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The "duration" of a port is how many degrees it is open in one full engine cycle (one full crank revolution). When you measure the deck (mm from top of cylinder to where the top of the piston is at TDC) when the engine is cold and use that in the calculation you get the cold port durations, the durations when the engine first starts up. When the engine reaches its full operating temperature the connecting rod will elongate a bit which lessens the durations. When you use a pipe calculator it expects the hot durations because you don't race a cold engine, only a hot one, fully warmed up. There are different ways to determine cold port durations but only one way to determine hot durations. Measuring the Hot Durations Measure important distances and then figure out the piston deck and then use those numbers in an online port duration calculator to find them out. This is the best way. Here's how: A. Measure the distance from top of port to top of cylinder. B. Find the piston "deck" (Piston top edge to Top of Barrel at TDC) by measuring the distance from top ring mark on the cylinder (see pic below) to the top of the cylinder and then subtract from it the distance from top of ring to top of piston edge (while making sure the ring is resting on the bottom of its groove).  In this photo you can see where the top ring kept the cylinder from having any combustion deposits. If the deck number turns out to be negative then that is because the piston top is above the cylinder top at the top of the piston stroke. That is OK because the online port duration calculator can handle that.  C. Now if you don't know the eye-to-eye length of the connecting rod then go here to look up the center-to-center length or just assume it is twice the length of the piston stroke. (Being off by 10% only nets a .5% difference in port durations.) Now use all these numbers at the online calculator to find out the port durations. This sequence applies for both the exhaust and the transfers and the boosts. Here is a good method to use with the caliper to find the port heights. If you have fingernails you can use them in place of the screwdriver. I prefer this method to the method of uing a metal rod that the piston can press against because due to the curve of the piston dome you can't get the caliper end to touch the far outside of the piston top.  Finding the port durations is important and should be done with great care, especially for the transfers since an error there gives 2 times the degrees difference than with an error for the exhaust. Generally you can subtract 2 degrees from the exhaust and 4 degrees from the transfers when converting from cold durations to hot durations. Click here to download my Deck Calc spreadsheet that is a helper for determining the deck distance. HOME |