Torque talk Trevor...

Trevor, to get more torque out of your engine, whats required? raised compressions? more/less surface area on piston (domed piston/flat top). shorter conrod (not stroke). just looked at the standard 175 engine spec 12.6hp @6000 rpm.

using this formula torque = hp x 33000/2# x rpm comes in at 11ftlb ish ( I think )

Rexs 175 engine 20 hp@ 7500.... 14ftlb ish

seems like maybe each extra 750rpm has given Rexs engine 1ftlb of torque and 4hp extra/750 rpm

while the engines bhp has increased from 12.6 to 21 bhp the torque value has only increased by 2 ftlb.

would the combustion chamber position/shape over the piston, being more to the front of the piston area, help with forcing of the piston back down the bore with greater power?

Love the topic title Nigel!

If you look at Rex`s engine spec you can see it is very modestly tuned, just the sort of data that does produces low revving, torquey power delivery, but for 186cc, not a lot of power. There is one exception though, the compression ratio is very high for this type of engine. I would suggest that this is giving the extra torque. But taking the tuning to typical race engine spec in an air cooled barrel will create heat problems that will only be contained with a lower cr. I don`t run much higher than that on the w/c engine! So Rex may have a problem just waiting to happen?
You could run the engine on 20:1 if the cylinder filling was very poor, little gas, little dynamic compression, so squeeze the hell out of what you do have. The more mixture that you get into the chamber the less static comp the engine can tolerate.
Yamaha flirted with an off-set combustion chamber on an early TZs, but for just one year! As soon as riders went just a little bit away from the factory set up the thing dettoed to destruction. A Bantam would do the same. If I was doing a head I would use a low, flat roof chamber with a suitable full depth radius to the edge of the 9mm wide squish band, of minimal clearance to use in conjunction with a 3mm, max height piston crown. Shorter flame travel, inter-active squish, shorter, more complete combustion, all are plusses, and all will give more torque.
The less the surface area, the less heat that is absorbed by head, barrel and piston the more remains to produce power; heat is power is heat. One caveat to this is that the cylinder/head finning must be open to lots of cool, free flowing air, and have sufficient surface area for the cooling to be effective? It is essential to have head fins rooted directly to the exterior of the combustion chamber, and the exhaust duct to be as short as possible!

Shorter con-rod, yes I would definitely do that! Combustion would benefit and heat transfer to the piston, etc, would be less because of the reduced time the piston would have at tdc. Angular, rotational thrust to the crank will be higher and absorb less energy in doing so. Transfer flow would benefit from the longer stay of the piston as it swings around bdc. All gains in torque.
Whilst the control of rules will not allow it, a retarding ignition would absolutely transform torque production. Being able to dial in a good degree of advance at low rpm and then retard at peak torque, is to introduce more efficiency, more torque and more reliability! Now, who doesn`t want that to be available to all, equality in a democracy?
Lastly the pipe, Rex`s pipe is doing him no favors, but he gets good results in despite that. The RS dyno graphs show that a longer tuned length than is perhaps ideal for absolute peak can offer improved torque below peak rpm.
Sadly, a static timing forces huge compromises none of which are good!

Hope that helps a bit, Trevor

If we only have 4 degrees. Of advance , what. Do you guess would be the best advantage of this I set to 0.070" static at the momemt

Nigel,
.070"=1.8mm, for a 58 stroke with a 125 rod that equates to 18.5* btdc, with a 4* retard that would then be 14.5* btdc, that seems pretty good as a starting point. So much depends upon the individual engine, may need a touch of adjustment either way. Pity you can`t dial in another 10* advance at the bottom end, that would make a tremendous difference to torque production.

Trevor

yes Trevor, programmable ignition would help. Although I think maybe as long as the system can only have 3* retard why couldn't we mechanicaly adjust the back plate at required rpm's. I havnt looked at rules for a while but I cant remember it stating when the 3* retard stops and starts

Nigel,
Back in the day, one rider I knew of had an adjustable contact breaker back plate that rotated when controlled manually by Bowden cable from a clip-on mounted lever. He reckoned it worked ok, at least when he remembered to work the thing! Perhaps the rules of today are there to interpreted and circumvented to some advantage?
Trevor

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Nigel,
       Just re-discovered this picture, it shows the impact a decent ignition system can make to an engine. No other alterations were made here other than plotting in the new ignition points and both were timed to 14* at 10,500 rpm. the far left line represents a few small improvements that could be made as a consequence of the top-end retard facility. The big surprise was the power past the old peak, again retard gave that!

Trevor

Manual advance and retard is very traditional, and since we now race with the Historic Racing crowd,
I think it should be acceptable.

I think you are correct John, no electronics involved, but doing a similar job for which there is historic precedence!
Go to it you guys!!!!.......win, win?

Trevor

Trevor, thanks for reply's and yes its such a difference I can see from your graph. Maybe the Hondas ignition curve is available to see on line somewhere, would be interesting to see the values that particular bike uses. I think Alan Brown had a two curve ignition option on his 125  bike for launch control off the line and then up and running situations.  ( in fact page nine in the timing , time thread shows the ignition curve details for aprilia)


edited again.. would be interesting to se a 175 engine, like perhaps michaels, set up with programable ignition.

Here you go Nigel

Not sure There's any movment or interpretation in the rules as regards ignition, its a fairly sort and sweet " ignition. Must be fixed " I would have thought the word. Fixed is fairly unable to be interpreted in more than one way m maybe ?

