Flywheel

What is it

Not nessecarily, you need to physically work out the compression by measuring the parts. Some of the data in the MR V6 manual is incorrect, for instance, if you look at the crank journal data, it says you cannot regrind even fire cranks, and hence gives no data for it, yet in the column next to them, it puts a couple of the Z7X engine numbers in the regrind list

Without looking, I believe it's making a distinction between the crankpins and the journals.

It does show the 72.7mm crank engines to all have a CR of 7.6:1 whioch I know is bogus now

, and also on the piston crown data, it says the 7.6:1 pistons are 'flat' and the 9.6:1 pistons have a 1mm dome.......If it had even a 1mm higher piston (so more overall volume than a 1mm dome), it still wouldnt increase the compression ratio to 9.6, it would be approx 8.4:1.
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Ah now I've been though this today...

Firstly the table is somewhat misleading - it states dome height versus compression for each engine type (basically 88, 91 or 93mm bore i with an exception made for the Z7U). but it doesn't state that all the pistons are the same, merely that the dome height above "flat" is how you determine the CR. It's not a guide to making pistons, but a guide to identifying them.

In other words, you can't take the dome off a 9.6:1 compression piston and make it 7.6:1 - precisely correct.Apparently it's actually 1mm off the top of the flat part of the 9.6 piston, and speaking to someone who's tried it, skimming the dome and a further 1mm off the top of a stock 9.6mm piston result sin a squashed piston in about 5 minutes.

If you remember Martin, the turbo pistons have a bigger dome than listed for the 2.8 pistons, yet the piston overall height is different, so don't take the dome height listed as being gospel for the compression ratio.

Hold on, which engine are we talking about? - if the Z7U then that's a whole different ballgame - shorter stroke, (much) fatter pistons and still lower compression. I don't know this for sure but I believe the heads are unique to the Z7U compared to all the other 91mm engines with a deeper combustion chamber. Also I don't think the pistons hit the top of the stroke as high as the 73mm stroke engines.

Basically forget everything you think you know about the 91mm engines where the Z7U is concerned - except that the block is identical to the Atmo and DeLorean!

Ever seen a Z7X turbo piston (I haven't) but I'm more than prepared to believe it has a flat top

What is it

It's a magic door.

What is even more confusing, is Renault list a different part number for the Laguna V6 pistons to the Safrane V6.

End of the day you can get a set of forged pistons in any spec for about £600 so its not really a problem, or if possible, take some meat out of the combustion chambers..

I don't know this for sure but I believe the heads are unique to the Z7U compared to all the other 91mm engines with a deeper combustion chamber. Also I don't think the pistons hit the top of the stroke as high as the 73mm stroke engines.

..the combustion chamber is the same as the 2.8 atmo so its most likely the same head apart from not having the hole for the dizzy and different diameter valve grind diameters.

Easy way to tell if the pistons hit the top of the stroke the same, is to measure the Z7U piston against the Z7W/X and see if its 5mm taller overall (there abouts, taking into account the compression ratio)

What is it

It's a magic door.

What a guy !!!

..just to confirm...

The A610 pistons came as piston and sleeve kits, part No.77 01 467 556. These are completely flat, without valve reliefs. The gudgeon centre to crown height is 5mm less than a GTA Turbo piston to accomodate the 10mm greater stroke.

When you think about it, the Z7U, even though its more restrictive design than the later Z7X, is probably far more suitable for serious abuse.

The crank has 10mm less stroke which makes it better for revving and reliablilty, the piston liners are thicker being a smaller bore, so less likely to split, and the pistons have 5mm more height to them above the gudgeon pin. So, in practice, with forged pistons, even though they would have slightly more mass, could have a nigh on indestructible thick crown to them.

So, the best of both worlds...? A 2.5 bottom end with Z7X heads... You can even have maximum lift cams, owing to the fact that your pistons have so much metal above the pin that can be machined for relief, and still have maximum strength...

Having bought and measured up a set of 93mm A610 Z7X pistons to put into a turbo motor I am building, I have found that this combination of flat topped pistons and GTA Turbo heads yields a comp ratio of 8.6, which is what a Z7U is supposed to have going by the literature. Except of course you now have another half litre of capacity, now 2975cc, when an even fire Volvo 760 crank with 10mm more stroke is introduced.

I have put bigger valves in the ported Turbo heads, and will be using the higher lift Volvo 760 (B280F motor) cams as well. I believe that the extra capacity has to achieve greater power and torque everywhere in the rev range. Looking to run .9 - 1 Bar of boost, for reliability and reasonable power.

This "Flywheel" thread is now an interesting "Piston" thread.

Having bought and measured up a set of 93mm A610 Z7X pistons to put into a turbo motor I am building, I have found that this combination of flat topped pistons and GTA Turbo heads yields a comp ratio of 8.6, which is what a Z7U is supposed to have going by the literature. .

How do you think its 8.6 Alan? I would have thought, that seeing that the A610 is 7.6:1 with 49.8cc combustion chambers, and you have Z7U heads which have 53.3cc chambers the compression ratio is going to drop (which does coincide with what the Venturi Atlantique has using 2.8 heads and a 3.0 bottom end - 7.3:1)

Wow - lots to reply to - so much that I'm actually writing this in notepad so I can paste it in!

