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
