Compression Ratios and Forced Induction
 

This thread is meant to be a general information thread, please think before you post and post only logical information/arguements/opinions. Thanks.

The arguement of Static and Dynamic compression ratios has long been raging in the world of turbocharging. Some say run 8.0:1 or even as low as 7.5:1 and crank up the boost. Others say that 11:1 CR with conservative boost is the way to go.

Right answer? There isnt one. It all depends on your long and short term goals for the car. Building an all out drag car? Dish out those pistons and crank the boost up! Want a reliable street car with low down umph and considerable top end as well? I would stay north of 9:1 and run less than 15 psi.

My personal preference is to raise the static CR. To me, if you are going through the trouble of overhauling the motor to make more power, its better to increase your engines performance and efficiency off boost. Most tuners will recommend 93 octane to turbo'd cars anyway, so you arent really sacrificing anything on that end. The upside to this is that you make more power off boost, your engine has higher egvs and will spool the turbo quicker, allowing you to run a larger one and pick up even more HP/L. The downside to this is that its difficult to tune and it gets more and more difficult as you head north of 15 psi, sometimes requiring 100 octane or greater to do so.

However, an 8:1 car at 20psi is not going to make the HP of an 11:1 car at 15 psi. And even if they were making the same peak HP to the wheels, the 11:1 car is going to have a better powerband and is going to be faster in the 1/4 mile.

Really, the biggest issue with static CR is the margin of error. It is much easier for the average Joe to run a lower CR and high boost because there is a much larger margin for error than there is with high Static CR. But you will lack the efficiency and the HP on low boost.


When advising others in their turbo builds, I normally advise that the stock CR is kept, simply because of the information/number of people running that setup allows for a wide knowledge base and less of a blind spot. And you arent sacrificing that performance down low, you will see improvements all the way across the RPM range and still be moderately safe.

Any imput/arguement is welcome. Keep it civil, we arent ricers or muscle heads, we are Miata owners.

Thanks JC rotor, he started this thread per my request in another thread.

First, I agree with most of that. A higher static compression ratio is going to make for a more efficient setup. More HP/lb of boost and more power out of boost.

However, it's not common to see high compression boosted engines. They are out there, but they are few and far between. Most reduce static compression. This cuts down on heat of compression.

When we compress the air with a turbocharger, it increases the density of the air. That is, there is more mass of oxygen per unit volume. However, it also raises the temperature of the air. We can use an intercooler to remove most of the added heat from the charge. This makes for denser air that is close to the same temperature as ambient. (assuming the IC works well)

Then the air is mixed with fuel and goes into a piston/cylinder arrangement. Here, it is further compressed before the combustion process occurs. Heat of compression causes the air to get hotter. How much is dictated by the compression ratio.

IMO, if you are an amateur DIY type of person that wants to make big power, you want to lower static compression, not raise it. You want to move as much air through the motor as possible with as little heat as possible. This is because the lower compression will give a "cushion" against detonation. That is, the mixture is less likely to auto ignite from too much heat. This gives you a safety margin of error while tuning.

If you are an amateur tuner, and are more interested in reliability at big power levels than efficiency at similar power levels, low compression, high boost is the better option. This means you will run more boost to make a similar power of a lower boost, high compression engine. However, your low comp. motor will have lower peak cylinder pressures, lower EGT's, less chance of detonation, and be easier to tune. The trade off is less bottom end grunt out of boost and overall less efficiency. I will chose power over efficiency. If I wanted efficiency I'd buy a new corolla. :)

Theres only a small gap in your logic here.

Yes a lower CR will result in lower EGTs initially, but as soon as your start adding boost to the equation, your EGTs will begin to rise as well. The reason the EGTs rise is because of the increased pressure in a higher Static CR car. ΔPressure=ΔTemperature. As you increase the dynamic CR, that is, the amount of pressure building before the piston reaches TDC, caused positively by boost or negatively by short duration, high overlap cams, you increase the pressure in the Combustion Chamber and increase your EGTs.

An intercooler works the same way on a 11:1 turbo car as it does on a 8:1 turbo car. Its not only about cooling the intake charge, its about keeping the CC as close to optimal operating temp as possible without passing over the threshold for detonation (premature ignition of CC mixture, before TDC). How the engine coolant is routed, if there are oil squirters, the thermal properties of the manifold and turbine housing all play a part in this equation.

To get this straight, Im not telling anyone to raise their CR unless you can do it safely! (See below for main point!)

Now, I do not disagree that lower CRs offer a larger margin of safety, I actually said that earlier. But I dont think that there is any logical reason to lower the ratio below 9:1 on the miata, as the fairly good engine design allows pretty high boost at this CR. (many run 15psi on stock internals)

Your time and money would be much more well spent concentrating on controlling heat under the hood and ensuring proper cooling components (alum radiator, coolant reroute) to increase your margin of safety.

