ok now i hear a lot of talk on piston speed and everybody afraid of stroke ok here it is maximum piston speed piston & danger of piston speed faliure is a average of 5600 fps now using a 5.25 stroke with a stock evo 7.440 rod lenght your talking 6500 rpm & a 4.25 stroke lenght with a twincam rod lenght 7.660 average piston speed of 5600 fps is 8000 rpm a 4-3/8 stroke does not hit this untill a little better than 7700. how many people here run their street bikes this hard piston stability was a issue with a 3.5 bore with strokes larger than 4-5/8 due to shortening of skirts & pin placement to crown with a 7.440 rod but as the rod lenght increased to 7.660 it allows for .220 longer skirt min and as bore size goes up you are increasing the useable load surface verses the old shovel and evo stock bore stuff and with the twin cam motor with the piston oilers you now have oil for the skirts that was not available before & that reduces heat build up and friction which killed the old style strokers and a 4-3/8 is only .125 longer than the evo and has the same rod stroke ratio of 1.75 so angularity is not a problem. ok anybody else want to chime in this or am i just venting.:hmmm:
Piston Speed
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Hello Otto, there was an article on the web directly related to this that I read quite some time back where they stated for the same capacity the square bore, over square and under square produced almost undiscernable differences in power output.
I have run these combinations through the desk top dyno that I have and have found the article to be correct.................and just which part did you want to discuss?
Where you wanting to discuss fatigue failure of pistons????
Happy new year........Ozzie
I have run these combinations through the desk top dyno that I have and have found the article to be correct.................and just which part did you want to discuss?
Where you wanting to discuss fatigue failure of pistons????
Happy new year........Ozzie
Otto! It is a pleasure to see you here. When I get some time, I'd like to discuss some engine math with you--specifically de stroke to 3 3/4" and boring to 4" to achieve 95" (94.2?), If you can manage to find time between your drag bikes and upcoming season. Hoping you have another successful year at the strip!
Ozzy! Happy New Year!
Ozzy! Happy New Year!
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Abelated Merry Xmas and a Happy New Year to you too Kelly...............Ozziekelly said:
I think maybe his point is that he has been seeing some posts that are worrying about excessive piston speed with stroked Twinkies. Stroking twin cams for street use is a non-issue in this regard. Correct me if I'm wrong Otto.
By the way, I know his 113" TC X-Mod dragbike motor is shifted at 7700 rpm, I'll bet that has some impressive piston speed numbers! of course that is track only and not subject to street reliability requirements.
By the way, I know his 113" TC X-Mod dragbike motor is shifted at 7700 rpm, I'll bet that has some impressive piston speed numbers! of course that is track only and not subject to street reliability requirements.
Matter of debate here is i see way to many people disregarding adding a little stroke to a TC (part availability custom parts for destroke} pros and cons. the big thing dependability. ok let start with the first thing i am seing happen we are having people with bottom end failures {bearings} with stock flywheel ass. is this a overstresed part we are talking a machine as forged flywheel ass verses a machined billet peice & are the wheels flexing and shifting thus edge loading the rollers at low rpm 1500-2500 area where everybody is trying to make them run where a more rigid unit like the se or jims wheel will maintain there True in this area underload. second piston speed issues on a street engine are a non issue at the operating levels we are currently using. i have done 103 engines production and build up motors where everything is the same exept for the flywheel ass and the build ups make more TQ & HP
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Happy Holidays Otto. As stated in the shovehead days piston speed was an issue (at least for me it was the short skirts mainly) Not sure about now, however there have been some piston failures with S&S's 106" kits. Skirts seem short, and may have contributed to the failures. Not an expert, but I would be more concerned with piston design rather than the speed most street engines are being run at these days. Personally I like square, or over-square motors better than strokers. Not because of piston speed, but because of more vibration with the stokers.
but so called vibration is power tremble and do to balance of the wheels 4-5/8 {106} stroke is max with a fittable in stock frame with the rod lenght used and the and 3-7/8 bore is small by comparison but again that same stroke with a 4, 4-1/8 bore would be much more stable reducing piston rock. trust me i am a big bore fan but i believe there are more advantages to replacing the stock flywheel ass.
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Yea, I agree a bigger bore "should" be more stable, provided the "design" of the piston is favorable. (adequate skirt lenght, etc.) I recently went with the SE 113" bigger bore set up (103 flywheel assy) and it's very smooth. As good or better than when it was 95", so yea, increasing stroke (up to a point) can be very beneficial, with out affecting reliability and comfort. (IMO).
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