Cam duration, overlap, valve closing and all that jazz

[Gaanja]

I was just comparing the Honda CBR 600 and a Suzuki GSX R. They have the following attributes:

CBR 600:

Intake:
Valve open: 21 deg BTDC, Valve close: 45 deg ABDC, duration : 245 degrees
Valve Diameter: 27.5 mm
Max Lift: 8.3mm

Exhaust:
Valve Open: 40 deg BBDC, Valve Close: 5 deg ATDC, duration: 225 degrees, valve overlap: 26 deg
Valve Diameter: 22mm
Max Lift: 7.2mm
Compression Ratio: 12.2:1
Bore x Stroke: 67 x 42.5mm

GSXR:

Intake:
Valve Open: 38 deg BTDC, Valve close: 66 deg ABDC
Duration: 284 deg
Valve diameter: 27.2mm
Max Lift: 8.2 mm

Exhaust:
Valve Open: 57 deg BBDC, Valve close: 29 deg ATDC
Duration: 266 deg, Overlap: 67deg!
Valve Diameter: 22mm
Max Lift: 8mm

Compression Ratio:12.8:1
Bore x Stroke: 67 x 42.5mm

All the specs of the 2 engines seem to be some what similar except the maximum valve lift of the exhaust vale and the valve timings.

As far as my understanding goes, these engines which are meant to operate in the high RPM range (>10k) usually have a bad volumetric efficiency and torque in the low and mid range speeds and also causes lumpy idling due to 2 reasons (especially when restricted)

1) When the intake valve closes later it pushes some of the intake charge mix back into the plenum when the piston comes up.

2) Overlap compounds this by hanging the exhaust valve open and allowing reversion from exhaust to shove more intake charge back into the plenum

The usual remedy for this in FSAE is to reduce the overlap period and to close the intake valve faster.

When comparing the valve timings of the GSX and the CBR, the GSX has 41 deg more overlap than the CBR and the intake valve closes 21 degrees later.

Either this should imply that the a restricted GSX must be way worse than a restricted CBR and cam mods are necessary to get even a reasonable amount of performance from the GSX at the low and mid range and make the car drivable, or I have understood it all wrong and have missed something big.

I was also of the opinion that these cam timings differences should also reflect in the stock dyno curves when the engines are of comparably similar geometry. If i see the stock power and torque curves of the two engines the curves are almost similar.

Would be great if someone could clarify this for me.

[Mbirt]

Ah yes, you have discovered one of the greatest difficulties in comparing camshaft specifications--not knowing the lift at which the valve events are measured. I found the same numbers for a 2006 GSXR600 at http://www.aperaceparts.com/tech/06gsxr600.html. If you look closely, you will notice that the event angles were measured at .3 mm lift. Valve events are usually given at 1 or 1.25 mm lift, not .3 mm. The GSXR600 event timing is likely much closer to the numbers you gave for the CBR600 at 1 mm of lift.

[Gaanja]

@Mbirt:

Thanks for the info, but what you just told me raises more questions:


If you look closely, you will notice that the event angles were measured at .3 mm lift.

Does that mean the ramp up from actual 0 mm lift to 1 or 1.5 mm lift is more gradual on the crank angle march for the GSX than the CBR (i am guessing it should)? If yes, does that have any significant influence on the gas dynamics? I also noticed that the Honda uses a pent-roof combustion chamber design and the GSX uses a twin swirl combustion chamber. Do any of the differences in the cam attributes help in induction in the two different chambers? If so is it possible to figure out which part of the cam lobe influences flow in what way in the 2 types (by just looking at the lift vs duration plot)? I would just like to know if certain things about the cam lobe profile need to be kept in mind when changing cam profiles for the two engines.

Thanks again...

[Jay Lawrence]

It doesn't necessarily mean it's more gradual. It just means that within that difference in duration there is about 0.7mm lift unaccounted for. This 'could' mean that the cam profile on one is more aggressive than the other (has more follower acceleration for a given RPM) which 'could' affect high RPM stability (valve float etc.).

Your valve lifts as they are are probably too large. We found significant advantage by running lower lift (same duration from memory). Lower lift can help with idle and knock resistance (it did for us). I don't know anything much about chamber roof profiles, except that they are shaped in a certain way to deliver the right combination of squish volume and swirling, and i guess Honda and Suzuki have different ideas.