Laust,
there was never any condescending tone in my posts toward you. If anything, I couldn't believe your tunnel vision and that might have come through in my posts.
The problem is that because you read some literature, you think you are now the pioneer of WI on this forum, and how could anybody else possibly give you any info that could benefit you.
I've been working with supplimentary injection since before most of us even knew this forum exsisted. So yes, these are my opinions based on facts and real research in everyday driving. I didn't get it right in the beginning, but I didn't lie to myself thinking: "well, based on all the info, it must be working". One has to dig a little deeper to massage the concept of WI.

I can't believe you still think you are maintaining "sound engineering principals" (this is becoming a famous phrase) when you are clearly asking your turbo to do something it physically can't do. This doesn't even have anything to do with WI, by the way.

To address your items even though they were sarcastic, and not sincere:

1)I will not benefit from these articles. You, however, will benefit from an open mind to other sources of information.

2)All I know is that it's not correct to have huge backpressure (read: hot exhaust gases left in combustion chamber) and then try to neutralize it with WI. As was said before, we really don't know for sure what happened to your engine, except that it wasn't detonnation. So, what could it be? It probably wasn't a coincidence that you have a badly engineered system and the engine went bang.

3)Most manufactuerers use port fuel injection because of better evenly distributed atomized fuel. Yea, so what of it?; and they do not draw parrallels to WI. The WI people draw parallels to the port fuel injection people, and that's a problem, right there. Water is not fuel.

4)Fuel has the ability to take out 40F. of heat with proper air/fuel ratios. Horsepower is irrelevant.
With a properly sized turbo (that has to be in place first) you should be using anywhere from 15 to 35 ml/minute of water per cylinder. There is no typical amount; depends on alot of factors. I doubt your port water injection nossles are that small (you'd have a hard time seeing the hole without a magnifying glass)

5)a)100% humidity? That's not realistic.
And what temperature? That's important.
Let's go by an average low temp. summer day. 951 engine will ingest about 150ml. of water per minute (give or take) at full power.
b)The amount of water produced by the combustion process is of little interest to us. You think this means something because you read it somewhere (I read it as well).This is after the piston has done its work. All we care about is the water content at the time the spark plug fires.

6)Torque: you can probably achieve 375 crank torque with any kind of proper WI system
Horsepower: 300 crank horsepower with or without water injection.

 

Laust, I too was a bit surprised by your replies to this post. You asked a question and when you didn't get the answer you wanted you went out of your way to prove everyone wrong. It probably would have been easier and better taken if you just proved you were right and didn't ask for replies.

Got Me A Porscha's block was broken on the #2 cylinder. I've also seen a powerhause 3.0L turbo motor with a crack on #1. The 3.0L motors as you know are strethened to preven cylinder wall flex so I don't think these sudden failures are a defect in the blocks. Chris Cervelli ran MANY Gt 25 turbo's @ 25 psi properly tuned and within the efficency range that made over 400 RWHP all day long. Here it is 4 years later and I've never heard of 1 cracked block running that combo. You make mention that you didn't hear any detonation but everyone knows that just because you can't hear it, doesn't mean it's not happening. I just hope that you chose this upgrade path out of your own quest for 951 developmant and someone wasn't talking you into it because they wanted to try some R&D. Take it for what it worth I guess and to each his own......

 

But as I think Danno pointed out it doesn't look much like detonation in the pics, and Laust said that there was a polished area on the HG, where the block was moving, this brings us back to fatigue through pressure, thermal shock or a mixture of the two.
Tony

 

In one of Laust's old posts he calculated back pressure at 5000rpm and 20 pounds of boost to be 57psi. How much pressure is each piston generating as it pushes the exhaust gas out the exhaust valves? Probably a lot more than 57 psi, I dont see how this would be an issue. Perhaps if he had a bad exhaust valve seal on #4? What is the volume in the combustion chamber at TDC?

 

Disclaimer, I'm sure there are deficiencys in the following calculation but as a fast estimation of the effects of back pressure it will give some information.
If we assume a compression ratio of 8:1 then residual exhaust gas with a volume of 1/8 of the cylinder volume when the piston is at BDC will be left after the exhaust stroke. If the pressure of these exhaust gases are double that of the boost pressure then when you expand the exhaust gas 1/4 of the volume will be occupied by exhaust gas not 1/8. In other words you will loose 12,5% of available volume. This is with a exhaust to boost pressure ratio of 2:1, if you push a K26/6 you will probably have even worse ratio? The bad effect also goes beyond volume loss, you will retain more heat inside the combustion chamber, making the engine more prone to detonation, and the dilution of the air/fuel charge will probably slow down combustion.

 

Exhaust scavenging is dependent upon cam-timing and RPM.

 

Yes, exhaust scavenging is dependant on cam-timing, rpm, port dimensions, port shape, valve size, header dimensions, compression ratio and backpressure (I'm sure I forgot something here).
But if you double back pressure and all other factors are remaning as before then exhaust scavenging will be much worse.

