Activa boost - A detailed explanation

XantiaMan · Unread post by **XantiaMan** » 21 Jul 2008, 19:26

After much searching on the internet i could find very little on the subject of boost on a 2 litre Activa, be it maximum boost, when it hits full boost at low revs and how the boost modulator does its job.

This evening i plumbed in a temporary boost gauge just before the ECU.

My car is believed to be untouched in this area and the results i got were to be expected.

In lower gears, maximum boost of up to 7.5psi was seen, hitting this maximum at around 2000rpm. In the higher gears, peak boost of 9psi was seen once, but generally this was around 8 to 8.5psi maximum, dieing off more over 4500rpm to about 7psi

So basically not alot of boost for an engine that runs a low compression ratio!

Alot of the 'flat' feeling i was getting which i understand is a trait of the engine was the boost in the mid range reaching no more than a peak of 5-6psi, after a few seconds boost would then climb a bit higher to 8.5psi but its really all over the place with the standard boost control valve.

I am running unleaded and i reckon this contributes to the flat feeling, as audible (to my ears) pinking can be heard at low revs, the ECU must be detecting some knock when it comes on boost, deliberatly tells the boost modulator to keep maximum boost low and retarding the timing, once this point has past the boost would then rise to maximum again.

As for boost building up low down, its not bad at all, 5psi could be seen from about 1800rpm in most gears so i left the actuator well alone rather than nipping it up to help with the boost build.

Once i was confident of the amount of boost my car was running, i removed the BM valve pipes, keeping the valve plugged in and tucked into the corner of the engine bay, in its place is a manual boost controller which i got from ebay for less than a fiver 8) I refined it somewhat by reducing the relief hole size to .8mm and cutting one coil from its spring.

The BM valve relief pipe which goes to the induction piping (the standard valve is basically an electronic bleed valve, bleeding excess pressure back into the inlet system) was then plugged off with a bolt and jubilee, and i then proceeded to do some testing...

After a few tweaks i was seeing a peak of just over 12psi, MBC's spike a little especially if they have not bedded in, after a few miles i noticed a difference and boost would quickly climb to 11.5psi and stay there till well over 4500rpm.

The end result is quite a bit more poke in the lower gears, and more mid range, its performance on 95 unleaded will never be that good as you can still feel the ECU advancing and retarding but once it gets its act together there is a useful increase. It now feels more like a Rover Vitesse, a car thats very similar in weight, but has 180bhp, its easily a second quicker to 60mph.

I dont see much point increasing the boost closer to the boost cut off, for one there is a grey area on the integrity of the gudgeon pins, the engine is definatly capable of alot more but the turbo is just too small for outright power and thats not what i'm after. A modified turbo with bigger compressor housing and wheel would see large gains and getting rid of the car would help further too.

I am getting 31mpg without trying too hard and the improvements of the boost coming on a bit sooner may help that more, increasing volumetric efficiency and making it pointless going over 3k in normal driving to make good progress.

It will never be a rapid car, its weight and long gearing are one of the main reasons but the simple addition of a boost control valve has helped release some more of the cars potential.

Gareth

Unread post by **CitroJim** » 21 Jul 2008, 20:19

That's an utterly brilliant piece of work Gareth

Excellent, a thouroughly enjoyable and insightful read!

Firstly, it was very pleasing for me to know in retrospect that the engine in yours is healthy and was producing the expected level of standard tune boost.

Secondly, it explains completely to me the characteristic of the engine in standard tune. It seems to modulate the boost all over the place

I'd be interested to fit a boost gauge to mine and see iof the MK1 does it any differently.

Your Activa definitely has a different characteristic to my new one, I double-checked this evening that my eyes were not deceiving me and that the MK2 throttle body does not have the big "shaped lump" on the throttle butterfly as found in the MK1. Mine is altogether more genteel in its power delivery and whilst it has the urge when the throttle is open, it is a lot more progressive then yours. I'm still trying to work out exactly what effect that lump will have. It's on the lower inner part of the butterfly and covers about half the area of the lower half. I'll take a picture of it in a bit. Perhaps you can throw some light on what it's real purpose is? I have one of each so a swap may be carried out one of these days to see the difference.

If you wind the boost up much further, with such a small turbo, you run the risk of surge. Are you going to fit a recirc. valve?

