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[Albatross]

I have a 3.2 which has 4 cats; a pre-cat and a main cat, and 4 O₂ sensors. this is 2 of each per bank.

There is a known issue on the 3.2 and 2.6 DBW engines that codes 0420 and 0430 come up to tell you that the pre-cats are "tired", this is a measurement of the difference of the exhaust mixture from the primary O₂ sensors in the series to the secondary.

Moving the secondary O2 sensors "tricks" the ECU, by cleaning the air through the main cats.



I have done this. When I had my stainless steel "cat-back" exhaust system fitted I had them add some O₂ sensor bosses behind the main cats and blanked out the original bosses with plugs. This has solved the initial problem of the 0420 and 0430 codes.

My question is this:

My car effectively no longer needs the pre-cats; would I gain much, if anything, in the way of power by replacing the front section with those from a 3.0 thereby removing the pre-cats?

My thoughts are that the pre-cats must be causing a degree of back pressure and I would have a freer flowing exhaust system without them. That is ultimately why people de-cat track cars and fit "sports cats" to their cars isn't it?

[feeutfo]

i guess this comes down to fully understanding the exact issue with the pre cats? I have a mental picture of a collapsed pile of honeycomb sitting it the exhaust. However from what i have read this not the case i believe. More that the cleaning effect of the cat is no longer within limits...?
I also believe that if the exhaust was, in effect, blocked to any noticeable degree it would throw a code? Not confident in my understanding of all this though...

Ps. Is it poss to slide a scope down the 02 sensor boss and physically see the pre cat condition?

[Albatross]

Personally I don't think that the pre-cats are faulty and I don't care really. That's not this issue.

The question is: Now that I don't need them will I gain a bit of performance by removing them?

[nabsim]

You can remove all the cats but it may cause emission problems, also I believe there are legalities involved.

My understanding is the pre-cats do the heavy gas scrubbing to make life easier for the main cats so removing the pre-cats will shorten the life and efficiency of the main cats. I am sure I have read where people have removed the pre-cats and it made no performance gains but can't remember which vehicles this related to.

I would say the best option would be to change all the OEM cats for sports cats, not sure what the cells are in Omegas but you can get sports cats down to 100 cell. I know this was a big difference to vecs.

Does the Omega actually suffer from poor inlet and exhaust though? I know there are big gains to be had on vecs but I thout the Omegas were much better balanced and more efficient?

[Albatross]

You can remove all the cats but it may cause emission problems, also I believe there are legalities involved.

My understanding is the pre-cats do the heavy gas scrubbing to make life easier for the main cats so removing the pre-cats will shorten the life and efficiency of the main cats. I am sure I have read where people have removed the pre-cats and it made no performance gains but can't remember which vehicles this related to.

I would say the best option would be to change all the OEM cats for sports cats, not sure what the cells are in Omegas but you can get sports cats down to 100 cell. I know this was a big difference to vecs.

Does the Omega actually suffer from poor inlet and exhaust though? I know there are big gains to be had on vecs but I thout the Omegas were much better balanced and more efficient?

Sounds like the answer to my question is "Leave it well alone".

Thanks; I think I will do.

Nathan

[nabsim]

Wait for someone with knowledge specifically of Omegas, only had mine a couple of weeks so my knowledge isnt on these Albatross

[Kevin Wood]

My understanding is the pre-cats do the heavy gas scrubbing to make life easier for the main cats so removing the pre-cats will shorten the life and efficiency of the main cats. I am sure I have read where people have removed the pre-cats and it made no performance gains but can't remember which vehicles this related to.

We're talking about replacing the front pipes with a single cat version from an earlier model, not just removing the pre-cat, so this shouldn't be an issue.

I would say the best option would be to change all the OEM cats for sports cats, not sure what the cells are in Omegas but you can get sports cats down to 100 cell. I know this was a big difference to vecs.

Me too. If you're going to play, might as well do a proper job. Having said that, I'd say the manifolds are probably the weakest link.

Kevin

[nabsim]

OOOOOh, manifolds. Had given up even thinking about them cos of the cost on a vec. Can you actually get them for a 6 pot omega without a mrtgage?

[Kevin Wood]

OOOOOh, manifolds. Had given up even thinking about them cos of the cost on a vec. Can you actually get them for a 6 pot omega without a mrtgage?

There has been talk about them. A guy on the AutoBahnStormers site was looking into getting some made. They are certainly available for LHD cars, but I suspect would foul the steering box on a RHD car. A nice tubular manifold would make a real difference, I reckon.

Kevin

[nabsim]

There is someone makes them for the vec v6's, Milltech did them but I think they had to be bought through Courtenay.

I know Davey Lad at Grassmere vauxhall has done a lot of work with manifolds on the vec, (I think with a 3 litre fitted). Would have thought it easier with the omega being inline, is it the sides of the bulkhead/tunnel fouling thats the problem?

[Albatross]

Would upgrading / balancing the manifolds really help that much?

I'd happily let a place like long-life exhausts build me one if it were going to make that much difference.

Do you think they are the right people or is it better done by an engine tuning specialist?

I'd get the manifold and then front section (with sports cat) done at the same time.

Would they need to lift the engine to fit it in do you reckon? When they did my "cat-back" system, they built it straight onto the car.

[Albatross]

Would upgrading / balancing the manifolds really help that much?

I'd happily let a place like long-life exhausts build me one if it were going to make that much difference.

Do you think they are the right people or is it better done by an engine tuning specialist?

I'd get the manifold and then front section (with sports cat) done at the same time.

Would they need to lift the engine to fit it in do you reckon? When they did my "cat-back" system, they built it straight onto the car.

Any advice on this from anyone?

[Martin_1962]

I think this is a Kevin Wood, ABS, & MDTM subject

[Albatross]

Would a set of pre-facelift M-Reg 3.0 "mid-sections" (From the bottom of the manifolds to the rear of the main cat) fit straight onto a 3.2?

I've been offered a free pair!

Thanks

Nathan

[Kevin Wood]

On the subject of manifolds I would say it needs ssomeone who knows what they are doing to get the best out of them, but anything would probably be an improvement.

The standard manifold is untuned, in that the exhaust ports for each cylinder just terminate into a common plenum with the pipe connected to one end.

The big advantage of a tubular manifold is that you can tune the lengths of the primary pipes to scavenge the cylinders more effectively and isolate the effects of adjacent cylinders. The length of these pipes depends on the engine speed at which you want them to work (there will be speeds where they are poor and speeds where they work well), and the exhaust valve timing.

Kevin