De-catting

I'm off to Powerflow in the morning to ask their advice on getting a de-cat pipe made up for my 156 V6. It currently has a SuperSprint backbox and a new mild-steel centre pipe. There is only one lambda probe (1998 V6), which is before the cat, so technically removing the cat shouldn't cause any ECU issues. Obviously it will go back on for MOT testing..

Couple of questions:

1) Will I notice much in noise/boom from the exhaust? Now the cat on the V6 is massive, it looks more like a silencer, is there a silencing effect "built in" to the cat?

2) Will I lose back pressure and therefore lose torque low down in the rev range. This isn't ideal as I come from a JTD and the V6 does lack torque for my liking..

3) Will I notice any significant power gains top end?


Cheers,

Dan

Pud,you should get a huge improvement however it is always best to get a before and after dyno test so you really know.Your ECU will take a 100 miles or so to fully adjust so I would drive it for a while before you retest it.

Cheers Kanga. The engine is already running a Superchip (for a 2.5 V6) and a Dastek Unichip in series which is mapped for the bigger displacement V6. Confusing or what!

I'd love to get it dyno'd before and after but it is quite a bit of extra expense for something I should be able to feel.. I'm thinking a de-cat pipe should cost me only £50 or so..

Some cars react well to de-catting and some do not. I'm not entirely sure for your application. Either way for a road car the gain/loss is going to be pretty negligible (Say 2-5 bhp either way and not a massive peak or trough in torque).

For a race application it would be removed anyway as their are other factors to consider, and if a loss was made, it could be compensated with improvments to the rest of the system.

Don't expect your car to be transformed or ruined following this procedure, the effects are probably going to be incredibly minor. You may gain a touch extra noise, i'm not sure, i'v only got experience of a standard system being switched for a complete front to back race jobby so the difference was stark.

Sam

Thanks Sam, interesting you think there will be only a minor improvement, I thought this originally but got my hopes up after Kanga mentioned there will be a huge improvement!

He did notice a great improvement removing the cats on a 159 3.2, but I'm sure the 156 was far less restricted to begin with than a 159.

Like i say its dependent on the car. If Kanga has a 159 then thats a newer car so i'd imagine the exhaust to be more restrictive than the 156. The only way you'll know for sure is to RR it before and after but like you say thats an expense you could do without.

Exhausts are an absolute minefield on N/A engines and whats good for one car may not be for the next. Unless you're gonna spend big bucks on a pulse tuned gas scavenging all singing all dancing affair then the result is going to be hard to determine until you've got it done!

Good luck anyway Sam

Pud,Monstro if I remember correctly said de-catting his 147GTA made a huge difference and de-catting was a lot better than hi flow cats.For 50 pounds you really have nothing to lose if the gain is insignificant and a lot to gain if it is.I would do it.Also I did not de-cat I just replaced the origional cats with high flow cats and that on its own transformed the car so I will be surprised if you don't get a big gain,but I am no expert so maybe I am wrong.

I just replaced the cat section on my GTV 3.0 V6 with a straight-through decat 2 days ago. It seems more eager at low revs, but idle is less smooth (hope it will settle as ECU relearns). I have standard middle section with 1 silencer and straight-through rear (0 silencers instead of 2) so that car was already pretty noisy. However, with the decat it is much louder and I don't know if I can live with it. I am hoping I have not got a proper seal and a bit of tweaking may help, but I think it is just much much louder.
Weight of the decat is much lower too - I would say at least 4kg less.

Fantom, did you have yours made up, or is it readily available?

cats are very effective at damping noise, in fact for Belcar racing cars the rulebook suggests to fit the cat if the car fails noise tests...

Originally Posted by phil-gtv6

Fantom, did you have yours made up, or is it readily available?

Off the peg from Alfaholics. Comes with mounting holes for lamba probes. I think they do a CF1, CF2 and CF3 version. Mine is CF3.

Originally Posted by Cuore_Sportivo_155

cats are very effective at damping noise, in fact for Belcar racing cars the rulebook suggests to fit the cat if the car fails noise tests...

Oh dear. Sounds like I am going to have to revert to cat or go deaf. I prefer the former.

I have a 98 V6 Pud. I went from a stock exhaust system to a complete stainless de catted system in one go. I noticed no power loss or grunt in the low gears and a marked improvement in 4th, 5th & 6th gear. It just wants to pull & pull.

