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Old 09-23-2008, 07:39 PM   #21
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Originally Posted by patsmx5 View Post
Are you basing your numbers and thoughts on two identical motors that have different static compression ratios, but different boost levels to maintain the same dynamic ratio? Because that's my argument and what I am comparing. Low comp/high boost and high comp/low boost.

I don't believe spool will be improved much, and definitely not 1K RPMs. More compression makes the burn more efficient. More thermal energy is used to push the pistons. Whether or not this corresponds to more, or possibly LESS going into the exhaust to power and spool the turbocharger is questionable. If you think it makes a 1K difference in spool, please explain HOW. I say if it did increase spool, it would be no more than 12%, and probably less than 12%.

And how is it gonna make more peak power at the same dynamic ratios? Or are you saying at the same boost, and only changing compression ratios? Cause I am clearly comparing high compression/low boost to low compression/high boost. Not just lower compression.

So why or how does the 4% rule change for FI motors?
This is the scenario my boss (I work at an import performance shop) gave after explaining this thread to him:

Identical setup with only change being the CR, between a 9:1 and a 8:1. The HC motor will spool 1k rpm sooner and make more peak power because it makes boost sooner and faster. The ONLY advantage for a LC motor would be for a high horsepower motor, cause you are limited in how much boost you can run and still make power on a HC motor. Now you can run 120octane and the HC can run the big power, too.
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Old 09-23-2008, 07:49 PM   #22
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Hey first off I'm not trying to **** you off. I'm just looking for a good discussion with reason. Even if you dyno'd your car and made huge power at 15PSI, I wouldn't just assume it was from X, Y, or Z. Also, I'm not an engineer. I've done some reading and research on motors, but I'm no expert. I'm hear to learn more than anything.

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Originally Posted by jc_rotor View Post
Did you read the chart?

Yes those pressures are right, at BDC. Once you start moving towards and hit TDC, the numbers will change. The pressure that initially started as "38.7 absolute" turns into a 21.06:1 Effective CR due to the compression of the piston+boost.

The pressure that started as 29.7 absolute turns into a 22.22:1 effective CR due to the higher compression of the 11:1 motor. Assuming that the tune is correct, this will make about the same HP and spool faster.
So wait. Which would make more power? The higher comp motor or the lower comp motor?


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Originally Posted by jc_rotor View Post
30% more air in the combustion chamber does not equate to 30% more power.
Is that so? This is new info to me. I'm not saying it's 30.0000%, but it should be close, no? I mean, we get power from burning air and fuel. I've always thought if we double the amount of air/fuel an engine burns, it's power output is gonna more or less double.


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Originally Posted by jc_rotor View Post
"A full point of compression ratio is said to add about 4% more HP"

This is at atmospheric pressure, correct? And it really depends on the size of the motor, original efficiency, size and shape of the combustion chamber, and so forth. You cant just make a claim like 4% more power.

Plus add boost to the equation. The effeciency of the motor ( higher EGVs, more off boost power) allows you to run a less restrictive turbo and make more power per PSI and have the same spool curve as the smaller turbo on a 8:1 car.
The 4% number is a "rule" I learned. Seen it in several books and other sources. It's more of a racers math number. Maybe there is some mathematical proof for it, but I think it's more or less just the general accepted value found empirically. FWIW, I've heard it's "4% at best", and it can be less depending on many things, some of which you yourself mentioned. That's why I used 4%

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Originally Posted by jc_rotor View Post
The static CR of the motor affects more than just the % of HP output. There are countless other things that are now areas you can make HP on the 11:1 motor that wont make much of a difference on the 8:1.
Like what? Just curious.

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Originally Posted by jc_rotor View Post
Not to mention you are losing power with the lower CR pistons. Assuming its ONLY 4% loss, probably more, you go from 110 whp (1.8) off boost to ~105.5, but I bet its more than that.
Suppose your 11:1 motor makes 8% more than stock, so that would put you around ~118.8 whp, and that's 13.3 more HP than me at 8:1. However, that's assuming we are out of boost at 7K RPMs.... Not the case. Really these numbers are only significant out of boost, from say off idle to say 4K. I'm betting the difference is less than 10 whp before boost builds.

