Flywheel

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)

The GTA Turbo pistons are domed, but I have not measured the capacity of the dome, whereas the A610 pistons are flat. I measured the capacity of my Z7U cylinder heads and measured the crushed headgasket and the 496cc capacity of each cylinder and did the math. I measured the chamber with a graduated burette, but I have not kept the figures. I have also gained compression through compressing the extra 86cc per pot.

I can though read your figures, and it would seem I have it all wrong. I will in the future remeasure it all again to be certain.

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.

That's interesting to know. Have you also noticed the Z7U's heater matrix outputs come off the back end of the heads which to my mind increases flow around the liners. The Z7X doesn't use them., although the ports are there. FYI the DeLorean uses one side, the other has the temp sender in it. On the TT DeLorean I was able to unscrew one barb and replace it with the temp sender.

Kevin at GTO has a 600hp 3 litre odd-fire and the bottom end is unmodified so I guess there's something screwy going on. I'm less worried about cooling as you know because the DeLorean has an open engine bay and excellent air flow over it.

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

Fairy Nuff. Our lot may come back and tell me I can't get the 60 part but we've succeeded through them to get Alpine parts that were apparently "unavailable"

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 .

But none of this is "way out there" - the chargecooler is just an oem intercooler jacketed, the engine is a stock 3 litre turbo, the boost is "only" a bar - the biggest gain is probably not having to map for stochiometry (no cats)

Youve only been out in a standard GTA - maybe you need to go out in an A610 to compare Tony's to it..

Any volunteers? I'm sure Dan'd be happy to return the favour in his Esprit!

The late 25's were still 2.5l, just 8:1 compression, same as the GTA Lemans..

Not according to Kevin - it's what Venturi used in the 280, and those pistons are like rocking horse sh!t. Renault did make a 2849cc turbo.

Apparently

WOW, all sounds good if you are into "modified" engines, but I prefer to run my car in an un-modified class,

Well you could cheat You can do a 3 litre without the balance shaft and with an A610 intake (only a trained eye would spot the difference!) - everything else will bolt right up

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.

Even with that though the throttle pot variance is a pain the bum to set up correctly, it's all about differentials of sudden deflection mapped against a percentage increase in injector cycles for a specified period. Very tough to get right! Everything else just uses it as a limit switch anyway (when throttle is greater than 97%, consider it fully open etc)

Not according to Kevin - it's what Venturi used in the 280, and those pistons are like rocking horse sh!t. Renault did make a 2849cc turbo.

Apparently

Yeah, but he probably heard it off somebody else..

The late 25 turbo is 2.5, 8:1 compression, slightly longer duration cams, lambda controlled ecu (and reprogrammable), amyl valve on the actuator with 5v throttle potentiometer instead of switched so the car runs low boost on part throttle and full boost only when the throttle is above 95%

Renault never made a 2.8 Turbo.

A company called AlpineCentre (not related) made some GTA specials which were 2.8 capacity and 8.2:1 compression. This used Z7W/Delorean height pistons but dead flat (no 0.8mm dome) with the 53cc chamber 25/GTA heads to acheive the 8.2:1 ratio. These ran 1 bar of boost and had WI. This engine design was than later bought by a company called EIA (the engine was then known as the PRV-EIA) and then fitted into the Venturi 260s. It made roughly 260bhp/300lbft.

Then later they switched to the 3.0 bottom ends with flat pistons again to acheive the 7.3:1 ratio and 281 BHP at 1 bar.

I am obviously very interested in all this. This 2.8 with GTA Turbo heads had 8.2 comp ratio with flat pistons, and yet a 3L with flat pistons has 7.3. Is the 3L head used that much bigger in the chamber? Somewhere back in this thread I thought the 3L had a slightly smaller chamber of 49cc or so.

My 2975cc with flat pistons and GTA Turbo heads I had calculated out to be 8.6, which by the first statement sounds like its on the money.

