Piston Speed is the limiting factor?

[racerrad8]

I think you'll find piston speed is the limiting revs factor for a road going FJ engine.

Noel

[racerrad8]

Here are the specific questions asked of you Noel following your post above.

Randy - RPM

Yep, I was wondering about that....Questions for Noel:

1) FJ Piston speed: How fast is too fast?
2) If the stroke is 63.8mm at 11k rpm the FJ piston speed is 4605 FPM, is that too fast? Why?

I'm open to learning....

I'll add a couple more questions for Noel:

3) Why is piston speed the limiting factor of a FJ engine?
4) Have you ever observed a FJ engine failure due to excessive piston speed?
  a) If yes, what failed due to the excessive piston speed?
5) Valve "bounce" versus valve "float"; which comes first?
6) Why does one come before the other?

I too am churning for learning...

Randy - RPM

[CutterBill]

I've looked through my reference books and I can't find it, but I seem to recall reading somewhere long ago that an average piston speed of 5000 ft/min should be considered as an upper "practical" limit for a piston engine. The reason escapes me, but I think it had something to do with ring flutter, cylinder wall lubrication failure and other such dastardly events. Obviously, highly-tuned racing engines (which our FJ motors certainly qualify) could be run at higher speeds, but this 5000 ft number was more of a practical design guide, rather than an absolute limit.

I thought for sure that it would be in my copy of "The Design and Tuning of Competition Engines by Philip H. Smith, but I haven't found it yet. Maybe it's in one of my WW2 piston aircraft engine design books.  I'll keep looking...
Bill

[Pat Conlon]

Kevin Cameron is my favorite technical writer and in this article he mentions 4000-4500 FPM as a traditional limit..
https://www.cycleworld.com/2016/01/25/plight-of-the-modern-motorcycle-piston-kevin-cameron-insights/

[Tuned forks]

Good article.

Joe

[fj1289]

FYI - with the stock 63.75mm stroke - here are the piston speeds at different RPMs (these are average speeds - max speeds will be more, but during mid stroke - not at the critical top of the stroke):

9500 RPM ~ 3,974 fpm

10,500 RPM ~ 4,392 fpm

11,000 RPM ~ 4,601 fpm


That's said, very few FJ/XJR motors have the mods needed to make any useful power at those higher RPMs ....

[Pat Conlon]

Chris, on a stock FJ engine,  do you think my guess correct, that valve float would be the first power limiter?
I see 3 frontrunners

1) Valve float (loft) or
2) Poor Volumetric Efficiency (head flow limitations) or
3) Durability (piston or rod failure)

[Tuned forks]

I'm curious about the rpm that peak horsepower occurs at?

Joe

[Pat Conlon]

How much is a bag of groceries? Depends on what's inside....

For our stock bikes the magazine folks already measured for us..
84/85 https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1100%2084.htm
86 https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1200%20%2086.htm
87  https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1200%20%2087.htm
89/90 https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1200%20%2089.htm
91/93 https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1200%2091.htm

[fj1289]

Pat, would it be possible to comb through the site and cherry pick the dunk sheets that have been posted along with the mods?  Would be a good one-stop thread for general info.  Unfortunately not a lot of before and after sheets that I recall, and each dyno on any day will read a bit different, but things like peak power or peak torque rpms, etc would be good info

[Tuned forks]

How much is a bag of groceries? Depends on what's inside....

For our stock bikes the magazine folks already measured for us..
84/85 https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1100%2084.htm
86 https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1200%20%2086.htm
87  https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1200%20%2087.htm
89/90 https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1200%20%2089.htm
91/93 https://www.motorcyclespecs.co.za/model/yamaha/yamaha_fj1200%2091.htm

Thank you Patrick.  Seems there is little reason to rev a FJ beyond 9k.

Joe

[Pat Conlon]

....Seems there is little reason to rev a *stock* FJ beyond 9k *while riding on the street*

FIFY ^^^

I've learned at my age, it's all about bladder control....

[Tuned forks]

^^^^^
Sphincter control too. 

Joe

[racerrad8]

Thanks guys for doing a good amount of work, but the questions are still there for Noel to answer. He is the one who made the statement that the limiting factor of the FJ engine is piston speed.

I think you'll find piston speed is the limiting revs factor for a road going FJ engine.

Noel

All of this research is just speculation without Noel's basis for his statement and the answers to all of the questions posed to him as follow-up.

Randy - RPM

[Pat Conlon]

Welp, by cunningly adding the words "I think..." it qualifies as non declarative sentence ...