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MG MGB Technical - tuning questions

hi, im new here, ive got an 1967 mgb roadster.

its a fantastic car and i use it everyday after i resprayed it, the rebuilt engine has done 5000miles

generally the car is in good order and runs near perfect.

i wish to give the car higher top speed, on the motorway at 3400 revs i can press the accelerator and it feels like it could pull another gear, so my first question:

what options are there on raising gearing? higher diff, higher overdrive ive got a set of 16in wheels with mg pattern hubs (its got wire wheels) i wonder if i can fit these wheels with lower profile tyres and increase overall diameter by about 1in to give a higher top speed.

second question, an old man gave me a cylinder head from his failed rebuild, it was a recon unleaded unit but now its fitted the car seems infinitely more powerful, the only distinguishing feature is a massive centre rib between plugs 2 and 3. is this a performance head or have i finally found the accelerator :D

anyway i'd appreciate some input and knowledge. i thank anyone who replys
d buck

Hi.

Your new head should have a casting number which would make identifying it relatively simple, please take a look and let us know.

The new head may be performing better because :

1) It has been gasflowed and ported.

2) It has larger valves.

3) It is producing a higher compression ratio.
(beware of pinking).


Don
Don

At 3400 rpm in OD 4th you should be doing about 76 mph i.e. 22.3 mph/1000rpm, about 61 in straight 4th i.e. 17.9/1000. I doubt you will get a higher geared OD without a lot of work, and then you will lose the benefit of OD 3rd. The usual way of getting a higher geared rear end is to change the rear axle ratio. The standard was 3.909:1, the auto used 3.7:1, the MGC had 3.307:1 or 3.071:1, and the V8 had 3.071:1 which gave 28mph/1000rpm in OD 4th. However all these will reduce acceleration in all gears and cause the speedo to under-read unless you fit an alternative item, as will over-sized wheel/tyre combinations. It may feel as though it could pull another gear, and it probably could on the flat, but come to any incline or the need to accelerate and you will have to change down a ratio by whatever means, unless your engine is producing significantly more torque than the standard. Before doing anything I'd have some torque measurements taken to see if it *is* more powerful than standard, and if so by how much. If you increase the gearing by a greater percentage than the torque you will probably lose out.
Paul Hunt 2

D-
Yes, you can raise the rear axle ratio by fitting different crownwheel and pinion gears. A 3.7:1 gearset @ 6,000 RPM will give a top speed of 113 MPH in 4th gear and a top speed of 141 MPH in 4th gear with overdrive.
Steve S.

Has anybody changed to a 3.7 diff whilst still using an MGB engine? I have a rather powerful engine and a 3sync box, which means I can hardly use 1st gear at all.

Would changing the diff from 3.9 to 3.7 make a lot of difference in the rev drops between gears?

looking forward to your responses,

Alex
Alexander M

The 280z 5 speed has a higher 5th gear. --.745 as I recall.
Barry
Barry Parkinson

Alexander-
Because the gear ratios inside the gearbox would remain the same, changing the final drive ratio would have no effect upon RPM changes between gears.
Steve S.

Steve:

I don't mean to be facetious but I don't think the MGB will do 141 MPH in free fall.
J.C. Weidner

JC-
It's all just a matter of having enough power. A well-tuned Rover V8 conversion could do it. A bone-stock B Series engine would run out of breath on even mild grades with such a final drive ratio. However, the figures are really just for comparitive purposes. With a well-tuned engine the car would cruise sedately at 70MPH.
Steve S.

Steve,

But wouldn't the 'total' gear ratios change? I mean by this: the ratio of the gear in the gearbox multiplied by the ratio of the diff.
I would think this would give the same result as a stock diff with gears that are further apart and of a smaller ratio (thus enabling higher speeds to be attained in the same gear than with a standard car).
I could of course be totally wrong here :-)

Alex
Alexander M

Alexander-
Higher road speeds attainable in any gear would be effected by the final drive ratio, but as long as the gear ratios in the transmission remain unchanged, the change of engine speed when changing gears will remain the same.
Steve S.

Of course... (the flu seems to have affected my head...)
Alexander M

There's a wee bit more to it than this. It is a well known fact that top speed is directly related to the BHP of the engine and there are several other important factors as well such as rolling resistance, drag coefficient and so on. The MGB delivers its max torque at somewhere between 3000 and 3500 rpm at which point the car would be travelling at just under 8o mph. Now I accept that the car will exceed 100mph but by this point the torque and hp will have dropped to a point where they cannot exceed the drag etc. It is for this reason that many cars used to have a higher top speed in direct 4th than in overdrive 4th. No amount of lowering rear axle ratios will therefore make any difference. If you want more speed you need more horses and this is where a tuned V8 will come in. You can then lower the diff ratio and still enjoy the extra top speed atlower revs.

