Streamlining CBR600 for Bonneville & Vetter Challenge

[renault_megane_dci]

Originally Posted by redyaris

The simple way to reduce duration and valve over lap is to increas valve clearance.

Wich is only really suitable on engines with rocker arms.
I believed the CBR 600 was a DOHC with the cams on top of the valves.

Anyway, cam phasing sounds like it has more potential (angle wise) and is not detrimential to durability wich valve clearance increase can be.

Of course cam phasing needs the cam sprocket to be trimmed wich is not always possible and always more fiddle than enlarge valve clearance on a rocker arm setup.

Originally Posted by redyaris

The stock final drive is 15T/44T, the furthest [available] from that is 17T/34T.
At 70 mph the change in final drive will drop the rpm from 5200rpm > 3500rpm.

That is a nice range of ratio !
I wish I could have such a potential (15/43 to 16/37 tops)

[serialk11r]

b

Originally Posted by renault_megane_dci

Wich is only really suitable on engines with rocker arms.
I believed the CBR 600 was a DOHC with the cams on top of the valves.

Anyway, cam phasing sounds like it has more potential (angle wise) and is not detrimential to durability wich valve clearance increase can be.

Of course cam phasing needs the cam sprocket to be trimmed wich is not always possible and always more fiddle than enlarge valve clearance on a rocker arm setup.

I think reducing lift isn't that big of a deal since at idle you're pretty screwed no matter what in terms of charge motion and bikes run at pretty high rpm otherwise.

The cams are chain driven right? I guess the teeth are quite big then. Bike cams have a giant amount of overlap, so you could probably get away with just shifting the cam position by one entire tooth, but if that one entire tooth is like 20 degrees then the later intake valve opening could induce quite a bit of pumping loss at higher rpm. From looking up CBR600RR cam pictures on google though, I'm counting like 36 teeth or something on the cams, so one entire tooth might not be so bad at 10 degrees. The later valve closing would push the power peak up quite a lot though since bike cams close pretty late already...might not be bad for top speed run

[renault_megane_dci]

36 tooth sounds familiar for cams in general.

I might be wrong but I think whatever you do on a motorbike engine is detrimental to top end, since those engine are very optimised.

Delaying the inlet would have the effect of "atkinsoning" the engine though, wich should be good for MPG but bad for high speed.

QUESTION :
Is your goal to vastly improve FE while not affecting top speed ?
OR
Is your goal to reach a given MPG (100 IIRC) and try to get as much speed as possible ?

If you are looking for the second option, the compromise is gonna be trickier I think ...

[serialk11r]

Originally Posted by renault_megane_dci

36 tooth sounds familiar for cams in general.

I might be wrong but I think whatever you do on a motorbike engine is detrimental to top end, since those engine are very optimised.

Delaying the inlet would have the effect of "atkinsoning" the engine though, wich should be good for MPG but bad for high speed.

QUESTION :
Is your goal to vastly improve FE while not affecting top speed ?
OR
Is your goal to reach a given MPG (100 IIRC) and try to get as much speed as possible ?

If you are looking for the second option, the compromise is gonna be trickier I think ...

I don't think this is true. Anything with fixed cam profiles isn't optimized anywhere because it's a compromise for everything. With phaseable cams you can have 1 optimal operating point (for power), and an optimal "curve" for efficiency. For example the CBR600RR has a 15k+ rev range, peak torque at 11k, peak power at ~13k. BMEP at power peak is actually somewhat poor considering how much overlap is available.

Car engines with close to zero overlap at the top end can achieve higher BMEP at peak power (typically 100Nm/L ish), though car engines don't rev as high so it's a little easier to hit higher BMEP, though if you look at liter bikes the story is still the same. By retarding the intake cam on a bike, the "midrange" (which is like the 7-11k range or something lol) will suffer a bit but the very top end probably gains a little. All in all, a peakier powerband, but possibly more peak power, and less torque down low but better combustion stability from less overlap. At least that's what I think should happen.

Bike engines have so much duration that they're essentially already "Atkinsoned", but there's way too much overlap for good part throttle efficiency.

[sendler]

Power band and over lap is not just a matter of cam timing. Cam duration is the same regardless of where you time the cams and could only be optimized for a lower rpm with a re grind. Changing the timing could help though.

[redyaris]

[QUOTE=renault_megane_dci;338709]Wich is only really suitable on engines with rocker arms.

Not true[/COLOR]
Cam phasing done right requires more time and effort than I am willing to spend on this project. It would be cheeper and better to get a cam regrind with less overlap and duration. I doubt I will do either...

Doing what I can with what I have is the objective.
Mostly with stream lining...

[Michael Moore]

Originally Posted by serialk11r

I'm counting like 36 teeth or something on the cams, so one entire tooth might not be so bad at 10 degrees.

The cams turn at 1/2 engine speed so 10 cam degrees is 20 crankshaft degrees.

It usually isn't difficult to slot a bolt-on cam sprocket to allow timing adjustments, and there are aftermarket sprockets for many sport bikes that come pre-slotted.

cheers,
Michael

[serialk11r]

Originally Posted by sendler

Power band and over lap is not just a matter of cam timing. Cam duration is the same regardless of where you time the cams and could only be optimized for a lower rpm with a re grind. Changing the timing could help though.

Sure, but the fact is that the lift and closing points appear to be much more important than the overlap or else car engines wouldn't be making much power. Scavenging helps but the closing point is matched to the intake harmonics which has a bigger influence on volumetric efficiency.

By moving the cam the idea is obviously not to optimize it for a lower rpm, it's to reduce the negative effects for efficiency and combustion stability at lower rpm of having large overlap (which possibly further trades off more low end torque), and possibly increase top end power (would depend on specs).

[renault_megane_dci]

What a crowdy audience about this engine tuning thing !
Good thing RedYaris tolds us he doesn't want to mess with the engine internals.

[redyaris]

I suspect that we can agree that a 4 cylinder 600cc, 85hp @ 11,000rpm motorcycle engine is not the best choice for a fuel economy bike, and no amount of internal engine modifications will do all that much good without spending more money than it is worth. One would be much further ahead buying a used bike that is more suited to the task.
As I own the bike allready and my choices are sell it for peanuts, watch it rust away, or streamline it and see what that does, I have chosen to streamline it to see what a sport bike would perform like today if the FIM had not baned dust bine fairings in 1957...
If I can get close to 100mpg in the Vetter challenge, and get into the 150mph club at Bonneville then mission accumplished.