). Fuel also runs over the supercharger vanes, which in the PTFE lined versions, can cause wear quite quickly. They also run HOT, so you'd need to consider radiator size and potentially an extra fan. There's a reason you see so many kits go up for sale second hand on Facebook.
I've been around turbo minis a fair few years now, and can honestly say it is by far the best option. Off boost, the cars drive great and delivers good MPG when cruising. And depending on your turbo size, you get a progressive increase in power up the rev range. You also have the option to adjust the boost depending on how far you want to go - plus BOV noises!

This may seem like an exhaustive list, but you need to be honest with your budget. There are 3 things to consider with a mini engine:

- Power
- Reliability
- Cheap

You can only pick 2 and sacrifice the 3rd.

You used to run the sw5. What cam are you running now Steve? My kit should be ready next week and I want to run the same cam for the rest of this year before engine out over Xmas

[Steve220]

 

To expand on what Isto has written, there's a few things to take into consideration if you're intending to go forced induction later on.

- Compression Ratio (CR) - Due to the nature of the 5 port having leaner outer cylinders, it is more susceptible to knock (detonation. If you see pre-detonation, don't take their advice, there's no such thing). The way to overcome that in old school terms is lower the CR into the 8's (usually around 8.5 is a sweet spot, lower will effect off boost and higher you'd be limited to the amount of igntion timing you could have before knock).

- Cam choice - You don't necessarily have to chose between a NA or FI cam. Ones such as the Swiftune SW5 of Kent 266 work great for both. I ran 150bhp/150lbft on the SW5 and 14psi of boost in my MPi turbo for years!

- Gearbox - This often leads to heated discussion, so i will tell you what i've learnt over the 7 years of turbo mini ownership. The standard gearset and diff can be fragile at low rpm torque (usually with a supercharger), they've been known to strip teeth clean off gears. As a minimum go for a cross pin diff and something like a 3.1 final drive (FD). I ran the standard gear set at 150bhp for 2 years without an issue, but i was sympathetic in low gears. If you want to take it up a notch, then the Minispares Evo helical gearset is very strong! I'm running c190bhp / 19psi now on this set up and give the car very little mechanical sympathy.

- Pistons - If you're going for a rebore then consider a set of MED cast pistons with the 10cc dish, you can then do the maths for CR based on your cylinder head pocket. These are very strong pistons for cast, and will serve you well!

- Cylinder head - As a minimum, i would consider changing your valves. The standard size are 'ok' but they will be restrictive with boost, they could also melt on boosted applications. Upgrade to a set of stainless steel and double valve springs.

- Fuelling - To do a forced induction set up, you'll need a high flow pump. Most just put an MPi tank in and run the standard pump to a carb regulator in the engine bay. But you can run an external pump and put it somewhere like your rear subframe. If you go VMAX supercharger, it'll come with the carb, but if you go turbo, you'll need a modified HIF44 carb so it can pressurise the bowl and provide the correct fuel flow.

- SC or Turbo - **this is Steve's opinion based on fact, and what i've been witness to, over the years** - The VMAX kit is a cheap way to get power. HOWEVER. Many have faced fuelling and running issues based on the carb being so far away from the inlet valves. When you put your foot down quick, there is a significant delay getting the fuel into the cylinder. Imagine putting your foot down and the engine gulping in all that air, yet the enrichment fuel is still a way behind; you get a serious lean event and can make the car bog. The charge temperatures are also in the region of 60-90 degrees C, this will limit performance significantly, and often most VMAX kitted cars only get around 120-130bhp (rolling road dependant). Fuel also runs over the supercharger vanes, which in the PTFE lined versions, can cause wear quite quickly. They also run HOT, so you'd need to consider radiator size and potentially an extra fan. There's a reason you see so many kits go up for sale second hand on Facebook.
I've been around turbo minis a fair few years now, and can honestly say it is by far the best option. Off boost, the cars drive great and delivers good MPG when cruising. And depending on your turbo size, you get a progressive increase in power up the rev range. You also have the option to adjust the boost depending on how far you want to go - plus BOV noises!

This may seem like an exhaustive list, but you need to be honest with your budget. There are 3 things to consider with a mini engine:

- Power
- Reliability
- Cheap

You can only pick 2 and sacrifice the 3rd.

You used to run the sw5. What cam are you running now Steve? My kit should be ready next week and I want to run the same cam for the rest of this year before engine out over Xmas

 

Avonbar Ph2 cam, however i've tweeked the timing for the torque to be higher up the rev range.

[IronmanG]

I'm looking to change to the PH2. It's supposed to be the best cam for road use right?which turbo are you running and when does it start coming on boost now?

[Steve220]

I'm looking to change to the PH2. It's supposed to be the best cam for road use right?which turbo are you running and when does it start coming on boost now?

Genuine reconditioned Garrett GT1752. It starts to see positive pressure around 2300rpm, but at 19psi around 3700rpm (14psi at 3300rpm)

[iDemonix]

Will look in to the PH2 as well as SW5.

 

Overall I think I'm going to aim for the bullet point list I put up before, but with some head work (or a different head?) later down the line when it's turbo time to bring the compression down.

[Steve220]

Will look in to the PH2 as well as SW5.

 

Overall I think I'm going to aim for the bullet point list I put up before, but with some head work (or a different head?) later down the line when it's turbo time to bring the compression down.

Ph2 cam is a turbo cam mate.