COMPRESSION RATIO: HOW to CALCULATE, MODIFY and CHOOSE the BEST one - BOOST SCHOOL #10

Summary

This video script delves into the concept of engine compression ratio, explaining its impact on performance and efficiency. It outlines how to calculate and modify compression ratios, and the importance of choosing the right ratio for specific applications. The script also addresses the limitations of high compression ratios, such as increased risk of knock and higher emissions, and provides practical advice for balancing power output with drivability and reliability.

Takeaways

• 🔧 Compression ratio is the ratio between the largest and smallest volume of your cylinder, influencing engine performance and efficiency.
• 🛠️ Higher compression ratios compress the air-fuel mixture more, bringing molecules closer together and enhancing combustion efficiency.
• ⚙️ Spark-ignition engines benefit from higher compression ratios as it facilitates heat transfer and more effective combustion.
• 🔥 While higher compression ratios can increase power output and efficiency, they also raise combustion temperatures, which can increase nitrogen oxide emissions.
• 🚫 The main limitation of high compression ratios in spark-ignition engines is 'knock,' where the air-fuel mixture ignites prematurely, potentially damaging the engine.
• 🔩 Compression ratio is determined by factors such as engine bore, stroke, head gasket thickness, piston-to-deck clearance, and combustion chamber volume.
• 📊 Modifying compression ratios can be achieved through adjustments in bore, stroke, head gasket thickness, machining, and piston top design.
• 🔄 Increasing bore and stroke generally increases compression ratio, while thicker head gaskets or dished pistons decrease it.
• 💡 Choosing the optimal compression ratio depends on the accuracy of the engine build, the hardware and software used, and the specific goals of the build.
• 🚗 For naturally aspirated engines, higher compression ratios are generally better, whereas turbocharged engines may benefit from lower compression ratios to accommodate higher boost levels.

Q & A

What is the compression ratio of an engine?

The compression ratio of an engine is the ratio between the largest and smallest volume of the cylinder when the piston is at 'bottom dead center' and 'top dead center', respectively.

How does the compression ratio influence engine performance and efficiency?

A higher compression ratio allows for more compression of the air-fuel mixture, which can lead to more effective and rapid combustion, thus increasing both power output and efficiency of the engine.

What is the significance of the air-fuel mixture compression in spark-ignition engines?

In spark-ignition engines, a more compressed air-fuel mixture facilitates heat transfer from one layer to the next, enabling more effective and rapid combustion, which results in a more thorough burn of the mixture.

Why can't we increase the compression ratio indefinitely?

While higher compression ratios can improve engine performance, there is a sensible limit due to factors like increased combustion temperatures, which can lead to higher nitrogen oxide emissions and the risk of engine knock.

What is engine knock and why is it a concern?

Engine knock occurs when the air-fuel mixture inside the cylinder becomes so hot from compression that it ignites before the spark plug initiates the flame front, causing an uncontrolled explosion. Knock can damage the engine and should be avoided.

How can the compression ratio be calculated?

Compression ratio can be calculated using a formula or more conveniently, by using an online compression ratio calculator that takes into account factors such as engine bore, stroke, head gasket bore, and other engine dimensions.

What factors determine the compression ratio of an engine?

The compression ratio is determined by the engine bore, stroke, compressed head gasket thickness, head gasket bore, piston to block deck distance, piston dish or dome volume, and combustion chamber volume.

How can the compression ratio be modified?

Compression ratio can be modified by changing the engine's bore and stroke, using different thickness head gaskets, machining the block deck or cylinder head, grinding the combustion chambers, or modifying the piston top design.

What are the considerations for choosing the optimal compression ratio for an engine application?

The optimal compression ratio depends on the build accuracy, the hardware and software capabilities for knock control, the engine's displacement and power goals, and the specific application requirements such as street driving or racing.

How does forced induction affect the compression ratio?

Forced induction engines are more limited in compression ratio because the compressed air entering the cylinder adds heat, increasing the risk of knock. These engines typically require lower compression ratios to avoid this issue.

What is the relationship between boost and compression ratio in terms of power output?

While a higher compression ratio can increase power output, boost has a more significant effect. A single point increase in compression ratio typically results in about a 4% power increase, whereas 1 psi of boost can increase power by around 7%.