..just to confirm...

The A610 pistons came as piston and sleeve kits, part No.77 01 467 556.

Just checked, this part is invalid on the Renault computer, the Alpine number still works.

When you think about it, the Z7U, even though its more restrictive design than the later Z7X, is probably far more suitable for serious abuse.

Definitely no argument from me on that one!!! The thing I keep coming back to is that I think I'm one of the few people to have been out in Tony's GTA after tuning and it is in a completely different league compared to a stock GTA. A stock GTA turbo is fast, this thing is ballistic. Dan (my business partner) has a chipped Esprit GT3 putting out something in the region of 280 at the flywheel at a bar and Tony's would poo all over it. Plus donor 3 litre engines are a lot more common than Z7U's! Although it is interesting that uo could put a set of pistons and cams in a normal 91mm block and build one, if you're goign to spend that kind of money you may as well go 3 litre because you get better cams to start with

The crank has 10mm less stroke which makes it better for revving and reliablilty,

...and have less torque

the piston liners are thicker being a smaller bore,

This is incorrect. All the liners are the same thickness, the seat in the block is different across the three sizes, and liners are not interchangable (this was an expensive trap I fell into once apon a time!)

with forged pistons, even though they would have slightly more mass, could have a nigh on indestructible thick crown to them.

That bonkers turbo DeLorean I posted a while back had forged pistons and rods made for it and he managed to lose 150g per piston. He can reproduce the same pistons/rods for $2200

So, the best of both worlds...? A 2.5 bottom end with Z7X heads...

Would you trust the head gasket to seal? Again, the od of the liners is not the same. It's one thing to put Z7U heads on a Z7X bottom end because there's plenty of face area on the head, but putting the bigger heads on the smaller liners I'd worry about sealing.

Having bought and measured up a set of 93mm A610 Z7X pistons to put into a turbo motor I am building, I have found that this combination of flat topped pistons and GTA Turbo heads yields a comp ratio of 8.6, which is what a Z7U is supposed to have going by the literature. Except of course you now have another half litre of capacity, now 2975cc, when an even fire Volvo 760 crank with 10mm more stroke is introduced.

Just an FYI: Aside the fractional difference between the 2963 and 2975 cranks (.3mm of stroke equals less than a 10th of a mm in piston height, or less than a layer of carbon!) ALL even fire PRV cranks are the same, except the Z7U

For my money the cheapest approach to greater capacity would be to make a set of conrods 5mm shorter and drop a Z7X crank into a Z7U engine. Presto 2849cc like the late 25 turbos but without forged pistons.

Should have worded it differently, not 'thicker' liners, but thicker for the diameter..and cooling. The A610's have a problem with cooling on the outer cylinders and these are the ones most common to fail. This is never a problem on the Z7U. The 610 liners get very close to the walls of the block.

Headgasket wise, you wouldnt be using standard gaskets anyway...

The part number for the pistons/liners is correct as this is the one Alan used to order his..

As for Tony's car against a standard GTA, well of course its going to be faster, bigger turbo, more boost, new ecu, chargecooler - hardly a fair comparison . Youve only been out in a standard GTA - maybe you need to go out in an A610 to compare Tony's to it..

When I said about the forged pistons having more mass, - more mass that forged Z7X application pistons due to the fact of them having a thicker crown - not compared to the originals...

The late 25's were still 2.5l, just 8:1 compression, same as the GTA Lemans.. Its not worth going to the effort of having forged rods made and using second hand old pistons. Even though they have a 5mm taller piston, the domed section is still quite thin.

WOW, all sounds good if you are into "modified" engines, but I prefer to run my car in an un-modified class, otherwise I'm up against all sorts of modified wizard tricks costing more than arms and legs. Something an old block full of half used parts will struggle against without a doubt.

So I'm sticking with my Z7U block, but happy to consider some other options like exhaust manifolds and cams. Maybe next year?

WOW, all sounds good if you are into "modified" engines, but I prefer to run my car in an un-modified class, otherwise I'm up against all sorts of modified wizard tricks costing more than arms and legs. Something an old block full of half used parts will struggle against without a doubt.

So I'm sticking with my Z7U block, but happy to consider some other options like exhaust manifolds and cams. Maybe next year?

Yes, truthfully, things like capacity upgrades, bigger valves etc, are all pretty academic if you havent got the basics such as the correctly matched turbo, intercooling, fuel and ignition map, and overall fueling correct. The ancillaries are far more important than the internals. No point thinking its great having half a litre more capacity, if you can't pick the correct turbocharger for it, or lose 30bhp by not having a clue about intercooling...

If you are going to go the full hog, be 100% sure you can make full use of it with the right matched components, and don't scrimp on trying to use second hand parts as you said.

I am a firm beleiver in that the whole standard package is one hell of a big compromise. The throttle pot has 2 settings (idle hardly counts as a setting) and the ECU just retards the ignition when it thinks danger might be approaching.

It all seems to be a different kettle de poisson with a mappable ECU.