ESE's works engine tuner

[SS90]

O.k

I can't find the piston I am looking for, and will have to wait for the boy to come on Monday.

Yea, I know, my fault, but it's nice to have a scapegoat when you can't find things!

However, I "feel your pain" in regards to cracked pistons when you relieve that area, I wrecked a few in my time for that very reason!

I have attached a picture of a polini 133cc piston, as an example of what "appears" to be the maximum you can safely go on a piston before it gets dangerous.

Looking at other manufacturers, no-one I have seen has exceeded that shape, and with my experience (and yours), that seems as it is for good reason!

But I had a thought today, when thinking of the problem.

I have to cut an intake window into the skirt of my pistons, which weakens the skirt in that area (of course), but if I don't modify the piston in any other way, all is well.... But..... if I cut the transfer cutaway in the piston further up, (to unshroud the transfers, which is a problem when I use a spacer under the cylinder when I "stroke" the engines... (I don't need a spacer unless I do, so it is unshrouded nicely otherwise)) it would always crack between the intake window and the higher transfer cutaway (if I increased the transfer cutaway in the piston too much that is),

I suspect in a similar way to yours.......

What do you think to the idea of instead of taking lots of material out from the piston to unshroud the transfer (which you know you cannot do), just drill a series of small "bleed" holes in the skirt in the area you want to remove.... (or perhaps 2 larger ones)? Which would not compromise the integrity of the skirt as much as simply removing material!

The holes, can simply "bleed" fuel air mixture into the cylinder transfers at B.D.C (a little bit like the Mallossi system, which it'self is a copy of an old Royal Enfield design from the 1920's) but instead of above the gudgeon pin, it only "bleeds" below the pin, (which is otherwise unused)..... and of course you don't need to put holes on the cylinder to facilitate this, as you have a spacer, there is an increase in transfer port area, equal to the height of the spacer....


I am unsure how much effect just drilling the holes will have, (compared to the power gains (particularly Torque) you would make by completely removing the "offending" piston skirt area), but, from personal experience, unshrouding this area just a small amount brings huge gains.

I am sure you agree, currently you are essentially trying to "post an elephant through a mailbox" when you compare the available area (cylinder liner cutaway), to the piston transfer cutaway.....

Anyway, simply removing the piston skirt in this area just causes failures anyway!

While holes in the skirt don't completely remove the problem, even a 30% gain in transfer area yields surprisingly high results....

Personally, I have never seen this done (the hole concept in this area), so it is only theory, but Science does back this theory...... so I feel it has merit!

The Polini pictures I have attached are commonly "spaced up" 5mm under the cylinder when tuning, and you only have to increase the "transfer cutaway" a small amount when doing so, as the manufacturers have thought of this when designing the piston...... I have never personally seen a failure on these pistons... they are however very heavily engineered in the thickness of the skirt walls....

The 4th picture is of a modified 136cc Mallossi cylinder (I can't find a picture with the piston in, sorry, but I assure you that the piston cutaway is matched perfectly to the cylinder at B.D.C) it has the C.V.F system, as you can see by the "extra transfer ports"

It has the "reed valve piston window induction" conversion fitted, and I used an aftermarket stronger piston.....

You will notice that the transfer area has been "relieved", and it is NOT SYMMETRICAL... this is because when I relieve the piston cutaway, I leave more material on the intake window side....as this is a weak point on all "piston window induction" cylinders..... However, this is an old engine I did years ago, and I have since changed my cut out design on the Mallossi conversions I do, and can now fully "open" the cylinder transfer cutaways (on the piston as well)

This particular engine was 17 P.S (with a 28mm carb).....to get over 20 P.S...... these cylinders start to look "a little different" (to say the least!)

The last picture is of an original Malossi 136cc cylinder.... you can see how much material you have to remove... even with the C.V.F system.... gains are still made in this area!

Of course, on my aluminium cyliders, it all looks different again.........

If you don't feel that the "bleed hole" concept is benificial......you could always grind a series of "boost ports" into the cylinder wall (fed via small holes in the piston skirt, just below the ring groove, just as Aeromachi did in the late 70's)..... If done correctly, this will certainly scavenge the area under the piston much more.......I have done this a few times myself (on older styled engines), and the gains have been fantastic.... I am doing one this week at work, and if you like I could post a few pictures as the process pans out...

Remember the piston you are using has such small cutaways, because it is designed to be used in a cylinder that has boost ports, negating the need for large transfer cutaways in the piston, as the boost ports utilise that Fuel/air mixture!

[TZ350]

I am doing one this week at work, and if you like I could post a few pictures as the process pans out...

Remember the piston you are using has such small cutaways, because it is designed to be used in a cylinder that has boost ports, negating the need for large transfer cutaways in the piston, as the boost ports utilise that Fuel/air mixture!

Thanks for the info I will look at it carefully. I would be pleased to see the progress. I had allways wondered about the small cutouts and not thought about the re-direction to the boost ports.

