Ghibli de Hongrie [ENG]

C'est ici qu'on présente nos voitures
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Froggie
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Re: Ghibli de Hongrie [ENG]

Message par Froggie »

alpa a écrit : Check again chamber cc. If they are that different then check piston crown thickness. If you have let's say 5mm of thickness then remove some material from the top to increase the cc. You don't care about pistons, at the next rebuild they'll be worn.
How is it possible to have such a 5 mm difference on new pistons' dimensions?
It seems huge to me...
spacecadet a écrit : If I change the pistons on only one side, won't that cause imbalance because of the different weights on the two sides?
Not a specialist but that will certainly substantially impact the weight of each piston and hence the overall balance.
Might require a rebalancing of all the pistons/rods system...
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Re: Ghibli de Hongrie [ENG]

Message par alpa »

spacecadet a écrit :That's exactly what I was afraid of, you already wrote above that these engines run lean (wonder why though).

If I change the pistons on only one side, won't that cause imbalance because of the different weights on the two sides?

I though the lack of competent people is only a local problem for me, but seems like it happens globally, which sounds quite bad.
Already explained: they run lean for turbo engines, they run stoichio in boost, because extra-enrichment is mainly for the charge cooling. These engines have excellent cooling system so they don't need extra-cooling.

In theory you'll have an imbalance. In practice you'll remove few grams but will get back combustion balance. While now the balance is (should be) good but combustion is imbalanced which is not good. A v6 engine is never fully balanced and especially an odd-firing one, it's all about compromises.
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Re: Ghibli de Hongrie [ENG]

Message par alpa »

Froggie a écrit :
alpa a écrit : Check again chamber cc. If they are that different then check piston crown thickness. If you have let's say 5mm of thickness then remove some material from the top to increase the cc. You don't care about pistons, at the next rebuild they'll be worn.
How is it possible to have such a 5 mm difference on new pistons' dimensions?
It seems huge to me...
That's not what I said.
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Re: Ghibli de Hongrie [ENG]

Message par Froggie »

alpa a écrit :
Froggie a écrit :
alpa a écrit : Check again chamber cc. If they are that different then check piston crown thickness. If you have let's say 5mm of thickness then remove some material from the top to increase the cc. You don't care about pistons, at the next rebuild they'll be worn.
How is it possible to have such a 5 mm difference on new pistons' dimensions?
It seems huge to me...
That's not what I said.
I understood we were talking of a 3 cc discrepancy (38.5 compared to factory 41.5), so a thickness to be erased from the top of the piston of close to 5 mm and about 10 g (if in aluminum alloy).
I would not be happy to machine off the piston(s) such a thickness of material by trial and error and play with the overall balance of the mobile train.
But if this is the only option left...
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Re: Ghibli de Hongrie [ENG]

Message par alpa »

82mm Bore -> 4.1x4.1x3.1415 / 10 = 5.28cc/mm
3cc -> 0.57mm off
If the crown is 5mm thick then removing 0.6mm in average does not hurt. If it's 2mm then it's a problem.
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Re: Ghibli de Hongrie [ENG]

Message par Froggie »

Sorry, my mistake, 0.5 mm.
Still need to find a good machining shop
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Re: Ghibli de Hongrie [ENG]

Message par alpa »

No need for shop, a Dremel is far enough.
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Re: Ghibli de Hongrie [ENG]

Message par spacecadet »

alpa a écrit :Already explained: they run lean for turbo engines, they run stoichio in boost, because extra-enrichment is mainly for the charge cooling. These engines have excellent cooling system so they don't need extra-cooling.
Ok, understood now
alpa a écrit :In theory you'll have an imbalance. In practice you'll remove few grams but will get back combustion balance. While now the balance is (should be) good but combustion is imbalanced which is not good. A v6 engine is never fully balanced and especially an odd-firing one, it's all about compromises.
What about slightly reshaping the combustion chamber. It's much harder to do consistently and measure precisely, so not a good option in practice, but would it work in theory?

Custom head gasket would also only work by using two different thicknesses on the two sides, 2 cc difference -> ~0.4mm which also doesn't sound like a good option.

I really don't understand how could they make such a mistake... :x Looks like I have to learn machining (and bodywork) too. Last time it was painting, at least preparation, sanding, primer, the most important what they usually are lazy to do properly.

