Partial Throttle Full Boost...

[RedRex02]

When I had my WRX, I know it was a problem when you had a mbc and this happened. Put a mbc on, and at say partial thrttle, it would climb to full boost, and alot of guys said it was terrible for the car. Good way to melt pistons.
In my 2g now, same thing could happen it looks like. One of my buddies told me it was ok for it to happen in this car, but I am not fully convinced. It is kind of a pain to either be wot or way under boost when driving.

I do have my car tuned with the safcII and was just wondering if it's ok(NOT HARMFULL) to be driving and say at 50%throttle or less and be close to full boost?
Jon

 

[andymoraitis]

I agree that this is perfectly normal. If I'm on the highway in a high load gear (5th) and roll on the throttle at 50%, I build 25 psi in an instant. The correct amount of fuel is being injected, as others have mentioned, since the airflow is rising commensurately with boost levels.

As a matter of fact, it makes for a great "passing gear" when you need to move out the way since flooring it in 5th isn't such a great idea.

Hope that helps,

Andy

 

[RedRex02]

Sorry if my babbling is confusing you RedRex02. Listen to the wisemen and ignore my babblings.



If it doesn't make a difference, then 50% throttle would give you the same torque to the wheels as WOT. I can't quantify the pressure drop across the butterfly, but at 50% throttle, it won't be zero, so you'll have less air (either hotter, lower pressure or both) entering the motor. Its whole purpose is to reduce or "throttle" the air to the motor. So I'd have to say that it would reduce flow for sure by decreasing the density of the air that enters the cylinders.

If you consider a chunk of air (some number of molecules), it will have some average pressure, temperature and volume before the TB and some different set of these three afterward. The end result is that the volume this chunk of air molecules fills when it gets into the IM will be higher and the density, therefore, lower. It's the whole PV=nRT ideal gas law thing. In this case, n (number of molecules) and R (ideal gas constant) are constant, so you can only decrease density by increasing V. But you can do this by an infinite combination of changes to P and T. If P doesn't change, T has to go up. If T doesn't change, P has to go down. In reality, both change (P goes down, T goes up), but I don't know the breakdown.

In an extreme case, if you have, say 5% throttle and this, hypothetically, produces enough exhaust to drive the intake system up to 17psi boost in the IM, then you might have such a hot charge that you see knock or hopefully retard timing, because the turbo would maybe be putting out 30psi boost upwind of the TB. Now this is probably too extreme, but illustrates my point (I hope). At less extreme cases, it would still hold true, just not as drastically. This would be a case of maybe wearing your turbo out prematurely. But none of this would happen if his MBC was reading off of the compressor or atleast somewhere between it and the TB.

I'll just think nonstop about this for a week and have some sort of epiphany about some stupid notion in my brain that's keeping me from seeing the light.

Aside from that, if he has the correct AFR and timing for the whole pertinent airflow and RPM range, his motor shouldn't run lean or knock, but he might be overworking his turbo if his MBC is reading off of the IM.

Did that make any sense?

Did it make any sense, since I felel bad you wrote that thing to try and explain, I will say yes.
I will just either go WOT and see all my boost, or just stay under vac.

 

[kenamond]

Did it make any sense, since I felel bad you wrote that thing to try and explain, I will say yes.
I will just either go WOT and see all my boost, or just stay under vac.

Where is your MBC boost source coming from?

 

[MyBeatGSX]

I agree that this is perfectly normal. If I'm on the highway in a high load gear (5th) and roll on the throttle at 50%, I build 25 psi in an instant. The correct amount of fuel is being injected, as others have mentioned, since the airflow is rising commensurately with boost levels.

As a matter of fact, it makes for a great "passing gear" when you need to move out the way since flooring it in 5th isn't such a great idea.

Hope that helps,

Andy

Exactly, my car acts the same.

The point you're (kenamond) missing, is that in a turbo car, the amount of air entering the engine does not directly correspond (sp?) to throttle position. 50% throttle does not mean 50% of possible airflow, because 50% at 10in/hg is WAAAY less air than 50% 20psi. You also don't seem to believe the fact that fuel is delivered based on airflow. Its why we have a MAF.

This part throttle not equaling part load is only a problem when running the bike only Alpha-N based engine management (I don't know of a car that uses it). In that case the engine can't directly see how much air is entering the engine and has to use throttle position as one of the factors in measuring it. Obviously this isn't a problem with our cars. I think we're fairly well covered with the MAF and MPD (which is basically a MAP sensor). The TPS in our cars is only used for transient fuel enrichment (when you punch the gas it keeps it from going lean before the MAF can respond to the air increase).

