[Inn-Tune]

I had a discussion with another tuner I'm friends with recently about intercooler efficiency and its relation to horsepower. I ran the numbers for an STi and I thought I'd share that information with the IWSTI community.

Someone on IWSTI asked how big a turbo you CAN run on a TMIC and the answers made me realize a lot of members don't seem to understand why people switch to FMICs. The answer is simple...the FMIC stays cooler on the street because it's not baking in the engine bay, it's bigger, and it has access to more airflow in most cases.

Now I'll explain how intake charge air temperature relates to the horsepower your car produces at a given moment. I'm going to make this as Subaru specific and realistic as possible so it has more meaning for you folks.


Lets start with a 70 F day because I know most people feel that when it's not real hot out, their TMIC works great. This car makes 525 crank hp. I'm using the APS D/R 725 intercooler simply because they have posted values for this piece.

Here are the numbers:
hp = 525
ambient = 70 F
compressor outlet = 415 F
intake manifold (post IC) = 105 F
Pressure Drop Across Core and all Ducting (psi) = 1.5 psi


Now the math:

Figuring out the efficiency of this intercooler:
Compressor outlet temp minus ambient temp:
415 - 70 = 345 F {this is the temperature gain from ambient to what comes out of the turbo}

Temp gain from turbo - temp dropped by intercooling + ambient temp = intake manifold temp.
345 - X + 70 = 105
X = 310 {this is the temperature dropped by intercooling}

Now that you have these numbers you can calculate the efficiency of the intercooler under these conditions:
Temp gain from turbocharging * cooling efficiency = temp drop from intercooling
345X=310
X=.898 = 89.8 % efficient
At almost 90% efficient this is working nicely.


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Now...how does this affect power? The density charge affects power and density is related to temperature.

Here we go:
Dc = Density change
Dt = Compressor outlet temp
It = Intercooler outlet temp
Dc = [ (Dt + 460) / (It + 460) ] - 1
Dc = [ (415 + 460) / (105 + 460) ] - 1
Dc = (875/565) - 1
Dc = .549 (rounded to 3 places)

This means the intercooler made a 54.9% increase in density over the non intercooled charge. Some of the power increase this denser air causes is negated by pressure drop and flow restrictions in the core and piping do play a role as well. That said, this IC only has a rated drop of 1.5 psi across the core and piping under these conditions (26 psi, above listed temps, etc.).

For anyone wondering where the number 460 came from, it's used to convert our Fahrenheit temps to Rankine for this equation. More info here:
Rankine scale - Wikipedia, the free encyclopedia


NOW THE BIG QUESTION!!! How much power am I going making because of this intercooler?
HPr = Rise in HP
Dc = density change from the intercooler
Ap = Ambient Pressure (I'm using 14.7 for sea level)
Bm = Boost pressure at the manifold (26.1 psi)
Bc = Boost pressure at the compressor (27.6 psi)

HPr = Dc + 1 – [ (Ap + Bc) / (Ap + Bm) ]
HPr = .549 + 1 - [ (14.7 + 27.6) / (14.7 + 26.1) ]
HPr = 1.549 - (42.3 / 40.8)
HPr = 1.549 - 1.037
HPr = .512

That's a 51.2% increase in power which basically means you'd have around 350 crank hp if you removed the intercooler, but had similar flow restriction and pressure drop. Of course that's not realistic, but you'd still be under 400 hp without the intercooler. Sound too good to be true? It's not...as long as my Sunday morning math isn't off.

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Now lets say this intercooler was heatsoaked because it's now a top mount sitting over the engine in the staging lanes before an autoX. Again I'm going to simplify here and say the post IC intake temp is all that changes from this. Instead of a post IC temp of 105, lets say the core is at hotter than before and it's not quite as efficient, so it only cools the air to 200 F.

