This slightly confuses me.. The Turbo Legacy's use charge coolers but the Imprezas have TMIC's ?? What is the logic behind this, even with the bonnet vent SURELY the charge cooler is far more efficient?

I've got the larger intercooler from a late classic to fit on the other halfs early classic, but I'm even more tempted to go down the charge cooler route.

 

Davey

All really depends what you want to use the car for. Charge-coolers are efficient, but only if you can ensure that the coolant stays cold. What often happens is that the coolant gradually heats up and the charge-cooler becomes less efficient.
Top mounts are not a lot of good if you're are in a situation where the ic can heatsoak ie. traffic etc, but once on the move a well designed system will always have cold (or at least ambiant) air passing through it. Water sprays are good, as evaporation will drop the temperature of the ic further, but you have to be careful that too much water is not sprayed as the ic will then clog and efficiency reduce.

On a standard(ish) road car, a well designed topmount will be fine, a front mount is better (if you accept the trade-off with potential lag-increase/expence of aftermarket ecu etc). A water mist will further improve both cases....

Hope this helps....

 

But surely a chargecooler with a large volume front mount rad and heavy duty pump is not only more efficient but cheaper than a TMIC or a FMIC??

 

A lot of people will have different views on this one. I know of several people from my GT4 days that upgraded to charge-coolers, but I also know people that made some impressive horsepower on topmount air/air with a lot less complexity and weight.....

 

It's a good question. My old Clio turbo ran a chargecooler, my Scoob runs a TMIC. Put it this way - if it's the sort of job where you NEED a chargecooler, then run a chargecooler. If you have the room for a big well designed FMIC then that will give good results, regardless of any slight extra lag. A TMIC is kind of a bodge that doesn't really do anything well, but works adequately.

So when do you NEED a chargecooler you might ask? Well if your front end is already jam packed with no sensible way of running a decently sized IC. If you think about putting a smallish IC up front cos that's all that'll fit, you're putting up with the disadvantage of long, convoluted pipe runs, which not only cause bigger pressure differentials, but also pick up heat from all over the engine bay unless you lag them. But you don't have the advantage of greatly increased cooling capability. All in all, you get the expense, the lag, and the additional heat pickup but no real benefit.

In this case a chargecooler is a great alternative. Put the chargecooler body right in between the compressor outlet and the throttle body is perfect, just where the TMIC is now. That's about as little pressure drop, lag and heat pickup that you'll ever be able to get. But you get the benefit that a chargecooler's water pump can be put anywhere really and it's only the size of a can of coke. The pair of water pipes running down to the chargecooler rad are only the same bore as standard oil or water pipes at most and can be tucked away. The chargecooler rad doesn't need to be as big as the equivalent IC and again it can be placed very easily.

If you were to do a chargecooler conversion on a Scoob, I'd say you could get a decent chargecooler body exactly where the TMIC goes now. Heatshield the underneath to protect it from radiant and convected heat off the gearbox and turbo. The top is ok, the bonnet scoop will keep it stone cold and allow any excess body heat to be lost at standstill.

Run the pump full time, wire it off the ignition so it starts up as soon as you turn the key. The water reservoir doesn't have to be huge but keep it heat shielded along with lagging the water lines as well if you can.

Job done. Not a complicated system at all.

BUT as I say if you have the room for a big, WELL DESIGNED FMIC up front and don't mind the extra lag and the huge pipes everywhere, then that is a better option because although an FMIC can quickly get overloaded itself if you're on boost the whole time, it can dump the heat almost as quickly if the conditions are right. You avoid this heat overload problem by using the biggest, best IC you can afford.

A chargecooler takes ages to get any kind of temperature into it at all, but once it does start to rise, without a good enough water flow rate, it can take ages to dump all the heat.

 

I just come from the school of thought where a charge cooler is better.. And if it was good enough for Lotus when they built the LC (which had TONNES of room at the front for FMIC) it must have had an advantage?

For me the reduction in Lag over a FMIC is the biggest pay off..

 

I wouldn't say 'better' - just a different approach to the same problem. I've had both systems, and they both serve their purpose in different ways. I think really the deciding factor is the car, the room you have, and the type of driving you do. For something like say uphill towing, a chargecooler would definitely be preferable as the huge amount of heat dumped into an intercooler would not be dissapated properly, whilst a chargecooler would keep eating it up a long while after a straight air-air intercooler had overheated. For snatchy, on-off-on-off kind of throttle work a BIG intercooler can be very efficient. It just fails when you are on boost for long continuous periods and it doesn't ever get a decent chance to lose the heatsoak.

 

A good FMIC setup (ie purpose made from the start) need not add lag. My legacy chargecooler wasn't that efficient, I doubt it was any better than an Sti8 top mount intercooler.

Provided the car is always moving, the heatsoak issues with a TMIC are minimal, and it's more a case of airflow and core size.

 

A water/air intercooler will be no better for prolonged WOT than a good air/air system. To put things into perspective, a air/water system designed to cope with 550hp continuously, will need to flow around 80L of water per MINUTE through the heat exchanger (dependant on target final temp) to ensure steady state temperature. The heat energy put into the water is in the order of 80hp, but unlike an 80hp car, we want the water to be at least as cool as our maximum target inlet temps. With the energy levels involved, a moderate amount of water will heatup very quickly without a suitable rad. In the end you still need to dump the heat that goes into the water, back into the air, needing a decent sized rad. When comparing this rad to an alternative air/air intercooler, I am curious why you think an air/air intercooler can't dissipate the heat "properly"?

