Conrod length

[john.r.davies]

Please will someone tell me the length, centre to centre, of a TR6 conrod?

 

Thanks!

John

[Richard71]

146.05mm according to these guys..........http://www.maxspeedingrods.co.uk/high-performance-triumph-tr250-tr6-late-4340-en24-chrome-moly-forged-h-beam-connecting-rods-conrod-with-arp-bolts-x6pcs.html

 

Richard.

[TriumphV8]

Its 5.75" and its one of the worst rod/stroke ratios

of any stock built engine with its 1.53.

Thats why I increase it to 5.85 with great success.

[astontr6]

Andreas,

 

Please can you explain your comment, as I thought that any very long stroke is not a very good design?

 

Bruce.

[john.r.davies]

Thank you! The link Richard provided is to a set of high performance rods, in the Carillo style, and they are 146.05mm or 5.96" long.

So Andreas' 5.75" for OE rods, with his recommendation to make them longer may match others opinion on this.

Why did I want to know?

 

In my current assignment, I'm discussing mean piston speed (MPS) and acceleration, at high engine speed.

This will be old stuff to many, but the ideas are new to me

 

2L, 76mm stroke
At 6000rpm, MPS is 15.2m/sec
At 7000 it's 17.7m/s
At 7500 it's 19.0m/s

 

2.5L, 95mm stroke
At 6000 the MPS is 19.0m/s
At 6500 it's 20.84m/s

(This can be compared with engines that rev very high, for instance bike engines. A Honda Fireblade has a stroke of 55.1mm and is limited to 11,500rpm. So the MPS in that engine is 2112m/s, but that uses relatively exotic materials. The 2.5L engine relies on a crank that is 25% heavier than the 2L.)

 

This is merely the mean speed, and simply done from basic stroke and speed. But the piston must accelerate from rest at Bottom Dead Centre to rest at Top Dead Centre, so the speed in the middle of the stroke will be greater, as will the stress on crank, conrod and piston due to the acceleration. That calculation is more difficult, but a simple formula for piston acceleration, is:

 

0.000457 x Revs^2 x Stroke in inches x (1 + Stroke/2 x Conrod length in inches) (in ft/sec^2)

 

which for the 2L engine is 64167 ft/sec^2, and 81541 ft/sec^2 for the 2.5L engine. This is more than 2000, and 2500 times ‘g’, the acceleration due to Earth’s gravity.

 

No wonder the 2.5L needs a steel crank, special conrods, pistons etc.etc, to use revs higher than 6K for very long!

 

As you can see, conrod length is irrelevant to the mean speed, but included in the acceleration calculation. And it's below the line, used to divide, so a longer rod will mean less acceleration.

That may be the rationale for the opinions of Andreas' and the makers of the high performance rods Richard linked to.

 

John

Edited October 31, 2016 by john.r.davies

[RogerH]

Hi John,

haven't a clue what you are talking about.

Thankfully the motion of the piston and attached gubbins is sinusoidal and has quite a comfortable life - nothing to abrupt.

 

Sadly anything falling out of the sky at 1g has a sudden stop at BDC (bottom DEAD centre)

 

Roger

[Peter Cobbold]

In my current assignment, I'm discussing mean piston speed (MPS) and acceleration, at high engine speed.

 

2L, 76mm stroke

At 6000rpm, MPS is 15.2m/sec

At 7000 it's 17.7m/s

At 7500 it's 19.0m/s

 

2.5L, 95mm stroke

At 6000 the MPS is 19.0m/s

At 6500 it's 20.84

This is merely the mean speed, but the piston must accelerate from rest at Bottom Dead Centre to rest at Top Dead Centre, so the speed in the middle of the stroke will be greater, as will the stress on crank, conrod and piston due to the acceleration. Calculation 2 estimates this, which for the 2L engine is 64876 ft/sec^2, more than 600 times ‘g’ the acceleration due to Earth’s gravity.

Max acceleration of piston assembly is at TDC, because the load is reversing, and that implies an 'infinite' acceleration momentarily.

fig 8: http://www.epi-eng.com/piston_engine_technology/piston_motion_basics.htm

Peter

.

[Peter Cobbold]

Hi John,

haven't a clue what you are talking about.

Thankfully the motion of the piston and attached gubbins is sinusoidal and has quite a comfortable life - nothing to abrupt.

 

Sadly anything falling out of the sky at 1g has a sudden stop at BDC (bottom DEAD centre)

 

Roger

Not truly sinusoidal, TDC has a different geometry to BDC.

