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Some of this week's Letters to AutoSpeed!

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Re Top-Mount Trial

When comparing the mass of the Subaru intercoolers, shouldn't the RS Liberty one have been filled with water before weighing?

Patrick McAuliffe

An excellent point - of course, it should all be measured as part of the intercooler's thermal mass. The Subaru workshop manual states the RS Turbo's water-to-air system has an overall coolant capacity of 1.9-litres (which is distributed throughout the heat exchanger, front radiator, pump and pipes). We imagine about a third of this volume - and, therefore, a third of this mass - can be added to the thermal mass we quoted for the RS intercooler. Also, bear in mind that the coolant is being pumped through the system at all times and that heat will be dissipated into the coolant up and downstream of the intercooler body. Well spotted!

Re Top-Mount Trial 2

In regard to the article by Michael Knowling on the WRX intercoolers, I would like to say that the numbers were handled in a very neutral manner and all in all it was fairly well balanced. However, I feel that the results may be misleading in that there are a few simple tests that would have made it much more accurate.

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First of all, a big percentage of the flow restriction - as you noted - is caused by the piece of crap plastic hose fitted to the factory IC. It would have been good to see the overall flow of the factory ICs with an aftermarket silicon hose. In my opinion this mod alone will bring the factory IC performance up to and above the aftermarket ones - maybe not on a flow bench but in on-car testing.

The thermal mass is also a little misleading. Sure, a heavy intercooler has more mass to dissipate heat spikes, but only if it is cool to begin with. I have tested pre and post IC temps (MY00/ Version 5-6 WRX) with K-type thermocouples in all kinds of driving conditions, and the factory IC is able to get rid of 70 degrees Celsius of heat soak in about 30 seconds of moving at 60km/h - even quicker with a water spray. Double the intercooler mass and that time is doubled.

On the car, the pressure drop of the factory IC is negligible - I've measured less than 1 psi at redline with a silicon hose, as opposed to 2.5 psi with the standard plastic one. Compare this to the 3-4 psi drop I have seen across large front-mount ICs...

On the track with a compressor outlet temp of around 130 degrees Celsius, the post IC inlet temp never went above 45 degrees on a 22 degree day - and that was without a water spray. On the street at above 60km/h, the inlet temp settles at about 7-10 degrees Celsius above ambient, and never rises more than 5 degrees when given a spirited overtaking move.

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Of course, the factory IC will probably be out of its range when power goes above 140-150kW ATW, but for most of the Rexis on the street I'm a firm believer that the silicon hose is a much cheaper option and gives far, far more bang for your buck (except in the case of a Version 2, I suppose). If an aftermarket IC was fitted with the plastic pipe, you would be asking yourself why you spent so much money for no noticeable gain. It seems that for about $1500 you can get a silicon hose with an aftermarket IC thrown in!

This is just my 2 cents. I know it is impossible to do on-car testing with the different version ICs on the same car, but I do think on-car test results speak for themselves. Cheers.

Brett Turner

As stated in our Results at a Glance; "Adding the feed pipe to the intercoolers has an interesting effect. It's clear that those feed pipes with plastic convoluted sections flow very poorly - airflow through Version 2 to Version 6 WRX intercoolers dropped nearly 50 percent when the feed pipes were attached, while the Version 7 fell about 40 percent. The STi and aftermarket feed pipes show the gains that are possible by smoothing the path into the intercooler - the alloy/silicone Version 2 STi feed pipe more than halved the restriction caused by the standard plastic part."

Your points - and measurements - in regard to heat soak are very interesting.

As we said toward the beginning of the article, nothing beats on-road testing. Certainly, the top-mount core acts as giant underbonnet heat-sink while the car is not moving and (all else being equal) doubling the thermal mass of the 'cooler will virtually double the time taken to cool down after being saturated with heat. This can be a major problem for cars that are used mainly in stop-start driving conditions; the only way to combat this is to ensure air flows through the core even while the car is stopped. An electric fan arrangement on the underside of the core (similar to a kit produced by Japan's Zero Sports) appears to be the only way around this.

The pressure drops you've quoted across the standard IC and an aftermarket front-mount should certainly cause some discussion!

X-Trail Fighter

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Hey guys, love the website. I was wondering if you have come across any details of performance mods for Nissan X-Trails. Strange but true. It's a great engine, but a bit more grunt never hurt! If not - no probs - keep up the good work!

Bob Irwin

We haven't heard of anything off-the-shelf yet, but we'd start by looking a changing the air intake and exhaust (though we don't know how receptive the ECU will be to those mods).

Import Power

I'm looking at buying an import for between 15 or 20 grand - it has to be a Nissan turbo (of course, presently being a VL-T owner). The two cars I'm looking at are the SR20DET 180SX and the twin-turbo 300ZX. I like the 180SX because they're a little cheaper, but I'm thinking the 300ZX would be worth the extra bucks because it's such an awesome car.

What would you buy if you had to make the decision? (It'll make it easier for me). Thanks.

Matt Roper

Yes, the 300ZXTT is an awesome car - but they inevitably cost a huge amount of money to maintain. We've seen many ZXTTs needing full engine and twin turbo overhauls and it's a BIG (and, therefore, very expensive) job. The 180SX SR20DET 5-speed is a great, enjoyable car for the money and simple tweaks give it a heap more power. We reckon the 180 is the way to go.

Another Starlet Bargain

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In regard to your article on the EP71 Starlet. This car seems very similar to another Toyota produced for the Japanese market around the same era - the 1989/90 Toyota Corolla GP Turbo. This car is a bit different cosmetically with pop-up headlights and a slightly different interior (but with the same LED boost gauge and gaudy writing on the side). It also seems a little better optioned with 4-wheel disc brakes and the (I think) 1.5L 3ETE motor. At a similar price (if you can find one) this car also represents a bit of a bargain. There are some pictures of one for sale at

Just thought you might be interested in the comparison.

Glenn Smith

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