17 replies to this topic

[kkonstantinopoulos]

Hi all,

 

I know this topic has been covered somewhat before, but I'm hoping to get some opinions on my specific setup. Apologies in advance for the long post.

 

I'm currently deciding between two engine specs for my 1970 Mk3 850 and would really appreciate any advice.

 

A bit of context: I inherited the car after restoring it with my dad as a kid (I actually learned to drive in it), so I'm pretty attached to keeping it an original 850. I don't want to swap in a bigger engine or build something crazy fast, just want it to feel a bit more alive and usable while keeping that same revvy, engaging character.

 

The current 850 engine (original from the factory) is already quite fun. I love the revvy nature and straight-cut 3-synchro noise. It's great on twisty roads (a bit sketchy on drums, planning Cooper discs), cruises at ~60 mph all day, but really struggles on hills, especially when loaded. Found that out on a road trip with my three ~80 kg friends.

 

I was given a spare late 850cc engine (around 1978) and some +0.060 998 flat-top pistons and rods, which I'm planning to build into a separate subframe assembly so I can swap it straight in and keep the original matching engine and subframe untouched for a later restoration.

 

I'm also a university student on a bit of a budget, and the main goal here is to learn engine building, so I've been doing as much as possible myself (I do get some odd looks when people come over and see a gearbox in pieces on the dinner table). I've done all the head work on the university's milling machine (big thanks to AC Dodd's YouTube) aside from the valve seats which I outsourced since I didn't have the right tooling.

 

Here are the two options I've converged on:

 

Option 1: STD 850 with supercharger

• 12G202 head, 3 angle unleaded valve seats, 12G295 sized inlet and exhaust valves, ported ports and chambers per Vizard

• 997 Cooper cam (12G229)

• Lightened flywheel

• Std split skirt pistons (will measure to confirm usability)

• Std crank, std rods with uprated pinch bolt

• Suck-through VW 1.4 TSI supercharger with custom manifold, ~6 psi boost

• 4 synchro pre A+ gearbox, std helical ratios with 3.44 diff

• 8.5:1 compression

• RPM limit ~5500–6000

• Programmable distributor

• ARP hardware (rods, mains, head)

Pros:

• No need to prep the crank, rods, or source/machine an overbore, keeps costs down

• Wider, flatter torque band suits daily driving and hill work well

• Lower final drive (3.44) means more relaxed motorway cruising

• No need to buy the SCCR kit

Cons:

• VW supercharger won't look period-correct under the bonnet (causes problems with FIVA card?) not a Shorrock

• Likely to be quieter and less engaging than twin HS2s through ram pipes with a straight-cut box

• Suck-through, non-intercooled setup means high inlet charge temps, which could be a problem in hot summers

• Stock split skirt pistons may not hold up well under boost long-term?

 

Option 2: Overbored 850-based build

• Twin HS2s

• Same 12G202 head, 3 angle unleaded valve seats, 12G295 inlet and exhaust valves, ported ports and chambers

• 997 Cooper cam (12G229)

• Lightened flywheel

• +0.060 998 flat-top pistons, lightened later rods

• 998 block with 850 crank, zero decked ~0.131 in

• Std crank modified to AEG515 spec (tuftrided, lightened, balanced, cross-drilled as required, custom thicker thrust washers to fit block)

• 4 synchro pre A+ gearbox with Clubman SCCR and 3.76 diff

• 9.5:1 compression

• RPM limit ~6500–7000

• ARP hardware + centre strap

Pros:

• Period-correct look and feel

• More engaging to drive, especially with the SCCR and twin carbs, gives a more responsive, revvy character

• 998 block is a stronger foundation than the 850 block, 3 cam bearings too

• Stronger rods/pistons

Cons:

• Crank reliability is a real question. I've heard they can let go, though Vizard suggests a properly prepped AEG515 should be fine at the RPM range I'm after (6500–7000 max, not trying to build a screamer).

