| Author | Message | ||
     Steveford |
We've got a bike in the shop which has 11:1 compression and has a Spyke starter motor which is giving us problems. Does anyone know if a stock starter motor can function reliably with 11:1? We don't know if this aftermarket motor is the culprit. Thanks Steve Ford | ||
| Diablobrian |
My firebolt has 11.43:1 and uses a stock starter reliably. Maybe low of amperage from the battery causing trouble? How old is it? Of course I'm sure you checked for any corrosion or loose connections. | ||
| Jkhawaii |
my stock 12S has trouble with the starter cranking the engine. happend after about 5,000 miles. guessing the engine broke in and the cranking compression is higher. battery is ok (using midtronics tester) voltage power and ground was ok while cranking. there is just one point that the engine has a hard time turning probly when both pistons are on the compression stroke. | ||
| Pammy |
It depends on what cams are in the bike. "Realized" compression is what matters. | ||
| Pammy |
Sorry, I didn't see the date on this...I gotta get out more | ||
| Jackbequick |
Maybe not, maybe this is a chance for a good H-D mech to explain realized compression to me, I've never heard of that. And maybe Jason in Hawaii could use some help too. He has a Buell that cranks slow when both pistons are on compression at the same time. Sounds like you need to go to Hawaii and check that one out! Is that a common problem? :>  Have a good holiday season Pammy, I always look forward to your helpful inputs here. Jack | ||
| Justin_case |
When one cylinder is on compression the other is on exhaust. Radical cams i.e. long duration, lots of overlap, will reduce cranking pressure making for easier starting. | ||
| Paintballtommy |
when you say 11-1 compression ratio your are speaking about static compression. assuming both valves were completely open at the bottom of the pistons stroke and completely closed when the piston starts to head up it would achieve a compression ratio of about 11-1. however static compression ratio is of little concern to engine builders. what is more important is what is called dynamic compression which takes cam timing into account. generally speaking shooting for a dynamic compression ratio of around 9-1 is accepted for mild performance applications. http://www.kb-silvolite.com/calc.php?action=comp that is a calculator for automotive applications but the theory is the same just substitute the variables. |
