![]() If the heads are brand new, you can usually stop here unless you're blueprinting every variable, but if they're used heads and you suspect they've been worked, repeat this process on the remaining chambers. Once the chamber is full and free of air, check the burette for your chamber volume by reading the measurement at the bottom of the meniscus (or curvature of the fluid's surface due to surface tension). Fill the burette to the zero mark, position it over the head, and begin filling. Engine Compression Ratio (CR) Calculator This calculator requires the use of Javascript enabled and capable browsers. Mineral spirits or alcohol are preferred, but even water would work here. Heavy grease is applied around the chamber, but not inside of it, before a block of acrylic (included with many kits, but can be easily made) is placed with its fill hole at the top of the chamber, ensuring air can escape while the chamber is filled. First, the heads are set with a slight uphill rake. Say a you've got a 700- to 800-inch engine with a 100cc chamber and you want to run 15 to 17:1, that's going to be easier because the chamber volume relative to the cylinder volume will be quite small, and consequently, you get a better piston design out of it."Ĭombustion Chamber Volume: While head manufacturers supply this number at purchase, say you bought a used set and you want to double-check if there's been any work done to them cc'ing the heads with a burette accomplishes this. On a decent-sized engine, 400-inch and up, you start getting close to flat pistons to get 10.5:1. For pistons BELOW the block TDC - value in box must be a POSITIVE number (.000) For pistons ABOVE the block TDC - value in box must be a NEGATIVE number (-.000) DOME piston - value in box must be a NEGATIVE number (-0. The small engines, like a 300-inch motor, and put a big dome on it, that's not really good for flame travel. How-To-Use: Enter a value in the un-shaded boxes below as they pertain to your engine. As you go larger, you start to revert more to a flat piston, which has the best of the flame travels of all of them. "The cool thing about big engines is piston design becomes way more practical. ![]() If you look at the work being done, if you have to fire the engine 10 degrees earlier on the compression stroke, the engine is now going to have to produce more work because you've started the burn earlier to achieve the same peak ATDC," he said. On the older, junk cylinder heads, you may have 45 degrees of total timing (approximately a 60-degree time frame). The compression ratio calculator spreadsheat below allows you to easily enter measurements and select a shim/CR combination to suit your needs. "And with a good cylinder head, you're looking at a 45-degree time frame (approximately 30 degrees of total timing) from the time you ignite it. Looking at the combustion chamber's efficiency, you can begin to look at compression as a function of the timing advance. He's looking for peak cylinder pressure at 15 degrees after top dead center (ATDC), where the most power is made. With zero deck height it calculates to 11.33 For skunk, I calculate it to be 10.34 with zero deck height, 9.90 with 0.020 deck height. ![]() Thats using bore 4.030, stroke 3.75, head chamber 64, piston dome -4 (for valve reliefs), gasket thickness 0.058 (0.038 gasket plus estimated 0.020 deck height). General rule of thumb for acceptable dynamic compression ratio to run safely on pump gas is 8:1 maximum for engines with cast iron cylinder heads and 8.5:1 with aluminum cylinder heads.The reason for this, Duttweiler explained, is in the time it takes for the flame to propagate during the crankshaft's rotation. When I calculate your engine I get 10.79. However, that same 11:1 static compression ratio engine with the radical 259/269 duration camshaft would have a dynamic compression ratio in the neighborhood of 7.5:1, totally acceptable to run on pump gas. Therefore with the mild cam it will have a high dynamic compression ratio, probably 9.5:1+ which would be way too high to run safely on 91 octane gas. If you put a very mild camshaft (194/204 duration this cam will have an "early" IVC (intake valve closing point), and will "bleed off" less compression than a radical camshaft with 259/269 duration and a considerably "later" IVC. To give an example, lets say you're considering only pump gas for your engine, and it has a static compression ratio of 11:1. 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. Static Compression Ratio numbers are the ones you hear thrown around the most ("10:1 compression"), and it takes into consideration the full sweep volume of the cylinder in regard to its range of crankshaft stroke. This calculator will calculate both Static and Dynamic compression ratioĬompression Ratio is the ratio of an engine's cylinder volume vs. ![]()
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |