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.

Hey first off I'm not trying to piss 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?


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.


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%

Like what? Just curious.

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.

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.

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.

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.

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.

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.

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.

400WHP thats all that matters is at the wheels to me.

"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.

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?

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.

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).

Like I said output (and BSFC) only improves by 4% per point, or about 12% from 8:1 to 11:1.

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.

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.

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.

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.