Maddog

Firstly, apologies for posting this question in the 22B BBS, but I think the more feedback the better.

Over recent weeks/months, we have all discussed the merits and results of using various bolt on / replacement tuning parts and ECU’s for our cars whether they are UK or Sti’s and it’s all to good effect ‘cause it is helping everyone new to the world of tuning Imprezas, spend their money wisely on items that are proven to yield improvements in performance.

One area I don’t recall seeing any info, is in reducing the compression ratio to further enhance performance. Has anyone reduced their compression ratio to increase BHP ?

Years ago, I reduced the compression in the cars I was using to great effect as far as increased performance was concerned and I’m asking if it is possible to machine the standard pistons in the Imprezas or does it have to be replacement pistons ?

Does the Impreza have to have better head gaskets if I go this route, or will they withstand the additional pressure that will be placed upon them. I’m thinking along the lines of the Cosworth here, where one of the first things you should do when tuning them, is replace the head gasket with an uprated one, otherwise the gasket will blow.

Oh…anyone give me an idea of costs for such things as pistons, head gaskets etc……maybe direct me to a few sites / suppliers with prices ?

Cheers and stay legal

AlexM

Hi Maddog,

With respect to compression ratios, dropping your compression ratio will reduce performance, unless you increase boost to compensate of course. If the issue is detonation with high boost using pump petrol, you should only consider compression ratio changes if all else fails, e.g. revised mapping, water injection etc. A lower compression ratio usually reduces efficiency, and if it is done wrongly can reduce squish which can make pinking worse rather thsn better.

Head gaskets don't seem to be a problem on the Impreza - the metal ringed STI gaskets seem to be enough for pretty high boost pressures. Detonation will kill any head gasket (shortly followed by the piston crowns and and big ends) in short order, so controlling detonation is essential.

Cossies do have a fundamental problem with head gasket durability, so most engine builders will use a wire ring gasket and a retaining groove to increase the clamping pressure. If you are planning to go mad with boost, you would need to do some extensive work on the bottom end first in which case you might use better gaskets, but this isn't really a problem.

Cheers,

Alex

MorayMackenzie

Maddog,

Have you considered fitting a stroker kit? You can increase the capacity to 2.2litres and this may also reduce the compression ratio a bit... more capacity and lower compression should help your power and torque levels a bit.

Moray

Maddog

Hi Alex

Yep, I agree that lowering compression with no other changes will reduce performance and that efficiency will lessen, more so when driving off boost.

I should have said lowering compression to allow an increase of boost to be used, thus increasing the power output, if the ECU has been mapped to optimise the new set up.

I basically want to run more boost pressure for more power in my car, but at the moment, if I increase the pressure I would have to retard the timing to such an extent to combat detonation, that it negates the point of the exercise, hence I’m asking if anyone has reduced their compression either by machining or replacing pistons or know of any company that has done this to good effect.

If so what are the results and what other mods are necessary to keep the engine together ?

Maddog

Hi Moray

I’m aware of the stroker kits but I’ve not heard of anybody that’s had this done to their car and the results achieved in doing it.

I’ve been offered a 2.5 kit from my local tuner for approx. £2500 ( for the kit ), but I’m trying to find a spare engine to allow work to be done on that instead of my own, in case everything goes pear shaped when it’s built.

It seems there are not that many engines out there for sale unless you’re prepared to spend silly money ‘cause of their rarity in the secondhand market. (Unless anyone knows different of course).

The other thing is, the company haven’t built a stroked Impreza engine yet ( to my knowledge ), though they have considerable experience with Skylines. I know I could possibly be the first up here to do it, but I prefer to see proven results before wasting cash needlessly. ( That’ll explain the tight Scots bit in me then !!)

Cheers