MBC or not to MBC

[_Madman_]

Ok, I have finally connected my boost gauge, the maintenance is done, intake is done and MBC is sitting comfortably in the boot.

But... I after little driving around today I'm confused about if I should put the MBC in or not.

The thing is that my boost gauge goes all the way to 1.1 bar without a hick up when car is not heated up real bad, and that is with stocker BCS. When it's warm (after multiple 1/8 mile launches when UIC pipe is hot enough that you don't want to keep your hand on it for long) the boost drops to 0.6 bar.

Then the vacuum, the good vac should be -0.6 bar, as far as I understand from searching, but I'm idling at -0.7 and it goes to -0.8/-0.9 at deccel...

What the heck is going on here? Is the ECU adjusting wastegate operation to dump extra air out at high temperatures?

Do I have a boost leak? But then again, how can vacuum reading be lower than what it's supposed to be at stock?!

 

[_Madman_]

Common people... I have a drag tomorrow

OK, back to topic, I actually started to think that it might be the fuel that caused the problem with boost drop. I was filling 91 (95 EU) octane in my car all the time and today when I was driving on the track in extreme heat boost quickly fell to 0.8 bar. I don't know why it was 0.6 yesterday... Maybe I mixed something up.

The 93 is the best fuel for extreme heat and side mount right?

Now I filled the tank with 93 (98 EU) and I hope that it might fix it. Although I don't know how fast the ECU will relearn new timings. After fillup boost was dropping only to 0.9, although I wasn't on the track, only driving intensively in the city.

Should I connect MBC tomorrow before drag or not?

 

[crimsondragon]

On decel, the vacuum should drop below what you would normally have at a stop. For example, most DSMs at idle stop have 19 vacuum. On decel it can go as far as 25.

To answer why you have less boost when you're warmed up, basic physics here. Cold air is denser. Hot air is less dense. I assume you're running the T-25. You're running 16 psi which is practically the limit of the T-25. Any more increase in boost is blowing nothing but hot air. Your stock sidemount has been heatsoaked at this point and is not compensating enough for it. The turbo is pushing in hot air which is less dense and will be less pressurized because it has reached its max efficiency.

As for which fuel to use, the higher the octane, the better.

If you want full control of your boost, hook it up. If not just stick with what you have and run off the wastegate spring and BCS.

How did you manage 1.1 bar? Are you on the stock turbo or not?

 

[_Madman_]

Yes i think I have a T25. And it goes to 1.1 bar when cold just fine, I don't know how... Maybe autogauge gauges are pos? The boost drops at higher RPMs, but 1.1 is attainable.

The funny thing is that the exhaust is still mostly stocker with cat in it...

Then again the car is JDM, everything is written in Japanese, not a singe symbol I can understand The ECU is labeled "Enginnering sample" and some hoses on intake manifold are missing when compared to other DSM's. The turbo might be different, but I cannot check that now.

But how so, if the air is less dense, then turbo should compress it easier and attain same boost pressure in the end, isn't that so?

 

[delta448]

On decel, the vacuum should drop below what you would normally have at a stop. For example, most DSMs at idle stop have 19 vacuum. On decel it can go as far as 25.

Wow 19inHg at idle seems like a lot to me. Granted, my engine probably needs a rebuild soon; I'm seeing about 15 inches of mercury vacuum at about a 900-950rpm idle -and that's even with stock cams. The higher the idle rpm, the more vacuum there should be, stock idle should be 750-850rpms. Tim's engine must be above average in health.

How did you manage 1.1 bar? Are you on the stock turbo or not?

The funny thing is that the exhaust is still mostly stocker with cat in it...

The stock exhaust is so restrictive to flow that the T25 is able to reach 16-17psi because the engine isn't seeing a good volumetric efficiency and isn't even using all of the airflow that little turbo provides. If you drop the cat and catback you'll quickly see how more flow reduces the T25's peak boost level. You're not going to get both moderate flow and moderate boost with the T25, it's just too small.

The turbo might be different, but I cannot check that now.

This would be interesting to know...

 

[_Madman_]

As far as I understand I will have to look at intake wheel blades to tell which one it is right? There are no markings for a quick check, right?

 

[delta448]

The serial number on the turbo's compressor housing can tell you. The T25 I think says T25 right on it, but can be easily identified by the compressor outlet which has a bend in it that points the outlet toward the passenger side of the car. However, so does a T28.

Mitsu turbos, except for the 20G and larger, use a J-pipe to route air to the intercooler. The turbine housing is stamped with 060 if you have an unmolested 14b, and 070 if you have a 16g. The 14b has all compressor blades at the same level, equally spaced. A 16G has paired blades with a slight curve to them, the T25 has paired blades with a larger curve.

The compressor serial number should be something like 49178-0(XXX)0 if it's a mitsu turbo, where XXX is replaced by: 103=14b, 520=s16G, 420=b16G and 470=E316G.