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[JamesV6CDX]

The 2.5 uses a composite head gasket, the 2.6 a metal one.

I know the heads are the same between a later 2.5 and a 2.6

The metal gasket on my 2.6 seems a lot thinner than composite ones I'm used to fitting on 2.5 / 3.0

Does that mean the 2.6 runs at a higher compression?

I'm guessing for this reason, you couldn't fit the metal 2.6 gasket to a 2.5, as it would make the compression too high? Or likewise a composite gasket on a 2.6 would make the compression too low?

Any info gratefully received

[Diamond Black Geezer]

I can't claim any experience in this field, sadly. However, you stating the potential compression change made me think - have you therefore possibly discovered a cost-effective way of raising compression, thus power?

Stripping down an engine to replace a head gasket is a job anyone can do in a garage, skimming a head down by xxxthou. isn't. I know people have raised compression on the V6s, no idea for what gains, though.

[JamesV6CDX]

I can't claim any experience in this field, sadly. However, you stating the potential compression change made me think - have you therefore possibly discovered a cost-effective way of raising compression, thus power?

Stripping down an engine to replace a head gasket is a job anyone can do in a garage, skimming a head down by xxxthou. isn't. I know people have raised compression on the V6s, no idea for what gains, though.

There was a discussion about fitting 2.5 heads to a 3.0 (or 2.6 heads to a 3.2) to raise compression but it was deemed the compression may be too high for 95ron unleaded

[Diamond Black Geezer]

Ok... just to appear slightly idiotic, but I have no shame, so here goes 

Why would the fuel octane affect/be linked to compression? You're suggesting that fancy doo-dahh 99* would be required? I'd have imagined the opposite effect, so for a higher compression, ideally a lower octane, less volatile fuel be used. Like if I tried my V6 on ethanol/alcohol, it would rev and burn itself up pretty quick?

Just interested to lean, is all 

[Temetsy]

2.6 and 3.2 have lower compression ratio than 2.5/3.0.(10:1 vs 10,8:1 IIRC) Ethanol can be used if compression is so high that regular gasoline starts to tick. Ethanol has in fact higher octane than regular gas but it has to be injected ~28% more to get the optimal burning since ethanol contains a lot more oxygen molecules compared to gasoline. Not sure how goes fitting newer v6:s' heads on the older ones but fitting 2.5 heads to 3.0 do raise the compression ratio to something around 12:1 and then higher octane fuel OR ethanol conversion is most likely needed.

[Marks DTM Calib]

I worked out the compression ratio increase once and it took the 3.2 close to that of the 3.0 so no issues with excessive compression ratio (plus the knock sensor will adjust).

The discussion was about fitting late 2.5/2.6 heads to a US 3.0 (which have lower CR's) or late 2.5/2.6 heads to a 3.2.

All to recover the CR to the levels found on stock 2.5 and 3.0 engines.

[aaronjb]

Ok... just to appear slightly idiotic, but I have no shame, so here goes 

Why would the fuel octane affect/be linked to compression? You're suggesting that fancy doo-dahh 99* would be required? I'd have imagined the opposite effect, so for a higher compression, ideally a lower octane, less volatile fuel be used. Like if I tried my V6 on ethanol/alcohol, it would rev and burn itself up pretty quick?

Just interested to lean, is all 

As Temetsy implies, octane rating is the other way around - it's a measurement of the fuels resistance to pre-ignition, essentially.. so the higher octane is, in effect, less volatile. (Kinda, sorta, but you get the idea)

[Diamond Black Geezer]

Ahh, that makes sense. Always thought of Diesel as 'more primitive' fuel therefore less combustible - it's further down the 'chain' of what's made from crude oil. But of course diesel needing to only be compressed to combust means it's more volatile, not less. Now I'm remembering a program where they threw a lit match onto a pool of aviation fuel, and the fuel just put the match out. (totally different to every Bond film you've ever seen).

Oh, that feeling when a large jigsaw piece clicks into place! 

[terry paget]

As they all have knock sensors I imagine you can fit either gasket, but the engine will not run quite as intended with the wrong gasket fitted.

[Kevin Wood]

Ahh, that makes sense. Always thought of Diesel as 'more primitive' fuel therefore less combustible - it's further down the 'chain' of what's made from crude oil. But of course diesel needing to only be compressed to combust means it's more volatile, not less. Now I'm remembering a program where they threw a lit match onto a pool of aviation fuel, and the fuel just put the match out. (totally different to every Bond film you've ever seen).

Oh, that feeling when a large jigsaw piece clicks into place! 

Not necessarily. Diesel engines have a lot more compression than petrol.

[aaronjb]

Ahh, that makes sense. Always thought of Diesel as 'more primitive' fuel therefore less combustible - it's further down the 'chain' of what's made from crude oil. But of course diesel needing to only be compressed to combust means it's more volatile, not less. Now I'm remembering a program where they threw a lit match onto a pool of aviation fuel, and the fuel just put the match out. (totally different to every Bond film you've ever seen).

Oh, that feeling when a large jigsaw piece clicks into place! 

Indeed, Diesel is a lot less volatile than petrol, but has (if memory serves!) a higher energy content per unit of volume than petrol.. So much more compression is required (especially as, as you rightly point out, ignition is via compression only) to get it to go bang - which is why spraying Ether into a diesel engine might get it going but at the expense of very banana shaped con rods (Ether is much, much more volatile than either petrol or diesel and therefore goes bang far too early)..

[Diamond Black Geezer]

Ether in a diesel engine you say?

Hmmmm..... (now thinking of a-where I can buy ether, and b-are there any people who have wronged me in the past who drive diesel car?)

Just theorizing, you understand..... 

[Kevin Wood]

Ether in a diesel engine you say?

Hmmmm..... (now thinking of a-where I can buy ether, and b-are there any people who have wronged me in the past who drive diesel car?)

Just theorizing, you understand..... 

Just pull the glow plug relay and the AA will do the rest when they turn up.

[05omegav6]

Ether in a diesel engine you say?

Hmmmm..... (now thinking of a-where I can buy ether, and b-are there any people who have wronged me in the past who drive diesel car?)

Just theorizing, you understand..... 

Just pull the glow plug relay and the AA will do the rest when they turn up.

Ah, good old fashioned plausible deniability...

[aaronjb]