Technical Turbo prep - getting my fact straight

[MKoers]

This week I'm finaly getting my engine to the shop for refurb and preppin it for the turbo.
To be sure I want to get my fact straight and checked so I'm on the right way.
I have a punto 75 1.2 MPI engine.
I also have 1.2 16v rods.
As I want to run at least 1 bar I think the engine should be able to handle at least 1.3 to 1.5 Bars of pressure so I don't blow it up.
Now an earlier thread of me turned up I need a compression of 8.5 or 8.6 for this.
But to achieve this the engine has to be changed to lower the compression.
I do get forged pistons so there are two options I'm thinking about:
Option 1:
8v piston rods with forged pistons that lower the compression. Or even by increasing the camber in the cilinder head.
Down sides that I can think is a thin piston witch means hot spots are easy to burn through.
If the head is changed not low squish anymore.

Option 2:
16v rods with forged pistons.
The rods lower the compression. (not sure how much). The pistons can be thicker as there is more room.
Also they can have a wider side and deeper cup in middle to achief the low squish.
Is option 2 a good Idea or am I thinking of something stupid?

 

[woj]

Option 1:
8v piston rods with forged pistons that lower the compression. Or even by increasing the camber in the cilinder head.
Down sides that I can think is a thin piston witch means hot spots are easy to burn through.

Do not touch the head chamber with any machinery. I do not have concrete examples, but I have heard that anybody that tried that in PL blew their engines out.

Option 2:
16v rods with forged pistons.
The rods lower the compression. (not sure how much). The pistons can be thicker as there is more room.
Also they can have a wider side and deeper cup in middle to achief the low squish.
Is option 2 a good Idea or am I thinking of something stupid?

In general I say proper low comp pistons is your best option if you do not want to lower the compression "artificially" with a decompression plate. But then, you should really read again TurboCinqy1250's thread on his turbo project. Forged pistons (c.r. at 7.8 from what I recall) + spesso HG + turbo at 1bar = constantly blown head gasket for him, and he (seems to) know what he is doing. And lots of theories for him why this is happening. On the other hand I decompressed my engine with "poorer" quality decomp plate from PL + two metal HGs and just closed 21k km last Sunday on this setup.

 

[craigb244]

you have got mixed up somewhere! the 1242 16valve and 1242 8valve rods are the same length! its the dish in the 16valve pistons that lower the compression.

16valve pistons would be perfect for 1.5bar of boost.

just a note about using 16valve pistons.
they lower the compression a bit to much (which is why they will be good for over 1bar) but you will suffer with off boost power.

But if i was you id not spend any more money till you do some more research and take notes (sorry if i sound harsh... just dont want to see another person spend ££££ only to never have it run)

 

[MKoers]

OK, don't know how I got mixed up with the pistons.
I do not want to use the 16v pistons and the engine cilinder will get a rebore to create a smooth surface.
Also I want forged pistons as the can cope better with the pressure/heat.

With the 16v pistons you say they suffer off boots.
What do you mean exactly? Poor driving or will the pistons themself suffer?

I do get the engine running sometime but don;t have the tool to prep the engine myself.
That's why it go's to a machinery that overhauls the entire engine.
Gaskets, bearings, pistons, cilinders plus the valves will be machined for better flow.

 

[craigb244]

ahhh sorry i never realized you ARE going to used forged pistons.

as for the 16valve pistons i say suffer as in performance off boost... the compression is a bit low with them (you can easily raise it by skimming the head but ive never bothered)

the 16valve pistons are solid, ive ran them since i have had a turbo, done countless event and miles.... even drove once when the solenoid pipe split making the turbo run over 3bar of boost (my ecu wont read over 3bar and it maxed out lol)

 

[MKoers]

Ok that's clear then.
I have been searching, probably with the wrong words, but can't find how much the compression is normally with the 1242 engine.
And how much it is with the 16v pistons.

 

[woj]

The stock 1.2 Punto 75 engine is 9.8:1 (SPI is 9.6 according to my book). The ratio with the 16V pistons I was told around here is about 7.8:1. I personally find it hard to believe, but oh well, never tried it, measured or anything, so I have to trust more experienced folks around here.