Whatever way you want to cut the cookie or whatever way you want to get the air in is meaningless. All that matters is the Hz reading on the MAF.

As for destroying your turbo or overheating anything. If the wastegate is opening (which it is if you hit full boost) then you aren't going to destroy your turbo. That means the turbo has all the energy it needs and then some and isn't having problems meeting flow demands. If the gate wasn't opening and turbo is just blasting away like crazy to make ANY boost, thats when you're going to break something.

Think of a VW/Audi 1.8T. Those turbos are so small they're making full boost almost any time the car is under power. My friend's Jetta will hit full boost just gently reving in neutral. Why would VW design a system like that if its detrimental to the engine?

 

[xveganxcowboyx]

Did that make any sense?

Yes, it makes some sense, but I imagine the real world differences are so trivial that it's more of a mind-buster than anything else. Try not to ponder so hard you give yourself a head-ache.

 

[kenamond]

Exactly, my car acts the same.

The point you're (kenamond) missing, is that in a turbo car, the amount of air entering the engine does not directly correspond (sp?) to throttle position. 50% throttle does not mean 50% of possible airflow, because 50% at 10in/hg is WAAAY less air than 50% 20psi. You also don't seem to believe the fact that fuel is delivered based on airflow. Its why we have a MAF.

I didn't say that. Having a butterfly in the airstream decreases airflow regardless of whether that air is boosted or not. I agree that the airflow at higher boost will be higher than at lower boost. And I know that the ECU measures airflow regardless of TPS reading.

As for destroying your turbo or overheating anything. If the wastegate is opening (which it is if you hit full boost) then you aren't going to destroy your turbo. That means the turbo has all the energy it needs and then some and isn't having problems meeting flow demands. If the gate wasn't opening and turbo is just blasting away like crazy to make ANY boost, thats when you're going to break something.

But if the wastegate is being driven by an MBC which is reading from the IM and not the compressor housing, the turbo could be driving a much higher boost level than the MBC is seeing when you're at part throttle.

Please reread my post carefully. Sorry I wasn't clear.

 

[oldman]

But if the wastegate is being driven by an MBC which is reading from the IM and not the compressor housing, the turbo could be driving a much higher boost level than the MBC is seeing when you're at part throttle.

The question here is pressure drop due to the throttle plate being only halfe open. Another words, the engine is out flowing the half open throttle body creating a pressure difference before and after the plate. Interesting and not too hard to verify, hook up an extra boost gauge to pre-TB area and test run with two gauges side by side. Anyone? My car is stripped down at the moment but I will do this, if no one will, after it's back up and running since I have an extra boost gauge. In theory you should be right, all we need now is real life data to verify. Once verify, we can add this to the long list of why the compressor housing should be used instead of the BOV line.

I'm impressed, it takes someone who sleeps and eats DSM to go as deep as this one.

 

[MyBeatGSX]

Ok I get that now.

But it still didn't answer the question whether its bad for the engine or not, which is what this topic is about.

I doubt it would make a big enough difference. The TB causes a pressure drop even when fully open. The IC causes a pressure drop. Manifold runners, valves, ports, etc all cause pressure drops. Piping length and curves create pressure drops. Any little restriction causes a pressure drop. So I'm sure if you add it all up, the TB thing turns out to be insignificant.

So yea I guess we should be sourcing off the compressor housing. But I'll keep mine on the manifold, because I like consistant boost and I want 20psi IN THE ENGINE, not in the IC piping.

 

[oldman]

I want 20psi IN THE ENGINE, not in the IC piping.

Why would where you tap for your mbc change what you see on your boost gauge since it's tapped to the manifold? Are you reading your boost level from your boost gauge or your mbc?

 

[MyBeatGSX]

Well say in the situation here. I want 20psi and I want it now. If the turbo can make 20psi, then make 20psi IN MY MOTOR! Don't stop at 20psi in the outlet, which could be 18-16-15 whatever psi in the manifold. It is a proven fact that outlet or IC pipe tapped MBC's slow spool.

Couldn't temperature also effect actual manifold boost because of the intercooler efficiency (sorry I can't spell that word)? Like on a hot day, wouldn't 20psi at the outlet be less at the manifold than 20psi at the outlet on a cold day? Temperature effects air density correct? So less dense = less boost right? I could be wrong.

I like the SRT-4. It goes one better (more than one better actually) and shoots for an airflow target instead. If its hot outside or you're at higher altitude, it just turns the boost up.