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Originally Posted by jc_rotor View Post
And even if 11:1 Only makes 12% more power (say all other things equal other than pistons) if both motors are at 15 psi, say the 8:1 makes 275whp, the 11:1 makes 308.
Makes since. I agree. I think this idea would be better applied to out of boost scenarios though. I'm arguing I'll be at more like 350 whp (arbitrary number, but one higher than yours) when I get to 24 psi and have the same dynamic compression ratio as you.
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Old 09-23-2008, 07:59 PM   #23
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The reason it spools faster has to do with the EGVs. Think about it, youre EGVs correspond to the pressure that is pushing the exhaust gases out of the CC after the combustion stroke. Higher CR equals higher pressure to move these gases out of the same size port. Higher pressure from the same volume of gases is going to create a higher velocity when forced through the same size opening. Also you have to look at the density of the gases before and after the burn. Are you saying a more complete burn will result in a less dense gas? Also, dont you know what happens to the density of gases when they are pressurized?
Makes sense. I never considered EGV's. F=1/2 M V^2. If you speed up the gasses, they are gonna do a lot more work on the turbine.

Density is mass/volume. So increasing mass while holding volume constant will increase it's density. Is a more complete burn more or less dense than one that is less complete? Or are they the same? Would you say the higher compression ratio motor has a more dense gas? Since mass is conserved, and density is mass/volume, I'm assuming volume is somehow decreasing in a higher compression motor and therefore increasing the density? I guess the density stays the same.
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Old 09-23-2008, 09:03 PM   #24
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ok i read some of this and im still scratching my head. Most of it make sense. heres my question can we see some real numbers????? like i know that 600 hp miata what cr and what boost is he running? same with the stp escort what kind of cr and what kind of boost? those are the only to big hp BP's i can think of i know im probably missing some i just want to see some real world numbers.
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Old 09-23-2008, 09:12 PM   #25
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Originally Posted by Hot_Wheels View Post
ok i read some of this and im still scratching my head. Most of it make sense. heres my question can we see some real numbers????? like i know that 600 hp miata what cr and what boost is he running? same with the stp escort what kind of cr and what kind of boost? those are the only to big hp BP's i can think of i know im probably missing some i just want to see some real world numbers.
"proof" is hard to come buy. Build quality, tune, engine setup, fuel used, weather conditions, and countless other things affect the numbers. It's not a clear cut case. That's why we are discussing it. I could probably look around and find 5 cars that make big power on low comp setups, and then the other side can show me 5 cars that run higher comp and less boost and make the same numbers. Neither one of us would be "wrong", but we would have learned and proved little. It all depends on your setup and goals. That's why I define the systems as being the same with the only difference being compression ratio and boost used. Trying to bring up information on both sides to better understand the relation between compression/boost/power and reliability.
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Old 09-23-2008, 09:47 PM   #26
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My 1.8L 1mm overbored motor at 11:1 CR made 140HP to the Wheels and 140LB/ft of torque with an SAFC (piggyback injector controller) and 330 RX7 injectors, and stock miata timing. Also with PS and AC.

1mm is not significant enough overbored to account for all the extra HP, especially with the crappy tune.

Now, 3 years later that I have a programmable ECU, I bet I can get it up to 150whp without any boost.

I wont know until i get the built motor back in, a disconnected stock motor is sitting in her right now so I can build the kit design around stock fitment.

Just so you know, my target HP goal is 400+ HP at 14psi.

The higher CR responds much better to breathing mods, like a more free flowing exhaust and better intake manifold design.
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Old 09-23-2008, 09:57 PM   #27
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We are probably at similar power levels right now, though I have no dyno proof. However I have the factory bottom end with quite a bit of head modifications. Enough to bump compression from 185PSI to 205 without shaving the head or deck. In fact I removed enough material from the head that I'm probably closer to 9:1 now. Hell it would barely run on the stock ecu after it went to open loop.

Are you looking for 400+flywheel HP or wheel HP? I'm looking for about 350whp myself on a stock block.
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Old 09-23-2008, 10:02 PM   #28
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Originally Posted by patsmx5 View Post
Makes sense. I never considered EGV's. F=1/2 M V^2. If you speed up the gasses, they are gonna do a lot more work on the turbine.

Density is mass/volume. So increasing mass while holding volume constant will increase it's density. Is a more complete burn more or less dense than one that is less complete? Or are they the same? Would you say the higher compression ratio motor has a more dense gas? Since mass is conserved, and density is mass/volume, I'm assuming volume is somehow decreasing in a higher compression motor and therefore increasing the density? I guess the density stays the same.
Once again, simple answer, the higher CR is due to slightly less volume in the CC at TDC, due to domed pistons. This increases the density of the gases in the CC.

I would assume that a more complete burn would result in a more dense gas. I havent done the chemistry mole for mole but you are exchanging ambient air (N and O2) and trimethylpentane for N2, H2O vapor, and CO2.

