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Ghibli de Hongrie [ENG]

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Re: Ghibli de Hongrie [ENG]

Messagede Froggie » Lun 3 Déc 2018 11:22

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]

Messagede alpa » Lun 3 Déc 2018 14:55

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]

Messagede alpa » Lun 3 Déc 2018 14:56

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]

Messagede Froggie » Lun 3 Déc 2018 16:51

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]

Messagede alpa » Lun 3 Déc 2018 17:59

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]

Messagede Froggie » Lun 3 Déc 2018 18:10

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

Messagede alpa » Lun 3 Déc 2018 19:41

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

Messagede spacecadet » Lun 3 Déc 2018 20:26

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]

Messagede spacecadet » Lun 3 Déc 2018 20:27

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]

Messagede alpa » Lun 3 Déc 2018 20:44

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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