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The Big Heatsoak Debate

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Lets see how this goes!

I have always been a bit sceptical regarding forum posts on induction kits and semi open filters suffering from heat soak.

I understand peoples theories but where is the actual proof on what impact it has on performance?

Are you saying its a power loss across the whole rev range or that it creates a flat spot? I know my Focus 182 is quicker with my open ended K&N in the factory box. It feels like the car is being strangled with the factory filter refitted.

If heatsoak does hinder the performance then why does the new RS have a semi open box? Surely if it was a real issue they would have fitted a sealed unit to get more power?

Lets hear everyones 2 pence worth on the issue.:smile:



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The bonnet seals the top of the RS air box, that's why it's so much taller and has the rubber seal.

The only extra bit of intake is the open bit at the front which should mostly pick up cool air coming in from the bumper and massive openings on the RS.

The open top of the RS air box is there for dyno runs with the bonnet open.

The reason your car feels freer with the k and n is not the open end but the reduced restriction and lower filtering capacity of the filter over the stock paper one. The stock paper filter is good for 350PS as otherwise they wouldn't use it in the RS. The k and n just provides better response but at the cost of lower filtration.

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Fair point on the reduced filtration Alex but i was more interested in where all the talk of heatsoak affecting performance comes in to it. The RS was just a car that came to mind due to the design of its factory airbox. If any one has any power graphs to show the negative effects of heatsoak with an induction kit that would be great to finally show facts over theories.

Im not planning on fitting a full on induction kit by the way. Its just a topic that always gets brought into discussions and i would like to try and put an end to it if possible one way or the other.

Ive always seen big power cars running open filters. Is that because a boxed set up cant flow enough air so the potential heatsoak (if its a real issue?) of the open filter is an unavoidale side effect in order to get the air in?

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Well that was the worlds smallest big debate!:laugh:

I thought there might have been a few more opinions on it seeing seeing as it gets brought up everytime someone mentions an air filter. Oh well. :smile:

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1 hour ago, alexp999 said:

Don't know of anyone that's tested the theory that's the thing.

And yet it gets thrown into threads all the time on car forums! I just wondered if its a people jumping on the bandwagon theory thats all Alex. 


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2 hours ago, WES180 said:

Well that was the worlds smallest big debate!:laugh:

Bit of a disappointing mass debate there Wes... :whistling:

I have seen proof of other cars having heatsoak issues on the dyno and losing power overall, but not on the Mk3 Focus so thought I'd leave my 2p out lol.  It was done by one of the big remappers a few years back but can't find it now.  There was a 4bhp peak difference between standard intake and a cone without CAF. 

In reality it won't matter on a turbocharged engine, a few extra degrees is nothing compared to what the turbo does to it, I doubt you'd see any difference in temp after the intercooler anyway...

But on an NA with reasonable power (GTi6 in my case) the cone does noticeably lose some compared to standard.  It sounded cool but after a week I went back to the original!  

I also dynod my Golf with both a paper panel and a foam panel a few years ago...Absolutely no difference in power lol.

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