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[Diamond Black Geezer]

However, if you can get around this (be it rejigged ECU, altered valve timing, or I'm just talking genital-located spheres) then if you get a 3.0, you will want to flog your cats to help fund the engine swap, as these are a bit sought after (2.6/3.2 not the same, much cheaper made) 

[elltee]

To say I'm cocking the whole thing seems harsh. I understand that most NA engines are designed with some back pressure requirements . Just thought I'd ask.

[mantaray]

I think that you're actually cocking up the whole thing. Massive difference between the CIH straight 6s of Carltons, versus the V6s... the V6 was designed to have cats from day one. The straight 6s weren't. Something to do with how the back pressure of an engine does actually help combustion, and there whole thing is tuned, with exhaust valve opening times and so forth,  to work with the reduced flow that having cats fitted causes.

It is said that Omegas (Cavs, Calibras, Vectras, Signums) run worse when decatted. 

Don't know about the above, but I am running a 2.5 V6 in my Manta A. It is a standard engine and ECU. EGR removed, lightend fly wheel, custom headers and exhaust, G cams.
Last time I had her on the rollers, she was booked at 215bhp.

[biggriffin]

Dbg does get a bit carried away some times, you won't need cats on it, with two middle boxes and a tail pipe, be enough back pressure,.
Use 3.0L ecu., you will need the key transponder, and ring, too, unless you have access to tech2.
There's a complete set-up here in hoof-land. Contact serek.

[Diamond Black Geezer]

eh? 

Genuinely thought that the idea of de-catting the V6 was a well-trodden topic, every bit as much as crank sensors etc, and the final consensus was leave the cats where they are. In fact I seem to think I was one of the people that asked this, and the idea was refuted as being inadvisable.

Apologies to elitee and others if 'cock it up' was too strong a phrase, that was not the intention, I literally just meant it's meant to affect the running.

[anV6]

3.2 inlet with injectors or fit 3.0 and the FPR from a 2.6/3.2 (slight increased rail pressure, worth having as it costs nothing)
3.2/3.0 inlet divider plate
2.5/2.6 heads which must be ported to match the inlet divider (2.5/2.6 heads have smaller ports than the 3.0/3.2) and 3.0/3.2 exhaust valves (sodium filled)
3.0 cams
3.2 bottom end
2.6/3.2 exhaust manifolds
2.5/3.0 front pipes and cats
3.0 electronics including DIS pack ecu etc with EGR blanked/removed and SAI removed


I have seen the above "recipe" posted several times. Has anybody actually done it? What was the bhp and torque gain? Bottom torque affected?

[anV6]

Should I guess no for an answer to my above question, given the silence?

[Diamond Black Geezer]

Possibly no-one has gone to the trouble of a complete engine re-construct, however all of the above has been done (at least in part) by more than one member on here, with good results.  I appreciate your point though  - "ummm... so no-one actually done it, then guys??" 

Depends on your starting point, of course. Start with a 2.5 and you've got most of the above to do.... start with a 3.2 and you've already got most of the above.


Ok, breaking it down as to the reasons why each is suggested as the 'best' of its type...

3.2 inlet with injectors or fit 3.0 and the FPR from a 2.6/3.2 (slight increased rail pressure, worth having as it costs nothing) - as per
3.2/3.0 inlet divider plate - bigger ports to match the heads, see below
2.5/2.6 heads which must be ported to match the inlet divider (2.5/2.6 heads have smaller ports than the 3.0/3.2) and 3.0/3.2 exhaust valves (sodium filled) - to match the divider plate, see above
3.0 cams - higher lifting - same as Courtney's high lift 'racing' cams
3.2 bottom end  - the biggest displacement
2.6/3.2 exhaust manifolds - better flowing pressed steel, versus rough cast
2.5/3.0 front pipes and cats - better quality, more metals, no extra 02 sensors which do break
3.0 electronics including DIS pack ecu etc  - 'proper' HT leads, not separate coil packs 1 per bank as on later engines
with EGR blanked/removed and SAI removed - SAI is pointless, most have already binnned it, EGR much the same, later cars do away with it.


