bansheeqc Posted October 10, 2008 Report Share Posted October 10, 2008 Hi, I just want to know how much compression you useally run in your drag bike (alky or gas please specify). I actually test my new engine and i think my engine get retarted by high compression.... I'm actually at 215 psi and i run av gas or c12. I see many compression number on this site...150-160-175 psi of compression on alky bike.. If any as try different compression for track time or dynoed the same bike at different compression, let me know... Thank you.. Quote Link to comment Share on other sites More sharing options...
bansheeqc Posted October 15, 2008 Author Report Share Posted October 15, 2008 any ??? Quote Link to comment Share on other sites More sharing options...
J-Madd Posted October 15, 2008 Report Share Posted October 15, 2008 I like around 175 - 185 psi on alky, and that seems to be to concensus of some others that I race with. I'm about to bump up the compression in my 392 cub...going from 20 cc (160 psi) to 19 cc domes (?? psi). I'll let you know if it helped or not. Quote Link to comment Share on other sites More sharing options...
locogato11283 Posted October 16, 2008 Report Share Posted October 16, 2008 175 is about right. youre puttin a lot of stress on parts with 215 lbs of compression. Quote Link to comment Share on other sites More sharing options...
Bansh-eman Posted October 16, 2008 Report Share Posted October 16, 2008 either you have TINY domes or your compression tester isnt accurate more then likely. 215 psi is very high! Most guys run from 160-185ish... Quote Link to comment Share on other sites More sharing options...
dajogejr Posted October 16, 2008 Report Share Posted October 16, 2008 What is your squish, and how wide is your squish band? There's a lot more to the equation on what fuel type, how much compression you can run, etc., other than dome/head cc and cranking PSI... Also... Compression ratio...that is the key. You could have a motor that has 145PSI cranking that MUST have race fuel, and a 170PSI motor that runs on pump gas...based on port heights and timings.... Compression ratio is what you should figure out. Quote Link to comment Share on other sites More sharing options...
bansheeqc Posted October 16, 2008 Author Report Share Posted October 16, 2008 how to mesure compression ratio on my motor? And i think squish it's the think i mesure with a little tin wire, into the cylinder (by plug hole) and i measure the tin wire tickness after (piston clearence with the piston)but if it's not this think, how to measure this think and what it's squish band? thank you Quote Link to comment Share on other sites More sharing options...
KEVSWS6 Posted October 16, 2008 Report Share Posted October 16, 2008 The PSI gauge has its place but when your drag racing you should be using this with it. The "Uncorrected Method" (sometimes referred to as the Geometric or European method) which compares the volume above the piston at Bottom Dead Center (BDC) to the volume above the piston at exact Top Dead Center (TDC). How to calculate? Quite simply, it is (volume of cylinder at BDC + volume of combustion chamber at TDC) divided by (volume of combustion chamber at TDC). The volume of the cylinder is easy....... (radius of bore in millimeters) X (radius of bore in millimeters) X (3.14159) X (stroke in millimeters). Then divide your answer by 1000 to get the cylinder volume in cc's. To check the "Flat Plated Volume" (FPV) of the combustion chamber, start by scraping the head gasket surface clean of gasket material, cleaning the combustion chamber of excess carbon deposits and the like (gently with a wire brush) and installing the normally used spark plug. Position some wood or similar supports under the head so that it is combustion chamber up on a bench with a slight tilt in one direction referencing the gasket surface (not level). Apply a narrow border of grease about 3 mm's from the edge of the combustion chamber totally encircling it on the gasket surface. Using a piece of plexiglass (should be round and big enough to totally cover up the combustion chamber, at least 1/4" thick or more) with a 3/8" "fill" hole in it at one edge, position the hole at the "high" side of the tilted combustion chamber and press it firmly against the gasket surface smashing the grease and creating a seal. Make sure to press it firmly so the grease does not become a spacer. Now carefully fill the chamber with Marvel Mystery oil from your burette again noting the beginning reading so you'll know how much has been used to fill the chamber after your done. Write your reading down. This is your chamber's FPV. To figure out the "Head