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TORQUE not BHP

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thermidorthelobster:
I understand the question, but I don't know the answer, sorry!  I'm OK with the theory side but I have very little idea about actual engines  :(

But I'm going to keep watching this thread, because I've often wondered the same thing.

Bulli:
i dont believe in BHP

what about the tooth fairy?

Right Rolla answered your question and if you understand all the above as you say then you should have more knowledge than the rest of us.

The main differences between the Rover SD1 and the v8 as fitted to the same year LR's were the cams and the pistons. Cams are the heart of the engine and really do play such an important role.
In that year there is no pulse width as they are both Carb engines!! so no fuel injection so there is little difference in the way they were fueled.
The cams in the SD1 are higher lift and longer duration. This is less fuel efficient but allows a larger charge to be drawn into the combustion chamber...hence a bigger bang.
The compression ratio was much higher in the SD1 engine which means the mixture is burnt more efficiently as this forces the flame front across the piston. Im also 'guessing' that the SD1 engine will run a little more advance.
The higher revving the engine the more advance is required, my bike runs upto 40 degrees advance at the redline of 11k , v8's are around 5.

The cams and high comp pistons combine to more the torque curve up the rev range, lowering bottom end torque but freeing up bhp a the top end.
You gain more bhp by allowing the engine to breate more efficiently. Thats why stock v8's run out of puff quite quickly.Well that and the hydraulic tappets....

Terranosaurus:
As far as I've grasped over the years it's down to what revs the engine is optimised as, all the things already mentioned, inlet and exhaust tract lengths, cam profiles etc all change the optimum revs of a particular engine.

Using different combinations of cams and lengths etc it is possible to produce engines that mange to get greater than 100% filling of the pistons at precise revs by using the harmonics of the inlet track to pulse air in when required.

High reving engines just haven't got the time to fully fill the cylinder but they make up for it by having more combustion cycles instead. This is one of the reasons for high compression ratios on these engines - the dynamic cylinder pressures aren't what they should be cos the cylinder wasn't full anyway.Also the long duration race cams mean that the pressure from the combustion is only being used to push the pistons for a relatively small l% of the cycle.

So on a race engine each small push from the piston means there isn't an awful lot of torque but the fact you have so many of them means that there is plenty of power there.

On the torque optimised engine the high % cylinder filling means compression ratios don't need to be high to get a good burn, and the short duration cam and low revs means each combustion can be used to it's max, this gives good torque but the small number of cycles means lower torque, as there is simply less fuel being used so less energy input.

Personnally I (fly in the face of convention) see no reason why a higher reving powerful engine can't be used off road provided the gearing is low enough. Low gearing multiplies torque, it does not multiply power though

Eeyore:
Also, at greater spark advance you need to run quite rich (up to 25% in some cases) otherwise you end up spitting combustion into the exhaust and melting stuff. The exhaust products'll hit 1100+degC - which isn't healthy! The solution to overheat is to over fuel and use the fuel itself to lower the temperature and pressure in the cyclinder (you'd be trying to run at about 850degC in a road engine)

Optimal torque is made when the moment of highest cyclinder pressure corresponds with the sector of the crank being horizontal - quite literally the force is pushing down on the longest effective lever. Flame propogation in the cyclinder happens at a fairly constant speed, so inorder to optimise the pressure in the piston with the crank as the speed of crank rotation increases, the spark has to be produced earlier and earlier- the higher the revs, the earlier the spark is required otherswise you still get maximum combustion when the crank has exceeded bottom dead centre, which drops the power somewhat.

Engines - seemingly so simple yet, so darn complex!  :shock:

It's one o' them subjects that needs someoen with a pad and pen and a lot of knowledge - and that ain't me!!  :lol:

This won't have helped!

Cheers
 8)
Eeyore

rollazuki:
A high revving motor with lots of gearing will work fine(try a zuki) but is annoying, and hard on the engine. Believe me, the difference between a stock zook motor(1.3 l, revs to 6000) and a diesel(peak torque at mebbe 2000) is sooooooo much nicer to drive off road. Easier on the ear as well!

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