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Discussion Starter #1
The sti's compression ratio is 8.2:1 running 14.5 psi and the evo is 8.8:1 running 19.5 psi

Why is low compression better for a Turbocharged Engine?
You make horsepower by how much air you move through the motor. A high compression 10:1 engine is more efficient than a 7:1 engine, so the 10:1 engine gives you more bang for the buck. However, because the lower compression is not as efficient, it will move more air through it. So, at 15 PSI of boost, the 7:1 engine will have an effective compression ratio of 14:1, will not be into detonation, and be moving more air, making more horsepower than the same conditions for the 10:1 engine. That engine will be in self-destruct mode, have detonation, and an effective compression ratio of 20:1!
This is why the racers only run 5:1 or even 6:1. All of this is great for a drag car, but because the static compression is lower, you will not have much bottom end torque either. So, since most of us don't drag race every place we go, a good compromise would be 8:1 or 8.5:1 compression. This way you don't loose too much bottom end for driveability, and if you don't run too much boost, say 10 to 15 PSI, you stay away from the gray effective compression area of 15:1 and up.

Remember, that the shape of the combustion area, cam, type of fuel, etc. all play a part of when the engine starts to detonate. It comes down to start with low boost, and sneak it up from there until you run into problems.

Thought this might be an interesting tidbit for everyone.

Please feel free to chime in.

Dan
 

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Those are interesting comparisons you have made. 15psi with 7:1 is equal to 14:1 comp. How do you make that comparison.....I was wondering about that the other day.

I think this is very good news for us. It looks at though the EVO engine is pretty tapped out already while we have quite a bit of room for more power.

Using your "boost = comp ratio" chart, what kind of boost do you think we can run safely with the comp ratio we have?
 

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Discussion Starter #3
Ok there are ALOT of variables that will effect the final compression rate. But I will give you an example.

I assumed the atmosphere for all these figures was 4500 feet which is where the bonneville racetrack is.

Evo
Stock 19.5 psi at 8.8:1 compression would end up with a final compression rate of 20:47.1 :eek:
If you boosted that up to 21.0 psi then you would end up with and adjusted for atmosphere ratio of 21.37:1 :eek:

Sti
Stock is 14.5 psi at 8.2:1 would end up with a final compression ratio of 16.29:1
if you boost that up to 19 psi then you end up at 18.80:1

Again there are other variables that I cannot account for but this is a good Ball park!

Dan
 

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Good info. I always wondered the benefits and how the compression affected things. Thanks for the explanation.
 

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CloNeGTS said:
I think this is very good news for us. It looks at though the EVO engine is pretty tapped out already while we have quite a bit of room for more power.
For some reason, I feel the evo still has some fight left in it. *cough*4g63*cough* :D
 

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Discussion Starter #7
chawklit said:
CloNeGTS said:
I think this is very good news for us. It looks at though the EVO engine is pretty tapped out already while we have quite a bit of room for more power.
For some reason, I feel the evo still has some fight left in it. *cough*4g63*cough* :D
The engine is good but with it already running 19.5 stock it doesnt have much "safe" room to grow with the stock internals. Thats the point. But the sti will probably push around 400 before you have to replace the internals. But that is just speculation based on whats in the block.

Basically comes down to how much compression you end up running. The sti runs way lower than the evo so it will be less stressed out. Which equals better reliability in the long run.
 

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So the question still is: Why did Subaru develop a brand new 2.5 liter engine for the USDM STi? Since the JDM & European versions get a 2.0 liter the same model year, one might assume that the extra .5 was needed for CA emmisions. However, the EVO seems to be getting the job done with just the 2.0 liter. I think room for growth is a great thing, but if that were the only reason, then I would think that Japan and Europe would be getting that engine as well. Do the Subaru engineers know something the Mitsu engineers don't (or possibly even the other way around)? I know there have got to be other factors involved, but the whole thing is buggin me! :-?
 

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Bigger engines get bigger taxes in Europe and Japan. But in the "bigger is better" American world, it was a no-brainer.

And the Evo is already pinging on California gas in stock trim, which seems like it isn't good news for street-driven tuned Evos. But the 4G63 has been around long enough that I'm sure someone can figure out how to bring on the power with 91 octane.

By the way, how are tuned WRXs doing on CA pump gas?

P.S. On the subject of compression ratios, remember the Dahlback Golf? 10.5:1 with 45 psi. Pure insanity, but obviously not on pump gas, and with "unknown" internals. Just goes to show what can be done if you have the dough and an iron block.
 
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Using a simple example as a rule of thumb:

Atmospheric pressure is 14.7, or approximately 15 psi. That is what your normally aspirated car sees.