The biggest help you could ever do to your turbocharged car is to quick dicking around with crappy piggyback ECUs and get a stand alone, and have an expert tune the car.

Take two identical motors with the same charge intake temperature T with an appropriate tune. Both at 300whp. Only difference is one is running 8:1 compression and an arbitrary value of boost X to get to 300whp, and the other motor is running 11:1 compression and an arbitrary value of boost Y to make 300whp.

Which motor has higher EGT's? Which has higher peak cylinder pressures? Are they the same?

That would be referring to the static compression ratio, no? Or am I'm confused? Or is dynamic compression ratio more like the total compression ratio? Back to thermodynamics HW. (seriously! :bowrofl:)

No, static CR is merely the size, or volume of the CC at TDC relative to the Size at BDC.

Dynamic CR is either compression gained or lost due to a change in pressure while the piston is travelling towards TDC. Static CR does not ever change, it is a mathematical ratio. Dynamic CR can be positive or negative. Boost can result in a positive CR, say 11:1+5 psi boost=12:1 (not that simple but just relating it) or 8:1+12psi=12:1, or 11:1-15psi due to improperly overlapped cams=10.5:1. It all gets really confusing, but basically an increase in Static CR results in power because of a more complete burn of the mixture=more efficient. And increase in Dynamic CR increases power because of an increase in mixture volume, aka more dense. Both make power, both have side effects. Increase in Static CR>Increase in Dynamic CR to a point.

Im really tired so ill get to the other part of the question in the morning, but basically they are different animals when it comes to ignition timing, and knowing the ins and outs of each is like having the golden key to the tuner city.

discuss:

http://members.aol.com/solomiata2/BoostCompRatio.gif

(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.

Yeah and then you also have to look at the numbers when you get around 12-14psi, which are where you could realistically run and still be within a decent margin of safety, if you know what youre doing. At 11:1 and 14psi the cylinder pressures (or static CR eqiv, whichever) is 21.5. You have to crank the boost up to over 24psi to even get close to that on 8:1. 9:1 is around 20 psi. So once again I have to say, it does not rationally make sense to lower your CR on a miata unless you are planning for 25+ psi.

EG: I helped build and tune an Integra B18C5 motor that is 10.5:1 at 18 psi. It makes 470HP to the wheels, and reaches full spool at 3K rpms. It beats 660cc motorcycles from a dead stop, and its front wheel drive.

We could have ran 8:1 CR, 27 Psi, and it might make the same HP but it would be a slower car.

Sorry!
The chart does not say that!

Edit: I looked it up and calculated effictive Dynamic CR under boost and this is what the chart is representing.

"You are running 5 PSI of boost at an altitude of 0 feet. Your motor's static compression is 11 :1. At this boost level and altitude your effective compression ratio is 14.74 :1"

You are running 12 PSI of boost at an altitude of 0 feet. Your motor's static compression is 8 :1. At this boost level and altitude your effective compression ratio is 14.53 :1"

sorry, my bad. I didn't mean to say the chart said that. I meant to say that generally speaking it's easier to make power with boost than with compression (my opinion)

Fact: It is easier to tune cars with high boost and low compression:bang:

Myth: Low compression, high boost cars make more power:hustler:



I would highly suggest that everyone do some research on ignition timing. Timing plays a vital role in cylinder pressures/temp/detonation.

There is a short but sweet article on the integra forums. LINK: http://www.team-integra.net/sections...?ArticleID=235

Keep in mind that there are differences in the way the mazda BP and the honda B18 operate, EG honda uses a distributor, mazda uses CAS. But it should give you a pretty good idea.

much easier to raise boost than compression. you can actually find your knock limit incrementally with boost. try doing it with compression. ...at least outside of a RON test rig.

The bottom line is that it's easier to remove heat from the charge pipes than it is to remove it from the compression itself. That's why it's easier to tune low CR, high boost motors - because there's less heat.

ding ding ding!!!

I love dicking around and street tuning my home built piggyback.

In your example you compare an 11:1 motor at 14 PSI and an 8:1 motor at 24+ PSI.

I KNOW an 8:1 motor at 24PSI ill make more power than an 11:1 motor at 15PSI. Do you seriously believe otherwise? A full point of compression ratio is said to add about 4% more HP. So your 11:1 motor would make 12% more power AT BEST. Fact is though, that's an estimate that could vary depending on just how aggressive and perfect your tune is. My guess is maybe 10% more power.

Consider the following.

Two motors with the same dynamic compression ratio using your numbers.

Your motor: At 1 atmosphere and add 15 PSI boost, that's 29.7 absolute pressure going in the motor.

