I am reading and learning. Love to have some practicle examples and results.

Impeller erosion
 

Just thought I would try and post some images of the turbo I just pulled out. It took a while to figure out how to get descent resolution images.

Richard I will send them to you by email if you could post them up as I have no place to host them.

Keep in mind a couple things:
1. On casual examination the impeller looks just fine, its only when you look close that you see a bit of errosion at the outer impeller tips.

2. I suspect I was over spraying (3%) mass air flow, and I also experimented with spraying at high idle speeds (very low throttle setting). I discovered that this would act to cool the intercooler prior to drag racing.

(note to self, very good idea for post turbocharger pre-intercooler injection point . If you could manually trigger this pre-intercooler spray prior to drag racing, or Autox event all the evaporation takes place in the intercooler and quickly cools the structure of the intercooler. Due to the engines air flow you completely eliminate any need for external fans to cause cooling of a heat soaked intercooler while the car is stationary)

I suspect during these experiments I may have had some liquid water streaming down the walls of the turbo inlet pipe. I believe, this could be a possible cause of the erosion at the compressor impeller tips.

IF that proves out to be the case, it might be useful to put a trap of some sort in the inlet tube that would re-suspend any surface water in the air flow. This could be in the form of a ridge that would stop the droplets that tend to run down the surface of the inlet tube and cause them to be ripped apart by the high speed airflow as they try to climb the lip.

3. Last but not least, it is important to note that I also made about 6 drag strip runs with this turbo using no air filter of any kind, to determine the intake air restriction the filter creates, and its effect on turbo boost. This was through a cold air intake system that had the intake in front of the right front tire and directly behind the front bumper. At the strip I run at there is a section of gravel/dirt you must drive over as you pull off the strip. I know I sucked up some dirt on at least one of those events. Shortly after that experiment, I pulled the discharge off the turbocharger and found small amounts of aluminum dust in the turbo to intercooler tube and some very fine dust. I wiped this off and it did not return after that. If you look closely at one of my pictures you will see a clear "meteor crater" impact about a quarter mm in dia. on the leading edge of one impeller blade from a piece of grit.

My personal judgement is most of the errosion is due to these no air filter tests, and the effects of the water were minimal but I cannot prove that case. After I run my current turbo for a comparable period of time I will pull the intake tube and check for similar wear without having ever made any filter free passes.


Larry

Here are the pictures of larry's turbo. Picture restored - lost and found.


http://www.aquamist.co.uk/forum/compressor9eb-s.jpg

Old Saab99 Turbos had a pre-turbo injection system factory stock. It was common after 100K miles or so to see some pitting in the compressor blade. But at that point, the turbo was going to be replaced anyway. I've never heard of it causing any problems.

Adrian~

Thanks
 

Thanks Richard!

Saab tuner
It was also used on 10's of thousands of military aircraft without serious problems.

There is however an urban legend sort of belief in the performance community accompanied with horror stories of "some kid" etc. etc. that ruined his turbo in a matter of minutes after trying a pre-turbo setup.

The main thing I wanted to demonstrate is that it does not cause immediate catastrophic damage. In my case I considered this turbo to be expendable as I already had its replacment on the floor beside my desk. After seeing the condition of the compressor and considering what I put this puppy through, I had not qualms about using pre-turbo WI on my new setup.

Larry

Like in those NACA reports.

I just wanted to point out that it was used on automobiles, which are usually at least perceived to have longer service intervals. (Whether they actually do or not.) Of course, it was used on Saabs, which are usually perceived as having short service intervals and low reliability.

http://www.saabcentral.com/trivia/mileage.php

Oh well. :P

Pre-turbo injection is certainly much much cheaper. When I work out a WI system, it's going to be multi-port, and I'm going to do my best to get it CARB Exempted. Otherwise I'll fail emissions. (Some people pass, but not if you end up at the notorous "test only" stations. Then you're toast. I have quite a bit of experience with that ... dammit.)

