Team Motorcycle battle plan

[alvaro84]

Is there a way to tell if there's an overlap without taking the engine apart and analyzing what's inside? Just to know if our bikes are infected with this diabolic disease...

[redyaris]

Almost all engines designed for rpm's above 1000 have valve overlap. I have modified cams on small Honda GX160 industrial engines that are designed to run at a constant 3600rpm. the problem they had was that with the govener removed for Kart racing, they would only rev to around 6000 - 6300rpm before they went flat on power. This was designed into the cam timing so that if the govener failed the rpm would be self limiting. After modifiing the cam I could rev to over 7500rpm and gain about 25% more power. Unfortunatly with the splash lubrication system they would go BANG after about 5 hours of operation when the conecting rod would brack due to the lack of lubrication at the crank shaft jurnal!
The basic problem is matching the cam timing to the engine speed to optemise the air/fuel flow through the engine. It is posible to design an engine that has very high rpm, large amounts of valve overlap and very good fuel economy. but it would only be good for a narrow range of rpm...
If you wanted to design a motorcycle engine for better fuel economy you would start with a long strock engine that did not need very high rpm to produce the power you needed, so that there is less rpm change with change in road speed, so that the engine stayed very close to its optimum rpm for FE most of the time...

[renault_megane_dci]

Originally Posted by redyaris

If you wanted to design a motorcycle engine for better fuel economy you would start with a long strock engine that did not need very high rpm to produce the power you needed, so that there is less rpm change with change in road speed, so that the engine stayed very close to its optimum rpm for FE most of the time...

You're basically summing up what a Harley Davidson engine is ...

Correct me if I am wrong but there is a direct link between :
the speed you require / the fuel you are using whatever your engine type is.
The link is in the losses.
Pumping loss if your throttle is opened less than 3/4
Friction losses inside the engine.
Thermal losses.
Aerodynamic losses that are to be overcome
Gear reduction that is required to match the speed you require.

I believe the optimal is an engine that hit posted speed limit with 3/4 of throttle in top speed with as low an rpm as feasible and as good an SCx as possible.

Ideally the "as low an rpm as possible" item is treated in such a way that you are not too far of peak torque (which is also an indication that you are close to the best engine efficiency) which ultimately requires smaller duration.

[alvaro84]

Originally Posted by renault_megane_dci

You're basically summing up what a Harley Davidson engine is ...

Or a Honda NC700 engine (I2, 73mm bore / 80mm stroke, 670cc, compr. 10:1). I'd like to try and compare it to the BMW F650 (single, 100mm bore, 83mm stroke, compr. 11.5:1), which can give me pretty nice FE. The new Honda has a very tall 6th gear (like 3000rpm at 100km/h), and works well at low rpms, as I heard. The engines are quite different, the bikes have many similarities. It would be a nice match.

Or a Yamaha Virago 250 (V2, 49mm bore, 66mm stroke, 249cc, compr 10:1)! That, being smaller, with a properly programmed EFI, would be a very good start for FE, I guess.

[Electictracer]

Originally Posted by redyaris

The basic problem is matching the cam timing to the engine speed to optimize the air/fuel flow through the engine. It is possible to design an engine that has very high rpm, large amounts of valve overlap and very good fuel economy. but it would only be good for a narrow range of rpm...
If you wanted to design a motorcycle engine for better fuel economy you would start with a long strock engine that did not need very high rpm to produce the power you needed, so that there is less rpm change with change in road speed, so that the engine stayed very close to its optimum rpm for FE most of the time...

Excellent point. Bikes like my Buell do this rather well, up to 60mpg out of 1200, same with BMW 1200s.

This cam idea would not work well for town driving for the reasons you describe above, but people like myself who do most of their miles could potentially see good gains in the rpm range that they cruz at. Like you said, the problem is getting that rang right.

Could a stock cam be ground to achieve the desired effects or would you need to start new with a blank?
If It could be ground, it seems like it would be worth while research to play around with this and possibly come up with a formula that could be duplicated on a new cam.

Just an idea...

[renault_megane_dci]

The issue is bike were dedicated to performance above anything else and this wastes fuel.

What we are doing (or attempting to do) is change the target altought the DNA of the bikes does not help us.
For example there is almost always a match between top speed and max power rpm.

Things are changing (Honda's NC700) but from the upper side of the tange (a NC 700 still has a very favourable power to weight ratio).

Redyaris's problem with the mixture of his streamlined VT 500 is a very good example of this.

I don't know if we can find a solution in the camshaft alone.

For me the biggest issue is the potential in gear reduction is very poor.
But I have a GN and I live in Europe so ...