| Author | Message | |||
     Deanh8 |
Is it just the pistons? If I put a 1050 kit on my xb12 it would just be a high compression 1250 right? | |||
| Sticks |
I think you're right, both 3-9/16 but it'd be 12.5 to 1 comp. | |||
| Natexlh1000 |
you're putting a 1050 kit in your 1203 to make a 1250? My brain hurts.  I know it must make sense from the bore/stroke but ouch. | |||
| Syonyk |
... 12.5:1 on pump gas seems a bit risky. | |||
| Rotor |
check out valve clearance. I think it's a bad idea, if this work i don't know why Rev Perf sells two different kits ! | |||
| Bike_pilot |
I can't see how it'd be different from using 9 pistons in a 12 really. I'd imagine a quick call to rev would sort it though. | |||
| Ericz |
The only difference between the 12 and 9 pistons is the volume of the dome. Both cranks bring the piston pin to the same height relative to the cylinder head at TDC. The cams and valvetrain are the same between the 9 and the 12, so as long as the 9 pistons work in the 9, they will work in the 12. Most of the kits that are sold, i.e. 1050 and 1250, are just named after the resulting displacement. Since the XB9 and XB12 share the exact same bore, cylinder heads, and cams, the same pistons can be used for both motors, just yielding different compression ratios. There are several combinations of pistons and head gaskets that can be bought off of the shelf to fit both the XB9 and XB12 to give a variety of compression ratios. The combustion chamber can even be modified to give you more flexibility with the CR. | |||
| Deanh8 |
Well I have XB9 pistons in my XB12, so naturally I would buy the 1050 kit to keep the same compression ratio and get more displacement. Just wanted to make sure. Thanks Eric | |||
| Blake |
>>> if this work i don't know why Rev Perf sells two different kits ! As noted above, the pistons are different, and most folks are not willing to run the significantly higher compression on a street bike that XB9 pistons would create in an XB12 engine. | |||
| Slaughter |
Just a note: one weekend, on two bikes, melted two pistons on 91 octane pump gas on 10.5 compression. That was the LAST time I used less than 100 octane. Now am tuned for 112 leaded. YMMV | |||
| Deanh8 |
My bike runs great with XB9 pistons in my 12, rode it an hour and a half to the track today (lots of traffic) and raced it and rode it back home on 91. No issues. need a good tuner! (Message edited by deanh8 on November 17, 2011) | |||
| Rotor |
running XB9 pistons on a XB12 is not the same than running a XB9 big bore kit on a XB12. other compression ratios and other piston domes | |||
| Blake |
Good point Rotor! Though the effect is the same as the heads remain the same. But the compression ratio of 10.5 for the kit is higher than the stock 10:1 CR. I have a spreadsheet. Wanna see it? Here it is.
Absolutely no guarantees as to accuracy or validity. Just something I did some time ago for fun. I have no idea why the engine displacement numbers are low by 1 cc. | |||
| Blake |
Well heck, I don't even know for sure if they use the same routine on the heads, I think so, but maybe not. | |||
| Fast1075 |
If you are running stock cylinder heads, the combustion chamber volume is the same for both engines. The difference in stroke (given equal size bore) is the mechanism that determines the difference in gross displacement. A cylinder in the engine with the longer stroke would at first glance seem to have the higher cylinder pressure. This is not always the case due to things that effect pumping efficiency, and cam timing. Confused yet? The difference is swept volume and trapped volume are never the same. And to make it more confusing, the trapped volume percentage changes under different operating conditions. An engine using long duration camshafts with a lot of overlap (racing cams) trap less volume at low rpm, giving less cylinder pressure...(why does my 12.5 to 1 cylinder kit with my new ginourmous cams have low cranking pressure?) The opposite and deadly problem is a high compression kit installed with stock cams with little duration and overlap. The trapped volume is high. At low rpm, heavy load, the volumetric efficiency of the short cams traps more volume. With the large dome of a high compression piston, two gremlins emerge. One gremlin is gross cylinder pressure. On a two valve, bathtub cylinder, you have limited quench volume which leads to irregular fuel distribution and atomization. (you get really rich areas and really lean areas). The rich areas are very prone to uncommanded ignition. The size of the piston dome and the shape of the combustion chamber dictate the progression of the flame front. Now you have one flame front from the uncommanded ignition and a second flame front from the commanded ignition (spark plug). This is the cause of the symptom of cylinder knock that is usually described as "detonation". The sledgehammer blows of detonation physically damage the parts. The lean spots burn hotter, If the lean spot is well below stochiometric, the oxygen will find SOMETHING to mate with (burn). Most commonly seen as a "holed" piston, or burned off lands, usually on the exhaust side. The flame propagation problem is the reason we see DUAL plug heads used a a method of improving the flame propagation across the obstructive high piston dome and the deep cylinder head combustion chamber. One you add the other variables into the mix, it truly becomes a bag of worms. Bottom line....a kit that is "10.5 to 1" compression in a short stroke motor, will have a much higher cylinder pressure when used in a long stroke motor, all other things being equal. It can be done, if you compromise the tune enough, or run fuel with enough octane to overcome the detonation. The old 2 valve motor has it's limitations. If the XB motor was 4 valve/pent roof/open combustion chamber/with centrally located igniter, it would be a whole different ball game. There are advantages. The pumping and thermal efficiency of the motor give it great fuel economy... | |||
| Blake |
XB9 cams and XB12 cams are the same. >>> An engine using long duration camshafts with a lot of overlap (racing cams) trap less volume You meant "traps less air/fuel charge"? The volume is constant expect for effect of crankshaft, rod, and piston distortion under load. >>> The rich areas are very prone to uncommanded ignition. You meant "lean areas" I think? I thought the squish/quench of the XB chambers was pretty good. Not so? Seems to me that two valves would offer more area for squish/quench than four, no? Looks like my spreadsheet has some errors. Will fix and update later. | |||
| Fast1075 |
I should have said "trap less volume at cranking speed"(as in measuring "compression"). In a high performance engine the intake closes "late" and the exhaust opens early to take advantage of the tuning variables such as intake and exhaust tract lenght, desired peak rpm, etc. On the subject of "good" quench, compare the XB cylinder head to an 1190 cylinder head. Because of the shallow valve angle and 4 valve configuration, a very high compression piston has virtually no dome at all. There is virtually full perimeter quench. This is the mechanism that allows modern 4 valve engines to live happily on "pump" gas. The design (due to the excellent flame prpogation needs less ignition lead. On an engine with poor quench and resulting uneven fuel distribution, it is indeed the "rich" areas that are the most prone to spontanious ignition. But it is the lean areas that burn hotter. Regardless of fuel or design of the reciprocating engine, it is cylinder pressure that produces the power. The higher the cylinder pressure, the more the output. All a power adder regardless of type does (at a given rpm) is increase cylinder pressure. Nitrous is a good example, as is forced induction. Since a fuel contains "X" btu per pound, the more fuel air mixture you move through the engine along with thermal efficiency determines the potential output. You either pack a lot of mixture into a fairly large low rpm engine (say an XB engine), or you take a smaller engine (for the sake of argument, a GSXR 600). You pack much less mixture into those little cylinders each cycle, but you do it more than twice s often...the result is the small engine makes more power because it has more BTU of heat energy to work with because the 600 moves more CFM of mixture. |
Compression Ratio, Chamber Volume, & HP Estimator