• Higher final drive (3.76 with SCCR), higher rpm cruise

• Cost adds up: sourcing a 998 block, overbore (~€400 if outsourced), straight-cut gears, crank modifications etc.

• If I try the overbore myself at uni there's real risk of getting it wrong

I've considered overboring the original 850 block for the aforementioned 998 pistons (I know it's been done, see the attachment) but I'm afraid to risk it, so a 998 block feels like the safer foundation. I did also briefly consider a small turbo instead of a supercharger as it would probably suit the setup better, but I'd rather keep things simple and easy to swap; adding a return line, rising rate FPR, HIF44 setup, etc. would add downtime I'd rather avoid.

 

Mainly I'm trying to figure out which makes more sense for a daily that's a bit more lively without losing its character. I'm slightly biased toward the overbore since I enjoy revving the car out as-is, but the pros/cons above show it's not a clear-cut choice.

 

Any advice or experience would be really appreciated. I've attached some documents that led me to these options.

Thanks in advance, and again sorry for the long post.

 

 

 

Attached Files

•  850bigoverbore.pdf   1.12MB   5 downloads
•  downtown.pdf   1.12MB   2 downloads

[timmy850]

Those sound like some fun ways to spice up an 850, but I think in reality without a decent budget and a definite need to stay with the 850 crank there’s some potentially better options, especially for a daily driver.

If you were sourcing a 998 block and pistons, then to me it makes a lot of sense to just get a complete engine (which can normally be found pretty cheap) and make a 998 based engine. They are nice and revvy too, and obviously starting with over 15% more cubic capacity it’s a good head start. A 50hp 998 would probably be a lot nicer to drive on the road than a 50hp 850 engine

The 1098 engine is another great option, although perhaps a bit less revvy than a 998, but better low down grunt which is good for scaling hills with a full car.

Externally an 850/990/1098 are nearly identical so they will all look period correct

I never had the 850 engine in my Mk1 to compare to, but the stage 1 998 was a lovely revvy engine that was otherwise stock, and it was super reliable and a lot of fun. My current modified 1098 makes a lot more grunt & revs higher but doesn’t have the same zippy feeling and due to the high compression (I over did the engine build a bit) it’s a bit more high strung and difficult to get right

[gaspen]

Off-topic but I am curious for the interior of that car 

[kkonstantinopoulos]

Thanks timmy850 for the reply, that makes a lot of sense.

 

I’m a bit limited by my setup as I don’t have a workshop, so everything is being done out of my apartment, with parts being taken to university for machining. Because of that, sourcing and handling a complete engine isn’t really practical at the moment.

 

The spare 850 I have is already stripped (I originally took it apart at a friend’s place, which I can’t use anymore), so that’s what I’m working with. The idea behind a 998 block would just be to have a better and safer base for machining the overbore, and it’s something I can realistically transport in the boot of the Mini and carry upstairs.

 

As mentioned, a complete 998/1098 engine would likely be the best route, but given the constraints and parts I have, I’m somewhat limited to building from what I’ve got.

 

Budget is also a factor, but I’ve saved up from work and I’m building this engine slowly, so spreading parts purchases over time should be manageable. I’m aware this won’t give the best cost-to-power ratio, but I’m hoping many of the parts can be reused later when I have the space and facilities for a more 'sensible' build.

 

I have also thought about a stroker-style setup using a 1098 crank with 948 Sprite pistons to gain some extra torque, which might suit the car better for hills and daily use.

 

Given my constraints, these are the two options that seem most feasible, so I’m trying to decide which would work best for a reliable daily. Any advice, potential issues, or things to watch out for with these setups would be really appreciated.

[kkonstantinopoulos]

Off-topic but I am curious for the interior of that car 

 

I don’t have any good photos at the moment, and it’s currently away getting undercoated for rust protection, so I can’t take any right now. I should have it back later this week though, feel free to PM me and I can take a few for you.