Mine: At 1 atmosphere and add 24 PSI, that's 38.7 absolute pressure.

That means I'm moving 30% more air through the engine than you are. You might be 10% more efficient from running higher compression, but you will never catch up to my 30% gain, much less surpass it. Low compression high boost will make more power for a given dynamic compression ratio.

Am I right? Prove me wrong or support your argument.

Consider your 4% theory, and that you are adding 8% more power going from 9:1 to 11:1 (2 full points), and that you are going down 4% from 9:1 to 8:1. So now the LC motor is down 12% from the HC motor. Then consider that the HC motor will spool earlier, probably a full 1k rpm sooner, and thus make more power under the curve, plus higher peak power cause it is more efficient. And the 4% is for NA motors, not FI. FI would net more power with higher compression than a NA car would.

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?

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.

30% more air in the combustion chamber does not equate to 30% more power.

"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 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.

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.

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.


So you are partially right, yes, all other things equal except pistons and PSI, the more boost will probably end up making more power. BUT the higher CR allows you to make more power off of a less restrictive turbo, makes more power off of a properly designed manifold, and makes more power off of appropriate cams.

You cant just swap the pistons and call it a day.

Look, I can sit here and tell you why all day long, but its not going to make a difference to you until I prove it to you with empirical data. Im in the process of building the turbo kit and when I get done youll see what kind of power it makes, and then we can continue the discussion from there.

This was typed for jc rotor, but if anyone else wants to take a stab at it I'm quite curious.

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?

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.

I dont see the logic in that? The higher CR is going to have a more linear power curve and the Low CR high boost is going to have more of a bell curve. That would make a high CR low boost application better for a daily driver/track car and the low CR high boost application better for a drag car.

And Jason the 4% rule as far as ive seen only applies to cars running on atm pressure (14.7). I dont know where you get the 3psi reference but that certainly isnt the case on a honda.

Once again the only definitive way to test the theory is to put it into practice.

So we'll see what the dyno has to say in the next few months.

jc rotor, did you see my post 33? Give me your opinion on my math. Is it correct? Or is it close? Do you agree with the 1/3, 1/3, 1/3 rule? Is an 11:1 motor more than 43% efficient at making chemical energy into mechanical energy?

No way. Even very efficient power plants work hard to get that kind of efficiency from coal-fired steam turbines, with very sophisticated heat cycle controls and schemes. They are much more efficient overall at extracting energy than piston engines are.

Think more like 20% for a gas piston engine.

Thermodynamics. It will be true whether or not the car is FI'ed. Think about it.

If the thermal efficiency of an Otto cycle is say, 20% at 9:1, at 10:1 the output will increase by 4%, for a new thermal efficiency of 20.8%. I mentioned "Otto cycle" in my post. As an M.E. you should have recognized the term.

I pointed out that 8:1 -> 11:1 is a 12% gain. 12% gain on a 15 psi setup is like a 3 psi gain.

I pointed out above that my car did NOT lose bottom end going from 9.5:1 to 8.4:1 - dyno measured. And spoolup IMPROVED.

The point you are missing
 

jc_rotor,

You seem to think that equivalent pressure at the top of the stroke is all that matters. It isn't. JasonC has pointed out the dramatic difference in temperatures (and hence the likelihood of detonation).

To that, I'll add that the lower compression engine makes more power at the same peak cylinder pressure. Why: The volume/quantity of high pressure gas in the LCHB (low compression, high boost) engine is larger than with the HCLB (high compression, low boost) engine. So, while the chamber pressure may be the same at the top of the stroke, the LCHB engine has more gas to distribute through the cylinder during the power stroke than does the HCLB engine. Using 8:1 vs 11:1, the 8:1 engine will retain roughly 15% more pressure/torque/power mid stroke and 30% more pressure by the time the piston reaches BDC.

"So if all other things are constant, which is makes more power? That's my question."

If all other things are constant, and both are tuned perfectly and they have the same effective CR, they should make the same similar power, the lower CR from the more air/fuel, the higher CR from higher efficiency.

You are looking at it differently than I am, but you also have to consider that a 24 psi charge is goinng to be hotter than a 14 psi charge, and the heat from compression should be the same according to pressure in the CC b/c they have the same dynamic effective CR. I cant say for sure because I havent done extensive research but they should be very similar in power.

Exactly. There is more "stuff" in the LC/HB motor. When combustion begins, it's going to release more energy. That simple.

ok i was hoping to copy a proven set up but i cnt find the specs on stp's egt or the 616 hp miata. is there a simple chart to figure out estimated hp in relation to cr, boost, octane, and what other veriables im missing? the question sounds like a noob question sorry, but ill leave the details to the engineers and experts i just want to know what piston cr i should get? i know your going to say well how dependable and how much do you want to spen? well i want the car to be some what dependable it will mainly be a weekend warrior and maybe once a year i may drive from chicago to ohio. i dont care about running low octane i always run 93 or better.

