Saabscene Saab Forum - Saab Technical Information Resource banner

1 - 20 of 36 Posts

·
Registered
Joined
·
262 Posts
Discussion Starter #1
As some of you maybe aware, I have recently installed the Speedparts Stage 1 to my LPT. For a days last week I had been inadvertantly running with a loose hose from the turbo housing, the result of which meant that the turbo was boosting as high as it would go as the APC was inoperative at the time and remained open. Now that the hose has been fitted back on, I am slightly dissapointed with 1st and 2nd gear acceleration. I know that the APC controls the amount of power in the first two gears to preserve the life of the gearbox, however I would just want a little faster spool up on a standing start. I am seriously considering fitting an MBC instead of the APC system. Would I face less problems due to the fact I already have a re-mapped ECU which would allow higher levels of fuelling? Also what do you guys think about MBC's in general and how many of you have them fitted to your own vehicles?
 

·
Registered
Joined
·
4 Posts
I use one admittedly not on a saab turbo, but I have supplied them for use on various other vehicles and find they work well, so I don't see why not on a saab as they seem to use std wastgate actuators and garett turbo units.
The newer prv type valves are a big improvement over older bleed type valves.
I think it is best to run with the factory boost controller disconected and alway use in conjunction with a boost gauge.

cheers
Scott
 

·
Registered
Joined
·
293 Posts
I have an MBC on my 900 Aero LPT and have gained 40 BHP. It has totally transformed the car in terms of driving - it was fun before, now its fantastic. I tend to take it easy in 1st and 2nd anyway - its too easy to wheelspin even in 3rd, but the boost really kicks in at 2200 RPM and keeps pulling through all the gears. It was dynoed recently at Emerald and there were no problems with fuelling.

Welcome to Saabscene Scott
 

·
Registered
Joined
·
14,305 Posts
Saab's APC and later Trionic systems are quite simply the best and safest turbo boost control systems in the world.

If you fit any sort of MBC, you will reduce the ability of the APC system to lower boost under knock, and thus risk damage to the engine. We do know of at least one engine that has been totalled this way.

That's the extreme end. Most Saabs, especially the older DI/APC ,cars will yield a reasonable increase in performance by using an MBC sensibly- but it is a calculated risk.

C900s can generally yield more performance by tweaking the APC box, which can be done for a lot less money than the cost of an MBC, and still reatins protection.

Later model Trionic cars repsond best to a re-mapping of the ECU.

So, in summary, fitting an MBC is crude but effective, but significantly raises the risk of damage to your engine.
 

·
Registered
Joined
·
2,277 Posts
Welcome to Saabscene Scott.

In response to Andyvo, I have used a MBC for the last eighteen months or so including a trackday.
The boost has been set with the aid of a boost gauge to 1.15 bar, around the same as Abbot would set it to for their ECU's one of which my car is fitted with. The only slight hiccup is that when changing up a gear under hard acceleration a boost spike can cause the fuel cutoff to engage briefly, but only if the driver stamps on the throttle pedal rather than squezzing it on. I only discovered this problem when a friend drove my car (he's used to kicking down his auto' CDE), I've never had the problem as my driving style is somewhat different. This may only be a problem with my car rather than a general fault of using a MBC as I havn't heard others complaining .

Nick.
 

·
Registered
Joined
·
262 Posts
Discussion Starter #6
Thanks for your replies guys and welcome Scott
. I already have Stage 1 so was looking to the MBC to speed up the spool up of the turbo and remove the boosty limit in first and second. I am already happy with acceleration in the higher gears so am not looking to wind up the boost to too high levels.
 

