HarleyTechTalk

Hi guys been a while since I posted the bike put on a lot of miles this summer.
Anyway I am running a TC A 106 stroked 4.5" motor with other bells and whistles fueling pump and plate. Since may I have put almost 11000 miles on it I have the revlimiter set at 6000 and shift light at 5800.
Question I have is about piston speed specifically I don't understand the limits I know I am up over the 4000 ft/min mark but how bad is it. If someone could point me in the direction of good reading material or please explain what kind of detrimental forces I am exposing my engine to I would greatly appreciate it

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With out  variable cam timming you are stuck with around 4000 revs , so the cam has alot to do with your piston speed, A motor,say if your cam starts to produce @1800revs then it is a 6000 rev limiter or 6200.shift light should be at nearly peak ,sounds right being around 5800.id leave rev limiter on  A @6000 unless your cam is still producing @5800 . dyno chart would be nice , and gearing of bike , cheers Barny

At 6000 rpm your piston speed is 4500 ft/min
At 5800 rpm your piston speed is 4350 ft/min

At high piston speeds you get problems with ring seal and ring flutter. Thin piston rings are one of the ways out of that problem but I suppose they would wear more rapidly?

It is possible to calculate the acceleration forces felt by the piston, and even on ordinary bike engines they are surprisingly high! Yours will, of course, be even higher... To calculate it you need to know the stroke and the rod length, rod-ratio might be enough but as I can't remember the equation that is no help to you. I have a notebook 'somewhere' with all this stuff in it. Can I find it? No.

Meanwhile, yes you are running very very close to the edge, yes your engine will break, but you know that anyway. Now, is it fun? If it is then rebuild it when it breaks and keep on doing it. Do you have a better understanding now why big-bore 4" stroke is a really good idea?

A really good book that tells you just about everything you ever wanted to know about the internal combustion engine is the two-volume set by Charles Fayette Taylor. The Internal Combustion Engine in Theory and Practice Volume 1 Thermodynamics, Fluid Flow, Performance, Volume 2 Combustion, Fuels, Materials, Design. I have the Revised Edition dated 1985. Published by the M.I.T. Press ISBN 0-262-2005-1 or 0-262-70026-3

I've been thumbing through the books. Volume 2, page 399 quote "Let it be assumed that, after studying this table (table 10.13 p401), the designer feels confident of securing adequate racing reliability at 4400ft/min piston speed."
By 'adequate reliability' he means a good chance of the engine finishing the race in one piece knowing it will have to be fully rebuilt for the next race!

Barny is right, a dyno chart would be a big help. Are you still making good power at 5800rpm?
Knowing your gearing and the torque curve means you can calculate the best shift point for each gear. Having said that, on all standard Harleys the best shift point for max acceleration is at the red line! That is because the gearing is wrong for a performance bike, and you might want to have a look at the Baker 7 speed.

Very good PC Hater....you did your home work....good answer.
Harley wants your pistion speed to be under 4300fpm for longeveity of your motor however 4500fpm will not make it or break it....you are still in range.
Enjoy and your motor isn't going to break at 4500fpm anytime soon.
Doc

Hey thanks alot for the info so far. I have attached one copy of the last dyno run but I just noticed it is not marked in rpm I will go talk to Walt today and get a copy printed out with the rpm scale but may take a day since the dyno and his computer are at his house.

Anyway the whole process behind my thinking is looking for a new cam/head combo this winter and I know I could just ask for advice on combos but the more I have learned about building this motor the more I want to understand about what's going on inside of it.

Also the gearing is stock and I haven't looked to hard at the Baker 7 but that's not completely out of the question which leads me to another question what should I be looking to do next gearing or the motor wish I could do both.
Thanks


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The house (all of it) needed a bit of a tidy, I can find things again, including the elusive notebook.
the max piston acceleration is calculated using the following equation:-

crankshaft rpm x crankshaft rpm x stroke in inches              1
------------------------------------------------------------   x (1+  ------  )   feet per second per second
                              2189                                                 2R

R is the Rod Ratio = 1.91675 based on 4" stroke and 7.667" rod length
2189 is one of those 'magic' numbers that are needed when you use Imperial units for proper engineering calculations that should be done in SI units.

