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Old 02-06-2019, 08:44 AM   #1
tomasss
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Default MBC general questions

The boosted world is still not very clear to me in some points. When I had S70R some time ago, the wastegate hose had burst and as the pressure was leaking I bent all five rods. That set my mind to be very careful when it comes to boost.
Now I want to put an MBC on my B204GT. From what I understood there are generally two engine limitations on the stock setup while increasing the boost:

- jetronic fuel cut, caused by too high reading of the AMM. Can be solved by updating the firmware (chips)
- lean mixture, caused by injectors not capable to deliver more fuel to keep the AFR. Can be solved by bigger injectors/pump.

But still, even if I have chips and bigger injectors, there is third limitation...the mechanical strength of the engine itself.

Can we talk some numbers? From other topics I understood that on stock engine (13mm rods) it is safe to run around 14-15psi of boost, with the chips and injectors it goes usually somewhere around 20psi. Is that the boost where it becomes mechanically dangerous even if it has good mixture? Other threads I have been reading says 30psi..

Also I remember that the mechanic reparing my S70R told me that in low RPM the engine can take less boost while on high RPM it can take almost whatever boost. But the MBC has no idea about the RPM...is there a risk if let's say I tune it for 20psi and then I hit the gas in low rpm?

The object of the questions is B204GT '90...
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Last edited by tomasss; 02-06-2019 at 09:22 AM..
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Old 02-07-2019, 12:03 PM   #2
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How mucho psi can stock support?....get ready for this
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Old 02-07-2019, 01:58 PM   #3
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Well you have to think about the engine as a system. I would first be kind of clear as to what goals for power you have in mind. The mechanical imitations are kind of a known factor but your desired usage, budget, skills will help people here make suggestions. Then you can optimize the boost controller to what setting will work for you.
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Old 02-07-2019, 06:55 PM   #4
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Well...the question was so general and theoretical intentionally.
What I am curious - suppose the late redblock engine itself is stock (rods, pistons etc), then what is the risky boost for the low rpm. Suppose the dummy user hits the gas from the idle.
Is it safe from the mechanical point of view to run the MBC set to those 15 - 20psi? When bad things starts happening?
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Old 02-07-2019, 07:48 PM   #5
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In a general answer it would be no. That is not a safe area of boost at low rpms.

With the stock setup it's generally accepted that if you run 93 premium gas and your engine is stage zero you can go to about 11-12psi max.

You can buy helpful things like a wide band oxygen sensor to help you figure out where your limit of boost is.
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Old 02-08-2019, 04:46 AM   #6
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Sure, wideband will help me make sure I am not running lean mixture, but not with the boost itself.

If safe boost at low rpm's is as you're saying 11-12psi, does that mean that all the people here running MBC set at 14-15psi are so aware of this that they always hit the gas only once they reach certain rpm? And if they coincidentally forget that once, they bend their rods?
Or is it the case that at low rpm the turbocharger doesn't build enough boost anyway, so no matter how hard you step on the gas pedal it will be safe around idle?
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Old 02-08-2019, 05:50 AM   #7
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The turbo on my B230FT 940 (16T) generally doesn't build up sufficient boost under like 2.500-3.000rpm anyway. If I go WOT at low RPM's, there's no boost available until this point. I don't really know top of my head where this exact point in terms of RPM is, but around there somewhere.
I do try to stay away from boosting at low RPM's however, let's say I'm in 4th going 60kmh and I need to make a quick acceleration, I will downshift to 3rd. If I just tap the pedal lightly, I don't worry too much about this, since I never go into boost anyway.

I'm running around 0,90bar (13psi) with the 16T.

I think it helps being cautious about this and keeping that in mind, at least I do.
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Old 02-11-2019, 11:27 AM   #8
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Ok, I see. The stock T3 turbo on my engine builds up the boost sooner, but still should be ok. Once it rolls I will keep an eye on my boost gauge.
Thanks
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Old 02-11-2019, 02:05 PM   #9
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I actually had a bit of a closer look at the boost buildup yesterday and I was a bit wrong initially. The boost on my car actually does start to build up a bit sooner, like around 2.100-2.200 and is at full 0,90bar at around like 2.5-2.6k.

But like I said anyway, I try to not boost too much under 3k generally. Not based on anything, just my gut feeling on how the engine "feels" and sounds under a certain RPM range.

Don't take my words for granted though, I know very little about engines vs. boost compared to a lot of folks here, so maybe someone else can chime in with their knowledge. Just telling my experiences
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Old 03-11-2019, 06:44 PM   #10
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B204GT with stock turbo won't build noticeable boost under 3000rpm. So you'll be safe there. As you're ditching electronic boost control for mechanical, you'll probably loose a bit of spool there as well, but it will come harder once it does.

I've run mostly stock B204GT's at around 1.6bar with enough fueling (remember what happened to your S70!) for a long time and they didn't mind. Though stock turbo runs out of efficiency and starts acting like an air heater above ~1.3 bar.
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