Compression and Turbos

[Leatherneck]

Since I am among other problems trying to lower my boost down I did a search on compression related to boost and found this. Most of you guys probably knew this but alot of people looking to build there first might not. I know I did not know which was better or easier to tune.

A debate that often is pondered by not only Honda enthusiasts but all import performance enthusiasts is whether one wants to go the N/A route, or boost with a turbo or supercharger. While each has their own benefits, debating this is out of the scope of this article. This article is about how boost adds power, beyond the saying "it adds more air to burn". We all know that motors use compression to make power. For example, a B16A makes 10.2:1 or 10.4:1 depending on the year. This is known as static compression. In this article, we will introduce what is known as a motor's effective compression and explain the differences in choosing a proper static compression ratio for boost.

Effective compression is the sum of the static compression, plus the additional compression added to the cylinder by a turbo or super charger, or any other forced induction tool for that matter. Effective compression is defined by the following formula:
E = C((B / 14.7) + 1)
Where E= Effective Compression, B= boost psi, and C= Static compression. Also remember that 14.7 is equal to 1 bar of boost.

Let's do an example. Let's say we have a B16A bone stock with 10.4:1 static compression who slapped on a Drag Gen III turbo kit and is now boosting 7psi. That takes care of our variables. Let's do the math.
E = C((B / 14.7) + 1)
E = 10.4((7 / 14.7) + 1)
E = 10.4((.476xx) + 1)
E = 10.4(1.476xx)
E = 15.35xxx
As you can see, we come up with an effective compression ratio of 15.3 or so. A motor in this effective compression range is easily daily driven with proper fuel and timing adjustments/upgrades.

Why will a forced induction care always make more power at the same compression level? This answer is easy to see after doing the math. Your engine will always see the effective compression level. If you are N/A, you have no additive. A B16A N/A will still make the 10.4:1 static compression, while the boosted B16A will be over 15:1 from the effective compression ratio.

When building a motor, we are all after a higher effective compression ratio. So which is better? In the next part of this article, we will weigh the pros and cons of the following combinations of static compression ratios and boost pressure:

* Low static compression / High boost
* High static compression / Low boost
* Medium static compression / Medium boost

Low static compression / High boost

Tuning plays a big key in all boosted setups. As the static compression ratio gets higher, it gets harder to tune. Lack of proper tuning leads to detonation, which leads to blown head gaskets, thrown rods, and cracked cylinder walls. Since this setup involves low static compression ratios, it is easy to tune. Just crank up the boost a little more to make up for the effective compression that is lost from the lower static compression. This is the easiest way to get a car boosted with the least amount of tuning. This set up, however, lacks in the low end torque department due to the fact that it relies on the turbo for most of its power.

High static compression / Low boost

This setup is harder to tune than the above setup, but at the same time, its output is overall higher, due to the higher static compression. This eliminates a lot of the low end torque/turbo lag problems that the above setup has, due to the fact that the higher static compression creates more power from the engine, and relies less on the source of the forced induction to create the higher effective compression level. Proper turbo size also plays a factor but, for the sakes of argument, we will simply discuss the motor's properties.

Medium static compression / Medium boost

This setup takes the best and the worst features from both sides. It will give a little more bottom end, but makes it a little more difficult to tune. A lot of people choose this route for VTEC engines. Dropping the compression down to say 9.5:1 and running around 9psi creates this medium zone that most people who boost tend to fit in.

So why does boost always make more power than N/A?

It's a simple explanation. In order for an N/A car to hang with a forced induction car, it would have to be running 15+:1 compression ratio; a ratio that's simply not useable on anything less than 125 octane gas.

[Kenny2428]

Leatherneck, great post. I will make it a sticky. Great info.

[david58]

Looks like you have been busy Leatherneck. Nice post.

[MarioVelotta]

Nice write up, thanks for sharing.

[Odyknuck]

Great post. I am in the middle of setting up my static compresion on my Woods buggy 2332. The deck Height of .120 (using a .060 copper gasket and 51 CC chambers. I end up with 9.1 compression. It appears that If I keep my boost below 10PSI then I would be good to go. Because its a woods buggy I need the compression for when Im not on Boast. Anybody running this combination with out any issues. BTW it allso will be using CB Performances EFI

[mike thompson]

It seems to me the forced induction engine at the same effective compression ratio as NA would make more power if it has the same efficiency overall.
Simply because more fuel and air is being burned.

