SaabCentral Forums banner
1 - 18 of 18 Posts

·
Registered
Joined
·
290 Posts
Discussion Starter · #1 ·
The boost gauge that I have installed by cutting into the dash gauge feed is reading 10 InHg when the standard gauge is just on the line between white and yellow, it doesnt really matter what gear either.
I thought the turbo would be creating some boost pressure by this point?

Does the reading seem correct to you guys?
 

·
Registered
Joined
·
3,938 Posts
Transition from vacuum to boost will be dramatic. Unless the throttle position demands boost you are going to see vacuum from that feed line. The throttle plate creates a vacuum even if the boost pressure is positive. The boost gauge reads intake plenum pressure NOT boost pressure.
 

·
Registered
Joined
·
290 Posts
Discussion Starter · #3 ·
Thanks for your reply, however I dont really understand what your telling me?
Have I plumbed it into the wrong place?
I just want an accurate boost pressure with which to set the MBC.
Thanks
 

·
Registered
Joined
·
3,938 Posts
You will need to plumb upstream of the throttle plate to read actual pressure delivered by the turbo compressor, however that may show the blips as the pressure spikes and drops momentarily when the throttle position changes suddenly.

However, you actually need to know only the manifold pressure as that is what the intake valves experience which is what fills the cylinder and affects knocking. To get the actual manifold pressure you use the standard mounting point downstream of the throttle plate. This shows the actual pressure effect in the manifold of the boost being produced but not the outlet pressure of the compressor.

It is difficult to see how a MBC can improve on SAAB's automatic system as the automatic Trionic system already delivers the maximum boost the engine can utilize. Raising the boost pressure from there does not result in more power but less.
 

·
Registered
Joined
·
590 Posts
Posted by a1damuk: The boost gauge that I have installed by cutting into the dash gauge feed is reading 10 InHg when the standard gauge is just on the line between white and yellow
Sounds about right (approximately 10 psi) for little or no pedal depression - atmospheric 14.7 psi is a little way into the yellow.

However, what you could do is compare the 2 gauges as the pressure rises above atmospheric (0 Bar), effectively calibrating your car's boost gauge. Here's a simple method...

Disconnect the hose with the 2 boost gauges, connect a tin of ordinary compressed air from any hardware store to this hose, and note the readings on each gauge as you gradually release more pressure into the line.

Then, use the MBC to increase the boost by 3-5 psi - any more and you risk fuel cutoff.
 

·
Registered
Joined
·
3,938 Posts
OK, but how does the MBC beat the Trionic? As I understand it, the Trionic already allows maximum possible boost without detonation so if you increase that level you will damage the engine if the Trionic system is bypassed and succeed only in getting the same boost if Trionic is still active. Since the Trionic is a continuous feedback boost pressure control I fail to see how the MBC can possibly work.
 

·
Registered
Joined
·
590 Posts
Posted by Superaero: OK, but how does the MBC beat the Trionic?
Well, the MBC is certainly not a Saab invention since it circumvents all that Saab tries to do with Trionic.

In my experience, Saab (and Maptun) have built a comfortable safety margin into the ignition/boost controls, because they don't vigorously push turbo owners into using high-octane gas, and they don't want expensive warranty repair jobs.

The MBC, in effect, gobbles up these safety margins, which seem to be about 3-5 psi of boost. The wastegate opens more slowly, the car gains about 20HP and accelerates like a bat out of hell at 3000 rpm. It's a very real effect.

Since I know you use high-octane fuel, put a low-cost ball-and-spring MBC in the APC-to-wastegate line and you will be astonished. For others who use regular (are your ears burning, SaabKen?), the MBC is not a good idea.
 

·
Registered
Joined
·
3,938 Posts
But can you do 300,000 km with a MBC?

Frankly, my tires won't hold the torque my Aero already delivers!

