Preamble:
Last week we had a glimpse into the philosophy behind the design of Tyres for F1 Racing Machines vs a Stock Car.

Considering the fact that your tyres are the only means of your communication with the road you drive on, they can either save you or kill you – depending upon what care you take in choosing AND maintaining them.

Here’s some more on how to look after your tyres…

Tyre Pressures – Do they Matter?

YES - they more than do! Here’re some basic facts:

1. The Tyres that come with your Car are the best suited for the purpose it’s intended for.
2. There’s an uncalled for mania in our Country to run down such OE Tyre specs, especially on the 80Bhp+ Cars.
3. When I say uncalled for, check out multitude of street cars in developed countries where none dare ‘up grading’ OE tyres coz then their Insurance Policies will become invalid and a crash out there, whether coz of tyres or not, can set one back by thousands in hard cash – assuming no life is lost!
4. Every ‘chosen’ Tyre has a ‘safe’ load bearing capacity - say 1/4th the ‘weight’ of a Car.
5. The Weight of a Car varies - from ‘Kerb’ to ‘fully loaded’ - for understandable reasons. This difference can be as much as 500 kgs!
6. The ‘ideal foot-print’ of a Tyre is a function of its inflation pressure when ‘cold’, AS WELL AS the load on it.
7. While in motion even a ‘correctly’ inflated tyre ‘warms-up’, coz of internal heat generation due it’s inevitable ‘flexing’ AND friction with the road surface. Consequently, the ‘cold’ pressures go up by as much as 15% on a high-speed cruise after some time.
8. All this is taken care of when carmakers stipulate ‘cold’ inflation pressures. They, however, take a medium route - by specifying cold pressures for ‘part load’, which is usually the case, and strike a compromise between fuel consumption - higher the cold pressures the better - and ride quality - lower the better.
9. Therefore, when going on a long trip, ‘fully loaded’ as is usually the case, inflation by +10% over the recommended cold pressures is highly recommended. The ‘effects’ of various degrees of ‘inflation’ are illustrated in the sketches below:
   
10. It’s obvious that one has to aim at correct pressures to give the best ‘foot-print’ between the tyre and the road.
11. Looking at above sketches, it may be noted that ‘under-inflated’ tyres will wear out faster from their outer edges and over-inflated ones from the middle. Only properly inflated ones can ensure even tread wear.

So how do we go about it!

Actually, it’s not that difficult, considering ‘Cold’ means when the tyres are at ‘Ambient’ temperature – ideally the first thing in the morning before one rolls off. Now that’s where crux of the problem lies. To overcome it, this is what you can do:

1. Ideally, you need a Tyre Pump and a reliable TP Gauge of your own - as most at the roadside are out of ‘synch’. If you find a foot pump bit of a bother, there are Battery operated ones available for ~ Rs: 500/-, which plug into the cigarette lighter socket.
2. Similarly, a good ‘dial type’ TPG can be had for ~ Rs: 100/- and a digital/self-calibrating type around Rs: 300/-. It’s strongly recommend that you buy your own tyre pressure gauge. Armed with this, you could also fill-up air at any pump, using your TPG.
3. What one can do and it works fine is that you get your Pressures corrected to, say (+) 10% per your Gauge’. Next morning before rolling off, check again with your TPG and let off excess pressure, if any.

Remember that Tyres under-inflated by as little as 4 psi can raise your fuel consumption by as much as 6% - besides seriously jeopardising their lives and thus your safety!

Next week we’ll discuss the importance of ‘Wheel Balancing’ and the need for their ‘alignment’.

Preamble:
Considering the fact that your tyres are the only means of your communication with the road you drive on, they can either save your life or kill you – depending upon what care you exercise in choosing AND maintaining them.

With a new car, the first part is taken care of beyond doubt, as other wise it won’t qualify as ‘street-legal’ for sale to public at large. Trouble comes when people mindlessly try to ‘upgrade’ them – leave alone neglecting them by way of properly maintaining their inflation pressures round the year, not to mention their ‘concentricity’, dynamic-balance and alignment.

So let’s first talk about the philosophy behind the design of our stock car tyres as explained to me by a Sr. Exec of a prominent MNC Tyre Co., when I picked his brains with the following questions posed by an enlightened visitor…

Questions
Regarding tyres, I’ve seen on TV and in Books that F1 tyres have 3-5 plain treads around or are completly plain, having full contact to the track. But in real life for our cars, we have lots of gaps, in the form of different tread designs. If these are worn out, i.e. when the tyre becomes bald we change it, which is in contrast with the idea on F1 machines where they run mostly on smooth tyres. Why can’t we use smooth tyres provided we run our cars only on metalled roads and not off-roads?