I am off to the dyno this morning, hopefully get some useable data. I have several bikes to test, all setup slightly differently with respect to port timing and chambers.

.

[Buckets4Me]

[TZ350]

While we are still on pistons. I photographed these ones at Johns shed.

1st pic is of a Honda RS125 piston showing a polished area around the squish band. 2nd pic is the RS piston at the BDC pisition in a RM Barrel. 4th pic is why you don't use a different brand of oil in your RS just because its cheaper, sold by a friend, sponsored by your local dealer, Etc., Etc., stick with what is known to work.

.

[Buckets4Me]

While we are still on pistons. I photographed these ones at Johns shed.


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what where you doing at Johns one wonders

[TZ350]

what where you doing at Johns one wonders

Ok. I was at Johns to dyno the bikes and yours turned out to be the best!!!! OK. and mine was the worst.

All four bikes were run up and it turned out that the more Hotted-Up they were the worse they performed.

The best one (yours) is running a RG250 pipe and has 1.75mm turned from the top of the barrel and "o" ringed, with no other mods. Real simple and ran the best on the initial tests, it also confirmed what we have seen on the track.

As the testing went on there were a few more surprises. I need to draw some graphs from the data, then I can post a report. Speedpro turned up, so now I can't gloss the results.

John and Carl working the dyno, Bikes:- Buckets4me's RS/GP125, Chambers GP125, Nedkellys RS/GP125 and TZ350's No 14.

Many thanks to John for his time and help.

.

[TZ350]

What do you think to the idea of instead of taking lots of material out from the piston to unshroud the transfer (which you know you cannot do), just drill a series of small "bleed" holes in the skirt in the area you want to remove.... (or perhaps 2 larger ones)? Which would not compromise the integrity of the skirt as much as simply removing material! The holes, can simply "bleed" fuel air mixture into the cylinder transfers at B.D.C

I am unsure how much effect just drilling the holes will have, (compared to the power gains (particularly Torque) you would make by completely removing the "offending" piston skirt area), but, from personal experience, unshrouding this area just a small amount brings huge gains.

I am sure you agree, currently you are essentially trying to "post an elephant through a mailbox" when you compare the available area (cylinder liner cutaway), to the piston transfer cutaway.....

While holes in the skirt don't completely remove the problem, even a 30% gain in transfer area yields surprisingly high results....

Trying to "post an elephant through a mailbox" I like that. Yes I think this is a very worthwhile area to seriously look at. Thanks for your help.

I saw on the dyno today after we got the bike dialed in, the graphed results of my old school porting compaired to the new generation approch and can see why you know its the way we should go.

Both graphs peaked about 19.5 Rw-Hp but the other bike with the new generation ports had a much longer flater higher torque curve.

.

[SS90]

I saw on the dyno today after we got the bike dialed in, the graphed results of my old school porting compaired to the new generation approch and can see why you know its the way we should go.

Both graphs peaked about 19.5 Rw-Hp but the other bike with the new generation ports had a much longer flater higher torque curve.

.

Yea, I thought that would be the case..... While, of course the HP results should always be considered a little abstract, I can assure you that as long as the conditions are the same the next time you do some runs, (after mods) the results will be comparable.....home made dyno's get a bad reputations (un deserved actually), as the only real difference between inertia dyno's is the software used for corrrection factors.... most home made dyno's don't even use correction factors for each run, so if it (the dyno) is at roughly the same atmospheric pressure, and altitude, the results are always reliable)

Well, as far as consistency goes anyway!

It regards to the boost port concept, I have included an attachment from an old piston port cylinder (Vespa big frame) from the early 60's.

I simply ground 2 "channels" into the back wall of the cylinder, with the top of the channels about 1mm higher than the transfers. I didn't take a photo of the piston on this job (I don't know why I didn't !, but I am doing another one this week, so photograph that one).

Just below the ring groove I drill two 6mm holes (that line up with the cylinder grooves), and at B.D.C, the piston crown uncovers the top edge of the grooves and fuel from under the piston feeds through the 2 holes, up the grooves, and into the combustion chamber.....

A set of "low tech" boost ports!

This system works very well on the older style engines,That, because of design faults had badly shrouded transfers anyway, but I modify the transfers HEAVILY, (extra spiggot windows (in the liner and piston) and get gains there, but with the "boost port" system, these gains are increased somewhat more.

Of course, this "system" is fed via fuel from under the piston ("scavenged"), where as most modern boost port systems feed straight from either the crankcase, or in the case of a reed valve, straight from the inlet manifold into the combustion chamber, or, sometimes from the reed block straight into the transfer ports, as well as the combustion chamber, and the fuel under the piston is not so important, as it is well compensated for by the inlet manifold fed boost ports......

[F5 Dave]

Having just reacquainted myself with a GP cylinder; the Briggs & Stratton type transfer port (as you say low tech boostport is a simple groove) is actually already std in the back of the barrel. I've never seen this in a Jap engine before. It would be interesting to find out the development process the GP went through to be so different to it's their designs.

