Hello

I haven't been on here for a few years. Last time I was on here I wanted to build a turbo cinq. However, a house purchase and a wedding put an end to that!

Now that I have a bit more money, I'd like to continue and build one.

I've read lots of conflicting information about what is needed to build a high boost mpi cinq so this is my list that I have found. Please correct anything that is wrong.

1242cc MPI engine
Turbocharger from Saab 93
Megasquirt
Larger injectors
Pistons (or conrods?) from 16v 1242 to reduce C/R to ~8.5:1
Blow off valve
Intercooler
Various other hoses, pipes for oil to turbo, and boost pipes.

I know the information exists on the forum somewhere but a thread with some clear, concise information on parts for building a 'reliable' high boost mpi cinq would be great

Thanks

Stuart

What conflicting information? :devil:

I think the piston/conrod (yes, you need both) setup from 16V will drop the CR well below 8.5, a quote I remember from here (I was asking this question myself when I was building mine) is around 7.8:1. You could of course use MLS head gasket to compenstate. I was also being convinced by a certain person once that despite dropping the CR very low 16V pistons make the engine non-non-interference. Aledegely this is because of the shape of the piston crown, but I do not have anything to back up this claim.

As for reliability I propose you go through my thread and see what I did. Mine is now past 55k kilometers and I have not had a single failure related to turbo (well almost, loosing turbo to manfold nuts should probably count as one). From my point of view the key to the success is double check everything, ask here about every single thing that you have doubts about. And proper engine control and tune up is a must. MS is a reasonable option, one however I would never go for myself, but I am "different"

there is no set list that is why you read different stuff.
but you need a turbo
a manifold that will hold the turbo
turbo downpipe
something to control fuelling (either a piggyback system or a standalone aftermarket ecu, injectors and things will depend on how you control it i.e. extra injectors or uprated)
you need to decomp the engine (again multiple ways of achieving that, 16v pistons will work in a 1242 8v engine or you could get decomp plate)
a boost controller, either bleed valve type or electronic
intercooler from something
uprated fuel pump
fuel pressure reg
3 bar map sensor
a plenum chamber

The list is pretty big, there is more than i've listed for sure but thats what comes to mind atm.

Edit: Of course if you fancy a seiTurbo you could just make me an offer for mine, don't really want to sell it but i'm financially fubar'd atm so offers considered - link to thread in my sig.

Thanks woj, your thread is interesting reading.

I however, am looking to steer clear of the decomp plate as I feel this introduces a weak point to the engine.

So are you saying that the 1.2 16v pistons cannot be used with the 8v conrods?

I'll try to find the source that mentioned using the 16v pistons / 8v rods (or 8v pistons / 16v rods) alters the compression ratio due to the distance between the gudgeon pin centre and piston top being different on the 2 pistons.

Has anyone else heard this?

Cheers

Stu

thats news to me.. i thought you could use either rod, and either crank so long as you use the 16v pistons in a 1242 block. The P75 block from the mk1 has oil squirters to cool the piston skirts whereas the p55 and p60 blocks don't so ideally you want one of them.

not sure about 16v 1242 bottom ends and these squirters, i have one but its not at home so can't go check for these squirters. Pretty sure though you can just get a complete 16v bottom end and bolt the 8v mpi head to it and not worry about swapping pistons at all - if you know its an ok bottom end that doesn't need a rebuild.

Why do you feel a decomp plate introduces a weak point? On the contrary, the experience of Polish Cento users is that a decomp plate with two mls hg-s is almost impossible to blow, if assembled right. It does remove a safety feature of hg-s. That is, people blew pistons before the decomp plate setup gave in, but that's another story.

stup86 said:

T

So are you saying that the 1.2 16v pistons cannot be used with the 8v conrods?

Click to expand...

I think the gudgeon pins are a different size. In any event, they're of the truly horrible penny pinching non floating type (retained by a heat interference fit) so are not to be messed about with lightly.

OK. Got it.

Looks like i'll be scrapping the 16v piston idea then...

I have run a decomp plate with no issues at all on a cento

I could post a pic of Quite a few Blowen hg and decomp plates

And to add to your list blu

A bag full of money

Decomp plates mess up Squish and reduce detonation resistance, however probably no more than another set of pistons from an engine not designed for turbo use... Two gaskets is a PITA though and torque settings will need to be revised. Gasket alignment and decomp plate quality will be critical. You've also added height to the head that just minced your cam timing unless you spend your Wonga on an adjustable pulley.

