ESE's works engine tuner

[Condyn]

Why is it that most twins come from the factory as oversquare engines? Is it an effect from packaging constraints since the engine is already wide, they don’t want to go taller? Or is it a piston speed thing? The reason I ask is the engine I am working on designing is oversquare by 2mm. I would be within the allotted cc limits if I were to stroke it square. Will it pay off enough to go through the hassle of squaring it up?

[wobbly]

Oversquare is an old fashioned precept that simply doesnt work.
Its all to do with the way you have to go about generating port area , and on the other side of that coin is the percieved idea that you can rev a short stroker harder due to lower piston speed.
But all physics aside , look at what history tells us.
In GP racing Yamaha steadfastly stuck to 56x 50.6 and were never even close to winning 125 or 250 titles untill they finally ditched corporate pride and went square in 2000.
That year they blitzed 250GP with Jaques and Nakano.
Due to the port timing limitations , the short stroke 4DP series TZ could never rev harder than the 54 square Honda or Aprilia - as the theory says it should.

There is a completely differing set of reasons for 250MX singles to be very undersquare at 72 stroke - where outright power isnt the issue, its a flat torque curve from low rpm.
But then look at the power results gained by Honda with its RS500V that was 68 x 68.8 square.

The worst example of oversquare idiocy would have to be" the great leaders " V twin GP500 Aprilia.
Its only claim to fame being McWilliams pole at Philip Island , and that was the rider on that day , not the engines power as was predicted ( never actually seen in reality ).

[husaberg]

Oversquare is an old fashioned precept that simply doesnt work.
Its all to do with the way you have to go about generating port area , and on the other side of that coin is the percieved idea that you can rev a short stroker harder due to lower piston speed.
But all physics aside , look at what history tells us.
In GP racing Yamaha steadfastly stuck to 56x 50.6 and were never even close to winning 125 or 250 titles untill they finally ditched corporate pride and went square in 2000.
That year they blitzed 250GP with Jaques and Nakano.
Due to the port timing limitations , the short stroke 4DP series TZ could never rev harder than the 54 square Honda or Aprilia - as the theory says it should.

There is a completely differing set of reasons for 250MX singles to be very undersquare at 72 stroke - where outright power isnt the issue, its a flat torque curve from low rpm.
But then look at the power results gained by Honda with its RS500V that was 68 x 68.8 square.

The worst example of oversquare idiocy would have to be" the great leaders " V twin GP500 Aprilia.
Its only claim to fame being McWilliams pole at Philip Island , and that was the rider on that day , not the engines power as was predicted ( never actually seen in reality ).

My theory was that Yamaha was tied into the oversquare design as it was an offshoot of the 500 team and the 500 program never had the room as the bore centers were to close to have decent transfers so they ran 6 transfer ported oversquare cylinders, did the move to 54x54 also includes move to 4 main transfers or did they do that prior?

[Schumae2]

Oversquare is an old fashioned precept that simply doesnt work.
Its all to do with the way you have to go about generating port area , and on the other side of that coin is the percieved idea that you can rev a short stroker harder due to lower piston speed.
But all physics aside , look at what history tells us.
In GP racing Yamaha steadfastly stuck to 56x 50.6 and were never even close to winning 125 or 250 titles untill they finally ditched corporate pride and went square in 2000.
That year they blitzed 250GP with Jaques and Nakano.
Due to the port timing limitations , the short stroke 4DP series TZ could never rev harder than the 54 square Honda or Aprilia - as the theory says it should.

There is a completely differing set of reasons for 250MX singles to be very undersquare at 72 stroke - where outright power isnt the issue, its a flat torque curve from low rpm.
But then look at the power results gained by Honda with its RS500V that was 68 x 68.8 square.

The worst example of oversquare idiocy would have to be" the great leaders " V twin GP500 Aprilia.
Its only claim to fame being McWilliams pole at Philip Island , and that was the rider on that day , not the engines power as was predicted ( never actually seen in reality ).

Wobbly; with respect to 125GP engines and 125cc KZ/ICC kart engines I was wondering your opinion on connecting rod length. With karts it seems many KZÂ’s have settled on either 110mm-115mm yet some engines such as the Modena MKZ is 106mm and the Honda RS125 is 104mm. I understand how rod length affect piston acceleration and dwell time at TDC / BDC, but in your opinion is there a specific reason why most higher performance 125cc engines utilize a 110mm connecting rod, while less use 115mm and even less around 105mm?