A boost of 15 psi is equivalent to 15 psi atmospheric + 15 psi boost = 30 psi, or 2 atmospheres.

Therefore, an engine with a compression ratio of 8.5:1 and 15 psi boost will effectively see (8.5X2):1, or 17:1.

That might be a little easier to remember in terms of the numbers.
 

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I'd like to help

ScoobyDo said:
Using a simple example as a rule of thumb:

Atmospheric pressure is 14.7, or approximately 15 psi. That is what your normally aspirated car sees.

A boost of 15 psi is equivalent to 15 psi atmospheric + 15 psi boost = 30 psi, or 2 atmospheres.

Therefore, an engine with a compression ratio of 8.5:1 and 15 psi boost will effectively see (8.5X2):1, or 17:1.

That might be a little easier to remember in terms of the numbers.
I am stunned by the clarity you have brought to this subject... I can actually quickly do the calcs using this info. Good Job!!!
 

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huh... I don't think the STi 2.5 L engine is really better than the EVO engine. And the more room to grow thing does not always depend on displacement.

First of all, we all know that the EVO engine is detuned by the ECU, you can basically get JDM spec power (which is 300hp+) simply by chipping the engine.

Some thing to keep in mind though is that few years ago, they had problem with the EVO engine at Australia becuase the octane is not high enough and the gas is not clean enough.

So, the question for the USDM EVO would be, can they get JDM spec power with US gasoline after you chip the EVO.


For the 2.5 L STi engine. No body knows the true potential of the engine yet. Maybe the Engneer at STi knows. But for us, we have to wait til some brave ones to actually try it, dyno it, drive it and to prove it.

I had get the STi in a heart beat still because the car is lighter, more power in stock trim, DCCD and that the USDM EVO as more like EVO VI than VIII
 

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This is all very true.

And while chipping the EVO will probably take place, I have a feeling it won't be a plug and play by any means. With all the "American" additions, we'll have to see how long it takes.

And of course gas is becoming the big question. The part about the EVO engine that makes me think it's in trouble, regardless of engine control, is that it is already at 19psi (and knocking on 91) to get 271 hp, if that is the true rating. On the other hand, we have 300 hp while only adding 1 psi max boost to the WRX. 1 psi is not taking advantage of the strengthened internals, so that just leads the general thought to be there will be room to work. And, however small the room to work, we are already 29 hp in front of the game.

Time will certainly show what can be done to both engines. I'd like to think I can be pretty neutral when thinking about the two cars as both have very clear advantages. Something about 19psi, only 271hp and knock turning back the timing in STOCK FORM.....that doesn't sit well with me.

And I really wouldn't want to have to buy 100 octane in order to take on some serious tuning.
 

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New pictures on Edmunds.com

CloNeGTS said:
On the other hand, we have 300 hp while only adding 1 psi max boost to the WRX
I have noticed by the posts on other threads that some people are talking as if the WRX and STi have the same turbo. Doesn't the STi have a totally different turbo than the WRX? I thought the turbo was bigger on the STi compared to the WRX, so comparing the max PSI to the WRX should not matter.
 

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Yeah, the turbo will be different. They aren't sure what type yet, but it isn't the rumored VF34, but probably a new IHI ball bearing turbo.

Regardless of what is making the boost, think of it from just the engine side. Pressure is pressure, regardless of what is making it. Compare the compression ratios and the boost pressures...that's all that should matter.

The power comparisons are being made based on the engines having similar compression ratios and boost levels.
 

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Hey everyone, while your all talking about all this great tech stuff, throw out some links. It would help me research some of this stuff on my own.
 

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Hey prost893, that how stuff works sight is great, though I don't really spend much time reading about car related info, because there's just to much other wacky stuff to learn about.
Thanks again.
 

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The maximum boost pressure that can be safely run for a particular engine is dependent on a number of factors aside from compression ratio. These include: turbocharger size, intercooler efficiency, intake port & combustion chamber design, air/fuel ratio, coolant temperature, fuel composition (octane rating), cam timing and duration, exhaust backpressure after the turbo, etc. Also, the way in which the boost curve is programmed has an influence....it may be possible to achieve a high peak boost pressure "spike" as long as it tapers off immediately before EGT's get out of hand. A manual boost controller that cannot taper boost off in the higher rpms is likely to require a lower maximum boost pressure then an ECU-controlled boost solenoid with proper programming. Regardless, the manufacturer is definitely going to allow a generous headroom in terms of boost mapping to account for the unexpected.

