ESE's works engine tuner

[Frits Overmars]

Wonder why RSA ran the domed piston? Frits? Ignition, triple port instead of bridge?

Simple answer: the RSA made more power with a domed piston. The ignition was optimized for each test and the triple port had already been proven better than a bridged port, so all following tests were carried out with the triple port.

[husaberg]

Simple answer: the RSA made more power with a domed piston. The ignition was optimized for each test and the triple port had already been proven better than a bridged port, so all following tests were carried out with the triple port.

Could it be that say a Honda RS125 with it's less curved transfers, bridge exhaust perform better with a flat top piston on account of these factors.
Conversely say a RS Aprilia with more curved transfers performs better with a domed piston and bathtub as they are a better combination of complimentary characteristics.
Than it would be mixing and matching. (I hope that makes sense)

[Frits Overmars]

Could it be that say a Honda RS125 with it's less curved transfers, bridge exhaust perform better with a flat top piston on account of these factors. Conversely say a RS Aprilia with more curved transfers performs better with a domed piston and bathtub as they are a better combination of complimentary characteristics. Than it would be mixing and matching.

Aprilia tested all combinations of Aprilia-style and Honda-style transfers, exhaust, piston shape and combustion shape. That was a lot of mixing and matching.
It could well be that a Honda RS125 with it's less curved transfers, bridge exhaust performs better with a flat top piston. But in any case it performs worse than an RSA.

[Grumph]

Right. In fact it's two DEA cylinders; the left and center pictures show a 2013 cylinder; the right pic shows a 2014 cylinder from the DEA superkart tandemtwin with RSA-based cylinders. The notches in the bore surface are supposed to retain some oil for the benefit of the exhaust bridges.

They look a bit more than just notches...Do they go through to the transfer ducts ? When i saw them in the pic I wondered if they matched holes in the piston skirt at BDC for a little under piston cooling/mixture moving through but assumed not for reasons you'd already mentioned.

[trevor amos]

Is it possible that the more compact chamber, having reduced surface area , shorter flame path, full toroid chamber profile and so on, for a similar comp ratio, are responsible for the performance step up rather than the flat top piston per se`. Or are these comments merely thoughts over reality?

Trevor

[seattle smitty]

Something that has surprised me (and pleased me, since it seems to contradict the widespread idea that our 2-strokes are doomed to disappear) is the number of little shop operations fabricating new state-of-the-art cylinders and complete engines for various sorts of kart racing, sled racing, scooter racing, and so on (including the two outboard engine builders that I already knew of). Many or most of these efforts seem to involve attempting to duplicate the last Aprilia design as nearly as possible. Also attempting this are some builders of direct replacement cylinders for Aprilias, one of whom is a fellow Netherlander of Frits and Jan Thiel, and was mentioned here by Frits, who also seems to have some level of personal acquaintance with some of the kart engine fabricators, if not the sledders.

With all of these parties (whose photos often show some beautiful work being done!) putting great effort into producing the best 2-stroke engines in their chosen sports, one might expect that one or more will eventually surpass the work of the masters. For one thing, some of them, maybe most, can work from a blank sheet of paper, whereas Thiel and Overmars have explained that they were constrained somewhat by having to build for an existing engine.

So, if, repeat IF this is something upon which you care to comment, who among the new engine-builders does our esteemed Professor Frits see as the next Luke Skywalkers, matching and/or exceeding the masters now or in future?

(There's a big caveat here, which is that the karters, sledders and boaters are currently showing only their "conventional" designs. In the matter of the FOS engine, a whole different thing, surely it will be a case of the master exceeding his own previous best work . . . ).

[wobbly]

I dont think its a case of the young pretenders exceeding the efforts of the long established masters, but more a case of taking the brilliant ideas
that have been developed in the past and learning from the results gained.
A perfect example of this would be that Jan let on that he had a huge issue at Aprilia with holing pistons once a power threshold was reached.
Jetting, ignition, compression changes had no real effect, except to reduce power.
The fix was to cut away the head insert to allow water close to the plug threads, thus better cooling the exposed plug body.
He wondered why the hell he had not thought of this 20 years earlier.
So, what do we learn here - when you have an issue with piston damage, look at the parts that can maybe ameliorate the problem, by cooling them.
KZ2 engines have always been limited in so many areas, as they have a straight line ignition.
This show up time and again as deto eating away the piston edge due to the trapped end gases in the squish being overheated by the excess timing at high rpm.
So whats the fix, cool the squish band properly.
None of the Italian factories have done anything about this, and implementing the extension of Jans idea doesnt make me a 2T genius, but does make me real grateful to the man
that passed on the knowledge.
Thats only one small example - but for sure if someone were to implement a large number of the ideas that were proven experimentally by people like Jan and even going back
to my hero Mr Fath then really big jumps in power could be had even in KZ2 where so little is allowed to be modified, I know, as its happening as we speak.

[F5 Dave]

With that lead in I'll post a pic of my engine & wish I'd cleaned the gasket goo off first as if to polish the proverbial turd.

The reed inlet I don't have a crank web handy but I've bunged a protractor in there. The skirt stub allows passage to the transfers but I'd been having ring seal issues with it largely ground away.

Honda didn't provide a case reed in 1978 so I've had to add one. This facilitates fitment in an NF4 frame so it was worthwhile just for that. The C port entry is visible from the Devcon drizzling out of it (yet to be ground back).

I'm wondering if I should trial without reed stops . . .

The skirt has no hole (Metal sprayed in the inlet port).

[speedpro]

You need a longer rod so you can raise the cylinder and un-mask the path between reeds and crankcase, or build up the inlet tract roof so it directs the flow down into the opening below the cylinder, though that's a bit limited with having to allow for the C port.

Do you have much of a problem fitting a carb to the intake? I had issues with my case reed TS100 with the carb hitting the case.

[F5 Dave]

I have to use an RGV carb nothing else will fit. Do have one bored to 35 to try again which it loved before. There is space around the sides to the transfers.

[peewee]

not sure if this amounts to anything good or bad or maybe it amounts to nothing at all but the piston travels a slightly shorter distance per crank degree with a longer rod attached. im sure frits or wobbly could better answer if this has any benefit or not. ive always thought its best to use the longest rod you can possibly fit

[husaberg]

not sure if this amounts to anything good or bad or maybe it amounts to nothing at all but the piston travels a slightly shorter distance per crank degree with a longer rod attached. im sure frits or wobbly could better answer if this has any benefit or not. ive always thought its best to use the longest rod you can possibly fit

It also dwells longer at tdc and bdc and lowers piston acceleration and allows for more crankcase volume

[ken seeber]

Peewee
The spreadsheet shows, for a 54 crank, the piston travel for a 100, 110 & 120 rods. This is based on a bore axis in line with the crank axis, ie no piston pin offset.
Longer rod offers more crankcase volume, more entry area for the transfer passages, less piston side thrust, lower peak acceleration and forces, less movement particularly towards the TDC & BDC positions. The shorter rod the opposite.
I'm sure that Wobbly & Frits could provide more subtle reasons for and against, based on the application and its limitations.
Ken

[speedpro]

The piston travels the same distance in any engine with the same stroke, irrespective of the rod length. The amount the piston travels per degree of crankshaft rotation will differ at various crank positions, sometimes more and sometimes less with different length rods. It's all down to geometry.
With longer rods the variations in velocity/acceleration are reduced which if other things are equal will allow higher revs for the same peak loads on the piston. With an infinitely long rod the piston velocity would vary at a pure sinusoidal rate and have the lowest variation in velocity.
Kawasaki ZX750 race kit rods were 2mm longer if I recall and later model FZR250(3LN) rods were 2mm longer and the pistons were 2mm shorter than the earlier 2KR models. The bore and stroke remained the same. In fact the cranks, cylinders and heads were interchangeable except that the 3LN head had bosses for the top engine mounts to the spar type frame.
Putting a 400 crank into a 350 chev creates a very torquey motor. Partly I think due to the longer stroke( and bigger engine at 388?) but also due to the short rods used which with the resultant crank/rod/piston geometry favours torque over revs and hp. Sorry, not well explained, probably 'cause I've got it wrong.

[Frits Overmars]

If this is something upon which you care to comment, who among the new engine-builders does our esteemed Professor Frits see as the next Luke Skywalkers, matching and/or exceeding the masters now or in future?

I won't comment, Smitty. There are several promising people around but I can't claim to know all of them. Naming names means excluding names and that may well do unjustice to some. Who am I to know who will rise and shine in the future?
Both Jan Thiel and yours truly are trying to spread our two-stroke knowledge with the intent that you guys take over where we sign off (although I have no intention of signing off just yet).

The notches in the bore surface are supposed to retain some oil for the benefit of the exhaust bridges.

They look a bit more than just notches...Do they go through to the transfer ducts ?

No Grumph, notches don't. This picture shows under what angle and how deep the cutter went in.

Is it possible that the more compact chamber, having reduced surface area, shorter flame path, full toroid chamber profile and so on, for a similar comp ratio, are responsible for the performance step up rather than the flat top piston per se?

Here are two combustion chambers with identical compression ratios, squish areas and squish clearances. Which one do you think has the smallest surface area?
And where do you see a performance step-up?

the piston travels a slightly shorter distance per crank degree with a longer rod attached.

So over 360° a shorter rod will shorten the piston stroke?

It also dwells longer at tdc and bdc and lowers piston acceleration and allows for more crankcase volume

Sigh.... a longer rod dwells longer at TDC and shorter at BDC; it accelerates the piston less at TDC and more at BDC.

The piston travels the same distance in any engine with the same stroke, irrespective of the rod length. The amount the piston travels per degree of crankshaft rotation will differ at various crank positions, sometimes more and sometimes less with different length rods.

Thank god someone is awake.

Yeah, I know, I sound like a grumpy old man. It's the weather, you know. Cold wind, rain... Wish I were in NZ right now.