| Author | Message | ||
     Docktor |
Hi, I am a Bueller from Australia. I am doing some work on my xb12r and I am trying to find out some info and have been directed here. I need to drop my compression a touch by using a thicker head gasket to 8.5 or 9:1. I am using a online comp calculator to help me but I am getting it wrong somewhere. Perhaps someone in the know can help me out? I do apologise for my first post being a question but my bike is in bits and I am starting to stress a little .  I am using a comp calculator to work out some stuff.. http://www.angelfire.com/fl/procrastination/motor.html Bore : 88.9mm Stroke : 96.8mm Number of cylinders : 1 Pistons are flat tops : 0 Head Chamber : 62cc's (from what i can work out) Gasket Thickness : Who gives a rats ? (not enough to worry about) Why does my bike have a comp ratio of 2.45:1 ? Doc | ||
| Glitch |
Number of cylinders : 1 Two cylinders? Stroke : 96.8mm That matters? What value did you use for Cylinder Diameter (mm)? You may want to post this in the engine section of the Knowledge Vault also. | ||
| Slaughter |
9.7 to 1 Piston Area = 6207.2 mm^2 Stroke = 96.8 mm Volume = Area x Stroke = 600853 mm^3 (or close to 601cc) Chamber vol = 62cc = 62000 mm^3 CR = Volume / chamber volume = 6000853/62000 = 9.69 Looks like you're close to where you wanted to be. Hope this worked (dang the coffee must be working now, it's early in the morning here) Ride well! | ||
| Slaughter |
After doing the above, I went to their site and tried. Dunno what their assumptions are about piston shape but I also came up with similar numbers using their calculator and your engine sizes. If you know your chamber volume (62cc) - then the piston area and stroke ARE what determine the volume. The CR is the ratio of the two. I'm too lazy to search that site to see how they came up with their table. I'm sure that you and I missed something in piston dome height, or something... who knows. All I know is your motor as described is reasonably good compression. Can still run pump premium grade fuel... such like that. What are you working on??? Enquiring minds wanna know? (Message edited by slaughter on August 02, 2006) | ||
| Glitch |
Would doing this allow you to run a lower octane? Go, Slaughter go!  | ||
| Slaughter |
at a compression ratio of 2.something to one, you could probably run something like a special blend of ether, nitromethane, and alcohol (for the rider, NOT the engine) ..... (KIDDING about the alcohol and the rider - no flaming please)  | ||
| Glitch |
What about 9.7 to 1 ? Would that allow you to run mid grade pump gas? Not thinking of doing that myself, but it might be an option for people that can't get the high test. Like the guy that's in Brazil. Or Dean, I think he's the guy in Alaska that has no access to high test. | ||
| Docktor |
Ok, I am impressed.umm Glitch, We only do 1 cylinder at a time coz hopefully the combustion chambers of the 2 are NOT connected. . And Slaughter I have no idea who made their calculator either.But it should be shot.Thank you for your calculations.We are just putting a turbo on the thing and heading for 150 hp to see what happens to it.i will post pics of progress if you like.(may be a little boring for you people but in Australia it hasn't been dont to a XB as yet.).I may have just found out that stock pistons have a 15 degree dome on them............. Soooo would that take the compression from 9.69:1 up to Buells claimed 10:1 ? As for the 2:1 ratio ,If anyone can make it start I will have what they are on . I am still stunned that people replied actually | ||
| Docktor |
Oh OH, thinking some more, If i decrease stroke then it stands to reason that the combustion size will go up as well right? Or in simpler terms barrel / piston height will increase thus throwing my calculations out again. I need to know what volume decreasing stroke by 1 mm will do to compression. I pulled 2 mm off the stroke using Slaughters calcs and dropped the comp by a mere .2 which we know cant be right because I didnt take into account the increase of "chamber size". My head hurts............... | ||
| Slaughter |
It's 5PM and I don't do math now. Leaving for a ride up to Newcomb's ranch for dinner on Angeles Crest. Brain re-starts tomorrow. | ||
| Fullpower |
my stock XB12 has a cranking pressure of only 150 PSI, so it is pretty tolerant of low octane fuel. if you are running much over 175 PSI you may well need the premium fuels, especially in hot weather. On the other hand, the sportster has a cranking pressure of 205 PSI, and runs best on various blends of methanol, 100LL avgas, 114 octane race fuel, acetone and premium pump gas, and is just barely tolerant of straight automotive gasoline. For stock production machines this is not required. for turbo or supercharging you may want to consider water injection, and aftercooling to help supress detonation, and also using forged blower pistons with a bit of a dish might be a good idea, if you want the motor to last long enough to get dialed in. using a thicker head gasket to acheive a reduction in compression ratio is a particularly bad idea, you will give up the built in "squish" or quench zone around the circumference of the combustion chamber, which in itself is the main impediment to detonation built into your engine. | ||
| Docktor |
I can use a thicker base gasket instead. that wont balls with the squish area at all right ?. I am happy to go with blower pistons if someone would be willing to ship them over to me , that would make life much easier indeed. Still need to know what to ask for, Do the lower the gudgeon height or are they a dished piston. Fuel doesnt bother me here, we can get 120 octane straight out of the pump . Restart ya brain in the Morn after a good gassin Slaughter. Cant beat a good ride Doc | ||
| Drfuyutsuki |
Alright if Im looking at this right in my mind adding 2mm in hieght using aforementioned numbers would essentially shorten the stroke and increase chamber slightly ~178 mm3. Using those assumptions I come up with 9.46 I assume this is the .2 drop you mentioned. But then again I never got into compression ratio's and all that math. In another post some one mentioned retarding the timing by 3 degrees to better compesate for lower octane fuels. http://www.badweatherbikers.com/cgibin/discus/show.cgi?tpc=32777&post=710697#POST710697 Fuy (Message edited by drfuyutsuki on August 03, 2006) | ||
| Docktor |
I have no drama at all with fuel, Its all good here, $1.50 a litre but its good. Fullpower, I am aware of the squish zone and the turbulence caused by it which causes detonation. Why cant I leave the head alone and drop compression by raising the barrels instead of the head ?. Meaning a thicker base gasket and leave the head gasket the same. That will decrease compression as well right?. All I need is to drag it under 9:1 and all will be good.Its just working out how thick of a gasket I need to do this............. Doc | ||
| Dale |
The compression area is equal to 1/10 of the volume of pie times 1/2 bore squared times stroke if the compression ratio is 10 to 1. Use this information to figure out how much thicker the gaskets have to be to get 9.1 to 1. pie = 3.1416 | ||
| Docktor |
Dont suppose anyone is still interested in a few pics of the monster created down under? Doc |
