Tuning Part 2: Proven results, or a waste of time?
Darrell continues his dyno testing of a few well-known and time-honoured traditions when it comes to tuning two-stroke engines.
There are some well-known and widely used two-stroke tuning traditions, but are they all being judged and applied within the correct parameters? If not, then ‘tuners’ may be wasting a lot of time, effort and (customers’) money! Darrell Taylor continues his research on the matter…
In last month’s article I shared the results of a collection of modifications to the lower cylinder transfer area on a Gori 175cc kit. This was a piston-ported, simple, four-port cylinder with just two transfers (like the original Innocenti design). Each of the transfer ports is fed from a single case entry area/section of the duct and directed out of a single cylinder-wall exit area. These are all fed from (in that case) a 28mm carb and silenced by a Clubman exhaust. The total power from the package sat at around 17hp. The same previous article showed that the result of applying the well-known and widely used tuning modifications was little (almost zero) benefit, especially for the effort put in. Though at least it was a small gain, as it could quite easily have been a small loss, such is the fickle nature of tuning a two-stroke where the only sure way to gauge results is to try each modification and test before/after, for better or worse.
Baseline tests…
With that in mind I chose to try another test on what is a more ‘thirsty’ motor, this time using a well-tuned RB252 motor, formed by using the stock RB250 mono-exhaust cylinder on a 62mm stroke crank, with a ‘fast road’ tune. This unit is at the furthest end of the scale away from the previous test on the small-block Gori. Could the results be very different in this example? The motor at 252cc has four cylinder-wall transfer ports (two each side) fed from the cylinder case base. It also has three boost ports in the rear of the cylinder wall, with two extra reed-fed inlets often referred to as Boyesen ports, and a single exhaust port. The Gori was piston-port inlet, whereas this unit is reed-valve induction, and fed from a good-sized 35mm carb into its large ‘W-block’ reed assembly. The base test for the tuned RB252 motor before the ‘test work’ commenced showed the motor with a healthy 43bhp and all done on a standard SIL 200 engine casing with no
The accompanying images illustrate the extent of the mismatch which exists between a packer plate which doesn’t match either the RB cylinder or the SIL engine casing. When stacked with a further 1mm shim on top, the entry into the cylinder transfer is more like a staircase.
porting carried out at all to the heavily mismatched and partially masked case transfer entry areas. The accompanying photographs show the standard transfer base entry of the RB cylinder and how the fitted packer plate used on this build heavily masks the RB cylinder duct entry size, pretty much blocking entry into the second transfer port from the casing and also from the Boyesen reed-inlet port, which feeds into the base from the reed… thus creating both inlet and transfer design restrictions.
Let tuning commence…
So with the ‘baseline’ dyno test completed, the cylinder is quickly removed from the engine and work on both the cylinder packing plate and the SIL engine casing can be done. The accompanying images illustrate the extent of the mismatch which exists between a packer plate which doesn’t match either the RB cylinder or the SIL engine casing. When stacked with a further 1mm shim on top, the entry into the cylinder transfer is more like a staircase. The images also show the type of quick blend that was done (in situ) to the crankcase. Zero swarf contamination occurs thanks to using very sticky adhesive foil tape which masked around the con-rod and crank webs. The packing plate was used as a template to scribe on to the case and simply hand-ported as a transition cut. Also shown in the images is the amount marked off the packer to remove all the steps and match case to packer to cylinder. Before and after images show the tuning work which was quickly completed for the tests. Due to the metal removed during tuning, the
The base test for the tuned RB252 motor before the ‘test work’ commenced showed the motor with a healthy 43bhp and all done on a standard SIL 200 engine casing with no porting carried out at all to the heavily mismatched and partially masked case transfer entry areas.
transfer base entry size has now increased from 28.22mm x 17.54mm (494.97 sq mm) to the ‘opened up’ size of 31mm x 20.5mm (635 sq mm).
The result of tuning…
So with the tuning work done to open up the engine case transfer, to match the packer plate, to match the 1mm shim, and also uncover the secondary transfer and Boyesen port, we were ready for the clean-up and reassembly. Everything was cleaned down with brake cleaner and then air-lined off before the top end was reassembled with quick-drying sealant and then left to cure. This test is quite representative of big transfer versus little transfer and was limited to these changes only, in order to identify benefits of this tuning method… should any benefit exist! No piston or entry radius changes occurred, as were done in last month’s Gori test; this time we only carried out the work that most tuners would consider doing to a badly matching case/base/gasket/ packer combo, that have some harsh steps, and small/mismatched case transfers that mask important areas of a motor.
With the engine fired up and brought up to a stable temperature (as per the base test) the set of three runs were made, which also assist in showing pipe temperature effects. The dyno results are seen in the images; the dyno runs are all done from as low as possible and revved out to the max, to show a good overlay graph. The result is… there’s nothing in it. Before and after graphs show 43.35hp to 43.21hp at peak and the power lines of the graph both laying over each other from start to finish, with just very minor bumps along the way that you would see anyway from any test-to-test result. Having discussed the results with colleagues and Dan the editor, the resounding question and reply that I hear is: “Why? This contradicts the information we glean from tuning books, surely it should work better.” I also asked why, but my question is different. It’s: “Why would you think so, when you don’t know so.” To test is to know and this is the important point in this whole exercise… nothing can be concluded from any tuning practice without the before and after tests being carried out to prove its worth. This type of testing can be done in many ways, not just the dyno, so on a race track we might see improved lap times, a sprint track might display an improved timing ticket, a speed event could show a new top-speed. But… the dyno makes short work of otherwise lengthy testing tasks, and does so in a repeatable and precise manner which can (if done correctly) eradicate a lot of other external factors (wind resistance, weather changes, human error etc.). Equally important is to repeat the tests on various engine types, as that itself can throw up many surprises. What works well on a liquid-cooled, 14000rpm, disc-valve 50cc may not work on an air-cooled, 7000rpm, piston port 175cc motor. With the last two articles as examples of that approach to testing, it has allowed me to share this data. So how about I answer the “Why?” part in more depth, and find out if and when these well-known and widely practised ‘tuning’ methods might actually work. Well… I have my theories and calculations that I feel support the test results, and I will share these with you in the next instalment of this series. Stay tuned.
Dan the editor, the resounding question and reply that I hear is: “Why? This contradicts the information we glean from tuning books, surely it should work better.” I also asked why, but my question is different. It’s: “Why would you think so, when you don’t know so.” To test is to know and this is the important point in this whole exercise… nothing can be concluded from any tuning practice without the before and after tests being carried out to prove its worth. This type of testing can be done in many ways, not just the dyno, so on a race track we might see improved lap times, a sprint track might display an improved timing ticket, a speed event could show a new top-speed. But… the dyno makes short work of otherwise lengthy testing tasks, and does so in a repeatable and precise manner which can (if done correctly) eradicate a lot of other external factors (wind resistance, weather changes, human error etc.). Equally important is to repeat the tests on various engine types, as that itself can throw up many surprises. What works well on a liquid-cooled, 14000rpm, disc-valve 50cc may not work on an air-cooled, 7000rpm, piston port 175cc motor. With the last two articles as examples of that approach to testing, it has allowed me to share this data. So how about I answer the “Why?” part in more depth, and find out if and when these well-known and widely practised ‘tuning’ methods might actually work. Well… I have my theories and calculations that I feel support the test results, and I will share these with you in the next instalment of this series. Stay tuned.