Jump to content
LegacyGT.com

Study of Hood Scoop Effectiveness and Intercooler Airflow


Recommended Posts

The question of how much air flows through a subaru tmic intercooler has bothered me for a while. I can’t find any sort of real world flow information for our setups, so I’ve decided to gather some.

 

Hood Scoop Design

 

First let’s look at the scoop:

 

IMG_0869.thumb.jpg.baf29017fe0cb8878f524f198fce002a.jpg

 

Our scoop is about 21in x 1.6in, so ~33in^2 in area.

 

IMG_0872.thumb.jpg.044416dfd22132c225bcf44803fc7967.jpg

 

Our intercooler is about 10in x 10in, so ~100in^2

 

Core_duct_shape.JPG.7542f64d6e0596d1823a244a9adbfdff.JPG

 

Per Bell’s Maximum Boost, you want to have a duct inlet area to be smaller than the intercooler frontal area, but more then 1/4 the intercooler area. The 4th gen legacy is at ~33%, so the design is within the recommended range. Note that out intake duct has a vent area for cooling the turbo that’s letting some pressure escape, but we’ll ignore that for this testing.

 

Intercooler Dynamic Pressure Delta

 

Next we’ll measure the dynamic pressure across the intercooler core while moving. To do this, I’m going to use a differential pressure sensor from the pitot static system of an autonomous plane wired to the right TGV. This uses a MPXV7002 sensor with a range of +/- 2KPA and is temperature compensated up to 60*C. Transfer function works out as Vtgv -2.5=P(kpa).

 

IMG_0867.thumb.jpg.6130aff72f3378bfe6a4b663205c471d.jpg

 

The lower chamber line is taped to the air conditioner pipe under the intercooler. This is about 2in below the core but likely the pressure gradient isn’t huge here. The upper line is positioned where the tip is at one of the air channels. Both line are perpendicular to the flow so should provide the static pressure at each point.

 

IMG_0872.thumb.jpg.044416dfd22132c225bcf44803fc7967.jpg

 

The sensor didn’t do as well as expected, likely due to bad electrical connections and bumping up to its temperature limit. But with a bit of filtering and data processing I was able to get some interesting results. I have a hard time visualizing pressure in metric, so I’ve converted this to inH2O.

 

492454363_Screenshot2017-05-0821_29_33.thumb.png.1b95398e5d9be8b4ce01c28c8383e87a.png

 

So we can see two things here: The pressure sensor was not zeroed correctly, and that pressure rises with the square of speed. This is to be expected as if you pull out your fluids one book cf183f4329b2ddd38828e731f104587366742f3b.png.945b4be216f7d020a3c0116f956d17b2.png. So at 80MPH we’re seeing something like 0.75 in-H2O dynamic pressure. Looking at more raw data you can see that the pressure isn’t that smooth, so the intercooler flow is quite turbulent.

 

1176880014_Screenshot2017-05-0821_30_11.thumb.png.7cb11247ae9608b140fa5dfbabb4a44f.png

 

Intercooler Flow Speed

 

So we have some idea of the pressure, what about the air speed? Well I picked up an Anemometer on eBay and tested it. Note so the splitter didn’t kill the sensor I had to place it at the very bottom of the intercooler as seen below:

 

IMG_0892.thumb.jpg.f6a45fd8300e3041483ec5621cc48905.jpg

 

Unfortunately this didn’t have any sort of analog output I could pipe into the ECU, so I had to record a video of the Anemometer display while driving. I called out the speeds while driving in the two videos below.

 

<MPH video>

[ame=http://www.youtube.com/watch?v=BIe7teIOaUE]Legacy GT Hood Scoop Test - Ft/Min - YouTube[/ame]

<KPH video>

[ame=http://www.youtube.com/watch?v=8lTEMxQb0Pk]Legacy GT Hood Scoop Test -Km/Hr - YouTube[/ame]

 

And that gives me this nice little chart:

 

1867325327_Screenshot2017-05-0821_30_48.thumb.png.137bc9323c9f8471972be5a2bd1782e7.png

 

So air speed is liner with vehicle speed. Per mph vehicle speed, the intercooler air speed increases by 20 ft/min or 0.22mph.

 

So all and all it looks like our scoop is fairly effective.

 

More testing to come, and I have a few things I want to try to boost the flow.

core_duct_size.JPG.8110cb2dd972bba2f4aa523bb157155c.JPG

1886680193_Screenshot2017-05-0821_39_28.png.a4e6cc0849e40dc3a61776e87b9478f1.png

45-85 filtered.csv

Edited by utc_pyro
Future
Link to comment
Share on other sites

Good work!

The turbulence should be no surprise given the directional changes the air has to make.

 

I'd really like to see a comparison with cars like a GRB and a BM9. Volunteers?