The only proper way is a set of bespoke pistons otherwise either way will get you a half decent running motor with a half decent tune. I have the 16v pistons, which I'll probably upgrade to forged turbo spec pistons at some point, if I ever get round to actually finishing the car.

Most premature engine deaths are caused by crap fuelling, over advanced timing and an urge to crank up the boost too much.

stup86 said:

OK. Got it.

Looks like i'll be scrapping the 16v piston idea then...

Click to expand...

Why? Just replace pistons and rods as a set -- or, if buying new pistons, get someone who does it everyday to swap them over.

jatkinson said:

I could post a pic of Quite a few Blowen hg and decomp plates

Click to expand...

Of a setup with two MLS HG-s? Because a decomp plate with one soft gasket is indeed easy to blow...

Honeymonster said:

Decomp plates mess up Squish and reduce detonation resistance, however probably no more than another set of pistons from an engine not designed for turbo use...

Click to expand...

All true, but for this kind of engine and small time tuning mostly irrelevant.

Honeymonster said:

Two gaskets is a PITA though and torque settings will need to be revised. Gasket alignment and decomp plate quality will be critical. You've also added height to the head that just minced your cam timing unless you spend your Wonga on an adjustable pulley

Click to expand...

Only PITA because the block and the head need to be skimmed. Tourque settings are well known and tested by probably close to 100 users back in PL Also, mind you, my setup was assembled by myself and it still runs. It was first (and so far last) head job I ever did (oh I wait for jokes on this ). So it cannot be that difficult.

Also the height difference, technically true, in practice it makes 2 degress of advance difference or so (I calculated it once). Stretching of the belt would produce more. Also I did mount an adjustable pulley - playing with changes of +/- 4 degrees did not make any change whatsoever for this engine running turbo (it seems it's all in the spark advance). So unless you are not up to doing heavy advancing/retarding you can do without a pulley to compansate the decomp plate.

woj said:

All true, but for this kind of engine and small time tuning mostly irrelevant.

Click to expand...

I think both these options are OK in most scenarios, however if building from scratch and a funds are available a set of forged pistons should be on the shopping list.

woj said:

Only PITA because the block and the head need to be skimmed. Tourque settings are well known and tested by probably close to 100 users back in PL .

Click to expand...

Yup, getting the block skimmed is a PITA no? I guess no biggy if you're doing full rebuild though. Out of interest which head bolts/studs did you use?

woj said:

It was first (and so far last) head job I ever did.

Click to expand...

Well it seems you did well, must of done your research, many make a mess on their first go.

And if you want to try again just head to one of the shows and look for the chaps eating Quavers... :devil:

woj said:

Also the height difference, technically true, in practice it makes 2 degress of advance difference or so (I calculated it once). Stretching of the belt would produce more. Also I did mount an adjustable pulley - playing with changes of +/- 4 degrees did not make any change whatsoever for this engine running turbo (it seems it's all in the spark advance).

Click to expand...

Makes sense I guess since with turbo's we use mild overlapping cams. I have a vernier fitted an will see if tinkering makes any measurable difference when it's on the RR.

Now you best get back to perfecting your head work.

my old cinq with the decomp plate never had an adjustable top pulley, car did at least 70k in turbo guise, maybe more - got upto an indicated 98k miles on the clocks. Never had issues with HG or anything engine wise tbh. Only thing i ever had issue with was the alarm that was on it which could kill the engine.. That car was running 1.8bar of boost which is more than double what my sei is running.

Honeymonster said:

I think both these options are OK in most scenarios, however if building from scratch and a funds are available a set of forged pistons should be on the shopping list.

Click to expand...

Well, indeed, my next turbo project, if it ever happens, is going to be on forged stuff, no doubt. And then insane boost levels :devil:

Honeymonster said:

Out of interest which head bolts/studs did you use?

Click to expand...

IIRC standard Reinz bolts for this engine, stuff that my engine builder bought to rebuild the engine. I still have the HG from the set floating around somewhere.

There are no off the shelf aftermarket head bolts, although ARP will make you up a set. Thread is 9mm, which probably rules out using sensibly priced high tensile cap heads.

Biggest weaknesses are probably the rods and the pistons. If going forged you could specify shorter rods which would allow you to specify bigger piston ring lands, which would really help.

The old MR2 has gone a long way to convincing me that higher boost isn't the way forward (except on the dragstrip). Dual entry ceramic turbine gives it throttle response which very nearly matches a supercharged car. Up the boost and the turbines become fragile, though.........

Everybody knows you just need a Hoooooooooooooj turbo. :devil:

lol, agreed. More boost solves everything possibly imaginable