I have read quite a few articles on this in TKart magazine, and the consensus is 110mm is a good trade off based on track length but, thereÂ’s been conflicting evidence in other articles in regards to balance factor and engines that donÂ’t have a power jet, that the shorter rod keeps the pipe hotter due to the less complete of burn around TDC and wasted heat energy being transferred to the pipe.....IÂ’m not sure about this but, VORTEX newest KZÂ’s (RSZ and RTZ are basically the same engine with 2-cylinders and a jug spacer, so crank assemblies and top ends can be switched track to track for both utilizing 110-mm & 115-mm lengths.....VORTEX claims the 115-mm is best for longer tracks).

I would greatly appreciate any insight you would be willing to provide (thank you!!!)

[wobbly]

Husa , na , the 56 bore compromised the transfers even more.
All to do with corporate pride and who was in charge of R&D at the time.
The dual boost port , big bore was a cockup in every direction , but untill managment changed in 99 and they bought a Czech CNC anemometric flow visualizing rig , suddenly they
went square and changed to 5 port designed on the new machines results.
Susan , as was his nickname at the time , the new head of the race dept told me they could accurately predict a Hp curve shape purely off the flow results , and didnt need to dyno dozens of
cylinders anymore - and his new 5 port design was easily way better than anything else they tested.

Schumae , if you really want to look at a positive direction re rod length then as I said before look at historical results.
The RS125/250 Honda had a short rod forever , untill Honda finally got serious as they wanted desperately to win the final season of GP racing.
Suddenly the new bike appeared with 109 rods and beat the Aprilia due to being just fast enough with Aoyama on board , and the Italian boys all stealing points off each other..
Jans RSA went to 120 , as part of getting a really big case volume , that RVs work well with.

The short rod KZ Modena has won nothing - the TM has always had a 109.8 ( an old mistake in CNC programming they never rectified ) very close to 2:1 and has easily the best record over time.
I wanted to go to 113 with the new homologation R1 but Franco flattly refused , and told me to shutup and stick to pipe design.
The 115 rod Vortex does make more top end power ( probably due in main to reduced bore friction ) but the issue with a reed engine is that you have a catch22 regarding case volume and the reed thickness needed.
Big cases under 1.3 ratio ( due to a longer rod ) do work better with aggresive pipes , but the bigger case has a Helmholtz trend that needs thinner and thinner stiffness petals - these quicly loose control due to
the first harmonic frequency - and thus power drops.

In my opinion people raving on about short rods , powerjets and incomplete burn , are trying it on with techno babble rubbish.
The heat transferred into a pipe is completly dependant upon the combustion variables of compression and ignition advance being optimised to suit the Ex ports interaction with the pipe length being used.
Yes , short rods dwell at TDC differntly than long ones , but both can be optimised to give the best result for that set in stone design variable.
And yes a Powerjet can make it easy to reduce the natural tendency of a venturi carb to go progressively rich over the top of the powerband.
But we have a whole bag of tricks to achieve the same effect in a KZ without one - easily able to hit 680*C in the header at 14800 , if the fuel will allow it.

[Schumae2]

Husa , na , the 56 bore compromised the transfers even more.
All to do with corporate pride and who was in charge of R&D at the time.
The dual boost port , big bore was a cockup in every direction , but untill managment changed in 99 and they bought a Czech CNC anemometric flow visualizing rig , suddenly they
went square and changed to 5 port designed on the new machines results.
Susan , as was his nickname at the time , the new head of the race dept told me they could accurately predict a Hp curve shape purely off the flow results , and didnt need to dyno dozens of
cylinders anymore - and his new 5 port design was easily way better than anything else they tested.

Schumae , if you really want to look at a positive direction re rod length then as I said before look at historical results.
The RS125/250 Honda had a short rod forever , untill Honda finally got serious as they wanted desperately to win the final season of GP racing.
Suddenly the new bike appeared with 109 rods and beat the Aprilia due to being just fast enough with Aoyama on board , and the Italian boys all stealing points off each other..
Jans RSA went to 120 , as part of getting a really big case volume , that RVs work well with.