[timmy850]

You wouldn’t necessarily need to get a complete engine, really just the 998 block, crank and rods. You’d replace the rest of the parts during a rebuild anyway

If you want an engine that’ll be reliable and relatively cost effective then the closer to standard the better. When you start trying to push a lot of power out of a tiny engine, or have to make custom parts or get expensive machining work then it obviously gets more complicated and expensive and more potential for problems

Adding the 12G202 head (and getting the compression ratio right), some twin HS2 or a single HS4 and a mild cam to your existing 850 engine is another option and it would pep it up quite a lot. The basic 850 twin carb tune up kits in the 60’s could cut 9 seconds off the 0-60mph time

[kkonstantinopoulos]

Adding the 12G202 head (and getting the compression ratio right), some twin HS2 or a single HS4 and a mild cam to your existing 850 engine is another option and it would pep it up quite a lot. The basic 850 twin carb tune up kits in the 60’s could cut 9 seconds off the 0-60mph time

 

This could work quite well and probably achieve what I’m after, especially with the parts I already have. I could build the spare engine to that spec and swap it in, giving the current 850 and 3-synchro box a break, particularly as it hasn’t been rebuilt and the gearbox is a bit crunchy. I found this article online with a similar spec:

"The tuned Morris Mini 850 modified by V.W. Derrington Ltd of 159-161 London Road, Kingston-upon-Thames, Surrey. The car tested, 754 PBB, was a 1960 car owned by Vic Derrington, the firm’s boss. The cylinder head had been gas-flowed while retaining the standard valve sizes, and the compression ratio raised to 9.3:1. The camshaft was the hotter 88G229 as used on the 997cc Mini Cooper, and a performance exhaust system was used. Carburation was provided by a twin choke Weber. The performance parts added around £52, but gained the buyer a 0-60 time of 14s and a 90mph top speed – yet retained a flexible engine."

 

Would a SCCR kit suit this type of build? I’m considering the Minispares C-STN76EVO gear kit while keeping the 3.76 final drive. At the moment, especially on hills, the gear spacing feels too wide. I end up downshifting and revving it to ~5000 rpm, but as soon as I shift up it bogs down and I have to drop a gear again. Would the closer ratios help keep it in the power band and compensate for the low torque, or would that be less suited to a daily? I know 1st is quite long, but I’m having a hard time judging how that would feel in normal 'around town' driving.

 

Edited by kkonstantinopoulos, 06 May 2026 - 05:08 PM.

[timmy850]

I think there will always be a compromise with a small engine that’s limited to approx 6000rpm. You don’t have the low down grunt to pull a higher gear, and don’t have the RPM to leave it in a lower gear.

If you use a close ratio gearset it’ll make 1/2/3 gears taller to make them closer to 4th, which is OK if you can get off the line with the taller 1st gear. If you use a lighter flywheel this could add to the problem

You’ll also need to consider the power band of the engine. Ideally if you’re racing through the gears you’ll want to change at redline and be above the peak torque rpm in the next gear.

Straight cut gears are also obviously very loud in 1/2/3 gears. You can cut close ratio gears that aren’t straight cut, and they’d be a lot nicer for a road car

There are some gear comparison websites you can use
https://www.classicm...chnical/gearing
https://www.guess-wo.../Tech/ratio.htm

[kkonstantinopoulos]

I’ve used those sites before, it’s just hard to translate the numbers into how the car will actually feel on the road.

 

I can somewhat compare it to the current gearbox, which is the 3-synchro remote straight-cut type with a 3.76 FD. The ratios are fairly similar (if not identical) to the 4-synchro pre A+ box I currently have apart, and to be fair 1st gear is already basically just for getting moving.

 

At 2000 rpm the SCCR setup only seems to increase road speed by around 5 km/h compared to the current gearbox, but again it’s difficult to picture how noticeable that would actually feel in the car. The cam’s power band is roughly 1200–6000 rpm.