Im less into the chemistry and more into the physics and thermodynamics as you can see.
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Old 09-23-2008, 10:04 PM   #29
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400WHP thats all that matters is at the wheels to me.
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Old 09-23-2008, 10:12 PM   #30
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Originally Posted by patsmx5 View Post
Hey first off I'm not trying to **** you off. I'm just looking for a good discussion with reason. Even if you dyno'd your car and made huge power at 15PSI, I wouldn't just assume it was from X, Y, or Z. Also, I'm not an engineer. I've done some reading and research on motors, but I'm no expert. I'm hear to learn more than anything.


So wait. Which would make more power? The higher comp motor or the lower comp motor?


Makes since. I agree. I think this idea would be better applied to out of boost scenarios though. I'm arguing I'll be at more like 350 whp (arbitrary number, but one higher than yours) when I get to 24 psi and have the same dynamic compression ratio as you.

"So wait. Which would make more power? The higher comp motor or the lower comp motor?"

At the same Dynamic CR, the high CR motor should make more power because of its higher efficiency.

the High CR will make more power per PSI than the low CR all day long.



"Makes since. I agree. I think this idea would be better applied to out of boost scenarios though. I'm arguing I'll be at more like 350 whp (arbitrary number, but one higher than yours) when I get to 24 psi and have the same dynamic compression ratio as you"

Not likely, no offense. At a given dynamic CR, the more efficient motor will make the most power. However, as boost is added, the gap in efficiency will get smaller.
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Old 09-23-2008, 10:39 PM   #31
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Quote:
Originally Posted by jc_rotor View Post
"So wait. Which would make more power? The higher comp motor or the lower comp motor?"

At the same Dynamic CR, the high CR motor should make more power because of its higher efficiency.

the High CR will make more power per PSI than the low CR all day long.



"Makes since. I agree. I think this idea would be better applied to out of boost scenarios though. I'm arguing I'll be at more like 350 whp (arbitrary number, but one higher than yours) when I get to 24 psi and have the same dynamic compression ratio as you"

Not likely, no offense. At a given dynamic CR, the more efficient motor will make the most power. However, as boost is added, the gap in efficiency will get smaller.
That still doesn't make sense to me. I don't see how the low comp motor could move 30% more air and fuel and make less power than the higher comp motor just because it's more efficient. I hear what you are saying, but I find that very hard to believe. I think we are not accounting for something. I find that very hard to believe. Rather I don't believe it. I mean, even though the dynamic ratios would be the same, I would have 30% more air/fuel to burn. Granted you can burn it more efficiently, but it still seems like the low comp/ high boost setup would come out ahead in power, though down from an efficiency standpoint. I need to come up with some numbers to support what I'm saying I suppose.
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Old 09-24-2008, 01:10 AM   #32
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That still doesn't make sense to me. I don't see how the low comp motor could move 30% more air and fuel and make less power than the higher comp motor just because it's more efficient. I hear what you are saying, but I find that very hard to believe. I think we are not accounting for something. I find that very hard to believe. Rather I don't believe it. I mean, even though the dynamic ratios would be the same, I would have 30% more air/fuel to burn. Granted you can burn it more efficiently, but it still seems like the low comp/ high boost setup would come out ahead in power, though down from an efficiency standpoint. I need to come up with some numbers to support what I'm saying I suppose.
It doesnt seem to make sense, but you are assuming all other things are constant and this is not the case.

If all other things are constant, sure, the more volume will make more power. But the higher CR is making more HP for each point of that effective dynamic CR. And thats just from an efficiency standpoint, it doesnt include other factors.

Intake charge temp, cam selection, timing adjustment, turbo size, intake manifold design, CC shape, and numerous other things can come into play.
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Old 09-24-2008, 01:25 AM   #33
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Originally Posted by jc_rotor View Post
It doesnt seem to make sense, but you are assuming all other things are constant and this is not the case.

If all other things are constant, sure, the more volume will make more power. But the higher CR is making more HP for each point of that effective dynamic CR. And thats just from an efficiency standpoint, it doesnt include other factors.

Intake charge temp, cam selection, timing adjustment, turbo size, intake manifold design, CC shape, and numerous other things can come into play.
I would assume we hold other things constant. IE, you don't run a bigger turbo, i don't run nitrous, etc. I know you can do X, Y, and Z to your high comp motor and make more power. That's not the point and that works both ways anyways.

So if all other things are constant, which is makes more power? That's my question. If you are saying, "Well see, I can run a bigger turbo, better manifold, and a less restrictive turbine and still have the same spool, so I'll make more HP", then that's not a fair comparison. I want to look at the physics of it. I seriously don't see the low comp/ high boost motor moving 30% more air/fuel and falling short on power to a high comp/low boost motor when they both have the same dynamic compression ratio. I have some reasoning, but I need to put it to paper to show the numbers. To me, for your argument to be valid, you would have to be a whole lot more efficient, and I don't see that happening.

