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Engine, Exhaust, Transmission Discuss the FR-S | 86 | BRZ engine, exhaust and drivetrain. |
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01-25-2015, 07:39 PM | #1 |
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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 |
01-25-2015, 08:14 PM | #2 | |
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01-25-2015, 08:51 PM | #3 | |
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01-26-2015, 09:00 AM | #4 |
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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.
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01-26-2015, 09:00 AM | #5 |
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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
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The Following User Says Thank You to celek For This Useful Post: | ChrisD (01-26-2015) |
01-26-2015, 09:01 AM | #6 |
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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
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01-26-2015, 09:02 AM | #7 |
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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
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01-26-2015, 01:16 PM | #8 |
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Your compression ratio should be determined by the engine builder in consultation with the tuner.
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01-26-2015, 01:52 PM | #9 |
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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.
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01-26-2015, 02:01 PM | #10 |
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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.
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01-26-2015, 02:04 PM | #11 | |
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http://www.xcceleration.com/cr-boost%20101.htm Read that^
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01-26-2015, 02:11 PM | #12 |
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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
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01-26-2015, 02:15 PM | #13 | |
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01-26-2015, 02:24 PM | #14 | |
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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). |
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