Outlines

00:00

🔧 Understanding Compression Ratio in Engines

This paragraph introduces the concept of engine compression ratio, explaining its theoretical basis and practical implications. The compression ratio is defined as the ratio between the largest and smallest cylinder volumes, determined by the piston's position at 'bottom dead center' and 'top dead center'. The higher the compression ratio, the more the air-fuel mixture is compressed, leading to more effective and rapid combustion in spark-ignition engines. However, there are limits to how high the compression ratio can be due to increased combustion temperatures and the risk of knock, which can damage the engine. The paragraph also touches on how compression ratio can be calculated and modified, setting the stage for a deeper dive into these topics in subsequent sections.

05:04

📏 Calculating and Modifying Compression Ratio

The second paragraph delves into the practical aspects of calculating and modifying the compression ratio of an engine. It outlines the factors that determine the compression ratio, including engine bore, stroke, head gasket specifications, piston top design, and combustion chamber volume. The speaker uses his 1.6 liter Toyota 4AFE engine as an example, demonstrating how to input these values into an online compression ratio calculator. The paragraph also discusses how to modify the compression ratio through changes in engine hardware, such as bore and stroke adjustments, head gasket thickness, machining, and piston top modifications. The importance of precision in these modifications is emphasized, especially when aiming for high power outputs.

10:04

🏎️ Choosing the Optimal Compression Ratio

This paragraph focuses on the factors that influence the choice of optimal compression ratio for a specific engine application. It discusses the importance of the build quality, the hardware and software used, and the goals of the engine build. The speaker explains that higher compression ratios demand greater accuracy and can increase power output but also raise the risk of knock. The choice between high boost and high compression is explored, highlighting the trade-offs between power output, engine responsiveness, and drivability. The speaker also considers the specific challenges of his mid-engine application, such as intercooling effectiveness and increased heat, suggesting that a lower compression ratio might be more suitable in such cases.

15:05

🏁 Balancing Power and Practicality in Engine Design

The final paragraph wraps up the discussion on compression ratio by emphasizing the need for a balanced approach. It acknowledges that compression ratio is always a compromise between power output, engine responsiveness, and practical considerations like build accuracy and intercooling effectiveness. The speaker shares his personal strategy for choosing a compression ratio that balances performance with drivability, particularly in the context of his mid-engine car project. He also advises viewers to consider their own power goals and the specific challenges of their applications when making decisions about compression ratio, concluding the video with a reminder of the importance of making informed choices.

Keywords

💡Compression Ratio

Compression Ratio is the ratio of the volume of a cylinder and its combustion chamber when the piston is at the bottom dead center (BDC) to the volume when the piston is at the top dead center (TDC). It is a fundamental concept in the video, as it directly influences engine performance and efficiency. The script uses the example of a 100 CC to 10 CC volume change to illustrate a 10:1 compression ratio, emphasizing its importance in how much the air-fuel mixture is compressed, affecting combustion and engine power.

💡Spark-Ignition Engine

A Spark-Ignition Engine, also known as a petrol engine, is an internal combustion engine where ignition is initiated by a spark from a spark plug, as opposed to compression ignition. The script mentions that in such engines, the combustion process is supposed to occur as an evenly spreading flame front, which is influenced by the compression ratio, affecting how effectively and rapidly the air-fuel mixture burns.

💡Knock

Knock is an undesirable phenomenon in internal combustion engines where the air-fuel mixture ignites prematurely due to high pressure and temperature, causing a knocking sound. The script warns that a higher compression ratio increases the chances of knock, which can be particularly damaging in forced induction engines where additional heat is added to the system, and it should always be avoided to prevent engine damage.

💡Piston

The Piston is a crucial component in an engine that moves within the cylinder due to the pressure of combustion. The script explains that the position of the piston at BDC and TDC determines the largest and smallest cylinder volumes, respectively, which are used to calculate the compression ratio. Modifications to the piston, such as changing its top shape, can alter the compression ratio.

💡Combustion Chamber

The Combustion Chamber is the part of an engine where the air-fuel mixture ignites and burns. The script discusses how the size of the combustion chamber can be altered to change the compression ratio, with a smaller chamber leading to a higher compression ratio, thus increasing the power output and efficiency of the engine.

💡Bore

Bore refers to the diameter of an engine's cylinder. The script mentions that the engine bore is a determining factor for the compression ratio and that overboring the engine (increasing the bore size) can increase the largest cylinder volume, thus affecting the compression ratio.

