2.5 V6 24V more capacity | Pistons etc. - Alfa Romeo Forum
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Old 04-11-13 Thread Starter
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2.5 V6 24V more capacity | Pistons etc.

Hi all,

I have been crawling through the search and google to find an answer on possible capacity upgrades for the 2.5 V6 24V upgrades but I wasn't really successful.

The question is;
What can I do with the 2.5 V6 engine to increase the capacity without dropping all the compression?
(Don't get me wrong, I know the 3.0 plug&play upgrade and both engines are actually sitting in front of me :-) Anyway, I wanna do something special not many people have)

I first thought o.k let's re-bore the block to fit the 3.0 pistons but that seems to decrease the compression and is more something for turbo projects I guess.

What about the 3.2 pistons in a re-bored 2.5 block with the 2.5 crank and 3.2 Cams? Is anyone here that has experience with this who can tell me what options I have to increase the capacity with original engine parts?

Thanks
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Old 04-11-13
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I would use the 3.0 and bore that out to a 3.5, much better end result and more special. Ahm do kit but it's still a lot of work to get up and running.
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Old 04-11-13 Thread Starter
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Thanks for your reply...

Isn't there a single option for the 2.5? I love this engine :-)
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Old 05-11-13 Thread Starter
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What about 3.0 pistons in 2.5 block with 2.5 rods and a "thinner" head to get the compression ratio back? Does anyone have experience with this?
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What about 3.0 pistons in 2.5 block with 2.5 rods and a "thinner" head to get the compression ratio back? Does anyone have experience with this?
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Keeping the 2.5 crank, you can take the engine to 3.3L
For a start, you don't "rebore" the block, it is a wet liner engine and needs larger liners and the crankcase machined to suit.
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Hi, thanks for explaining

Do you mean keeping the 2.5 crank with 3.0 pistons in the 2.5 block or switching to the 3.0 block with a 2.5 crank?
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Quote:
Originally Posted by cciech View Post
Do you mean keeping the 2.5 crank with 3.0 pistons in the 2.5 block or switching to the 3.0 block with a 2.5 crank?
Either way is the same result, just using the 2.5 block means a lot of complicated and expensive machine work.
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Quote:
Originally Posted by cciech View Post
I know the 3.0 plug&play upgrade and both engines are actually sitting in front of me :-)
Don't try to do something that no one did it before, it will be too expensive and time consuming, just fit 3.0.......
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What I am going to do is to take the GTA intake cams, letting the 3.0 valves fit onto the 2.5 head and work the whole intake and exhaust system.

That should bring me above 200HP. Wanna stay with my 2.5 and if there is no other way to increase the capacity I will just replace the bearings and maybe lighten the flywheel in the block.

If anyone is sitting on a oversize kit or similar please ping me.
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Crankshaft data | different bore sizes

Here some data about bore and crank

2.5 v6 24v
Bore 88mm
Stroke 68.3mm
Capacity 2492ccm
23KG part-nr 96412856 <-- same as 3.0

3.0 v6 24v
Bore 93mm
Stroke 72.6 mm
Crank 23KG part-nr 96412856 <-- same as 2.5

3.2 v6 24v
Bore 93mm
Stroke 78mm
Capacity 3179ccm
Crank ONLY 14,3KG part-nr 96412916

GTA crank fits into 2.5 block and gives about 0.2ccm more if I got this right

I might have an offer for a GTA crank for 250EUR. Hard to imagine the Alfa engineers saved 11Kg on the crank but it is listed like that in two different alfa part stores. Imagine the effect of such a crank in an 2.5 engine when going from 0-100 or 60MpH
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Sorry to herald the bad news, but they are all ~23kgs, inlcuding the GTA one.

Btw there is no reason to stick with 2.5 unless you like her so much you would merry her ... (or it's a some motorsport division requirement)

Btw2. 2.5 and 3.0 are not the same

2.5V6 96412856
3.0V6 96412855
3.2V6 96412916

Last edited by yan.ko; 05-11-13 at 19:09.
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Old 05-11-13 Thread Starter
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What source are you using? Some of the part numbers I checked back with the GTA camshafts did not match up at all :-)

Till now I used eper.fiatforum.com but they seem to su.. badly.

Can anyone tell me if both, the 3.0 and the 3.2 crank increases the stroke in any way or not? I still did not find any answer to that.
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the latest ePer.

Sure the 3.0/3.2 camshaft increases stroke over the 2.5 one. You already post it above.
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ah o.k so I am finally clear in all things :-)

Thanks a lot to everyone who replied and helped me! It is much appreciated. Not many people wanna invest money in a 2.5 so I am very happy to have all data now :-)

Big Thanks
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Old 09-11-13
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Keeping the 2.5 crank, you can take the engine to 3.3L
For a start, you don't "rebore" the block, it is a wet liner engine and needs larger liners and the crankcase machined to suit.
Quote:
Originally Posted by cciech View Post
Hi, thanks for explaining

Do you mean keeping the 2.5 crank with 3.0 pistons in the 2.5 block or switching to the 3.0 block with a 2.5 crank?
Neither.
I mean if you keep the 2.5 crank, you can machine the case to take oversize pistons & liners.
The same size pistons with a 3.0 crank would give you 3.5L
And with a 3.2 crank would give you 3.7/3.8L

Obviously an expensive route...
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So if I got you right I can leave the block, liners and pistons original but put a 3.0 crank into the 2.5 engine which will give me a 3.5l engine?
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No.
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Old 10-11-13
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No, then you would get 88mm bore and 72.6mm stroke (resulting in 2650cm3). That's the very basic calculation. And you'd like to mod your engine?

Last edited by yan.ko; 10-11-13 at 13:54.
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I just wanted to get confirmed the 3.0 crank fits plug and play ;-) The rebuild has already started and I will post the results once done
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You can't just fit a 3.0 crank with your original 2.5 pistons & rods....... The longer stroke would result in the pistons hitting the head...
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I thought using the rods and crank from the 3.0 with the pistons from the 2.5
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there's no telling where you end up in compression ratio if you mix'n match like that. you'll know if you compare pistons between the two as there could be a height difference.
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Old 11-11-13
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The converse can work too if you are interested in a nice turbo application. Put the 2.5 Crank in a 3.0 block will give you a low compression (9.5:1) ratio 2.8L motor. But you have to lengthen the conrods by 4.3mm (the difference in stroke), the other way - by shortening conrods by 4.3mm you end up with 10.9:1 ratio

Last edited by Gertie; 11-11-13 at 07:07.
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10.9 ratio would be nice for an atmospheric motor with high duration cams
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