Your engine's static compression ratio is easily calculated if you know six measurements or volumes:. If you want to take the formulas with you and put them in your tool box, here they are. Bore inches Diameter of cylinder Stroke inches Distance piston moves Head Gasket thickness inches Measure your head gasket tthickness.
It's usually 0. Deck Height inches Move piston to TDC, and measure the distance between the flat top surface of the piston not the top of a dome or the bottom of a valve relief and the top of the cylinder. Enter a negative number if the piston's flat surface sticks up past the top of the cylinder. At TDC the piston will "rock" so be careful in your measurements.
ORCAs are. Piston top volume cc's You'll have to calculate this value the hard way i. Enter a negative number for a domed piston. Enter a positive number for a dished or dimpled piston. If you have a flat piston with no valve reliefs enter 0. Combustion Chamber cc's Get out your ml burette, your light, non-nasty grease, your handy dandy plexiglass or lexan sheet, your Marvel Mystery Oil and a six pack and get ready to party.
Don't forget to cc the combustion chamber. Static Compression Ratio:. Definitions: Your engine's static compression ratio is easily calculated if you know six measurements or volumes: Bore-- The diameter of the cylinder bore Stroke-- The distance the piston travels in the cylinder Deck Height-- The distance between the top of the cylinder bore and the top of the piston, when the piston is at TDC Top Dead Center, or at its highest position Compressed thickness of the head gasket-- Usually 0.
Combustion chamber volume-- How much open space is in the head above the cylinder. Head Gasket thickness. Measure your head gasket tthickness. Move piston to TDC, and measure the distance between the flat top surface of the piston not the top of a dome or the bottom of a valve relief and the top of the cylinder. Piston top volume. You'll have to calculate this value the hard way i.Illinois Finest Automotive Machine Shop. The compression ratio is a single number that can be used to predict the performance of any internal-combustion engine.
It is a ratio between the volume of a combustion chamber and cylinder, when the piston is at the bottom of its stroke and the volume when the piston is at the top of its stroke. The higher the compression ratio, the more mechanical energy an engine can squeeze from its air-fuel mixture.
Higher compression ratios, however, also make detonation more likely.
Basic Engine Building Specifications
The ratio is calculated by the following formula:. Ford Motor Company small block V8 engines include the following:. General Motors small-block V8s include:.
Big-block Short block Long block Cylinder head V8. Chevrolet 's small-block V8 is one of the most famous automobile engines in history. This engine is still in production today at General Motors Toluca Mexico plant, but is no longer offered in current model year vehicles since the year Its production numbers were impressive, with more than 90, built.
Fromthe small-block engine was known as the "Turbo-Fire V8". The original design of the small block is broken into two distinct families: the first design in whose family is known as SB1 and a redesign in known as the SB2. The SB1 design was available in each displacement. The major internal changes by year are listed below.
There was no provision for oil filtration, dooming the longevity of this engine even with frequent changes. Previously only certain years of the engine could be bored safely to 4-inches. The oil fill location was moved from a tube on the timing cover to a cap on either side valve cover. The troublesome rear main seal was changed from a 2-piece rope design to a 1-piece design that used a mounting appliance to hold it in place.
Also changed were the mounting angles of the center 2 bolts on each side of the intake manifold and the lifter bosses were increased in height to accept roller lifters. The cylinder heads were redesigned in terms of the casting quality of the valve bowls and runners and the ports were moved slighly.
This change became known as the Vortec series and resulted in significant power increases. While the engines produced today are only for off-road or marine applications, the changes will retrofit to older engines. The cylinder heads were redesigned and the lifter bores were offset. The little engine went from drawings to production in just 15 weeks. Besides its compact dimensions, the small-block was known for its novel green-sand foundry construction process. Dimensions were oversquare - 3. The small-block's 4.
It was a pushrod cast-iron engine with solid lifters and a 4-barrel Rochester carburetor. The first two production years of this motor had no provision for oil filtration, and as such the engine is undesirable to all but period collectors. Power was up a bit each year for, and The engine featured Ramjet mechanical fuel injectionallowing the engine to produce 1 horsepower per cubic inch, an impressive feat at the time.
It was the product of placing the 3-inch stroke crankshaft from a into a 4-inch bore block. This engine was only used in the first-generation Camaro Z Engine bore was 3.Engine Displacement & Compression Ratio - Tutorial/Explained "Tagalog"
Chevrolet never produced a high-performance version of this motor.By definition, the compression ratio CR is the total swept volume of the cylinder with the piston at bottom dead center BDCdivided by the total compressed volume with the piston at top dead center TDC.
The tuning window widens and provides more of a safety envelope in the event of a fuel pressure or delivery problem, or even a bad batch of gas.