I'd be interested to know though if Surrey is exempt from the National speed limit !!
Iain MacKintosh

Iain,

I agree, that is why I asked if this modification has been done before. As my car now has 124 hp at the wheels (and a lot more torque as well), I think a lower diff ratio might work...
Alexander M

Yes exactly and it would work for you and you would reap benefits, but we don't know what the originator of this thread has done to his motor if anything and he may still have the original 68bhp at the wheels. That being the case his gearing is correct.
Iain MacKintosh

Alexander - as I recall that is about the power from the factory V8 *engine*, so the MGC/V8 diff of 3.071:1 at least would definitely be better (overall) than the standard MGB.
Paul Hunt 2

thankyou to everyone who replied.

my car has got less silencers than before and this head, which im 1/2 certain gives me more grunt.

regards to some suggestions on my shopping list should be either:

automatic b crownwheel/pinion

mg c crownwheeel/pinion

i still wish to clear a few more items, can i use 15in mgc back wheels??

and what vehicles/axles do the 3.9 3.7 diffs come out of (even though i probably wont use one !)

again, thankyou everyone.

ps the speed limit in surrey is very well imposed!
d buck

Paul Hunt has detailed the source of these alternatine diffs as MGB Auto, MGC and MGB V8 but take care here. I don't think you want to change the axle ratio as this will alter the ratio of all the gears and for a quick get away first will be starting to get a bit high. Yes I know that second could do with being a bit higher but first would be my concern. Added to this you could very easily get to the stage where the car won't pull the skin of the proverbial. Have a look at Peter Burges's book Tuning the B Series engine and see what he estimates your engine might be producing and I doubt if it is much more than 75 HP at the wheels. If you fit a higher diff ratio you will pull the performance back again to maybe the same or less than a standard car and if your coppers are as active as you say then I can't see the benefit as the improvement would all be in the illegal ranges.
Iain MacKintosh

well what im really getting at is a boost in fuel economy, to my mind first gear is far too low, and the jump to second is massive, in an ideal world i would have a slightly higher first gear.

i do understand what you are saying regarding the bhp etc, i think i really need to check what head im running, but from a general drivers perspective i can be sitting at 3800 rpm going up an incline in top with od engaged, hit the throttle and the car begins to accelerate quite rapidly, certainly it can outgrunt the average car. is this normal??

i think a slightly higher first would mean you wouldnt have to change gear as soon as the clutch is bought up.

d buck

hi , i had a look outside but cant remember where the code is on the cylinder head.

at the moment i'm looking ofr ome 15in wheels or an auto b diff, which upon first impressions appears to be as rare as rocking horse poo :D

does anyone have one i could purchase?
d buck

D,

Just an idea but perhaps you could put your car on a rolling road to have an idea of the power you have available? This might help in your decision.

Secondly, you can fit MGC wheels, no prob at all (I did this last year). I do however seem to recall (this will be in the archives somewhere...) that the added inch doesn't make a huge difference...

hth,

Alex
Alexander M

Wasnt the topped out speed for the MG 140mph If I remember correctly? Dont know what rpm that is at though
James

James,

The MGC tops at around 120 mph (http://www.mgcars.org.uk/mgc.html) but the MGC speedo goes to 140 mph (as opposed to the MGB speedo which goes to 120 mph, the mgb tops at about 110 mph iirc).

D,

I just remembered: you can fit MGC wire wheels to a MGB wire wheel car, the MGC 'steel wheels' have a 5 bolt pattern while the MGB has a 4 bolt hub.
Alexander M

this is true but i run wire wheels and the hubs just must be the same on B and C, can anyone correct me, is there a difference in spline??


also where should i be looking for a code on the cylinder head, is there one set place?
d buck

I'm confused now. In another thread you seem to be into slipstreaming vans !!!!
Iain MacKintosh

to save fuel slipstream vans.
d buck

D,

As a posted: yes, you can use MGC wire wheels on MGB wire wheel hubs without a prob.

I see your point about saving fuel, but you have got to know that old British cars are bottomless money pits? ;-)
Alexander M

Because most HP at high speeds is dissipated as drag, hp requirements increase with the cube of the speed(assuming a constant drag coefficient. That means if you want to go 10% faster (100mph to 110mph) you need 33% more HP. If you have twice the HP, you theroretically can go 26% faster (100mph to 126mph).
CDD Dewey

Hi D

You will find the casting number under the rocker cover, it will be in raised figures and will probably start '12H....' (but might not).

Don
Don

cheers don, i will remove the rocker tonight.

its not really about more speed, its fuel efficiency, well a combination of both if im honest.
d buck


I am seeking the same thing,more mpg.I have a 1980 GT with overdrive and sunroof. I hav returned the car to standard rostyles to correct speedo error and found an added bonus of a more comfortable ride. Keeping the car at about 3300 to 3500 revs on the motorway is giving me about 32mpg. I would prefer to improve on this. A higher ratio diff would be my way to go. I understand that a sports manifold KN filters and rejetting the carbs helps economy. Can anyone comment
G Withnell

Hi all.

It is worth noting that an overdrive (or 5th gear on a more modern car) is not ideal for improving fuel economy, as it introduces extra gear pairs.

