Tuning the C20LET - Breathing/Intercooler

One can only feel contempt for those who decided to place the stock intercooler where they did.

If their aim was to protect it from the airflow and the elements, then they succeeded - hidden behind the headlight it's free to act as a heatsink, absorbing as much heat as it can from the engine bay. Not that it can absorb much heat anyway, with it's miserable dimensions and pathetic plastic endcaps.

It has only one place, and that's in a bin - a small bin will do nicely...

But how much better can a proper Front Mounted InterCooler be?

Below are plots from two dyno runs on my car. They are a year apart, but the only difference is the FMIC. Fuel pressure and fuel quality are the same. Boost is the same. Nothing was altered, the car wasn't even remapped. It's the same 4x4 dyno with the same operator, so comparisons can be made:

The improvements are obvious. They are all over the rev range, not the usual swapping of 10bhp here for 5 bhp there! The previous torque graph would peak momentarily at 270lbft at 3800rpm and then rapidly decline below 200lbft at 5500rpm (heatsoak - the tiny stock i/c is overwhelmed quite easily)

The new torque is 20-50lbft higher throughout low and midrange, peaking at just above 300lbft and staying there for almost 1000rpm. At around 4600rpm it's 60lbft higher! And the turbo itself wasn't getting any younger either, it had started to smoke!

It has to be noted that in both cases the car was doing 20psi overboost, so the extra intercooling effect is more pronounced than what it would have been on a stock car. Still, the dyno operator couldn't believe it was just an intercooler change. They've got lots of Evos and Scoobies (and Fords!) down there at PowerEngineering and none of them has such a ridiculous stock setup.

This is the powergraph from an otherwise stock engine with a proper FMIC. 20% more power, not bad at all...

If there is a single modification one is to do to this engine, it's certainly fitting a FMIC.

 

Location, location, location

There is only one location for an intercooler, and that's outside the engine bay, a place where fresh airflow is abundant.

When stuck in traffic, the engine bay heat fills the space and you can see the hot air rising from the bonnet exit vents. If there's an intercooler in there, it will act as a heat-storage brick, absorbing the engine bay heat and releasing it later into your inlet manifold. Not nice.

The intercooler has to be outside the bay, perhaps somewhere front, so that the rising heat won't affect it. That's one of the reasons Front Mounted InterCoolers (FMICs) can have such a dramatic effect on these engines (the other is size)

If it has to fight for space with oilcoolers, radiators or condensers, the intercooler has to win every time - it has to be right before anything else. If there are loads of other airflow users behind it though, it's best if the core is not as thick so that they, too, can have some flow. Remember that too many obstructions behind the intercooler reduce it's efficiency, as it needs low pressure behind it.

People wonder if having half of the core unexposed to fresh airflow is a problem. It's not ideal, but don't lose sleep over it. As long as the fresh air is ducted (with no other place to go except through the intercooler fins) it will do it's job even if up to two thirds of the core is 'hidden' from direct airflow

A mesh is essential, or else the road debris will damage the turbulators and knock the paint off

It doesn't have to be too thick, just enough to keep the beez and the small animals at bay:

Actually the mesh above was from B&Q, cheap'n'cheerful. After a trip to Scotland it looked more like this: (click for larger image)

A stainless or plastic purpose-made mesh would be more appropriate...

 

Size

Even for stock boost the size of the stock i/c is barely adequate. Yes, size is important, but it's not everything. It's just one of several factors to be considered.

Catch of the day:

	From the Skyline forum. That's a mother of a cooler, best left for 700bhp+ engines though...

Here is a very rough idea of what an intercooler manufacturer recommends for various flow rates.

I've seen claims of "8.1", "9.0" or "12" litre units, and people fall for the hype. Hey, the intercooler helps you make power, so a BIG one will make more power than a smaller one, right?

Well, not necessarily. First of all, what "volume" are we referring to? total external, core external, total internal, what?
If I had to compare volumes, the only volume that would be at all meaningful would be the core internal, and no-one quotes that.
Design of endcaps, pressure drops at fixed airflow points, turbulator airspeed efficiencies are more important, but hey...gimme more (brochure) litres than the other guys.

Volume (internal) and Mass are the major factors in intercooler efficiency. So is the type and density of the turbulators (fins) both inside and out. Length of the tubes is important, as they influence the pressure drop - longer tubes need reduced density in internal turbulators though. Not too straightforward is it?

What is not widely known is that volume is also needed in order to slow down the air. At full boost the air is going too fast through the pipes for any meaningful heat-exchange relationship. A large core will act as a sudden increase in pipe volume - pressure goes up, speed goes down. Better heat exchange versus risk of blowing it up. That's why build quality matters a lot, good intercoolers cannot be cheap. Good cores must be robust and well-made on the inside as well, all seams have to be right, all metals have to be of the correct thickness and quality. That costs money.