On the flip side, if the CR is lower than the allowed maximum, then the racer is giving up horsepower. Knowing the CR will provide a strong foundation for the tuning strategy.
In the old days, calculating the CR meant getting out the slide rule really long time ago or working through a set of formulas on a handheld calculator. Today, finding online calculators that quickly spit out the results is only a Google click away.
But as the old saying goes, a computer is only as good as the quality of information it gets. There are a couple of high-tech calculators online that ask for even more, such as rod length and distance from the first compression ring to the top of the piston.
Online calculators generally offer a choice of entering all measurements in either inches or metric, except for the combustion chamber and piston dome volumes, which are always entered as cubic centimeters, or cc. Is it Decked? The deck height is the one measurement that the engine builder will have to make for an accurate calculation. Even with a new cylinder block, new rods, and new pistons, there can be a significant difference with adding up the deck height and trying to subtract half the stroke, rod length, and compression height.
This is critical to accuracy of compression ratio since a difference in deck clearance of 0. Again, the CR is calculated by dividing the total swept volume by the total compressed volume. All of the factors must be in the same numeric value. The online calculators can also figure out the gasket volume with the correct thickness and bore, but many gasket manufacturers will provide this information in their catalogs or on the packaging. Identifying Speaks Volumes Again, performance aftermarket companies usually supply the required numbers with new parts.
However, it never hurts to confirm with your own measurements. Assuming and not measuring almost assures an expensive and messy outcome. Experienced engine builders have the proper tools for taking all the required measurements, such as a bore gauge and dial indicator.
The most tedious measurements are the piston volume and the combustion chamber volume. A burette, colored liquid, and task-specific fixtures are required, as noted in the accompanying photos. Starting with a 0. Rounding out the rotating assembly will be 6. The chosen cylinder heads have cc combustion chambers, and the head gasket has a bore of 4. The manufacturer says the gasket volume is 9. With that deck height and rotating assembly, there is 0. Plugging all those numbers into an online calculator we get If the engine had a new block with a standard 9.
Expressed as a negative volume because piston shape is domed. If the piston were dished or flat-top with valve reliefs, it would be expressed as a positive. With those numbers we add up the swept volume as 1, The compressed volume is 0. Dividing the swept volume by the compressed volume we get Static Versus Dynamic Compression On a final note, these calculations will compute the static compression ratio of the engine.
There is also the dynamic compression ratio to consider, which is relevant to the camshaft timing. A high-CR engine will lose some of that compression pressure if the intake valve remains open after the piston starts the compression stroke.Gain extra benefits by becoming a Supporting Member Click here find out how!
Compression Ratio. Send Private Topic View Profile. Later motors post 86 can stand higher compression because of head design, tighter oil control inside the chamber oil inside the chamber causes bad pre-detonation problems when burned and aluminum cylinder heads.
In my 66 owners manual it states that all V8 engines other than the requires premium fuel. Execution time: 0. All times are GMT Pacific. Current time is AM Top. Attach Photos to Posts. Contact Us. My Cookies. Frequently Asked Questions. Forum Rules. Username Post: Octane vs. Is there a rule of thumb as to what octane gas one should run for a certain compression ratio? In other words at you can run 87 octane, at 9. What does it depend on? Jim www. In his book How to Port and Flow Test Cylinder Heads, he states that he shoots for cranking psi when he runs on 93 octain.
Compression Ratio Calculator
And for every octain rating lower, he lowers the cranking psi by 5 psi. I'm not trying to prevent detonation with this question. It has more to do with a stock engine and what Chevrolet says its octane requirements are. What about a HP at What does Chevy say run in it?
Or is this a question that can only be answered by trial and error? I'll check out the links.How to use: simply fill in all the boxes below with the requested figures and click on "Calculate CR" to find your engine's compression ratio. The compression ratio of an engine is a very important element in engine performance. The compression ratio is the ratio between two elements: the gas volume in the cylinder with the piston at its highest point top dead center of the stroke, TDCand the gas volume with the piston at its lowest point bottom dead center of the stroke, BDC.
There are two ways of calculating the compression ratio for an engine. Firstly, you can make a math calculation as accurately as possible, or secondly - the more popular method - uses an empty spark plug socket with a pressure gauge inserted. We're looking at the first of these ways here. This technique is suited to someone who is in the middle of putting an engine together and has the right tools, or someone whose engine is already in pieces.
Stroke in. Chamber Volume cc cu in. Gasket Thickness in. Gasket Bore Dia in. Piston to Deck Clearance in. Piston Dome cc cu in. Piston Dish cc cu in. To find the compression ratio CR you divide the total swept volume with the total compressed volume. This is how you find out what these totals are:. All of these elements must be measured with the same units. If you're doing a manual calculation, that usually means using cubic centimeters cc. You may also be interested in our Power to Weight Ratio Calculator.
Compression Ratio Calculator. This compression ratio calculator can be used to work out the compression ratio of your engine. Definition of Compression Ratio The compression ratio of an engine is a very important element in engine performance.
CR Calculator of Cylinders Bore in. Compression Ratio : to 1. Working it out To find the compression ratio CR you divide the total swept volume with the total compressed volume. Currently 3. Join with us.Think all small-block Chevy heads are the same?
Think again. With so many different combustion-chamber shapes and sizes, you must measure to really determine an accurate volume. Valves can also have an effect. Notice how the valves in this photo have a small recess The right way to measure combustion- chamber volume is to invert the head on a workbench and install a pair of valves and a spark plug in the chamber.
Then you can use a colored liquid blue food coloring in rubbing alcohol works well to fill the chamber from a graduated cylinder or burette. That same plastic plate can be used to check the actual piston and crevice volume. Use a depth mic or dial indicator to determine the piston depth below deck and then, following the example we described in the story, you can determine total piston and crevice volume.
All this leads to more accurate compression ratio figures. Any time you bore or add stroke to an engine, the compression ratio will increase. For example, with everything else remaining the same, building a from a standard-bore changes the compression from 8. Boring the cylinders 0. Dished pistons like these need to be measured in the bore in order to determine their exact volume.
If you measure the piston at its true deck height 0. Deck height is the distance of the piston just above or below the block deck surface. The further the piston is below the deck surface, the more volume this adds, which reduces compression. Most engine builders shoot for as close to a zero deck height as possible. Remember that you must maintain a minimum of 0.
With a zero deck height, the only clearance is the head-gasket compressed thickness. This is what the Performance Trends compression-ratio screen printout looks like.
The basic inputs are the same as those used in any compression-ratio program. You can choose from flat-top, flat with reliefs, dished, or domed piston volumes, as well as change the gasket thickness and deck height. The more advanced program does more but you have to buy that one.
The outputs on the right are fairly self-explanatory. The volume contributions are expressed in percentages of the total.
Performance engines are built to push the envelope. A bigger cam, a single-plane intake manifold, a monster carburetor, and large-tube headers all contribute to making more power. Everyone also wants to push the compression as well. If we measured a small-block with a cylinder volume of 45 ci at BDC and a cylinder volume of 5 ci at TDC, then 45 divided by 5 equals 9, giving us a compression ratio of While this sounds simple, it takes accurate measurement and some patience to come up with these values.
There are several volumes you must measure to determine compression.
The largest is the volume of the cylinder. The other volumes that affect compression include the combustion-chamber volume, the deck height the height of the piston above or below the block deckthe head-gasket volume, and the piston design that will either add volume a dished piston or subtract volume a domed piston from the combustion chamber. All of these components have a direct affect on compression. For example, as the bore becomes larger, compression increases.
If you use heads with a smaller combustion chamber, the compression will increase. Even changing to a thinner head gasket will increase the compression ratio. The equation for the volume of a cylinder is i.The following specifications are general guidelines offered to aide in building an engine for street use. For more detailed specifications regarding your specific application or for "strip only" use, please consult a professional engine builder. Forged pistons recommended for all applications.
Cast and hypereutectic pistons can be used but should be limited to lower horsepower approx hp applications. For pump gas octane applications, a compression ratio of 8.
Higher octane fuel will allow you to run higher boost levels, approximately 1 psi for every 2 points of octane.
To determine the maximum boost level for your compression ratio using pump gasrefer to the enclosed compression ratio chart. The same rules for normally aspirated engines apply to supercharged motors.
Higher flowing heads will help generate more horsepower than stock heads. Supercharging produces a percentage gain in horsepower; by starting with more base horsepower a modified motor will receive a larger total hp gain from the same percentage gain.
Porting, especially on the exhaust side is recommended. Aluminum heads will allow you to run approximately 1 psi more boost than cast iron heads due to their ability to dissipate heat.
Lobe separation: to degrees Split pattern: Exhaust duration and lift approximately 10 degrees and. Install cam straight up. Contact a cam manufacturer for lifts and durations that best suit your application.
Headers are recommended. The size of headers are dependent on whether you are wanting to create more low end torque or high rpm horsepower. Dual planes are recommended to improve low end torque, however may require staggered jetting for good fuel distribution with carbureted applications. Your fuel pump must be capable of supplying the proper amount of fuel flow at the maximum operating pressure.
To determine maximum operating fuel pressure requirement, add your maximum boost pressure to your initial idle fuel pressure. To determine required fuel flow, multiply your total expected horsepower by a bsfc brake specific fuel consumption of.