 

Not sure what you are getting at but I think the pressure when the valves are closed and not leaking (on the compression stroke) will be much higher, especially when there is some hard to squash water mixed in.
Tony

 

Thomas, now I get it! Thanks! I wasnt sure how backpressure mattered, but it does since not all of the exhaust gas is evacuated on the exhaust stroke, so the higher pressure it is at, the more volume it will take up and heat up on the next intake stroke. I finally see how a given turbo is limited, it is by the amount of flow it can take through the hot side, the bigger, the more it can flow, but the slower it will spool.

 

Yes Mark, that's correct if you look at the exhaust side of the engine and turbo. To further complicate things, the compressor side of the turbo also indirectly contributes to backpressure. When the compressor flow increases beyond the efficieny peak it requires more power to drive. You can see in the compressor diagram how the rpm lines bends downward and getting closer to each other. You need to spin the compressor wheel much faster when it goes out of efficiency. To drive the inefficient compressor requires more power from the turbine. This power need is fulfilled by the wastegate which tries to keep boost constant and therefore closes and routes more exhaust through the turbine and therefore causes more backpressure.

Tony, exhaust backpressure is not nearly of the same magnitude as the pressure of combustion as you pointed out. It's just that it makes the engine inefficient, increases the heat load on the combustion chamber and makes the engine more prone to detonate.

Tomas

 

Yea, that's right, Tomas,
Incidently, that's why I'm not a fan of aftermarket wastegates that take away the opportunity for the back pressure to contribute to the opening of the wastegate. The stock boost control/wastegate is an ingenious effort by Porsche to keep the boost pressure/back pressure relationship in check. People are kidding themselves if they're holding boost all the way to redline on a smallish turbo. They're not really making any more power; it just looks good on the boost gauge.

 

>>>1) Please read a few SAE papers on this and closely related subjects, for example:

a) “Prediction of Nox Reduction Rate Due to Port Water Injection in a Di Diesel Engine ”
b) “Impingement Spray System With Direct Water Injection for Premixed Lean Diesel Combustion Control ”
c) “Fuel Consumption Improvement and Operation Range Expansion in Hcci By Direct Water Injection ”
d) “Significant Nox Reductions With Direct-Water Injection Into the Sub-Chamber of An Idi Diesel Engine ”<<<


All these papers have aluminum open deck motors as their test subjects (so to speak)? I was under the impression that diesel motors were typically, uh, how shall I say it, on the stout side.

 

With the stock wg Porsche accomplished two things. As you pointed out they kept backpressure at reasonable levels and by that they also kept the compressor within it's efficiency range. The other thing is the possiblility to regulate boost down to a harmless level in case of engine problems. These two things are both the result of the spring choice Porsche mede for the stock wastegate. I don't know which was more important for them.

Although I recently installed a Tial 38 mm wg I basically agree that aftermarket wastegates are overrated. It's better to save up to a new turbo.

Tomas

 

"Tony, exhaust backpressure is not nearly of the same magnitude as the pressure of combustion as you pointed out. It's just that it makes the engine inefficient, increases the heat load on the combustion chamber and makes the engine more prone to detonate."

Yeah, initially when the exhaust valves first open at the beginning of the exhaust-stroke, the combustion pressure will easily overcome the pressure in the crossover. This higher-pressure is also what drives the turbine. But high-RPM cylinder-filling is very sensitive to the overlap period on the cam at the end of the ehxaust-stroke. Optimally, you'd have a large overlap to use exhaust-scavening to pull in the intake mixture. But the higher backpressure in turbo cars result in a backwash of exhaust. Thus the reason for "square" cams on turbo cars. This results in great low-end torque and reduced turbo-lag, but really hurts high-RPM revving ability and HP.

So with the goal of building high-RPM/high-HP engines, you DO need to retard the intake-valve closing and increase the overlap for efficient high-RPM operation. But this makes the system more sensitive to exhaust backpressure, so you have to pick your turbo-specs very meticulously to work with the rest of the system.

"Although I recently installed a Tial 38 mm wg I basically agree that aftermarket wastegates are overrated. It's better to save up to a new turbo."

Yup. Also keep in mind that the higher the power-levels and higher boost-levels you want, the less the wastegate has to open. If you've got a big GT35 turbo running 15psi boost, that wastegate's gonna be dumping a lot of exhaust! But if you're running it at 25psi for 450rwhp+, that wastegate's is going to be opening barely at all from 3500-5000rpm. There's really no need for a wastegate that's much larger than the stock 39mm one. Even a Tial 38mm doesn't open more than 3-8mm to regulate boost in the majority of installations on our cars.

 

Header design will probably also be of importance when aiming for high rpm. The goal must be a well matched combination of camshaft headers and turbo (combined with a matched intake system).

And the higher boost you run the higher will the flow capacity of the wastegate be. More pressure will be able to push more flow through the wastegate valve. Large wastegates are mostly needed on high volume low boost engines.

Tomas