Technical Question: how much boost can the stock engine & turbo handle?

how much boost can the stock engine & turbo handle.

If you are looking for a simple answer to this questions you might as well stop reading now, no such luck today. Typically I prefer to answer questions that can be answered in black and white, but that is just not the case with this one. To start with the stock long block is the same as the STi, so we have a great advantage here. The new WRX may have a 2.5L motor in it, but it does not have the same pistons, heads, or injectors as the STi, among other things. This means that anything you see the STi guys doing you should be able to do also. The turbo, IC, and possibly exhaust manifolds are different, but you are going to have to upgrade those to push the motor anyway.

The Motor

The stock turbo is pretty small. With sufficient octane you could just pull the wastegate line and it will not make enough boost to break the stock motor. Please read that again, I said with sufficient octane. DO NOT PULL YOUR WASTEGATE LINE ON PUMP GAS! Also, let me take a moment to reinforce that increased boost does not mean more power. Many factors play into determining the most power you can make. As an example running a larger intercooler may allow you to make more power with the turbo farther out of it's efficiency range at a higher pressure ratio. So, with proper tuning and fuel you can run as much boost as the turbo will make with the stock wastegate. To take this a step further you could weld the WG door shut and see how much boost you make. I suspect you will only make 21 psi or so.

Unfortunately boost pressure is relatively meaningless. As an example I would bet large amounts of money that pulling the wastegate line of the stock turbo on pump gas with no other modifications would put an end to your motor within miles. If we assume that it makes 21 psi it could be said that the stock motor is only good to 21 psi. You might think that, but I had 10K miles on my motor at 23 psi before it's demise. We have run 28 psi on a stock long block in a STi, does that mean that it's limit is 28 psi?

The truth is that most people loose their motors on pump gas due to detonation. Detonation is not solely caused by boost. For a given fuel detonation will occur at a given pressure and temperature in your combustion chamber. Any time you exceed that temperature and pressure the remaining charge in the cylinder will explode. This explosion is what breaks ring lands and/or pounds the bearings out of your motor. So how do we control temperature and pressure? Temperature is partially a function of the mechanical setup of your motor, we will not address this since we are talking about a stock motor. The other main way to control temperature in the combustion chamber, for a given fuel, is with the air fuel ratio.

Many people on this forum make statements like, my tune is too lean, or too rich, but does that really mean? If every molecule of fuel found it's oxygen partner you would run 14.7:1 AFR to get peak power out of your motor. However you have to take into consideration the random distribution of molecules and the time available for every molecule to find it's partner. The molecules have to bump into each other before they can combust, so getting every single molecule to bump into it's partner takes some serious time. At 7000 rpm that time is just not present. The best way to consume every last oxygen molecule is to add more fuel. Theory and practice show that running about 12.5:1 AFR for most fuels will yield peak power. The problem is 12.5:1 AFR will increase the cylinder temperatures well above the point at which detonation occurs for the cylinder pressures that occur with turbo motors under boost. Normally aspirated motors can typically run this AFR. Adding additional liquid to the combustion chamber can lower the cylinder temperatures. The liquid flashes to steam in the combustion chamber and absorbs heat in the process. This lowers the cylinder pressure and the efficiency of the motor. At this point you have to give up some efficiency in order to not destroy the motor. Despite the lower efficiency you may find a combination that yields greater power. So running too lean or too rich is relative to the point at which you make peak power, not a stoichiometric ratio.

Pressure is the other main factor that determines the point at which detonation occurs, for a given motor. The number of molecules in the combustion chamber and the temperature of those molecules at a particular crank angle will determine the cylinder pressure. Leaner AFR means hotter, hotter mean more pressure. The point at which you strike off the air charge with respect to the crank angle determines peak pressure, up to a point, firing the spark plug farther and farther before top dead center increases peak cylinder pressure. At a given efficiency of the turbo increased boost will increase the number of molecules in the combustion chamber. Once the efficiency starts to fall off for the turbo it will make more pressure by heating the molecules more, not by moving more molecules, at this point you will not make more power with increased boost.