EDIT: not to mention my clutch slips at 2,700 rpm in fifth when the boost gauge reaches the end of the red zone! :cool:
 

·
Registered
Joined
·
307 Posts
im boosting to 18psi, with my t7 apc.
but while my t7apc wasnt working, i was using an mbc, at set it to boost at 18psi, the car seemed to boost quicker, and also held the boost at 18psi (spot-on), where-as the t7apc, fluctuates, (eg. when i hit 18psi it goes down to 16psi then back upto 18psi).

just my two cents
 

·
Registered
Joined
·
3,486 Posts
im boosting to 18psi, with my t7 apc.
but while my t7apc wasnt working, i was using an mbc, at set it to boost at 18psi, the car seemed to boost quicker, and also held the boost at 18psi (spot-on), where-as the t7apc, fluctuates, (eg. when i hit 18psi it goes down to 16psi then back upto 18psi).

just my two cents
Does it fluctuate on a constant throttle position and at a constant rpm.
Does it only fluctuate once you get close to the target speed.

That would make sense to me when using a BPC valve controled by Trionic.
 

·
Registered
Joined
·
590 Posts
Posted by Superaero: But can you do 300,000 km with a MBC?
I'll let you know in a a few years' time. So far, 200,000 and going strong.

How did you get to the end of the red zone without some tinkering? That's MBC territory.
 

·
Registered
Joined
·
3,486 Posts
But can you do 300,000 km with a MBC?

Frankly, my tires won't hold the torque my Aero already delivers!

EDIT: not to mention my clutch slips at 2,700 rpm in fifth when the boost gauge reaches the end of the red zone! :cool:
Yes, I have put a stage II T5 suite tune on my 93 9000griffin, I understand what you mean about the tyres not holding the torque.

They are ok with the original 200hp tune.

So I will have to drive more carefully, it only starts to really get going from about 2/3 throttle.
 

·
Registered
Joined
·
3,938 Posts
When I switched to platinum plugs I saw more boost on the uncalibrated stock "gauge".

Not only do I get clutch slip but I also hit the fuel pressure cut off now and again.

The effect was more pronounced with the 7 heat range. However, I found that the 7 heat range displayed symptoms of fouling after around 12,000 km (they don't look fouled but they still misfire) so I switched to one heat range hotter: 6 and these work better.

I drive at 1,000 meters elevation which will affect the boost gauge since it does not show absolute pressure but only relative pressure. For example, on my 2001 Audi S4 the aftermarket boost gauge shows about 3 psi negative pressure with the engine off. That is approximately how much lower ambient air pressure is at 1,000 meters.
 

·
Registered
Joined
·
86 Posts
Interesting...ive got clutch slip, and im hitting fuel cut with a T7 APC on a stage 3 tune. So, whats causing it? Too much power (if there is such a thing)? Could it be a faulty WG Actuator?
 

·
Registered
Joined
·
3,486 Posts
Without an accurate auxillary boost pressure gauge it is hard to tell exactly what is the issue.

I would suspect something wrong in the boost department, leading to too high boost pressure.

With a hand pump check the pressure the actuator starts to move so you are making sure it is not stuck.

not sure how you check the T7 valve to make sure it is operating properly electrically.

Will have to find that our as I am about to fit one.
However if the boost pressure is right, then a good gauge will show you what you are getting at max boost.

Hope this helps
 

·
Registered
Joined
·
86 Posts
I have got a boost gauge fitted (if it makes it easier to diagnose). I think fuel cuts out at 1.54 bar, boost rises very quickly, and then fuel cut (which is poo )
 

·
Registered
Joined
·
3,486 Posts
I will have to check, and the experts on the ECUProject will know, but:

I think that must be very close to what is supposed to happen.
If it was my car, I would check what the fuel cut out is set to in the map, and make sure the boost request was lower so it could not reach the cut out.

My cars are only set to 16.9 psi mx boost.

I have only reached fuel cut on a test when the car was set to unlimited boost.
That is wastegate shut tight.

Also - a dodgy APC valve can cause boost spikes, do you know if it is a good one. Check Norman Lovies sticky on preparing for an ECU upgrade.
 

·
Registered
Joined
·
86 Posts
My car has had the T7 APC valve fitted, i cant imagine it is that as they are super reliable. Im confused. My brain hurts!
 
1 - 18 of 18 Posts
Top