| Issue: 210 | Section: Technical Features | 10 December, 2002 |
Top-Mount Trial - Part TwoFlow bench testing and looking at the thermal efficiency of ten Subaru top-mount intercoolers...By Michael KnowlingClick on pics to view larger images |
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Done a few basic mods to your Subaru WRX? Not interested in the hassle and expense of going for a front-mount 'cooler? Well - following the five top-mount 'coolers tested in Part One of this article - we'll now continue with the five remaining intercooler and give you the overall wrap-up...
Version 5-6 (MY99-00) WRX Intercooler
| Flow | 286.3 cfm |
| 134.8 cfm with standard Y-shape feed pipe attached | |
| Mass | 3.8kg |
| Core Measurements | 37.5 x 14.5 x 6.2cm (3371cm2) |
As mentioned in Part One, the Version 5-6 intercooler has the same core dimensions as the previous model but has much denser external cooling fins and revised internal fins. This contributes to a big 21 percent increase in mass over the Version 4-5; we can only guess Subaru suddenly decided to get very serious with charge-air cooling.
This focus on improved cooling has come with a trade-off, however. On the flow bench, the Version 5-6 WRX intercooler dropped to nearly to the performance of the Version 2 WRX intercooler. With just 286.3 cfm flowing at 28-inches of water, this is certainly one of the most restrictive Subaru top-mount cores.
Airflow continues to decline when the standard Y-shape feed pipe is bolted on; this part is identical to that used on the Version 3-4 and caused a similarly massive flow loss. The result - at 134.8 cfm - is the worst flow figure of all the 'cooler/feed pipe assemblies.
Version 7 (MY01-02) WRX Intercooler
| Flow | 293.1 cfm |
| 134.8 cfm with standard Y-shape feed pipe attached | |
| Mass | 3.5kg |
| Core Measurements | 41.5 x 15 x 6.2cm (3860cm2) |
Surprisingly - despite its larger dimensions - the current Version 7 WRX intercooler has 9 percent less thermal mass than the unit before. On the other hand, its 15 percent larger core volume - with a completely different cooling fin design - redeems its overall charge-air heat exchange potential. Airflow through the Version 7 intercooler is only slightly improved over the previous Version 5-6 - at 293.1 cfm, though, it's still a fair way behind the flow of the Version 3-4.
The Y-shaped cast alloy/plastic feed pipe is slightly different to those used in previous models and its restriction isn't quite as severe - adding the Version 7 feed pipe reduced airflow by 37 percent (down from about 50 percent in the Version 3-4 and 5-6 WRX).
Aftermarket Version 7 (MY01-MY02) Intercooler
| Flow | 349.5 cfm |
| 231.7 cfm with silicone aftermarket Y-shape feed pipe attached | |
| Mass | 8.3kg |
| Core Measurements | 48.8 x 17.5 x 11.2cm (9565cm2) |
Boy is this thing heavy! The MRT bar-and-plate replacement for the Version 7 WRX 'cooler is well over double the mass of the original part; it's so heavy, in fact, we reckon you might need to strengthen your intercooler mounting brackets! Much of this extra weight comes from an 80 percent thicker core, which - in addition to contributing to a massive 147 percent larger core volume - also enables air to flow more freely. The airflow through the MRT Version 7 core is exceptional - its 349.5 cfm result equates to 19 percent more flow the stock unit.
As part of the MRT upgrade a two-piece silicone Y-shaped feed pipe is supplied. Thanks to its smooth internal surface, this pipe gives minimal restriction - adding it to the intercooler reduces airflow by a relatively low 33 percent. Note that this particular feed pipe won't mate to your factory Version 7 intercooler (again, damn!).
BG (MY97-98) Legacy Twin-Turbo Intercooler
| Flow | 314.3 cfm |
| 255.7 cfm with twin cast alloy feed pipes attached (not including plastic pipes to turbochargers) | |
| Mass | 3.6kg |
| Core Measurements | 37.5 x 14.5 x 6.2cm (3371cm2) |
Hmmm, these figures look familiar. It seems the imported BG Legacy twin-turbo intercooler core is the same as used in the Version 3-4 STi - that's not surprising considering they're from the same timeframe and are used on engines making similar power. On the flow bench, the TT intercooler flowed around only half a percent less than the Version 3-4 STi job - this margin is so miniscule, however, it's likely to be variation of the flow bench.
In terms of heat exchange performance, the Twin-Turbo intercooler is around half a kilogram heavier than the Version 3-4 STi - though we suspect this is only because of some extra brackets and a metal core garnish. The core volume is identical to the Version 3-4 STi and WRX; that means, amongst the OE intercoolers, its core volume is beaten only by the Version 7 part.
BF (MY92-93) Liberty RS Water-to-Air Intercooler
| Flow | 221.1 cfm |
| 211 cfm with standard RS feed pipe attached | |
| Mass | 3.9kg |
| Core Measurements | Unable to measure, but smaller than Version 2 WRX (at 2852cm2) |
First, we must point out that the 3.9-kilogram thermal mass quoted for the RS water-to-air heat exchanger is a little misleading in the context of air-to-air intercoolers. Some of this mass can be attributed to the external water jacket with two water necks and the alloy feed pipe that's cast as part of the intercooler body. Oh, and - since it's contained within the water jacket - we were unable to measure the volume of the water-to-air heat exchanger core.
And how did the RS intercooler flow? Well, we recorded 221.1 cfm but - again - this figure is misleading. As mentioned, the RS intercooler body integrates a large section of the feed pipe; this inevitably causes extra flow restriction compared to the rest of the 'coolers, whose feed pipe connect at the base of the rear end-tank. In other words, the flow figure for the bare RS intercooler is at a disadvantage.
On the other hand, only a short length of plastic pipe (which incorporates a blow-off valve fitting) is required to link the beginning of the alloy feed pipe to the turbocharger. Adding this to the intercooler saw just a 5 percent flow loss; the relatively small loss from fitting this part helped bolster the flow of the complete RS intercooler assembly beyond that of the Version 2, 3-4 and 7 WRX!
The Results At a Glance
This flow graph shows that the two aftermarket intercoolers have easily the least restriction. The MRT unit delivers 19 percent more flow than the standard Version 7 'cooler, while the AVO Version 2 job picks up 16 percent over the stocker. The acclaimed STi intercoolers put in a fairly unimpressive performance. Both the Version 2 and 3-4 STi parts out-flowed the standard WRX intercoolers by less than 2 percent - bugger all. Note that the Legacy Twin-Turbo intercooler flowed every bit as well as the Version 3-4 STi product.
You can see the run-of-the-mill WRX intercoolers have been undergoing continual development. The progression from Version 2 to Version 3-4 brought a considerable 16 percent flow increase, but - thanks to a focus on improved heat exchange - the Version 4-5 slid back to within a couple of percent of the original. These days, the Version 7 WRX intercooler walks the middle ground - it flows better than the Version 5-6, but not as well as the Version 3-4.
As mentioned, the flow figure we recorded for the bare Liberty RS water-to-air intercooler is a bit pessimistic (thanks to the majority of the feed pipe being cast into the intercooler body). Once the short plastic feed pipe extension is brought into the picture, however, the RS 'cooler assembly out-flows some of the WRX gear.
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 of the standard plastic part.
This graph clearly shows the superior mass of the aftermarket intercoolers - the large bar-and-plate core constructions are much heavier than the factory cores. In contrast, the STi intercoolers have no thermal mass advantage over the base WRX versions, but - as a trend - the 'coolers become heavier as the models progress. The only exception to this is the Version 7 unit, which is lighter
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