Answers
Our compliments to you on your observations and conclusions. Regrettably, what’s good for the Goose is seldom so for the Gander as well! Consider the following:

The ‘Road-Grip’ a tyre provides is directly related to the ‘Friction’ generated between road and the tyres. And ‘Friction’ is directly proportional to the ‘Coefficient of Friction’ and the ‘area’ in contact. Worn out tyres thus have more ‘area of contact’ on ‘flat’ roads whereas ‘CoF’ varies with road material (Tar, rubber mix or concrete), its finish (smooth, coarse) and of course tyre tread rubber ‘composition’ AND condition.

Tyres exposed to strong Sun/UV light for long or aged over time makes their rubber hard, leading to reduced CoF. CoF also reduces if it has just rained and again increases if it continues to rain for some time and water gets drained out from the road surface.

Let’s break your query into two now:

Q1. Why do the stock tyres have treads while F1’s don’t, and
Q2. Why is it not recommended to use worn out tyres i.e. that have no tread left.

A1.
F1 tyres for dry tracks do not have any tread for the reasons stated above i.e. max contact area and the fact that F1 tracks are clean - no mud, no dust etc. However, F1 tyres for wet surfaces do have grooves i.e. ‘Ribs’ which run circumferentially on tread, to channel out water so that there’s no water trapped between tread and road. If dry/flat tyres are used on wet tracks, there will be ‘hydro or aquaplaning’, which will result in skidding/loss of control. In reality, public roads are not exactly that flat and free of loose material. There is loose dust, sand, small stones and gravel all over the road.

So in stock tyres, we have to have tread patterns to assist channeling out of water, dirt and air (to reduce ‘rolling noise’) as in reality, we have to drive the same car/tyres in rainy season, on roads with dust particles, surface irregularities, and of course off the road some times. One more thing - to judge a worn out F1 tyre, expert inspection is needed, whereas in real life, any one can see if a treaded-tyre is worn out, tho’ most penny-wise pound-foolish types ignore it and keep on driving until steel wires come out or worse still, till the tube itself starts showing up!

A2.
A worn out tyre has better braking and traction on plain clean roads but it is not recommended to use worn out tyres on Stock as well as F1 vehicles because the roadside tyre-wall thickness reduces considerably. After such a degree of wear, chances of hydroplaning increase manifolds and even if there is no water on road or other obstacles, a tyre can burst due to sheer centrifugal force alone at high speeds.

Further, in Indian road conditions, due to their vulnerability towards cuts and bruises, worn out tyres have more chances of moisture seeping into steel belts, leading to their rusting and consequent loss of adhesion with rubber, causing belt separation and ultimately leading to tyre burst.

To sum-up, for F1 racing, all that matters is ‘grip’ and they’re no bothered about the costs of changing even up to 4-sets during one race that may not last even a few hundred kms. In real life, at average speeds that may not even be 1/10th of the F1, ‘cost per km’ to the owner is the overriding factor and a pax car tyre in India is expected to last at least 40-50 kkm - besides expectation of ‘reasonable’ grip under all possible surfaces that an average motorist is likely to drive on – hence a multitude of ‘tread designs’, each one claiming to be superior than other!

Preamble
A couple of months ago, I wrote about the Electrical Systems of present day cars and how they are a far cry today from the Amby’s and Fiat’s of yore. While the advent of ‘Alternators’ solved a lot of problems, but as was to be expected, people got confronted with new ones they hadn’t faced before.

Well, as usual some intelligent fellow asked me some more Q’s on the last week’s writing. This is how it went with him this time…

Question:
As per what this Indica gentleman had to say, he observed that the engine temp. gauge gradually increases (to the red zone) and then decreases. If Voltage Regulator is defective, shouldn’t it remain ‘high’ all the time? VG/Mumbai.

Answer
Having personally experienced Voltage Regulator malfunctions on M800s more than once, the Alternator over-voltages are ‘transient’ in nature and occur almost ‘instantaneously’ for all practical purposes. These manifest as brightening up of head lamps, if driving at night, and shrilling up of the horn - which are on day and night in our scenario, accompanied by ‘consequential and pronounced’ drag on a small engine like the M800’s. The latter is caused by the Alternator imposing greater load on the engine, as it’s called upon to push more Amps due to the over-voltage condition, through whatever loads it’s connected to at that time.

