Tuning Proven results, or a waste of time? There are some well-known and timehonoured traditions when it comes to tuning 2-stroke engines… but are they all being judged and applied within the correct parameters?
There are some well-known and time-honoured traditions when it comes to tuning two-stroke engines… but are they all being judged and applied within the correct parameters? If not, then a lot of people may be wasting a lot of time, effort and money!
What is tuning? Look it up on the internet and you will find this description: Engine tuning is the adjustment or modification of the internal combustion engine or Engine Control Unit to yield optimal performance and increase the engine’s power output, economy or durability.
For most in the scooter world an increase in engine power would be perceived as the endgame for such modification work. But what actually achieves that end result, from the multitude of potential areas for improvement? Will you achieve useful gains or just waste time, effort and money? Historically, many claims have been made for certain types of modification work, some claiming huge horse-power gains or tremendous torque attributes.
Transfer tuning
So for the first instalment we will look at what many would consider to be the most basic and commonly perceived area for improvement, which is matching and flowing of the aftermarket cylinder kit transfers to the crankcase. Many an internet forum post or discussion group on social media will see groups of owners discussing the merits of such work, with many who haven’t carried it out during fitting displaying their regret of not doing so, and discussing the resulting loss of performance they are now suffering. But how do they all
know this to be the case? Well it’s obvious really… it’s what two-stroke tuners have done for years, it’s in the tuning books, and obviously the kit manufacturers/designers would not design their kits with the bigger transfer entries if it did not yield useful gains. Hundreds of internet posts and pub discussions have said the same for years beforehand, providing this conclusion to how the two-stroke engine works. Then, add to this that anyone who has seen a good performance motorcycle cylinder design – be it for motocross, road racing or karting use – will tell you about the huge transfer entries they have and therefore conclude that the way to obtaining such a similar power or torque must require a similar design.
Put the theory to the test
So to put it to the test we have many development engines that we undertake back-to-back testing on, to evaluate the effects of minor changes, and to see how each engine or cylinder responds to various modifications. It becomes very addictive and hugely time consuming, but the results and information gained are well worth the effort. Strangely, it’s the areas you find that lose power that become your greatest learning curve, as they not only keep the power up by not doing them, but they also save you lots of unnecessary work in the future.
The test engine
At the time we did this, the Italian manufacturer Gori had just released a range of cylinder kits for Lambretta and at a reasonable price. We had the 175 cast cylinder TV replica kit for the small-block 125/150 case. I found it quite appealing as my latest ‘tuning puzzle’ to amuse myself with, so snapped one up and we decided to set about a new testing project. The engine had a new ignition fitted, along with a 28mm carburettor, and ran on a new but standard 58mm stroke, 107mm rod crank, freshly built with new bearings and seals fitted etc. So it was a healthy base motor, quite typical of many out there. The base motor came in at around 11bhp so quite a good result for the kit, probably picking up a bit of power from the good ignition and 28mm open carb, and gives an insight as to how well the TV175 performs, as that level of power is closer to that of a later (standard) Stage 4 SIL 200 engine unit. The testing then progressed through various tuning steps; exhaust duration changes were made, along with inlet timing and transfer timing changes, all leading to its final power output figure of 17.6hp, while running on a modified Clubman exhaust that we had been playing with too. The development stage had got to a point where torque had just started to drop off in exchange for horsepower gains so had reached/gone slightly past its natural stop point for me when tuning a road-going engine, so I had concluded that the testing was over at 17.6hp.
But! Before we finished off and called it a day, I then considered that as we were still on a standard case and cylinder gasket face (it’s how we do most of our builds) it would be a great opportunity to test the remaining areas left on the table, such as enlarging the case transfer feeds, along with piston modifications that were seemingly causing flow restrictions, plus some small detail changes normally associated with Grand Prix cylinders, and use these as a back-to-back test.
It’s not over till it’s over…
To start with, we began the comparison test with a set of base runs to set a datum, taking time to ensure that the motor was up to temperature, tyre pressure set, seating method comparable each run, etc. This is so that every variable can be matched like-for-like, as closely as possible, for each run. Then the cylinder was removed from the test chassis and work began on the cylinder, as can be seen in the step-by-step photo guide, making the various changes to the transfer entry areas. This test was quite interesting as the kit had the early type piston design, the one which appears to restrict flow quite badly into the transfer ducts, so not just a ‘feeling’ of tuning a cylinder, but when visually compared to what appear to be better designs, it looked like we were effectively de-restricting the
design (like taking the tube out of a moped exhaust header pipe). Surely these obvious restrictions would yield great gains! As it turned out, we ended up with quite a list of changes made, most of which are commonly done by a lot of tuners. Alterations implemented:
1. Cylinder transfer entry ‘square casting’ made radiused.
2. Cylinder transfer entry lowered to below base gasket level, to tie in with transfer flow over piston edge at BDC.
3. Cylinder spigot transfer cutaways widened.
4. Cylinder bore side transfer radius as per works RSA GP design.
5. Radius to spigot corners.
6. Piston skirt strengthening rib removed to open flow path into transfer area.
7. Piston cutaway radius chamfer added to inside and outside edges.
8. Piston cutaway widened to match spigot cutaways (see point 3).
The proof is in the pudding!
So with the tuning work done it was all refitted and the gasket face sealants left to dry. Forgive the final finish on the ports but this whole test from first test to porting to being reassembled took just one-anda-half hours to complete in between other jobs. The first road test carried out using the ‘seat of the pants’ dyno couldn’t detect any major change which is normally quite accurate, but the graphs will let us know the exact details, and if our labours will prove fruitful!
Upon first glance of the graph I could see there was hardly any change with barely 0.2hp difference at peak, which is practically nothing at all! I’ve added some markers to provide a start and end point to the graphs as I try to start the dyno run at the lowest rpm as possible. So only 0.2hp gain may feel like a small increase for all the work done but when you check the detail as on the enlarged picture you can begin to extract some further useful info. Between 55mph and 67mph the motor is up approximately 0.75hp in places, whereas from 30mph to 55mph there are no gains at all so the improvements are greater at higher rpm where flow through the motor is more critical.
Conclusion
On this example, a 175cc piston ported engine tuned to 17.6hp, we are seeing very limited gains from all the additional tuning work carried out, compared to the standard supplied design. Although the results are hinting to us that had this been a higher rpm race/sprint spec motor, where every last bit can count, it may have shown a bigger improvement. So for the average road-going scooter build, the message would be: don’t worry about enlarging or matching your cylinder to the case as the results from such effort won’t be noticeable. It’s a waste of time, effort and money (if you are paying for this work to be done).
Is that the end?
This is not the end – next month let’s see how a set of big transfers vs standard transfers change the outcome on a tuned long-stroke RB250 kitted motor putting out 43bhp on standard cases! Surely at this level we can make some gains… can’t we? Find out in the next instalment. Stay tuned...