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

In today's video we're talking about your engine's compression ratio. First we'll explain the theory behind the compression ratio, so what it is and how it influences the performance and efficiency of your engines. After that we will dive into the practical side of things and we will see how to calculate and modify your compression ratio and finally we'll talk about choosing the best compression ratio for your application. So, let's get started.

Now when we say compression ratio we're actually referring to the static compression ratio of the engine, and that is the ratio between the largest and smallest volume of your cylinder. In other words it's the ratio between the cylinder volume when the piston is at bottom dead center and the cylinder volume when the piston is at top dead center.

Your compression ratio, as the name implies, determines how much the air and fuel mixture inside your cylinder gets squeezed and compressed. The higher your compression ratio the closer the air and fuel molecules are brought together which means that we allow combustion to occur more effectively and more rapidly which ensures that the air fuel mixture is burned more thoroughly. Iin general a higher combustion ratio is achieved either by reducing the size of the combustion chamber or by bringing the piston closer to the combustion chamber. By doing this we of course bring the piston closer to the heart of the cobmsution or the source of energy which allows more of this energy to be transfered onto the piston and turned into piston movement or mechanical energy. In other words a higher combustion ratio can improve both power and efficiency.
So the more the better right? Well as with all things there's a sensible limit and you can actually have too much of a good thing.

Because a higher compression ratio contributes to a more thorough burn of the air fuel mixture it also increases combustion temperatures. The more compressed the mixture the better it burns and the better it burns the hotter it burns. The upside of this is of course more power potential and more efficiency but the downside is that the engine will run hotter and will have increased nitrogen oxide emissions.

Higher combustion temperatures lead to more nitrogen oxide emissions which is one of the main reasons why more modern diesel engines that have a EURO 6 emissions standard run on average lower compression than their predecessors from a decade or two ago.

But one of the main limiting factors when it comes to compression in spark ignition engines is of course knock. When you compresses gasses they heat up, air is of course a gas and if you compress it too much it can get hot enough to ignite gasoline fuel before it's actually reached by the expanding flame front created by the spark plug. This is called knock. Obviously a higher compression ratio increases the chances of knock and thus limits the ratio of compression a gasoline engine can have. This is especially true for forced induction engines which send already compressed air into the engine.

Okay, so that's the basic theory now let's move onto the practical side of things. So what determines your engine's compression ratio?
It's actually seven things:
1. Your bore
2. Your stroke
3. The thickness of your compressed head gasket
4. The bore of your head gasket
5. The distance between your piston top and your block deck
6. The volume of your piston dish or dome
7. And your combustion chamber volume

So how do you calculate it? Well there are formulas but the advent of the internet allows us to be lazy and just plug everything into readily available free to use online compression ratio calculators.

When it comes to changing our compression ratio here's a basic run-down.
Increased bore - increased compression ratio
Increased stroke - increased compression ratio
Thicker head gasket - reduces compression ratio
Thinner head gasket - increases compression ratio
Decking the block - increases compression ratio
Cutting the head - increases compression ratio
Removing material from the combustion chamber - reduces compression ratio
Domed pistons - increase compression ratio
Dished pistons - reduce compression ratio

A special thank you to my patrons:
Daniel
Peter Della Flora
Daniel Morgan
William
Richard Caldwell
Pepe
Brian Durning
Brian Alvarez
Dave Westwood
Joe C

#d4a #boostschool #compressionratio

00:00 What is compression ratio and how it works
04:20 How to calculate compression ratio
06:42 How to change it
09:37 Choosing the optimal one for your application

[da]

Sorry that my voice is a bit nasal in this one. I keep catching every bug from my son's kindergarten 😅A capable E

[sethblocker]

D4A has to be the most in-depth, informative channel on YouTube when it comes to engines. The amount of time, effort, and attention to detail that goes into every single video is insane, and I feel like whether this man had 5 subscribers or 5 million he’d put in the same amount of time and care because it seems like he is genuinely enthusiastic about teaching people about every little detail of how a car works. Keep up the good work homie 🙏

[clunt]

Crazy how a concept that has eluded me for years was explained so easily in the span of about 16 minutes. Ive been wanting to get into this type of mods and you’ve helped me immensely, thank you.

[overstack]

Just a little detail :
It isn't the friction generated by the bumps between the air molecules that generates the temperature, but the bumping IN ITSELF that IS the temperature of the air !
Really loves your videos, i would go as far as to say that this is the single best car and ICE channel on the entire youtube !

[FriendlyPhoton]

There's no mention here of dynamic compression ratio, which is the true defining factor of how much the air/fuel mixture gets compressed. Compression in the compression stroke doesn't begin until the intake valve closes, which sometimes doesn't happen until the piston is well on its way up. A change in cam, or a change in cam degreeing can dramatically change the dynamic compression ratio, and thereby determine how much power and heat will be generated. All of this can be changed without ever touching the static compression ratio

[GamingKeenBeaner]

I am impressed that not only is he doing this in English, but also making more sense than most people who are native speakers of it! Great channel. I love the presentation. Better than many college courses.

[lorriecarrel]

The highest compression ratio is achieved by my finger clicking on your videos

[brocluno]

OK, now you should do a video on dynamic compression. Or what the engine really sees with the available valve timing, etc.

[sins]

one thing to add to this is choosing a knock suppression system either in the form of auxiliary injection or a more knock resistant base fuel can leave room for a higher compression ratio. I will be running a 9.5:1 2.0l that should make about 6-700 awhp.

[CoryRoy]

Great video as always! I love the MR2. My neighbour bought one when I had just got my driving license in 1986 and let me drive it in exchange for washing it - best trade ever - and it was a very fun car! I can imagine that the car would be a handful with 3x the power though!

[LVPIERCED]

This is literally the best breakdown I’ve come across to date. Will be rebuilding an engine for my first time solo soon here and increasing compression and air flow is already on the top of my list

[john-martin]

On a 4 cylinder I made 409whp w/ 9:1 Compression @17 PSI and 511whp w/ 8:1 Compression @ 22.5 PSI both setups on 91 octane Valero gas.

[marianlech]

I find your lessons very solid, comprehensive and digestible, Sir. Always a pleasure to learn something about cars, especially when taught so well. Thank you so much!

[Dashamuulam]

Someone read his description, its a whole story about compression ratio.
I love this channel❤️

[Djou-Karl]

I learned more than I expected on compression ratios! I'm planning to build an N/A someday, this will be useful knowlesge to have, expecially concerning the machining part.

[jackenglefield]

Very helpful. I was always curious how to apply compression ratio as I have never built or even planned on building an engine. I'm sure one day this information will come in handy for some baseline knowledge before delving into the specifics of my engine. Thanks for making this video :)

[kolourblind]

Instant subscriber. This video has helped me tremendously. Building a lq4 6.0 currently and I’m shooting for 11.4:1 for a mild na street car. Thanks for making this! I’ll be sure to watch more

[juangraniello]

one of your best videos from your library, great job !!! I cant wait to see the entire process

[vitis]

Dude, I don't even care about cars and mechanics and somehow all I watch now it this channel!! What?!...why is this so awesome?

[Karnonful]

Excellent video as always! I am currently having my 1.6L built (forged) for higher boost, as you are doing, and this week I was just asking myself about CR, and here is your vid! Thanks! :)