Here is a cheap'n'cheerful approach, using one from a SAAB 9-3 aero

Unfortunately this intercooler doesn't look much better than that of a Frontera. Not surprising, as the SAAB is a low-pressure design, so the intercooler isn't meant to flow as much as a tuned LET

Frontera intercooler fitted on Astra MK2. Looks neat, and the thin profile of the intercooler allows only 1" of bumper to be trimmed off.
Closeup of the MK2 intercooler. Such a thin unit from a diesel is not the best choice for high-boost applications, but here it will be in FWD and space is at a premium, so it fits the bill quite well. It's also quite cheap from breakers, and much better than the standard LET joke

Below is an example of an intercooler that's just too big for this engine (if it's doing less than 30psi that is!) The trouble of fitting it won't be worth the extra cooling (compared to a core half this size). Neither will it be worth the blocking of all air intake into the engine bay.
We do want something bigger than the std EDS "8.1litre" unit, but not this big.

Even bigger... some people just don't know when to stop!..

 

This one's a bit smaller, but again it's a bit too long, resulting in encaps that are too restrictive. The lower rows of this core will not flow the same as the upper ones. This encap design would be fine for flow of liquids, but not pressurised air. One to avoid.

 

DIY intercoolers

Here are some hints on using intercoolers from different kinds of vehicles:

A huge, thick and dense intercooler might be good for a big, slow earth mover, but won't do a 150mph car any favours. This is a closeup from a large diesel truck

The turbulators inside the air passages are not too bad, although they would be much more efficient in a low-speed, high air volume application like the one they were designed for.

As mentioned before, there is an optimum airspeed for efficient heat exchange, and that stands for both internal and external airflow. If the external (cooling) air moves too fast or too slow (for the turbulators) then the cooling function can be reduced considerably. Same with the internal airflow, that can be almost supersonic after the compressor. An intercooler meant to be on a 200cfm engine (say a diesel) will experience extreme airspeeds if fitted on a 600cfm engine. Not only will heat exchange be rubbish at full boost, but there is the risk of internal turbulators breaking off and there is only one way for them to go: through the engine.

In the tube and fin cores, tubes are more expensive to manufacture than fins. So an el-cheapo core will be mostly fins and short on tubes. This will give the impression of VOLUME but pressure drop will be high and heat rejection low. The rest of the engine bay will be deprived of fresh airflow for no good reason!

The endcaps are very important - sloppy and angular designs are cheap to manufacture and buy, but they flow badly, and they might not spread the airload evenly through the tubes (increasing backpressure for no good reason). The endcap right after the compressor is a bit more important, as the air will be very hot and very fast and it has to turn AND split into different passages. The FRONTAL section of the inner core is very important too, as that's what the ultrahot air first 'sees'. If the surfaces are sharp and vertical they increase backpressure with no added benefit. However the internal turbulators are meant to upset the airflow so that it swirls and hits the edges of the tubes giving up energy. That's why it's important to look at a section of the core before the endcaps are fitted.

Bar and Plate core designs are considerably more rare and expensive. I don't think they're worth the extra money in an application like this. They are better when space is severely limited and every drop of intercooling efficiency is essential (say on a motorbike)

My first choice for a replacement OEM intercooler would be from an engine of similar displacement and power as the one I'm building. That's why I floated the idea of the EVO intercooler some time ago, and it has caught on. I remember being at Power Engineering (Uxbridge) and they had 3 stock EVO intercoolers piled aside because their owners bought bigass aftermarket ones. I felt that they would go for peanuts, keeping in mind that they are proper designs, unlike the Vauxhall jokes.

Here Simon Morris displays an example of fitting an EVO intercooler onto a Calibra.

Here are the steps for fabricating an intercooler capable of flowing 1000Hp (twin turbo Corvette)

Custom single intercooler for a twin-turbo application, courtesy of toohighpsi. The two opposing inlets are not allowed to mix too much - compact and nicely executed.

 

Leaving the stock intercooler as well

In a nutshell, NO

Heat exchangers operate with maximum efficiency when the temperature differences are large. When the turbo chunks out air at 150C and the ambient air is at 10C, that's a 140C difference - the intercooler operating at (say) 70% efficiency will reduce the temp by 70%*140C= 98C.

But if the compressed air is at 30C the difference from ambient (10C) is just 20C. The efficiency now is much lower, below 40% perhaps, so this same intercooler will only reduce the temp by 40%*20C = 8C. Still, 2% power increase, but the extra pressure drop will take some of this back.

This is always the case with intercoolers (or chargecoolers) connected in series. They both share the same ambient air temp, but the first one in line experiences the large temp difference, and does 80% of the work. The next one breathes in the cooled-down air, and can only bring a modest improvement - but it doubles the pressure drop nevertheless. (Strictly speaking it only doubles the flow resistance, because the pressure drop also depends on temperature, but let's keep it simple here).