Gasket Volume" (HGV) simply use the same formula as you used up above to figure out the cylinder volume, just substitute the radius of the head gasket ID (usually BIGGER than the bore, so measure it!) and use the thickness of the gasket (preferably the compressed thickness from a used head gasket) as a substitute in the above equation for "stroke". Divide your answer as before by 1000 and you'll have the HGV. Write this down also. The "Deck Height Volume" (DHV) is again calculated using the same basic formula. But you must either note the Deck Height during disassembly or put the piston temporarily back on the rod, slide the cylinder down over the piston (onto a fresh base gasket but you don't need rings) and use a couple of head or base nuts to pull it down firmly against the cases. Bring the piston up to TDC and use the depth measuring probe from a vernier or dial type caliper to determine how much below or above the top of the cylinder the piston crown edge is. VERY IMPORTANT! Check it inline with the wrist pin so the piston will not tilt on its' wrist pin axis during measurement. In the formula, use bore size again and substitute the deck height for the stroke. If the deck height was ABOVE the cylinder at TDC, put a negative sign (-) in front of your calculated answer. If the deck height was BELOW the top of the cylinder at TDC, leave the calculated answer as is (positive). Record this number as DHV. To determine the "Piston Crown Displacement Volume", (PCDV) On Banshees just use 7cc's Example: The dome volume of Banshee piston is approx 7cc (6.5cc)The cylinder vol is 184.745cc on a 66mm boreThe head vol is 15cc (with a 15cc dome).So, if you take the cyl vol + the head vol - crown vol you get 192.745cc.. This is your full stroke volume of the cylinder and the head, taking into consideration the piston volume.OK... So you need to figure the trapped volume .. The trapped volume is the actual volume present when the piston is at TDC. So.. this would be the 15cc (head) - the 7cc (piston)so 15 - 7 = 8cc Trapped volume....So now it becomes a simple ratio.. The total cylinder volume / the trapped volume so.. 192.745 / 8 = 24.093 = compression ratio of 24.1:1 I might not be dead on with some of this but it gets you in the ball park. I did not write this but save good info and post it for others. Quote Link to comment Share on other sites More sharing options...
speed-racer Posted October 17, 2008 Report Share Posted October 17, 2008 The PSI gauge has its place but when your drag racing you should be using this with it. The "Uncorrected Method" (sometimes referred to as the Geometric or European method) which compares the volume above the piston at Bottom Dead Center (BDC) to the volume above the piston at exact Top Dead Center (TDC). How to calculate? Quite simply, it is (volume of cylinder at BDC + volume of combustion chamber at TDC) divided by (volume of combustion chamber at TDC). The volume of the cylinder is easy....... (radius of bore in millimeters) X (radius of bore in millimeters) X (3.14159) X (stroke in millimeters). Then divide your answer by 1000 to get the cylinder volume in cc's. To check the "Flat Plated Volume" (FPV) of the combustion chamber, start by scraping the head gasket surface clean of gasket material, cleaning the combustion chamber of excess carbon deposits and the like (gently with a wire brush) and installing the normally used spark plug. Position some wood or similar supports under the head so that it is combustion chamber up on a bench with a slight tilt in one direction referencing the gasket surface (not level). Apply a narrow border of grease about 3 mm's from the edge of the combustion chamber totally encircling it on the gasket surface. Using a piece of plexiglass (should be round and big enough to totally cover up the combustion chamber, at least 1/4" thick or more) with a 3/8" "fill" hole in it at one edge, position the hole at the "high" side of the tilted combustion chamber and press it firmly against the gasket surface smashing the grease and creating a seal. Make sure to press it firmly so the grease does not become a spacer. Now carefully fill the chamber with Marvel Mystery oil from your burette again noting the beginning reading so you'll know how much has been used to fill the chamber after your done. Write your reading down. This is your chamber's FPV. To figure out the "Head Gasket Volume" (HGV) simply use the same formula as you used up above to figure out the cylinder volume, just substitute the radius of the head gasket ID (usually BIGGER than the bore, so measure it!) and use the thickness of the gasket (preferably the compressed thickness from a