The short rod KZ Modena has won nothing - the TM has always had a 109.8 ( an old mistake in CNC programming they never rectified ) very close to 2:1 and has easily the best record over time.
I wanted to go to 113 with the new homologation R1 but Franco flattly refused , and told me to shutup and stick to pipe design.
The 115 rod Vortex does make more top end power ( probably due in main to reduced bore friction ) but the issue with a reed engine is that you have a catch22 regarding case volume and the reed thickness needed.
Big cases under 1.3 ratio ( due to a longer rod ) do work better with aggresive pipes , but the bigger case has a Helmholtz trend that needs thinner and thinner stiffness petals - these quicly loose control due to
the first harmonic frequency - and thus power drops.

In my opinion people raving on about short rods , powerjets and incomplete burn , are trying it on with techno babble rubbish.
The heat transferred into a pipe is completly dependant upon the combustion variables of compression and ignition advance being optimised to suit the Ex ports interaction with the pipe length being used.
Yes , short rods dwell at TDC differntly than long ones , but both can be optimised to give the best result for that set in stone design variable.
And yes a Powerjet can make it easy to reduce the natural tendency of a venturi carb to go progressively rich over the top of the powerband.
But we have a whole bag of tricks to achieve the same effect in a KZ without one - easily able to hit 680*C in the header at 14800 , if the fuel will allow it.

Wobbly; thank you for the detailed reply, much appreciated!

IÂ’m working on a Honda CR125 based kart engine (the class is called mod-Honda) and the rules are very open in terms of what can be changed...basically the cases must be 97-99 cr125 (modification allowed) and the cylinder can be any CR125 from 99-02.....the ignition, pipe, carburetor, and rotating assembly are all open to modification / replacement.....my current setup is an 02-cylinder that has been extensively ported / welded / filled to be as similar to an RS125 NX4 as possible, and the combustion chamber has had brass inlay added with anti-detonation ring to RS125 dimensions, associated VHM RS125 piston, 38mm non-power jet kehin PWM carb....many pipes / ignitions have been tested and it feels theres more development needed there as well as the reed pack (your input was very helpful, I will put my focus on the pipe, reeds, and ignition opposed to considering a rod length change and all associated changes that would come with it, at this time).

[Condyn]

Excellent information Wobbly. The next issue I am running into is the potential of too much case volume. Stock, this engine came with a 125mm rod and is 66x64. If I increase the rod to a 132 for 2:1 and square the engine I will be adding quite a bit of volume, and it is a reed engine. I will have to find a piston with a high pin and need a spacer. Once i finish welding on the case reed blocks, I will collect the ccr as stock form and see what I am working with as a baseline. But that is all besides the point for right now. 66x66 is where it will end up one way or another.
Ps Schumae, this site does not use apostrophes. I had to adapt myself.

[andreas]

Going from 56x 50.6 to 54x54 gives 2.9% area increase. Sounding a little negative there I realize- just to mention there might be less money&labour intense alternatives.

[wobbly]

The TM is 54 x 54.43 stroke FYI.
Mod Honda , didnt know that class still existed - having won SKUSA Stock Honda 3 times I can tell you that all of the approved class pipes were rubbish.
I tested all of them and looked into making a new approved pipe with alot better power and spread , but the die sets were around 20K I didnt have at the time - so Pro Circuit got that job.
The best power increases came from making a header manifold like the 250GP , with no steps at either end , and I found nearly 2 Hp in the reeds.
First thing to change tho would be the inlet manifold , there is 1.5 Hp in a dead straight late model RS125 rubber.
If you can run a programmable then using the short powerjet carb would be a huge advantage.

[Schumae2]

The TM is 54 x 54.43 stroke FYI.
Mod Honda , didnt know that class still existed - having won SKUSA Stock Honda 3 times I can tell you that all of the approved class pipes were rubbish.
I tested all of them and looked into making a new approved pipe with alot better power and spread , but the die sets were around 20K I didnt have at the time - so Pro Circuit got that job.
The best power increases came from making a header manifold like the 250GP , with no steps at either end , and I found nearly 2 Hp in the reeds.
First thing to change tho would be the inlet manifold , there is 1.5 Hp in a dead straight late model RS125 rubber.
If you can run a programmable then using the short powerjet carb would be a huge advantage.

Yep, its not a popular class anymore (nether is stock Honda as of last years SKUZA rule changes) most everyone that has a shifter kart is running a KZ or the 175-SSE by IAME or a VORTEX-ROK shifter.....many tracks mix all the engines together in the same class to keep the Kart count high, but run them at different weights to try to attempt to keep some performance parity. I have a ROK, but the mod-Honda is mainly a project to me, to see how far it can be developed, and see what I can learn during the process (hopefully with it ending up being competitive).