·
Registered
Joined
·
4,215 Posts
i used a MBC for over 50 k miles and have just removed it on my last upgrade
I found that it did speed up spool up and gave a lot of low down grunt..
I fitted a dual stage gated mbc from an turbosmart which allowed me to run at low boost (with quick spool up but no risk to the engine/turbo)
and high boost if a porsche was annoying me ...
with a big red switch in the cabin.. to switch it over .
I saw now ill effects from this although it was probably putting more strain on the turbo
my first turbo failed at 145 k so it did not owe me anything
I know of people who ran their MBC at the max all the time and ended up melting their pistons !
this was probably due to the fuelling not being able to keep up with the 315 hp it was putting out !!
I think that if you are sensible and use a "gated type" rather tan a simple bleed valve and put it into the line from the APC to the actuator then you retain some apc function but get the quick spool up
 

·
Registered
Joined
·
4 Posts
Cheers for the welcome always looking in to other turbo cars as well as my rover tomcat.
The saabs seem to make pretty good power only trouble is they weigh too much!
Like I say I have seen no ill effects from running a prv seen mine making a max power of 307bhp on the last r/r shoutout.

As long as you stick to a sensible boost your reliability shouldn't suffer.
If you are worried you can easily turn em down for day to day use and just turn it up for track use

cheers
Scott
 

·
Registered
Joined
·
4 Posts
yeah it is a bit of a handful now although I have ditched the torsion as I kept braking it and have got coilover shox all round!
I have managed to get decent power from it now after fitting forged pistons(cracked 3 of the originals!
Got it timed by redline magazine last week managed some good times
 

·
Registered
Joined
·
262 Posts
Discussion Starter #11
I remember when I was blown away by a 220 Turbo Coupe ( I was only in an 8v Calibra back then), think the result may be a little different now!
 

·
Registered
Joined
·
14,305 Posts
Originally posted by tomcat:
[qb]As long as you stick to a sensible boost your reliability shouldn't suffer.
If you are worried you can easily turn em down for day to day use and just turn it up for track use[/qb][/b]
That's the exact opposite of the advice I would give I'm afraid. In my experience, track conditions are far more demanding that everday driving, with higher temps etc and you are more likely to do damage on a track with prolonged WOT through running too much boost than in day to day driving.
 

·
Registered
Joined
·
247 Posts
Just thought I'd add my comments since some of the claims about people blowing up engines etc. appear to be directed at me Just to set the record straight, my car produced 311bhp with an a/f ratio of 12.5:1 with an MBC fitted. That's lean but not awful. Some people spent a lot of money on turbos, special maps etc etc and were not able to better this figure.

I blew the engine up because I insisted on upping the boost still further while feeling assured that the engine was protected using w/i. In short, I took things too far but I had fair warning. The big problem with the MBC is that it allows fools to make mistakes while Trionic doesn't. Used sensibly, however, it is safe.

Another serious issue which Ylee mentioned is the problematic way the boost comes in. It is brutal to the gearbox.

On the other hand, some people on Saabnet have used the MBC to great effect. It can be a very cheap and effective mod and, once my new intercooler and fifth injector are installed, I will be returning to the MBC. My estimate is that I will be looking at a reliable 320-330bhp for a total outlay of GBP1100 (incl. Speedparts stage 3).

Alanb
 

·
Registered
Joined
·
4,215 Posts
Thank you alan for setting the record straight..
I was aware of some of the facts (but not all )
My view is that a mbc can give you cheap power but take it too far it can be expensive
I for one miss the quick spool up it gives you and seriously considering trying it out with my stage 6...(johnnie will kill me !)
 

·
Registered
Joined
·
247 Posts
More than the ultimate BHP it allowed, it was always the incredible low RPM torque that I liked about the MBC. It was the instant retaliation to any have-a-go WRX/VR6/VTR pilot that was satisfying

Alanb
 

·
Registered
Joined
·
1,676 Posts
Well - for me, it was the other way round. The way APC is implemented on the older cars, it allows humungous low/midrange torque because it operates close to the detonation threshold there where non-electronically controlled wastegate systems will leave a fair bit of leeway there. The 'octane needs' of a petrol engine seem to be highest at or around max torque.
But at higher rpm levels (from ~3,500 rpm upwards) APC severy tapers off boost. Why? Because it simply isn't very good at detecting high speed knock. Therefore, the APC engineers have chosen to drop the boost ceiling progressively with rpm as not to rely on the knock detection circuits to a large extent.

For a standard or slightly modified car, this strategy allows for good driveability characteristics - there's lots of midrange torque for overtaking, so the car is faster in real-world terms than the hp rating suggest.

From a certain level of tuning, however, this interferes with the nature and limitations of a FWD layout and the classic 900 gearbox in particular. There's only so much torque the gearbox can take, and frankly taking the range of gearings available in c900 gearboxes in mind, you couldn't transfer much -if any- more torque at the wheels to the tarmac in the lower three gears anyways (so much for spending $$$$ on 'bulletproof trannies' then ). When that point is reached, you want to concentrate on extending the torque/power curves towards the high end of the rev range instead of creating ever more wheelspinning, gearbox-breaking low/midrange torque.

That, and in my case APC was never very consistent or reliable, even after swapping out several control units, solenoid valves and knock sensors. I dearly wanted to take out one variable out of the equation and so decided to ditch APC and settle for slightly less boost than APC would allow for using a Forge bleed valve type MBC. This gives me for instance an initial 1.05 bar peak boost, settling at 0.95 bar up to the rev limiter. APC OTOH would max out the boost gauge initially, then drop back to a more sensible level.

Instead of maximising turbo boost, I decided to optimise the mechanical side of things, particularly lightening/balancing rotating parts, and airflow at high rpm levels. This doesn't give me the biggest bang for buck if you consider the engine alone, but it allows me to maximise performance within the constraints of my car as a system .
 

·
Registered
Joined
·
682 Posts
The problem with the MBC is the ALL OR NOTHING when the boost comes on,these where a big thing 12-18 months ago on Subaru's with every man and his dog fitting one and being amazed at the extra power.
They failled to realise the implications of incorrect fuelling and wondered why so many engines went bang.As somebody has also said the boost in 1st and 2nd is restricted for a good reason. 'The Gearbox' cant take too much torque and subsequent gearbox failures proved this.

I agree £ for bang they are good but extremely crude.

Remapping is the safest way to go.
 

·
Registered
Joined
·
1,676 Posts
Well yes, but:

* I'm using a Forge bleed valve which offers quite similar spool-up behaviour to an APC system. Having a bigger turbo 'helps' too... ;

* There's nothing to 'remap' with APC as it's just a box of resistors, transistors and diodes spitting out voltage based on knock sensor, pressure transducer and rpm inputs; the 'map' is non-programmable and can only be tweaked with the trim pots and different resistor values to a limited extent - there's no way you can 'map' an exact boost curve based on WYSIWYG. APC doesn't know in which gear the car is, so there's no boost restriction in the lower gears.

What APC provides is optimalisation of boost pressure vs. knock in the low/midrange. It is not advanced enough to detect and act on high-speed knock, hence the tapering off of the boost curve to prevent the engine from entering the twilight zone between effciency and detonation at higher revs in the first place.
As previously noted, in my application I need max attainable boost under 3,500 rpm like I need a pain in the lower back.

* As we speak, the car is at Beek Auto Racing for fueling/DI ignition re-mapping (after the head and intake have been swapped).

We're not talking Trionic or anything similarly advanced (if such a thing exists) here...
 

·
Registered
Joined
·
1,299 Posts
By Eric Van Spelde:
 Instead of maximising turbo boost ...[/b]
and
 This gives me for instance an initial 1.05 bar peak boost, settling at 0.95 bar up to the rev limiter.  [/b]
I don't know Eric, but would 0.95 BAR boost with the Forge bleed valve type MBC not be producing a far more respectable percentage of boost maintenance at the rev limit compared to the APC? In which case you would have INCREASED turbo boost compared to "standard"? If so, then "maximised boost pressure" would be considered by many to have been achieved, even if it was not the intended objective.

Please correct me if I misunderstood you .
 
1 - 20 of 36 Posts
Top