Facts about Intercooling

[Turbo6man]

How does a larger intercooler affect throttle response. Does there come a point when the sizeof the intercooler would affect throttle response enough to make the car slower in certain circumstances?

Geea.

As I understand it the intercooler can act like a muffler in an exhaust system - if it is too large the air density will decrease as it passes through the cooler. This is similar to slowing down the air intake and so gives the effect of lag or slow throttle response. It will be overcome as boost and revs increase towards their maximum. So, like with all things there is a compromise, a balance that has to be found.

[aps]

It looks like there are quite a few valid questions about the role of an intercooler, its affect on the engine as well as the design criteria that make it applicable to an XR6T.

Given that an intercooler is supposed to extract energy (heat) from the charge air and present a minimal restriction to the charge airflow, one could design an intercooler with the following characteristics. Let’s begin with a couple of very simple scenarios.

1. A short straight but large diameter tube of say 300mm diameter. This design will result in a very small restriction to flow and even though the air speed through the tube will be quite slow, the amount of heat extracted will be very small if any at all. This satisfies one criteria but fails miserably in removing heat.

2. A very long and very thin tube that is wound around and around the front of the vehicle. This solution will result in a very large resistance to flow with huge pressure drop. However, the amount of heat extracted will be great. This again satisfies one criteria but fails miserably due to the extremely high pressure drop.

The optimum intercooler design lies somewhere between the two.

I haven’t touched on intercooler construction and won’t other than to say that of the two types, Bar and Plate or Fin and Tube, the Bar and Plate construction is preferable given that we have fairly strict options regarding the size and shape of the intercooler core in the front of an XR6T. The reason is that a Bar and Plate construction core will present a lower pressure drop and will be more efficient than a similar sized fin and tube core. The bar and plate also has a higher thermal inertia which is preferable in a drag race application since it uses it’s mass to store “cold” capacity. A similar sized fin and tube gets hot quickly in drag applications. However, bar and plate is expensive so it is rarely used when cost is considered more important than intercooler performance. That said, there are some instances where a larger fin and tube core is preferable to a smaller bar and plate – but with the XR6T we don’t have that option. More to the point it is effectively impractical to attempt performance comparisons between the two construction approaches.

Now, back to core size selection. The optimum size is one that takes into account the width, length, depth, frontal surface area and direction of flow through the core. To consider one aspect such as depth alone as Brian did is a sure path to blindness by missing the forest for the trees.

Frontal area, the result of length by width determines the amount of cooling air available for the intercooler. From there, the depth is important because as the cooling air travels through the depth of the intercooler, it continues to remove heat from the charge air inside the intercooler. In addition, the depth plays a large part in keeping intake charge pressure drop to a minimum.

Direction of flow of the charge air now comes into play. If one has for argument’s sake an intercooler that is 300 mm long and 100 mm wide (tall), passing the air from left to right means that the air must travel 300 mm through the intercooler and through say 4 passages. This will no doubt result in a good temperature reduction but a poor pressure drop. Conversely, if the flow is from top to bottom, the air passes through 4 times as many passages (12 in this example) and travels only 100mm through the core. This will result in an excellent pressure drop but perhaps not as good a temperature drop since it is difficult to get the air to flow evenly over the wider area – unless the end tank design matches the deeper core.

Now when the depth of the core is considered, it gives the designer some flexibility to improve the intake airflow by making the passages larger to not only lower the pressure drop, but also to slow the charge air speed through the intercooler and further improve the heat transfer rate. But, a very wide intercooler core makes it difficult to distribute the charge air evenly through the intercooler. So again, the designer is faced with more in-depth engineering to reach an optimum design.

Further to this, the end tank design becomes critical because it determines the attack and exit angles and volume profiles of the air entering and exiting the intercooler core. For example, the end tanks on the APS LS1 Twin Turbo system to be released in the US in the future has quite novel end tanks that I am sure will have “experts” scratching their heads over the shape of the end tanks and in particular a couple of “bumps”. Spreading the air flow evenly across the intercooler core internal passages is critical for optimum intercooler performance across the entire RPM range and air mass flow rates.

For example if length and width remain constant, going from a 70mm bar and plate core to a 90mm core of the same construction results in a 30% increase in total passage volume, a significant improvement in temperature reduction and most importantly in the case of and XR6T a drastic reduction in pressure drop. This results in a big improvement in intercooler performance on the XR6T – that has important ramifications on the thermal load on the engine. By drastically reducing the charge air temperature over a 70 mm core, the combustion temperatures and the likelihood of detonation are reduced significantly. Hence it is worth while for the intercooler designer to spend the time in reaching the optimum core depth and end tank design because it has a great impact on overall engine performance.

