Compression Ratios (Part 1)
In this article I would like to discuss compression ratios and what it means and does with air cooled motors. I am sure I am going to take some heat for this, so keep in mind this is my opinion and what I have found to be true in Northern California.
So what is the compression ratio? It’s how much you’re squeezing (compressing) the mixture before you fire it. The more you “squeeze” it, the bigger the bang; the bigger the bang, the more heat you get. Heat is energy and energy is power.
In simple terms here is what your compression ratio is:
Head volume: The amount of CC in the combustion chamber in the head.
Deck volume: The amount of CC between the top of the piston and the top of the cylinder.
Sweep volume: The amount of CC between when the piston is at bottom dead center and top dead center.
Once you have all these numbers, you can use this math to figure out what you have.
Now that you understand the different components that go into your compression ratio, let’s figure out your ratio.
To measure your head volume, you will need to make or buy a flat piece of Plexiglas. Take your cylinder and trace around the outside dimension, making a circle on your Plexiglas. Then cut it out so that it will fit down in the head where the cylinder fits. Drill a ¼ inch hole in the middle and an 1/8 inch hole about ½ inch from the side. Now, install a spark plug in the head and grease the edge of the plate to make a good seal. Put your head in a vise or on the bench and get it as level as you can. Place the plate in one hole. Push down and give it a spin so as the grease makes a thigh seal. Make sure your 1/8 inch hole is in the bathtub area of the combustion chamber (this is your vent hole). Now with a syringe or burette fill the chamber with water or better yet denatured alcohol. Keep track of how much your putting in. When its full, you have your number. Now one thing to pay attention to here, before you pull the plate off, look into your intake port and your exhaust port. If you see liquid here, your valves are leaking and you’ll need to do something about that because your number you just got is not a good number. Make sure you measure at least one chamber on each head. Never assume that both heads are the same.
Deck volume: You can measure this a depth mic and then with this math convert it into CCs.
Make sure that you have the cylinders torqued down as if the head was on. If you don’t, your number will be off enough to make you have changes that are not accurate. Measure each cylinder. I make a plate that has slots and holes in it for doing just that. I also use a dial indicator to make sure each piston is as far up as it can be. Once the piston is all the way up, measure from the top of the cylinder to the top of the piston. Write this number down. What I look for here is that the numbers are with in five thousands of each other from number 1 and 2 or 3 and 4.
Let’s say they are .060 on one .063 on two. .072 on three and .074 on four. Well this is ok in that we can shim 1 and 2 .010 and make all the number close. You can shim one side different than the other but you can not shim side by side cylinder different as you will cock the head and it wont seal. So let say we have .060 on one and .072 on two. Now what? Well the first thing you can try is trade rods and pistons around to get closer numbers. HUH? If you are using reconditioned rods, one can be shorter than the other. If you have a rod that has been machined 5 times and one that has been machined once then the one that has been machined 5 times is shorter than the one that has been machined once.
What about the pistons? As you know, a lot of pistons are coming out of China and there is a chance that the wrist pin hole can be in a different spot on each piston. Bottom line here is that we would like them to be as even as possible. This will insure we have an equal compression ratio in each cylinder. Makes for a smoother, longer lasting motor.
Sweep volume: As said earlier, there is not much you can do here. It is what it is. Here is the math to get your cc number.
Tune in to next month’s article to find out what ratio you should be running and how to get it there.
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Compression Ratios (Part 2)
Now that you know the different factors of compression ratios and hopefully have spent the time to figure yours out, I’m going to go over some different options of how to change yours as well as what I believe are good ratios for your style of car.
So why not have as much compression as you can get? Again, when you squeeze the mixture you create heat and we all know too much heat can be lethal to these motors. Plus, if you squeeze it too much, your mixture can light without the help of the spark plug, going off too early (pre detonation) and can break parts. So why not make the motor really low in compression? If you did that, you would have no power and you might as well ride your bike, you could get up the hill faster. So the trick here is to find the compression ratio that suits your needs. You really need to decide what this car is going to be for. Is it a daily driver and you want to go on long trips with it? Is this a Friday, Saturday night car that you want to take on some Hondas with from the stop light? Is this a drag car? Maybe a sand or dirt car? Keep in mind, the higher the compression ratio the fewer the miles you will get out of the motor before you need to take it apart and do some maintenance.
Head volume: If you want to raise your compression ratio, you can fly-cut the head and reduce the amount of CCs you have in the head. Note: After you fly-cut a head, make sure your fin area of your head does not bottom out on the fin of the cylinder before the top of the cylinder hits inside your head. If it does, you will want to fly-cut some of the fin area off to make sure you get a seating surface where its supposed to be. If you want to lower the ratio, you should think about increasing your deck volume.
Deck volume: This the best place to do some adjusting of your compression ratio. If you want to raise the compression then go back to head volume and fly-cut your head. If you want to lower the compression ratio then you can add a shim under the cylinder to raise your deck volume. In my opinion, I try and keep a min deck of .060 for a daily driver. That is measured between the top of the cylinder (torqued down) and the top of the piston at top dead center. On a drag car I will go as low as .045.
Sweep volume: There really isn’t much you can do here. It is what it is. What affects this is bore and stroke. The bigger the bore and or the bigger the stroke the more sweep volume we will have here.
The one thing I see a lot is thinking that if you go from a 85.5 cylinder to a 87 cylinder that your compression ratio wont go up. It does. Do the math.
So I did the math. What is a good compression ratio to have? Keep in mind that you can ask ten different engine builders and you will probably get ten different answers. So again, this is just my opinion and what I have found to work well.
If this is a daily driver then my magic number is 7.7 - 8.2 to one. This will allow me to run regular gas. (I have run as low as 7 to one when gas was red, but regular gas has changed since then) What I have found is if you run that low with todays gas, it will not burn all of it. Plus, remember ethanol runs cooler than gas. This ratio allows us to run regular cheap gas have decent power with not too much heat.
Now on a Friday/Saturday night car or a dirt car I will run between 9 and 9.5 to one, making sure I only run premium gas. This will give us more power but also brings the heat. We need the higher octane to insure we don’t have pre detonation.
On a drag car, we’ll get the compression as high as your parts will allow. Compression is power and we want all we can get. The higher the ratio the higher the octane needs to be. So break out your wallet, race gas is not cheap.
I find that if the motor is built right and all things are a good combo, we can get 100K out of daily driver motor before we should open it back up for a look. On a Friday/Saturday night or dirt car we usually open them back up 20 to 50K miles on it depending how hard and how high we rev it. Drag car motor should be opened up all the time or at the very least once a season.
So there you have it. Sounds like a lot of work and math, but is it worth the effort? Hell yes it is.