Adrian~

Still trying to get my head round the thermodynamics, but its a bit of a puzzle. The evaporative cooling takes heat out of the air and avoids the air gaining temperature and pressure. At first glance it is obvious that this means there is less energy being put into the air, but isn't it just going into the water instead? Or have I missed the point? I guess that the outlet pressure would be similar whether the charge was cooled during compression or later in an intercooler, so reduced temperature implies reduced volume, speed and KE. Is this where the gains come from?

The heat energy (kinetic) is being converted into entropic energy (liquid to gas transition--the heat of evaporation (heat needed to disassociate H2O molecules)).

Ideally at the moment the adiabatic heat is generated, it is absorbed by the water evaporation. Compression occurs with less heat imparted to the compressed air.

If liquid water traverses the impeller wheel without evaporating, we are no better off injecting before the turbo as after.

Now the water droplet will spend very little time traversing the impeller wheel. We what it to completely evaporate during its journey. Hence, the dialogue about droplet size. We can calculate this time frame with some precision, but the time to evaporate a droplet is a difficult one. Pressure, temperture, velocity, increasing surface to volume ratio, and methanol mixture, will make it a multivariable calculation, one best left to empircal determination.

Aquamist at 147 psi is our best bet for small droplet size. Smaller droplets can be made, but the pressure and nozzle sophistication to make them is untenable in automobiles with current technology. I know of no sensor that could easily be rigged to detect droplet exit from the compressor wheel (a spectrophotometer could work, but we are talking at least a $1000).

Hotrod has kindly displayed his turbo impeller after 7000 miles at 2.7% water to air mass. It shows some notable erosion at the edge (with extenuating circumstance). The erosion can be viewed as a cost of implementing the impeller injection, or we can wait for creation of a nozzle that expels smaller droplets. By injecting less water than hotrod, and only at high RPM and boost, I hope to reduce the erosion substantially (using a solenoid to inject only under the chosen parameters). None the less I expect some erosion to occur. If a new turbo is needed every 20,000 mi, that is a cost of running such a system. It remains to be seen if the benefit outweighs the cost. If a 10% increase in airflow can be achieved at high RPM, it would be worth about 40 hp; that is worth it to me.

Re: Impeller erosion
 

When I was careless (sorry, experimental!) enough to run my turbo (bike) without airfilter, that's the sort of wear I saw on the compressor blades.
Didn't take long to happen either... :roll:

Good info
 

Thanks, that backs up my circumstantial evidence and theory that that damage was from the "dirt experiment".

Really T'd me off at the time, I forgot there was a segment of dirt and gravel at the end of the strip where you turn off to pick up your timing slip. Lots of folks cut the corner short and thow dirt and sand up on the paved portion of the turn out. I took the turn out at about 30 mph and was in the crud before I realized it. My air intake sits right behind the front air dam and I figured the air dam passing close over the surface of the sandy pavement suspended a lot of crap in the air.

Like you said it only takes a single event. After that first pass, I took the turn off road very wide and slow and saw no more evidence of sucking dirt.

Interesting side note is that the damage is confined to the extreme outer blade tips, probably a combination of the swirl that develops ahead of the compressor inlet centrifuging the dust to the outside of the tube and the higher relative velocity of the blade tips vs the portion of the blades closer to the hub.

There is one guy that posts on another form that says he's run a pre-compressor injection for years with no sign of compressor damage.

Larry

Re: Good info
 

Way off topic, but I wonder would it be feasible to use this effect to create a centrefugal air cleaner? Vacuum cleaners using this effect claim no loss of suction as the 'bag' fills, presumably in our case that would translate to no increase in pressure drop across the filter with age? For this to work it would be necessary to recover the static pressure drop across the vortex, in an ideal world it would be possible to recover this by slowing and expanding the air to convert dynamic pressure back to static, as long as there was not too much drag within the vortex. Also, straying slightly closer to topic, by passing a water mist through a vortex of this sort, fine droplets might tend to be drawn through with the air while larger droplets woud be held back until they have evaporated, broken up or settled out on the walls. Probably a crazy idea, but do you reckon it would work? How much of a swirl would be required to separate out the dirt and water droplets of interest, and how would you go about testing it?

