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In the maintenance manual it give a road speed for a given engine speed, not in RPM but ft/min.

Something like 2500ft/min piston speed and 85 mph. Is this a recommended max crz. speed. For engine life.

If so I better ease of the gas a bit!!

 

Dave

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Dave,

" is this a max crz speed........for engine life"

That opens up several cans of worms.

 

2500ft/min is the mean piston speed. Roughly speaking max instantaneous speed will be about 1.5-fold faster.

 

Piston/bore friction, which will be a rough indicator of ring and bore wear, rises with piston speed:

https://www.researchgate.net/profile/Mohammed_Kamil_Mohammed2/publication/261171062/figure/fig3/AS:296856555933699@1447787526329/Figure-4-Piston-friction-behavior-with-engine-speed.png

So wear-wise lower rpm are beneficial ( within reason).

 

The rpm ( = piston speed) for best fuel economy is really interesting. Its not the slowest rpm, nor the fastest. And its not the smallest throttle opening. Its around three quarters max throttle at rpm that deliver max torque. So a TR engine will give its best " Brake Specific Fuel Consumption" ( bsfc) at around 3500rpm ( max torque) and 3/4 max throttle. Of course the car would then be making too much horsepower for a legal road speed and we'd back off the throttle. But that kills the bsfc. The relationship is given in an engine's 'performance map'. I have attempted explanations here:

https://supertrarged.wordpress.com/2013/04/14/tr6se-project-2-bsfc-performance-curve/

https://supertrarged.wordpress.com/2013/04/17/tr6se-3/

 

Peter

Edited by Peter Cobbold
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Hi Dave,

I think your engine speed will be about the same as the chassis speed and the person sitting behind the wheel. :P

 

Any major difference and you have problems :huh:

 

Roger (just helping)

Dave, But Roger would not see time passing at the same speed as you do, when you drive past him. Peter

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As we seem to have taken on a thermodynamics slant, don't you just love it when scientists find an even more fundamental law of thermodynamics only to find they've already named the existing ones first and second so they come up with the zeroth? What happens if an even more fundamental concept is uncovered?

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As we seem to have taken on a thermodynamics slant, don't you just love it when scientists find an even more fundamental law of thermodynamics only to find they've already named the existing ones first and second so they come up with the zeroth? What happens if an even more fundamental concept is uncovered?

 

They use imaginary numbers based upon the square root of minus 1.

Otherwise called i

 

Which is why Lucas's Pi causes so much head scratching.

Edited by Peter Cobbold
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TR6 not only deals with high piston speed, what is calculated

as a somewhat intermediate speed but also very high piston

acceleration due to the very short rods.

 

To overcome those problems it is a good idea to use special pistons.

Forged or hypereutectic ones can withstand that forces in the TR6

engine much better.

 

Coating of the piston skirt helps to reduce friction and allows to shift the

max piston speed upwards and the last improvement is reducing the ring

thickness where there is the rule that smaller rings allow higher revs.

 

That leads all in all to a simple solution, cheap and undestroiable:

It is the extended rod with piston pin offset and the VW piston!

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So. If I get pulled over by plod I just say that I am driving at the bestest bsfc possible in O/D top. Regardless of the velocity!

Yep I'm happy with that!

Trouble is those Phoenix twin boxes really bark when the revs get up near the red line.....but oh what fun!

Sounded good belting through that tunnel on the a3 the other day!

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When Peter ( or anyone else) has an odd five minutes going spare, try working out how far a TR piston actually travels up and down the bore over the course of a mile-assuming its doing a constant 3000 rpm in od top. It will make you re think how far your engine is actually travelling when the average mileage of a car is supposed to be around 10,000 miles per year.

 

hoges.

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3000rpm in od top = 90mph ( to make the sums easier)

I mile takes 0.66 minute

Total revs: 1980

Stroke: 3.74inch or 7.5 inch per rev

So total travel: 1240 feet, or 412 yards

The TR is going 4x faster than a piston.

 

6 cylinders = 2475 yards.

Pistons travel 40% further than the car.

10,000 road miles = 14,000 piston miles, in od top.

Approximately.

 

Peter

 

Curiously at 3000rpm the flame speed will be roughly 20m/sec or 45mph.

( slide 26 of IWE talk) - half the road speed. Answers on a postcard to....

Edited by Peter Cobbold
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3000rpm in od top = 90mph ( to make the sums easier)

I mile takes 0.66 minute

Total revs: 1980

Stroke: 3.74inch or 7.5 inch per rev

So total travel: 1240 feet, or 412 yards

The TR is going 4x faster than a piston.

 

6 cylinders = 2475 yards.

Pistons travel 40% further than the car.

10,000 road miles = 14,000 piston miles, in od top.

Approximately.

 

Peter

Nice

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TR6 not only deals with high piston speed, what is calculated

as a somewhat intermediate speed but also very high piston

acceleration due to the very short rods.

 

To overcome those problems it is a good idea to use special pistons.

Forged or hypereutectic ones can withstand that forces in the TR6

engine much better.

 

Coating of the piston skirt helps to reduce friction and allows to shift the

max piston speed upwards and the last improvement is reducing the ring

thickness where there is the rule that smaller rings allow higher revs.