I am obviously very interested in all this. This 2.8 with GTA Turbo heads had 8.2 comp ratio with flat pistons, and yet a 3L with flat pistons has 7.3. Is the 3L head used that much bigger in the chamber? Somewhere back in this thread I thought the 3L had a slightly smaller chamber of 49cc or so.

My 2975cc with flat pistons and GTA Turbo heads I had calculated out to be 8.6, which by the first statement sounds like its on the money.

Alan, ignore the dome. The actual height of the pistons are different which completely changes the compression ratio.

If your engine has a 3.0 bottom end (which if it had A610 heads would be 7.6:1), and you then fit larger capacity Z7U heads, then it will lower the compression.

There is no data on 'piston heights' in the engine manual, so you physically have to compare them to know whats going on.

Dear Alan & DG,

I recently cc’ed Laguna 3ltr heads and of the ones I measured they averaged 53.2cc. I have a number of the other head types as well and will post up those numbers if it helps.

Are you sure?. Renault themselves say the combustion chamber on the Z7X is 49.85cc and the Z7U and Z7W/Z6W is 53.3cc, which is a gnats off of what you have. Are you sure they were Z7X heads, or you have measured in a different manner to Renault..

Hi DG, let me check, one of the ones I cc'ed is under my bed in a bin liner.... must be why I'm no longer married - laugh! Head cast number: 7946000282, think you will find this is 3ltr n/a Laguna. Standard method: spark plug and valves fitted, head sealed to thick transparent poly plate with two holes, sealed to head and burette filled.

Plate straight to head, or with a headgasket too?

Can't find that part number on the Renault system, the 79 is strange as all the heads have always been 77 part numbers, apart from old R25 heads which some begin 79...

I am obviously very interested in all this. This 2.8 with GTA Turbo heads had 8.2 comp ratio with flat pistons, and yet a 3L with flat pistons has 7.3. Is the 3L head used that much bigger in the chamber? Somewhere back in this thread I thought the 3L had a slightly smaller chamber of 49cc or so.

My 2975cc with flat pistons and GTA Turbo heads I had calculated out to be 8.6, which by the first statement sounds like its on the money.

Alan, ignore the dome. The actual height of the pistons are different which completely changes the compression ratio.

If your engine has a 3.0 bottom end (which if it had A610 heads would be 7.6:1), and you then fit larger capacity Z7U heads, then it will lower the compression.

There is no data on 'piston heights' in the engine manual, so you physically have to compare them to know whats going on.

David you are right about the piston crown heights. I had assumed (yes I know)that the A610 pistons came to the top of the bore, but they are more like 2.5 mm down the bore, and so my equivalent comp ratio is much reduced and most likely the 7.3 you mentioned.

Perhaps that means I can get more serious about the boost with the new turbo. The new turbo (Garrett GT3071R 90T Ex .63AR comp .5AR) is quite a bit bigger physically than the original and presents a whole new set of problems with fitting of the pipework, and the engine cover hitting the compressor housing, and a larger "oven" around the turbine necessary.

Eh , no worries...

Good choice of turbo Alan. Go for a bag around the turbine housing instead of the old metal casing.

Which style of GT turbine housing have you gone for?. Please don't say the traditional 5 bolt so it can fit the original Renault wastegate housing can fit because you will be doing the turbo no justice.

Are you going for the CC? Now I have a single exit exhaust again, Im going for the largest unit I distribute, by dropping it down low where the old exhaust system used to sit, instead of on top of the engine with the smaller unit.

The turbo still has an internal gate, but you run a separate exhaust pipe away from the wastegate, and then join into the main pipe say a foot down the main exhaust away from the turbo.

I will also be using a single outlet 3" system, with a large oval straight through muffler.

I am thinking of fitting a water to air intercooler whilst I am doing so much other modification, particularly as it seems they are so effective.

What size main core is the chargecooler that you use to replace the std casting pipe to the air to air intercooler? 8" x 4" or 10" x 4" ?