Obligatory '[URL="http://legacygt.com/forums/showthread.php/2008-gh8-238668.html?t=238668"]build thread[/URL]' Increased capacity to 2.7 liters, still turbo, but no longer need spark plugs.
Link to comment
Share on other sites

I'd really like to see a comparison with cars like a GRB and a BM9. Volunteers?

 

Anyone that wants to try is welcome to borrow my rig. TGV wiring will be trickier for the pressure delta, but just the flow velocity alone should be useful.

 

I'm curious to see how a 4th gen with a Grimspeed/Process West style TMIC would do as the intercooler frontal area is so much higher vs the inlet area.

 

If you have a safe place to do the test, try 95 MPH+. In my experience, the aerodynamics of the car appear to cause airflow to shoot above and miss the hood scoop, causing the charge air temperature to increase.

 

I'll give that a try once I get an axle replaced. It's clicking when over 80 MPH, so I don't want to push things right now.

Edited by utc_pyro
Link to comment
Share on other sites

yeah, different model years and lifts would be intetesting. pre/post facelift, outback, wrx, etc...

 

cool stuff. love seeing/reading stuff like this.

 

Are you volunteering your Outback for testing?

 

PM me and I'll send you stuff to try.

Link to comment
Share on other sites

yeah, i'd be up for doing some testing. could have work get me the same sensors.

 

was thinking more about this. do you have your engine undertray? and should plug up the lower grill and see if there is a change. always heard people discussing how the undertray helps pull air through. would be cool to have conclusive data (or supporing data).

 

also, is that an oem tmic? looks AVO-ish. too square of tubing and those triangles on the ends...

Edited by Flinkly
* Build Thread * 26.53 MPG - 12 month Average *
Link to comment
Share on other sites

Yes it's an AVO. It's 3.5" thick along with being bar and plate so i wonder how much this is slowing the air down.

 

Stock undertray, but the front mounting points are broken so it sagged oddly. This is one area I'd really like to test on. The USDM cars came with the same undertray that the NA cars had, but the JDM/EDM is longer. One of the JDM versions actually has the same undertray as a '08+ WRX. The guy the sits next to me at work has one of those, maybe he'll let me swap the trays for a day.

 

I'm likely going to put on a beatrush under panel this summer to fix the mounting issue so it'll be interesting if that changes anything.

Link to comment
Share on other sites

dang, and while making some bfast i realized your logging from the ecu through the tgv inputs. haven't deleted mine yet for such usage, so would be harder to get the same data,, or good data.

 

That was only differential pressure, I measured velocity with a gopro then manually entered that into excel while watching the video. To figure out the effective airflow through the intercooler I think the velocity and area is all you really need. With 1in H2O increase in pressure the air density is 0.25% higher so that component dosen't really change the mass of air avalible to pull heat away.

 

Pressure is more useful if you’re trying to change something to create airflow or debug a problem (see my next post). It also had the side effect of showing how turbulent the flow was as the sensor responded much faster than the spinning blade Anemometer.

 

On a positive note I have a guy lined up to velocity test his ’14 WRX next week!

Edited by utc_pyro
Link to comment
Share on other sites

Now to take the conversation on a small detour…. When I started this testing the intention was proving if fan feeding the intercooler was a good idea or not. There has been a LOT of discussion over the years about this but not much testing on the aerodynamic side of thing. Some people posted before and after temperature deltas, but nothing in regard to the clam that a fan would be restrictive once moving. I had hypothesized that it wouldn’t be a restriction at any speed one would reasonably ever hit on the street, but with my tiny AVO intercooler airflow is pretty important:

 

[ame=http://www.youtube.com/watch?v=6sveQ7R95g0]2005 Legacy GT AVO TMIC - YouTube[/ame]

 

So with the pressure/flow profile above let’s compare that to the performance charts of two commercially available fans: a cheap ebay special (with more realistic specs) and a 130W Maradyne fan. These are “9in” fans with a shroud diameter of 9.7in, so take up ~0.51 ft^2 of the ~0.78 ft^2 intercooler core frontal area. Fans will become restrictive once the flow exceeds what their output at 0 static pressure. To figure out the static flow (when stopped or slow) you look for the intersection of static pressure and speed.

 

1324016559_IntercoolerFanTheory.JPG.838a8a28b3fffdfa21e6011a80d68ae7.JPG

 

So looking at this chart it’s not quite as bad as some implied, but it’s not exactly great ether. The cheap fan starts being an issue above ~60mph, and the high end fan will keep boosting flow through ~80mph. In exchange, the center part of your intercooler would have the same flow in traffic as if you were driving 55mph or 65mph respectively.

 

That’s handling only the center of the intercooler and thus isn’t very efficient, so what happens if you add a (theoretically perfect) shroud to cover the entire core?

