Forged Pistons - Bore and Compression Ratios
 

As far as I was aware, all of the GT86/BRZ/FRS engines are exactly the same (please correct me if I'm wrong) so why are there several different options for bore size when looking at forged pistons? For example see the bottom of this page there are options for 86mm, 86.5mm and 87mm: http://www.fensport.co.uk/Parts/Model_108/Category_12/

How do I know which size I should get? Is the stock size 86mm and the other options are just there for people who have had theirs bored out to a larger size?

Also, each of the piston options has a compression ratio and from doing a bit of reading it seems like I should go for a lower compression ratio as I am going to be using a turbo (which would be getting installed at the same time as the pistons). My question is what would be the better option between the 9.5:1 CR and 10.5:1 CR, considering I'm going to be getting about 300-350 bhp from a stage 3 AVO turbo? or would I need to know how much boost pressure it will be running before anyone could advise on that?

Thanks
Chris

The different size in pistons is to account for the overbore when honing out the cylinders to clean up any scoring. The compression ratio you go with would probably be a decision best left to whoever is building/tuning your engine.

What kind of fuel are you going to be running? If you are running higher octane fuel or E85 you can get away with a higher CR.

Just regular pump fuel (which is 95 ron or 98 ron here in the UK). I think I'm definitely going to have to go for a lower CR but out of the two (9.5:1 or 10.5:1 I don't know which would be better). I guess like Malt said above, I will just ask the people who will be installing it but I wanted to do a bit of my own research first rather than just asking one person.

I assume you are going turbo hence the forged pistons.
I would recommend factory 86mm unless you have damaged the cylinder walls and a 10:1 compression ratio
PS you may be able to contact Wiseco or JE directly for purchase if you wanted a custom ratio.

Wiseco is currently making my 89.5mm/13:1 compression set. It help with me in the USA though

OK so it should just be 86mm then? The engine only has 7K miles on it and has not had any work done on it other than normal services

Yeah I will be going turbo (I did mention that in the original post). What makes you suggest the 10.5:1 rather than 9.5:1 ? I'm trying to understand what would make one better than the other

Your compression ratio should be determined by the engine builder in consultation with the tuner.

You should NOT armchair this build.

-alex

I don't see the harm in me doing some research and trying to understand the effects of different compression ratios rather than just handing the car to someone and saying do whatever you want. I just like to get other people's opinions rather than putting all of my faith in the person that is making money from my choices (i.e. the builder/tuner), but yes of course if the person installing the turbo and pistons says that a certain CR would not be a good idea then I'm not going to just say do it anyway.

There was a good post elsewhere in the FI section recently about calculating the affective compression ratio with boost... I don'y have the thread link, but it linked to another site with a detailed explanation... here is the link

I don't think you need to go as low as 9.5:1 for the power level your talking about.... Heck you can probably get there with stock compression... depending on what dyno your talking about.

they wont tell you a lower CR is a bad idea, they will tell you how much boost youll be able to use on any particular fuel mixture though. The lower the CR the more boost you can run (on pump gas), the drawback is lower power off boost.

http://www.xcceleration.com/cr-boost%20101.htm

Read that^

Well from what I understand yes you definitely could do it on stock compression but it just isn't going to be as "safe" as having a slightly lower CR. The whole reason I'm getting forged pistons and rods is to minimise the risk of ever ruining anything in the motor, so I don't want to just stick with the stock CR and add boost and find that it causes reliability problems

Yup, check out the link we both posted above.. should answer your questions and help you get your head around the subject so you can have a more informed conversation with your tuner.

Thanks that link was very useful! To be honest I'd like to keep the throttle as response as possible, I don't want it to be terrible off boost and also don't want it to have a really big hit of power when it comes on boost (I'll be using this car for drifting a bit and that sounds like a recipe for spinning out).

So it sounds like going for the higher CR would be a good idea, and seeing if it can be mapped in the most progressive way possible (although I guess there's only so much that can do).