When I learned about VE ratios and RPM's per mile and BMEP was at Honda
motorcycle school. I had some preconceived beliefs about running engines
too slowly that were shaken up by Mr. HONDA'S brief quote "All other
factors being equal, rpm's are free." He went on to say that modern engine
design in air cooled motorcycle engines required waaayyy oversquare
configurations. That is where the bore is larger than the stroke, and more
small cylinders are better than fewer larger ones.
Examples were given to us in what was known as the Hailwood Sixes. Not
many of them existed, probably fewer than 100 were ever produced. None ever
ended up in privateer hands, they were exclusive to Honda Racing Division,
and factory riders. Mike Hailwood was one of them. Hence the nickname.
Those engines came "up on the cams" about 17,500 rpm, and redlined around
23,500 rpm. I had the opportunity to view those sixes in operation, they
made a sound all their own. More of a shrieking sound. The Yamaha 2 cycle
engines of the day were 2 cylinder and based on the RD street bike engines.
They realized about 9000 rpms and were prone to seizure if everything was
not perfect. On the starting grid, the 2 strokes were first off the start,
and that lasted until they ran out of rpms in first gear. At that point the
Hondas shot by them and just disappeared.
GMC application? The 403 is oversquare, the 455 is not. That is why
they like to rev, and the 455 likes to shake and not rev over 2900 or 3000.
Jim Hupy
Salem, Oregon
On Mon, Nov 30, 2020, 10:04 AM Matt Colie via Gmclist <
> I just have to get on and write this.... I didn't want to start the topic
> on my own for reasons you may understand or not....
>
> The 3.07 that was the original final drive in our coaches was wrong when
> they were built, but all the testing was done before the first Arab Oil
> Embargo. Speed limits were at least 70 everywhere and fuel cost was not a
> major issue. Then came 55 and all that went out the window. Even at 70,
> the 3.07 is a little low and GM knew that, but by the time they came up
> with the 3.42 number, nobody was interested.
>
> Many people have been stuck for years with the long standing preconceived
> misconception that running an engine slower is always better. Better in
> terms of both fuel consumption and engine life. Well, both are wrong and
> for pretty much the same reasons.
>
> Even in as little as the 50 years ago in my engine-engineering classes, we
> were taught two wrong things. One was that Piston-Miles were everything
> that mattered and conventional carburetor was mass-flow devices and so not
> subject to density issues. Well both were wrong.
>
> At that time, piston miles was the primary engine wear determiner.
> Consumer Reports in their car data used to publish the N/V (that amounts to
> turns
> per mile.) Well, I hope to convince you that this is just wrong. A great
> deal of the engine wear is from internal friction. Most of that friction
> is directly related to the BMEP (Brake Mean Effective Pressure) that is
> what makes the torque that is needed to go down the road. Higher BMEP means
> more wear on the rings and bearings. People that have been around engines
> can tell you that lugging an engine is just bad for everything. Modern
> trucks are all geared with “Torque Backup”. They are geared so at most
> road conditions they are operating over the torque peak. This provides
> the reserve so they don’t lose speed and have to downshift for every
> little hill. This works well for us too.
>
> First thing you have to wrap your thoughts around is that a more or less
> typical passcar has a road load of about 10~12Hp. I do not have numbers, but
> from decades of vehicle testing I would have to guess that the road load
> for 60~70MPH to be up there in the 30Hp region.
>
> If you start up with a ~3K# passcar, with a ~100Hp engine, you have a very
> different situation than we do with a 10~12K# with an 250+Hp. Though that
> may sound equitable, you forgot the 30Hp road load we still have to
> manage. If that passcar engine is running below its torque peak, who
> cares? Just
> bump the throttle a little. If my coach is dragging on the hill and I
> bump the throttle, two things are happening. First is that the BMEP is
> going
> to climb putting more load on all the wearing parts. Then, the carburetor
> will probably move into the power enrichment part of its operation. There
> goes the fuel economy!
>
> Ok, so what is the loss for spinning the crank faster? Pumping losses.
> An internal combustion engine is really just an air pump that burns fuel.
> As
> it happens the torque peak is closely associated with the peak Volumetric
> Efficiency (VE). When you spin it faster or slower, (with a fixed camshaft)
> that efficiency goes down. For an engine that is not set up for all out
> maximum power output, that curve can be relatively flat. That means that
> there is little penalty for spinning faster and, then there also is less
> wear on all the parts that you have to replace at some time.
>
> Real fuel consumption is all in #/Hp-Hr. Notice that there is no element
> of crankshaft speed there. This will be on the engine map, and it will not
> be a straight line. It will have a little hump at the VE peak, but for
> engine like ours, that will not be any big issue.
>
> Now we go back to the input side. I (personally) have to shift gears here
> because in engine labs we don’t have vacuum, we have manifold pressure.
> The two are inverse. I will try not to confuse these.
>
> Whatever you fuel delivery control is, you want to deliver the minimum
> amount of fuel to create the horsepower you want. While the vacuum is high,
> there is not much charge in the cylinder, so not much heat is created even
> though the leaner mixtures actually burn hotter. Economy wise, this is all
> in your favor. But, when the vacuum gets lower and you have more charge
> in the cylinder that heat can be damaging, so the fuel system richens the
> mixture to keep the temperature in check. As backwards as this may seem
> to some, you would want to make road load horsepower with the vacuum as high
> as possible. That means that the engine is doing less work and making
> less friction all the way trough the driveline. That is a Win-Win for you.
>
> Let’s take a minute to look at carburetors. This piece of nineteenth
> century technology survives even to this day because it is relatively simple
> and inexpensive to manufacture. It has been displaced because it is not
> known for its overall accuracy. Everything it does is a compromise. So, if
> you change air density, the calibration will probably not be ideal. Why
> does this matter in this discussion? Because the less manifold vacuum that
> the carburetor is seeing is also the most varying mass flow. That varying
> mass air flow means that the carburetor calibration will have a much harder
> time metering the fuel input. I would like to go into this more
> completely, but just to explain it adequately requires more time and
> mathematics than
> I have patience for right now. Suffice it to say that the steadier air
> flow of the higher vacuum will give better results.
>
> I am going to break this off here because I have other things to do
> today. If there is some part of this where I lost you, please say so
> because you
> are probably not alone.
> --
> Matt & Mary Colie - Chaumière -'73 Glacier 23 - Members GMCMI, GMCGL,
> GMCES
> Electronically Controlled Quiet Engine Cooling Fan with OE Rear Drum
> Brakes with Applied Control Arms
> SE Michigan - Near DTW - Twixt A2 and Detroit
>
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