💡Stroke

Stroke is the distance that the piston travels from the top to the bottom dead center within the cylinder. The script explains that the stroke, along with the bore, affects the compression ratio, and modifying the stroke can be done through stroker kits to change the engine's performance characteristics.

💡Head Gasket

The Head Gasket is a component that seals the combustion chamber by sitting between the engine block and the cylinder head. The script discusses how the thickness of the head gasket and its bore size can influence the cylinder volume and, consequently, the compression ratio.

💡Turbocharging

Turbocharging is a method of forced induction where exhaust gases drive a turbine, which in turn drives a compressor to force more air into the engine, increasing its power output. The script uses turbocharging as an example of how an engine can achieve higher power levels, but it also highlights the need for careful consideration of the compression ratio to prevent issues like knock.

💡ECU (Engine Control Unit)

The Engine Control Unit is the electronic brain of an engine that controls various parameters for optimal performance and efficiency. The script mentions using a standalone ECU with advanced features like knock monitoring and engine protection strategies to safely increase power output while managing a high-compression, high-boost engine setup.

💡Nitrogen Oxide Emissions

Nitrogen Oxide Emissions are pollutants produced by engines, particularly at high combustion temperatures. The script points out that higher compression ratios lead to increased combustion temperatures, which in turn can increase nitrogen oxide emissions, a concern for modern engines designed to meet strict emission standards like the Euro-6 norm.

Highlights

Introduction to the concept of engine compression ratio and its impact on performance and efficiency.

Explanation of static compression ratio as the ratio between the largest and smallest cylinder volumes.

How the piston's position at 'bottom dead center' and 'top dead center' determines cylinder volume.

Compression ratio's role in the compression of the air-fuel mixture and its effect on spark-ignition engine combustion.

The relationship between higher compression ratios and more effective, rapid combustion.

Methods to increase compression ratio by altering combustion chamber size or piston position.

Limitations of high compression ratios due to increased nitrogen oxide emissions and knock.

The concept of 'knock' in spark-ignition engines and its potential to damage the engine.

Factors determining compression ratio, including engine bore, stroke, and other hardware specifications.

Demonstration of using an online compression ratio calculator with example values.

Practical modification of compression ratio through engine building phase hardware adjustments.

The effect of bore and stroke changes on compression ratio and engine performance.

Cost-effective methods to modify compression ratio, such as head gasket changes and machining.

The impact of piston top modifications on compression ratio and cylinder volume.

Guidelines on choosing the optimal compression ratio based on build goals, hardware, and software capabilities.

Considerations for naturally aspirated engines and the benefits of higher compression ratios.

Striking a balance between power output, engine responsiveness, and practicality for street use.

Special concerns for mid-engine applications and the effects on intercooling and intake air temperatures.

Conclusion on compression ratio as a compromise and guidance for making informed choices for different applications.

Transcripts

What is up Engine heads?

Today we're talking about the  compression ratio of your engine.

First, we're going to explain the  theory behind compression ratio, 

so we're going to see what a compression ratio is, 

and how it influences the performance  and efficiency of your engine.

After that, we're diving into  the practical side of things.

And we're going to see how to calculate  and how to modify compression ratios.

And finally, we're going to  be talking about choosing the  

optimal compression ratio for your application.

So, let's get started.

Now, when we say compression ratio, 

we're talking about the static compression ratio of your engine.

And that is the ratio between the largest  and the smallest volume of your cylinder

The largest volume of your cylinder is determined  

by the position of the piston  at 'bottom dead center'

So when the piston is at bottom dead centre, this is your largest cylinder volume.

Your smallest cylinder volume occurs  when your piston is at 'top dead center'

So the compression ratio is the ratio  between these two cylinder volumes.

So if our largest cylinder volume, when the  pistons at bottom dead center, is 100 CC

And our smallest cylinder volume, when  the pistons at top that center is 10 CC

Then our compression ratio is 10:1

It's that simple.

Now, your compression ratio, as the name sort of implies, 

determines how much we compress the  air-fuel mixture inside the cylinder.

The higher the compression ratio, 

the more we compress the mixture, And the more we compress the mixture, 

the closer we bring the air  and fuel molecules together.

Now, this is especially important  in a spark-ignition engine

Because in a spark-ignition engine combustion  occurs as an evenly spreading out frame front.

At least it's supposed to occur that way.

And it means that the first  layer that gets ignited, 

increases the temperature of the next layer, and then combust the next layer, and so on.