Such over voltages last for a few seconds and then settle down, only to happen again unpredictably - due to the malfunction of the IC-controlled/in-built all solid state Voltage Regulators.

However, since the Temp Gauge and the Fuel Gauge now a days are of ‘viscous-damped’ type, the former being ’stay-put’ in addition, they don’t respond to the over voltage condition ‘instantaneously’ but will register a ‘well-damped rise’ to their max position – something like the fuel gauge after you’ve filled up to ‘F’ from ‘E’ and drive off. .

Likewise when the over voltage condition disappears after a while - entirely at the sweet will of the VR. In petrol Mpfi’s, the ECU may shut off too, due to its in-built over voltage protection. In that case, it’ll switch over to ‘limp-home’ mode as programmed, with the engine/RCU light coming up on the Dashboard. However, I’ve yet to experience such a condition either on my 7 yr+/80Kkm old Santro or ~ 4 yr/12K km old Baleno.

Preamble
A couple of months ago, I wrote about the Electrical Systems of present day cars and how they are a far cry today from the Amby’s and Fiat’s of yore. While the advent of ‘Alternators’ solved a lot of problems, but as was to be expected, people got confronted with new ones they hadn’t faced before.

Keeping in mind that the present day Alternator in a car mainly comprises – a) Stator, b) Rotor, c) Slip-Rings/Brushes, d) Diode Plate and e) a built-in Solid state Voltage Regulator, out of which the first three are relatively trouble free, it’ll be easier to understand the most commonly faced problem by motorists from the following real life QnA…

Question
I have an indica DLS/2001. Some times, the temp gauge needle goes upto the red mark. After few minutes, it comes down to above but not 100% normal. In addition, I find that when the temp gauge rises, the headlights become brighter and the engine also slows down as if over loaded.

Answer
To me it’s quite clear that your Alternator ‘Voltage Regulator’ is malfunctioning i.e. producing over voltages erratically - as evident by the headlights becoming brighter and the temperature gauge, which is a voltmeter of sorts, showing higher reading. Since the Alternator is getting into an over voltage mode, it imposes a greater load on the engine also, resulting in a feeling of drag on it. Such malfunctions are not uncommon in 30 kkm+ old cars - especially ones fitted with Lucas-TVS Units.

To sum-up, please get your Alternator serviced at either a Tata Motors or its own authorised service centre, depending on its make. Further, it’s advisable to change BOTH its ‘Diode Plate’ AND the ‘Voltage Regulator’ at one go. If you try and save on the Diode Plate now, even if found ok, you’re most likely to find it giving way soon thereafter, coz it’s already been overstressed by the malfunctioning Voltage Regulator.

For your advance information, Diode Plate failures first appear typically as the Alternator/Charging Light on the Dashboard not going off fully but remaining a-glow. This can have a snow-balling effect as more and more diodes fail, out of a minimum total of six, leading to correspondingly reduced out put from the Alternator, which in turn will lead to flat a battery one fine morning!

Incidentally, one of my enlightened readers asked further questions on this QnA. For want of column space, we will discuss them next week.

Preamble
Week before last week we had a look at as to how Power Steerings appeared in India and then sampled a somewhat an ‘extortionist’ kind of a problem faced by one in real life.

Well, I thought it’d end with that but it didn’t, as one well informed car owner bowled another googly at me. This is how it went…

Question
A small doubt SKG: You have said that ‘power assist’ in such steering systems (hydraulic or electric) is cancelled out at road speeds higher than 40Kph. Is it true for all cars, especially those with hydraulic power-assist? I’m asking this coz in my Indigo which has hydraulic power steering, I noticed that if I give even a small twist to the steering wheel at expressway speeds, the car suddenly swerves, which could unnerve you, if you are not prepared for it! In fact, the corrective action you may take on reflex could cause an over-shoot in the opposite direction, making the car go zig-zag at high speeds and thus create a dangerous condition, especially if there are other vehicles near-by or trying to overtake. VG/Mumbai.

Answer
Thanks for the input/feedback VG. Let me try and rise to the occasion:

Although the EPS/HPS fellows positively claim ’speed sensitive’ power-assist, it’s relatively easier to achieve in EPS’s, as it’s a simple matter of programming the EPS/EPROM that takes a ‘road speed’ signal from the main ECU of the vehicle.