To reach the optimum design however, the most critical information is that which pertains to the engine dynamics, target power levels and turbocharger mass flow rate. Without a deep knowledge of the requirements one has little chance of specifying the correct length, width, surface area and depth of an intercooler.

With regard to engine response and the affect intercoolers have on it, there seems to be a good deal of confusion out there. I can only assume that this has come about through experience with small capacity turbocharged engines (more so from old designs). Let’s first-off agree to use intercooler dimensions that are sane and reasonable. To have an intercooler that affects the response on an XR6T engine it would have to be very large indeed and before anyone gets silly, let’s state up-front that a cubic mile of intercooler core will create some lag. Now let’s get back to sanity and use the above example of going from a 70mm core to a 90mm core of the same design and construction type. Better intercooling allows for more ignition timing. Any tuner worth his salt will have seen this repeatedly regardless of the engine capacity. When ignition timing can be advanced without engine detonation, the engine response is improved significantly – across all throttle positions and engine RPM. As long as we stick to sane intercooler sizes in relation to engine capacity and turbocharger mass flow rates etc. this will always hold true. So, in an XR6T engine, the optimum intercooler system (total system) will result not in degraded engine response, but improved response. Conversely, a less than optimum intercooler system will be see engine response (and power etc) suffer.

On the issue regarding intercoolers and the affect they have on air flow to the radiator, this is a relatively well known and straight forward issue. There is no doubt that an intercooler placed in front of a radiator will impact upon the air flow to the radiator. What is less known though is that it is actually the intercooler void area that is the critical factor – not so much the core dimensions. For example, an intercooler with closely spaced and fine external fins will have a bigger impact on the air flow to the radiator than one which has straighter and wider spaced fins. Again, it comes down to the desired engineering goals that the intercooler is to achieve. If for example an intercooler must reduce charge air temperature from say 350 deg C to 50 deg C, it must have many fine fines. If however a temperature drop from 200 deg C to 50 deg C is desired, then a more open construction can be utilized with a resultant improvement in air flow to the radiator. What this also means on the XR6T is that a larger bar and plate core that is deeper and more efficient than a smaller tube and fin component can be specified to reap the benefits discussed above and also utilize a larger void area to improve the air flow to the radiator. Conversely, a smaller intercooler core that must utilize fine finning with smaller void area in an effort to improve the intercooling capacity will result in a great hindrance to air flow to the radiator. So much so that it is not uncommon to see a smaller intercooler place a higher load on the radiator than a free flowing larger and deeper core. To answer Brian’s question this is exactly the case.

So after this war and peace effort, I trust that all XR6T enthusiasts are now armed with information that at first might be difficult to digest, but will prove to be beneficial when wading through the volumes of “fact” (and I did mean to use the inverted commas in a facetious manner) found about intercooling and its application to the XR6T.

George

APS Web Team … and other stuff

[aps]

Oh, and while we’re at it in the later discussion, debunk Brian’s furphy regarding the relationship of core thickness to engine coolant temp.

George

Furphy, is it.

So you’re telling me that no matter how wide (thick) the Intercooler core, and therefore how much you block air flow to the radiator, there will be no impact on engine temperature whatsoever.

I also note that on your APS web site you recommend in fine print that after fitting the APS stage 3 with bigger intercooler “For motorsport applications other than drag racing: fitment of a large capacity engine coolant radiator and engine oil cooler system”. I read this as meaning that if I do track work with this intercooler that a bigger radiator is recommended.

Just for your information your web site claims “APS shoehorned the largest intercooler core possible into the XR6 Turbo”. Your core is 89mm. I have seen a 100mm core fitted, without any bar cutting required, as I was looking at that as a future option.

Brian

Brian, let me save you some heartache.

To place a 100mm deep core in an XR6T it means that the air gap between adjacent cores (intercooler and air conditioner) will be of the order of 5 mm, which is something to definitely be avoided at all costs. You will end up with severely restricted flow through the intercooler core and hence into the radiator. It’s not the depth of the intercooler so much but the inadequate air gap that is critical in this case.

