406oc.co.uk

Hi everyone, ive just got hold of a Xantia Activa and joined this forum because engine wise, its pretty much identical to the 406 Sri Turbo and i have a number of questions that need answering.

Firstly, my idle is a bit lumpy. i'm told this is normal, and because of the size of the valves and the low compression not surprising, but the spark plugs dont look that recent so i intend on changing them. What is the O.E part number? I am fan of NGK plugs and dont believe in all these fancy electrodes on such a basic design.

Although it goes ok and pulls well from 1500rpm indicating the turbo does actually exist, i want to plumb in a boost gauge, what boost can i expect to see, and from what rpm? i.e when does it hit maximum boost.

I will be removing the boost modulator valve in favour of a manual boost controller, hoping this will help the turbo spool a little earlier and increase the maximum boost, i've read 12psi is a good limit, i assume the ECU will compensate for any extra fuelling required at this level?

Thanks for your help in advance.

Gareth

Xantiaman wrote:Hi everyone, ive just got hold of a Xantia Activa and joined this forum because engine wise, its pretty much identical to the 406 Sri Turbo and i have a number of questions that need answering.

Congrats on the buy - I adore that engine! The whole car's pretty much identical to the SRI and Exec Turbo from what I understand, apart from the suspension.

Firstly, my idle is a bit lumpy. i'm told this is normal, and because of the size of the valves and the low compression not surprising, but the spark plugs dont look that recent so i intend on changing them. What is the O.E part number? I am fan of NGK plugs and dont believe in all these fancy electrodes on such a basic design.

I just replaced mine (when I had one) with a set of Bosch from Halfords which worked fine. The idle on them is a bit unsteady, but it can get iffy with time. A good thing to do is attack the idle control valve and piping with some carb cleaner. It's the cylindrical thing bigger than a spice jar smaller than a can of Lynx that's probably in front of the throttle body, near the radiator.

Although it goes ok and pulls well from 1500rpm indicating the turbo does actually exist, i want to plumb in a boost gauge, what boost can i expect to see, and from what rpm? i.e when does it hit maximum boost.

You'd know it if it weren't boosting - out accelerating a double decker bus becomes a challenge. The thing with this engine is it dies flat at about 4250rpm - if you're flooring it, change at 4k. The boost is low and will probably barely register on most gauges, but the lump's useless without it

I will be removing the boost modulator valve in favour of a manual boost controller, hoping this will help the turbo spool a little earlier and increase the maximum boost, i've read 12psi is a good limit, i assume the ECU will compensate for any extra fuelling required at this level?

I don't know anything about this, apart from the fact that the engine left stock will last forever (when mine went to the scrappers at 180k the engine was just as good as it was when I bought it at 100) but overboosting and changing the turbo will drastically reduce its life

Thanks for the reply.

I hooked a boost gauge up tonight and was seeing a very max of 9psi but 8 to 8.5psi average, boost is all over the shop with the BMV under instruction from the ECU to keep it low under some circumstances, i am running unleaded and some pinking can be heard so i suspect this doesnt help with the smoothness.

Saw good boost from low rpm as to be expected with such a small turbo, 8psi around 1800rpm.

I removed the valve though and fitted a MBC, much better boost response throughout the rev range and increase in torque, maximum steady of 11.5psi and small spike to just over 12psi when it hits full boost, should be safe enough from what i have read, the compression ratio is definatly low enough to cope with that boost but the turbo is just too small to make power and just blows hot air so bhp is not its forte!

I will have a look at the idle speed control valve, is it usually quite noisy as i can hear it hissing quite loudly?

What should the spark plug gap be? My plugs are in good nick but wanted to make sure they were gapped within tolerance.

Gareth

I'm no expert on the 2.0T but I remember Niz saying that the boost varies according to load! in 1st and 2nd you get 6psi (for example) and 3rd, 4th you get 10psi etc.

By all account the boost control is pretty fancy on these, I think it works on what speed your doing and deduces you must be in first gear accelerating etc etc.

For sure, i understand the boost will vary to flatten the torque curve but the end result is it feels like its being constantly strangled, fortunatly, its now much better!

Xantiaman wrote:
I will be removing the boost modulator valve in favour of a manual boost controller,

any info on what you used and how it was setup? an easy mod?
cheers

I just used the cheapest controller off ebay. Just set it to what i wanted and left it. Had to tweak it a bit recently as the cooler weather gives slightly higher boost. Its running 12psi happily.

Xantiaman wrote:I just used the cheapest controller off ebay. Just set it to what i wanted and left it. Had to tweak it a bit recently as the cooler weather gives slightly higher boost. Its running 12psi happily.

I've been running 12psi for the last 18,000 miles without issue. I've yet to fit my AFR gauge but I believe the car is mapped at all loads up to the max resolution of the MAP sensor so I can't see there being any problems, providing you keep up regular maintenance. I also fitted a temp sensor just before the throttle body and even in the middle of summer, that much boost wasn't lifting intake temperatures hugely high, so the intercooler is coping just fine.

I had a Xantia Activa a while ago, got it very cheap, changed a driveshaft and away I went, my impression on it was the turbo wasn't particularly noticeable but from when I read up on them I gather you can up the boost pretty easily giving around 200BHP.As a preferance looking back on it now as thier similar money I would go for the newer shape Saab 900 Turbo.

Xantiaman wrote:I just used the cheapest controller off ebay. Just set it to what i wanted and left it. Had to tweak it a bit recently as the cooler weather gives slightly higher boost. Its running 12psi happily.

"CT" = Stands for Constant Torque.

The engines limit boost due to the notion that it provides a constant torque spread across the rev range - It's not one of those "boyracer" turbo skyline pssssstch psssstch engines.

Think more Saab "Ecopower" when it comes to power delivery!

XUD wrote:my impression on it was the turbo wasn't particularly noticeable

You evidently never ran the car in limp-home mode or with the turbo otherwise non-functional. The car's barely able to move without it

mjb wrote:

XUD wrote:my impression on it was the turbo wasn't particularly noticeable

You evidently never ran the car in limp-home mode or with the turbo otherwise non-functional. The car's barely able to move without it

no perhaps not, mine just didnt feel very quick though, but in fairness i got a dog, i much prefer the Saab FPTs for cheap turbocharged motoring

To be honest the delivery of the turbo's so smooth that the only time you'll notice it (very hard driving excepted) is when it's wet and you want to join a road/roundabout sharpish from a standstill - it can bite you with a hefty wheelspin if you're not sharp enough to catch it