However, in HPS’s, as can be imagined, the things are a little more complex. As far as I know, there’s a ‘Pressure Regulating Valve’ of sorts that’s interposed between the steering wheel shaft and the actual steering gear/linkage.

This PRV has 2-slotted discs or cylinders wiping against each other and in their ‘reset’ i.e. ‘straight-ahead’ position, the slots are more or less offset, thus allowing minimum fluid flow thro’ them. Since one side of the PVR is tied down to the front road wheel side of the steering system and the other thro’ a ‘torsion link’ to the steering column, it can be visualised that the ‘through-put’ of Hydraulic (fluid) Pressure thro’ the PVR can be controlled by the difference of torques that it’s subjected to on its 2-sides.

In other words, if the effort on the steering wheel side is significantly higher than the road wheel side of the PVR, it’ll sense it as an attempt to turn the vehicle at low road speed and thus allow higher HP thro’ it, which in turn will result in higher ‘degree of assist’. The opposite is true when the effort-differential is low, meaning a higher road speed and therefore, lower the degree of assist.

So as can be seen, the design AND ‘calibration’ of the PVR is crucial in controlling not only the overall degree of assist but also in imparting a ’speed-sensitivity’ to the system. That’s how one keeps reading in professional road test reports of ‘over or under assisted’ systems on a particular vehicle, thereby losing that ‘feel of the road’ or vice-versa. In some HPS’s, the setting of the PVR is ‘service-adjustable’ but in most they’re not.

Coming now to your real question, since I haven’t lived with an Indigo, I really can’t tell, except that there seems to be something amiss with its PVR, either by way of design/calibration or later malfunction. What you can do is to carefully check as to whether the effort required at the steering wheel is nearly the same at crawling as well as cruising speeds. If so, then it means that the PVR is working, as it should. On the other hand, if it gets lighter as the vehicle road speed increases, then the PVR is not ok i.e. it’s not as speed sensitive as it should be - assuming that it’s there in the first place in a cost-cut Vehicle!

For more on HPS/Variable Assist systems, you may like to browse thro’:
http://auto.howstuffworks.com/steering4.htm

As it provides detailed graphics, which is not possible in the present format. This will make understanding of the whole concept a lot easier.

Preamble
Last week we had a look as to how Power Steerings appeared in India and the existing divide between ‘Electronic’ and ‘Hydraulic’ Systems. Given its relative simplicity and thus the ease and cost of manufacture, coupled with the fact that it hardly requires any maintenance ever, EPS’ are gaining popularity today.

All out R&D efforts are on towards refining them to give that hitherto missing ‘road feel’ and manufacturers have met with considerable success in the recent past also, to the extent that the world famous Toyota-Lexus switched over to EPS a few years ago.

EPS’ can be basically of two types – one where the power-drive electric motor is steering-column mounted and the other where it’s on the Steering Rack in the engine compartment, in the form of a ‘cassette’ assembly. The former finds favour with smaller cars like the Alto/W-R where it can be easily accommodated under the dashboard. The latter becomes inevitable for heavier cars, like the Swift.

Notwithstanding, even today there’re appreciable high-end car makers like the Merc and BMW who continue with HPS’ in most of their models. Nearer home, while Maruti have more or less switched over to EPS, Hyundai have also made a switch to EPS on their ‘i10’ to begin with.

And like any other complex/belt-driven hydraulic System, one can have problems with an HPS. Sample this real life QnA, for an insight into the matter…

Question
I have Maruti Esteem-vx 1997 model with power steering. Recently I got it serviced from a Maruti Authorised Service Station. In their service report, they’ve mentioned that power steering oil leak needs repairing. They’ve also informed me that tho’ the oil level is not reduced but it needs urgent repair. They have quoted a whopping Rs. 6,500/- for the repair work. They’ve also stated that if it is not repaired now, then later on it will have to be replaced at a cost of Rs. 30,000/-! Should I get the job done? What if I do not get it repaired, whether steering will stop working? Can I get the same repaired from some good workshop at a lower price?

Answer
In Hydraulic Power Steering Systems, there can be such a thing as an ‘Internal Leak’ in the Hydraulic Pump - some thing like excessive ‘blow-by’ in engine cylinders due to worn-out piston/rings, resulting in loss of ‘compression’ and therefore, loss of engine power.