If you are prepared to cut the 20mm spot weld ledge at the rear of the front impact beam then you have a chance of fitting a 100 mm deep core into the front of your XR6T and maintain an adequate air gap between cores. This however is something I personally would not attempt because of the adverse affect this modification may have on the structural integrity and air bag actuation in the case of an accident. Up to you Brian, but I bet that if you didn’t take the air gap into account, you would have blamed your boiling engine on the deep intercooler core – which will almost certainly not be the case depending on the core design in question.

It's no accident Brain that the APS intercooler is specified at a particular length, width, depth and construction and the term "shoehorned" is appropriate in the case of the XR6T. It's a free world though Brian and if you want to build a boiler, give me a yell and I'll drop over with the corn cobs and weaners - and we'll have ourselves a good time. <--- for the humour impaired

On the issue of specifying a larger radiator in motor sport applications (but not Drag Racing) in this case, engine heat load is a function of the long and sustained period of time spent at high RPM and wide open throttle – ie it is continuous high power that places the thermal load on the engine cooling system. Contrary to what you are trying to imply, the larger APS intercooler in these applications aids engine durability by maintaining lower charge air temperatures, thus reducing the thermal load and likelihood of engine detonation. In addition, being of a high void/open fin bar and plate construction, the airflow to the radiator is as high as possible and by far the best combination.

George

APS Web Team … and other stuff

[hello]

not changin the subject.... buts that's one hellofapost

[ZAP]

Thanks George and APS for this information. I was a 2lt Turbo owner and the rule of thinking was always the bigger the better when it came to intercoolers and turbos. This would cause our little milk carton motors to have massive LAG as you would bolt on the T04 turbo, HUGE intercooler and then floor it, count to 10 then hang on for the ride. I did find most of the intercoolers that were available for the Jap cars were very thin (<60mm) and only the really expensive ones like HKS were bar and plate, but these usually were over $3k. It became a never ending cycle of upgrading to compensate for the smaller engine size and what the mods did to the overall package.

It is nice to see someone who take pride in their products, spending heaps of time answering our questions and developing new go fast bits for our cars.

[Guest jteale]

I agree, very informative ... if a little long :lol:

Thanks for the effort George

Cheers, Jason.

[aps]

Alas, as an engineer I suffer from the same affliction that all engineers suffer from. That is, as the upholders of truth and fact, verbose communication is the norm.

Unlike my Father who can not only upturn your thought process with a concise and provoking sentence, but in his twilight years enjoy watching you squirm.

George

APS Web Team … and other stuff

Edited 28/07/04 07:54 AM by aps

[ktford]

Unlike my Father

to whom we say thanks for giving us George

Scotty

[curious]

Its refreshing to read the enthusiasm within the APS team and their search for maximum gains.

Imagine what you boys could do with cold fusion and a change in the laws of physics.

[Guest Maniac]

I've got to hand it to you George. You have explained yourself clearly and I'm sure it has shed some light on the people who have doubted your products as well as your knowledge of intercoolers.

APS seems to have their head screwed on straight as it is very rare where you receive such live responses to questions of such degree. Normally companies would ignore customers questions and add it to the "pile" of things to answer in their FAQ on their web site.

Great customer service and great support. It's good how you guys are truely informing the community in "how things really work". These days any 20 year old is simply following what they hear from their friends without even knowing what they are doing, which is causing people (like some friends that I know) to be working just to support their car.

Normally people these days simply buy a VL, Slap a T04 on it with a massive Hybrid / HKS / Trust front mount, buy a boost controller a boost it to 12 pounds in hope for "250 RWKW" without knowing exactly what they are doing. Having said this APS have explained what they do to the car and why they do it, which justifies each modification. This is a rare quality as many people will sell you on the product but not tell you exactly what the capabillities of the product are and why it's their for. Why it is used etc..

Judging by the sound of things George, you're greek? I'm a bit of a wogboy too being (being Greek and all myself).

I suggest everyone takes George's post to extreme thought when modifying their XR6T as it has been proven over and over again that APS have the best XR6T Kits available on the market and this is coming from someone who has delt with APS and it's competitors. I can tell you now, APS is one company you can't go wrong with.

Seriously,

Which other company educates people on their products the way George has? Not many and I can't stress out how good this is. IT is very rare.

Well done George, and thanks again. You have finally cleared the air not only about your intercoolers but about what an intercooler "really is" all about.

Regards,

George

(how convinient we have the same name too?)

Edited 28/07/04 03:35 PM by Maniac