Compression ratio for 500hp turbo build
 

I’ve spoke to 5 engine builders/tuners and got 6 opinions. Thought I’d open it up for debate:

Aim is 500+ engine HP at 7500 - 7700 rpm on e85. We can leave the support mods and whether this is a good idea for another thread. Specifically want to focus on compression ratio.

School of thought 1. 10.5:1
By adding this much boost to make the HP, you need to lower the compression for mechanical sympathy and to stop knock at high revs. Even with e85 and direct injection there will be knock above 7500rpm reducing engine longevity

School of thought 2. 12.1:5
The DI and e85 works so well that conventional wisdom may no longer apply. The inherent knock resistance of e85 allows this compression ratio, adding to the power meaning you need to run lower boost and a smaller turbo which all helps the engine. The lower flow rate means you don’t need to do lots of head work and larger valves saving some money.

I’ve got plenty of experienced tuners swearing by both methods who will readily point to examples. I have no reason to believe any of them are misleading me – they all just approach things in a different way. Keen to hear more about experiences good and bad.

oldschool brute force: drop compression and push tons of boost.

New school efficiency: high compression, moderate boost.

Notice that OEMS are all the latter now. They're not stupid; OEMs invest hundreds of millions of dollars trying to increase power efficiently. The trend is toward high compression boost for a reason. Even for NA cars, notice how high compression is nowadays, compared to a decade or two decades ago. 14:1 compression NA engines that run on low octane are commonplace now.

At the end of the day, when you want huge hp/liter numbers, you will run into other hard limitations that have not yet been solved for the FA. Do whatever it takes for you to get to your goal with the shop/builder/tuner of your choice.

Ls6

No.

Call Outfront Motorsports, and delicious tuning

People are already doing that much hp now on stock engines, until the rods go at least, so I wouldn't consider dropping compression. Part of the reason for the old school rules with static compression was because the dynamic compression ratio was also fixed due to the cam timing being fixed. With variable cam timing, the negative effects of very high compression can be mitigated by altering the intake cam timing and overlap.

If you're keeping stock compression would there be even a need to upgrade your pistons?

Are the stock pistons good enough when paired to forged rods?

I've spoken to Outfront and they are strong supporters of the 12.5:1. Lots of customers making big power. Looking to buy a bunch of internals from them as they clearly know their stuff.

Haven't spoken to Delicious as I'm not considering buying products or services from them, so don't feel I have the right to ask for 20 minutes of their time.

This is a great question. I'm definitely in the 'buy once, cry once' frame of mind and am not pulling this engine anytime soon, so am doing pistons 'cause.

Engine will almost certainly need a light hone anyway so why buy 86.1mm OEM pistons?

I run a Delicious Tuning flex kit and tune, I have over 700whp and no problems, I highly suggest that you take a second look at their kit and tunes.... Like you said "buy once" I also run a 12.5 Outfront motor

I'm running a Motec ECU so they can't really help with the tune. I'm working with a local tuner with decades of experience on Motec and high performance engines but not a lot of experience on this platform.

I'll be grabbing most of the internals from Outfront and having them work with my local engine builder on the details of the build.

And it will be happening at 12.5:1......

Lots of experience doesn't mean it translates to this platform.

It's like saying a high end shop with decades of experience tuning/repairing CRT televisions can do the same with LCD televisions.

Specifically, unless they have significant experience with DI and dual injected platforms, they're basically starting from scratch like everyone else.

I recommend you talk to as many local tuners as you can, and pick one based on their experience with this platform, as well as their other experience.

Any recommendations for forged pistons with stock compression?

I'm not able to find any except for SuperTech

Outfront have made a batch CP Carillo custom made pistons in 12.5:1 in stock. 86.25mm so just big enough for a light bore and hone without using too much of the cylinder wall thickness.