JE878 gone bad

[dandywarhol]

I would agree with the weight loss from the pistons - every gramme becomes a helluva lot more at 8000 rpm

At 8000rpm the piston travels up and down the bore 133 times a second!! With a giant 448g piston the force/stress on the small end journal must be considerable with that level of acceleration/deceleration. No wonder lubrication is critical. It is a miracle that the internal combustion works at all, never mind lasting hundreds of thousands of miles.

Now go and work out the piston speed @ 8000 rpm Mind you, the stroke is quite short on these motors so it isn't humungously high but still v. fast at mid stroke.

Then consider that the piston travels less distance in the last 90 degrees than the first 90 degrees of crank rotation and you then understand about non-sinusoidal vibration

[cobbadiggabuddyblooo]

When I talked to a few engineering shops about balancing, even though there workshops where full of race cars being rebuilt, they all have me the same answer" Yer bring it in we can do that" but not much more was said. The old fella tuning the head who works from under his home put me on to another old dog working from the back shed from his house and balancing cranks is all he does. The first thing he asked me how I'm tuning the motor . He requested the specs from the headwork to find what range of the RPM Spector its tuned to and he will balance accordingly to suit. As for tollerence that's just my sense of humour and play on words . Cheers

[Tarwetijger]

One more thing about the piston weight and stuff. As I see it, the original piston pin looks quite sophisticated. Especially compared to the JE pinston pin, which just looks thick. Now I was wondering: how stupid would it be to use the original piston pin with the new JE pistons?

[cobbadiggabuddyblooo]

The JE pins are 2mm longer..

[Tarwetijger]

The JE pins are 2mm longer..

So..?


Okay okay, I already knew it was not going to work but I was curious why.

[misterdimwiddy]

One more thing about the piston weight and stuff. As I see it, the original piston pin looks quite sophisticated. Especially compared to the JE pinston pin, which just looks thick. Now I was wondering: how stupid would it be to use the original piston pin with the new JE pistons?

I am having the same thought myself at the moment.

Using Yamaha pins would bring the weight of the JE and the Yamaha piston assemblies very close without the need to machine the JE pistons to shed the 40g difference.

1mm at each end doesn't sound a lot in terms of bearing surface area.

Was reading last night that forged pistons are a lot more dense than cast and that you can have a reduced crown and skirt thickness with forged items. The JE however are pretty thick internally below the oil ring (compared to stock) which is where I am considering losing some metal. I'm leaving the crown alone but may look at the region around the pin as well.

I have not got the courage to be the one who sticks Yamaha pins into JE pistons just in case there is a good reason why they are longer and thicker

Could always reduce the thickness of the JE pins either through the full length of at the piston bearing ends leaving the centre full thickness? Gotta be easier than machining within the piston casting.

My hope is that this will be my final TRX engine rebuild so I am not talking any chances!

Keep us posted with the route you decide to take.

[cobbadiggabuddyblooo]

https://www.google.com.au/search?q=mach ... &gws_rd=cr

You'll find info on here from Carillo and JE piston design

[Kenny]

The more pin clearance at the curclip this gives the pin the opportunity to hammer the clips and cause premature failure.

[misterdimwiddy]

https://www.google.com.au/search?q=mach ... &gws_rd=cr

You'll find info on here from Carillo and JE piston design

Good info thanks Cobba.

I'm now going off the idea of piston machining , especially in the undercrown region after the finish OD has been machined because this may relieve stresses in the forging that could change the skirt profile and ovality. Mixed opinion on this online but makes sense to me. Heat treatment after machining is probably a good move but I am not going to these lengths.

May just have to live with he extra 40g's.

Bored JE pins still seems the most simple way to lose weight..............

[dandywarhol]

Omega pistons (I used them on a race Kawasaki 984 engine) used teflon mushroom pads to keep the gudgeon pin in place - no circlips.

[misterdimwiddy]

Reading my way through '4 stroke performance tuning' by A. Graham Bell and stumbled upon the following equation to calculate piston acceleration at TDC.

a=0.000457*N^2*S*(1+(S/2L))

where

a = acceleration ft/s^2 (horrible imperial units)
N = rpm
S = stroke (67.5mm or 2.657")
L = rod length (5")

Putting some numbers into this, and if I have done my calculations correctly, the indication is that over revving has an enormous effect on piston acceleration because rpm is a squared term in the equation.

Revving an extra 1000rpm to 9k results in a 27% increase in acceleration which then means that using F=ma (Force is proportional to acceleration), Force on the rod (small and big ends) also increases by this % amount.

For a given level of acceleration, if piston mass increases (as the JE does being 40g heavier than stock), the force F increases even further.

Need to double check my calcs but I reckon over revving to 9k with 448g JE pistons combines to give a 39% increase in force on the small end compared to a stock piston at the red line.

Even with a perfectly healthy lubrication system, this has to be a major threat to effective small end lubrication (much smaller bearing surface area than the big end) which doesn't even have pump pressure to maintain the seperation between rod and pin surfaces.

Pickup on my rods/pins is not dreadful but I am now fairly confident that the damage is my own fault through (usually unintentional) over revving and that keeping to 8k (which then only results in a 10% Force increase due to the 40g extra piston weight) should give me a reliable motor.

Bore and ring wear has resulted from running the JE top end with a knackered crank back in April.

Thinking of using a programmable ign unit (again thanks to Sandune51 who pointed these out to me) to enable me to set a reduced rev limit. You could simply say 'don't over rev it' but on track I find it difficult to focus entirely on the tacho and it revs like a good'un with all the goodies attached.

May all be Sunday morning nonsense but at the moment it seems a plausible explanation to me.............