[TZ350]

TZ's Dyno Report. Rear wheel HP curve charted against road speed (Kmh), very handy for setting up the gearing.

Before we ran the bike up, John told me that most bikes that have been tuned on the track are usualy over advanced and over rich.

We warmed the bike up and it sounded sharp and loud, but the exhaust fumes were something else, they were eye watering.

The bike sounded sharp and I had high hopes. The good 4-strokes at Mt Wellington are pushing 18-20 RW-HP.

Run 1 returned 15.8hp at 78Kmh, curve-1. Well not a happy camper.

Over the next few runs the ignition was retarded a bit at a time untill we ran out of adjustment. The ignition was retarded from 20 degrees BTDC to about 15 and with each step the power went up, the eyewatering fumes went away and the sharp exhaust note softened and became mellow.

Then we started leaning the main jet out, going from 117.5 to 105 a step at a time. With each change the power improved. Even changing the needle clip one notch showed up on the dyno. We stoped when adjustments started making little difference.

The last run curve-2 returned 19.5 RW-HP peaking at 84Kmh. Just right for Mt Wellington. The runs were done in batches of five, John pointed out that the bike went better as it got hot and interpreted this to mean that the tuned length of the pipe was a little to longe for the state of tune of the exhaust port. That is probably right as the RM125 spec pipe was originaly intended for an exhaust opening 86 degrees ATDC and we were running an exhaust which was opening at 81 degrees ATDC.

Typical temperatures taken during a hard run were. Head-57, Barrel-98, Header-250, Pipe Midsection-170 degrees C.

The last thing John showed me was my last 19.5 RW-HP run compared to engines that have had the modern porting treatment, curves-3 and 4. The features are my old school graph was very good in an old school way and initialy had better torque curve but the modern porting gave no more Hp and it had an initial a steeper start but a much longer power band. The modern curve looks very much like the curves in the SAE paper posted earlier.

.

[SS90]

Curve #2 looks the best to me....... big long power curve, and obviously the torque curve would be longer than all the others as well.....

Cool!

[SS90]

Having just reacquainted myself with a GP cylinder; the Briggs & Stratton type transfer port (as you say low tech boostport is a simple groove) is actually already std in the back of the barrel. I've never seen this in a Jap engine before. It would be interesting to find out the development process the GP went through to be so different to it's their designs.

It was Aeromachi that pioneered that design, and yea, it seems to work.... I am quite surprised that the GP has it, like you, I have never seen a Jamanese engine with one.....and looking back at a early picture of Teezee's piston, there could be a gain in drilling a hole higher up the piston (above the existing one)

Actually, no, I looked again, and the hole is high enough...... the only gains you would make would be in the angle that the boost port enters the cylinder.......55 Deg (maybe) would be an angle to consider.......

Now that you are making almost 5 PS more...... I can't help but wonder what you will gain with the changes you have outlined previously.....

Experience tells me that you are unlikely to make more peak power, but rather with the things we have talked about you will increase "the width of your power band" quite dramatically, as well as increasing the low RPM torque......

I keep thinking back to the idea of holes in the skirt to increase the relief area now.......

[TZ350]

Barrel Pic's.

Pic's 3 & 4 show an early 70's TM barrel with the traditional "Two" transfers.

Pic's 1 & 2 show a brand new late 70's GP125 barrel with rear gully port and two secondry ports.

The GP's secondrys look like after thoughts when you look at the ugly way they have been siamesed into the main transfer port ducts, pic-2.

I was thinking of opening these up and adding dividers between the mains and secondrys.

.

[TZ350]

Now that you are making almost 5 PS more...... I can't help but wonder what you will gain with the changes you have outlined previously.....

Experience tells me that you are unlikely to make more peak power, but rather with the things we have talked about you will increase "the width of your power band" quite dramatically, as well as increasing the low RPM torque......

I keep thinking back to the idea of holes in the skirt to increase the relief area now.......

Yes 5 PS for just retarding the ignition and leaning it out a bit.

Like you I don't expect any/much more Hp but I am excited by the possibilitys of a wider power band. Thanks for your help with this.

I am going to have to formulate an action plan. One of the problems I have is working out how to get a main port that is angled up at 10 degrees to 25-30 degrees. I can't just file away at the port, it still has to retain its timing.

.

[SS90]

Hoho!
O.K, I see what you mean..... I believe that because of the "crowded" design, you must be careful that the streams don't "short circuit". That said, changing the angles of the secondaries will be of HUGE gain on this design.

Personally, I would treat the secondary transfers ports like boost ports (as far as their angles are concerned., particularly in relation to the angle they point to the combustion chamber... and have them enter at quite a high angle into the cylinder.... possibly the same angle as the rear boost port (which I would possibly raise to 55 deg...). have them work in symmetry......

Not 100% sure on the angles, some experimenting is needed.

The whole design really is a compromise though..... I am confident there is real gains scavenging what is left under the piston, especially from what I see of the port set up......