If you think that the compression with 16V pistons is going to be too low (and Craig is very right about poor off-boost performance I guess) instead of skimming the head you can always use a thinner 2-layer metal head gasket from the newer 1.4 8V engines (they are fully matching, the Fiat part number is 55183371). This will drop the head about 1mm resulting in a reasonable compression ratio. But then again I have not done any real calculations. Also, I guess a metal HG is less likely to get blown. Works for me anyhow Note however that you need to have both the head and the block perfectly flat for this, so they both need to be machined, but that should be no problem in your case

 

[MKoers]

No that's no problem.
As said I try to do as much as I can myself but the internals of the engine (crank, rods, pistons) and the head (valves, cam, valveclearence) are not things I do usualy.
As I don't want to blow the engine I leave that to the pro's haha.
I will assemble the engine as far as possible as putting the bottom and top together, sump, waterpump and so on.

A drop of 9.6 to 7.6 just by the extra valve cuts on the 16v pistons?
Sounds like a lot.

What would be the best compression for high boost plus normal driving.
Around 8.5 i guess?

I use a Garret GT1750 turbo so it commes in nice and slow and delivers good flow at the high rpm rates.
Want to set the rev limit to 6500.

 

[woj]

A drop of 9.6 to 7.6 just by the extra valve cuts on the 16v pistons?
Sounds like a lot.

It does sound like a lot and that's why I find it difficult to believe, but this is what I was told here. But it is not only because of the valve cuts, the whole piston is lower. I also heard from another source, and this is also something I find difficult to believe, is that 16V pistons make the whole construction interference prone. This is supposedly because of the valve cuts being in wrong places.

What would be the best compression for high boost plus normal driving.
Around 8.5 i guess?

That's about right. This is what I was aiming for myself, how close I got I am not sure. I boost 0.8bar with not much problem, I could do 1 bar with no problem either, just getting the ignition advance and keeping the EGT within limits gets a bit more tricky then.

 

[MKoers]

To cope with that I replace the original ECU and put in a KDFi.
That's a factory build ECU based on megasquirt.

Also I got 4 injector from an volvo T5. That's and 2.5 liter engine so each injector can handle 0.5 liters.
It are the same injector used on the cinq of fordcosworth.

 

[ycming]

The stock 1.2 Punto 75 engine is 9.8:1 (SPI is 9.6 according to my book). The ratio with the 16V pistons I was told around here is about 7.8:1. I personally find it hard to believe, but oh well, never tried it, measured or anything, so I have to trust more experienced folks around here.

If you think that the compression with 16V pistons is going to be too low (and Craig is very right about poor off-boost performance I guess) instead of skimming the head you can always use a thinner 2-layer metal head gasket from the newer 1.4 8V engines (they are fully matching, the Fiat part number is 55183371). This will drop the head about 1mm resulting in a reasonable compression ratio. But then again I have not done any real calculations. Also, I guess a metal HG is less likely to get blown. Works for me anyhow Note however that you need to have both the head and the block perfectly flat for this, so they both need to be machined, but that should be no problem in your case

Cheers for the metal gasket in the 1.4 8v Thanks!

Ming

 

[fingers99]

I would go for forged pistons to the 8v size gudgeon pin, but to 16v pattern, if you follow me. You can always specify that the piston deck height is higher (and no valve cut outs!) if ordering forged ones. The guy building your engine should be able to spec/measure the combustion chamber and piston cut out volumes in order to get the CR right.

Running much over 1 bar boost is a cheap and nasty trick -- most turbos will be outside their peak efficiency much past 1.2bar and you'll just be stuffing the engine with hot air (which will cause detonation). The trick is in the specification of the correct impellers for the turbo.

A solid copper HG won't need the block machining, will be re-usable and much tougher than a standard HG. Thye same cannot be said for the steel gaskets in the SuperFIRE engines.

 

[MKoers]

A mate had his 1.1 engine prepped by the same company. Also forged pistons.
No valve cuts in the pistons just a nice dish in the middle.

HG will be spesso.

 

[fingers99]

Spesso are amongst the best "standard type" HGs. But they're not a patch on solid copper ones for forced induction engines.

You might talk to your engine man about using fully floating gudgeon pins.