convert cranking psi to compression ratio

the stroke length of the piston. Generally, when you increase compression past a certain level, you need to increase the fuel octane requirement in order to combat detonation. Ok, so say your engine is running at 140psig. For example if a 2009 4.6L Ford F150 has a compression ratio of 9.8:1 and its intake pressure is at 17 in.Hg, then you can expect a cylinder pressure in the 170 PSI range (17 x 9.8 = 166.6, which I then round to the nearest 10 and call it 170). The rings and valves checkout fine during a leakdown test. the size of the camshaft in relation to the compression ratio of the engine could very well create 110 psi in an otherwise healthy engine, with the only problem is that the cam is entirely too big. A 1:1 ratio is equal to 0 PSI. Octane Requirements. I shoot for 200 and then some. Instructions. For a premium pump fuel combo cranking compression needs to be a max of about 190 psi for your chosen camshaft specs. C. This reveals our engine was running an effective compression ratio of over 15:1. Step 1. Determining the % contribution of various dimensions on the overall compression ratio. I have it mounted on a homebrew engine cradle - see enclosed fotos. Calculating cranking compression pressure and dynamic compression ratio. More compression will improve power throughout the engines rpm range, which makes for snappier acceleration and increases fuel mileage because the engine is more efficiently converting energy into power. Calculating Bore/Stroke and Rod/Stroke Ratios. I use cranking compression to verify cam timing, or checking severe cylinder sealing issues but not as a measure of performance. We started the compression ratio measurement process by setting the head upside down on our work bench. Plugging the variable numbers into the formula will get the compression ratio , such as 10:1 . If the whole build is baloney and they used builder parts, all he can really do is tear into it. The following example goes over the steps required to convert a compression ratio into pounds per square inch (PSI). The first step in determining the psi is to determine the compression ratio. In this case, the compression ratio is a simple 2:1 = 2/1 = 2. Next, determine the pressure that the engine is working at. 1 Atmosphere = 14.7 PSI. Now a third example, a 268 degree cam on 106, IVC is 60 degrees, that means 120 degrees of crank rotation compressing the mixture. The static compression ratio is approximately 10.94:1, intake valve closes about 60 degrees after bottom dead center and compression pressure is 190 PSI. #3. The formula that is used is: Effective Compression = ( (Boost PSI / 14.7) + 1) * Current Static Compression Ratio. 210@ .050 / about 8.5 to 1. compression ratio. The compression ratio is the ratio between the volume of the cylinder and combustion chamber in an internal combustion engine at their maximum and minimum values.. A fundamental specification for such engines, it is measured two ways: the static compression ratio, calculated based on the relative volumes of the combustion chamber and the cylinder when the piston is Divide by 16.4 to convert to cubic inches. Higher compression ratios can affect cranking compression, but the two are not related. As I mentioned the cam does affect the overall effective compression ratio. X:Y is the compression ratio. With 95, shaved heads, etc it is now ~185 ccp. Vintage Air, AGR steering, Corbeau GTSII seats, 700R4 transmission, 12-bolt w/Eaton 4.11:1. Likewise a reduction in compression ratio from 11:1 to 7.0:1 should result in a 12.3-percent decrease in power. This gives a way to estimate cranking cylinder pressure. When the engine is running, it sees a dynamic compression ratio. The cylinder compression test will give a reading in PSI on the gauge , such as 135 PSI . My cranking compression is 200 psi, so I figure it is real close to a 10:1 engine. 88 GT CHI headed 351w 10.8 @ 125 - still figuring it out. IVC is now at 70 ABDC, so there is now only 110 degrees of rotation compressing the mixture. For example, if the atmospheric pressure is 14.7 psi and the compression ratio is 11:1, the equation to solve for the psi is (14.7*11)/1. Effective D Ratio is 19:1, and the following is from the rebuild section of the manual: "Compression of a standard new engine cranking at about 300 rpm is about 300-350 psi (2070 to 2415kPa) prior to Spec P and 350-400 psi (2415 to 2760 kPa) Spec P and later. Any cylinder with this compression (or lower) value will misfire. This is only in a perfect cylinder where valves close exactly as the piston reaches the bottom and stays closed the whole way, and if no air bleeds out from the valves, or between the piston and cylinder wall. variable. And 175 psi with a cam with about 240 degrees duration at 0.050" lift. Compression ratio is nothing more than a mathematical ratio of the total volume of a cylinder at bottom dead center compared to the volume at top dead center. Step 1. That's one of the reasons I want to increase static compression. Your bike's engine has a compression ratio of 9:4:1 here is a guide frm compression ratio to psi :-8>1 = 102.9 psi 8.5>1= 102.9 - 110.25 psi 9>1= 110.25 - 117.6 psi 9.5>1= 117.6 - 124.95 psi If a compression of Cranking Compression Calculator. You can't estimate cylinder pressure by multiplying atmospheric pressure (14.7 psi at sea level) by the mechanical compression ratio. Bore: (diameter) The rule of thumb is that the cranking compression should fall within a range of 15.5 times piston compression ratio and 21.5 times the piston compression ratio. 15 Votes) Generally, readings below 90 PSI for modern two-stroke outboards indicate a problem; most will produce cranking compression readings in the 90 to 105 PSI range, depending on the engine year, make, and model. Compression will be higher. mm Cylinder Head Volume: cc in. I add the g because t is measured with a gauge ignoring atmospheric pressure. Instructions. Effective Compression Ratio Static Pump Gas Race Gas Compression Blower Boost Pressure (psi) Blower Boost Pressure (psi) Ratio 135789 10 11 12 15 6 6.4 7.2 8.0 8.9 9.3 9.7 10.1 10.5 10.9 12.1 14.7 = Atmosheric Pressure @ Sea Level (psi) CR = Engine Compression Ratio Might be a repost, I dont know! If your motor is normally aspirated boost is 0. The compression ratio of an engine is a very important element in engine performance. 1 Bar = 14.5 PSI. So perhaps the compression ratio is not that high, and the effective displacement (at cranking speed) is likely to be a little greater. Get your engine's optimal compression ratio and total displacement in no time flat! Calculate Engine's Compression Ratio (CR) Calculate Engine's Compression Ratio (CR) Get accurate compression without the guesswork! The following formula is used to convert a compression ratio to PSI. To invert this looks like: (CR * 1 ATM) + 1 ATM = 160 due to the fact that the compression tester is adjusted to not show the 1 ATM that is already calculated into the gauge. NHRA quit using them years ago because they aren't accurate. That would indeed be a radical race cam with lots of valve overlap. Just multiplying the static CR by atmospheric pressure gets you 161 PSI, but the actual CR is lower than 10.94:1 and the compression result is much higher. The high-speed value would be the 8.55:1 effective compression ratio x 14.7 psia, or 125.69-psia. 327, Edelbrock E-streets, hooker, DUI performance distributors, ultradyne cam, stewart, TRW etc. When reading the pressure with a gage, there are too many variables involved to accurately calculate the compression ratio. OnlineHelp. fujimook. This calculator provides the new clearance volume and the new compression ratio of the engine based on changes made. Or maybe it will go by cranking compression test pressure instead of compression ratio. Compression Ratio to PSI Calculator. Engine Building. In a 460 combo, juice cam with intake duration @ .050" spec of: 200@ .050 / 8.0 to 1. I was searching a way to determine the cranking pressure(psi,compression test) you can get at various compression ratio and I found this calculator!!! 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, TDC), and the gas volume with the piston at its lowest point (bottom dead center of the stroke, BDC). Effective Compression Ratio (ECR) = [(Boost / 14.7) +1] x Static Compression Ratio. Your dynamic boost compression ratio, reflecting static c.r., cam timing, altitude, and boost of 0 PSI is 7.64 :1. That post said "I'm pretty sure I'm at 9.5 cr and the shop told me that 165 psi cranking pressure isn't 9.5" My point from the beginning is that cranking compression doesn't tell you anything about static compression. stroke. mm Stroke: in. Compression Calculator Simply fill in the form below to calculate your compression ratio Bore: (diameter) in. Are there any published charts out there to show how much turbo psi is safe to put on a certain compression ratio engine with a certain octane level? 04 Mach 1 12.7 @ 109 mostly stock - sold it. You could have 20:1 static compression and close the intake valve at 90 ABDC to get really low cranking pressure. You can even add a boost controller to increase abit without any problems. There's formulas to figure compression ratio based on variables such as piston type , combustion chamber volume , valve timing and overlap . In general, 215 psi is 10.0 to 10.5 with a stock cam. The one thing I did not do is hold the throttle open, so my readings are probably low. My stock 04 ccp was just under 130 psi at 3300 feet elevation. 14.7 PSI is equal to a 2:1 ratio. Nov 8, 2004. ANSWER: 149 PSI. The compression ratio for any engine or individual cylinder is defined as the ratio between the swept volume of the cylinder with the piston at bottom dead center (BDC) and the volume with the piston at top dead center (TDC). what you need is a cylinder leakdown test to tell you exactly what is wrong with it. Only show this user. Given the compression ratio, I would think the max achievable cylinder pressure would be (9.2-1) * 14.7 psi above ambient atmosphere. I'm too tired right now to test it, if someone want to play with it an give us some feedbackit''ll be great!! So anyway: To answer your basic question, I doubt that "too much cyl pressure" is the cause of your problem. a compression test wont confirm engine work that needs to be done. The whole idea of doing the test is to determine the state of the piston rings' sealing ability (a wise procedure when carrying out modifications to avoid costly repairs later on). Dec 7, 2007. The short answer: Among other factors, cranking compression is not so much a matter of static compression ratio but effective compression ratio: cylinder volume at IVC (intake valve closing) vs. cylinder volume at TDC. Re: translating compression ratio into psi. Unlike Static Compression Ratio, Dynamic Compression Ratio takes into account camshaft timing by considering the intake valve closing point in relation to the piston position. That means less "dynamic" or "swept" compression. Compression Ratio to PSI Formula. Number of Cylinders : Bore in Inches : Stroke in Inches : Rod Length in Inches : Static Compression Ratio : (:1) Inlet Valve Closes ABDC : I am pretty sure that compression is normally recorded in PSI so there should not be any conversion unless its some sort of dynamic compression ratio but I doubt anything would change. Changes can include a reduction in the gasket thickness or decking of heads or block. I worked these calculations for a 1600 engine (1588cc) with stock 8.3/1 CR and a very optimistic 140 psi gauge pressure at cranking speed. Is 150 compression good? description. Examples: Suzuki FA50 compression ratio is 6.5:1 (6.5*14.696/1 = 95.524 PSI) Sachs A However, the reason we talk about compression ratio is it is a measure of how well compressed the inhaled mixture will be. It is all the same thing just using a different unit.