How to measure air-flow under the bonnet...?
Discussion
Bit of an odd one this.
I have an old impreza p1 with a front mount intercooler fitted. I have fitted a 9-row air to oil cooler in the original top-mouint intercooler position (against the bulkhead) with the idea that the bonnet scoope will feed air to the cooler.
However, the temps are still getting a bit high on track, well, higher than I wanted anyway - 112 degrees.
A bit of research reveals a lot of internet hearsay that there is very little airflow through the scoop at higher speeds becuase the air comes off the front of the bonnet and misses the scoop. Hence the higher scoops you sometimes see.
I've been thinking of trying some kind of experiment to measure the airflow. We've got a weather station thingy with a wireless anemometer. Is this likely to work, fitted under the bonnet in front of the oil cooler? Any other suggestions?
Cheers,
Iain
I have an old impreza p1 with a front mount intercooler fitted. I have fitted a 9-row air to oil cooler in the original top-mouint intercooler position (against the bulkhead) with the idea that the bonnet scoope will feed air to the cooler.
However, the temps are still getting a bit high on track, well, higher than I wanted anyway - 112 degrees.
A bit of research reveals a lot of internet hearsay that there is very little airflow through the scoop at higher speeds becuase the air comes off the front of the bonnet and misses the scoop. Hence the higher scoops you sometimes see.
I've been thinking of trying some kind of experiment to measure the airflow. We've got a weather station thingy with a wireless anemometer. Is this likely to work, fitted under the bonnet in front of the oil cooler? Any other suggestions?
Cheers,
Iain
Pitot tubes! 
Should be easy enough to knock a few together to get measurements at different points. Recording the manometer readouts would be the trickiest bit.
http://www.engineeringtoolbox.com/pitot-tubes-d_61...
Should be easy enough to knock a few together to get measurements at different points. Recording the manometer readouts would be the trickiest bit.http://www.engineeringtoolbox.com/pitot-tubes-d_61...
Selotape short pieces of string to the bonnet around, and on top of the scoop. Drive at speed and observe (using go-pro or similar if necessary). String will align to "flowlines"!
Also, 112 degC oil temp is absolutely fine! Most modern semi/full synth oils will run continuously at >140degC without degradation. (OEM "HST" durability tests for example are done at ~150degC bulk oil temp)
Also, 112 degC oil temp is absolutely fine! Most modern semi/full synth oils will run continuously at >140degC without degradation. (OEM "HST" durability tests for example are done at ~150degC bulk oil temp)
make a U-tube manometer from a piece of clear plastic tubing and stick it to the windshield. You can use it to measure small pressure differentials anywhere the tube ends will reach.
10mmH2O ~= 0.1kPa
Keep the ends of the tubing perpendicular to the expected airflow to approximate static pressure. An end pointing into the airflow direction will give you total pressure on that side of the U-tube.
If you measure your static pressure differential across the intercooler, you'll quickly see roughly how air flowrate through the rad behaves with road-speed.
10mmH2O ~= 0.1kPa
Keep the ends of the tubing perpendicular to the expected airflow to approximate static pressure. An end pointing into the airflow direction will give you total pressure on that side of the U-tube.
If you measure your static pressure differential across the intercooler, you'll quickly see roughly how air flowrate through the rad behaves with road-speed.
Edited by AER on Friday 13th December 05:00
crossy67 said:
I've always thought you'd get better flow with the scoop facing the other way (backwards towards the screen). This way all that lovely cool air you're cramming into the engine bay will have an easy passage to escape, especially at high speeds and towards the top of the engine bay.
Like a "cowl induction hood" on US muscle carsLots of good ideas, thanks all.
On the road, even driving pretty hard on deserted roads in the west highlands, the oil temp never got above 99. On track it very quickly got to 120, peaking at 112. That may well be a safe number but I found myself backing off, worrying about oil temps.
The car has a modine and I know plenty of people who track Imprezas without a secondary cooler but I like the safety net tbh.
On the road, even driving pretty hard on deserted roads in the west highlands, the oil temp never got above 99. On track it very quickly got to 120, peaking at 112. That may well be a safe number but I found myself backing off, worrying about oil temps.
The car has a modine and I know plenty of people who track Imprezas without a secondary cooler but I like the safety net tbh.
Fleckers said:
When I had my scooby I removed the blanking plates under the bonnet vents
Found the car ran much cooler as it let hot air out
In traffic you could see the heat escaping especially on a cool day
If your oil cooler is against the bulkhead surly there is no air flow through it ??
Thanks, I've already removed the blanks. The core actually sits about 3" forward of the bulkhead. I'd even considered some ducting from the TMIC scoop to the rad but want to check how much air is getting to it first.Found the car ran much cooler as it let hot air out
In traffic you could see the heat escaping especially on a cool day
If your oil cooler is against the bulkhead surly there is no air flow through it ??
If I binned the ac I could mount the cooler at the front but the p1 is a road car and ac is very nice to have. I could ditch the driving lights and mount a bigger, square cooler behind one of the bumper "holes" for the driving lights...but they're useful fit winters back roads.
A cheap-ish Magnehelic pressure gauge and reading some of the ideas about using one on www.autospeed.com should do the trick.
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