I have done some of the things you mentioned...Im running V-force-4 reeds (designed for the RS125) with 2-stage petals, and a straight billet inlet manifold that is even a bit shorter than the original RS manifold (the carburetor outlet is about 5-mm from the reed stuffer.......the header manifold is a kart piece from a stock Honda (I will need to look into a making one without steps as there is one at the cylinder but it flows smoothly into the pipe). I have programmable ignition though its a reprogram of the stock CDI from Swedetech for Mod-Honda (looking into changing that soon)......the pipe that has worked best so far is the RLV-R5 pipe, that is meant for road racing (Ive shortened the primary header length by 19mm to get the tuned length to make peak power closer to 13,000 RPM and the stinger diameter has interchangeable pills to reduce the inner diameter [currently at 23.5mm])......I wish I could use a carburetor with a power-jet but, the rules say that all fuel must enter the carburetor through the main jet, so that has limited my options (the only carburetor modification is I have a spare that was oval bored from 38-mm to 40mm in the vertical orientation [i didnt see much of a difference swapping back to a standard unit]).......Ive also played with the combustion chamber CCs bit, but that was more an effort to make the 2-set ignition curves work well with my setup.

Thank you again for the helpful insight! Ive lurked on this thread for a year or so, and have read many of your, Jans, Fritz, and Husas posts over that time, which has been a great supplement to the books I’ve been reading, and now beginning to play with modeling software.

[Schumae2]

Excellent information Wobbly. The next issue I am running into is the potential of too much case volume. Stock, this engine came with a 125mm rod and is 66x64. If I increase the rod to a 132 for 2:1 and square the engine I will be adding quite a bit of volume, and it is a reed engine. I will have to find a piston with a high pin and need a spacer. Once i finish welding on the case reed blocks, I will collect the ccr as stock form and see what I am working with as a baseline. But that is all besides the point for right now. 66x66 is where it will end up one way or another.
Ps Schumae, this site does not use apostrophes. I had to adapt myself.

Thank you Condyn, for the tip on the apostrophes! Im new to posting on this site, and wondered what I was doing wrong.

[andreas]

Hi Schumae, do you know how much hp it's making?

[Schumae2]

Hi Schumae, do you know how much hp it's making?

Hello Andreas;
Im not sure in its current configuration? Based on its relative lap times compared to the VORTEX-ROK, I would suspect that its of somewhat similar power to that engine, though its parity to that engine depends somewhat on track size / gearing differences. VORTEX advertises the ROK shifter to make 43-44hp, but Im unsure what dyno or correction factors are being used to generate that number.

[andreas]

Ok, cool.

[jamathi]

Oversquare is an old fashioned precept that simply doesnt work.
Its all to do with the way you have to go about generating port area , and on the other side of that coin is the percieved idea that you can rev a short stroker harder due to lower piston speed.
But all physics aside , look at what history tells us.
In GP racing Yamaha steadfastly stuck to 56x 50.6 and were never even close to winning 125 or 250 titles untill they finally ditched corporate pride and went square in 2000.
That year they blitzed 250GP with Jaques and Nakano.
Due to the port timing limitations , the short stroke 4DP series TZ could never rev harder than the 54 square Honda or Aprilia - as the theory says it should.

There is a completely differing set of reasons for 250MX singles to be very undersquare at 72 stroke - where outright power isnt the issue, its a flat torque curve from low rpm.
But then look at the power results gained by Honda with its RS500V that was 68 x 68.8 square.

The worst example of oversquare idiocy would have to be" the great leaders " V twin GP500 Aprilia.
Its only claim to fame being McWilliams pole at Philip Island , and that was the rider on that day , not the engines power as was predicted ( never actually seen in reality ).

In 2000 the Yamaha France crew-chief Guy Coulon copied the Honda cylinders, I don't know if this was done with permission of Yamaha or not
Later Yamaha-Japan did the same with the 500cc fours.
They even published a SAE paper about it, incredible!!!!!!!
In 2003 a longer stroke was tried at Aprilia: 53mm bore
The result was less power and more detonation----
Then 52 bore was tried, even less power, and after some tests the piston oin was found, inside the carburetor.
That was the end of Aprilia's long stroke experiments....