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.

I did that a year ago when I started this journey and made my choices. It comes down to trading off a little direct experience for the fact they are easily accessible.

I'm inside a 10 minute drive of arguably the best Motec tuning house in Australia, Holinger's R&D and manufacturing plant, Motec's head office and the engine builder who has the Subaru contract to do all their engine warranty work. Are they all experts on this platform? Maybe. But with some help from others we're making it work as best we can. They all know each other and work together routinely. Hell, the Mainline Dyno head office and software development team sublets the upstairs floor my Tuners shop!

The best part, is that with a Motec, you can live, tune, and can accomplish more in 1 hour, than a full day of Ecutek tuning! :party0030:

Sounds like you're in great hands.

Yes. I had a software issue with Motec and they couldn't reproduce it on their simulator. I drove there and took the developer who does the software for a drive in my car, expained the issue and they fixed it. That sort of accessibility is very helpful.

I've been giving this a fair bit of thought lately as well. I think one thing that is missing from the discussion is the availability of E85 and how that factors into the decision.

In my case E85 is pretty much impossible to find so I'm stuck with 93 pump gas. Most of the higher power builds I've seen are only able to achieve 400+ RWHP with stock compression using E85. Has anyone seen a good counter example?

I'm close to the point where I'm either going to pull off my SC and sell/trade in the car or go for an engine build and install a small pulley on my Harrop.
-Matt

Agree. This conversation is premised on e85 being the predominate fuel. I have, and will keep, a 98 octane map with the flex fuel as e85 is not exactly readily available in the city/suburbs, but you can do it if you plan ahead. But it is impossible to find in the bush. I'm OK with little/no boost and very conservative ignition timing on 98 to get me home. It's already saved my bacon once.

Yeah, my challenge is that I'm currently running a supercharger. Turbo systems have the advantage of being able to run a high boost/high power map for E85/FlexFuel then a lower boost map for regular pump gas. When you go with a supercharger boost control isn't really possible. If I want the most versatile configuration possible I suspect a lower CR build coupled with a high-boost pulley (~14-15 PSI is the max, I believe). In that case I can tune for max power on pump gas and maybe a bit more on E85 just through ignition advance.

A good example is TJ Hunt's car. He ran a built engine with stock CR and the a 14psi pulley on a Edelbrock SC. He made 420RWHP on E85 but had to pull so much timing to keep avoid knock on pump gas it only made 270RWHP. I bet if he could have dropped the boost down to 10-12PSI or so he could have made 300RWHP or more on the built motor on pump gas.

So, will an Edelbrock SC make 400RWHP or more on pump gas? Probably not. If you lower the CR to keep the engine safe you can't increase the boost enough due to the minimum pulley size and max RPM of the SC. But could you maybe make 350-370RWHP on pump gas? Maybe... Wish there were datapoints for this type of configuration.
-Matt

Actually, you can boost control with a supercharger as well. Harrop even has a kit for it!

I've also helped many people do this with a centrifugal to meet p/w requirements on racecars.

hmm.. interesting. Do they just vent through the bypass valve? I've seen the systems that drive the supercharger with a CVT to provide a wide range of boost levels and maximize boost at low RPM.
-Matt

Yes.

CP or JE.

I was under the impression that the piston was shaped a little odd, which makes it hard to fit aftermarket rods to it. Unless you shave the piston and make the rod fit.