Pete 'crazy ideas' Humphries (and a green V8S)

I want to say, I'm perfectly willing to accept that Hotrod's impeller wear is from dragging with no filter. At 150,000 RPM, that's 2500 RPS, so 2 seconds of dust, dirt and you have 5000 turns of the impeller. You could do a lot of damage with 6 runs.

I'm only trying to express the possibility of wear, erosion. I would not want anyone reading these threads to go into pre-impeller injecting without this as a concern.

air filter
 

They already exist, industrial equipment (cat tractors diesel trucks etc.) use them as pre-filters ahead of the normal filter element to extend its service life.


Larry

This may be a dumb question. Isn't the main heat generated by the turbo is friction between the air/impeller surface? The same can be said with moisted air.

Shouldn't really compare it with the space shuttle entering the earth's atmosphere and picks up speed and heat rises as the craft speeds up, but pretty close to the air sliding passes the impeller at 150,000rpm.

I'd say that probably constitutes a big portion of it.

Also when the air enters the scroll, or vane, diffuser section the velocity pressure is converted to static pressure by slowing the air back down. This involves a lot of small eddie currents in the air associated with fluid vorticity. Generally vorticity ends up dispersing into heat.

Adrian~

"eddie current" of the air is another word for the air molecules rubbing against each other in a circular fashion and the friction between the molecules generate heat?

Sorry to look at things in a more simplistic description.

Some of the heating corresponds to inefficiency in the compressor, and I imagine most of this comes from small scale local variations in velocity around the compressor which are eventually damped out by friction in the air, as suggested previously. However, turbo compressors are relatively efficient so this effect would only account for a small part of the heating effect. The rest of it comes from adiabatic heating. The mechanism causing this occurs at the molecular level, where gas molecules bouncing off the inner surface of a container rebound with increased speed when the container shrinks (think of tennis balls bouncing off a moving racket). This results is increased speed and momentum in the molecules, which is essentially what heat is.

Turbos are rated in terms of efficiency of compression, but they should really say efficiency compared to adiabatic compression. At best a turbo in a narrow RPM range is 80% efficient with respect to adiabatic compression. So, there is 20% of the heat that is not merely the result of compression, but comes from somewhere else--friction, turbulance, heating of the turbo materials. As the turbo moves toward stall speed (the speed of sound) the efficiency drops off exponentially. As mach 1 is reached, not just heat but castistrophic sound waves disrupt the whole stability of the spinning wheel.

All the talk above in this thread about isothermal compression, is compression without the generation of any heat. Isothermal is a theoretical idea that cannot be achieved. However, if we can cool the air with water droplets as it is being compressed we may achieve a quasi-isothermal compression. At the outer limits of the impeller speed (near mach 1) the efficiency can drop to 50% or less of the adiabatic value.

For the sake of arguement let's say we could "recover" all the energy lost to heat at these near maximum speed. We could realize a 50% increase in compression with no heat loss. Effectively, our turbo would be 50% larger. That is a major accomplishment for the injection of a little water.

Practically, we will never cool compression this much, but even a 10% or 20% gain would make a huge difference in the amount of air mass available for combustion. For a 300hp engine, that's 30-60 hp gain. Many folks would kill for a gain like that.

Ideally, I'd like to experiment with the turbo off of the car. Compare the compression with and without water injection pre-turbo and see if compression is increased (i.e. air flow) and if post turbo temperature is reduced. Also in this ideal set up, we would want not liquid water (droplets) to survive traversing the turbo (not that that "overspray" as it is called is a bad thing, we are just not measuring the change in compression/heat only).

In the real world the best we can do is measure post turbo pressure and boost with and without injection of water. My hope is that I'll see more compression (higher boost) and lower temp (adiabatic and "friction losses" absorbed my water evaporation).

We are basically talking about running the turbo outside of it's efficiency range, using water to compensate for adiabatic and turbo losses. Like, the fear of water shut down that may destroy the engine, similarly the turbo may spin out of control if water were suddenly removed.