That is the basic recipe for tuning. You have to balance the AFR, timing, and boost values for a given motor and fuel to find peak power for the varied environmental conditions that the vehicle will operate. You should also take into consideration that the fuel will vary on a tank to tank basis. Even at 10 psi you can blow up a motor. All you need to do is use the timing and fueling values to exceed the detonation threshold for the fuel and motor. There is a point that running too much boost will always detonate. If you push the turbo so hard that reducing the timing and making the AFR richer still will not eliminate detonation. Since the boost pressure does not exclusively determine the number of molecules in the combustion chamber and timing and AFR play significant roles in reaching the point at which detonation occurs you can not say what the safe limit for boost. There is no single answer.

The turbo

The turbo is a little easier to discuss than the motor. You can look at a compressor efficiency diagram for a given turbo and you can see at what point the compressor wheel falls off. If you fall off the peak efficiency of the compressor wheel you will not make more power. You still have to look at the hot side of the turbo. If you have a small hot side and a HUGE compressor wheel you will never be able to reach the point where the compressor wheel falls off. Small hot sides limit the amount of air flow able to escape out the exhaust, and can ultimately limit power production. Finally you have to look at the shaft speed. If you have an intercooler capable of over coming the lack of efficiency being run on the compressor wheel you will still dramatically shorten the life of the turbo. Basically that means that you may destroy the bearing assembly with too much boost even if you are making more power while you do it. Most people seem to agree that 18-19 psi peak tapering to 13-14 at redline is about the most boost you should run from the VF40 on the LGT 2.5L motor, particularly on pump gas.

Jarrad

If you have an intercooler capable of over coming the lack of efficiency being run on the compressor wheel you will still dramatically shorten the life of the turbo. Basically that means that you may destroy the bearing assembly with too much boost even if you are making more power while you do it. Most people seem to agree that 18-19 psi peak tapering to 13-14 at redline is about the most boost you should run from the VF40 on the LGT 2.5L motor, particularly on pump gas.

So I'm resurrecting this thread from the dead...

Does this mean that if I run a Perrin intercooler with a stock turbo, it will make my turbo explode really soon? Regardless of the tune?

he's talking about over tuning the car. you can run a Perrin TMIC just fine on stock turbo as long as it's tuned for it and you don't over boost.

I thought you didn't need a tune for Perrin TMIC>>>hmmm. Enlighten me.

So does that mean a stg II tune from PDX with boost somewhere around 18-19, could handle the TMIC well enough without having to go back and get another tune (or etune)?

if you tell them you are planning on upgrading your TMIC, they will tune for it. you don't have to get tuned again, but as long as you are running a basic stg 2 tune, you should be fine. otherwise, just running bolt ons without tuning will cause all kinds of problems!

That's what I thought, as you know I am runnign stg II now, but I think I will call Jared and confirm before I install it. It's just sitting in the garage...calling my name.

also, going back and reading the info, i also believe jarrad is talking about over boosting with a TMIC on a stock VF40. he's saying that you can go up to 18-19 psi, and taper to 13-14 psi, but that should be the max. i know some people will try and boost up to 20+ psi and that will probably detonate your stock turbo. that's what i'm understanding from his post. so if your tuner understands this, he's not going to allow the car to boost over 18/19 psi. i know when i ran my stg 2 set up, i was boosting up to 18 psi and tapering to 14.

and remember, booster made one pass in his stg 2 LGT with the wastegate cut off and he was boosting up to 21 or 22 psi on his 12.85 run. he only did it once just to prove a point, and never again afterwards because he knew if he did that on a normal basis, his turbo probably would go boom.

"boom goes the dynamite!"

[ame=http://www.youtube.com/watch?v=W45DRy7M1no]YouTube - Boom Goes the Dynamite[/ame]