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! )

 

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:



(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

Myth: Low compression, high boost cars make more power



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?