Comparing the EVO specs. to the STi is not going to tell you the whole story. The EVO uses a front mounted IC which is not as prone to heat soak as the top mounted IC on the STi. The 4G63 also has slightly better combustion efficiency then the Subaru. Many tuners are shocked to find that the WRX engine makes best power at ultra rich mixtures in the 10.5-11.5:1 range. So as you can see there may be a lot of reasons why the big differences in compression and boost.

Also, an engine that would be a total dog without the turbo attached to it is going to still be a dog until the turbo has time to spool up. Lower compression ratios are usually a tradeoff in low speed performance in order to be able to run big power up top when the turbo hits its stride. For drag racing there may be enough clutch slip and wheel spin for an engine to never have to run at lower revs so super low compression may be an option. A manufacturer designing a road car, however, will never go that route because they want a car that drives smoothly and with some grunt in all areas of the power band. With the STi vs. EVO...the STi has more displacement and therefore can, in theory, generate more torque with less compression.

I am sure there will be plenty of ways to improve on the power output of the STi when it comes out. On the WRX, we have been able to achieve over 285hp/285lb-ft torque with just an improved exhaust and ECU tuning...running 93 octane and 16.5psi of boost on the tiny stock turbo. So it seems to me that the STi should have some additional room for improvement even with stock components. I think exhaust and ECU tuning will provide good power gains.

As for the turbo on the US STi....the larger displacement is going to call for an appropriately sized turbo when compared to the 2.0L STi. Its likely IHI will put together a VF34 with a larger compressor or maybe they will even use the P20 turbine housing from the VF22 with a larger compressor. It will be interesting to see.

--Dave
RalliSpec
 

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Let me try and shed some light on the whole power issue between the STi and the EVO. An earlier posting of mine seems to have been deleted...

In engine design, a common number used to compare engines is the brake mean effective pressure, or BMEP, of an engine. This is the constant pressure that would be present in an engine during all four strokes in one engine cycle to make the power or torque it advertises. This allows people to compare engines of all sizes on an apples-to-apples comparison. Here are the values for the STi and Evo, using SI units. Please note that the maximum BMEP value occurs at maximum torque.

Engine Specifications Used:
STi - 300 HP (223.7 kW) @ 6000 RPM, 300 lb-ft (406.8 N-m) @ 4000 RPM
Evo - 271 HP (202.1 kW) @ 6500 RPM, 273 lb-ft (370.1 N-m) @ 3500 RPM

Equations:
BMEP [kPa] = 6.28 * 2 * T [N-m] / Vd [liters]
BMEP [kPa] = P [kW] * 2 * 1000 / Vd [liters] / RPM * 60

STi:
BMEP(max) = 6.28 * 2 * 406.8 [N-m] / 2.5 [liters] = 2043.8 kPa

Evo:
BMEP(max) = 6.28 * 2 * 370.1 [N-m] / 2.0 [liters] = 2324.2 kPa

So we see the Evo is being pushed harder, even though it produces less power (as can be seen by the high boost of 19.5 psig). Anyway, modern engines can withstand up to about 3300 - 3500 kPa BMEPs before they need to be beefed up. This falls into a produciton safety factor of about 1.5 for the high performance engines above and something like 2 for detuned engines. On a side note, Formula 1 engines in the early 1990s were already pushing BMEPs of 3500+ kPa, but each engine was designed to complete one race and be rebuilt after each race, so they weren't as beefed up as a production engine needs to be. They need them to be as light as possible.

So, if we say the STi peaks at about 3300 kPa BMEP since it has an aluminum cylinder head and block and assume the Evo peaks at about 3500 kPa BMEP due to its cast iron materials, we can make the following calculations. We have to note that the BMEP at maximum power is about 10% lower than the BMEP at maximum torque.

Equation:
Max Power [kW] = BMEP @ Max Power * Vd * RPM / 60 / 2 / 1000

STi:
Max Power = (3300 * 0.9) * 2.5 * 6000 / 60 / 2 / 1000 = 371.3 kW
Max Power = 497.9 HP

Evo:
Max Power = (3500 * 0.9) * 2.0 * 6500 / 60 / 2 / 1000 = 341.3 kW
Max Power = 457.6 HP

Please understand that these are the theoretical, maximum values these engines should see without having to rebuild the bottom end and strengthen the cylinder head. If there were no material stress problems, it would take much more work on the Evo to achieve it's maximum power due to it's higher octane requirements and it's larger intercooler requirements since it will need to boost the intake air quite a bit.

Anyway, referencing the above numbers, I would guess to say it would be possible to make 425 - 450 reliable horsepower in the STi and 375 - 400 reliable horsepower in the Evo without having to use exotic engine parts to rebuild the major engine components.

JDiesel
 
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