Project 100mpg

[Pinhead]

I have driven a 1979 Honda CB650 SOHC4 (626cc) motorcycle pretty much every day for the last 3 years or so. My average gas mileage since day one has been 55mpg. IMO this is absolutely pathetic considering the horsepower level (~60 hp). Peak torque is at 8000 rpm and peak HP is at 9000 rpm. With my current sprockets, 70mph is 4500 rpm.

In the bike's current state, it smokes, rattles when it's cold, has to be choked to start, hesitates off idle, and runs rough under load below 2000rpm. Needless to say, it needs some work. However, even in it's current state it gets the same MPG as it did when it was new.

Here are my plans and goals:

1) Increase spark power.

--I'm going to build a DC-DC converter/regulator to increase the voltage going to the coils to between 16 and 18 volts. Temperature will be monitored to make sure they don't melt. I'll also have an extra set just in case they give up the ghost.

--Currently the bike employs a transistor-switched Kettering system with no vacuum advance. I may consider a programmable ignition (with vacuum advance) if it's not too terribly expensive. DV has suggested that a proper vacuum advance curve could net up to 20% better mileage.

2) Increase fuel atomization.

--DV has written an article on modifying side-draft carbs to emit a "fog" instead of droplets of fuel. Hopefully I'll be able to adapt the concept to my Kei Hin's.

3) Increase port vaporization and reduce wet flow.

--Mike Holler (MPGMike) developed "PowreLynz" which greatly enhance wet flow characteristics of a relatively un-developed intake port. The ports on this bike, while better than decent, can still use some work in the wet flow department (especially when cold).

--Also, the engine has the typical old-school round ports; filling the floor just before the short-turn and roof above the valve, and widening just around the short turn radius should help with port efficiency and hopefully wet flow.

--"Surface Turbulence" valves will be employed to help further.

4) Increase in-chamber turbulence.

--This will require the most planning and fore-thought (as much as the port work); clearances seem to be pretty tight in the chamber. However, the Hemi-style chamber is practically devoid of turbulence until the RPM's get up. This can cause pinging with raised compression, and definitely attributes to the lack of low-end torque. I'm going to try to use the perimeter of the piston and chamber as a squish ring to help with in-chamber turbulence.

5) Raise compression to increase thermal efficiency.

--Normally when raising compression, one has to worry about pinging and overheating. The above mods should reduce detonation tendencies and also decrease heat soaked into the engine.


All of these mods do two important things: decrease detonation and increase lean-burn limits. My starting point for mileage is 55mpg at 70mph. My goal, as stated in the title, is 100mpg at 70 mph with no loss of horsepower. Considering that there are many cars out there that produce more horsepower in a heavier car with more wind resistance and still get 50 mpg, the goal should be doable.

I've discussed topics similar to this at SOHC4.net, however most (if not all) of them are still stuck in their bikes era; they don't think any gains are possible, even to the point of completely dismissing the possible gains of a simple vacuum advance curve.

I am open to suggestions to what else I should add.

[Pinhead]

More crap from the "experts" at SOHC4.net with regards to vacuum advance:

First and formost is what works for a water cooled iron block engine doesn't always work for an air cooled aluminum engine

It doesn't seem like I'm going to get any good pointers or even any suggestions over there, so I'm just going to have to hope someone over here can give me some pointers.

[automotivebreath]

To get the mileage up you will need the complete the burn sooner. You are on the
right track, can you post pictures of the piston and chamber?

[Pinhead]

Click each picture to see a larger version.

[automotivebreath]

Poor surface to bore ratio. Does the piston dome match the chamber well enough
to create squish area? If so what's the piston to head clearance at assembly?

[Pinhead]

Originally Posted by automotivebreath

Poor surface to bore ratio. Does the piston dome match the chamber well enough
to create squish area? If so what's the piston to head clearance at assembly?

The outside edge of the chamber and piston already create a slight squish area. However, it looks very narrow. I don't know the piston-head clearance yet; I haven't yet disassembled the engine. The simplest thing I have found that I can do is get a thinner head gasket; losing half-thickness (0.6mm) will raise my CR from 9:1 to 10:1 and bring the piston closer to the chamber for better squish.

[automotivebreath]

Thats what i would do, you can use clay to see how well the piston matches the
chamber and make adjustments as needed. In addition I would cut one groove in
each piston on the opposite side from the plug from the parameter of the bore
on an angle where it points between the valves.

[Pinhead]

Have you used piston grooves? I was always under the impression that for some odd reason, the groove only worked on the cylinder head.... ??

[automotivebreath]

Yes, cut the groove so that it divides the squish area opposite the plug in half and aim
the groove just between the valves.

Another thing you should consider is to regrind the camshaft to reduce overlap.

[Pinhead]

Originally Posted by automotivebreath

Another thing you should consider is to regrind the camshaft to reduce overlap.

Wouldn't that be detrimental to power production??