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MG ZR ZS ZT Technical - Full Power

I've been giving some thought about power of my car - 105 bhp. This is how the cars are sold, based on their respective power outputs. On the MGR website, I then checked the statistics to see that I get -

Max Power 103(Ps) @ 6000 revs and Max Torque(Nm) 123 @ 4500. Performance is then less at other engine speeds to some degree.

Then I looked at my daily driving, studying the speedometer and the tachometer. 70 mph in 5th gear gives about 3250 revs. So 4000 revs in 5th gives about 90mph. Most of the hard revs are done in 1st or 2nd, to clear a junction, or grab a gap on a roundabout, and only in these gears would I get up to 4000 revs. All my driving seems to be between 1000 and 4000 revs.

Does this mean the potential power of my car is never fully used, and I never get what 103Ps really feels like?

What about the most efficient engine speed for fuel economy?

Any thoughts? I'm always interested!

Gary
Gary

we're at opposite ends of the spectrum; my ZS180 does very little below 3000 and often catches me out by hitting the rev limiter in 2nd, such is its willingness to rev and keep on revving. When feeling lazy I tend to leave it in 3rd which is good for anything from 15 mph to well over the national speed limit apparently.
David

Ps is a smaller measurement (slightly) than bhp, therefore there will always be slightly more Ps than bhp on a given engine. This is why manufacturers use it, it makes thier engines appear (slightly) more powerful (and makes it easier to cross compare with other EU cars)

If your engine has 103 Ps then it has slightly less bhp, probably high 90s (can't be *rsed to work it out).

-------

In answer to your question Gary, yes if the peak performance at a given point in the rev range is higher than you use then you don't get to feel the full performance of your car.

I'd suggest a track day if you want to try to get the most out of it, or maybe a 1st Lotus driver day training course (which is great fun, loads of spinning out and finding the handling capabilities). :-)

If it is any consolation, most people on the road have little idea of the performance of thier vehicles, the closest they get is when overtaking.

SF
Scarlet Fever

Unless pushing hard, the full power potential is never explored in day-to-day driving. Your fuel bill and maintenance bill would be horrendous! Never mind the speeding tickets!
The 105 monicker is the max bhp, which is the same as the now European adopted DIN Ps of 103. In terms of KiloWatts as used in some Commonwealth countries, the 105bhp would be about 77kW.
Torque, as created by the pistons forcing down the conrod and creating a turning force on the crank increases up to a point (on the 105 as you state at 4500rpm). However, the gearbox determines what amount of torque is fed to the wheels for turning effort. Depending on the engine design and fueling etc, the torque curve maybe fairly flat. However, the power is a measure of the ability to do work in a certain time, and consequently James Watt used this to sell his steam engines and equated to the ability of horses to lift weights, hence horse power. Horse power is measured from the torque and the revs, so as the revs continue to increase, the power continues to increase up to a point where the torque becomes significantly less and so the power starts to decay. Max power is at 6000rpm. The actual power transmitted to the wheels is probably about 75-85% of the power generated at the flywheel because of various losses in the transmission and drivetrain. However to maximise performance, the idea is to drive between peak torque and peak power, another words keep the revs between 4500 and 6000.
This gets the maximum potential out of the car, but for daily use is inappropriate.
For acceleration from standstill, power is a reasonable comparison, but for maintaining speed or even accelerating from 50-70, torque is a better figure.
Diesels have lower power outputs generally becauste the revs are generally restricted by comparison to a petrol engine, but they have far greater torque outputs. In diesels it is usually a case of driving it at max torque revs to get the best consumption. However in petrol engines, it is not necessarily the case that driving at peak torque revs will result in the best fuel consumption. In 5th gear this would equate to in excess of 90mph (sorry, Officer!). The resistance to motion is a factor and if you double your speed you increase the wind resistance by 4 times. So keeping speed down to a steady 60-70mph in 5th gear, with light acceleration in 5th for overtaking will usually result in best economy. The downside of gearboxes, although they efficiently multiply torque at the wheels, is that you have five or six fixed gears which means that to accelerate within a gear you have to use the revs. Although an old concept, more and more modern cars are offered with infinitely or continuously variable gearboxes. In this instance, there are no fixed steps in the gearbox, in fact there are an infinite number of potential ratios between two points. The logic is that the engine turns at a fixed speed, and the gearbox matches the ratio most suitable to the speed you wish to drive at, and increases the ratio as you wish to accelerate. Ideally it means that engine designers can lower the max torque speed to a lower level allowing the engine to run at constant revs at torque peak most efficiently with reduced fuel emissions and improved wear and tear etc.

Hope that helps.
Martin

BHP is a *LARGER* unit of measurement than PS, therefore you get *LESS* BHP than PS on a given engine.