So, for a TC88 at 6000 rpm the max piston acceleration is 82,943 feet per second per second or 2,591g if you want to think in terms of 'g' force and gravity
for your bike at 6000 rpm assuming you used the same rod it is 117,443 feet per second per second or 3,670g.

for a Norton Commando it is 52,057 ft/sec/sec
for a Harley 45 it is 45345 ft/sec/sec @ 4500 rpm

All that extra loading from the piston acceleration stresses the piston, stresses the con rod, and puts more force through the big end and main bearings.

Quote from: flyingfish427 on November 12, 2011, 05:29:32 AM
Anyway the whole process behind my thinking is looking for a new cam/head combo this winter and I know I could just ask for advice on combos but the more I have learned about building this motor the more I want to understand about what's going on inside of it.

What sort of performance do you want from your motor? That helps to define the cam/head combo.
What sort of performance do you want from your bike? That helps define the gearing to aim for.

How often are you on full throttle wanting more?

The waterfall diagram for my TC88 is attached as a pdf. 84 ftlb max torque, 74HP max.
See the big gaps between the gears? I have to redline it in every gear to get max acceleration. I rarely do that of course, occasionally in 3rd I hit the redline. I do hold the bike on full throttle but not often enough to want to pay money to do anything about it. The performance is very definitely 'adequate'!

[attachment removed after 60 days by system]

At high piston speeds you get problems with ring seal and ring flutter.

Good point! The 4.5" stroke 106 has very short skirts and this would be my concern. After all poor ring seal (flutter) not only looses power due to compression loss but causes poor heat transfer. IME with this motor in the early 2000s era they were short lived.

While running the numbers for piston speed and acceleration are fun.. The motor important thing to worry about  may not be related to the numbers.. You mention the stroke is 4.5 inches and it's 106.. What's more important is the actual cylinders and pistons you used. The most common 106 is SnS which does 3.875 in stock/se cylinders. The piston spigots on the these cylinders are short for this kind of stroke and piston pull pretty far out of the spigot at BDC so they can rock a bit.. I'm not sure if it the length of the spigot or the design of the piston itself but the pistons and rings on these have been reported to come apart at about 20 to 30K regardless of how the motor was run.. What is interesting is that the current 4 3/8 stoke 103 motor seems to work just fine.. So BDC may not be an issue but piston design.    or possibly a combo.. Remember adding that 1/8 inch of stroke (4 3/8 vs 4 1/2) means that the piston needs to come down 1/8 inch more when not changing cylinder length.

Max

Your are right on track there 

OK. So what's a good max piston speed/rpm on the 120r for long life when used in a touring enviroment?
Ron

I would not get too wrapped around the axle about it. The important factor and bigger picture is they shortened the rod and have an improved skirt plus longer spigots by 5/8". That combined with the Ti coated 1.5mm twist rings and the nickel coated skirts all add up to improved ring seal, less ring flutter, better heat transfer, and improved ring life.

FWIW,

Deweysheads is talking about the 120R motor not the 106 motor.. Just a clarification... Max

Quote from: Deweysheads on November 12, 2011, 07:55:34 AM
I would not get too wrapped around the axle about it. The important factor and bigger picture is they shortened the rod and have an improved skirt plus longer spigots by 5/8". That combined with the Ti coated 1.5mm twist rings and the nickel coated skirts all add up to improved ring seal, less ring flutter, better heat transfer, and improved ring life.

Not getting too bent out of shape with it since I'm limiting mine to 5800-6k anyway on my softail.   I thought shortening the rods was the wrong direction to go due to increased rod angle. Did I miss something?
Ron

RE: the dyno chart, torque and HP always cross at the same RPM....5250 (from memory)......

Piston speed is commonly used as a reference to the limits that a reciprocating engine will turn without serious harm.  A factor not mentioned so far is piston/rod stretch and compression. As the engine turns over the piston and rid follow an elliptical orbit of acceleration and deceloration. Where as piston speed is the total distance traveled decided by time.