[Rod.s]

Reading my way through '4 stroke performance tuning' by A. Graham Bell and stumbled upon the following equation to calculate piston acceleration at TDC.

a=0.000457*N^2*S*(1+(S/2L))

where

a = acceleration ft/s^2 (horrible imperial units)
N = rpm
S = stroke (67.5mm or 2.657")
L = rod length (5")

Putting some numbers into this, and if I have done my calculations correctly, the indication is that over revving has an enormous effect on piston acceleration because rpm is a squared term in the equation.

Revving an extra 1000rpm to 9k results in a 27% increase in acceleration which then means that using F=ma (Force is proportional to acceleration), Force on the rod (small and big ends) also increases by this % amount.

For a given level of acceleration, if piston mass increases (as the JE does being 40g heavier than stock), the force F increases even further.

Need to double check my calcs but I reckon over revving to 9k with 448g JE pistons combines to give a 39% increase in force on the small end compared to a stock piston at the red line.

Even with a perfectly healthy lubrication system, this has to be a major threat to effective small end lubrication (much smaller bearing surface area than the big end) which doesn't even have pump pressure to maintain the seperation between rod and pin surfaces.

Pickup on my rods/pins is not dreadful but I am now fairly confident that the damage is my own fault through (usually unintentional) over revving and that keeping to 8k (which then only results in a 10% Force increase due to the 40g extra piston weight) should give me a reliable motor.

Bore and ring wear has resulted from running the JE top end with a knackered crank back in April.

Thinking of using a programmable ign unit (again thanks to Sandune51 who pointed these out to me) to enable me to set a reduced rev limit. You could simply say 'don't over rev it' but on track I find it difficult to focus entirely on the tacho and it revs like a good'un with all the goodies attached.

May all be Sunday morning nonsense but at the moment it seems a plausible explanation to me.............

Awesome, now we're talking, love your work now factor the pressure taken to force the oil form the gudgeon pin surface and you are on the way to finding out the best oil to use for high RPM! So is the pin designed to rotate in the piston or is it static?

To keep to your red line, fit a rev limit light (my choice) or a rev limiter. The light should be mounted in your peripheral vision allowing you to keep the track in view and not over rev…..
Rod

[dandywarhol]

Good stuff Mister - I used to do that sort of stuff with my HND students - kinda forgotten most of it now!

I don't know of any bike engines with oil pressure going to the small end - all splash fed of the pressurised stream from the tiny hole in the bottom of the rod. What has been a trend in the last 30 or so years is to run the pin directly on the rod - no bush, but there again, many engines had the pin secured in the small end and running directly on the piston surfaces (but they tended to be lower revving)

[Rod.s]

Good stuff Mister - I used to do that sort of stuff with my HND students - kinda forgotten most of it now!

I don't know of any bike engines with oil pressure going to the small end - all splash fed of the pressurised stream from the tiny hole in the bottom of the rod. What has been a trend in the last 30 or so years is to run the pin directly on the rod - no bush, but there again, many engines had the pin secured in the small end and running directly on the piston surfaces (but they tended to be lower revving)

Yep the gudgeon pins are not pressure fed, we all know that, but the oil is under pressure on both strokes, it's this pressure that the oil needs to withstand to maintain a film over the contact points. Oh and buy the way, my high revving 11000-12000rpm Honda has pressurised oil feed to the gudgeon pins, quite common really

[cobbadiggabuddyblooo]

Good to here you have a copy of the bible A wealth of information in there . It's one of those books once you start reading everything really starts to make sense.
How often have we all started on something, pulled it out of the box ,began to put it together and then pick up the instructions after half way through..
There are a few fundimentals to just keeping everything in balance and once you look at what you really want from the motor you can make the most by complimenting each modification.

Interesting reading on timing in there, I've been supplying a few of the lads here with programable ignitions and dynatec coils .Straight away they notice an overall improvement.
Go for a ride behind a bike with a stock ECU and then stick the prog ignition inplace and listen to the difference straight away just on the default settings.
The stock ECU is programmed to cover all sorts of fuel octane ratings, emission, noise restrictions and laws and with the capability to take your TPS it gives you a constant 3d mapping on that basic map to start. Even your aftermarket ECU's like ones from Over have a basic map that is programmed on their Over race spec including cams, pipes, headwork and carbs ect.
Then you TPS gives you mapping over this one program.

The beauty of the programable igition is you can taylor your own map to the amount of mods to your own motor and where it is tuned to perform,then save this preprogramme as your own personal in amoungst the other number of presets in there.
There is graph mapping to show through the rev range or just utilise the 100rpm incriments within the program to change the timing where you find that dip in HP from your dyno results.
Something the others don't have because your 3d mapping from the TPS in the stock and other units is from the one and only progam set in default.
Also if your FCR's dont have a TPS ,you can just turn that function off through a simple program. Plus as you've been made aware, you can set your own rev limit on the limiter. Excite the dwell on the spark to suit coil needs.
Straaight away following Alex last weekend up the goat track I pointed out to him there where no more big puffs of black smoke coming out of his exhaust when on the throttle. He noticed the colour change in the pipe too.
Just getting the correct fuel / air mix ,and not loosing that density through heat and a correct burn is a fundimental to getting the most out of whatever you do otherwise you start to fight against the laws of nature and it's all about the nature of the beast between your legs.. Maybe I should rephrase that..
Have a read on the section on exhausts too as this is a complimentary to it all on how your pulse scavanges .
You will then apreciate the beautiful nature of an Akrapovic system and the abundance of low/midrange torque with just under 6H/P gains on a stock motor alone around 4500rpm.
Best mod anyone can do to a TRX with a budjet is to just change the stock ECU with a programable unit.
Sounds like it's all making sense to you and your on the right path.. It's great to see your making the effort and taking the time to understand how and why. Rome wasn't built in a day..
Well done for putting the research in to get the result you want