Engine management to prevent catastrophic damage is essential, since the computer can react much faster than we can. All attempts to operate at the edge of tuning involve some form of risk. Sensors, redundant pumps, auto shut off fuel or boost blowoff, would all lead to a safer system. This is one of the reasons I was so excited about the now indefinitely postponed EcuTek-Aquamist offering. For now, I'll be using the Element Hydra to design a system as best I can (one I'm sure will not be as good as a professional product).

"As the turbo moves toward stall speed (the speed of sound) the efficiency drops off exponentially. As mach 1 is reached, not just heat but castistrophic sound waves disrupt the whole stability of the spinning wheel."

I think what you are referring to is termed the "choke line".

As a rule boost pressure is related to the square of the compressor speed. The ultimate boost limit is not related to the speed of sound, rather by the heat at the compressor discharge. Aluminum compressors become weak above 300F degrees, too much boost and the wheel is weakened significantly.

Also, remember that the speed of sound is an entirely localized phenominon. The speed of sound on the backside of the compressor blades is much less than the speed on the front side. Even at a relatively low speed the backside surface of the compressor blades exceeds the local speed of sound.

Furthermore, a few simple calculations will yield that most compressors operate at a speed which exceeds the speed of sound at 1 atmosphere of pressure.

The choke line is more likely related to cavitation on the backside of the impellor intake side fins. Air cannot flow inside the cavitation. As the compressor speeds up the cavitation size grows for any given pressure ratio. Eventually the increase in flow generated by a higher compressor velocity does not make up for the decrease in area due to the expansion of those cavities.

Cavitation is worsened with increased flow because the faster the air flows through the compressor at any given static pressure ratio, the lower the internal pressure ratio vector in the direction of the cavitation as the flow is perpendicular to the direction of cavitation.. (Bernuelli Principle essentially.)

edit: Another possibility is that the choke limit is when the boundary layer between air and compressor is no longer stable. This is probably the most likely reason. Increased speed and flow both contribute to a destabilized boundary layer of air. Vorticity will become immense when the boundary layer is disturbed and the air will get very hot, and not flow well.

Anyhow ... sorry for that topic tangent.

Adrian~

Don't be sorry for the diversion, I truely enjoyed it, and it was intimately related to the topic of tuning. I did imply and desire to turn back to tuning, but all your comments and others are welcome information.

The extent lf my fluid dynamics knowledge is weak at best, so I am aprreciative also of the more thorough explication of choke (I always confuse the two sides of the compressor maps, my apologies). I find all of your explanations highly probable, cavitation, heat, and laminar distruption--all probably play a role in destablizing air flow.

Please let us know if you think the argument still stands, that water/methanol will stretch the choke line by reducing heat and possibly slowing the compressor. I would think that heat radiating from the impeller would be a profound cause of disruption of the boundary layer in particular.

I'm putting a thermocouple in after the turbo to measure temp, a pressure transducer would be nice too, but they are too much dough and I already have the boost gauge and engine MAP sensor to follow pressure.

Thanks again for your detailed input and data!

I've read through this thread, and this statement is really the crux of what's being discussed here. However, what I have not seen is any evidence or argument to suggest that this effect would be any different from the heat absorbtion that occurs when injecting water after the compressor. I.e., that it isn't accompanied by a proportional drop in pressure. I don't mean to be challenging, just looking for an explaination if there is one. It seems to me that there is a fairly large logic leap involved in the supposition that injecting water at the compressor inlet will increase the MASS at the compressor outlet (that is, a drop in temperature without a drop in pressure). It would be fairly easy to test. Anyone with a WRX + DeltaDash could gather this data from their MAF/MAP sensors. However the theory of why it would happen would be interesting as well. In order for the turbocharger to be "effectively" larger, it would need to flow a greater mass of air at a given PR with the same efficiency, and I cannot get my head around the logic of that.

The data you seek do not exist for our little turbos, but for prop jets and turbine engines the evidence is ample. Look under "wet compression" of "fogging" or "overspray" or "swirl flash" and you'll find the papers and studies.

In addition we have the testimony (the word comes from if you lie, your testicles are lopped off) of HotRod and some others that you can improve the efficiency of compression itself.