Power Conversion: ps to bhp

50 PS = 49.3 BHP
100 PS = 98.6 BHP
120 PS = 118.4 BHP
150 PS = 147.9 BHP
180 PS = 177.5 BHP
200 PS = 197.3 BHP

You will note that 1.0 PS = 0.98632 BHP and therefore if you measure your engine output in PS you get MORE than if you measured it in BHP.

Therfore, 105 is the power output in PS rounded up to the nearest integer. IT IS NOT THE OUTPUT IN BHP.

103 PS equates to 101.59 BHP.

Therfore:

ZR 105 = 103 PS = 101.59 BHP
ZR 120 = 117 PS = 115.4 BHP
ZR TD = 101 PS = 99.62 BHP
ZR 115 TD = 113 PS = 111.45 BHP
ZR 160 = 160 PS = 157.81 BHP

ZS 120 = 117 PS = 115.4 BHP
ZS TD = 101 PS = 99.62 BHP
ZS 115 TD = 113 PS = 111.45 BHP
ZS 180 = 177 PS = 174.58 BHP

ZT 160 (1.8 Turbo) = 160 PS = 157.8 BHP
ZT 160 (2.5 KV6) = 160 PS = 157.8 BHP
ZT 180 (Sports Auto) = 177 PS = 174.58 BHP
ZT 190 = 190 PS = 187.4 BHP
ZT CDTi = 116 PS = 114.41 BHP
ZT CDTi 135 = 131 PS = 129.21 BHP

TF 115 = 116 PS = 114.41 BHP
TF 135 = 136 PS = 134.14 BHP
TF 120 Stepspeed = 120 PS = 118.36 BHP
TF 160 = 160 PS = 157.81 BHP

(all PS figures taken from MGR website)

Hope this clears up this (very) common missconception.

SF
Scarlet Fever

Booger, my mistake, getting my conversion factors the wrong way round. Apologies!
Martin

many thanks once again for such informative and detailed answers. I wonder how customers feel when taking delivery of brand new cars, with a tight new engine, and instructions to keep revs below 3000 for the first 1000 miles. Perhaps a lesser engine and different gearing would provide more useable torque and power for general daily use. I wonder about that because I gave back a 1.4 - 8 valve K series and in a Metro it was a real cracker. But I have no torque and bhp figures for that car, and it didn't come with a tachometer. I think also it would explain why my 105 ZR gets better the faster you go and what is not good about that is our motorway speed of 70mph when my car would rather be doing 90mph.
Gary

105PS = 103.56BHP

Looks like Rover have been caught out (not for the first time) by confusing PS and BHP. The ZS180 is also quoted as 177PS... 180PS = 177.54BHP.

Ralph
Ralph

still intrigued - where can I get power and torque curves, and through the gears in mph? We've read the stats, but is the 1.8 VVC really better than the 1.8 MPI in terms of daily average revs if you always drive between 1 - 4000 revs?
Gary

The brochures used to give the torque and power curves for each engine. Not sure if they still do this. For the average driver needs, a VVC is probably overkill, I'd agree.
Martin

In the real world, on various tours, there is little to seperate the models. On a recent Treffen we had 17 MGFs of various guises (including a couple of Trophy 160s) zipping round Belgium on a run and the only issues were people taking wrong turns. Last year's Le Long Weekend tour saw a splinter group consisting of my mdified MPi (147 bhp at flywheel), a VVC, 2 Trophy 160s and a TF 160 heading off for some high speed stuff, again no one had trouble keeping up.

Imperically there are very real differences, out there on the road, the differences narrow to an extent that it isn't an issue (just as well really as any tours would be a nightmare with the front group having to wait for the others to catch up).

1.6 and 1.8 may be more noticable, but to be honest, it hasn't cropped up on one of these tours yet.

SF
Scarlet Fever

still on the subject of power, and taking the 1.8 as an example, there are two 160 versions in 4 cylinder form. The VVC, as used on the TF and ZR, and the turbo, used on the ZT and ZT-T. Not entirely sure if they use exactly the same 1.8 as a starting point, as there is the 120 version, and with some modifications, such as high lift cams, comes the 135 which with the VVC mechanism then becomes 160.

But compare the differences -

1.8 turbo
Max Power 160 Ps @ 5500
Max Torque 215 Nm @ 2100

1.8 VVC
Max Power 160 Ps @ 6900/7000 (TF/ZR)
Max Torque 174 Nm @ 4700/4500 (TF/ZR)


So, on the turbo, the torque figure is a lot lower down. Every one passes the 2100 revs mark all the time, so is there any point to a VVC except at high spin?

Anyone had the experience of both?

I suppose, put together, turbo plus VVC would produce some mean machine!

cheers everyone
Gary
Gary

yes, quite a degree of experience of all the current engine ranges in most of the available body styles.

A point not mentioned is that the 105, 110, 115, 120, 135, 160, 180, 190 and 260 designations are all simple marketing titles and only 'point' to the engine power. Of interest only the 260 actually corresponds to BHP, (264Ps) but that is of little real interest, the 302lbs ft (410Nm) of torque is of more importance.

The V8 engine too demonstrates an apparent peaky nature with peak power quoted as happening at 5250rpm and peak torque at 4000rpm. However, the reality that over 90% of the peak torque is available for most of the rev range from not much above tickover upwards to the max of 6000rpm.

the engine is only part of the equation, vehicle weight and gearing are the other major influences. Here the K 1.8 engines demonstrate two completely different characters for two completely different roles.

The 160 VVC delivers it's peak at 6900rpm with a quite respectable torque peak at 4500/4700rpm. The VVC enables the engine to continue to breath heavily beyond the point where the non VVC engines are starting to tail off. Therefore to gain true advantage needs the engine to be worked in that specific area. IOW revving its n*ts off!

The 1.8T and plain 1.8 engines share the same block with all other K4 engines as a matter of interest, 1.1 to 1.8 litres. There were a couple of months ago three different liners used, 75mm for 1.1 and 1.4 litre versions and two 80mm liners, one for 1.6 and 1.8 non turbo, and another uprated liner for the turbo. The 80mm liner was to commonise on the turbo type to reduce complciations, don't know if this has happened yet.

Pistons vary accoring to use and all non VVC engines share the same head, a plain simple small valve spec head with plastic manifold and a 5500rpm peak power for most 6750rpm for the TF135 spec with cam induction and ECU changes. VVC engines peak at 6900 or 7000rpm depending on when the engine was made. Torque peaks are varied with 4500/4700 for VVC engines and anything up from 2750rpm for the saloon 1.8s to 5000rpm for the TF135. Driving the cars though and you would be hard pressed to notice these apparent huge differences in peak torque. By comparison the 1.8T peaks at 2100rpm and has boost to give it a a solid 20plus % bonus. The differences in the other 1.8s is quite small and then enhanced or masked by gearing to suit the weight and style of the vehicle it powers.

Gearing is decided from a number of different requirements and the chosen gearing is effectively a torque (and power) multiplier, hence the numerically higher lower intermediate gears create a greater resulting number reflected in the difference seen when accelerating in second gear and in fifth gear from the same rpm point. Gearing is also chosen to help move the vehicle mass so a heavier vehicle is better suited to an engine that produces lots of low speed torque (power) and this is helped by the gearing.

The character of each engine spec is also very different in the rather frenetic operation of a VVC compared to the ZT 160 1.8T which is somewhat laid back and does it's best work below 5000rpm. However these characteristics are designed in and so it is technically quite feasible to turbo a VVC and operate with much higher boost pressures. Quite whether that on paper spec is any better than a carefully tweaked current spec 1.8T is another matter though!

Character is what makes a car and here the engine is a major input in that area. The differences in the 1.8 K series are deliberately chosen to enhance that effect, rather than they turn out engines and the effect is a side issue.

In terms of a VVC in a ZT and you need low gearing and to move the car any better than the new 120 model, operate it in the 5000 to 7100 rpm area. Not ideal since such use will reflect in poorer fuel consumption. Ease off and then there is no real advantage over a base 120.

The 1.8T in a TF or ZR though and the top speed would drop below the VVc engined version unless you lengthened the gearing to match the lower rpm thresholds of the Turbo engine. Longer gearing reduces that torque multiplier effect and so to stay ahead you need a higher torque number delivered by the engine. Here 215Nm is enough to maintain this lead and so the car would be quicker in acceleration and match top speed. However the sporty nature that makes up an importnat part of the cars character is changed due to the laid back delivery of the 1.8T. This of course takes no account of the prospect of altering the power output of the 1.8t, nor of any deliberate change in it's character to suit the different application.

In the real world Scarlet mentions differences in performance have to be huge to see any really worth while on road differences. The on paper differences are largely immaterial unless this as at least an additional 50% more of both power and torque, ideally delivered by a large displacement engine, now where did I put the keys to that 260 V8??

Rog
Roger Parker

........i can see a bedroom chat Between Mr Parker and Mrs Parker on the subject of 8 cylinders......SOON!

Mega
Mega

Why do you do this Megsy? You have nothing of any value to add so you decide to try to belittle the previous poster who by my reckoning has come up with one of, if not the most informative postings i've seen on the Zed forum to date.

Not funny.

:(
Smiley

lol
Eric

This thread was discussed between 03/09/2003 and 23/09/2003

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