 

[oldman]

Well say in the situation here. I want 20psi and I want it now. If the turbo can make 20psi, then make 20psi IN MY MOTOR! Don't stop at 20psi in the outlet, which could be 18-16-15 whatever psi in the manifold. It is a proven fact that outlet or IC pipe tapped MBC's slow spool.

Couldn't temperature also effect actual manifold boost because of the intercooler efficiency (sorry I can't spell that word)? Like on a hot day, wouldn't 20psi at the outlet be less at the manifold than 20psi at the outlet on a cold day? Temperature effects air density correct? So less dense = less boost right? I could be wrong.

You seem to forget the fact that your mbc is adjustable, what is stopping you from turning the knob a couple of turns so you see 20 psi in the manifold immediately?

What is being discussed here is only one of many, a less significant one in fact, reasons to tap the compressor. I will give you one important and a very likely situation: Let's say a leak or leaks developed somewhere in your intake tract, what do you think will happen to your turbo if you're tapping the manifold? I got even more situations for you if you're tapping the BOV line.

 

[MyBeatGSX]

Yea ok you got me. But as for turning the boost up and down. I don't want to do that depending on what throttle position I'm at. I can't reach my MBC that's zip tied to the radiator fan while driving at WOT (or driving period).

 

[oldman]

But as for turning the boost up and down. I don't want to do that depending on what throttle position I'm at. I can't reach my MBC that's zip tied to the radiator fan while driving at WOT (or driving period).

You mis-understood me, what I said was to adjust your mbc till you see 20psi on your gauge and leave it alone.

But it still didn't answer the question whether its bad for the engine or not, which is what this topic is about.

To get back on topic, this has already been answered many times. It will not hurt your car because it proper amount of fuel will be injected according to the MAF reading and tapping the compressor, the way it should be anyway, will eliminate the possibility of pressure drop ending up hurting the turbo, I will still do the dual gauge test just for the sake of knowing.

 

[CanadianTSi]

Couldn't temperature also effect actual manifold boost because of the intercooler efficiency (sorry I can't spell that word)? Like on a hot day, wouldn't 20psi at the outlet be less at the manifold than 20psi at the outlet on a cold day? Temperature effects air density correct? So less dense = less boost right? I could be wrong.

Yes and this is why it is best to get the source for your MBC at the IM, more consistant boost.

And I checked on the way home tonight, 24psi at 37% throttle

 

[MyBeatGSX]

I knew it! Thanks.


That's what I meant about constantly changing the MBC setting, oldman.

 

[oldman]

That's what I meant about constantly changing the MBC setting, oldman.

Constantly changing your mbc? You truely think that there will be a noticeable difference between a 90* vs. 70* days? If so why aren't we all on CAI? The most I would expect is maybe 1-2psi between winter and summer which probably applies either way. I honestly never have to adjust my mbc at all between last winter and this summer, I think you guys are making a big deal out of a small issue. If you want to talk about consistant boost, do a search on boost spike and see how many are resolved by simply switching from BOV to compressor housing due to short route pipping. With that said, it's your car and you get the last word.

You still have not answer if you're tapping the BOV line? If you insists on using the manifold pressure, at least leave the BOV line alone for the sake of the BOV, find another manifold source aside from the BOV, FPR and brake booster lines, then perform boost leak test on a regular bases.

 

[MyBeatGSX]

No I'm not tapping the BOV line. Neither my MBC nor the BOV are T'ed off of anything, they both have their own dedicated lines.

And yes I agree, using the outlet as the MBC reference does stop boost spike. But it stops it for the same reason it slows down spool. So like you said its personal preferance. I don't mind making the turbo work extra if it gives me the boost I want when I want it. That's what its there for. And don't forget, most of us don't even have nipples on the compressor outlet or J-pipe, so we couldn't run it that way if we wanted to.

 

[oldman]

I'm not understanding this slower spool at all?

Most Garrett compressors comes with 1/8npt tap straight out of the factory to be used as mbc reference source, it will also take no more than 30 mins to tap one yourself.

 

[MyBeatGSX]

When the outlet reaches set boost, which will be almost instantly, the wategate will open. Meanwhile, the IM still isn't up to full pressure. So the turbo is slowing but the entire system isn't fully pressurized yet. It isn't like WAAAY slower spool, but it is noticable. With it tapped in the IM, the wastegate doesn't open until the last part of the system is up to pressure, which as you said can cause an overshoot of the set pressure, but I'll deal with that.

