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Originally Posted by MikeSTI
I guess I'll just agree to disagree simply because I think you limit the thinking process by running back to a smaller motor to get the horse power just after putting the smaller motor down. I would think if I was going to get a new short block I would want a 2.5 or bigger 2.8 as a replacement rather then a 2.2, to me it sounds like closed deck is more important then the size it self?
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Mike, you have to keep in mind that internal combustion engines are incredibly complex machines, with dozens and dozens of variables to account for. Each variable, when changed, causes domino effect changes to many others. Part of my inability to properly explain my points to you is that I'd have to bring you up speed on everything I've learned researching Subaru motors the last few years, and I can't. I'll try to limit this post to just a few key (hopefully, the most relevant) points to illustrate what I mean.
First, Forged pistons aren't "stronger" in terms of being able to handle more power/boost directly. Forged pistons are harder to destroy through detonation and pre-ignition damage. The reason they've fallen out of favor with me personally lies partly with the fact that the hyper-eutectic cast pistons in the STi are much stronger than standard cast pistons- in fact, they're very very nearly as strong as forged pistons. They're light and fairly detonation resistant, and they're less expensive.
Here is a good link that explains the differences between the 3 kinds of pistons. The STi crank, rods, and bearings are plenty strong for building a usable track or race motor. Forged pistons vs. regular cast don't actually allow you to make more peak power- they just give you a little more margin for error with detonation.
This link explains what detonation and pre-ignition actually are, and is one of the best primers for understanding tuning I've found.
Back to your last post. Displacement is important, but it's not the end-all be-all of turbo motors. In fact, within certain broad limits, motor size effects your peak horsepower in only one way- consider the following. A turbo, for example your stock VF39, has a certain range of air flow vs. boost pressure ratio that it is thermally efficient in. when people talk about compressor maps, this is what they mean. Example, using the stock WRX TD04 (sorry, nobody has published any IHI compressor maps yet, or I'd use the VF39):
See the small font numbers in the center of the graph, ranging from 60-75%? Those numbers indicate the thermal efficiency of a TD04 in those ranges. Thermal efficiency is basically a measure of how good the compressor is at not heating the air moving through it beyond what the pressure increase itself should create. Looking at a pressure ratio (basically, boost pressure, but not exactly) of 1.8 and a flow of 200 CFM, you see that it's in the peak efficiency range. If you look at 1.8 PR, at 350 CFM, you see the efficiency is barely 60%, meaning the air coming from the compressor is much hotter. you know what hotter charge air leads to.
Now, back to motor size. I said that motor size doesn't matter, right? This is because I can make up for air flow with higher RPMs. Obviously, there's a practical limit to just how high I can rev a motor. Current F1 motors see close to 18 or 19k rpms. That's way more than we need for this discussion. Lets say for sake of argument, the ratio of displacement needs to be matched by ratio of rev limit for the smaller motor. I.E., 2.0 is 20% smaller than 2.5, so it needs 20% more revs to flow the same amount of air. no problem. The EJ257 revs to 7k stock, and has been safely taken to 8k by quite a few people, with no mods. 20% more than 8k is 9.6k- call it 10. So, to match the flow of an EJ25 at 8k, an EJ20 spins to 10k. This relationship is why turbo size, and ONLY turbo size, determines how much torque you can make, with no octane limitations.
Man I'm wandering. See what I mean about having too much to explain at once to prove one point? I'm trying to keep it concise, honest.
Okay, so I've established that displacement is not crucial to making more peak power. What added displacement does is move your peak torque lower in the rev band, where it's more usable with reasonable gear ratios. I'd take 400 pounds of torque at 4k RPMs over 400 pounds at 7k. That's the advantage of displacement- not more power, just power lower in the rev band.
So, why did I say that you'd have to go to an EJ22 for over 400 whp? Well, I think you said something about closed deck- and that's it. To make over 500 hp at the flywheel, you need extremely high cylinder pressures- 25+ psi of boost on top of what the compression makes. I haven't stumbled on the equation for determining dynamic CR with FI yet- maybe Austin can pick me up on that later. anyways, it's EXTREMELY high- 3-5 times higher than your car has right now. Now, given high enough octane fuel, and good enough tuning to prevent knock, your pistons will still hold up to this, What can't hold up is the cylinder wall itself. it will start to flex, stretch, and warp at some point. This has several catastrophic effects. One, it causes head gaskets to blow. Two, it causes pistons to bind, bend, or excessively wear either their skirts or the cylinder walls. All 3 of these will result in near-instant engine failure at high torque outputs. The closed deck EJ22 is better able to resist the warping and bending forces that result from high boost pressures. That's why it's better for really high hp applications than the EJ25; it's not the forged pistons. A closed deck EJ25 would be better still, because it would be just as strong, but would move the power band back down while making the same amount of torque.
I guess that's enough for this post. If you want, I'll summarize the exact differences between the EJ207, the EJ205, and EJ257 and tell you what makes each one suitable for it's application. Then, I'll go into what makes the boxer motor so different from an inline 4, particularly the 4G63.
Edit: red X?