In other words, combustion occurs in the  layers that spread out evenly outwards.

And by bringing the air-fuel  molecules closer together,  

we facilitate the heat transfer  from one layer onto the next one.

In other words, we make it possible for combustion  to occur more effectively, and more rapidly.

And by doing this, we ensure that the  air-fuel mixture is burned more thoroughly.

In general, a higher compression ratio is achieved  by reducing the size of the combustion chamber.

And/Or by somehow bringing the piston  closer to the combustion chamber.

In both cases, we're bringing the piston  closer to the heart of the combustion, 

to the source of the energy.

And by doing this, we're allowing  more of this energy to be transferred  

onto the piston more effectively, and be turned into mechanical energy.

In other words, by increasing  the compression ratio,  

we can increase both the power output  and the efficiency of the engine.

So, if higher compression ratios are better,  

we should all run infinitely high  compression ratios on our engine.

Well, of course not.

As with all things, there is a  sensible limit to a compression ratio.

And you can actually have  too much of a good thing.

Now, a higher compression ratio contributes to  a more complete burning of the air-fuel mixture.

But this as a consequence, has  increased combustion temperatures.

The more we compress the air-fuel mixture, the better it burns.

And the better it burns.

The hotter it burns.

The upside of this is, of course,  increased power potential and increased.

But the downside is that high combustion  temperatures increase nitrogen oxide emissions.

This is one of the reasons why  more modern diesel engines, 

for example of the Euro-6 emissions norm, 

actually on average, have lower  compression ratios than their predecessors.

But the greatest limiting factor when it comes  to compression ratios in spark-ignition engines, 

is called the 'Knock'

Now, when we compress gases, we bring their molecules closer together, 

so they bounce off of each other more,, which increases their friction, 

which increases the temperature of the gas.

Now, air of course is also  a gas, so we compress it, 

we heat it up.

And in fact, if we compress the  air too much inside the cylinder, 

we can get it so hot, 

that it can ignite the gasoline  fuel inside the cylinder,

before it's reached by the evenly expanding  flame front initiated by the spark plug.

When this happens, we have 'Knock'

In general, knock has the capacity  to kill an engine pretty fast.

And it should always be avoided.

Of course, a higher compression ratio  obviously increases the chances for knock.

This is especially true for  force induction engines, 

which are sending the compressed  air into the cylinder.

Inevitably adding heat into the system

Which means that force induction  engines are even more limited, 

in the compression ratio that they can run.

Okay, so that's the basic theory.

Now, let's move on to the  practical side of things.

What does actually determine  your compression ratio?

Well, it's actually seven things

Your engine bore

Your stroke

The thickness of your compressed head gasket

The bore of your head gasket

The distance between your  piston top, and your block deck

The volume of your piston dish, or dome

And the volume of your combustion chamber.

Okay, so that's what determines it.

But how do you calculate it?

Well, of course, there's a formula.

And we can do it manually.

But the internet allows us to be lazy instead.

And we're just going to plug everything  into a readily available, free to use,  

online compression ratio calculator.

Like this one.

Now for the sake of the example, I'll be plugging  in values from my 1.6 liter Toyota 4AFE engine, 

which I'm planning to  turbocharge to 300hp on pump gas.

And install into my Toyota MR2 Mk1

So let's start with the engine bore.

Obviously, that's diameter of our cylinder.

Now, in my case, that's 81.5mm.

Now in stock form, this engine  actually has 81mm of bore.

However, I have overbored the  engine to install oversized pistons.

So now my bore is 81.5mm

Our stroke is the distance that the piston  covers from top to bottom dead center.

And in my case, that's 77mm.

My head gasket bore is 83mm.

Now, some online compression ratio calculators,  

actually don't have an input  for your head gasket bore.

In general, these calculators will  assume that your head gasket bore  

is equal to your cylinder bore.

And will give you a slightly  higher compression ratio value,  

than calculators that do have this input, 

because in general, your head gasket bore is  a tiny bit larger than your cylinder bore.

The thickness of my compressed  head gasket is 1.4mm

And the volume of my  compression chambers is 36.5 CC

Finally, we have the distance between  the piston top and the block deck.

This is obviously measured at TDC.

And if your piston protrudes above the block deck,  then this value should be entered with a '-' sign.

If the piston is perfectly  flush with the block deck, 

then the value is 0

And if the piston top is  slightly below the block deck, 

then the value should be  entered as a positive value.