Compressor outlet temp minus ambient temp: (THIS STAYS THE SAME)
415 - 70 = 345 F {this is the temperature gain from ambient to what comes out of the turbo}


Temp gain from turbo - temp dropped by intercooling + ambient temp = intake manifold temp.
345 - X + 70 = 200
X = 215 {this is the temperature dropped by intercooling}


Now that you have these numbers you can calculate the efficiency of the intercooler under these conditions:
Temp gain from turbocharging * cooling efficiency = temp drop from intercooling
345X=200
X=.58 = 58 % efficient


how does this affect power? The density charge affects power and density is related to temperature.

Here we go:
Dc = Density change
Dt = Compressor outlet temp
It = Intercooler outlet temp
Dc = [ (Dt + 460) / (It + 460) ] - 1
Dc = [ (415 + 460) / (200 + 460) ] - 1
Dc = (875/660) - 1
Dc = .326 (rounded to 3 places)

So now the intercooler only bumped the density charge up 32.6 %


The power difference?
HPr = Rise in HP
Dc = density change from the intercooler
Ap = Ambient Pressure (I'm using 14.7 for sea level)
Bm = Boost pressure at the manifold (26.1 psi)
Bc = Boost pressure at the compressor (27.6 psi)

HPr = Dc + 1 – [ (Ap + Bc) / (Ap + Bm) ]
HPr = .326 + 1 - [ (14.7 + 27.6) / (14.7 + 26.1) ]
HPr = 1.326 - (42.3 / 40.8)
HPr = 1.326 - 1.037
HPr = .288

Now the intercooler is only making 28.8% more power than having no cooling at all (again assuming other things being equal).

The difference between the intercooler adding 51.2% power or 28.8% power is sizable in this case...about 75 hp.

I've kept these numbers realistic for a common situation many of you face. Now imagine it's 80 degrees out with a track temp of 120 F and you're at an autoX or drag event where you're hot lapping. Just think how enormous your losses can be when that intercooler gets hot!!!!


If you want to read more about intercoolers, check this other thread out:
http://www.IWSTI.com/forums/engine-p...ercoolers.html

[RONIN]

Wow...vote for sticky!

[Tortfeasor STi]

CN: A better more efficient intercooler creates a denser air charge that is less likely to pre-ignite (detonation) upon compression. This creates a more oxygen rich charge in the cylinder requiring more fuel. The combination of more power and more fuel per combustion means more horsepower.

By the way, kudos to the OP for doing all that ****ling physics math that calculates why a hot intercooler and intake charge makes less power.

[Turbineguy]

+1 for a sticky. I may rethink my top mount setup.

[Szumita]

sticky for sure.

[modaddict]

winnAr!

one reason I have a fmic.

one reason why I have the largest fmic on the market!

[toogood]

sticky ftw!

[jasv11]

Thanks Mike, I will be throwing my aps725 thats up in my attic on my car for my new setup this winter. I have rethought it and the advantages hugely outway the 30lb weight gain from it.

oh, and I have a headache now....to much thought for a sunday morning

[WXRTSI]

Very interesting numbers. But there is one thing that is very important thing that you forgot to mention that is proven fact: FMICs look sweeeet. haha jk, but seriously really good info out of all of this.

[berdugo]

Originally Posted by modaddict

winnAr!

one reason I have a fmic.

one reason why I have the largest fmic on the market!

+1

[berdugo]

Inn-Tune,

Does humidity affect the power too? Lets say 80-90% humidity, if it affects the power at all. Thanks

[Chinky_Sti]

nice info, but now my head hurts from all that math..

[r12rex]

This is definitely some good info. Numbers. Nice. Looks like you did the math correctly to me. Not bad for Sunday morning. It only makes sense though that a FMIC equipped car would produce more power than a TMIC equipped car. How much a gain could one get from a stock turbo STi and a FMIC set-up? I can't really see it being too much, but I'm sure the tempatures will be quite a bit lower.

I'd love a FMIC set-up, BUT...to keep it cop-friendly, I think I'll stick to my TMIC. Of course this could change in the future...


Kind Regards,
Ryan

[520]

Very nice!

[boxtwo]

Excellent information! Thanks!