Here's an overly simplified example, but perhaps it will explain my reasoning.

Generally speaking, at best, 1/3 the air/fuel mixture is converted from chemical to mechanical energy. So out of 100HP worth of air/fuel, the motor makes 33HP to the crank.

But say you are high compression, and your motor is 12% more efficient. Then you will make 12% more of 33. So 1.12*33=37

But I accept the standard 1/3 rule, but move 30% more air/fuel instead. so 33*1.3=43.

So I just made more power. Feel free to tear my argument apart. I know it's oversimplified, but I think yours is too.

EDIT: continuing...

So for you to make more power than me, you will have to be a lot more efficient. Specifically, you will have to be greater than 30% more efficient than my 8:1. So you are saying that your 11:1 motor is more than 30% more efficient at turning chemical energy into mechanical energy? Ie, your motor can turn more than 43% of the chemical energy into mechanical energy by use or increased compression?
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Old 09-24-2008, 03:05 AM   #34
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Quote:
Originally Posted by y8s View Post
discuss:



(I think the middle numbers are cylinder pressures or equivalent static compression ratio but I forget and it's not labeled)

the chart says basically you will get more power from boost than you will from compression ratio.

example: 11:1 motor running 8 psi will have the same cylinder pressures as a boosted 7:1 motor at 22 psi.
That chart is only mildly useful.

I typed a dissertation on this in MF...

Basically detonation is most strongly a function of temperature at the top of the compression stroke, less so of pressure. A 15 psi boosted 7:1 motor will NOT reach the same temperature as a n/a 14:1 motor; the latter will require higher octane. The temperature is a function of c/r only, and not boost (assuming the same intake air temps). However pressure at the top of the compression stroke also makes the mix more detonation prone, thus more boost = more tendency to ping.
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Old 09-24-2008, 03:09 AM   #35
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jc_rotor,

From thermodynamics (otto cycle), an increase in c/r by 1 point (e.g. from 9:1 to 10:1) is an increase in output by only 4%.

From 8:1 to 11:1 is ~12%. On a ~15 psi motor, that's worth only about 3 psi, or like from 13.5 psi to 15.5 psi. However 11:1 is gonna need some serious octane.

I can tell you right now that with a miata motor at 10 psi and 9.5:1 c/r, 91 california craptane isn't good enough. On a hot day it needs ignition retard to prevent ping. (and loses power due to backing away from MBT).
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Old 09-24-2008, 03:11 AM   #36
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Originally Posted by patsmx5 View Post
So for you to make more power than me, you will have to be a lot more efficient. Specifically, you will have to be greater than 30% more efficient than my 8:1. So you are saying that your 11:1 motor is more than 30% more efficient at turning chemical energy into mechanical energy? Ie, your motor can turn more than 43% of the chemical energy into mechanical energy by use or increased compression?
Like I said output (and BSFC) only improves by 4% per point, or about 12% from 8:1 to 11:1.
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Old 09-24-2008, 03:14 AM   #37
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Also, it's only at low RPMs where the motor doesn't make boost that a high c/r motor will have more output (4% per point).

However due to higher EGTs, a low c/r motor will tend to have a lower boost threshold (and more boost in the spoolup region).

I went from 9.5:1 to 8.4:1 and saw a datalogged improvement in spool.
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Old 09-24-2008, 03:17 AM   #38
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Dynamic compression ratio is a function of VE. It is also knows as BMEP, which is proportional to torque per liter per atmosphere of MAP.

If you have 2 cams, one with a peak VE (peak torque) at 4000 RPM, and a 2nd at 5500 RPM, the dynamic compression *is the same*.

*However* the cam that makes peak torque at higher RPM can run a higher static c/r because pinging happens more easily at lower RPM (more dwell time for charge), all else being equal.
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Old 09-24-2008, 03:20 AM   #39
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I have a theory as to one advantage that high c/r has that I have not seen in a reference. It will have *slightly* higher VE because the clearance volume (c.c. volume at TDC) is smaller and thus:
a) leaves a smaller amount of dead exhaust gas in the cylinder and
b) the initial "suction" on the downstroke is stronger leading to more suckage of intake charge / more initial velocity.

(a) may be extra significant with high exhaust backpressure i.e. turbo.

This may be responsible for some of the gains people see with higher c/r. Plus maybe the piston tops of hi perf hi c/r aftermarket pistons make for better flow and thus VE than factory pistons which are designed for emissions too.
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Old 09-24-2008, 10:09 AM   #40
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I think lower CR is better for the daily driver. You may have less bottom end but when cruising the streets at low rpm you are hardly gunning for it. When you do want to go for it you just keep in the power band so the extra power bottom end does not make as much difference.
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