Forged Pistons - Bore and Compression Ratios

[mav1178]

Originally Posted by ChrisD

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.

But that's just it...

I'm assuming you already have an idea of what you want with this upgrade. I am assuming you've already communicated what characteristics your new engine will have, according to your needs and preferences.

I'm assuming the engine builder and tuner will know what the various changes (compression, boost, etc) will do to address your needs.

Obviously if your tuner or builder can't answer these questions for you, they don't deserve your money, but my reference to "armchair this build" is really just saying "the answers are already there and should not be blindly provided by the tuner", assuming you know what you want and the tuner(s) know what they are doing...

Again, going back to square one: you need to figure out what deficiencies you are addressing with the engine build, and how you want to get to the end goal with any combination of CR change, boost, turbo sizing, IC piping and build, tune, etc etc.

Everyone wants 300HP. How to get to 300HP will vary drastically from person to person.

-alex

[wparsons]

There's nothing wrong with boost on the stock compression ratio. You'll get much better off boost power, better throttle/boost response, and make the same power with less boost.

Compressing air creates heat, the less you compress it the cooler the intake charge is going to be.

If you're going with forged internals, talk to your engine build and tuner about running 11.5:1 or higher and see what they say.

You can run more boost with lower compression, but you also need to to make similar power.

Personally, I would want 12.5:1 unless I was going for HUGE hp numbers on pump gas. 300whp on pump 98 RON should be reliable on stock internals with a good tuner, so doing nothing but making it stronger (same bore, c/r, etc) will only improve reliability.

[Malt]

Originally Posted by ChrisD

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

I don't see how you could possibly make that decision without first disassembling the engine and your engine builder making that call. While your engine may only have 7k miles on it, you have no idea what the status on the cylinders are without visually inspecting them. Many other posters have told you that you really need to consult with your builder/tuner and I agree completely with them.

Doing research is fine but unless you are doing the work yourself many of these decisions are best left to the people actually doing the work/tuning. That means leaving these decisions up to them unless you have experience with these tasks (and if that's the case you'd probably be tackling these tasks yourself). You might not want to leave the decisions up to them but ultimately you are the one paying someone to build the engine for you. If you don't trust their judgement/opinion when it comes to part selection then it makes zero sense to actually trust them with the build itself.

You seriously need to sit down with the builder and discuss your options as he/she is going to be the one doing the actual work.

[ChrisD]

Originally Posted by wparsons

There's nothing wrong with boost on the stock compression ratio. You'll get much better off boost power, better throttle/boost response, and make the same power with less boost.

Compressing air creates heat, the less you compress it the cooler the intake charge is going to be.

If you're going with forged internals, talk to your engine build and tuner about running 11.5:1 or higher and see what they say.

You can run more boost with lower compression, but you also need to to make similar power.

Personally, I would want 12.5:1 unless I was going for HUGE hp numbers on pump gas. 300whp on pump 98 RON should be reliable on stock internals with a good tuner, so doing nothing but making it stronger (same bore, c/r, etc) will only improve reliability.

Yeah I spoke to the people who would be doing the work and they agreed - they're still on the stock compression ratio on their 400 bhp GT86 (although they're running race fuel). The plan now is to get the turbo and exhaust installed and have the turbo on low/mid boost levels with a fairly conservative map, but leave the pistons and rods alone so stock compression ratio. In theory that should give around 300 bhp, so I'll see how I get on with that and if I want more power then we'll start looking at higher boost and possibly lower compression ratios with the forged pistons. The guy I spoke to said he's driven a GT86 with 9.5:1 ratio and you could barely notice any difference in throttle response (off boost obviously) compared to the stock 12.5:1, so I don't think it is going to be that much of a big deal either way in the end.

[wparsons]

I call BS on a 9.5:1 motor responding anything like a 12.5:1 motor off boost. It'll make way less power off boost with lower compression as well.

The rest sounds like a good plan, 300whp should be safe on good fuel with a good tune.

[ChrisD]

Originally Posted by wparsons

I call BS on a 9.5:1 motor responding anything like a 12.5:1 motor off boost. It'll make way less power off boost with lower compression as well.

The rest sounds like a good plan, 300whp should be safe on good fuel with a good tune.

I dunno, in the article someone else posted it said that going up a full point (i.e. from 9.5 to 10.5) would only give you a 3% increase in power at absolute best. So dropping by 3 points probably wouldn't be quite as noticeable as people make out... probably realistically only giving you a 6 or 7 percent decrease in power, and I think a lot of people agree that anything less than 10 percent is not really noticeable. I know when I had one of my previous cars re-mapped to give it an extra 15% power, I could hardly tell.

[Malt]

Originally Posted by ChrisD

I dunno, in the article someone else posted it said that going up a full point (i.e. from 9.5 to 10.5) would only give you a 3% increase in power at absolute best. So dropping by 3 points probably wouldn't be quite as noticeable as people make out... probably realistically only giving you a 6 or 7 percent decrease in power, and I think a lot of people agree that anything less than 10 percent is not really noticeable. I know when I had one of my previous cars re-mapped to give it an extra 15% power, I could hardly tell.

That 10% figure is definitely not accurate. Going from a stage 1 tune to a catless header plus stage 2 tune is probably 10hp at best and it's certainly noticeable.

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