31 mpg is good Gareth and it is good to see that the fact that a 2.0CT in good fettle can be reasonably economical. Mine is proving to be just about the same, economy-wise. I've been thouroughly enjoying mine these past couple of days

So, when is a bigger turbo and Emerald management going on then

You're right, the Activa is never going to be a real road burner or a threat to a Scoobie but that is not it's niche. They're really a true Grand Tourer, able to cover distance quickly and in quiet comfort, something it does very effectively

XantiaMan · Unread post by **XantiaMan** » 21 Jul 2008, 20:32

The lump could be either a deliberate restriction, or something to do with emission control. SU carbs on some models used to have a poppet valve for this purpose. The throttle cam will also make a significant difference to the feel of the engine, i've already taken the slack out of the cable on mine to help with response.

Although the turbo is small i dont see surge as being a problem, if i did fit a DV it would only be a recirc but i doubt i would notice too much difference in normal driving as truth be told, it wont be driven hard but its nice to know its there.

Emerald is a good bit of kit, but the only major expense i would consider is an LPG conversion.

It does make me laugh when i read about how rapid the Activa/406 Sri Turbo is, but it stands no chance against cars like an Impreza/Clio 182/similar quick car in a straight light. Corners would be a different matter altogether as the Activa suddenly loses about 500kg of its weight and becomes nimble and responsive

Gareth

MikeT · Unread post by **MikeT** » 21 Jul 2008, 20:57

Nice write up Gareth, good work. I assume this is all part of a bigger picture or something? 8)

When the head is off my xantia, I'll be looking more closely at how my turbo works and modifying it slightly in the same vein. I'd like to know more about surge, doesn't sound like a good thing - how does that work? Is it a risk of underfuelling or too much pressure?

One thing I have to question though - if you're getting pinking as stock, is it wise to then up the boost levels or are you confident in the ECU/knock sensor relationship?

I'd also like to know why the piston pins are a concern, does the engine have a reputation for failing?

Jim, my guess is it improves airflow at low engine speed making better torque.

XantiaMan · Unread post by **XantiaMan** » 21 Jul 2008, 21:08

The pinking is mainly to do with the mapping of the ECU rather than the boost level, as you get it low down and under load, at these speeds boost is not high.

Running a higher octane fuel should get rid of this, as Citroen actually recommend 98 ron fuel for all their petrol cars.

I often get a Tesco fuel voucher and using this to get 99 octane works out the same as getting 95 ron without, so i wil try next fill up or give it a dose of millers octane booster.

I've read about the pins being a weak point, but also read the engine internals are good for 270bhp! something that would not happen without a hybrid, head work, better exhaust and aftermarket mappable ECU. Thats not the direction i am heading with this, i simply want to untap some of the engines potential whilst retaining decent mpg and refinement.

Unread post by **CitroJim** » 21 Jul 2008, 21:15

MikeT wrote: I'd also like to know why the piston pins are a concern, does the engine have a reputation for failing?

No, just the opposite

They have a reputation for being practically bomb-proof in standard tune, on the bottom end at least. They also carry their head gaskets pretty well too. After all, it's just the good old XU engine and I believe the petrol bolcks are more or less identical to the diesel ones. XU engines are big, robust old-school engines and the basic design goes back a bit now. They first made an appearance in 1982 or thereabouts.

There is a legend, expostulated on other forums, that the bottom end is good for 250BHP but that needs to be taken with a pinch of salt Mike.

This is my understanding of surge in a turbocharger Mike.

Surge is where (and correct me if I'm wrong Gareth

) the turbo is producing a given (high) amount of boost and the compressor is unable to continue spinning at speed due to the pressure the boost generates downstream; it is unable to develop sufficient power to covercome the downstream pressure it has already delivered. This slows up the compressor, boost falls, it speeds up again and so on and so forth. It can be damaging to the turbo as during surge it can overspeed and stall in a very short stretch of time and this can break blades.

A quick closure of the throttle can also stall the compressor and this is why dump valves are uesd in high boost conditions to dissipate boost on throttle closure and allow the compressor to remain spinning at speed.

Stalling is never a problem on diesels due to the absence of a butterfly but surge could be if you really wind up the boost. I reckon for all intents and purposes on a 1.9TD it will not be a problem.

MikeT · Unread post by **MikeT** » 21 Jul 2008, 21:57

XantiaMan583 wrote:Running a higher octane fuel should get rid of this, as Citroen actually recommend 98 ron fuel for all their petrol cars.

Ah, I see! I was going to ask if super-unleaded would cure it and would the ECU be able to take advantage of higher (than specified) octane.

MikeT · Unread post by **MikeT** » 21 Jul 2008, 22:23

citrojim wrote:...They also carry their head gaskets pretty well too.

