12 replies to this topic

[thomas36]

I recently bought a 1980 mini, having an engine swapped mg metro 1275 engine bored out to a 1301. I wanted to put a turbo kit on it, but wasn’t sure if this would put too much much strain on the engine block and crack it, as it has already been bored out.

Edited by thomas36, 24 November 2024 - 08:49 PM.

[Icey]

That doesn’t really make sense, you can’t take a 998 to 1.3, are you sure it’s not just been replaced by a 1275 lump? I suppose you can get kinda close with a 1098 crank and +100 bore?

[whistler]

That doesn’t really make sense, you can’t take a 998 to 1.3, are you sure it’s not just been replaced by a 1275 lump? I suppose you can get kinda close with a 1098 crank and +100 bore?

That takes you to just about 1200cc.

[Icey]

That doesn’t really make sense, you can’t take a 998 to 1.3, are you sure it’s not just been replaced by a 1275 lump? I suppose you can get kinda close with a 1098 crank and +100 bore?

That takes you to just about 1200cc.

What’s 100cc between friends?

[thomas36]

 

 

That doesn’t really make sense, you can’t take a 998 to 1.3, are you sure it’s not just been replaced by a 1275 lump? I suppose you can get kinda close with a 1098 crank and +100 bore?

That takes you to just about 1200cc.

What’s 100cc between friends?

 

I have just looked, and it has been replaced by a 1275. Lucky find. I did the dvla check and it said it was a 998, so must have been swapped at some point.

[Ethel]

Most turbo conversions will be rebored just to fit new pistons & recondition it in prep for the extra power.

 

Anything up to 1330 can just be considered reconditioning. Going any bigger for a turbo doesn't make a huge amount of sense as you produce more power than you can sensibly use with boost & larger displacement makes it harder to keep the compression ratio down.

[Steve220]

Unless its a 1380 and you risk blowing the gasket between 2 & 3, bore size doesn't make much difference. As long as the compression ratio and supporting mods are there for the extra power, turbo charging a mini is rather straight forward. A LOT depends on your budget.

[Cooperman]

To fit forced induction you would need to do a few things.

First of all measure the existing static compression ratio which means removing the head. Then get the piston part numbers to ensure that they are suitable for the increased compression loads a turbo-charger will impose. There is help on here, but bear in mind that you might need to strip it all down and even re-bore it again to fit suitable stronger pistons, such as the Omega ones which are recommended for forced induction. Some standard pistons are not up to the job and won't last long.

Once you have calculated the existing comp ratio for the pistons you have chosen, work out the new comp ratio required for the increased forced induction pressures and the increase in combustion chamber volume required. It will probably mean that a few cc's will need to be ground out from each combustion chamber in the head and each chamber will need to be exactly the same volume.

To get this all correct the maximum boost figure will be needed and if it's not a new turbo you may need to take it to a specialist to have it set up and calibrated. This is most important.

Then a distributor with the revised advance curve will be needed. Aldon Automotive is one specialist who can best advise and sell you the correct new distributor together with the suggested initial setting figure.

Fitting a turbo-charger is an involved engineering exercise. Get it wrong and the engine can 'expire' very easily. I once had to sort out a turboed engine which had not been properly engineered and it cost over £2,500 to rebuild it correctly when it 'blew up' after about 10 miles from being installed (it was running with a static CR of c.16.5:1 with a standard distributor and standard pistons!).

When fitting a turbo-charger or supercharger it is best advised to strip the engine and completely rebuild it to the correct dimensions and parts.

[InnoCooperExport]

16.5:1 static CR?! Blimey I'm surprised his old mate Rodney didn't come knocking! 

 

I would also consider getting a good electronic distributor like a 123 and then getting the curve setup properly on a rolling road. I would also think fitting a decompression plate between the head and the block is easier than die grinding the combustion chambers to lower the CR. 

[Cooperman]

On the one I had to rebuild, the head was standard and some gas-flowing and de-shrouding of the valves was necessary. Thus, with the Omega pistons which gave a bigger dish and they sat a bit lower down the bores I had to remove around 3 cc from each chamber This was easy and it was not necessary to fit a decompression plate. 

What had happened in this case, the owner had paid someone cash to build the engine and obviously the so-called 'builder' had absolutely no idea what he was doing. After it blew up, with a broken piston and some broken rings, the 'builder' would do nothing a he had been paid cash. That's how I ended up sorting it. 

The turbo was taken to a specialist company who checked it and set the max boost, also issuing a certificate of setting. I went from there with the necessary calculations. If buying a forced-induction engine, or a car with one already fitted, it's best to ask for the calculation sheet for the engine.

[Shooter63]

(it was running with a static CR of c.16.5:1 with a standard distributor and standard pistons!)..[/quote]

Can I ask how they managed to gat a compression ratio so high, doing a few quick calcs using flushed flat top pistons means you would need 16cc in the chambers, which is a fair old lump off the head face.

Shooter

[Cooperman]

(it was running with a static CR of c.16.5:1 with a standard distributor and standard pistons!)..[/quote]

Can I ask how they managed to gat a compression ratio so high, doing a few quick calcs using flushed flat top pistons means you would need 16cc in the chambers, which is a fair old lump off the head face.

Shooter

The compression ratio calculation is calculated on the maximum boost possible with whichever super/turbo-charger you are using. 

The CR is increased by a factor of the boost pressure to normal air pressure. Thus at a max boost of 5 psi the CR is increase by almost 33% when on full boost.

The one I had to fix had a max boost of 7 psi, as measured by the specialists, which increased the CR by around 50%. It was reduced to 5 psi max. Thus not only must the CR be set for the highest possible boost (hence the need for accurate calibration), but the distributor advance curve must be retarded to match to avoid detonation during acceleration.

The main point it that you can't just bolt a turbo- or super-charger on and expect it to work properly. It's a more complicated exercise which requires measurement and calculations.

[Shooter63]

[quote name="Cooperman" post="3776918" timestamp="1732883369"][quote name="Shooter63" post="3776837" timestamp="1732748852"](it was running with a static CR of c.16.5:1 with a standard distributor and standard pistons!)..[/quote]

Can I ask how they managed to gat a compression ratio so high, doing a few quick calcs using flushed flat top pistons means you would need 16cc in the chambers, which is a fair old lump off the head face.

Shooter[/quote]The compression ratio calculation is calculated on the maximum boost possible with whichever super/turbo-charger you are using. The CR is increased by a factor of the boost pressure to normal air pressure. Thus at a max boost of 5 psi the CR is increase by almost 33% when on full boost.The one I had to fix had a max boost of 7 psi, as measured by the specialists, which increased the CR by around 50%. It was reduced to 5 psi max. Thus not only must the CR be set for the highest possible boost (hence the need for accurate calibration), but the distributor advance curve must be retarded to match to avoid detonation during acceleration.The main point it that you can't just bolt a turbo- or super-charger on and expect it to work properly. It's a more complicated exercise which requires measurement and calculations.[/quote]

Yup there's plenty of things to take into consideration

Shooter

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