Using a legacy unit (on my old legacy) with a custom made radiator larger in frontal area than most FMICs I would still see throttle body temps of over 55deg. WIth a larger engine side heat exchanger I think it may work better, and of course a more efficient turbo charger that the one I tested with on my legacy, it will hopefully work better.

For drag racing I don't think you can beat the chargecooler if you go to the lengths of chilling the coolant below ambient temperature (using icewater for example).

 

It's not exactly a technical test by any means but even after a real thrashing the charge cooler on my Legacy never got beyond luke warm except for when the pump failed and it soon got very hot indeed!

A lot of people with Legacies (myself included) upgrade from the charge cooler to a front mount intercooler but the main reason most of them do it is so that the charge cooler doesn't become less efficient when they're going for more power upgrades. The Aussies are mad for Legacies compared with Imprezas and they've done a lot of stuff about the pros & cons of both setups over the RSLCF board over the years. Many of them go for the FMIC when they start to pursue bigger power.

I'm not sure which is more efficient out of the normal Impreza TMIC and the Leggy's charge cooler but as I said mine never got beyond luke warm so they must be fairly good I expect.

 

Some fair points there, but IMHO an AVERAGE chargecooler system can handle more punishment in general than a GOOD air-air intercooler. Of course there are all kinds of variables. But I'd argue that you can get the same heat dumping performance from the smaller, more distributed engine bay 'package' of a chargecooler system than you can from the larger intercooler that you'd need to get the same results.

When I talk about an air-air intercooler not dumping heat 'properly' perhaps I didn't phrase that right. What I mean is that on a long WOT run - and I'm not talking about a 12 second quarter mile pass - let's say a minute or two continuously foot to the floor, that's got to be one hell of a big, well designed intercooler to handle that amount of heat load and be able to reject it at anywhere near the same rate that it's accumulating. There's nothing you'd be able to do apart from slow down or take your foot off the pedal.

With a chargecooler, as long as you've got adequate water volume in the system, a high flowing pump and a good sized rad, will take a lot longer before it starts struggling, because you've not only got the rad doing the work, but also the water volume acts as an additional heatsink. Granted, once the hard work is done, and our maniac driver has took the pedal out of the carpet, the chargecooler will take longer to get back to normal running temp. But it's done its job in preventing a huge temp spike that IMHO would have happened using an air-air IC whose performance is simply correlated to road speed.

OK, it might be an unlikely situation, but it's something to think about. I am happy to agree that in most cases a well designed air-air IC does a great job. But don't rule out chargecoolers for road use. Rads can be upsized easily, as can water pumps, water pipes, chargecooler body etc. With an IC you're stuck with it. IMHO

 

I think the trick is to use an efficient intercooler in the first place. The sort of intercooler commonly fitted should keep temps below 40ºC for long periods of time. My induction temps would hit about 62º after about 40seconds WOT at 2 bar with over 550hp. For a 350hp engine at lower boost the thermal load is significantly reduced.

I guess we will see just how a good sized chargecooler system works when I get my car up and running again this season!

Paul

 

That's a bloody good result there to keep it to 62 under that kind of punishment. Will be interesting to see how you get on with a decent sized chargecooler. Or perhaps...I've always wondered about a dual system - running through an intercooler first, then a chargecooler right before the throttle body. Best of both worlds maybe - temperature stability and cooling volume of a CC system with the almost instant heat dumping capabilities of an IC. Lag needn't be a major problem if the cores and pipework were designed well.

 

Lotus had been chargecooling for years prior to the LC with Esprits and always got incredible torque and power from fairly small capacities.Must say as an engineering principle the chargecooler seems to be the more sound of the two.

 

And the other thing is that as a cooling fluid, water is something like 12 times better at removing heat than air is IIRC. Hence why it's used to cool an engine and air isn't, on any modern car. Even Porsche finally dumped air cooling a decade ago.

Of course you've still got to take the heat out of the water through a normal water-air rad up front, but water presumably gives up its heat at the radiator interface just as well as it picked it up in the chargecooler.

The main use of chargecoolers however is still where due to packaging problems, a properly-sized intercooler and associated pipework just won't fit.

 

i have a sti8 with tsl chargecooler on it runs with less lag more power due to a 3 degree ignition advance and looks lovely chalk and cheese with sti8 tmic
martin

 

Glad to see some other scientific views!

I think its down to "street cred", the bonnet vent or a massive FMIC just look "cooler" to most

 

I just dont get this 'lag with a FMIC' that everyone bangs on about. Totally understand the theory but in practice i dont get it.

I have a cheapo hybrid FMIC on my car at 405 BHP. Previous to that i had a STI8 top mount on .

Brfore taking it off i did a few low RPM 4th gear runs, to check spool. Did the same with the FMIC after mapping and you cant tell the difference, either in feel or by looking at the boost gauge and Tacho. You could argue it may be 100 RPM or so later but thats about it.

The difference in charge temps however is massive. could easily get to 50 deg with the TMIC on a WOT run within 1/2 a mile or so, only get to about 25 max over a lot longer distance with the FMIC.

 

That's fair enough. There's a lot of reasons for changing to a front mount and not very many for keeping the top mount, if you're really wanting to make a difference to your charge temps. And as you've found, you got a BIG difference there - I'd much rather have charge temp at 25 than 50. Who cares about a tiny bit of lag in that case?

 

im about to fit a chargecooler from a legacy on n my 95 impreza,have the pump, pipes, cooleer and the rad for the front of the car, but how is the rad to be e m,mounted, as the legacy has a slighhtly different shaped bracket where it bolts on,the impreza doesnt,anayone any picss ?