 

Regarding the straight-cut noise, I actually really like it, it’s probably one of my favourite things about the Mini. I can always shift into 4th to quiet things down a bit when cruising anyway.

 

I am quite tempted by the supercharged build in the end. The Shorrock setups seemed fairly popular on 850s back in the day, and using a more modern Eaton-style charger should improve efficiency and reliability quite a bit (not to mention avoiding the oil-burning total-loss lubrication system). That way I could probably keep the standard ratios, maybe fit a 3.44 diff, and hopefully use the extra low-down torque to solve a lot of the drivability issues.

 

Packaging the supercharger also shouldn’t be too difficult, as I can fabricate a manifold fairly easily using the university’s metal 3D printing facilities along with some flow-driven generative design software for the CAD side of things. In theory this should outperform a manifold made from more conventional fabricated tubing and welded construction. It's the stock pistons I'm worried more about...

Edited by kkonstantinopoulos, 07 May 2026 - 09:17 AM.

[Cooperman]

Several points if I may.

First of all, going to forced induction is costly and complicated. Personally I would not use a splint skirt piston for a blown engine. The loads on pistons and rods will increase. A 6 psi boost pressure will give just over 11:1 effective CR which is a bit high for those pistons.

A SCCR gear set will give a very high first gear and result in high clutch wear and poor acceleration off-the-line.

Your best bet is a 998 bored to plus 60 thou with a 12g295 or well- flowed 202 head. Set the CR at around 10.3:1 and always use 97 or 99 Ron petrol. Get the distributor recurved to suit the cam and CR, Use a better carburettor, an HS 4 would work well, and a Mini Spares Evo 001 cam would be ideal. With a 3- branch exhaust manifold and a 1.625” bore exhaust pipe you should get a reliable 60 to 65 bhp engine.
A standard 4-synchro gearbox would work fine with a 3.44:1 FDR.
Oh, and yes, you will need better brakes!

[kkonstantinopoulos]

Thanks for the input, Cooperman.

 

I do realise tuning the 850 is somewhat futile from a cost-to-power perspective, but I’m mainly trying to see what interesting and usable setup I can build from the parts I already have.

 

Interestingly, I had spoken to Stuart from Vmaxscart and Tomshop in France regarding the supercharger route, and both suggested the stock pistons could likely survive at low boost with a moderate CR. That said, I completely agree there’s very little real-world data on supercharged 850 reliability nowadays.

 

Sourcing a good 998 block and crank may be difficult where I am, but with some patience I could probably find something. A 998 Cooper-style setup like you described would likely make a really nice road engine, especially as I already have the head, exhaust, and HS4 setup for it.

 

Machining costs are another factor. I could potentially attempt the overbore myself on the large milling machine we have at university using an offset boring bar or custom tool, but I’m unsure how accurate the result would realistically be. I also still need to properly measure the pistons and ring gaps, as they do appear somewhat worn.

 

I’m definitely planning to upgrade the brakes and suspension on the new subframe regardless of which route I take, as the drums can already feel fairly sketchy when the car is loaded.

 

Would 7.5" discs work well for this type of setup? I was looking at the Minispares C-AJJ4028MS kit, potentially with 4-pot calipers and maybe Minifins at the rear. I’d also like to run slightly wider tyres and replace the slightly bent rims, possibly with something like a period-looking Blockley tyre, while keeping the stock look with chrome hubcaps and without pushing the wheels outside the arches. Would 10x4.5" rims sit too far out, or would something like a 10x3.5" with wider tyres make more sense? The current wheels and tyres look incredibly skinny on the car, my friends often joke that the tyres look like they belong on a bicycle

 