 

That leads all in all to a simple solution, cheap and undestroiable:

It is the extended rod with piston pin offset and the VW piston!

Does the length of the rods directly influence piston speeds or is it more a reflection of the throw of the crank?

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The area of bore scraped by the rings over 1 mile is interesting.

In a mile requiring 1980 revs ( post #16) the six cylinders scrape past a area of bore=

Bore 3 stroke 3 ( roughly ! ) Swept area= 2 x 3.14 x 1.5 x 3 = 28 sq in per sweep

So for 1980 revs two sweeps each, total swept area= 112,000 sq inch

= 770 square feet per mile.

 

Oil consumption say 1 pint per 1000 miles

1 pint per 770,000 square feet swept.

Thats - in metric - 500ml per 71,000 sqmetres = 142 sq meter per ml

Or 1 ul of oil burned per 0.142 sq meters of swept volume.

Since 1ul is the volume of oil in a cube of side1mm. So that is burned for every 142,000 sq mm of area swept

So oil film thickness that is burned is 7 x 10^-6 mm, which is 7 nanometers.

Or roughly 70 times the diameter of a hydrogen atom.

 

 

Peter

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Does the length of the rods directly influence piston speeds or is it more a reflection of the throw of the crank?

 

The middle of the speed (do not know in English precise) is the same but two

important things are different with shorter rods:

1.) The acceleration of the piston leaving TDC and BDC becomes faster

2.) The time of the Piston staying in TDC is shorter and with that the burning

process of the fuel differs. It has less time to build up pressure because the

piston is starting to go downwards. So these engines produce higher torque

at low revs and less power hat high revs.

 

From experience rod/stroke ratios should be in limits.

Ford BDA for example made the expensive step for getting homologation

for longer rods. That is the proove that there must be some serious

difference compared to the original rods.

TR6 has the worst stroke/rod ratio of all engines built in numerous amounts.

Only Chevy big block is that bad.....

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TR6 has the worst stroke/rod ratio of all engines built in numerous amounts.

 

Which is one reason for supercharging as opposed to revving. :)

Peter

Edited by Peter Cobbold
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Yes, besides that pressure to the cylinder walls is pretty high, too

and we have Olympics, where doping is forbidden its a nice solution.

Turbo and supercharging is somewhat like doping to me.......

 

Supercharging or a turbo is a way to boost power but its not the way

of a nice artificial job but more a coarse work like engine swap

(that I would not deny, too as you know)

 

I built a 1300 Alfa Romeo Compressor from the Mercedes charger.

It was a single seater that ran in the 2 liter class and there was

really a winner! After the race a competitor came to my friend to

tell that it had sweet power and he showed the half popped off

tube at the inlet to show that he had only half the power availiable

(and still passed the most of the 2 litres)

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Yes, besides that pressure to the cylinder walls is pretty high, too

and we have Olympics, where doping is forbidden its a nice solution.

Turbo and supercharging is somewhat like doping to me.......

 

Supercharging or a turbo is a way to boost power but its not the way

of a nice artificial job but more a coarse work like engine swap

(that I would not deny, too as you know)

 

I built a 1300 Alfa Romeo Compressor from the Mercedes charger.

It was a single seater that ran in the 2 liter class and there was

really a winner! After the race a competitor came to my friend to

tell that it had sweet power and he showed the half popped off

tube at the inlet to show that he had only half the power availiable

(and still passed the most of the 2 litres)

There's no denying that a piston is a cylinder is poor air pump compared with a rotating pump.

So to me supercharging is the obvious way to get 30-50% more air, and mixture, into an engine. To do that with rpm is impossible.

The most impressively tuned TR was never seen in UK but was supercharged in Adelaide by an engineer Eldred Norman. After building and racing a twin-engined monoposto 'special', he imported the first TR2 in Southern Australia, and fitted a American blower. Summary here:

https://supertrarged.files.wordpress.com/2015/08/supercharging-trs-for-the-road-iwe-shorter-for-wordpress.ppt

slides 6-9. Racing or towing a trailer on the road, that engine could do it. In 1954. Coarse? I dont think so.

Peter

Edited by Peter Cobbold
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Not sure I would call forced induction as a coarse solution. In the day of TR production turbocharging wasn't the commonplace solution of today nor was supercharging as reliable an option.

 

As has been mentioned already the 2.5 six was limited by its long stroke design which ultimately limits its ability to rev without excessive forces on the rotating bits. The ideal engine for power output has a large bore and short throw which means lesser forces on the bottom end and ultimately the greater ability to rev, which equates to more air/fuel per minute and therefore more power.

Naturally there's a balance to be struck. Back in the early days of car tax in the uk wasn't it based on a calculation of horsepower that was related to bore rather than capacity which favoured long stroked small bore engines. The Triumph 6 perhaps inherited this configuration.

 

Thus if you want to build a screamer of an engine of a 6 beefing up and finely balancing the bottom end is vital for any kind of reliability.

Forced induction gets the power at lower revs so has its advantages, although 2.5 litres gives plenty of power as standard.

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