SOHC.co.uk Forums > CB750
How do bikes know when to break?
MarkCR750:
I had asked him to keep the revs down, because it's an old engine that I had not done any internal work on, however curiosity got the better of me and I asked him to give it one more run to 9k after the print out, it didn't produce any more power to speak of though, which makes sense with the red line being just over 8k i.e. it's tuned to make max around there, I'm still trying to figure out if Hondas quote of 67bhp is crank bhp?, if that's the case then mid to high 50's is what you'd expect at the rear wheel after drive train losses, depends which site you read but the majority say that Honda quote crank BHP, the bike feels quite quick but that could be down to all the weight it has shed, I am interested to know if it's making a lot less power than it should due to the restrictive silencer or if we're getting confused with crank bhp, has anyone dynoed a stock bike?.
Trigger:
A engine loses BHP the more wear the engine has. I thought I explained this when we met mate. I would say that the original BHP was done at the fly wheel, as Honda would pull engines from the production line for testing.
MarkCR750:
--- Quote from: Trigger on May 21, 2016, 10:12:11 PM ---A engine loses BHP the more wear the engine has. I thought I explained this when we met mate. I would say that the original BHP was done at the fly wheel, as Honda would pull engines from the production line for testing.
--- End quote ---
They do Trigger but not that much, provided there is no major wear and the piston rings are sealing etc, I had my 136k mile Alfa GTV dynoed and it made 152bhp, I had de-catted it but that's only good for 10bhp, when new Alfa stated 150bhp so it had hardly lost anything in all those miles , I just found this from a guy in the states that builds and races CB's
"I have not got my 836 engine on the road yet so it tough for me to make any comparison between the two. In top stock form a cb750 turns about 56hp while I've read dyno reports that an 836 will turn close to 70hp at the rear wheel"
So I recon mine is making pretty much stock power , it ties in with Greebos 56bhp at the rear wheel when dynoed, I still won't be fitting a louder/freer flowing exhaust though, it might give another 5bhp? but I value my eardrums 😀
PS your right about Honda and other manufacturers quoting crank BHP, I have now read that the rules allow them to quote "maximum engine power" so they use crank dynoes, therefore the figures they quote are only useful as "comparative" figures and aren't "real world" as they ignore frictional losses in the drive train which typically are between 10 and 14 percent .
royhall:
Most automotive companies use crank output so can't see Honda being any different as they would seriously loose out on the PR game. My CB1300 has a full titanium Akrapovick (how do you spell that?) system with power commander that has a custom map, and that only just managed stock output. So I just gained back the transmission losses proving they are crank rated.
MarkCR750:
Yes I've since read that Honda do indeed use crank bhp, what was throwing me was my XJR1300SP which is quoted as 106 bhp by Yamaha , I had it dynoed and it came out at 104 pretty much bang on, so either yam are being strangely honest for once or CMC calculate back to crank HP, I'll have to ask them , I subsequently changed the intake rubbers for FJ1200 items and it made 112, total waste of £70 for the rubbers cos the bikes all about torque not BHP, but I was younger then (2 years ago 😀), Sam's Dyno is rear wheel BHP (thankfully), anyway it's running nicely that's the main thing, been out on it since 7am must go home some time today!.
Navigation
[0] Message Index
[#] Next page
[*] Previous page
Go to full version