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MG MGF Technical - A Question of Speed

I noticed something kinda starnge over the weekend.

i have a lovely 1.8i

and would you all agree that max speed in 2nd and 3rd gear is about 60 (possibly 62) and 90(possibly 92) well thats what i thought, and thats what it was doing on the way to work. red lining at about 7100.

But all weekend the rev limiter seem to cut in slightly higher ( i think) but what i am sure about is i could get just over 65 in 2nd an just over 95 in 3rd.

now whats going on there. the only difference i could think was that over the weekend the hood was down.

but who knows. wierd stuff, whats anybody think?

7100 with an MPi ??
Dirk Vael


90MPH on a public road? What sort of message are you trying to give out to us younger members of the BBS?

You are a disgrace and should be banned from this BBS.

Very worried,

now to your problem. A : remove the rev limiter.
B : when the hood is down surely the car has a higher Cd and therefore will go slower in each gear unless the engine has enough power to keep on going?


Was the rev limiter comming in it 7,100.

Dirks right, the 1.8i red lines at 6 thousand and something and the rev limiter should come in well before you see 7,000 revs.


did I forget to mention, I was only joking :-P
so what speed have you hit?

Check my FAQ for some detailed MPH per gear, check the 1.8i vs VVC section


Greg Hilton

ECM max permitted engine speed is 7000rpm achieved by fuel cut off. This is reinstated when engine speed is detected at 6990rpm or below.

Remember that readings from both the tacho and speedo are unreliable and will have significant error.

Roger Parker

and if you weren't joking steve, who said i was on a public road.

rev limiter does cut in, but its definetly over 7000, and he needle is sittingly nicely IN the red.

Steven, your Cd comment is right, thats why i was confused, the car was definetly going quicker , noticably. acel seemed pretty much the same. generally all i could think though was how dam good the car felt and how well it was running.

maybe somebody snuck in one night and put a VVC engine in my car for the weekend, i just dunno

Don't assume that the car is slower with the top down. Aerodynamics can do funny things especially when you get turbulent vortex shedding at certain speeds. A smooth surface is not necessarily the slipperiest one!

Also atmospheric conditions can change the performance of the engine considerably. I seem to remember something about every 1mb pressure increase giving a 0.1% drop in output power - same for ambient temperature. (numbers probably wrong). This is one of the reasons why aircraft taking off from say Denver (1000+m) can carry less load than identical aircraft leaving from Schiphol (-50m).

N837 OGF

Whoops - on previous posting read 'decrease' for 'increase'. Don't know my + from my -.
N837 OGF

All good stuff Hugh, and it all crossed my mind (I'm a flight test engineer (for one more month only - thank god)) but for those to be the reasons, other poeple would have noticed them by now, especially if the hood down aerodynamics did finky things. and i doubt very much that the pressure was particularly special over the weekend.

If you look at dodgyrevs/stalling, i think thats what the threads called, well that problem appeared after the motorway journey when i noticed that my car had been sneaking the odd steriod here or there.


well as a pilot in training i can agree with hugh on the power losses at higher altidutes. the airs just thinner. but unless you live in a very high altitude mountainous region i doubt a car would notice it much.

btw, matt, i bet it was a public road :-P

Re aerodynamics - I was told authoritatively that pre launch testing (at Kylami) showed 10mph improvement in top speed with hardtop on vs hood down.
John Thomas

This last week I’ve experienced the same thing with my 1.8i as Matt It’s feeling quicker and it runs easily over 60 in 2nd gear. We have had very good weather in Sweden this last week (+25 C), could the temperature have any influence or is the performance or is it just an imagination due to an unusually early and warm spring?
Mikael Fällström

Well, I don't know about 2nd gear. But I can say that in fourth, i achieved 115mph with the top down and carrying a passenger, up hill (about 7% gradient) whilst giving a BMW and a Mercedes a run for their money!. In case you wondered, they both pulled over for me. I'm not sure what model the merc' was, but it wasn't a small one and it had a 'Kompressor' badge on it!



the warmer the air entering the engine the lower its performance will be. nice cool, dense air will result in the most power as the difference in air in/air out (i.e. exhaust gas) relates to the power produced. hence why some vehicles have inter-coolers, to cool air that has been heated due to the effects of compression.


So the aerodynamic are better with the hardtop. Anyone got any figures for the softtop? If noise = wasted energy, I would suggest that the softtop is a bit of a dragchute......

N837 OGF

Now this is getting silly.

agree - hood down is more draggy with the windows down.(specially with my head sticking out over the windscreen). John, do you know if your figure of 10mph is windows up or down. i have noticed that windows down the car struggles at high speed, a big and noticable difference.

hopefully you will all have had a quick look at the dodgy revs thread, cos i think its connected.

On the way home last night in second i was touching 68 at least, a comfortable 65 at 6500rpm. which is a shocking increase really. and at the same time the dodgy rev problem appears. which seems to be connected to throttle position not revs, because letting of the gas ust before my foot comes of theres a little lumpy patch, and again when i put my foot down.

this morning, back to normal, 60ish in second as it was and should be.(hood was down aswell)

admitadly the times when it does happen i am hammering it and red lining a lot, but its strange.


I noticed a sudden decrease in power and top speed. The car struggles to get 90mph in top (I borrowed Tiffany Dell's runway to check this ;-)) and takes about 8.1 minutes to get to 60mph. I also seem to be having problems getting the roof to fold down, the brakes are practically non-existant and the car wallows round corners. The slightest side-ways breeze blows the car across to the neighbouring lane.

Oh, did I forget to mention that I have been given a Nissan Micra as a courtesay car by the company repairing the damage to the rear of my car?

Isn't it strange how you can go weeks without seeing another MGF and then, when you're driving a Micra, suddenly 2 come along at once? So if you see a Micra having the rowlocks thrashed of it on the M1, M10, M25 or M2, trying to get MGF performance out of it, then spare a thought for me.

Ralph Gadsby

You can thrash it all you like, but you won't catch me, cos i got a super MG

I'm a poet and i didn't know it

>Don't assume that the car is slower with the top down.

I disagree *g*
hurried from East to West this afternoon on the motorway.. 290 km in 2h 20 minutes (only one trafic jam occured near Cologne)

Roof down and I got only top speeds about 210km/h. With roof its much easier to reach the 220++ and 6800 revs.
Arta windstop installed and drivers side window half raised. Co-drivers window closed.


Roof up and roof down. I had a real bitching session with Motor Mag (now Autocar) and makers quite a number of years ago because they did their comprehensive road tests and issued lots of glorious figures, but they all applied with the hood in place.

I made the point that it would be a valid addition for all cabriolets to have additional testing and results obtained for both performance and economy with hood up and hood down.

This issue was raised after I had been testing my MGB V8EFi at Bruntingthorpe test track and with the hood up I passed throught the timing gate at 142.6mph still slowly accelerating. With the hood down the same run peaked at just over 10mph less before the gate. The buffetting was like having someone standing behind and smacking the back of your head with a shovel!

I would suggest that around 8 to 10% reduction in top speed is a very reasonable estimate based on comparitive experience with the F and other soft tops. The effects of the greater drag should appear in a steadily increasing degree above 60mph and the fuel consumption will drop in a corresponding way too.

Of further note was some wind tunnel testing, again on the MGB, which showed that the hard top had additional advantages over a hood due to the air pressure deformation of the hood material whilst the hard top was a rigid structure. Hood flap was also a negative point, but this isn't such an issue with the F hood.

The specific shape of the MGF means it's results will be individual to it, but the previous experience gives a good indication of what will be applicable. I am certain that somewhere there is aerodynamic performance results for the F, if anyone knows where it is.

Roger Parker

Roger, i ahgree it would be interesting and useful for mags to get figures like that. but then thye would also have to discuish between hood down, with windows up and windows down. IMO with the windows up the difference isn't huge, speed wise, but with the windows down the reduction is very noticable. as for fuel consumtion, i've only been watching it for a few months, but you could plot it against temperature and get a nice oposite trend.

I forgot just how many miles i do when the sun is out, and they are mostly hard driven miles aswell.

over speed problem hasn't appeared for a week now by the way, neither has the rev problem. might have to try and induce it so that i can show it to the garage, otherwise it will be a case of, if we can't see a problem we can't fix it.

cars going to quick again, i tried it hood up and hood down - no difference. went onto my private race track and was nearly get aton in 3rd. very strange. acceleration flt v good as well, but that might just be my imagination running away with me. I am going ot do a proper job of plotting the speed/revs/gears and see how it all looks, see if theres a pattern

Why should aerodynamics (i.e. hood up or hood down) matter if we are just talking about maximum speed in a particular gear given that the rev limiter always cuts in at the same number of revs? Surely the same number of engine revolutions per second in a particular gear always results in the same number of wheel revolutions per second, which always results in the same speed (unless there's wheelspin)? It may take longer to get there if the hood is down, but the max speed should be the same.

I'm sure I must have overlooked something however...


my one goes similarly brilliantly. Don't need vvc

The dial showed nearly 130 with the hood down the other day.
Roger's shovel man was on the bootlid but phew it was exhiliarating.

Some F's just seem to be quicker than others.
Also as my one has no cat K&N and Trevor Taylors "English Electric Lightning" Exhaust The sound was there too!!



Scot, Aerodynamics plays a big part in top speed. A car will accelerate as long as the driving force is greater than the sum of the frictional forces. However, air resistance (drag) is a function of speed and the aerodynamics of the car. As the car's speed increases so does the air resistance and when the friction forces equal the driving force (which is a constant value) no further increase in speed can take place. Newton's 2nd law. So, the less aerodynamic a car is, the slower the speed requires to produce balanced forces.

Hope that helps

Tillman Kleinhans


Scot, i agree with you, Tillman, you are right aswell but the fact remains regardless of power, if the engine is spinning at x rpm, and the gear ration is y:w then the (assuming the same wheel spin) the speed is the same. aerodynamics will effect how quickly you get there, and in top gear will effect top speed, because you don't reach the rev limit so speed is limited by power pushing the car and resistance trying to stop it. so everybody is right.

which tells me that the 'problem' of the car going faster must be a speedo error, or for some reason the rev limiter is cutting in a 100 or so rpm late

but the car seems to be running fine still, no engine problems, no smoke, no leaks, so what the hell. i'm driving slowly now anyway cos i'm that skint that i have to try and cut petrol costs and trying to put off new tyres and brake pads (how long should they last by the way - standard rover)
Matt Lawrance


O.K. Lets add my two cents,

What was the wind speed and in what direction. As i have found in my little van, even a 10-20mph difference can have a major effect (obviously) if i've got the wind on my tail in the van i can get up to 110mph wheras in a head wind the best is 70-80mph. So if you've got your hood down with a tail wind your'e gonna go faster!

Nick ;-)

Hmmmm, that still doesn't account for how the limiter is behaving though does it? After giving it some thought there are no sensors to give any indication of loading on the engine (drag) to make adjustments to the limiter. The only answer i can think of would be not that the speedo is innacurate but maybe the rev counter?


I notice that my 1.8i's perfromance alters significantly, and is associated with a number of factors:

1. weather- cool+damp= more pep in the motor dept.
2. clean filter! Clean filter+cooling ducts that haven't been mangled= sheds more power, flexibility and fuel economy
3. previous driving- if I've just given the car a thorough thrashing, it feels far far better for it. I always attributed this to clearing crud from the spark plugs (probably erroniously). MEMS learning phenomena?

Hood down definately blunts top speed. Can't say I have noticed much difference between hood up and hardtop on.

Rob Bell

As long as there is sufficient power then intermediate gear terminal speeds will remain a function of max engine rpm. In top gear where the gearing is matched to the engine peak power rpm there will be a significant reduction in max speed because the resistances are so much greater. This assumes that aerodynamic resistance increase is not offset by gravitational advantage (downhill) or prevailing wind advantage. (Tailwind) Nothwithstanding this, specific fuel consumption will increase.

Rob's observations can be measured and confirm his feelings.

1, The difference between a cool damp day and a hot dry one can be between 5 and 10% power difference.

2, A blocked airfilter can significantly reduce airflow and this will reduce power. (Of contrasting interest, the provision of extra airflow over standard does not guarantee extra power!)

The clean airfliters extra power reflects extra efficiency. Most of the time you do not operate at full throttle but at a throttle opening to enable you to maintain a set range of speeds within the traffic. During htese times the extra efficiency means that a smaller throttle opening is required to generate a set power level and this returns better fuel economy.

3, Nothing more complicated that cleaning crud from the whole inlet tract, combustion chamber and extaust tracts. A similar but better effect on post 40k mile cars is achieved by using one of the 'on the car' special injector cleaning rigs. Even more effective if immediately followed by a rolling road run which is far harder than any road thrash.

Roger Parker

but why should the terminal speed change though, from one drive to the next sometimes. in the speedo/rev/limiter stuff are there any sensors that could suffer from a step change in tehir calibration for example. the change would only have to be small.

Just a quick thought i had the other day, on the cold air thing. this ais a general question and not specific. air con uses up power form the engine, could this not be reduced by using some of the cold air from the air con unit it chucking it in the intake, seems like a good idea to me, but i'm sure people will have things to say on that.
Matt Lawrance

I refer to terminal speed changes in top gear only, as there is simply a surplus of power available to overcome all resistances in the lower gears, up to max permissable rpm. The overall gearing is chosen to maximise the potential performance so that the vehicles maximum speed is a balance between the power available against all the resistances. Put simply if the engine produces 120bhp and the maximum potential calculated speed is 120mph then to achieve that speed you need the gearing to be such that the engine rpm where that power is produced matches that speed.

If that maximum 120bhp is produced at 5500rpm then the gearing needed to achieve that balance of power vs speed is 21.8mph per 1000rpm in top gear. These are the approximate figures that apply to the 1.8i. Now if you alter the aerodynamic shape of the car, which is like a dragging brake, then things change significantly.

The power required to achieve that original 120mph will now be something like 140bhp which the engine hasn't got. In addition the existing 120bhp it does have is produced at the same rpm, but the gearing is now too high so the car can't pull maximum power rpm in top gear. We would have to look at a graph of the power required against speed for the cars new configuration and reset the gearing to once again achieve the balance where max power rpm and max speed lines crossed again. In the meantime the unsuitable gearing would ensure that a somewhat slower than that maximum speed would be achieved.

Of note adding weight is of no consequence to maximum speeds, only the time it takes to accelerate to those speeds.

Roger Parker

Drove to Vincenza italy this weekend for work reasons. Got my 1.8i up to 225KM with top down and she still had a bit left.
One question though. I use my F as my everyday car. I drive her on the Autobahn alot and usualy drive between 180-210 KM. Should I be worried about perhaps driving my F to hard?

Adrian and Bucketm(HD K1256) in Germany
Adrian Ferrer

Hi Folks,

Now safely back from my weekend in Plymouth and with a further 600miles on the clock!

My postings under 'High flow filter' are now relevent on this thread. I can report that after moving the intake pipe from the side vent to underneath the car has had no difference except that it is much noisier!. In fact i have reason to believe that it may even have impeded power. The reason could possibly be that although airflow has increased, fuel has not. which may have resulted in a lean mixture. Fuel economy is good though, using only just over half a tank at an average of 90mph over 300 miles!?!!! I think i'll be putting the pipe back to the side very soon.


BTW one other small thing - downhill at 90mph foot right off the throttle and maintained constant speed until braking (forced air pushing throttle open??)

This thread was discussed between 08/05/2000 and 05/06/2000

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