In my case, the piston is just 1/10th  of a millimeter above the block deck.

So I'm entering the value with a - sign.

Okay, once we have all the values  in, we just click on 'Calculate CR'

And we get our result.

And as you can see in my case, this is 8.44 : 1

Now, before I explain why I went  this particular compression ratio,  

let's explain how to modify  your compression ratio.

Now, modifying an engine's static  compression ratio is really  

easy during the engine building phase.

But it's impossible to do it once  the engine is assembled and running.

And this is because to  modify the compression ratio,  

we must modify the hardware  that makes up the engine.

Let's start with the bore & stroke of the engine.

All other things being equal, 

increasing the bore and/or stroke of the  engine will increase the compression ratio.

And this is because, by either  increasing the bore or stroke, 

you're increasing the largest cylinder volume.

So, when the pistons at bottom dead center

While also leaving the smallest cylinder volume, when the pistons at top dead center, untouched.

On most engines, we're pretty limited in how  

much we can increase the bore  without major modifications.

In most cases, the stock bore, the stock sleeve can be increased by around 2mm on most engines.

Before you run out of material between the  bores, to support the construction of the engine.

On the other hand, stroker kits for example, 

allow you to increase the engine  stroke by pretty substantial amount.

As much as 10-15mm in some cases

Which leads to a pretty substantial  increase in the compression ratio.

The next thing we can change  is of course the head gasket.

And this is probably the most cost-effective and  simplest way to modify your compression ratio.

By changing the thickness of the head  gasket, we're changing the cylinder volume, 

which of course changes the compression ratio.

A thicker head gasket is going  to reduce the compression ratio.

While a thinner head gasket is going  to increase the compression ratio.

But be warned!

A thinner head gasket is less capable at absorbing  any sort of imperfections in your block deck,  

or your cylinder head surface, so you must ensure that  

everything is machined perfectly flat for a reliable seal with a very thin head gasket.

Since we're speaking about machining, 

that too is a great and inexpensive  way to modify your compression ratio.

However, machining can only remove material, 

which means that it can only increase, it  cannot decrease your compression ratio.

By machining away or removing material from  your block deck, or your cylinder head surface,

we're going to be decreasing our cylinder volume,

and increasing our compression ratio.

The only way to modify the volume  of your combustion chambers,

is to grind away material from  within the combustion chamber,

which will increase the size  of the combustion chamber,

and reduce the compression ratio.

The final thing you can do  is, modify your piston top.

Now, in most cases, this  means replacing the pistons,

so it's not going to be as cost-effective  as machining or a head gasket change.

But it's still going to be cheaper  than a stroker kit for example.

If we assume that we start  out with a flat top piston,

then replacing this with a dished piston  is going to increase cylinder volume,

and reduce the compression ratio.

While replacing a flat top  piston with a domed piston,

is going to reduce the cylinder volume,  and increase the compression ratio.

So here's a little overview.

And as you can see the general rule is that;

Anything that increases cylinder  volume, reduces the compression ratio.

While anything that reduces cylinder  volume, increases the compression ratio.

Now, that we know what it is

How to calculate, and how to modify it.

Let's discuss choosing the optimal  compression ratio for your application.

Now, doing this depends on three factors:

First, let's discuss The How  you're building your engine.

And this mostly refers to the degree of  accuracy you have incorporated in your build.

So, in other words:

Are you doing an enthusiast  level build with lots of DIY?

Or having a professional shop, with  a proven track record of building  

motorsport winning engines, do all the work for you?

In general, increasing the compression ratio,

reduces the margin for error,  and demands greater accuracy.

So for example, in my case

I have ground away material  from my combustion chambers.

And increased their volume from 32 to 36.5 CC

Now, although I have done all this manually

I have verified the volume and  I have measured it afterwards.

And I have done my best to ensure  all the chambers are of equal volume.

And although I'm confident that I managed  a pretty reasonable degree of accuracy, 

none of this really compares for example  to the accuracy of a CNC machine, 

or the degree of accuracy professional  volume measuring devices can achieve.

So, this means that in my case,

it's a good idea to leave a  slightly larger margin for error.

What you're working with, refers to your hardware.

And more importantly to your software.

Again, let's take my build as an example.

I have a 1.6 liter engine

with 8.44:1 compression,

and I'm trying to achieve around 300 horsepower

To put this into perspective, the  very popular RB26 and 2JZ engines  

have almost the same compression ratio, and realistically the same power output.