A driven gear pair is no better that 95% efficient (and may be significantly worse) and two gear pairs are usually involved. If you need proof, try the temp of your diff after a run, that heat had to come from somewhere.
It is better to go 'straight through' from the flywheel to the diff if possible.
If you look at a graph of fuel consumption in different gears for a typical car (don't know if / where there is one for the 'B') you will see that while going from 4th to 5th gear does improve fuel economy the improvement is significantly less than it is from 3rd to 4th.

I did a comprehensive trial of this with my old Cavalier, I found that 'hanging on' to 4th gear (ie refraining from going into 5th at the earliest opportunity) did not affect fuel consumption while improving flexibility. At motorway speeds 5th was used, of course. My 2.0 Cavalier regularly did 44mpg, BTW).

An engine is generally running at it's most efficient at the revs which produce maximum torque (yes, it is more complex than that).

Don
Don

JC, that has been done you know. An MGB in free fall. Not intentionally of course and they don't say what speed it reached on the way down. Of course the speedo doesn't go that high :)

http://www.mgb-stuff.org.uk/splat.htm (on Paul Hunts site).
Simon Jansen

I've oft seen it writ that OD makes no difference to consumption, but I would have thought that doing less bangs per mile would reduce consumption, unless one has to use a bigger throttle opening in OD 4th than straight 4th to maintain a speed, and I don't have to do that in either of mine.

Funnily enough economic driving is apparently to be part of the driving test for ecological reasons, including fuel consumption. It advocates getting into 5th as soon as possible and not hanging on to lower gears.

The only fuel consumption graph I am aware of for the B cab be found at http://www.mgb-stuff.org.uk/wn_engineframe.htm and click on 'Power, Torque and Consumption curves'. Bear in mind that all these are taken at full throttle, the engine speed being controlled by a brake.

"Of course the speedo doesn't go that high" It was pretty high when they pushed it out of the plane :o)

Paul Hunt 2

You may remember back in the 70's if you are old enough that motor manufacturers promised to take steps to improve fuel consumption in order to preserve oil stocks and so on. Little did we know that they were about to incorporate 5 speed transmissions at which point it was mission already accomplished. Whilst I agree with Don to a certain degree we cannot deny the fact that the incorporation of the 5th gear has achieved approaching 20% improvement in mpg on cars at that time. The overdrive will do the same.
Iain MacKintosh

Hi all.

I guess that the optimal solution is for the car to be in a 'straight through' gear for the majority of the mileage travelled.

So... is it time for cars being driven on modern roads to be fitted with gearboxes that are 'straight through' in 5th gear and go via the layshaft in 4th ?.
This might not be a good idea for small 'shopping cars' but might be suitable for larger cars like 'rep-mobiles' that tend to be driven for long distances on motorways.

Out of interest, does anyone know which gear is usually 'straight through' in cars with 6 speed gearboxes ?.

Don
Don

4th, or whatever gear is 1:1
greg fast

in rep mobiles i would pressume 5th as 6 speed boxes give more ratios inbetween, so 4th would become a gear between theoretical 3rd and 4th on a "normal" car.

as my friend with his ST220 mondeo says: "all about keeping the enigne in the sweet spot at 5000rpm"
d buck

Suprisingly not, if you look up the ratios of a few six speed boxes via Google. However the few I've seen don't have an exact 1:1 ratio anyway, although 4th is very close. 5th and 6th are overdrive gears, which makes one wonder how 6th pulls, but it all depends on the final drive ratio anyway. Maybe there are good reasons for undersizing the final drive and oversizing the gearbox ratios so as the end result is a close-ratio box with one overall overdrive ratio.
Paul Hunt 2

that is interesting, i wonder if its possible to fit a 6 speed box from something to a B
d buck

If the bellhousing, used for adapt the T9 gearbox to the type B engine, is adaptable to the Caterham gearbox, why not ?
The six-speed box has a direct drive (1:1) 6th gear.
I don’t know the price fot the six speeds Caterham gearbox built by Nova racing transmission.
The Quaife six speeds dog engagement with top gear 1/1 cost 3600£ without VAT (without bellhousing).
The Hewland SGT six speed cost 8000£ with H-gate,10000£ sequential, without bellhousing.
The MGB type B engine rev slow and he as a large “power” band, imho six gears it’s not necessary, however the OE gearbox plus overdrive equal six gears.
Regards
http://membres.lycos.fr/mgcontact/fileupload/uploads/1133874600_box6.jpg
michel

"however the OE gearbox plus overdrive equal six gears"

Two of them electric at the flick of a switch, which is why I've always regarded conversions to a 5-speed box a retrograde step.
Paul Hunt 2

i think a 6 speed with a 1.25:1 top gear ratio is more apprpriatte, otherwise itll be worse than stock with overdive
d buck

I'm not sure why you would want a 1.25:1 top gear because your revs at highway speeds would be even higher than they are now. The regular OD is about 0.82:1 . A lot of six speeds that I've seen have a 0.87:1 fifth gear and a 0.62:1 sixth gear which would be much better-- especially if your motor has higher output.
CDD Dewey

This thread was discussed between 25/11/2005 and 07/12/2005

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