The result of such an internal leak is that the hydraulic pump doesn’t develop enough pressure and therefore, ‘power assist’. Consequently, the steering feels appreciably heavier than what it should. Its other manifestation would be higher than normal noise emanating from the pump.

If neither of these two is happening in your case, then in all probability the Garage is trying to make a fast buck, under the pretext of changing the internal sealing rings of the pump and scaring you further with an imminent/high replacement cost if not heeded to.

Over and above these two symptoms, if you did not mention a ‘heavy steering’ in your schedule of ‘demanded repairs’, then you can ignore their suggestion. At best you can ask them to demonstrate to you on what do they base their diagnosis.

And btw, to get max life out of an HPS, it’s advisable to restore the steering wheel to ‘straight ahead’ position after parking and before switching off the engine. This way, the stress on the Pump is minimal when the engine is started next time. As can be imagined, such a contra doesn’t apply to EPS’s.

To sum-up, it’s unlikely that any Garage would turn up with such a recommendation w/o the owner having complained of a heavy or a noisy steering. Therefore, I feel you should take an independent second opinion, if necessary.

Preamble
Until late ‘90s, Power Steering per se’ was not provided by the carmakers as an OE fitment, at least in their entry-level hatch’s and sedan’s. But slowly things began to change when Hyundai first gave its option in their Santro.

Others naturally followed suit, with Maruti being the first to come out with ‘Electronic’ Power Steering in their smaller cars like the Alto and ‘Hydraulic’ ones in cars like Esteem and Baleno. So here’s an attempt to distinguish between the two systems and their pros and con’s…

A ‘Power-Steering’ is a ‘System’ which minimises the effort required to turn the Steering Wheel of a Car - which becomes more and more as the Cars get heavier and is worst during slow parking manoeuvres. Today they’re basically of two types – ‘Electronic’ and ‘Hydraulic’. Both eventually draw power from the engine - of the order of 3-5% - unlike power brakes.

The degree of ‘power-assist’ is designed to taper off as the car’s road speed increases, to maintain that ‘road feel’ at its steering and is, therefore, it’s designed to be max at standstill. It naturally comes at a price and then becomes another system in the Car to maintain - with attendant costs and vulnerability to failures - like any of its other systems.

An ‘EPS’ is relatively more ‘power-efficient’ than an ‘HPS’ and having lesser moving parts with no high-pressure hydraulics, it’s also less prone to breakdowns and maintenance. However, the flip side of it is that it requires a higher Capacity Battery/Alternator - especially for heavier Cars - thus indirectly adding to the overall cost of the System

On the other hand, an HPS runs off the engine directly. Eventually, both draw their power from the Engine only - which is the sole ‘prime mover on-board’. While HPS’ have been around since WW-II, EPS’ are < 10 yrs old and have yet to reach their full development, by way of providing that ‘feel of the road’ as compared to an EPS.

Here’s a real life QnA:

Question
For a car having low curb weight like Alto, Spark or Santro, is it wise to take a car without power steering? Does it affect the driving comfort significantly? Some people say that power steering makes a car more prone to accidents at high speeds.

Answer
For all practical purposes, cars tipping around 700-750 kgs can do w/o power steering. However, with increasing urban congestion leading to b2b parking and not to mention sissiness amongst men and women, power steering even for such cars is being looked upon more as a necessity than luxury.

As regards PS-Cars being prone to accidents at high speeds, it’s more of a myth since both the electronic and hydraulic ones are ’speed sensitive’, where the degree of ‘power assist’ is designed to go down with road speed. In fact the EPS guys claim that above 40 kph, the ‘degree of assist’ is zero i.e. revert to full manual.

On the other hand, with HPS, coz the system is always pressurised from both sides of the front/steering wheels, it infact resists sudden/involuntary direction changes at high speeds, to the extent that small unbalance in the steering wheels also doesn’t immediately result in a ‘wobble’, even at high speeds.

Preamble
The last two articles (Part 1 | Part 2) on the subject evoked some more, if not grilling, responses from my ‘regulars’.

Here is an interesting one from an Instructor level Marine Engineer – who has been around the world several times – if not ‘80’ - including on super-tankers as their chief engineer.