"Must be fixed" is pretty unambiguous. That's a pity - still it ensures a level playing field, and saves further complication.

the whole subject of rules has got me thinking

Rear Brake:    Must retain Bantam hub (may be modified) now this one has give me an idea, so I can in theory turn down the hub till its little more than a spacer, and use it in a nice alloy 36 hole hub and have a good picking of cheap alloy rims, after all it is meant to be a cheap class

also the rules don't say you cant have a switch to pick different curves or ignition timings etc, correct? hum..........


this one too
"Modifications to engine or gearbox are allowed, provided that conditions (a) to (i) below are met: -
(a) Bantam crankcases must be used, and remain externally unchanged, except for increasing stud centres to 60mm, on 52 and 55mm versions."
no where does it say the bantam engine has to power the bike, so pop in a rs125 engine, you must use the cases though, these could be screwed onto the side of the bike as some sort of crash protection ??

ok... must be fixed,  fixed to what? , IGNITION SYSTEM(CURVE) FIXED LINE , Ok, Oh but your allowed to let  it RETARD  MAX  4*  . doesnt say 4* atdc or btdc. Anyway I  should imagine the amount of heat produced would kill the air cooled engine, unless you used it only as a launch system
For me, i need to consentrate on staying on any  bike long enough to at least get around without having to think about pulling more levers or foot pedals

Rules are for the obedience of fools, and merely for the guidance of wise men, it is not for us mere mortals to question such things!

Not sure if that applies here, but some rules are forcibly adhered to even to the point of being detrimental to the original intention, desperately short sighted, and eventually, counter productive.

Like your thinking Nigel, conjures up all manner of comical and refreshingly surreal images! You seem rather like me, I can`t multi-task, just one lever at once and preferably in a repeating sequence. I can`t even work out which gear I`m in with the car, now that it has one more than I've been used to?

Your latest car has FOUR gears, Trevor?

Nothing but the latest, cutting edge technology for me John, I would never have believed it, four forward gears, what ever will the boffins come up with next?

Deleted as unnecessary......

Examining the exponential curve of the RS Honda ignition timing it is interesting to have a look at the final retard point of 5.1* btdc at 12,500 rpm, and apply its effects on non-specific engine behavior.
From the instant the plug discharges and gets the fire going in any meaningful sense, there is a delay, but the piston is still moving. Initial flame propagation is laminar (slow) in nature until sufficient turbulence and increasing flame-front spread develops to rapid combustion.
As a side note to this, squish action is at work well before the later, turbulent combustion, is fully underway!
Laminar flames have a very low, for our needs, propagation velocity of around 2mtrs/sec, mind you for the same fuel/air mix, detonation speed can be up to 2,000mtrs/sec, and unbelievable temperature, so better that you stay away from that!
What the ignition delay indicates is that 95% of combustion is happening with the piston descending. Other than simple mechanical compression there is little negative work hampering upward piston motion prior to this.
Gas entering the exhaust system is hotter and at a higher velocity, has expanded less so is more dense, both of which means more energy is available for the pipe to exploit later in the engine cycle. Torque is therefore maintained at higher overall levels and also for much later in the rev band. If more heat then goes into the pipe it follows that less can go into the piston, barrel and head, and for air-cooled engines that has to be a bonus! There is a finite amount of total energy to be shared around!
After effective completion of combustion there could be perhaps 300* of crank rotation available for external cooling, before commencement of the next ignition/combustion phase?

Allin0,0048ofasecond...

208.333 times up to TDC in a second --- phwew!

I am too old-fashioned for this -- I am told my computer is too slow at over 1,5 million bits (or bytes) per second -- everything is going too quick....

John,
It`s only after doing the sums, can a full appreciation of the incredibly short time scales that events in high speed engines occur at, the wonder is that the thing is able to function at all?
An endlessly fascinating subject, and at the same time, frustratingly complex!

Trevor

Too true Trevor.

A couple of years back I had the urge to do somrthing with bits of junk I´d accumulated. I made a model (single-acting) steam engine  because we had a steam-cleaner and it seemed the right thing to do.

I estimetad that it ran at a speed(?) of 0ne to 1 &1/2 revs per SECOND... and during each rev it would go to tdc quite slowly and then rush like mad on about half the expansion stroke. Everyone reading this is going to say, "Well what did you expect, you dozy old AP?"

Of course! No flywheel.

With a flyyweel fitted it needed much more  push on the crank ( actually then flicking  the  flywheel rim) to get it to run but it ran with less jerking motion at about 5 times the speed of earlier.

One could call it a nonsense exercise yet I needed to do it...

Perhaps this might generate some discussion on the merit (or NOT) of a large flywheel in the Bantam Racer....

Re flywheels - back in the 1960s I was in the IOM at TT week, and went to Jurby airfield to see if any of the factory teams were testing. Honda were there with the amazing 50cc twins. They were doing high speed carburation/plug checks. I was standing at the side of the runway near where the rider shut off and pulled in the clutch at full chat. The engine stopped as though it had seized. Not much flywheel effect there! Also it was surprising how noisy a motorcycle is with a dead engine at around 100mph. The rush of the slipstream, the hum of the chain, and the tyre noise were very considerable.

There was a rider there who had just swapped from a Honda contract to ride a rival two stoke 50 - I think it was a Kreidler he was testing, but can't remember who the rider was. He seemed to be determined to impress Honda with his new mount, but slid off in an embarrassing heap, turning round at the end of the runway!

With a crank of this tiny proportion, small wonder there was little apparent torque?
Exquisite workmanship, so typical of Honda.

Trevor