[SKIDDERWOLF]

posting this as there are some similarities as to my upgrades as I’ve recently gone down the Vmaxscart supercharged route with my stock 998 +30” +Kent 266 cam and I am really pleased with the results. It would be really interesting to see the results on an 850. I have no previous experience and did it because I wanted to and I learnt heaps on doing the work myself. I’ve also previous fitted 7.5” discs as it originally as twin leading drums and converted to Hilo all round, currently running on Dunlop D1 copies with Blockley 165 10” wheels. The head is a std 998 however I machined away the valve bosses to improve flow and enlarged the chamber to avoid fitting the decomp plate. I also installed the 123+ distributor which works well with adjustable advance curve and boost retard. Really interested to see how this progresses give me a message if you have any questions as I finished 2 weeks ago. Rich

[kkonstantinopoulos]

posting this as there are some similarities as to my upgrades as I’ve recently gone down the Vmaxscart supercharged route with my stock 998 +30” +Kent 266 cam and I am really pleased with the results. It would be really interesting to see the results on an 850. I have no previous experience and did it because I wanted to and I learnt heaps on doing the work myself. I’ve also previous fitted 7.5” discs as it originally as twin leading drums and converted to Hilo all round, currently running on Dunlop D1 copies with Blockley 165 10” wheels. The head is a std 998 however I machined away the valve bosses to improve flow and enlarged the chamber to avoid fitting the decomp plate. I also installed the 123+ distributor which works well with adjustable advance curve and boost retard. Really interested to see how this progresses give me a message if you have any questions as I finished 2 weeks ago. Rich

 

Thanks for your reply, Rich.

 

I’m in a pretty similar boat, this whole project started as a way to learn by doing, combined with a lot of reading (mainly Vizard’s book), forum posts, and YouTube videos. The cylinder head work and machining so far has also mostly been me teaching myself as I go along, and I think the current head spec would actually suit a supercharged setup quite well, of course with the compression ratio dialled in properly. I’ve deliberately left the combustion chamber CCs unfinished for now so I still have some flexibility before fully committing to a final spec.

 

The 123 distributor seems like a really nice piece of kit. Their workshop is only about 20 minutes from me, and from what I’ve seen I really like both the product and the flexibility it offers. I’d probably fit one regardless, even on the current engine.

 

As mentioned earlier, the main concern is really the pistons long term. That said, Stuart mentioned he had a customer running around 8.8:1 CR with 9 psi on stock pistons without issues, and Tom suggested something like 8.5:1 with ~6 psi using the smaller VW charger could work well on an 850 (likely much better than the larger Eaton M45).

 

From what I hear though it feels like it would probably be right on the edge, where a hot day or a bit of detonation while tuning could quickly destroy the pistons...

[SKIDDERWOLF]

I had the 123 fitted to my N/A engine first too and it was a big improvement over the old distributor. I like that you can make small adjustments in real time, which makes it easy to dial the timing in. I have mine running on stock pistons it is the high compression engine running around 8.9:1. I had issues with detonation but have managed to control it through the boost retard and richening the idle jets in the Weber. I’ve done tons of reading in this forum and tuning books too which has helped but I have found that driving the car and making adjustments in real time has helped bring it together.

[Ethel]

The chances of finding a Mini engine in bits must be about the same as finding one built up & likely cheaper. Even if it was assembled it could be possible to strip it enough where you bought it. If the Mini is your only option to collect it, a whole engine assembly would go in the place of the front passenger seat if you knocked up a platform from a pallet to sit it on & some securing off the seat belt mounting points.

 

As for tuning: all As are pretty asthmatic if we're honest, so forced induction can find power by just making up for the poorer breathing as the revs increase: not giving the pistons any harder of a time than they had where the unblown engine made peak torque. The threat to them is piling more stress on them in that peak torque range. Turbos have the advantage over mechanically driven blowers with that, but good control of the ignition timing can be more important with either. If you're tech savvy & have access to the machining tools, you should consider putting together some programmable ignition system, like Sparkduino. There will be some cost, but once you have built it you'll have it to use on future projects, A Series or otherwise.