The reversal of motion at TDC is the most extreme rpm-related loading.

Peter

Peter

[RobH]

Bruce - the thing Andreas is describing is the geometry of the conrod path, not the stroke. Of course the stroke is determined by the offset on the crank, not by the length of the conrod. For best geometry the rod should be long in relation to the piston stroke, so that the angle it describes during rotation is small. That means the force from the piston can be better transmitted to the crank - a short rod increases the angle from the gudgeon pin to the crank and hence sideways force, friction and losses are greater. However a long rod is bad for weight and strength and requires a tall crankcase, so a compromise has to be made. Andreas is saying that the stock rods are a bit short and the geometry can be improved by altering the rods and pistons but that does not change the piston stroke.

[Peter Cobbold]

Andreas,

 

Please can you explain your comment, as I thought that any very long stroke is not a very good design?

 

Bruce.

Bruce, As Rob says. The stroke is set by the crank journal throw. Changing con rod length doesnt affect cubic capacity, it has a subtle effect on the rate of change of piston velocity /crank rotation either side of TDC.

Peter

Edited October 31, 2016 by Peter Cobbold

[RobH]

Max acceleration of piston assembly is at TDC, because the load is reversing, and that implies an 'infinite' acceleration momentarily.

 

Er- no Peter. Fig 8 in your post shows that acceleration reaches a peak at the point where the motion changes direction but that does not imply infinity by any means. After all, F=MA - you cannot have infinite mass nor infinite force, it is very definitely finite.

[TriumphV8]

Andreas,

 

Please can you explain your comment, as I thought that any very long stroke is not a very good design?

 

Bruce.

 

Hi Bruce,

long stroke is one thing to bring more displacement into an engine.

As this is the base to get proper power just from idle and does not

affect driveability I like that way very much to tune an engine.

I dont thing there is something worse because we do not rev a street

engine that much that we limit the engine on top end due to piston speed.

 

Besides the mean piston speed the acceleration of the piston is

a thing to be taken into account.

With a short rod the piston is faster pushed away from TDC and

acceleration is faster.

That brings two things:

1.) The combustion must be fast to bring good results because

piston is quicker on its way downwards before combustion is fully working.

There is a different way how the fuel burns and delivers power

when the rod length is changed (what in practice will need a

different piston to compensate the length difference)

 

2.) On the other hand at low revs a short rod gives a better "transmission"

to bring the power to the crank and gives more low end torque

(for the price that the piston skirt is pressed more to the bore on the load side).

 

By empiric the rod length/stroke ratio is found about 2 to 1.75 for best compromise.

For special reasons this is sometimes expanded like with our TR6. The reason was here

that in those times it was difficult to make a shorter piston and was interesting to use

the existing rods because that is much cheaper.

 

So besides the small 2 litre cylinder head the rod ratio is a main problem

for higher revs. As the TR6 anyway will have its limits with piston speed

there must not be done much but a 6" rod in the TR6 would be wonderfull.

 

To point that out clearly: With a long rod the piston "stays" longer in TDC and

the combustion can build up pressure longer until the piston moves downward.

 

By the way: In the TR4 these longer rods are often in use in race cars.

In the TR6 I am limited to my excentrical gudgeon pin but as its the right

way I shall believe in it......

 

 

This is the Maxspeedingrod prepared for the VW piston with excentrical bearing.

Also in the picture is the stock rod with the 77.75 Mazda piston

Edited October 31, 2016 by TriumphV8

[john.r.davies]

Thank you, Peter. Of course, acceleration is greatest around TDC as it changes direction Doh!

But I agree with Rob - finding TDC with a DTI shows clearly that the piston is stationary for a period around TDC, which makes its precise measuremen quite difficult. It doesn't change direction instantly.

 

Thank you Andreas, I was puzzled by your reference to "1.53" but now I see it is the stroke/conrod length ratio.

 

John

[Peter Cobbold]

Max acceleration of piston assembly is at TDC, because the load is reversing, and that implies an 'infinite' acceleration momentarily.

 

Er- no Peter. Fig 8 in your post shows that acceleration reaches a peak at the point where the motion changes direction but that does not imply infinity by any means. After all, F=MA - you cannot have infinite mass nor infinite force, it is very definitely finite.

