Compression Ratio Explained - Roadkill Extra

hey it's freiburger here from roadkill and engine masters and I'm about to show you a sample episode of this how to build your first engine series that we've been doing on motor trend on demand it's me and Steve dulcet showing you just basics of assembling your very first engine we're doing them every Monday Wednesday and Friday on motor trend ondemand.com here's a sample episode this time you're gonna learn all about an engine's compression ratio what it does for you and how to calculate it or how to affect it i'm Freiburger this is roadkill extra and we've been doing a whole bunch of stuff with this small-block Chrysler engine that's gonna end up in one of our project cars for roadkill garage and this one is gonna be a little bit different of an installment than the rest that we've done because it's not really a succinct how-to hands-on stuff instead there's gonna be some theory here an explanation about compression ratio we're gonna talk about what it is how it affects performance and how you can change it or the things in your engine that affect it so let's start with what is compression ratio basically it is the volume of a cylinder with the piston at bottom dead center compared to the volume of the cylinder with a piston at top dead center so let's look at my handy dandy drawing right here so what we've got here is the engine cylinder that's this right here the circle where the pistons go up and down and up here indicated by this dotted line I've got the chamber in the cylinder head which is this right here obviously the cylinder head bolts to the top of the engine and this chamber caps off the cylinder where the Pistons moving up and down so right here I've got the piston at bottom dead center and so I'm looking at the volume of all of this in here is air or in fact it's air fuel ratio you've got a bunch of gasoline and air all floating around inside here now let's look at the same thing with the piston at top dead center it's taken all of this air all of this area and it's squeezed it compressed compression ratio and moved it so that all of these molecules are now crammed in really tight in this small space so what we're looking at again is compression ratio is all of this volume divided by this smaller volume again piston to bottom dead center versus piston at top dead center so that's what compression ratio is but why would it make power if you were to increase your compression ratio or decrease it well decrease in compression tends to hurt performance almost always increasing compression ratio up to the point of where you've got detonation which can be an issue with a high compression ratio increasing compression ratio tends to improve power why well it again has to do with all of these happy air fuel ratio molecules cruise cruising around inside your cylinder if you have a low compression ratio it is not squeezing the molecules together as much as a high compression ratio so imagine you've got a really high compression ratio say 15 to 1 that's big it is cramming all that air into that little chamber and all of those molecules are getting closer and closer and closer and what that means is when the spark plug goes off and it ignites the explosion all those molecules can hand the fire off to the next one much quicker and much more violently than they would with a lower compression ratio that isn't stuffing those molecules together as much so again it's the density of air and fuel crammed into the chamber that makes more power with a higher compression ratio than it does with a lower compression ratio so what are typical compression ratios well a small engine from the 70s can have a hideously low compression ratio like seven and a half to one that's horrible eight and a half to one also horrible getting into a street performance engine you're looking at between ten and eleven to one ten and a half to one's a real happy medium and then I call like twelve and a half to one compression ratio no-man's land because at that point you typically need to run race gasoline whereas at ten and a half to one depending on the camshaft you're using you can typically run regular pump gas so at twelve and a half to one if you've got to buy a race gas you might as well have fourteen after one go big so that's kind of the basics of what compression ratio is how it affects your power now let's talk about the elements in the engine that affect the compression ratio so basically it's going to be this anything that you do to increase the volume of the cylinder with a piston at bottom dead center will increase compression ratio because you're taking a bigger area and cramming it still into the same size chamber conversely anything that you can do to make the chamber smaller will increase compression ratio because again you're making a smaller space for the piston to cram all that air into so once again larger down here means higher compression smaller up here means higher compression now let's talk specifics the elements that can affect compression ratio are the elements that do affect compression ratio are displacements again this area right here it's affected by the cylinder bore how big around is this hole bigger means more displacement how long is the stroke stroke is how far up and down the piston travels in the cylinder so a larger bore increases displacement and increases compression ratio a longer stroke increases displacement and increases compression ratio the next thing is the piston configuration here's the piston that we've got going on in our small-block Chrysler it is a moly forged piston and it has two valve reliefs in it here which are depressed into the face of the piston they give room for the valves to open and close so this would be known as a dish this one is five cc's of volume so if you were to cap this off with a glass plate and measure just the volume of liquid that would fit inside this hole it would be five cc's now because that's a dish it reduces compression ratio because imagine the piston up here at top dead center once again this is your chamber size but if there's a hole in this that's down here that is increasing the total size of the chamber with a piston to top dead center conversely if this piston instead had a dome on it like this which makes me much happier you can see that that dome is now reducing the size of this chamber and making the thing have more compression I'm so good fire alarms are going off okay where was I piston configuration the next thing about the piston that you need to know is the deck height so deck height is at top dead center how the piston is in relation to the deck surface which is where the cylinder head bolts on we call it a negative deck