Oh, really?

That's upped the ante for a possible cracked or corroded head on mine then.

citrojim wrote:Stalling is never a problem on diesels due to the absence of a butterfly but surge could be if you really wind up the boost. I reckon for all intents and purposes on a 1.9TD it will not be a problem

Cheers, you second guessed my intention to modify my turbo but you know how quickly my projects develop!

I'm just gathering information when I can.

Toby_HDi · Unread post by **Toby_HDi** » 21 Jul 2008, 22:43

Very informative is this. Very interesting too.

I really do have the urge to sample an Activa

XantiaMan · Unread post by **XantiaMan** » 21 Jul 2008, 22:55

After a run out tonight in the damp evening air i can confirm the boost increase has made more of a difference than i first realised, it had no problem showing my sisters Fabia VRS how to take corners and then accelerate with the minimum of fuss, but alot of that comes down to my driving

I would suggest that anyone who owns an Activa CT to check the boost and get rid of that BM valve, it does nothing except slow the car down and make it feel like a gutless NASP engine. Even setting the boost to a constant 9-10psi would see the engine within standard specs but improve the response and feel.

Xaccers · Unread post by **Xaccers** » 21 Jul 2008, 22:57

Would it be an idea to leave the battery disconnected for a while to reset the ECU?

Unread post by **CitroJim** » 22 Jul 2008, 08:01

Gareth,

I (and I suspect several others) would be most interested in full details of the boost controller you are using. I guess you've left the old Boost Modulator in place doing nothing but keeping the ECU happy.

Toby_HDi wrote: I really do have the urge to sample an Activa

You'll be hooked if you do Toby! Gareth is so right about the car seeming to loose 500kg of weight as it goes into a corner

XantiaMan · Unread post by **XantiaMan** » 22 Jul 2008, 09:30

The boost controller i use is identical to this one -

http://cgi.ebay.co.uk/TURBO-BOOST-CONTR ... .m14.l1318

I didnt buy mine from this seller but i do know him and had a chat about the valve, having used many in the past and he really does use this valve on his own car.

Out of the box it should work fine for most, its important the valve replaces the original BMV and placed as close to the actuator as possible to reduce spiking.

The same valve fitted to my Maestro works well but suffers more from spiking because it has alot more piping, something which i will have to work on.

There are other valves available and i will stress these are not bleed valves even though they look similar, these are much better because they block the signal reaching the actuator, which helps faster spool up.

A normal set up will have the actuator seeing boost all the time and it will start to creep open a few psi before the actual boost setting i.e it will start to open at 3-4psi and fully open at 7psi.

In stopping the boost, the actuator will not open until the preset pressure of the valve has been overcome, in my case a smidge under 12psi.

In most cases boost holds longer than it does on a standard set up so you gain all over the rev range.

Here is another valve, but alot more expensive - http://www.gb-ent.com/index.php/Boost-c ... flyer.html

It does contain a ceramic ball bearing intended to speed up response, i've used a few of these in the past and cannot fault them, but then i dont think the extra price is justified.

Unread post by **RichardW** » 22 Jul 2008, 10:56

"Surge is where (and correct me if I'm wrong Gareth ) the turbo is producing a given (high) amount of boost and the compressor is unable to continue spinning at speed due to the pressure the boost generates downstream; it is unable to develop sufficient power to covercome the downstream pressure it has already delivered. This slows up the compressor, boost falls, it speeds up again and so on and so forth. It can be damaging to the turbo as during surge it can overspeed and stall in a very short stretch of time and this can break blades. "

That's pretty well spot on, Jim

On big process plant compressors I deal with from time to time, a lot of time and effort is spent on anti surge controllers (basically just a pipe flowing from discharge to suction with a valve in it) - the effects of surge on a big (we're talking several mega watts here

) compressor can be catastrophic!

Edit: Just remembered a practical demonstration - was on site in Pakistan where a 20MW 5-stage air compressor (think large house sized

) was being commissioned. Following surge tests there was a bit of vibration on one of the stages - eventually traced to damage to the liner in the inlet flexible bellows - now this bellows was in a 12" pipe, the liner was made of 1mm thick stainless, and once removed I could stand on the side of it without it collapsing - and yet the end 3" of the liner were crushed to half the size of the pipe, just like a coke can having been squeezed

AndersDK · Unread post by **AndersDK** » 22 Jul 2008, 15:30

How does it perform in London traffic ?

Quick anyone - where is the exit ?