This is how the QnA went with him…

Question
Sorry to butt-in SKG, but I think the question was: What rpm would give the most mileage.
I don’t know that much about cars, but on ships, it is a simple rule: the faster you go, the more fuel you burn! Although the specific fuel consumption (gm of fuel burnt per unit power per unit time) may be better at higher rpm, the total consumption is directly proportional to the power consumed. The faster you go, the higher the resistance (its a cubic curve), consequently the more the wastage!

Even with my car (an Indigo-D), I’ve found that if I keep a steady speed of 80 kmph (5th gear) on the Expressway to Pune (the recommended speed for best specific fuel consumption), I get an average of 16 km/litre. If, however, I chug along at a sedate 55 - 60 kmph, I have got between 19.5 to 20 km/litre! Of course, it’s really boring driving like this !! VG/Mumbai.

Answer
You’re most welcome any time, as it invariably gives me some more food for thought!

Anyways, what you say is quite right - as it’s elementary physics. However, as far as I know, besides ‘rated’ rpm, there’s a basic difference between automotive, marine and stationary power plant engines - in the sense that the latter have their fuel efficiencies ‘optimised’ for ‘near’ full load coz that’s where they’re expected to operate ‘most’ times.

Whereas an automotive engine is required to operate at different rpm’s ‘all’ the time, for reasons one can very well imagine and appreciate. Therefore, it’s ’specific fuel consumption’ is optimised for an rpm/bhp somewhere around 50-60% of its max ratings, to give it a reasonably exciting cruising speed as well as make it run as relaxed as possible and yet have a sufficient ‘reserve power’ for contingencies.

Another notable difference between automotive and stationary ICE’s is their rated rpm. Since the former have to run at various speeds and yet have weight/volume constraints that come with mounting on a car, the present day trend is towards higher and higher rpm capability - both for petrol and diesel. Where as for the stationary ones, the lower is considered to be better coz it not only results in longer life/in hrs but also, most times they’re burning near asphalt like fuels - as you know very well - that require longer full-burn times.

That given and said, it follows that the speed at which an automotive engine will consume the least amount of fuel is the minimum speed at which it can ‘cruise’ w/o any ‘hiccups’. This, if you’ve observed, is of the order of 30+ kph in the 4th gear and 40+ in the 5th for most present generation cars. In other words, nearly ‘no-throttle’ for an Mpfi/Crdi car!

In fact, the ‘record breaking’ FC’s achieved by various cars like the Ford-1.4D, Logan-1.5D, Verna-D etc. that one keeps hearing about in the media, of the order of 30+ kpl, are achieved under such speeds only! But then, as you’ve yourself said, it’d be awfully boring to be driving on the EW’s at such speeds for hours together. In reality, at such low speeds, at least my adrenalin also drops so low that I tend to doze off at the wheel!!

Preamble
The last week’s article on the above subject evoked some interesting responses from some of my ‘regulars’.

Here’s one from a power systems engineer by ‘training’ (like me!) and this is how the QnA’s went with him…

Question
SKG, what is the rpm that is recommended for the best mileage? I tend to ‘cruise’ at 2K rpm in my Wagon-R - not that one can really ‘cruise’ in a city like Bangalore but I do because I leave home at 5.15/5.20 am for the golf course for teeing off at 5.45 am and so I can actually ‘cruise’ at that hour. I choose the 4th or 5th gear to be able to drive at 2K rpm. Is this OK or should I do a higher or lower rpm while cruising? – NJR/B’lore

Answer
It’s somewhat difficult to answer your question with respect to today’s Mpfi’s, where the engine ‘performance’ can be manipulated at the design stage to a very large extent, by programming its ‘ECU’ accordingly. In the pre-EU-II/carbureted cars of yore, it was believed that an engine was most fuel-efficient at its ‘peak-torque’ rpm - which was generally around 2500-3000 rpm and max/bhp rpm’s were of the order of 5-5,500.

However, to me it appears that this is no longer true. Take the example of a 1600cc car like a Baleno, where its peak torque occurs at 3500 rpm and in its Vth gear at this rpm, it’d do close to 140 kph! Whereas its ‘red-line’ happens to be around 6000 rpm - with a top speed ‘capability’ of 185 kph. The corresponding figs for a W-R, peak-torque and max-bhp rpm-wise, are quite similar to a Baleno’s. I remember my one such ‘92/M800 with an ingeniously designed ‘on-board’ Tacho used to do 80kph in IVth (no Vth gear then) at 3000 rpm.