used head gasket) as a substitute in the above equation for "stroke". Divide your answer as before by 1000 and you'll have the HGV. Write this down also. The "Deck Height Volume" (DHV) is again calculated using the same basic formula. But you must either note the Deck Height during disassembly or put the piston temporarily back on the rod, slide the cylinder down over the piston (onto a fresh base gasket but you don't need rings) and use a couple of head or base nuts to pull it down firmly against the cases. Bring the piston up to TDC and use the depth measuring probe from a vernier or dial type caliper to determine how much below or above the top of the cylinder the piston crown edge is. VERY IMPORTANT! Check it inline with the wrist pin so the piston will not tilt on its' wrist pin axis during measurement. In the formula, use bore size again and substitute the deck height for the stroke. If the deck height was ABOVE the cylinder at TDC, put a negative sign (-) in front of your calculated answer. If the deck height was BELOW the top of the cylinder at TDC, leave the calculated answer as is (positive). Record this number as DHV. To determine the "Piston Crown Displacement Volume", (PCDV) On Banshees just use 7cc's Example: The dome volume of Banshee piston is approx 7cc (6.5cc)The cylinder vol is 184.745cc on a 66mm boreThe head vol is 15cc (with a 15cc dome).So, if you take the cyl vol + the head vol - crown vol you get 192.745cc.. This is your full stroke volume of the cylinder and the head, taking into consideration the piston volume.OK... So you need to figure the trapped volume .. The trapped volume is the actual volume present when the piston is at TDC. So.. this would be the 15cc (head) - the 7cc (piston)so 15 - 7 = 8cc Trapped volume....So now it becomes a simple ratio.. The total cylinder volume / the trapped volume so.. 192.745 / 8 = 24.093 = compression ratio of 24.1:1 I might not be dead on with some of this but it gets you in the ball park. I did not write this but save good info and post it for others. This is actually the static compression ratio, which means nothing about the fuel an engine will require. The dynamic compression ratio is what will dictate fuel needs. Calculating the DCR is a little more complicated and requires everything mentioned above as well as the rod length, stroke, and port timing. While an engine is running, the compression ratio will never remain constant and you can never acheive the static compression ratio. Static compression is what you would have if there were no valves (or ports on the two stroke). Changing the valve timing in a way that creates more overlap (drag bike) is really what dictates the need for a small domed head. All of this information together is what determines fuel requirement. I agree that a compression tester is not good way to determine the fuel needed, but it is good to compare to that of a stock bike (or one with a known dynamic compression ratio) to get an idea where you stand. Every guage will be a little, and sometimes a lot, different. Hope I didn't sound like an ass, this is not an attack on anyone. I just wanted to state that a compression test actaully can be useful. Mark Quote Link to comment Share on other sites More sharing options...
dajogejr Posted October 17, 2008 Report Share Posted October 17, 2008 I agree 100%. Stock or stockish motors with only cc'd domes or heads...compression tester is good for two fold, measuring the health/wear of an engine if done when new and again as used...and a good idea for fuel needs...but not bible. When you start to get into aggressive porting, aftermarket cylinders, etc....then you have to do a little more math to be safe! Quote Link to comment Share on other sites More sharing options...
locogato11283 Posted October 18, 2008 Report Share Posted October 18, 2008 When you start to get into aggressive porting, aftermarket cylinders, etc....then you have to do a little more math to be safe! no wonder my stanley compression tester said 275psi on my cub motor.. i forgot to take the extra stroke for one cylinder + the extra stroke for the other cylinder = 100 and subtract that from 275!! so i should be right around 175! thanks dave!! :beer: Quote Link to comment Share on other sites More sharing options...
bansheeqc Posted October 18, 2008 Author Report Share Posted October 18, 2008 Thank you for everyone for the precious information! KEVSWS6:thank for all the info and the way to calculate the compression ratio. I check all thing this weekend and i post news. bansheeqc Quote Link to comment Share on other sites More sharing options...
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