height if the piston is down in the hole save 50 thousandths of an inch and it's positive if the piston is coming up above the deck at all so here's how that affects compression basically here's the piston at top dead center well if the piston is down here at top dead center then this whole chamber is bigger if the piston is way up here at top dead center it makes the chamber smaller so again if the piston is below the deck surface you have lower compression if it is even with the deck surface that's about perfect and it's higher then you can run into some mechanical problems but it also increases compression ratio the next thing that is discussed sometimes is crevice area and basically that is the the crack that you see right here the distance between the piston and the cylinder wall and above the top ring a lot of people think that that's critical for compression ratio calculations honestly I leave that out but on some of the calculators that you'll see it will include the volume of that little crack all the way around there from the ring up but don't worry about that the next thing you do need to worry about though is your head gasket so the head gasket bolts between the cylinder head and the engine block so it's basically a spacer between the block and the cylinder head so there's two elements of this that affect compression one is the size of the bowl or in the gasket as an example this has a four point one eight inch gasket bore gasket bore is what that is known as and the cylinder itself is only four point oh four so since the gasket is this big and the cylinder is this big it creates a little bit of area in there which creates more volume with the piston at top dead center which reduces compression ratio ideally a performance gasket would be almost exactly the size of the bore itself so that you're not creating that little gap between the deck and the cylinder head which can also lead to problems but it is very typical to have it bigger the second thing that will affect your head gasket on compression or affect compression with your head gasket is what's known as compressed thickness when it's all bolted together how thick is this gasket again look at it as a spacer between the block and the cylinder head and you can see if the piston is only coming up to right here that if you make the gasket thicker you're increasing the volume of that chamber when the piston is up at top dead center therefore a thicker gasket reduces your compression ratio this happens to be thirty nine thousandths of an inch when it is all crushed if it was thicker I'd have a lower compression ratio if it was thinner I'd have a higher compression ratio next thing is chamber size so this is the cylinder head and this is the combustion chamber it's where the intake and exhaust valves are now you can see if I put my hand over this that this is all recessed it's like a heart-shaped bath tub in here this particular Edelbrock aluminum small-block Chrysler cylinder head has a 63 CC chamber a smog head on a small-block Chevy has a 76 CC chamber a big-block Chevy can have like 120 cc's of combustion chamber but it doesn't matter because the big-block has more displacement so anytime that you take this chamber and make it smaller you're going to increase compression ratio and anytime you make it larger you're gonna reduce compression ratio so I've just run through all the elements that you need to know to calculate your compression ratio and I'm going to blast through them very quickly again because do this you need to go online and just google compression ratio calculator and if the calculator doesn't ask you for everything that I'm about to tell you it's not good enough these are the elements that you need to be able to put in for an online compression ratio calculator to spit out your information you're gonna need to know your bore the diameter of your cylinders in this case 4.04 you're gonna need to know your stroke how far does the piston move up and down in this case three point five eight those two things affect the displacement of that cylinder and again bigger means higher compression smaller means lower compression next you need to know your piston configuration the piston manufacturer will tell you the CC displacement of any dish or dome that's on the top of the piston next you need to know your deck height go back to one of our other installments and you'll see us showing you how to use a deck bridge to determine what the relationship is between the top of the piston and the top of the deck to see if it's below deck or above deck the calculator will ask for a negative number for a dish or below deck and a positive number for a dome or above deck next I talked about the crevice area around here don't worry about that most calculators don't have it next you do need to know your gasket bore and your gasket compressed thickness some calculators will instead ask you for a volume of displacement in here when the thing is crushed both of those pieces of information or all three the bore the thickness or the CC total volume is available from the gasket manufacturer it's usually on the package next you need to know your chamber size in your cylinder head that is available from the manufacturer of the cylinder head or if you've had a stock head milled meaning material cut off of it or if you've changed the valves you may need to have a machine shop figure out what the total displacement is in cc's of your chamber so those are all of the elements that you need to know it was a lot but if you go back and watch this and think about it and go to an online compression ratio calculator and put in all the data you will get your number and for this engine we figured it all out so the little 360 mopar that's going on our challenger has ten point seven six nine to one compression and that makes us happy because that's right at the top of what we're gonna get away with with this camshaft on pump gas whoo that was a lot if you need more roadkill extra go sign up right now you

Channel: MotorTrend Channel

Views: 719,356

Keywords: roadkill extra engine-building series, what is compression ratio, engine performance, tech tips, how to measure, top dead center, cylinder heads, engine displacement

Id: 5qH_ThYLD6Q

Length: 13min 49sec (829 seconds)

Published: Sat Oct 28 2017

Reddit Comments

[u/yatlvcar]

TL;DW

CR : Volume of cylinder when piston is at BDC/Volume of cylinder when piston is at TDC

More compression ratio is good for power, but too much will cause knocking

(There is a lot more in the video than the info above)

[u/Aero06]

Very informative, funny how he basically spends ten minutes deconstructing the idea of hemispherical engines without even mentioning them by name.