 

510766270_IntercoolerFanTheoryshroud.JPG.4db674509d021d8dffd96cf353d14089.JPG

 

Yep, the restriction is real here! So the cheap fan starts being an issue over 40mph and the 130W fan above 55mph. So in exchange for more flow through the core when going slow, you’re killing performance right where you’d need it on twisty backroads.

 

So in conclusion it looks like I was wrong, a fan really can restrict things. What’s worse is these are all presuming the fan is running at 100% all the time, so it’ll increase electrical load and make a lot of noise. Hence why you don’t see OEM’s use them unless strange placement dictates the use (rear/mid mounted motors manly). That said an unshrouded high performance fan with some form of automatic control appears like it could help in urban driving.

Edited by utc_pyro
Link to comment
Share on other sites

I live in a suburb of Chicago and I run my fan 100% of the time unless I'm on the highway. Running a similar set up for over a year now. Since I do a lot of city driving I don't see over 60 mph unless I'm on the highway or know I'm going to cruising above 50 mph for awhile. The heat soak is near non-exist and the butt dyno notices the difference. It's not a prefect solutions but on hot days I don't feel an over heated TMIC. While you are seeing some restriction, don't forget that the TMIC is already somewhat cooled off. Edited by Schrei Wolf
Link to comment
Share on other sites

Well, I am in Charlotte now so maybe we can meet up at the Dragon some time and you can test my fan setup as I never got to do any kind of pressure testing. From what I have seen temperature wise, my setup has not reduced flow. I have mine always running as it is just much easier and some made mention that the front fans running at highway speeds with no issue or something like that.
Link to comment
Share on other sites

For comparison, I found where someone did a similar test on the intercooler of a Lotus Exige

 

inletstudy.jpg.8790fa70084e0a0c8d0265584ee732e7.jpg

Source: 2007 Exige S Intercooler Air Flow Study

 

They had a wider intercooler then we do, but it's shorter and has slightly less frontal area. You can see that the Subaru hood scoop is doing a lot better job than their stock roof duct, we have over twice as much airflow.

 

Infact some of them modify the ducting for more airflow:

919978619_Intercoolerairflowwroofduct.gif.0248acec49c54cf39d0939aaae3ec139.gif

Source: RLS CF Exige Window

This is better, but still less than what our hood scoops will do.

 

Hopefully my coworker with the GR WRX will have time this week to test his tmic. Still looking for a bm9 legacy to test. Silinc3r, I'm down to test your fan setup as well.

Edited by utc_pyro
Link to comment
Share on other sites

  • 1 month later...

So in conclusion it looks like I was wrong, a fan really can restrict things. What’s worse is these are all presuming the fan is running at 100% all the time, so it’ll increase electrical load and make a lot of noise. Hence why you don’t see OEM’s use them unless strange placement dictates the use (rear/mid mounted motors manly). That said an unshrouded high performance fan with some form of automatic control appears like it could help in urban driving.

 

Are you doing your testing with the fan on top of your IC or underneath?

Link to comment
Share on other sites

Are you doing your testing with the fan on top of your IC or underneath?

 

I'm not planing on putting a fan in to test at this point. There isn't enough room under the AVO intercooler if you have a 5EAT (wiring harness and breathers are in the way), and on top requires cutting of the divider in the scoop. Also the results from the testing show you need a very high power fan running full time to not restrict airflow.

 

I have a 9in fan on hand and it'd be fun to try, but I still need to test two more cars and finish 500 other things on mine before I can get to that.

Link to comment
Share on other sites

Hmmm, a grid of slim 120mm fans to pull air through. That would be interesting to fabricate. Wonder if you would have similar restriction problems; 120mm "pipes" of air pulled vs. the full surface area without additional restrictions.
Link to comment
Share on other sites

Hmmm, a grid of slim 120mm fans to pull air through. That would be interesting to fabricate. Wonder if you would have similar restriction problems; 120mm "pipes" of air pulled vs. the full surface area without additional restrictions.

 

Any "thin" computer fans (thinnest 120mm fans tend to be 25mm) don't have nearly enough static pressure to deal with the restriction a TMIC will place on it. You might be able to get away with a 38mm thick fan (some beastly Deltas out there, as well as some Sanyo Denkis, among others), but I wouldn't expect too much out of those, especially for a thicker-than-stock aftermarket TMIC.

Link to comment
Share on other sites

Yea I can see how a fan would restrict air flow on top. If the fan won't fit under your IC I don't think I'd have any luck under my GS TMIC.

 

Have you though about using high flow computer fans as a smaller option?