So it's not a good idea to fit a massive FMIC and keep the old chargecooler or the stock i/c connected as well. The chargecooler would be less of a problem actually, as it flows much better internally, but it's efficiency would still be greatly reduced, and all that weight and extra piping may not be worth the aggro.

 

Colour

According to Ali-G the rapper-tuner, there is only one colour for a heat exchanger, and that is

BLACK.

Not silver, nor chrome - but matt black. Shiny surface externally is not a good thing (doesn't catch the incoming air), neither is a thick coat of paint (insulates)

But if the intercooler is situated inside the hot engine bay, then the black colour will aid heat exchange you'd rather avoid - it will be more of an 'interheater' - but then colour is the least of your problems...

 

Material

Copper has superior heat transfer properties, why don't we use copper cores? Good question.

The story goes that copper cannot take easily the pressure and heat stresses from a couple of bar pressure and 150C at the same time. It would need to have much thicker pipes to compensate, but that would reduce the heat exchanging capabilities to the point where it's worse than an aluminium core of the same external size.

I wonder how it would be to have a steel (or aluminium) frame, with a copper core inside. The external frame would provide the structural integrity and flex-resistance, and act as an auxiliary heat sink as well. The copper core would then focus on pure heat-transferring tasks. Maybe a project for the future.

Here is a lot more on the automotive use of copper

Intercooler spray

On very hot days, the intercooler core will obviously be quite hot, and we know that it's impossible to cool the charge below that temperature. In fact we can't really cool it even at that temperature, efficiency can never be 100%

But if we get a bit creative with the cooling medium, we can go one step further. A well-known technique is spraying water onto the intercooler core. If the core is hotter than the water, then some heat will be absorbed above and beyond that absorbed by the incoming air cooling stream. However at high airspeeds water just won't stick to the core's turbulators, so it won't do much for power.

The water needs to be sprayed as a fine mist - it will then be able to stick on the core, having a better chance to absorb some heat. The water injection nozzles have to be carefully positioned, too. A spray that hits the middle of the core with the car stopped, might only wash the headlight as the car is moving at 70mph.

Other cooling media have been tried during futile attempts of further cooling down an intercooler. Even compressed CO2 is being touted as the king of intercoolers sprays, in the from of the N-tercooler

Alcohol seems like a good liquid to use, with a low boiling point and easy evaporation. Unfortunately real-life tests have proven otherwise.

A test-rig can be setup to test various attempts to increase efficiency of an intercooler core.

This one uses a powerful heatgun to simulate the hot air exiting the compressor. Although temperatures are approximated, the total airflow is nowhere near the real thing.

Checking on the temperature of the exiting air we can get an idea of the efficiency increase from the various intercooler sprays.

Reality is a bit more brutal - with a good setup, the bestl you can do is increase the intercooler's efficiency by 10%. That's not 10% more power unfortunately, but 10% more of the extra power boost given by the intercooler. So if the intercooler is responsible for 60bhp, it will now account for an extra 6bhp (while the spray is operating). Kits spraying NO2 are more effective while running (skyline and supra owners testify to this) but they are very wasteful on gas, compared to the modest power increases. Nitrous is meant to be injected inside the intake!

Not too impressive, but on a hot day every little helps!

Here is an article from "Performance Ford" used more as an advertisement for a commercial intercooler spray system based on CO2 (more effective than just water). Even that doesn't manage to get more than 4-5%bhp.
Still, it doesn't hurt.

 

Here is more on Chargecoolers

 

Here is a Water Injection setup on the LET

 

ECU air temp sensor

It sits on the top left of the throttle body and informs the ECU of the air temps, after all intercooling has taken place. If it gives off erroneous readings, the fuelling is unlikely to be accurate. Here's how to check if it works OK without the engine running:

The sensor's resistance varies with temperature. Colder temperatures should lead to higher resistance. If your sensor is way off these figures, it's best to change it for a new one.

First measure the resistance between the two terminals. If the engine is cold, then you can assume that the ambient temp is fine to use.

At 100C it should be around 200 Ohms

At 25C 2K - 2.5K

At 12C 3K ~ 3.4K

At 0C 14K - 17K (freezing temp)

Finally measure the resistance between the metal body and the terminals in turn. Should be infinity.

Finally...

To get an idea of the temperatures and efficiencies blend together, here's a handy little calculator I prepared earlier. It is unique in the business and comes in handy when tuning on the edge. This is the basic version and doesn't cope with water injection or NOS. Still, very handy indeed.

Some hardcore intercooler theory for those who can't be satisfied easily.

Fabricating your own intercooler is possible (but not too easy though) (local copy here)

Good reading on Water Injection, scooby alcohol injection here

An American site for one of the first production turbo cars (Buick Regal)

If you think that cooling the air is the only way forward, see how heating it up can make even more power

Mathematical analysis of aircraft intercooler design from the NASA archives

 

On to the Throttle body...