Karn,I thought that would be the case.People (JUG) keeps harping about needing back pressure otherwise you lose torque,it's nonsense,they wouldn't make hi flow cats and free flowing exhaust systems if that were the case.When I changed my system with hi flow cats and a free flow exhaust the car pulled like a train.

Originally Posted by Kanga

Karn,I thought that would be the case.People (JUG) keeps harping about needing back pressure otherwise you lose torque,it's nonsense,they wouldn't make hi flow cats and free flowing exhaust systems if that were the case.When I changed my system with hi flow cats and a free flow exhaust the car pulled like a train.

Jug is right (in theory anyway), if you don't have enough back pressure you do lose torque at low revs (to do with exhaust gasses not properly leaving the system or something)

High-flow cats and free flowing exhaust systems are primarily for track cars and performance cars, where it doesn't matter about low-end torque cause your always at high revs, where the free-flowing exhaust will improve power (by maintaining peak torque higher through the rev range)

The question is, without the cat is there still enough back pressure to maintain the torque at low revs? From Karn's response, it seems there might be. And bearing in mind mines a modifed 3.0 running through a (partially) stock 2.5 exhaust system, I don't think lack of cat converter will negatively affect torque at low revs (at least noticabley anyway)

But I've decided I'm going to wait until after MOT time (June) before I get this done

wether there will be enough "backpressure" or not after decatting, depends entirely on the cam timing. Do you know the cam timing for your engine? a lot of valve opening overlap will result ion less torque in low revs but more Bhp (and thus torque) at high revs, and most importantly it'll stop the power from making a nose dive after peak Bhp.

A restrictive exhaust will mimick the effects of small overlap...

Pud in theory correct,but in reality modern cars are very restrictive due to emissions and sound requirements.With pre cats in the manifolds,restrictive ceramic low flow cats(400 cells per square cm )compared to metallic metal hi flow(200 cells per square cm) and restrictive exhausts.An example can be had with the latest Aston Martin Vantage;LARINI make news cats for them and hp increases by 32,a free flowing sport exhaust and a further 26 hp increase.I would say nearly all road cars are restricted because of emission regulations and noise regulations.

Originally Posted by Kanga

Karn,I thought that would be the case.People (JUG) keeps harping about needing back pressure otherwise you lose torque,it's nonsense,they wouldn't make hi flow cats and free flowing exhaust systems if that were the case.When I changed my system with hi flow cats and a free flow exhaust the car pulled like a train.

Kanga,
the general rule is that restriction on the exhaust imposes pumping loss on the engine ao by removing it you increase the effective work of the engine. The exception on the rule though is that if you have an engine with large valve overlap then you would lose torque at low rpm as the A/F mixture would escape through the exhaust valve. Of course at high rpm this cannot happen and therefore you take full advantage of the reduced pumping losses.

Originally Posted by Kanga

Pud in theory correct,but in reality modern cars are very restrictive due to emissions and sound requirements.With pre cats in the manifolds,restrictive ceramic low flow cats(400 cells per square cm )compared to metallic metal hi flow(200 cells per square cm) and restrictive exhausts.An example can be had with the latest Aston Martin Vantage;LARINI make news cats for them and hp increases by 32,a free flowing sport exhaust and a further 26 hp increase.I would say nearly all road cars are restricted because of emission regulations and noise regulations.

This is a vintage 156 V6, so no pre-cats in the manifolds and only one metallic cat.. Centre pipe is unsilenced also.

Anyway, my mind is made up. I am going to de-cat but only after MOT-time

Pud,let us know the result when you do it.

Originally Posted by Kanga

Pud,let us know the result when you do it.

Will do. Need better brakes before I need more power anyway.

Originally Posted by Pud237

This is a vintage 156 V6, so no pre-cats in the manifolds and only one metallic cat..

Surely you mean ceramic cat?

Originally Posted by Cuore_Sportivo_155

Surely you mean ceramic cat?

I was under the impression the cat converter in the 156 was metallic, as in if you take a cross-section of the cat itself there would be a spiral-honeycomb sort of thing, made of thin wire, which got hot with time and combusted any unburnt or partially burnt hydrocarbons as they passed through..

Is that not the case?

Pud,it may or may not be,ceramic cats clog up where as metallic don't,due to price a lot of manufacturers would use the cheaper ceramic.When they are new they would be equal,however the manufacturers would be twice as restrictive;ie 400 cells compared to 200 and also smaller,hence less flow.