 

[xveganxcowboyx]

But if the boost gauge the pressure is set by is tapped later in the system, the waste gate won't open until the correct pressure is reached:

If the MBC is at the compressor and the boost gauge is at the intake manifold and you set the boost to 17 psi you are setting it to 17 psi at the manifold. The MBC may actually be set to 18 psi, but you wouldn't know that. The waste gate will open at 18 psi which will yield 17 at the intake manifold. (note - numbers are all hypotheticals)

 

[oldman]

If the MBC is at the compressor and the boost gauge is at the intake manifold and you set the boost to 17 psi you are setting it to 17 psi at the manifold. The MBC may actually be set to 18 psi, but you wouldn't know that. The waste gate will open at 18 psi which will yield 17 at the intake manifold. (note - numbers are all hypotheticals)

That is the point I was trying to make.

 

[boostedinaz]

If the MBC is at the compressor and the boost gauge is at the intake manifold and you set the boost to 17 psi you are setting it to 17 psi at the manifold. The MBC may actually be set to 18 psi, but you wouldn't know that. The waste gate will open at 18 psi which will yield 17 at the intake manifold. (note - numbers are all hypotheticals)

Exactly.

The only reason for tapping the MBC on the turbo housing it to get an accurate and stable reading of pressure. That pressure will be what you set the boost gauge too and that boost gauge it usually tapped into the intake manifold.

 

[kenamond]

Just got back from vacation.

So is this the conclusion:

- If you aren't worried about overworking your turbo, tap your MBC into the IM
- If you are worried about overworking your turbo, tap your MBC into the compressor outlet.

?

I see problems with both setups. I've been thinking of this analogy:

Consider a house (intake system) that is several rooms long, all lined up, with a radiator (compressor) in the first room and a thermostat (MBC) placed at one of two possible locations: in the room with the radiator or in the room at the other end of the house (IM). Also, you have a door (throttle) separating the last room from the rest of the house.

If the thermostat is in the same room as the radiator, that room will be kept at the temperature setting on the thermostat (compressor not overworked). However, the room on the other end of the house will be colder. If you care about the temperature in the last room most, then you could turn up the thermostat until you get the desired temperature in the last room as long as the door is open. But if the door to the last room is closed, you'll never get it warm enough. You can get the correct temperature in the last room by opening the door, but it will take time for the heat to make it there (lag).

If the thermostat is in the last room, that room will end up the correct temperature, but the rest of the house will be hotter. If you close the door to the last room either partially or fully, the radiator may be running full tilt (overworking the compressor), but when you open the door, the heat is right there ready to heat the last room (less lag), but there may be too much heat flowing in to the last room leading to a higher temperature than what you want, so the radiator will turn off, but the effect will take time to reach the last room, so you end up with a period where the last room is too hot (boost spike).

Now when you go back to turbochargers, another thing I see as a potential problem with an MBC off of the IM is surge. Since the TB restricts airflow, you could be demanding 20psi from your compressor at very low airflow which would put you into the surge region of the compressor map.

Furthermore, if you're running in the inefficient airflow region of the compressor (low airflow, high PR), you could heatsoak your IC or atleast add unwanted heat to the intake components which will lead to a hotter charge when you first go to full throttle.

I don't know which is better in certain applications, but those are my observations.

 

[MyBeatGSX]

^THANK YOU! That was a very well thought out post.

Do you understand what I'm saying now? Of course you'll eventually hit the boost you set it at, but it will take longer. The outlet will reach that boost long before the IM, thus opening the WG early. It will take a period of time (lag) for the IM to get to the same pressure. Whereas tapping the MBC off the IM, the wastegate will not open until the very last part of the system is up to full pressure, which reduces lag time. I can quote Corky Bell saying the exact same thing if you would like.

Now when you go back to turbochargers, another thing I see as a potential problem with an MBC off of the IM is surge. Since the TB restricts airflow, you could be demanding 20psi from your compressor at very low airflow which would put you into the surge region of the compressor map.

This is a very good point, and I had completely forgetten that I've actually experienced this. When I had my S16G and it was set at 15psi, I could hit 15psi very easily at light throttle in 5th gear, this caused insane surge that had the boost gauge rapidly bouncing between 10psi and 20psi. So yes this is definately a problem.

With my new turbo (Evo III) set to 20psi, I haven't encountered this at any combination of boost/throttle opening. I must have had it just right with the last setup.

 

[kenamond]

I can quote Corky Bell saying the exact same thing if you would like.

I'm starting into chapter 9. I'll get there eventually.

Anyway, it looks like you may be okay tapping the MBC off of the IM in some situations, but if you're running your T25 at 15psi, you probably don't want to do that. Folks need to understand that there are consequences for both setups, so they need to know the pros and cons and choose wisely.