But I'm thinking about getting a different used pair of heads. Which means I loose the newly installed valve guides. But would like to have a stable engine, which can run at least another 150k kms.
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Re: Ghibli de Hongrie [ENG]

Message par spacecadet »

alpa a écrit :No need for shop, a Dremel is far enough.
That I have :D
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Re: Ghibli de Hongrie [ENG]

Message par alpa »

spacecadet a écrit : What about slightly reshaping the combustion chamber. It's much harder to do consistently and measure precisely, so not a good option in practice, but would it work in theory?
I think these heads are much more valuable than pistons that can be easily custom made.
spacecadet a écrit : Custom head gasket would also only work by using two different thicknesses on the two sides, 2 cc difference -> ~0.4mm which also doesn't sound like a good option.
0.4mm is nothing. People use two composite heads gaskets with a copper gasket in the middle and then run 2bar boost or even more.
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Re: Ghibli de Hongrie [ENG]

Message par spacecadet »

Thanks for the suggestions. I will then also ask around for custom head gasket.
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Re: Ghibli de Hongrie [ENG]

Message par alpa »

spacecadet a écrit :Thanks for the suggestions. I will then also ask around for custom head gasket.
I really don't see the point: you can not reuse pistons yet another time and skiming them is simple and free. Custom gaskets are expensive so why would you do that ?
If you are scared to disbalance the engine then think that many of biturbo engines have up to 5gr weight difference across the stock pistons.
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spacecadet
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Re: Ghibli de Hongrie [ENG]

Message par spacecadet »

I wasn't aware of that. Also didn't measure mine. I was expecting tighter tolerances because of the 4 different weight classes of rods and crank. Although the pistons have a smaller rotational mass and more reciprocating I guess that's why they were less conscious about them.
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Re: Ghibli de Hongrie [ENG]

Message par alpa »

Yes there are classes for rods, they are heavy things.
Pistons and rods are MUCH lighter on Ghibli but the crank is the same as before ! We compared 2.8 18v and Ghibli cranks: same weight.
Don't tell me you are building a perfect engine while you have used 40 years old rings and you have a chamber disbalance.

If you are really worried about weight balance then remove some weight from the piston skirts on the other bank.
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Re: Ghibli de Hongrie [ENG]

Message par spacecadet »

Found the two topics http://www.maseratitude.com/viewtopic.php?f=7&t=1666 and http://www.maseratitude.com/viewtopic.php?f=56&t=5645, lots of very useful and interesting information.

Yes, I was planning for the perfect engine. I know that rings are a slight compromise, but I wasn't expecting this disbalance issue at all :|

I might find a second hand head pair here, still waiting for the answer. Based on that I'll decide which way to go.
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Re: Ghibli de Hongrie [ENG]

Message par spacecadet »

I'm pretty late with this, but after going back and forth wit the solutions, the cc volume mismatch was solved by minor adjustment in the cc itself. This brought everything in the required ±0.5 cm3 diff. And the heads can be reused in the future without having to touch any other part.

Image

One of the touched up cc

Image

As visible these were already touched up slightly in the factory, on some only machining marks are visible, but on some there are rougher hand made marks

Image

Heads are now installed, valve clearance measured. I have the new shims, they are just waiting to be installed in the coming weeks. Then set the timing and pretty much done with the engine.

Image
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Re: Ghibli de Hongrie [ENG]

Message par Konimino2 »

Beautiful to see :Maze
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Re: Ghibli de Hongrie [ENG]

Message par alpa »

Why not to machine the chamber. I'd rather unshroud exhaust valves instead of removing between them. These angles just above valves restrict the flow, removing them does not hurt.
But as I said I'd machine piston tops.
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Re: Ghibli de Hongrie [ENG]

Message par spacecadet »

You mean to bevel / blend the edge right above the seats? I guess that would help for the flow, but not for my nerves if doing this after the valve job (wasn't done by me but still) :) This was a simple and fast solution. Just like the piston tops as you said, but I didn't wanted to disassemble the block again.
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Re: Ghibli de Hongrie [ENG]

Message par alpa »

spacecadet a écrit :You mean to bevel / blend the edge right above the seats? I guess that would help for the flow, but not for my nerves if doing this after the valve job (wasn't done by me but still) :)
I don't see in what it would be different for your nerves from what was done.
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