The only advantages to injecting pre-turbo are 1) you want a bigger turbo but are not ready to role the dice, and 2) you have no intercooler, can't fit one, or don't want one, and 3) there may be a thermal advantage to not generating the heat early in the air flow (w/out heating the intake system).

To me number 1 is paramount the others. The quest for a greater dynamic range in turbos has been a long sought pursuit--variable vane turbos, twin turbos of differing size, larger turbos with less spool friction (i.e. ball bearing), external wastegates, and so on.

That said, water injection is still a marvelous means of increasing the density of the air, suppressing detonation, and permitting combustion with more power/push (leaner AFR), even if injectin post-turbo.

It's not the holy grail, but it's sure is fun thinking about, and pretty cool if you can make you 400 CFM compressor flow 500 CFM. Penny per HP it's a huge mod.

#2 can be achieved with WI regardless. #1 is still dependant on the compressor flowing greater mass when water is introduced. I find this all very interesting, and may revise my plans for adding a second water jet pre-intercooler to adding it pre-compressor. I did find a good article based on your search terms that was very helpful - http://www.caldwellenergy.com/pdfs/WETCOM.PDF particularly "With compressor inter-cooling, the compressor discharge temperature is reduced considerably allowing more fuel to be burnt in the combustor. A secondary increase in power output is due to increase in mass flow rate." . It all sounds too good to be true but highly attractive if it works.

My second concern is how to calculate water usage for a given jet size given an assumption there will be a variable vacuum present at the compressor inlet. I suppose I could relocate my boost/vac gauge there for testing.

Turbine's Power output is thrust related, the higher the mass on the intake regardless of either water molecules or air molecules, the higer the outout thrust.

Engine's output is mainly oxygen related, mass-increase as a whole in the intake may not yield extra oxygen available for combustion if a large portion of the mass is water molecule. This is an interesting topic to discuss if there is an actual oxygen increase.

If the engine can make more power by injecting a fine mist before the impeller, it can easily be verified on the dyno?

Indeed.. this is why the data I would really like to see on the subject is output of the engine's MAFS compared to MAP and RPM. If no more air enters the vehicle intake, the turbocharger has not been made "effectively larger." It still feels like a big leap to me to state that water injected into the compressor will directly increase the efficiency of the compressor (vs. just reducing the outlet temperature). It may be that this is just a peculiarity of centrifugal compressors, or it may be that details that affect one type of system are being inappropriately applied to a combustion engine's turbocharger. Dyno results can be dubious on these issues.

I think there is a bit more to it. The MAF is temperature compensated but wet air will give a different read on the hot wire type.

In order to verify the theory, we really need to find a relate simple method to get some result - it may not have to be vey accurate but the claimed 20%+ air flow increase should give some positive indications no matter how inaccurate.

I really like the idea behind the claim and I think it will work but may not be a much as 20% in oxygen increase. Any thing to reduce heat is a good thing especially on the turbo impeller.

Most turbo on the market is water cooled, some benifit for sure.

I have been watching this thread with some interest. I'm beginning to become inspired to do a little testing. Currently I am only using one of the TEC3r's (www.getfuelinjected.com) mapable outputs to map WI (using Aquamists 2C system), with 2 nozzles post IC. I have another spare mapable output - and I'm beginning to think that placing another HSV in a line supplying a pre-compressor nozzle might be a worthwhile experiment. This would enable me to play with the water delivery and compare results of varying switch on points and delivery amounts independently of water that is being delivered post IC.

The only thing making me think twice at present is I am running a Garrett Disco Potato turbo (GT28RS, 0.86 AR compressor 76 trim, 0.64 AR turbine 62 trim) - so it is if anything oversized for a 1.6L engine making 280bhp (sized for future plans).

Worth making the effort? I have full data logging software that will allow repeatable power/torque graphs to be plotted if using the same stretch of road.

I can send you a HSV for that purpose?

Sounds like a plan. I'll post results here once I've got them.

But the air at the MAFS should not be wet. The spray would still be well downstream of the MAFS (at least, on most engine types). The data I'd like to see would be whether this practice can affect a real efficiency improvement from the compressor. That is, at the same PR & flow, there's a greater mass of air in the charge. If more air-mass exits the compressor, it had to enter the compressor, and that should be suitably measurable by the MAFS. Or am I missing something?

