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

 

[oldman]

Folks need to understand that there are consequences for both setups, so they need to know the pros and cons and choose wisely.

Please explain the cons of tapping the compressor again.

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).

This anology doesn't really work well for several reasons.

1. Comparing pressure and heat transfer is a little unfair and mis-leading.

2. If the door to the room is closed, the radiator is automatically shut off like a turbo system. Since we're talking about WOT, we should assume the door is always wide open.

3. The combination of wategate and boost gauge doesn't function as one unit like a thermostate since the boost gauge is always tapped at the manifold, in this case the thermostate is always installed in the last room. Tapping the compressor in your anology would be like installing the thermostate in the last room with a temperature sensor hooked up to the radiator room connected to a safty switch that cuts power to the entire system when the sensor and thermostate shows a difference of let's say more than 2 degrees. This is to protect the system in case if someone left a window open in the hall way, BOOST LEAKS.

In a WOT situation, unlike part throttle and the usual system pressure drop which is consistant in both cases, whether you're reaching 20 psi on the boost gauge by leaving mbc at 40% (just a guess) and tapping the manifold or 42% and tapping the compressor, your turbo is working close to the same rate. The direct relations here is between the boost gauge and wastegate, pressure source and mbc adjustment are simply two middle agents representing two different routes. Bottom line, tapping the compressor offers the safest as well the more precise route, how many times do we have to convince someone that they can have boost leaks even when they're still hiiting full boost?

I would also like to clarify that the phrase "manifold pressure" being thrown around in this discussion is refering to a manifold source aside from the BOV, FPR or brake booster line. I think we can all agree that the BOV line should never be used as a pressure signal for your mbc.

 

[patty AT forge]

In regards to the original question of safety, the stock ecu uses airflow and engine speed to determine fueling and timing, TPS voltage isn't a concern (ecxept during tip-in, I would imagine). However, you have an S-AFC that does use TPS voltage to determine how it trims the airflow signal, so depending on how you have your Throttle Point set on the S-AFC you might have an A/F problem.

 

[RedRex02]

In regards to the original question of safety, the stock ecu uses airflow and engine speed to determine fueling and timing, TPS voltage isn't a concern (ecxept during tip-in, I would imagine). However, you have an S-AFC that does use TPS voltage to determine how it trims the airflow signal, so depending on how you have your Throttle Point set on the S-AFC you might have an A/F problem.

Tis is exactly my point. I have decided not to do anythign in boost past 5psi, unless I am WOT.

 

[kenamond]

Please explain the cons of tapping the compressor again.

If you have your MBC off of the compressor, when you go from partial to WOT, there will be a lag. I have no idea how bad of a lag it would be, but it would exist.

This anology doesn't really work well for several reasons.

1. Comparing pressure and heat transfer is a little unfair and mis-leading.

2. If the door to the room is closed, the radiator is automatically shut off like a turbo system. Since we're talking about WOT, we should assume the door is always wide open.

No. The door is the throttle butterfly. We're focusing on partial throttle here and transitioning from partial to WOT, not pure WOT situations. When the throttle is closed, the BOV is activated, but the turbo will still be pumping out whatever the turbine is able to produce; it is not shut off at all. Granted, at closed throttle, there will be nothing driving the compressor, but at partial throttle, there will be compression. And I'm sure that recirculating alters the compressor behavior, but we're looking at partial rather than closed throttle.

3. The combination of wategate and boost gauge doesn't function as one unit like a thermostate since the boost gauge is always tapped at the manifold, in this case the thermostate is always installed in the last room. Tapping the compressor in your anology would be like installing the thermostate in the last room with a temperature sensor hooked up to the radiator room connected to a safty switch that cuts power to the entire system when the sensor and thermostate shows a difference of let's say more than 2 degrees. This is to protect the system in case if someone left a window open in the hall way, BOOST LEAKS.

I didn't mention the boost gauge explicitly in my analogy. Apologies. This would be the person in the last room feeling too warm or too cold and figuring out what thermostat setting was the right one to get the correct condition in the last room (figuring out where you want your MBC set to give you the correct boost gauge reading). And the temperature sensor you mention IS the thremostat; there aren't two different items, they are one. The hypothetical thermostat I'm assuming in my analogy would read temperature and turn the radiator up or down depending on the reading. Assume it isn't turning it on/off so that it is analagous to a WG.

With those clarifications, I think the analogy does hold. It's a matter of having your closed loop feedback control mechanism be more- or less-tightly coupled.

In a WOT situation, unlike part throttle and the usual system pressure drop which is consistant in both cases, whether you're reaching 20 psi on the boost gauge by leaving mbc at 40% (just a guess) and tapping the manifold or 42% and tapping the compressor, your turbo is working close to the same rate. The direct relations here is between the boost gauge and wastegate, pressure source and mbc adjustment are simply two middle agents representing two different routes. Bottom line, tapping the compressor offers the safest as well the more precise route, how many times do we have to convince someone that they can have boost leaks even when they're still hiiting full boost?

The only thing I recall about my SAT scores was that I sucked at reading comprehension, so forgive me, as I am confused by this paragraph in places. Anyway, at 25% throttle, if you tap your MBC off of the IM and set it to 20psi, then your compressor might run up to 30psi to get 20psi in the IM (assuming there is a 10psi drop across the TB). I agree that there won't be a huge difference in MBC setting between the IM and compressor outlet options, and the compressor will be running about the same at WOT in both cases, but the compressor outlet pressure may be vastly different at partial throttle in the two cases. So I think I agree with what you said, but I'm not positive I understood all of what you said.

Here's an idea.

Run two MBCs: One (call it "A") off of the compressor housing and one (call it "B") off of the IM. Set "A" to the highest compressor outlet pressure you're willing to tolerate. Set "B" to the desired WOT boost pressure you want to see in your IM or on your boost gauge.

That way, at partial throttle, "A" will limit the possible runaway compression while "B" will cap the peak IM pressure at any throttle position.

You'd probably have to add a boost gauge to the compressor to set "A", but that's no problem. I'm not too sure about the plumbing between the two MBCs and the WG solenoid, but a check valve or two might be needed to act as a "boost diode". I dunno.

Seems like you need your two MBCs tied into your ECU to do this right so that you didn't see surge at low airflow rates, but I think that this would be better than just one MBC off of the IM. I know that the TECII we used on our college project race car in 1994 had a voltage output that you could program on its own map, so you could use this to control one of the MBCs (assuming you have some motor-driven MBC). Then you could program the MBC to stay to the right of the surge line by dropping the PR at lower flow rates. Perhaps newer ECUs have multiple, independent voltage outputs such that you could control both MBCs. At WOT, that would also let you run an optimal path across your compressor map with increasing RPM rather than the flat, horizontal line (constant PR) you get with a single, static MBC setting.

Yes, I have a problem with digression.

Don't get me wrong, I think that the MBC should go off of the compressor outlet, but somehow "preboosting" at partial throttle is still attractive if it can be made to work without causing other problems.