Choosing Your Power Plant

Here is some basic information that might help you to make an informed decision on the right engine for you.  Of course nothing is simple and there are lots of variables like gearboxes (or transmissions), final drives, chains, belts and shafts, tyre sizes and the type of driving you’re doing.  I’ll try and go through them and how they fit with your buggy build in subsequent articles.

Power and Torque

When choosing an engine it’s tempting to just look for the one that makes the most power but then you’ll hear “you want low down torque” or the famous “horsepower sells cars, torque wins races”.  Fact is power, torque and RPMs are related and interdependent:

Horsepower = (rpm x torque (lbft))/5252  or  kW = (rpm x torque (Nm))/5310

Checking out the maths you can see that power is related to torque by the RPM of the engine divided by a constant.  This EPI Inc. article provides a nicely detailed explanation of the relationship.

Different engines produce torque at different RPMs and this is a factor of their design, this in turn affects the amount of power they make and the location of maximum power in the rev range.  The most common and probably the easiest way to look at these relationships is on graphs or charts.  The article mentioned above provides some explanation on these as well.  The take away message here is don’t make a decision based on max power and torque numbers; you need to see the torque curve of the engine you’re looking at.

The Motus MV4R has a nice flat and fat  torque curve (there is lots of area under the torque curve on the graph meaning lots of toruqe everywhere in the rev range) and a resulting linear power curve.  This engine will not be peaky with nothing down low and everything up top or a sudden rush of power; this should make it easier to drive and drive fast.  It produces excellent torque from low in the rev range and is ideally suited for an off road buggy.  This engine is not from a hypersports bike so it’s worth looking at other bikes that may not have the highest max horsepower figures but have the fattest and flattest torque curves which is what we want for our buggy application.

Things to consider:

• We usually have low traction (grip levels).  Low revs and high torque are likely to result in more traction and acceleration than high revs and lower torque (similar power), especially when starting or at low speeds when there is a big difference between the engine and drivetrain/tyre speed.
• It can be very busy in the cockpit and you don’t what to be changing gears every 5 seconds to keep the engine pulling.
• Course conditions change constantly off road and you want to be able to accelerate at any time without having to change gears or having the engine bog down.
• Buggies are generally heavier and have more rolling resistance than bikes from which many engines are derived but are lighter than potential car engine donors.
• Jumping and rough terrain puts a lot of stress of the drivetrain components especially with large tyres.

Some helpful generalisations:

• ‘Hypersport’ bike engines are short stroke and produce high power by making high revs with lower torque.  They therefore tend to have ‘peaky’ power delivery with most of the action up the top of the rev range.  They have lots of gears and fast sequential gearboxes to mate with the engine’s characteristics and they are not super strong.  They don’t have reverse either.  This isn’t ideal for us but they are light and compact which is a big plus.  They are also very responsive to throttle inputs which is great for throttle control.
• Diesels produce high torque at lower revs, they can pull heavy things but not very fast.  They have very high compression and given this and the high torque levels are built strong, which translates to heavy.  They usually have fewer gears and the boxes are strong but slow.  Modern forced induction diesels are changing this but they aren’t really suitable for superlite buggies yet.
• Car engines are somewhere in between and smaller varieties can certainly be suitable.  They are usually bigger and heavier and their packaging is not as easy to adapt as motorbike derived engines.  Their gearboxes are usually bigger and heavier synchromesh types that are not as fast as the dog engagement sequential boxes on most motorbikes but they all have reverse.  Performance varieties are not as common as in bikes.
• Engines with longer strokes produce more torque for a similar capacity as they have longer levers to turn the crank with and have longer power stokes (the time where the combustion power is pushing down on the piston).  This also makes them lower revving.  Fewer cylinders with longer strokes (say V-twins) usually produce more ‘low down torque’ than in-line 4’s (L4) of a similar capacity for example, but also rev less and consequently often have lower maximum power figures.  These engines might produce more useful torque for buggies though.
• Forced induction engines produce more torque in a similar way to long stroke engines.  They pack more charge into the cylinders allowing more power to be produced as the burn is longer and hence the force pushing the pistons down goes for a longer portion of the power stroke.  The most common method of forced induction is turbo charging which has the downside of poorer throttle response and also less low rpm power production.  Modern turbos are much better but it’s there; direct driven supercharges offer a significant advantage in this respect.

My wish list for a buggy engine and gearbox combo:

ESSENTIALS

• Lightweight compact design.
• 6 speed sequential dog engagement gearbox or CVT or synchro box (in order of preference).
• High torque across the rev range or a fat and flat torque curve (or the most area under the torque curve).
• Packaging that suits buggy installation.  This will depend on whether you run chain, belt or shaft and a diff or not.  For my purposes its chain or belt.
• Readily available without the need for extensive mods for performance or installation.
• Strong engine and gearbox.
• Good parts and replacement engine availability.

REALLY NICE TO HAVE

• Reverse in the g’box.
• A dry sump.
• Factory supported supercharger or turbo options

Likely choices: