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  #1  
Old 11 Feb 2005
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BHP per Tonne vs Torque

Hi all

I read in Chris's book and other books about the minimum BHP per tonne required for a good overlander, but this figure does not consider torque?

I am restoring a 1963 FC Land Rover and it is fitted with a 4236 Perkins. Looking at the bhp figure for this engine it is under powered, but when testing the engine in a RR it pulled like a train. We loaded it up, added a fully loaded car transporter and it still pulled like a train?? Which raise the following questions:

Why is torque not considered in these calculation? Have Turbo engines made everyone think about revs and bhp? Which is better for desert driving, high torque at low revs or as on modern turbo engines, torque higher up the rev range? Do Jap turbo diesel engines suffer from reduced power before the turbo cuts in???

I know the limitations of the vehicle I am restoring and am only using it as an example. I am a big fan of simplicity and would like you views on the questions above bases on your experiences in the desert. I, unfortunatly have no expectence on driving on sand, unless driving on Ainsdale beach, nr. Southport counts?

Cheers Mick
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  #2  
Old 11 Feb 2005
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Broadly speaking power is speed, torque is pulling power. Sometimes, for example you need to build up speed to climb dunes or accelerate through soft sand, that's why overlanders need 50-70 bhp per gross ton. On the flat - power - or the lack of it is not so important. The 4236 as I recall was used in 7.5 ton trucks and with suitable gearing pushed them along at over 70 mph even in the early sixties. Don't know what version you have but it would probably eat the transmission in any overland or enthusiastic driving situation.

Andrew.

[This message has been edited by Andrew Baker (edited 11 February 2005).]
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  #3  
Old 12 Feb 2005
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Hi Andrew

thanks for your reply. It does not answer the questions, but does highlight why I asked it. The 4236 vehicle engine has a max 60bhp and it was fitted in 7.5 Tonne truck, which is 8bhp per tonne. Torque is far more important in these slow reving engines, but never discused???

Is speed the best aproach to accend dunes? or is slow and steady better?

Cheers Mick
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  #4  
Old 12 Feb 2005
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Quote:
Originally posted by Jabbawocky:

Is speed the best aproach to accend dunes? or is slow and steady better?

Mick,
It's not about charging at speed but building up momentum. Imagine a vehicle with standard tyres, unlimited torque and momentum going uphill in soft sand at a speed equal to the vector of gravity. How far would it be able to travel before sinking down?


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  #5  
Old 12 Feb 2005
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Hi Mick,

Gnerally it's good idea to tackle obstacles slowly to limit damage to vehicle and to maintain good control. These are both very good reasons, but it's actually kinder to your vehicle to go as FAST as you can, providing you don't risk losing control or doing damage. Momentum is like a free ride for the transmission.

Sand is perhaps the best example of where faster is better, subject to maintaining good control. If you go too slowly through a soft or steep patch, you will sink further into the sand, and the engine and transmission have to do ALL the work of ploughing you through.

Regarding torque vs power: Torque is what pulls you up the hills; power is how fast the torque can do it. A lack of torque can be fixed by lower gearing, but there is no solution for a lack of power. (Power = torque x revs, so an engine that delivers high torque at high revs is by definition a high powered engine).

A high, wide torque output is important only because you need not change gears as often, for a given power. (That is to say, a 100kW engine with 300Nm of torque over most of its rev range can stay in a higher gear than a 100kW engine that delivers only 200Nm). The challenge facing a low-power vehicle is that if he starts in a low gear, he can't build up much speed; if he starts in a higher gear, he runs out of steam halfway up.

The reason that torque is an important consideration for a 4x4 is that high powered engines for road cars often deliver their torque at high revs only. Since P = T x r, when revs decrease, T decreases also, so P falls pretty dramatically - a sort of double whammy.

Off-road, you want an engine with a nice flat torque curve - your revs will be changing all the time to suit the terrain, and you want the power to fluctuate as little as possible, especially at low revs. The actual maximum torque figure is not that vital, so long as the curve is flat, the gearing is well matched, and the power output is adequate.

HTH,

Michael...
www.expeditionoverland.com


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  #6  
Old 12 Feb 2005
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Is speed important to ascend dunes - yes - and therefore you need power.

An observation - new 300 Tdi 110 bhp vs 15 year old Land Cruiser both on 7.50x16's attempting steep dune climbs over 500 mile stretch. The LC beat the Tdi every time - same peak power, but the bigger engine had more of it available even at lower revs. So you don't actually need loads of power, but what you have you need available at modest revs, which the large capacity Land Cruiser engine delivers.

Andrew.
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Old 24 Feb 2005
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I've read this through a few time and I understand what you are saying, but which engines give a good flat power curve? Are diesels better than petrols? are N/A engines better than turbo engines? or is it down to CC ? Where can you find this sort of data?

I should explain why I am so interested. I have drivern Land Rovers all my life and eventually had a 300tdi Defender 90 which I thought superior to the previous vehicles. Then I got promoted and ended up with a Vectra. so out i went an bought a 2a Diesel, fixed the suspension and found out that the old diesel 2a can dig its way out of situation the defender would never get out of. Hence my interest in engines and how they effect the offroad performance of vehicles.
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Old 25 Feb 2005
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Jabbawocky,

Your dream machine should have a 1HD-T or 1HD-FT engine. It will give you all the torque and power you will ever need - without electronic engine control, oil leaks and the legendary LR reliability problems ;-)

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  #9  
Old 25 Feb 2005
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Jabbawocky, can you give me an example of where the 2A beats the 90 ? interesting
I spent many years driving 101's

Some of it - on sand is down to tyre size - if you are running 9.00x16 the taller tyre works much better than a 7.50x16 and you get more ground clearance.
The 2A has much deeper gearing which is also a help, and IMHO a vehicle with a 100 inch wheelbase (ish) is the best length - 90 bit too short, - 110 bit too long.
Thats why a FC101, Classic Range Rover and Discovery are so good off road.
Tyre Tread pattern and pressure make a huge difference. -obviously
Also vehicle weight - it makes an enormous difference if you keep the weight down

If youve driven a (200/300) LR you know that when you hit some soft sand you have to be in the correct gear to keep the turbo spinning - if you loose it you go back down through the box and loose your momentum - which makes the engine work even harder to regain momentum.

If you have a large capacity engine it does not rely on the turbo for peak torque/power so it is easier to drive - however you generally get poor fuel consumption.

The best combination is to have a larger capacity engine with a Turbo charger - best of both worlds - and a little economy , this is why a TLC is generally better than a LR in Sand.

However my 90 is kept very light and I have put it in places (in Mauritania on the PD Route) where a TLC on 7.50 XS could not go - I was on BFG Mud Terrains !!!
A lot of it is how you Drive/Ride
.....Forward Obsevation, Correct Gear/Momentum and Route Selection etc etc.

Cheers
Grif

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Suzuki DR650 Overlander
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  #10  
Old 28 Feb 2005
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I struggle to think of any reason a diesel 2A might outperform a 300TDi Ninety in any situation.

Series LR fans might argue that it has the advantage of simplicity, but in terms of "getting out of situations", the Ninety has similar dimensions and mass, better articulation, more power, more torque at all revs, better visibility and controls, and it's an easier drive. The 2A has lower diff ratios, but overall gearing on the Ninety is amply low (as low or lower than on a 2A).

I'd go along with Grif - a big diesel has oodles of low-down torque, but tends to run of revs too soon. A turbocharger complements this perfectly, doing nothing at the low end, but boosting performance at higher revs.

Rgds,

Michael
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  #11  
Old 1 Mar 2005
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Well it's almost all been said. If you want to keep the grunty great Cummins you have to accept a certain amount of leg and arm work to keep the thing in the right gear. IMHO you won't enjoy dunes like that.
You could of course stick in an auto box. Always in the right gear, continuous transmission instead of hurried gearchanges.
I would imagine that one of those huge-engined low-revving American heaps would have a box that could survive the load. Avoid ones with electronics for all the normal reasons.
Good luck with the conversion/renovation whatever you decide.
L
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  #12  
Old 2 Mar 2005
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Final Drive Ratios:
Land Rover 90 - 43.38
FC 2A - 49.40
FC 101 - 55.32

The FC deeper ratos offset by 9.00x16 tyres.

I always Remember driving a 101 and thinking it was awesome - even with 2 tonnes behind it in a peat bog ! - I'll have one on coils with a GMC 6.6 Turbo Diesel, Auto and that lovely PTO winch.
However Cresting steep Dunes in a FC is not for the fainthearted........

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Suzuki DR650 Overlander
..and Bloody Nice Bloke!

[This message has been edited by Gipper (edited 02 March 2005).]
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  #13  
Old 2 Mar 2005
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Thanks chaps for the replys. I though that might get you talking. I am a self confessed Series fan without a doubt, but I do believe the statement above.
Here are few pictures to prove that 2 1/4 diesels do have their uses.
http://img.villagephotos.com/p/2004-...71/2bvs101.jpg
http://img.villagephotos.com/p/2004-...110caravan.jpg
http://img.villagephotos.com/p/2004-...owingDisco.jpg
http://img.villagephotos.com/p/2004-...onDefender.jpg

What was that about 101's

Cheers Mick

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[This message has been edited by Jabbawocky (edited 02 March 2005).]

[This message has been edited by Jabbawocky (edited 02 March 2005).]

[This message has been edited by Jabbawocky (edited 02 March 2005).]
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  #14  
Old 2 Mar 2005
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That is down to the tyres - Bar grips are Useless - and the rest are on road bias.
Grif


[This message has been edited by Gipper (edited 02 March 2005).]
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  #15  
Old 2 Mar 2005
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Yeah, but by Christ they're ugly...

Sam.


That should get a response!!!
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