Without knowing your piston weight or rod weight I am sure the commonly used figure of 16,000# acceleration and deceleration at 6000rpm will be close to most HDs.  That is saying as the piston goes up the cylinder there is in a moment 8 tons of acceleration pressure acting against the crank pin. That will compress the piston and rod around .016" (lots of variables here to take it out 5 or 6 decimal places) The reverse is true when the piston is getting pushed or pulled back down the cylinder. That is a function of weight, modulus of elasticity of the metal and time.


Where things happen is when those forces find the weakest point in the piston or rod and separate.  Generally 4000' will allow for commonly made reciprocating parts. 5000' will allow for forged and high tencsil products and 6000' beyond is a limited life for exotic parts.


There are lots of factors effecting this as ring material, clearance, weight, tension, cylinder surface, clearance, lubrication.

Not getting too bent out of shape with it since I'm limiting mine to 5800-6k anyway on my softail.   I thought shortening the rods was the wrong direction to go due to increased rod angle. Did I miss something?
Ron
That's in chapter 2.

Remember adding that 1/8 inch of stroke (4 3/8 vs 4 1/2) means that the piston needs to come down 1/8 inch more when not changing cylinder length.

I thought it was split evenly between top & bottom?

Quote from: rbabos on November 12, 2011, 08:33:06 AM

Quote from: Deweysheads on November 12, 2011, 07:55:34 AM
I would not get too wrapped around the axle about it. The important factor and bigger picture is they shortened the rod and have an improved skirt plus longer spigots by 5/8". That combined with the Ti coated 1.5mm twist rings and the nickel coated skirts all add up to improved ring seal, less ring flutter, better heat transfer, and improved ring life.

Not getting too bent out of shape with it since I'm limiting mine to 5800-6k anyway on my softail.   I thought shortening the rods was the wrong direction to go due to increased rod angle. Did I miss something?
Ron

The benefits of the shorter rods far out weigh the deficit especially at the rpm range these motors run in

Actually the 1/8" longer stroke is made up of the crank going 1/16" deeper and higher in the cylinder. The piston pin location compensates for the increased height on compression stroke.  Shorter rids make for higher lateral thrust loading on the piston as compared to longer rods. Longer rods behave better under higher rpms.   Also the time spent (Velocity) at TDC and BDC change slightly with rod length.

Longer rods behave better under higher rpms

OK tell the rest the con side & frame your answer based on a 6,000rpm max only very occasionally and the majority of the time spent below 4k. The 106 and the 120r are a great compare and contrast to illustrate how the Smokey doctrine may not be the best for this motor especially when the skirts are too short and they punch out the bottom. In car circles the 420 SBC guys have learned this lesson. 

QuoteActually the 1/8" longer stroke is made up of the crank going 1/16" deeper and higher in the cylinder.

Technically you mean the connecting rod end  not the crank.. Max

Longer rods behave better at ALL rpms!
But then we get to define what we mean by 'better'.
A longer rod reduces secondary engine vibration - the vibes at twice engine speed. You might choose to ignore it on a rubber-mount motor but those extra vibes also mean extra stresses.
A longer rod 'softens' 'changes' the speed at which the piston reaches Top Dead Centre. That affects breathing, that changes the cam design for that particular engine.
As an end-user all you can do is choose from what is available in the market place. With a bit of background reading you can improve your chances of ending up with a powerful and reliable motor.
I like the RB Racing website. It makes me laugh. And their AFR gauge works on my bike so I like them even more. As the stroke lengthens on their Orca motors the piston pin diameter increases. With the info in previous posts about the increased stresses as the stroke gets longer you now understand why they do that. Do the pistons in 'your' long-stroke motor have large diameter piston pins? If not, why not?

A quote from 'Some Unusual Engines' by LJK Setright published by Mechanical Engineering Publications Ltd in 1975 ISBN 0 85298 208 9. If you don't have a copy, buy one or borrow one from a library. Chapter 5 page 99 "Can one imagine the internal combustion engine having a place in an ideal world? If that be possible, it is certain that if it had connecting rods they would be of infinite length."

As we are all now very old the UK crew will remember reading the LJK Setright column in Bike magazine.
Do get hold of a copy of the book if you can - it is a delightful read with enough technical info to be interesting without drowning you in stuff you can't understand. It was written as a result of an Institution of Mechanical Engineers meeting on interesting engines. Aero engines, ship engines, tank engines, racing car engines, plain weird engines.