[Odyknuck]

Interesting observation. This post has been up here several weeks and only a few responces to such a controversal subject. I figured it would have at least a 100 responces by now Hmmm!

[Wally]

Letherneck,
Interesting post, but I have not once read the word 'cam' or cam overlap in the write-up.
High static CR, but coupled with large duration cams gives average dynamic CR, which may be quite good for turbo'ing?

So, I miss cam duration in the whole compression ratio discussion.
Thought: couldn't you just use a larger duration cam to bring (dynamic) compression down and get the same results as when you use low static CR and mildly duration cams?

[SiQDiZ]

Camshaft grinds for turbo

For more information regarding the turbo camshaft subject.

[70dragbug]

Good article! Another thing to look at IMHO is chamber shape.A Honda 16V engine has a pentroof design,which in itself is designed for high compression,high rpm.Generally volumetric efficiency is much higher with a 16 valve head than an aircooled 8 valve head,at least in stock form.So the gains are much higher and more effective even with low boost.Camshaft and bore size are substantial to tuning with a turbo or a high compression NA motor.A large bore with high compression or boost will ping a lot sooner than a smaller bore.Although a large bore will require less boost pressure to make the same or more power than a smaller bore.A large bore engine is also much harder to tune because it will ping much sooner at a high CR.So chamber shape and camshaft character will dictate tunability.

[jweir]

so what effective compression ratio is the maximum a person can run with pump gas (92 octane) and does altitude effect the equation? I know the pressure in denver is 12.1 instead of 14.7, so can I effectively run a higher compression ratio? I have 8.5:1 compression and I am trying to see how high I can take the boost on premium gas here in denver. Is there an equation for effective compression versus octane rating? Do i change the 14.7 to 12.1 in the EC equation?

[mike thompson]

I would not try to run a VW at 15 to one effective ratio.
But if you stay in town you can run a couple more pounds boost than down here.

[Joe Perez]

With due reverence to the fact that I'm a newb posting in a sticky (and an old one at that) I think it's worth mentioning that there is a crucial dimension here which I think is being overlooked entirely. I will add that I am not a mechanical engineer nor an automotive designer. I'm just a self-educated guy who tries to take a somewhat scientific approach towards working with engines, and specifically with turbocharged and supercharged engines for the past 10 years or so.


Compression Ratio is an oft quoted measurement, and one which is fairly easy for most folks to wrap their head around, at least at a fundamental level. We all know that, up to a point and all other factors being equal, increasing the compression ratio of an engine improves the thermodynamic efficiency of the combustion process, thus allowing a greater fraction of the potential energy of the fuel/air mixture to be captured as actual kinetic energy, ergo: more power at the crank. The engine with the higher CR has a more optimal BSFC.


When adding forced induction, it's easy to think about the fact that, by increasing VE, you've essentially increased the dynamic compression ratio (and consequently, the chamber pressure at the end of the compression cycle), and this is in fact true. The fallacy lies in attributing the resultant power gains to this increase in effective compression.


Let's compare two hypothetical engines being tested on a warm spring day in San Diego (IOW, standard temperature and pressure). Both engines have 4 cylinders, with a total displacement of 2.0 liters, or 500cc swept volume per cylinder. The first has a static compression ratio of 20:1. The second has a static compression ratio of 10:1, but is fitted with a turbocharger that is 100% efficient, operating at a pressure ratio of 2 (or 14.7 PSI of boost). Assume all other factors to be equal and ideal.

If we instrument the two engines so as to measure the dynamic compression ratio and thus, the in-chamber compression pressure prior to ignition, we will find that they are both yielding identical measurements. However, we will find that BMEP is much higher on the turbocharged engine and, consequently, it is producing nearly twice the power when we attach a brake dyno to the crankshaft.


How is this?


Well, by focusing merely on compression ratio, we overlooked something fairly obvious and terribly important. The reason that the 10:1 engine produced the same dynamic CR numbers as the 20:1 engine is that the 10:1 engine took in twice as much air during the intake cycle. Forget about CR for a moment and consider the idea that volume (as expressed in cc) is not an absolute.

