boost creep

[Ender]

Originally Posted by JBurer

Boost,
My S5 has a ported motor, and this is exactly what I'm seeing. 2nd gear hits 11psi and then slowly drops to 5psi by around 8k rpms.

The statement I keep reading is that the turbo can't keep up - this makes no sense to me as the wastegate would completely close if boost pressure dropped below the target.

First gear is different for me, I see only 8psi of boost, which also drops to 5psi. Anyone have an idea whats going on here?
Best,
John

In my case at least, and I'm sure a lot of other guys use these, too, it's the manual boost controller that's part to blame. Unless electronically monitored I don't think there's a real smooth way for a boost controller to hit peak boost and simply keep it there unless it's set at or only slightly above the stock boost level. A bleeder valve simply isn't the best solution, but since it's very cheap we do it anyway. At 4000 RPM and 12psi there's a lot less air moving than at 7000 RPM at 12psi, that little bleeder valve can only release so much air so fast. What does this mean? It means the wastegate actuator ends up seeing more pressure as the RPM rises while boost is up, which will unfortunately bleed boost through the wastegate.

Another factor is the turbo itself. For one, the stock turbo is built for holding stock boost levels on a stock motor. It can perform better than that, but we can't expect too much from it. It can only be pushed so fast before it just becomes a heating fan and stops helping you make more power. What I've read is that the stock turbo is pretty useless past 12psi and many people have blown stock turbo's or apex seals by boosting higher on the stock turbo. You may help the situation a little with a FMIC, but not by a lot. The stock turbo simply can't push a ton of air at high RPM. You might look into BNR turbo's if you're looking for a cheaper turbo upgrade. I've heard a lot of people are happy with that route.

I have all my intake and exhaust ports mildly modified. The funny thing is that even if my boost peaks around 5000 RPM I can tell I'm making more power at 6000 RPM at ~3psi below peak boost than at 5000 RPM at peak boost. I'm aching to get a good electronic boost controller to see how much improvement I can make over the stupid MBC. I'll probably start roasting tires at the top of second gear...

[Boostmaniac]

Originally Posted by JBurer

Boost,
My S5 has a ported motor, and this is exactly what I'm seeing. 2nd gear hits 11psi and then slowly drops to 5psi by around 8k rpms.

The statement I keep reading is that the turbo can't keep up - this makes no sense to me as the wastegate would completely close if boost pressure dropped below the target.

First gear is different for me, I see only 8psi of boost, which also drops to 5psi. Anyone have an idea whats going on here?
Best,
John

Gimme a sec and I'll explain in depth. A lot of people see this and don't understand it.

Originally Posted by Ender

In my case at least, and I'm sure a lot of other guys use these, too, it's the manual boost controller that's part to blame. Unless electronically monitored I don't think there's a real smooth way for a boost controller to hit peak boost and simply keep it there unless it's set at or only slightly above the stock boost level. A bleeder valve simply isn't the best solution, but since it's very cheap we do it anyway. At 4000 RPM and 12psi there's a lot less air moving than at 7000 RPM at 12psi, that little bleeder valve can only release so much air so fast. What does this mean? It means the wastegate actuator ends up seeing more pressure as the RPM rises while boost is up, which will unfortunately bleed boost through the wastegate.

Another factor is the turbo itself. For one, the stock turbo is built for holding stock boost levels on a stock motor. It can perform better than that, but we can't expect too much from it. It can only be pushed so fast before it just becomes a heating fan and stops helping you make more power. What I've read is that the stock turbo is pretty useless past 12psi and many people have blown stock turbo's or apex seals by boosting higher on the stock turbo. You may help the situation a little with a FMIC, but not by a lot. The stock turbo simply can't push a ton of air at high RPM. You might look into BNR turbo's if you're looking for a cheaper turbo upgrade. I've heard a lot of people are happy with that route.

I have all my intake and exhaust ports mildly modified. The funny thing is that even if my boost peaks around 5000 RPM I can tell I'm making more power at 6000 RPM at ~3psi below peak boost than at 5000 RPM at peak boost. I'm aching to get a good electronic boost controller to see how much improvement I can make over the stupid MBC. I'll probably start roasting tires at the top of second gear...

Ender, you are on the money like usual, but there is one thing that needs to be explained better.

It all starts with separating the turbocharger and the engine in your mind. It will make this a hell of a lot easier. Also, I am going to make up a lot of numbers here to make this simple, so please don't flame for the false data.