But this is, to a degree very much a 'chop n change / pick n choose' list. If you just want to do a 2.5 with G cams, go for it, if you just want to do a 3.2 with earlier ECU/HTs, go for it, and so on...   

[anV6]

Nice breakdown. 

2.5/2.6 heads which must be ported to match the inlet divider (2.5/2.6 heads have smaller ports than the 3.0/3.2) and 3.0/3.2 exhaust valves (sodium filled) - to match the divider plate, see above

But the actual reason for the 2.5/2.6 heads is to increase compression?

It seems that apart from the cams and electronics, if one had both a 2.5 and 3.2 he would have all the parts. I would guess the 3.0 cams is easy enough to find. The 3.0 electronics I'm not sure.

And apart from the head porting, everything else seems plug and play or direct bolt ons.

But the big question is, how much more bhp and torque would these mods give. If we are talking like 10bhp then I wouldn't say it's really worth it. So it would be interesting to know or even see an educated guess.

[Diamond Black Geezer]

Ahh, yes I forgot that, 2.5 heads do raise the CR on a 3.2 block, yes.

In answer to your big question... I suppose this is where we wander into the grey area of theory and practice...it's a case of lots of small differences add together to make a...slight difference.  But by and large they're cheap/free and really doesn't hurt.


Right, from what I've picked up on here...
3.2 inlet with injectors or fit 3.0 and the FPR from a 2.6/3.2
3.2/3.0 inlet divider plate
2.5/2.6 heads which must be ported to match the inlet divider (2.5/2.6 heads have smaller ports than the 3.0/3.2) and 3.0/3.2 exhaust valves (sodium filled) - well, all the above is really in 'one package' all just mean larger intakes, easier breathing engine. The higher FPR does nothing, in itself. More fuel does NOT equal more power! However this is a 'free' mod which I suppose means you definitely won't be underfuelling at FT. The ECU will back off the injectors' squirting time to compensate for the higher fuel pressure until happy anyway. Some advocate leave the original FPR in, some say it doesn't hurt. Maybe it's pub car park bragging rights? Dunno.
3.0 cams  - these are the one true component that offers actual, real power increase. (though allegedly at the expense of lower end torque, no such thing as a free lunch)
3.2 bottom end - no replacement for displacement, as the saying goes.
2.6/3.2 exhaust manifolds - if you're freeing up the intake, paired with free-er flowing exhaust does make sense.
2.5/3.0 front pipes and cats - this may be a mod recommended for their longevity, rather than bhp-increasing properties?
3.0 electronics including DIS pack ecu etc - this leaves you free to fit multi-core super duper HT leads etc.
with EGR blanked/removed and SAI removed - just removing redundant systems, weight(!) and enginebay space-saving only.


the G cams alone move the (167bhp) 2.5 to the MSD / GSi Vectra spec (192bhp). With exception of the 3.2 block, all the above would theoretically bring that figure a shade higher, nudging 200bhp maybe, but not over. (there are those who claim over 200bhp... I've no desire to argue, because, I've done none of it, so it could well be the case, but remember the 3.0 produces about 207bhp with its 500 extra ccs.

As for starting with a 3.2 block in the first place, I can't really say, because most of the above is already there, hell, it might be 212bhp or something! Certainly if you stick with a 2.5 block it's more 'tunable', from 167 to 'about' 200bhp.... or, as has been said many a time, start with a 3.0/3.2 in the first place and you're 90% there. But there's also 90% less you can do.

Hope that's ironed out a few things. Might have raised more questions, though! 


Might be worth opening up the floor to OOFers who have actually done some of these, and on what engines etc, see what they have got/rolling road figures etc? 

[Doctor Gollum]

3.2 crank is a must as it's forged and therefore stronger...

