Pistons
Turbo Mini Forums

Hi,

Ive been offered a 4000 mile n/a motor that im considering buying and adding a turbo charger too.

Assuming that the CR / Cam and head are all OK, i have a questions concerning pistons.

The engine comes fitted with a set of 73.5mm Cast Omega pistons & Omega rings, are these suitable for use on a turbo or not? And if yes, what BHP / Torque are they considered reliable up to? Realistically i would like the engine to be putting out around the 150 bhp mark.

Is this realistic with a T2 Turbo on a Mirage manifold?

Regards,

Mike

If I'm not mistaken, the Omegas come with a number of different dish sizes, so you've really answered your own question when you say "Assuming the CR....... etc are all OK"

If the motor was built as a high performance N/A - and I would assume so as it's up at 1380cc - then it will probably have the smallest dish pistons rather than the larger dish you need to get the CR down for turbo use.

Schrödinger's cat - so which one am I ???

CR down = more boost

The engine that im looking at buying is a 1338cc 9000rpm Screamer with a destroked crank, cooper S rods, IDA weber, APT Cam ,BVH and a metro turbo Cup challenge gearbox.

Its not been confirmed, but its said that the engine was putting out 128 bananas at the last rolling road session.

So then, assuming that the Omega pistons have the smallest dish, it is possible to mill the head out to lower the CR down to something more socially acceptable?

Does a target of 7.5:1 sound about right?

Also, i would be aiming for around 150 bananas so do folk think i should change the pistons? Or would they be up to the job? im concerned about the additional heat generated from the turbo.

The last think i want to do is make an expensive mistake! I would rather get it right first time.

Ive only every modified n.a. engines before, so turbos are a whole new entity for me!

Mike

The fact it has a destroked crank (and the S rods to fit around the smaller journals) suggests it was specifically built, as you say, as a "screamer".

The only reason for destroking at almost the largest possible bore (and hence bringing the cc back down) is to go towards oversquare and hence high revs.

All the associated mods (CR, cam, etc) will be for that aim.... high revs.

Turbos genarally are built for high torque at much lower RPM.

Choice of cam for turbo is completely different to a "screamer" - do a search and you will see people's opinions vary as to which is best but the common choices all have completely different timings/durations than what is required for a high revving N/A.

Schrödinger's cat - so which one am I ???

Cheers,

Im not too concerned about high revs. I was more concerned with getting a cheap engine and box!

He only wants £1000 for the lot which seems very reasonable given the gear thats on offer.

My aim was to sell the carb, cam and head and replace them with something more suited to a turbo application. Im OK with cam choice (poss a kent 266) its just the pistons im worried about.

So you think that the idea is a goer then?

Mike

If you plan to change the head and cam, then it would be a goer.

I think that there are 1380 turbos on here with cast pistons running around 200hp.

I would aim for a CR of around 8.5:1. Then run around 12-15psi boost with a good intercooler to give 150hp.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Thanks Paul S, that is exactly what i needed to know!

Mike

The only thing I would add (if you haven't actually bought it yet) is to find out what cc (dish size) the current pistons are.

Then you can work out in advance how much you will need to take out of your new head to see if it is feasible. There is a limit as to just how much you can take out of a head to get CR down if you start with high CR pistons.

Schrödinger's cat - so which one am I ???

Rod is quite right.

You will need to find about another 10cc of combustion chamber volume if you want to drop from, say, 11:1 down to 8.5:1.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Ive just spoken with the seller. He reckons that they are a 7c dish.

To be honest hes advised againsts turboing the engine because of the small crankshaft, hes also recommended forged pistons but freely admits that he doesnt know much about FI.

I would suggest you start from scratch, decide exactly what you want the end result to be, and do your "shopping" once you have decided.

I won't get hung up by people saying you have to used forged pistons. For example, I am building with cast Hypatecs - so are others on the forum, and although yet to be proven in extreme conditions, I am reasonably confident they will be OK. Plenty of people in Oz say they are fine...

Do some reading (use the dreaded "search" button) and you will find out what is required for a turbo "A" series.

I did, and that's why I'm here now with a "nearly finished" one....

Schrödinger's cat - so which one am I ???

Yeah cheers Rod.

I have perused using the search button. Starting from scratch would be ideal. However, funds are tight and i thought that maybe i could use this engine as a basis for something tasty, rather than having to get an engine rebored, cranks reground etc.

Cheers,

Mike

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

Would it not be easier to start with a standard block and build it to exactly what you want?

Spending 1k on an engine, then replace the head, cam and pistons etc doesn't really make sense to me. Better to get a boring motor for a couple of hundred, then get it machine exactly to you're liking and add the parts you were going to buy anyway....

The value is in the gearbox and the bits he can sell.

You could easily spend £1k on refurbing a standard engine and box, but you would still have a standardish setup.

I've just spent nearly £4k to get anything special. Could have spent more.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

What do you reckon the gearbox is worth? My plan was to keep the gearbox and straight cut drop gears x pin diff , and sell the engine on with a STD gearbox for as close to the £1000 as i can.

Mike

To rebuild a box with new bearings, straight cut drops and gears and a x-pin diff would cost close to a £1000.

Saul Bellow - "A great deal of intelligence can be invested in ignorance when the need for illusion is deep."
Stephen Hawking - "The greatest enemy of knowledge is not ignorance, it is the illusion of knowledge."

I think you will have issues using this engine,

the cam will be to hot and need to e swapped out,

i would not have any major wories about the reliability of the destroked crank, but you are going to have a hard time getting the C/R down, asuming the stroke is 79mm and the pistons are 7cc dished the head will have chambers about 19-20cc. I would guess the head has had the chambers cut back a it and the head skimmed a fair amount to acheive this, so you are looking at another head with chambers around the 33cc mark (not impossible but very)dificult or swaping the pistons out (or machining thedishes bigger in them) and geting another head.

I would flog the engine, keep the box, and start fresh with a more standard engine.

Cheers mini13. That was my intention.

The 'box is a Metro Turbo Cup Challenge box with an extra long first gear. I think that it would be foolhardy to let it slip through my fingers!

Mike

I've got one of those.

If something is worth doing, it's worth doing half of.

How is the box minimadmotorman? is it any good? Whats the spares availability like? Are the ratios suitable for road use as ive been told that 1st gear is extra long.

Mike