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

 

[RedRex02]

Noone?

 

[Jeremoose]

When my MBC was set at 15psi (for about 3 months), half-throttle brought my boost up to about 10-12psi with no harmful effects...

 

[xveganxcowboyx]

I doubt it. I'm guessing the theory is that it would be running leaner and thus hotter. If you're not seeing a lot of knock it can't be too bad.

 

[RedRex02]

Hmmmm... So even with the afc hoked up, and if the MAF seeing more air, it still should compensate with more fuel even if it's only partial throttle right?

 

[oldman]

Where are you seeing the 50% throttle? Logger or guessing? The amount of fuel injected depends on MAF reading, not throttle position.

 

[kenamond]

What are you using as your boost source for your MBC?

 

[MyBeatGSX]

Like oldman said, the fuel is delivered based on airflow. What throttle position you're at doesn't matter.

If anything with the SAFC you'll be running richer instead of leaner. My low throttle settings have FAR less fuel pulled than my high throttles.

 

[RedRex02]

I get the throttle position by the SAFC.
So the fuel will be compensated with the extra boost, even if it's only partial throttle?
Just want to make sure before I melt some pistons. I would hate to see 15psi at like 40% throttle and something blow.
Jon

 

[oldman]

I get the throttle position by the SAFC.
So the fuel will be compensated with the extra boost, even if it's only partial throttle?
Just want to make sure before I melt some pistons. I would hate to see 15psi at like 40% throttle and something blow.
Jon

Well you have a SAFC so you should have a logger, you do have a logger right? Do a log at 50% throttle/full boost and see.

 

[xveganxcowboyx]

Where are you seeing the 50% throttle? Logger or guessing? The amount of fuel injected depends on MAF reading, not throttle position.

This means a (relatively) stable air fuel ratio no matter the boost level (assuming the fuel system can handle it) or throttle position, right? That would mean (relatively) consistent combustian temperatures is all those situations. This is how I understand it. Just like to confirm.

 

[kenamond]

I'm a bit confused here. At 50% throttle, there is a restriction at the TB that *will* drop pressure across it. If he's seeing full boost at 50% throttle, then his boost upwind of the TB *must be* higher than his full boost setting, right? That's why I asked where he tapped his MBC boost source. If he's tapping into the IM, then he'll be running much higher boost at the compressor than he might be aware of, because at, say 25% throttle, the pressure drop across the TB will be even higher. If his MBC is getting boost signal from the compressor outlet and is functioning properly, he should see less than full boost at anything other than WOT, because any more of a restriction will drop pressure at the TB.

What am I missing?

 

[MyBeatGSX]

You missed the fact that the engine is also flowing (for example's sake) 50% less air too. So the turbo doesn't have to work as hard to make the same boost.

 

[kenamond]

You missed the fact that the engine is also flowing (for example's sake) 50% less air too. So the turbo doesn't have to work as hard to make the same boost.

Still confused. If you consider two cases: one at 5000rpm, WOT, 17psig in the IM and one at 5000rpm, 50% throttle, 17psig in the IM. If the IM air temperatures are the same, the pressure is the same, and the rpm are the same, then the air flow is the same, and the car would act the same in both cases. I know this can't be the same.

First, the 50% throttle case will have a much higher pressure on the upwind side of the TB. The air will be hotter as a result of the compressor working harder (unless 17+psig is more efficient than 17psig) and the air being compressed more. Also, the TB restriction will further increase the entropy of the air charge in the IM. What this means is that if you compressed the IM air charge back to the pressure it was at before the TB, it's temperature would be higher than what it actually was before the TB. So you have a hotter, higher entropy air charge, and both of these factors reduce the density going into the motor, so yes, you will have less airflow.

My question is related to him getting 17psi in the IM at 50% throttle. The only way I can see this happening is if he has his MBC feeding off of the IM pressure rather than something upwind of the TB. If his MBC is off the compressor, he shouldn't be getting 17psi in his IM at 50% throttle.

Did any of that make sense?

EDIT: So I'd like to know where his MBC is hooked up. If it's off the compressor housing, I'd like someone to explain how he can get 17psig in his IM at 50% throttle which to me seems impossible.

 

[boostedinaz]

Anyone ever gone up a hill with the cruise control on?

You can be at a low throttle percentage but because there is more load on the engine it will build boost and pull you up the hill.

We don't really have enough to come to a conclusion as of yet.

 

[RedRex02]

Wow, now I am really lost.

 

[oldman]

Wow, now I am really lost.

1. Can someone confirm that the throttle reading on a safc is accurate?

2. What is the throttle reading at idle on the safc?

 

[RedRex02]

1. Can someone confirm that the throttle reading on a safc is accurate?

2. What is the throttle reading at idle on the safc?

The safc is hooked up to the tps wire from the ecu. I would hope its close.

All I am asking, is it safe to be spooling up and boosting more than 10 psi under partial throttle. 50% was just a estimate and an example.

 

[oldman]

All I am asking, is it safe to be spooling up and boosting more than 10 psi under partial throttle. 50% was just a estimate and an example.

The question here is why is it doing it aside from whether or not it's safe, I can see it happening at let's say 70+% but 50% is kinda unreasonable. This is why I wanted to know the accuracy of a safc reading, idle reading will help determine whether the TPS is set correctly in the first place.

 

[CanadianTSi]

I can get atleast 20psi with all of my mods at under 60% throttle. If I just sort of roll into it in 3rd and never go more than 60% it will still pretty much build full boost, although alot later in the RPM Range.

I'll try it today and check for sure that I can get full boost.

 

[RedRex02]

I can get atleast 20psi with all of my mods at under 60% throttle. If I just sort of roll into it in 3rd and never go more than 60% it will still pretty much build full boost, although alot later in the RPM Range.

QUOTE]

My question is, is what Canadian TSI and myself are seeing is safe for the motor?

 

[CanadianTSi]

I see no reason why it wouldn't be safe, the car is seeing all the all that is entering so it is still injecting the correct amount of fuel.

 

[xveganxcowboyx]

Anyone ever gone up a hill with the cruise control on?

You can be at a low throttle percentage but because there is more load on the engine it will build boost and pull you up the hill.

We don't really have enough to come to a conclusion as of yet.

I have experienced the same thing. The more open my exhaust, the more it seems to happen.

I still *think* it should be perfectly safe. Would love to hear a scientific explanation though.

Kenamond, I understand your point, but do you think the throttlebody being half-open is enough to make a drastic difference in flow, temp, etc?

 

[boostedinaz]

You should be just fine.

The MAF will read that more air is comming in and will supply the addtional amount of fuel.

If this was truly a problem turbo cars would be blowing up left and right.

 

[kenamond]

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

Kenamond, I understand your point, but do you think the throttlebody being half-open is enough to make a drastic difference in flow, temp, etc?

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?