A stock port turbo motor ingests less air than a ported motor, everyone knows that. So on to my fake math.

If you could measure the amount of CFM a stock 13B ingests as the revs increase, you will see a rise in CFM as it approaches its VE (volumetric efficiency) peak, which should also be it's torque peak (usually, but this is different from motor to motor.) If you measure the CFM of a ported 13B as the revs increase, down low they will be close to the same, but as the revs increase, the CFM goes up MUCH more rapidly. Basically, a streetport flows an ever increasing amount of CFM as revs increase.

Now, if you understand that, on to the turbo.

Our turbo's suck for what we want to do with them. The stats on your turbo are static. For any given PR (pressure ratio) they can only flow so much air. A lot... and I mean A LOT of people think that the power comes just from the boost, and that is what gets a lot... and I mean A LOT of people in trouble with RX7's. The CFM is actually more important to me in regards to the PR than boost is. But on to the WHY your car can't hold boost.

Example 1 : Stock car in third gear from 2k to redline, stock turbo, 3 inch DP and some other bolt on goodies.

At WOT (wide open throttle) the turbo will come online. Full boost is usually realized before 4k rpm. The turbo is spinning merrily and you sustain a solid boost number, we'll say 10 psi for ease of use. On stock ports and stock turbo, the extra exhaust energy from the increase in boost and the freer flowing exhaust utterly defeats the wastegate so the turbo starts spinning faster (boost creep). The motor can't ingest all the air the turbo is feeding so you will see an increase in boost. If left unchecked, the system will literally self feed itself and your boost will go to the choke point of the turbo. Hence, some aftermarket goodies on your car means your car has a tendency to creep unless you port the wastegate A LOT.

Example 2: Nasty streetport in third gear 2k to redline. stock turbo, 3 inch DP and other goodies.

At WOT at 2k, your car is producing a retardedly higher amount of exhaust energy due to the porting, so the turbo will start to spin up MUCH faster than stock. As the turbo comes online headed to the 10 psi peak, your exhaust volume and energy is increasing way more than the stock example above. Therefore the turbo hits 10 psi, your wastegate opens, but the turbo was accelerating so fast there is almost no hope of controlling it without an EBC. So you end up with what is normally a large spike at the midrange in the RPM, which also puts it dangerously close to the torque peak, which is where the engine can get closest to it's detonation threshold, but that is for another post later. Anyways, you spike to 15 psi, then the turbo comes back down to 10. As your revs increase, the engine starts ingesting more air until it reaches equilibrium with the turbo. (I'll take a guess and say it is usually around 5k plus for most of us.) Now something really cool and really bad starts to happen. The motor revs increase and it starts ingesting more air than the turbo can actually put out. The boost level starts to drop and it is sad faces all around. But here is what is actually really bad. Your wastegate cares not for IAT's or CFM's, it only cares about PSI. So when boost drops below threshold, it shuts the wastegate to play catch up. Guess what, your motor is still eating more air than the turbo will ever put out now and the boost still continues to drop. Your exhaust energy is now increasing the impeller speed of the turbo to pretty close to the choke point, which increases IAT's and wear on the turbo itself.

Now, example 3 : same motor as above with a BNR stage 3.

The revs increase, the turbo comes online a little later (takes more time to spin up a larger turbo). Motor starts eating the air and the turbo keeps thumping it out at 10 psi. BUT!!! at 10 psi this turbo flows more air than a stock turbo so your power output is going up drastically. Since the compressor wheel is larger it can flow more air at a higher psi and maintain it throughout the power band.

Conclusion, go get yourself a BNR stage 2 or higher and have fun in your car again. You have no idea what the difference will feel like if you have the fuel for it. Streetport with a BNR stage 3 at 15 psi = you rolling up lots of fools.

I'll try to get some more real data if there are any questions whatsoever.

P.S. Bridgeport and a 42R is what EVERYONE in this forum needs in their life.

That is all.

P.P.S. I missed one question.

The reason your car can't hit your target boost in first gear is because our turbo's suck and the engine load isn't high enough or long enough to get to your target boost in time, so the engine gets to the point where it is eating more air than the turbo can put out before the target is even reached which is why you see only 8, followed by a drop to 5. I bet every gear drops to around 5 by redline (please don't try in fourth or fifth gear, you may hurt yourself, your car, or someone's dog running across the road.)