Think, instead, about mass. Both engines took in 500cc of air per cylinder per cycle. However the density of the air taken in by the turbocharged engine was twice as high as the air taken in by the naturally aspirated engine, because it had already been compressed by the turbocharger at a ratio of 2:1. Twice the density equals twice the number of O2 molecules. In other words, the turbocharged 2L engine is breathing in as much air as a naturally aspirated 4L engine would (I know this isn't strictly true, but this is a hypothetical 100% efficient turbo.)

I think we can all agree that, all else being equal, a 4 liter engine will make more power than a 2L engine, yes?


That's really the key point I'm trying to convey. Trying to compare a turbocharged engine to a naturally-aspirated one on the basis of dynamic compression alone is a bit like trying to compare apples to fear. It's just a complete mis-matching of concepts. Yes, forced induction does act as a compression multiplier within the engine. However this is merely a side-effect, not the principle mechanism of action. The effect of increased CR on BHP, while not completely inconsequential, is trivial compared to the effect of increased mass flow.



References:

1: Internal combustion engines, theory and design By Robert Leroy Streeter, p. 65, etc.

2: Internal combustion engines, their theory, construction and operation By Rolla Clinton Carpenter, Herman Diederichs, p. 65-69, etc.

3: Variable Compression Ratio Engine by Roland Gravel, DOE Office of Advanced Automotive Technologies

4: The Gasoline FAQ by Bruce Hamilton, ch 7.2

5: Brake Mean Effective Pressure: An important performance yardstick from EPI, inc.

[klatin]

Hi Joe,

You are quite right in essentially saying that multiplying the compression ratio of the engine with the pressure ratio of the turbo does not make any sense.
The whole reason gasoline engines are limited in the possible compression ratio is the heat rise happening in the cylinder due to compression. When a gas is compressed, its temperature rises. If the temperature rises high enough to reach the ignition temperature of the gas/fuel mixture, we can get uncontrolled ignition -> knock.
What makes engines still work with high compression ratios is that high temperatures have to persists long enough to cause auto-ignition, even when quite above the ignition temperature. That's why engines are more knock prone at low rpm.

The temperature rise due to compression can be calculated approximately:
Tend= Tstart * ( (CR)^0.4 ),

where Tend is the end temperature (in absolute temperature scale),
Tstart is the gas temperature in the cylinder at the beginning of the compression stroke,
CR is the compression ratio.

Anything to lower Tstart, like an intercooler, also lowers proportionally the end temperature.

For example, with an 8:1 CR turbo engine without intercooler, and an IAT (in cylinder IAT) of 150 deg C (~300 F), the end gas temperature after compression would be about 700 deg C, or 1290 F. Well above the ignition temperature of gasoline of ~495 F. The only thing that saves this engine from exploding is that the 300 deg F IAT is only reached at high rpms and then there's not enough time for the autoignition to start.

If the engine is intercooled, and the intake charge is cooled down to 150 degF (in cylinder after intake stroke), the end temperature would be 940 degF. A change of 350 deg F in end temperature, which lengthens the time allowed for the mixture to be at the high temperature quite a lot.

For the calculations the pressure ratio of the turbo has only an indirect influence, as higher pressure ratios cause higher IATs. But to get rid of that is what intercoolers are for after all.

However, back pressure of the turbo will have a large influence, as high back pressure caused by for example low A/R numbers, will also cause the in-cylinder temperatures to rise. That's because more hot exhaust gas remains in the cylinder after the exhaust stroke, which mixes with and heats up the fresh intake gas. It also lowers the effective VE of the engine, as it takes up room in the cylinder. As a result you'll see a falling torque curve soon after boost onset.

[drmiller101]

compression ratio and boost are not the same thing at all. They do both run into detonation limits however.

Compression ratio means the charge burns quicker, which means pressure inside the combustion chamber has a higher average pressure pushing down on the piston. The engine is in effect "more efficient." Changing the compression ratio 10 percent does NOT equate into 10 percent more power in the engine.

boost is more of a replacement for displacement. When all done, an engine makes horsepower based upon a function of air and fuel put into the engine. The more air you put into the engine, the more fuel you can put into the engine, and the more horsepower the engine can make. Turbos push more air into the engine, which allows you to add more fuel, which allows the engine to make more power.

At the same time, when you cram more air and fuel into the engine, your effective compression ratio also ends up higher.

If you can cram 10 percent more air into the engine, and keep the same air/fuel ratio, you WILL make 10 percent more power.