It may be interesting to know that the present day tendencies of car designers are to have higher and higher peak torque rpm’s, to boost the accelerating capabilities of a car. Whereas for cars basically meant for pottering around town with minimal gear changes, such as a W-R, it’s desirable to have them around 3000 rpm - clearly a clash of interests.

To sum-up, I feel one can get the best out of a present day/petrol car - small or big - by limiting the highway cruise rpm’s to 3000-3500 from engine health/longevity points of view and to get the best ‘overall’ mileage, cruise between 80-90 kph in the Vth. The latter will correspond to 2000-2500 rpm for most petrol cars. For diesels, depending on their max torque and Bhp, these figs could be lower by, say, 500 rpm for the same road speeds.

Preamble
Early in October this year, I’d essayed on the fuel ‘efficiency’ vs fuel ‘consumption’ of a car, giving an example of an old Zen cruising at different speeds.

As a fallout of it, a friend posed me some further questions but this time, talking about the differences in engine rpm as displayed by the Tacho on Dash for different cars. This is how the QnA went…

Question
I was reading the other day an AutoExpress/UK review about a BMW-5 Series which does 100 mph @ 1700 rpm. My question to you is how do these guys make an engine perform at such low rpm’s at such high speeds, considering that my Verna-D though not comparable to the BMW, touches 100 kph only at around 2200 rpm in 5th.

So is there a specific logic to their engine/gearbox design that such cars work leisurely at much lower rpm’s whereas the rest are really pulling more than twice that number. I am foxed and hence this question to you. CM/Hyd.

Answer
Thanks for the teaser CM. After a long time some one has asked me an interesting question that sends my creative juices a-flow. Coming down to think of it, it’s not much of a brainer. So here we go:

We all know that an ICE basically produces a ‘torque’ i.e. a ‘rotary force’, like an electric motor and, therefore, has an Rpm vs Torque ‘Characteristic’. We also know that ‘torque’ is primarily responsible for the accelerating ‘capability’ of a car, of course in tandem with its gearbox/ratios.

Since ‘power/(b)hp’ i.e. ‘rate of doing work’ is a direct multiple of torque and rpm, it follows that - a) the bhp vs rpm ‘characteristic’ of an ICE closely follows the shape of its primary torque vs rpm characteristic and, b) that the ‘max power’ dictates the top-speed ‘capability’ of a car.

Now, the power required to move a car at ‘a’ speed basically depends on - a) its ‘road-rolling resistance’ and, b) the ‘wind resistance’ faced by it. At higher speeds, the latter is predominant and it goes up ‘exponentially’ with road speed.

Coming now to the root of your question. Assume for a moment that (a)+(b) above are the same for a Verna-D and a BMW-5D, for a road speed of 100 mph/160 kph, that a Verna is also quite capable of doing, AND that to move them at this speed, their respective engines must produce the same, say, 100 bhp.

Now comes the crux. Since the Verna can do only ~110 bhp/max, say at 4,000 rpm, to be doing 100 bhp for a 160 Kph speed, it’s engine will have to be spinning at least at 4000×100/110 = 3636 rpm.

On the other hand, the BMW-5 produces 190 bhp at 4,000 rpm. Therefore, for it to produce only 100 bhp to move @ 160 Kph, its engine stands run only at 4000×100/190 = 2105 rpm, or 58% of what the Verna has to do. And eureka, this is more or less what you’ve observed!

Please note that the actual bhp vs rpm figs in real life will vary, depending upon the ’shape’ of their individual bhp/rpm curves but for simplicity’s sake, I’ve taken them to be a straight line @ 45*.

Moral of the story, a high/max bhp engine doesn’t mean it can rocket a car to the moon BUT the ‘reserve’ power it has while cruising at an optimum or recommended road speed, so that - a) it cruises with the engine most ‘relaxed’ and more importantly, b) it has a tiger-in-waiting to literally ‘leap forward’ as and when required - either for over taking or just for the kicks!

It’s with this funda at the back that some of the current models of Rolls’ don’t have a conventional/rpm Tacho on the Dash. Instead, what they have is an rpm meter ‘calibrated’ in terms of ‘remaining’ engine power at a given rpm, showing ‘Percentage Reserve Power’ at any driving speed in any gear!

And coming down to think of it, not so long ago until the world at large started laying due or undue emphasis on max torque and max bhp even for high end cars as their USPs, ‘RR’ used to declare ‘Adequate’ against the Engine/bhp columns of its Specs!!