Yes but its also a less complete burn of the volume that is there.

My whole point the whole time of this, with or without arguing the extremes of either case or the (not likely) chance that everything but CR is different. Is that there is absolutely no reason to lower your CR lower than 9:1 unless you are going to run some ridiculous amount of boost.

The best CR for turbo cars IMO is 9.5-10.5:1

Im doing 11:1 because I want to see what happens to the miata motor specifically when running this high of a CR. All engines react differently to CR changes because of the cams, bore to stroke ratio and so forth. I think that 11:1 is going to be the sweet spot for boost on my motor because of the way im doing everything. Ive already reached 140HP dyno proven naturally aspirated, more than 8% power I "should have gained" from efficiency. So im very interested in how its going to run at 14psi and im anticipating 400+ to the wheels.

I'm definitely looking at it differently. So what is right? I used the 1/3, 1/3, 1/3 rule. Even if it was 20% as ZX-Tec suggest, the idea that you will have to be MORE THAN 30% MORE EFFICIENT from changing compression ratio's alone to make more power than LC/HB still stands. So is that reasonable to you?

I think my case is clear. I say that will never happen.

Hotwheels, go make yourself a thread and ask for opinions. We are talking physics right now, not answering phone calls.

Leave the stock CR of 9:1 if its an early 1.8. Get 1mm overbore pistons if you are going to build the bottom end with A-beam rods. Run 14-18psi. Should get you considerable power and be very reliable. Make sure you get fully programable engine management and get it properly tuned. Should get you north of 275 HP easily.

If you are using the 1/3, rule, then (assuming) you have 30% more power from the added volume, you take that 100HP at 30 and THEN do the efficiency. so instead of 100HP worth of air fuel, its 130*33.33%=43.329 HP from it, and the 11:1 is 12% more effcient its 100HP*45.33%=45.33 HP. Very similar as you can see.

These calculations are not accurate though, you are assuming too much.

Your math is wrong. Increasing the compression ratio will increase power to the crank by a max of 12%. That means the change in power to the crank will increase by 12%. You can't just add 12% to 1/3. 1/3 is an arbitrary value anyways as it is a constant in both scenarios.

Assume P=Power to the crankshaft, Q = efficiency added in %, R = increase in air/fuel, * represents multiplication.

We will let my 8:1 motor be the datum for which yours is measured off of.

So for me, Q is 1.00. For an 11:1 motor under ideal conditions, Q=1.12.

For me, R=1.3 as I will move 30% more air. For an 11:1 motor, R=1.00.

P = (R)*(1/3)*(Q)

For an 8:1 motor:

P = (1.3)*(1/3)*(1)= .433

For an 11:1 motor:

P= (1.0)*(1/3)*(1.12)=.373

Again the 1/3 is a constant in either case and is therefore arbitrary. Delete it from the equation and you will still get the same ratios.

All I'm assuming is that everything is held constant and we only change compression ratios. Assume whatever makes you happy, as long as it is the same for both motors. Only reason for this is to calculate the change in power from changing compression ratios.

What else did you change?

-- 14yr engine builder/tuner who owns an import turbo performance shop

Even if you changed the tune to compensate for the new lower CR, you wouldn't be able to spool faster unless you tuned it BETTER for the LC vs the HC. All things in the tune being equal, meaning they are as close to the line to knock as you can get, and tuned to 11.5:1 perfect AF all the way across, the HC will spool faster and make more power on less boost.

Nooooooooo... Get that "effective CR" out of your head, it's next to useless!!! (Dammit y8s!! ;) )


Try and think of this. Higher CR gives higher thermodynamic efficiency. END OF STORY.

Power output is proportional to efficiency and manifold absolute pressure, all else being equal.

Each point of additional compression adds 4% of output. END OF STORY. That piece of shit term "effective compression ratio" doesn't figure in this at all!

JESUS CHRIST I THOUGHT YOU WERE A MECH. ENGINEER????!!! :D

Minor DIY valve seat cleanup and 3-angle valve job.

Power is a function of BMEP. BMEP is a function of volume. It can be said that Power is a function of volume. If there's any doubt I'll type of the long fancy version tomorrow.

Power = BMEP * RPM.

BMEP is proportional to air consumed per cycle, and thermal efficiency.

jc_rotor,

AG Bell's book "4-stroke performance tuning" warns against the common mistake of raising compression ratio too high for the octane (whether FI or n/a). The temptation is to raise c/r, then retard timing to prevent knock. The latter will lose you more power than the raised c/r will gain you. High c/r is over-rated.