However, they have noticeably  more displacement than my engine.

In other words, I'm trying to achieve the  same power with the same compression ratio, 

with almost half the displacement.

Which means that I'll be running a lot more  boost than these engines did in their stock form.

To be able to do this, I'll  be running a standalone ECU

An AEM Series 5 / Infinity ECU.

Which has integrated knock monitoring.

And multiple engine protection strategies.

Now, it's dramatically more capable  and a lot more faster than the  

stock ECUs that the 2JZ and RB26 came from, which allows me to triple my horsepower output, 

with a pretty reassuring degree  of safety and reliability.

In general, the stronger your hardware

and the more capable your software.

The better your knock control

And the faster your ECU

And the more engine protection options you have...

The higher the compression ratio you can run.

What you want to achieve are of  course the goals of your build.

For example, let's say that maximum horsepower is  your absolute top priority on a boosted engine.

In that case, you want to run the lowest  compression ratio you can sensibly run.

And this is because boost makes  more power than compression.

As a general rule of thumb, a single full point of  

increase in compression ratio is going  to result in a 4% increase in power.

In contrast to this, 1 psi of boost  increase is powered by around 7%

So if we take a 100hp engine, and increase compression from 9:1 to 10:1

Which is a pretty substantial  increase in compression ratio

We can expect the horsepower  output to change to 104hp.

On the other hand, if we add  14 psi, around 1 bar of boost  

to that same engine, without  modifying the compression ratio, 

we can expect the new horsepower  output to be increased by 98%

So with 1 bar of boost, you can practically  double the horsepower output of the engine

However, all compression high-boost engines tend  

to be a bit unresponsive or  lethargic outside of boost, 

and then when the boost kicks  in it kicks in violently.

So these engines can be  

a bit challenging, or even outright annoying  to drive on the street, or through the corners.

So if horsepower isn't your top priority, but instead it's engine responsiveness,  

versatility, and fun factor on the  street and through the curves, 

then I'd say you should aim for  lower boost and higher compression

Now, if you want high boost and high compression, 

then you must ensure that  the accuracy of your build, 

as well as your hardware, and your  software is absolutely top notch.

Which sometimes simply isn't possible or  practical for an enthusiast level build

In my case, I try to strike a middle ground

I want that engine that packs a pretty big punch, 

but I also don't want it to be  absolutely horrible on the street.

For example, if I was aiming for 500hp from the same 1.6 liter engine 

I would probably have gone  for a 7:1 compression ratio.

On the other hand, if I was aiming  for around 200 hp with a smaller turbo

I will have gone for let's say a 9.5:1  to maybe 9.8:1 compression ratio.

Also if I had a two liter engine in the  same target horsepower level of 300 hp 

I would again run higher compression  let's say around 9.2:1 to 9.5:1

Because by having more displacement  I don't have to run as much boost  

to achieve the same horsepower output.

If my build was naturally aspirated,  

then I'll be running the highest possible  compression ratio I could safely run, 

with the build accuracy, knock  control, and fuel that I'll be using.

And this is because what natural  aspiration, a higher compression ratio  

doesn't have the potential downside  that it has on forced induction.

In general with natural aspiration,  the higher you can run, the better.

So in my case, I would be probably,  I would be aiming for around 12.5:1 

If I was naturally aspirated.

The final factor is your hunger for power.

If you're a power addict, obsessed  with straight line performance, 

and you get bored of a power level quickly,  

and always have the urge to  increase boost just a little bit,

then it's a good idea to future  prove the engine against yourself 

by leaving a bit more room for boost  by running a lower compression ratio

Any special concerns?

Well, I do have one.

And that's my mid-engine application.

And this isn't some modern hypercar middle-engine  thing, with giant intakes on the sides.

It's a boxy 80s mid-engine car,  with a single tiny duct on the side.

And although I can add  ducting and work around this, 

inevitably having the engine in the back,  

increases the potential to  complicate intercooling, 

and reduce its effectiveness, and  add overall heat to the system.

Which can increase in intake air temperatures.

So running a lower compression ratio  also leaves some room for that.

And there you have it

Compression ratio. Always a compromise.

But I hope today's video helps  you make the right choices, 

to strike the best compromise  for your application.

As always, thanks to all for watching  I'll be seeing you soon, with more fun  

and useful stuff On the D4A channel