 

Actually I think a fan on the GS intercooler might actually work better. It's wider and thinner. Our scoop is really quite small to feed an intercooler that big, so the average airspeed will be slower and thus possible for a fan to keep up with. use that data I collected and see what you come up with. If you look at some WRC cars they had a fan on the hot side of the intercooler without a shroud. I suspect this was help while staging with anti lag on, but obviously didn't cause enough issues at top speed for them not to use it.

 

computer fans really don't have the oomph for this, especially the waterproof ones. You'd need server garade screamers to keep up with the airflow, even with a oversized IC like the grimmspeed. They are much more likely to hurt than help.

 

Edit: SPAL makes a 7.5/8 in fan that's rated at 500 CFM. If you stick that under just one side of the GS intercooler is shouldn't be an issue untill into triple digit speeds. It will still need to be on to not restrict things though.

Edited by utc_pyro
Link to comment
Share on other sites

what if you built a duct that connected to the underside of the TMIC and put the fan at the lower end of the duct? Looking at my setup (PW, hta68, 5mt) there's a little bit of room to run ducting down the DP side of the transmission.

 

I never noticed much heat soak, however, I rarely drive in the city or slow for that matter. Only at autox did I have any trouble. There I put a 5lb bag of ice on my PW while waiting. One run, I dumped the ice on it and put the hood down. The ice was fully melted when I got done, so something happened. :-)

Link to comment
Share on other sites

what if you built a duct that connected to the underside of the TMIC and put the fan at the lower end of the duct? Looking at my setup (PW, hta68, 5mt) there's a little bit of room to run ducting down the DP side of the transmission.

 

I never noticed much heat soak, however, I rarely drive in the city or slow for that matter. Only at autox did I have any trouble. There I put a 5lb bag of ice on my PW while waiting. One run, I dumped the ice on it and put the hood down. The ice was fully melted when I got done, so something happened. :-)

 

That's likey to be more effective, you can use better designed fans that way then the crapo slim axial ones.

 

As the testing showed, our scoop does fairly well once moving even at moderate speeds. The only people that would really benefit are ones that drive in the city a lot, or compete in motorsports that involve a lot of staging at idle (autoX, maybe drag racing). If highway, fast back roads, or the track is your thing fans are as likely hurt performance as help.

 

Does your racewagon still have a TMIC?

Link to comment
Share on other sites

the silver bullet wagon (nee-silver eagle) is fmic.

Boxkita's wagon is tmic.

 

I have no data on the bullet. Boxkita's wagon was always cool at the end of a session even on hot days. At autox, I used the bag of ice to cool it off between sessions.

 

If I'd ever gotten serious about autox (classed as SM, so would have had to), I'd have built a dry ice setup on top of the TMIC with down craft pipe from the bottom. My only competitor has twice the horsepower and nearly double the width of tires. Rather than bitch about the classing, I quit. :-)

 

On track, as long as I took the straight in clear air, power was always fine.

Link to comment
Share on other sites

dang, and while making some bfast i realized your logging from the ecu through the tgv inputs. haven't deleted mine yet for such usage, so would be harder to get the same data,, or good data.

 

You have a tactrix listed in your build thread... do you do the tuning yourself?

If so, then you can just disconnect the TGV sensor and disable the codes in the tune. Even if your car has the motor/sensor as one unit then it could still work, you'd just have to disable a few more codes. IIRC the TGV's are normally open, so unplugging the motors wouldn't cause any problems.

Link to comment
Share on other sites

You have a tactrix listed in your build thread... do you do the tuning yourself?

If so, then you can just disconnect the TGV sensor and disable the codes in the tune. Even if your car has the motor/sensor as one unit then it could still work, you'd just have to disable a few more codes. IIRC the TGV's are normally open, so unplugging the motors wouldn't cause any problems.

 

not so on the 08/09 models. the earlier cars only had them closed till you used a little bit of throttle after startup. the late model cars use then on and off during a driving session, and no one really knows (or cares) to find out when they're active.

 

i've also got some JDM "TGV"s that i just need to install, but i keep flip/flopping about what i want to install and how many iterations i want. best case scenario would be installing and tuning for everything at once. but that just isn't goign to happen, and so i need to get over it and install a hefty chunk and just enjoy it.

 

there's also the rear O2 input too. easy to "delete" the rear O2 from the tune, but i'm hesitant to put too much voltage into that as it's only seeing max ~2V from the sensor. most people feed it a 0-5V signal (wideband).

 

oh, and so far i've done all my own tuning, if you can call it that. i'd consider myself an almost complete noob, and even though i'm "smart", i'll probably never try to learn it too much.

 

 

EDIT: re-read and i'd bet the TGVs are normally open still; spring loaded. i just need to get off my bum and swap them out, with some new ID1000s and new TS system...

Edited by Flinkly
* Build Thread * 26.53 MPG - 12 month Average *
Link to comment
Share on other sites

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


×
×
  • Create New...

Important Information

Terms of Use