Rob, My argument is based upon F going from positive to negative. But I could be wrong. Peter

[RobH]

The sign of the acceleration curve does not change at the peaks Peter and neither does the force - upward deceleration is the same 'direction' as downwards acceleration. The sign changes at the zero-crossing points where velocity is at a maximum and acceleration at a minimum.

[Peter Cobbold]

The sign of the acceleration curve does not change at the peaks Peter and neither does the force - upward deceleration is the same 'direction' as downwards acceleration. The sign changes at the zero-crossing points where velocity is at a maximum and acceleration at a minimum.

Then I dont understand why conrod bolts fail if their tensile strenght is inadequate. Isn't that due to inertia forces acting at TDC when the piston/ conrod reverses direction ?

Peter

[RobH]

Perhaps fatigue failure which is cumulative rather than a straight tensile overload ? Thinking about it, I was wrong that the force doesn't change direction, because where it is coming from changes. On the firing stroke it is compressive force pushing down on the piston whereas on the induction stroke it is the conrod pulling backwards in tension against piston inertia and friction.

Edited October 31, 2016 by RobH

[Andy Moltu]

 

 

Rod for VW piston01.jpg

 

This is the Maxspeedingrod prepared for the VW piston with excentrical bearing.

Also in the picture is the stock rod with the 77.75 Mazda piston

 

Presumably the reason the Maxspeedingrod fouls the bore is because of it's width - is that why you use the eccentric small end bearing?

[john.r.davies]

Gosh, all I asked for was help with my homework!

 

But continue the tutorial!

JOhn

Edited October 31, 2016 by john.r.davies

[TriumphV8]

 

Presumably the reason the Maxspeedingrod fouls the bore is because of it's width - is that why you use the eccentric small end bearing?

 

Its for expanding the working length of the rod

without buying an expensive custom made.

Stock is 5.75" this is about 5.85"

[Dave McDonald]

Wow, way beyond me, I think I'll have to read all this lot again to understand it!!!

 

On the other hand, perhaps not. I'll read Motorsport mag instead.

 

Good luck with your homework John.

 

Dave McD

[astontr6]

Hi All!

 

Thanks for all your responses.

 

Bruce.

[Richard71]

 

Hi Bruce,

long stroke is one thing to bring more displacement into an engine.

As this is the base to get proper power just from idle and does not

affect driveability I like that way very much to tune an engine.

I dont thing there is something worse because we do not rev a street

engine that much that we limit the engine on top end due to piston speed.

 

Besides the mean piston speed the acceleration of the piston is

a thing to be taken into account.

With a short rod the piston is faster pushed away from TDC and

acceleration is faster.

That brings two things:

1.) The combustion must be fast to bring good results because

piston is quicker on its way downwards before combustion is fully working.

There is a different way how the fuel burns and delivers power

when the rod length is changed (what in practice will need a

different piston to compensate the length difference)

 

2.) On the other hand at low revs a short rod gives a better "transmission"

to bring the power to the crank and gives more low end torque

(for the price that the piston skirt is pressed more to the bore on the load side).

 

By empiric the rod length/stroke ratio is found about 2 to 1.75 for best compromise.

For special reasons this is sometimes expanded like with our TR6. The reason was here

that in those times it was difficult to make a shorter piston and was interesting to use

the existing rods because that is much cheaper.

 

So besides the small 2 litre cylinder head the rod ratio is a main problem

for higher revs. As the TR6 anyway will have its limits with piston speed

there must not be done much but a 6" rod in the TR6 would be wonderfull.

 

To point that out clearly: With a long rod the piston "stays" longer in TDC and

the combustion can build up pressure longer until the piston moves downward.

 

By the way: In the TR4 these longer rods are often in use in race cars.

In the TR6 I am limited to my excentrical gudgeon pin but as its the right

way I shall believe in it......

 

Rod for VW piston01.jpg

 

This is the Maxspeedingrod prepared for the VW piston with excentrical bearing.

Also in the picture is the stock rod with the 77.75 Mazda piston

 

Andreas,

Sometime ago you posted some info/advice on the Mazda & VW pistons, could you re-post same said info or PM me please?

 

Richard.

[TriumphV8]

Hi Richard, do not remember what i posted.

What do you want to know about that?

I did both engines with MAZDA and VW pistons.

The MAZDA is very similar to the TR6 piston, the VW

a very light modern piston.

[Richard71]

Hi Richard, do not remember what i posted.

What do you want to know about that?

I did both engines with MAZDA and VW pistons.

The MAZDA is very similar to the TR6 piston, the VW

a very light modern piston.

 

Please see PM.