Engine Tuning

This page is here to dispel some of the myths associated with 2 Stroke engine tuning, without being too specific.

Engine tuning takes on many aspects.  Most commonly, in 2 Strokes, people assume porting to be the first and most effective method of 2 stroke tuning.  Porting takes on different methods i.e. porting can refer simply to modifying the crankcases to improve the flow of fuel/air from the carburettor to the cylinder.  Porting more commonly means to optimise the exhaust and transfer ports to flow more fuel/air into and exhaust gasses out of the cylinder.  Using this method of tuning has to be dealt with carefully for various reasons.  Sometimes porting would be the first and most effective method to gain extra performance from an engine but sometimes it would be the last and least effective, other aspects of the engine have to be considered first, ‘carb’ size and engine application to name but a few.  If appropriate and if there is scope to widen the ports, that is always the first step to take.  If not you have no choice but to increase the height.  Widening is a win win situation as there are only benefits to be gained.  Raising height inevitably results in a loss of power lower in the rev range.

The same principles apply to the transfer ports but additionally the transfer ports are designed to control the direction of the gasses entering the cylinder.  Changing the direction of the flow control is very unwise.  Transfer ports direct the fuel/air into the cylinder in specific directions, the idea being to prevent as much charge as possible escaping down the exhaust port and into the pipe.  Some gas escape is inevitable but the Transfer ports are designed to keep it to a minimum.  Transfer ports have various features all designed to manipulate the charge flow to achieve various power characteristics.  The “Cag” Chinese engines have a very crude and basic Transfer port design.  These engines are derived from chainsaws, the engines from which are designed to operate at constant throttle speeds, not on/off throttle applications.  When it comes to racing engines manufacturers and tuners employ more sophisticated designs.  Transfer ports in racing engines have angled port roofs, angled port walls, kickers and the port durations can vary.  The Blata water cooled engines are a good example of an engine that employs all the above Transfer port tuning tricks!

Tuning takes on many other aspects too,  “porting” is far from the only consideration.

I’m bored of the sound of my own voice banging on about the importance of getting the Multi Meter Carburettors set up.  There are however serious gains to be had.  I see it on the ‘dyno’ every week with bikes that have had a carburettor fitted that was acquired from a dubious budget source and the settings are nowhere near what they should be.  I have seen 3hp differences from the first to the last ‘dyno’ run! But, horsepower is only part of the benefits.  Even when there isn’t serious horsepower gained, the transition from low to high revs can be made much quicker and the power curve much smoother.  This has to be the first aspect of tuning, getting the carburettor set up properly.  If your budget is limited get this sorted first.  With carburettors acquired from budget sources the chances of it being calibrated to the requirements of your engine is slim, very slim!  It will inevitably cost you more in the long run or more likely you will simply give up!  I saw a genuine 0.5hp gain on the day of writing this article just from changing the atomiser bush in a carburettor.  This part retails at £9.34.

Something very rarely mentioned is the speed at which the engine gets to its power peak.  I’ve seen engines that made as much power as the best engines out there.  Trouble was the length of time they took to achieve it.  Most Dyno’s will measure not only Torque and Horsepower but also the time the bike or engine took to achieve it.  I place as much emphasis on this as I do on the final power output figures.  Very often I’ll even sacrifice some peak power to achieve a faster timed run.

Cranks are a more and more common tuning method.  The original Pocketbike cranks on Polini and BZM engines had a stroke of 39.2mm.  On the 50cc engines these left the engines below their permitted capacity and therefore the “red” and “gold” cranks were introduced.  Increasing capacity by this method achieves gains all round.  Its  what tuners often refer to as “free horsepower”  What they mean by that is that there are gains without drawbacks.

We also need to consider bolt on accessories of which there are numerous.  Some live up to their claims, some don’t!   Pipes are a common ‘bolt on’ performance tuning part.  The fact that a certain 50cc pipe worked very effectively on the 3 port 40cc BZM engines is pure coincidence.  It doesn’t necessarily follow that all pipes labelled “50cc pipe” will reward 40cc engines.  The clue is in its label, “50cc”!  They are primarily designed for the 50cc engines, they’ll work on a 40cc engine but might not be the best for their application.

As mentioned above, there are numerous ‘bolt on’ accessories from numerous manufacturers, BZM, Blata, Polini, Zocchi and more recently from GM Phantom and their “KO” race accessories and the “OR” engine range.  In short it’s about having the right combination of parts.  If, for example, you upgraded your crank and decided to splash out on one with a billet rod, you need to look at what those billet rods are designed to achieve and apply those principles elsewhere on the engine and bike.  Another, more simple example would be upgrading your ‘carb’ from a single meter 14mm Sha to a multi meter ‘carb’  There would be instant benefits but you might not gain the full benefits without considering the aperture in the manifold!  Then you’ve got to consider the reed cage and reeds.  There might be a 1.5hp gain from the ‘carb’ alone but there might be even more if a specific reed cage and reeds appropriately set up were used.

Tuning requires a great deal of thought and use of appropriate methods/parts.  This article/page is designed to give the reader some perspective into how an engine tuner thinks about such matters.   Many consumers are suspicious when tuners suggest upgrades, seeing their motives as financial.  I cant speak for other tuners but DMR is only interested making engines and bikes as competitive as possible within the consumers budget.  I want to make all engines as fast as possible.  Often the best way to approach a tuner is to state a budget limit and give them license to get on with it, an experienced tuner will be able to apply that to the individual in the most effective way.