You are correct on the MAF location in general but some aftermarket bolt-on systems do have the turbo before the MAF.

I think we shall see slowMX5's results to make this discussion more meaningful.

I'll be tuning soon as well and post numbers. We will see what we get. Maybe Hotrod can be presuaded to throw a thermocouple in his set up and run plus/minus WI. He should see definitely see a drop in temp, but will he see an increase in boost is the question.

It's a tough experiment to compare because of detonation.

temp and mass flow
 

I don't have the resources to do the thermocouple test, but I know the result. Several folks have run similar tests and the temp drop is on the order of 30 - 40 deg F for common WI injection rates. For straight water injection with no alcohol the temp drop will be about 20 -30 deg F, depending on the relative humidity.

As far as the boost, I already know the answer to that question, My electronic boost controller has a peak hold feature that tells me the maximum boost achieved on a given run. I had to turn down my boost controller to avoid over boosting after I installed the pre-compressor injection.

The mass flow through a centrifugal compressor varies directly with the absolute temperature of the inlet air. Lower the inlet temperature, mass flow goes up. Its the nature of the beast.

Larry

HI,

Arch my brain cell herts, na just kidding love this thread,

I was thinking that when you inject water post compressor, this cools the intake temp,

but does this cool it enough to reduce the volume (less heat less volume) when the injected water mass/volume is included,

if so are you not getting the same effect , ie compressor pumpping more air but is hot, then its cooled back down and dropped psi (less volume same mass+water mass) to the same mass/volume as the preturbo injection goes?. or fairly close to it, ie both will have water taking air space, and have lower temp-more air mass.


just a thought, sorry if this sounds dumb to you, but mayby has some relivance to this topic as far as how much more air will be moved with pre-compressor injection vs post compressor injection.

although preturbo would make for good mixing/evap of injected liquad.

and the lower temp in the compressor exit would make for less volume(same mass), so there would be less pressure for the compressor to fight at the same mass flow so less heating and more efficenecy , hmm O.K. I'll stop typing now.

Cheers
Ryan

I've been watching this thread with interest, thanks for such an informative discussion guys.

I don't have any practical experience to contribute, but a comment by Hotrod has resolved the main issue that had been puzzling me.

I couldn't see why it made any difference whether the air was allowed to heat up in the compressor and then cooled by downstream injection, or whether upstream injection was used to prevent the air from heating up as much in the first place. At first glance it should not make much difference since a similar amount of water is evaporated (OK maybe *slightly* more evaporation in the upstream case due to the turbulence inside the compressor) so the final charge temperature should be similar. I don't think anybody has suggested any reason why the compressor efficiency would be improved by upstream water injection, and I can't think of any.

However, Hotrod has pointed out that the compressor works more effectively when it has denser air going through it. By reducing the air temperature rise inside the compressure, the upstream injection increases the air density inside the compressor and therefore increases the effectiveness of the compressor. (I think the distinction between effectiveness and efficiency is important in this case.) This means the same compressor will produce a higher boost pressure and greater mass air flow with upstream WI than with downstream WI, which I think was the original contention. My mistake was to assume this resulted from increased efficiency; I don't think it does.

Does this make any sense, or am I still missing the point?

Good summary
 

That's a good summary of the situation.

It also has a secondary impact that is not often discussed. If you increase the effeciency of the compressor, it takes less power from the turbine side to do the same work. The result of that, is the turbo spools faster and at a give boost pressure it will produce less exhaust gas back pressure.

What you have is a situation were several small, and complementary incremental improvements in the performance of the entire system make a noticable difference.

On my car, since the stock TD04L-13G turbo on a WRX is undersized by the factory, to get good midrange response. Added to that since I live and race at 5800 ft altitude, the effects of operating at high altitude made the workload for the turbocharger even greater.

The seat of the pants improvement in turbo spool and improved midrange power and torque were very noticable.

Larry

Studying the thermodynamics is dizzying. There are so many variables, I just cannot predict the outcome of measurement. Evidence from multiple sources makes me believe that the turbo will actually compress more air, but proving this with heat in the mix will be difficult.

I see two ways to measure an effect. One is to tune to knock with only WI at the throttle body, and then turn the water on pre-turbo and tune again to see if more power can be had. This only establishes that more power can be had. Perhaps injection of water immediately post-turbo would have the same effect? I'm not willing to do that control experiment.

The second way is to measure pressure and temperature post turbo with and without WI. We might hope to rely on the ideal gas law to compute a gain over and above mere evaporative cooling. Since on a mass basis we are only adding a small percentage of water, the molar contribution to the gas's mass flow should be minimal when computing pressure, as well, any effect on the R value should be mininal as well.

Using PV=nRT, we will be solving for n, number of moles, or an increase in the mass of the air compressed. We will have to adjust any pressure reading by compensating for the compressed gas temperature post-turbo. If there is a difference of more than a few percent at near redline RPM we should see it. Any ability to increase power without knock will be only confirmational.

I'm buying a hypodermic thermocouple to stick into the hose that runs to the intercooler. I don't know where the stock pressure sensor for boost is, preferable before the intercooler. (Do you know hotrod? I'm in a WRX STi.)

Sounds good
 

Sounds good to me.

The MAP sensor on the WRX is on the throttle body passanger side right above the throttle shaft. It has a three pin connector and is held in place with 2 bolts.

Larry

Re: Sounds good
 

As far as I understand it, the theory is that upstream water injection enables the compressor to achieve greater mass air flow (by reducing the temperature rise and loss of density that occurs inside the compressor).

I don't think it is possible to prove this theory by comparing with/without water injection, because we already know that water can be beneficial wherever it is injected.

The only way I can see to prove the theory is see whether we get more mass air flow using WI immediately upstream of the compressor or with the same amount of WI immediately downstream of the compressor. By concentrating on mass air flow at the compressor I think we can avoid complications from other changes in the tune. To be conclusive, we would need to ensure that a similar amount evaporates in both cases. I think it might be possible to achieve this by using only a small amount of WI so that we get total evaporation in both cases, does this seem reasonable? Since mass air flow is the thing we are looking for, it would make sense to measure that directly if possible, rather than trying to infer it from changes in temperature and pressure or detonation, net bhp etc.

I think we agree. I'm talking about computing the number of moles (mass of air) and you are talking about mass flow, but we are both talking about mass.

In the end we are asking: does the compressor carry more air particles if cooling occurs during compression, than if cooling occurs after compression?

I believe the answer is yes, and here is my lastest thought experiment. It's gets right to the heart of the matter: number of air molecules compressed per rotation.

Imagine this case. The ambient air is -20C, when compressed this air, with each turn of the compressor wheel, contains more air molecules (cold is denser) than if the air were at 20C (less dense few air molecules/unit area).

Likewise, if during compaction, temperature (i.e. kinetic energy) is kept constant, the air being compacted will be more dense, and hence again with each turn of the compressor more air molecules are compacted.

The work that we are interested in is the work of compression, which is all about the pressure ratio given by the centrifugal forces than are acting on the air. Imagine that each time the impeller blade swings by is analogous to a shovel full of air. If each shovel full wieghs twice as much, then you end up with twice as much air shoveled per unit time. Similarly, with a pressure ratio of 2 (isothermic) each swoop of the impeller results in a compression of twice as much gas.

If without water the turbo is compressing air at a PR of 2, and we could completely suppress all heat during compression, we will have gained back all of the adiabatic heat loss and all of the compressor's inefficiency. The percent gain will be directly related to the PR. The larger the PR, the more gain can be had for each compression event--just like the shovels of air, 4X density air is more than 2X air.

As has been said before, the gain could be huge (40%+) if all the heat is absorbed by a small amount of water. To tie this back to yesterday's discussion. We would need a much larger turbo to compress say 40% more air, that was then cooled by WI post injection. It's all in the air density per rotation of the compressor--more dense air (cooler) leads to more air molecules compressed per rotation of the impeller.