And just to put the idea out there, how about rebuilding a 3.2 with 2.5 conrods and a supercharger...

[Nick W]

3.2 crank is a must as it's forged and therefore stronger...

And just to put the idea out there, how about rebuilding a 3.2 with 2.5 conrods and a supercharger...

It's only a must if the cast cranks are weak. Who has broken one? Take a 318 Mopar V8 and tune it to 500hp, which can be done reliably with a stock cast crank and rods. You can fit the dimensionally identical and stronger 273 forged crank, but the cast ones just don't break. It's only worth doing if you have one lying about.


The list of parts are the 'best' bits, but better seems to be largely subjective. Things like injectors or higher pressure regulators are only worth considering as part of a system. Any benefits are likely to be minute.


Has anyone done this 'engine build' properly and actually measured the implied improvements? Considering how many parts from different engines would be required, has anyone assembled one at all? Without free access to a huge pile of bits, it's probably cheaper to use properly developed tuning parts from companies like Kent Cams, and/or a competent head developer.


The only thing I've done to my 3.0l is to fit the later exhaust manifolds. They make the engine rev slightly quicker over about 3500rpm, but I doubt there's much of a power improvement. I fitted them because they were cheap when I was about to replace my head gaskets; the hassle of acquiring and fitting them outweigh the benefits on a road car, and I wouldn't bother again.

[Doctor Gollum]

Better is indeed subjective,  but re the cranks, it has previously been suggested that the 3.2 crank is slightly more durable for tuning purposes...

Tuning meaning anything mechanical that might make a two ton saloon a smidgen quicker

[anV6]

I'm having a bit of a hard time following this with all the abbreviations and also some of the technical terms. I guess I've been here too long and I'm forgetting some of the English terms. And to be honest lots about mechanics I have learned here so might never had become familiar with the English names.

the G cams alone move the (167bhp) 2.5 to the MSD / GSi Vectra spec (192bhp).

What are the G cams you speak of here?

As for starting with a 3.2 block in the first place, I can't really say, because most of the above is already there, hell, it might be 212bhp or something! Certainly if you stick with a 2.5 block it's more 'tunable', from 167 to 'about' 200bhp.... or, as has been said many a time, start with a 3.0/3.2 in the first place and you're 90% there. But there's also 90% less you can do.

These mods would be interesting to try only if I end up getting a 2.5 manual for not being able to find a 3.0 manual. Then I would definitely want to tune the 2.5 because IMO 200bhp is a bare minimum for a car the size of the omega. So if I'm tuning a 2.5 I might as well try these mods. But I think I would definitely get a 3.2 and use the 3.2 block. I don't want to stay with the 2.5 block just to struggle to get 200bhp.

But if I find a 3.0 manual I will be less inclined to try the above. Because I would then need more parts and maybe not even gain much after all the mods.  On the other hand, starting from a 2.5 and buying a 3.2 basically gets me everything on the list. So it would be easier to try, especially that I will not be happy with the stock 168bhp from the 2.5 no matter what. So even if the mods would not get me much I could always just swap in the whole 3.2 engine and get the 215bhp it produces. Although since when trying the mods I would be starting with the 3.2 block anyways I think the mods would not chop way from the stock 215bhp. I might just end up with 216bhp. 

Hope that's ironed out a few things. Might have raised more questions, though! 

Might be worth opening up the floor to OOFers who have actually done some of these, and on what engines etc, see what they have got/rolling road figures etc? 

Well, it's definitely opened to OOFers who have actually done the mods.  Unless I need to post somewhere special for them to see it?

But I'm thinking there might indeed be "considerable" power to be gained with the 3.0 cams and 2.5 head. I mean, the raised compression ratio from the 2.5 head must count for a few extra BHPs right?

Although if the 3.0 cams would mess with lower end torque I would rather skip it. Especially if the bhp gained is low. Lower end torque is what is needed most of the time, unless you need a motorway cruiser, which I don't. For around town driving lower torque matters more.