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MG TD TF 1500 - rear end ratio and tuning stage

Hello,

My 53 TD2 is too powerfull!

The engine is tuned to stage 2
9.3:1 compression ratio
Larger valve with stronger springs
Matching inlet and exhaust ports

However, I still have the original H2 carbs.
Original late camshaft as well.
The rear axle ratio is the original 5.125.

The car is really pulling hard on all gears and even in mountains. It s a real pleasure to drive. Top gear is usable at 5000RPM, climbing motorway slopes.
To maintain the engine below 5000 RPM on flat roads, I need to close throttle maybe half way.Once, I tried 5500 RPM. Then I slowed down because the engine was still accelerating.I can't reach the top speed with my configuration.

Has anyone the same configuration ? With the same results ?

What end ratio would be more convenient ?


Laurent.
LC Laurent31

I had that dream once too. I woke up in a cold sweat and went down to the garage and felt the radiator.
MAndrus

My TF is + 80 thou (1330cc) and Stage 2. 9:1, sports cam, modified head, std springs, lightened flywheel, shortened pushrods, MGB distributor, H4 carbs, sports exhaust. Etc. I am using a Ford Type 9 gearbox and 4.55:1 axle ratio. I don't rev it beyond 4000rpm much and it goes very well indeed. It sounds like you need more appropriate gearing.
Dave H
Dave Hill

Laurent, Bud has the Cedarstrand booklet that discusses rear end ratios on his site. It's well worth reading. My recommendation is the 4.3 from the "A" & early "B". I fitted the 4.3 but I'm yet to drive it. Cheers
Peter TD 5801
P Hehir

No wonder it goes like a rocket with the 5.125 axle. I'd certainly change the ratio to give the engine an easier life at cruise. My TF has one of Terry Peddicord's superchargers, 8.5:1 compression, fast road cam and +.080 pistons (forged, like the rods, and I have a new crankshaft). On the track, with a 4.55 diff, it held 5,600 RPM in 4th. Because it is basically a road car I changed it to 4.3, so it now does 100kmh at 3,500. It picks up well for hills and overtaking - it will happily do 70mph in 3rd, so that's also useful for overtaking.

Hope some of this may be useful. Good luck with your change - I'd say 4.3 unless you are racing it.

David
D A Provan


I have a standard engine with a T Lange blower, Datsun 5 speed trans and a Dave Clark 4.3 rear. It goes very nicely on the flat in 5th.

Jim

James Neel

I modified my early TD some years ago. It now has round water hole head and block. The head is 74.25mm thickness (ie -2.5mm), and the cylinders are 40thou over. I reckon that this gives a compression ratio of 8.9-9.0. It has the 0002 Crane cam and a gas flowed head with standard exhaust, 1.25 carbs with standard needles and shallow cone filters (KN 59-9335)- required to get enough air in and avoid a too rich mixture, .
After the engine I put in a 4.3 crownwheel and pinion with fingers crossed because several people advised it was better to use the 4.55 version as the engine might not be up to the longer legs.
No worries. Not only did the engine handle that change but I later put in the ford 5 speed box.
The setup works well because the 4th gear gives good entertaining driving with usable revs almost everywhere and the 5th gives sustained cruising at legal max speed at about 4000rpm. In my opinion this combination can't be improved on. On my car at least!

Anthony
AJ Lunn

That is the same gearing as mine and I an very satisfied with it.
Dave H
Dave Hill

James Neel: You said, "I have a standard engine with a T Lange blower, Datsun 5 speed trans and a Dave Clark 4.3 rear. It goes very nicely on the flat in 5th."

I have the same configuration as you (except for the blower). I am extremely happy with it for acceleration and cruise up to 4th gear. I do find in 5th gear I can cruise nicely, but rarely use it, but can't get any or much acceleration in 5th at speeds above 70 (which I'm rarely at). Am I experiencing the correct results, or is there something amiss?
L Karpman

For information;
Ford Sierra type 9 gearbox fifth gear is 0.82:1, so overall 5th gear ratio with;
4.3 diff is 3.53:1.
4.55 diff is 3.73:1.
Datsun 2T/3T T50 gearbox fifth gear is 0.861:1, so overall 5th gear ratio with;
4.3 diff is 3.70:1.
4.55 diff is 3.92:1.
R A WILSON


Larry,

I am chuckling at your "speeds above 70" remark. There are many on this forum who start to blanch at that speed in a TD. That being said, I found that having the correct carb needle for my setup made all the difference in acceleration especially in the upper gears. Of course having Len's roller cam and rockers may help the performance. I have not yet had the opportunity to go all out in 5th but I know there seems to be plenty of hp to reach terminal velocity.

Jim

James Neel

James, around here 70 on a freeway is very common, and I have no desire to exceed that speed. But there are times at that speed that acceleration may be necessary to avoid some of the idiot maneuvering by other drivers. Trying to accelerate from 70 in 5th is when I feel I have little, if any, juice left.
L Karpman

70??

http://www.youtube.com/watch?v=rpaKBpcceFc
L E D LaVerne

No 80mph for me, I used to cruise in the upper 50s..
P G Gilvarry

LaVerne,

Rumor has it - if that was a TC doing 80 - you would need to be on a motorway and have all 3 lanes to keep her strait :)

Rod Jones

That's about what I needed Rod on the one I worked on at 50 mph. And that was after I put in the VW steering box and new king pins. I should have kept it here longer and straightened it out.
L E D LaVerne

I don't know why, but I have found TC's that wander all over the road have excessive front wheel toe-out. With a proper toe-in (1/4") and a well set up front end, the TC tracks and handles very well. This is a mystery to me, why do so many cars have excessive toe-out?
Be well,
David

D. Sander

Racing TD's see 7000 rpm's quite often and top out over 100 mph at Watkins Glen with 4.875 rear ends. I'd like to put together a 4.3 and see what that would do.
Kurt Byrnes

When do you expect the Hornet to be on the road again,,
TRM Maine

I'm back,
Thanks all of you for the advice.
The Cedarstrand artcle is very interesting. It gives the drag power loss of the TD over the speed range in figure 2.
Combined with the engine power for tuning stage 2 available from the "Autocar" article from July 52, Performing midgets. I could build the equivalent diagram figure 4 of Cedarstrand for tuning stage 2. See picture.

As standard engine, the 5.125 seems to be the best trade off for top speed. Giving 78 mph at 5500 RPM. 165R15 tyres.
But this i my configuration today and the car still accelerates beyond this RPM.

So, my engine is a stage 2++ because of good porting,valve seats angle, bigger exhaust pipe, late cam (the article of Autocar may be based on the former cam in 52)... ?
Or the drag curve is erroneous ? I can't understand why there would be so much difference between TD and TF for drag strength. Maybe today's tyres require less power than the cross ply ?
Any idea ?

Lets assume that I run the 4.3, the surplus power available at 55 mph is 18 HP compared to 27 for 5.125.
Read from the diagram.

Simple physics law gives, for this speed, the maximum climbing slope at this speed with this surplus power : 5.8% for 4.3 ratio and 8.7% Quite an important slope for freeways here in France.
Mountain winding roads never allows such a speed.



Laurent.






LC Laurent31

Hi Laurant and others,

I think diff ratio in a road car depends greatly on the cars usual use. If you live in hilly country, ratios closer to standard might suit. If you live in flat country, a "taller" ratio might be better. A more powerful motor will change this, but if the power is due to a "hotter" camshaft, remember that high revs have to be used to get that extra power. If the power is due to a supercharger, extra torque and extra power can be achieved throughout the rev range, so go to a taller diff. (Maybe one day I will s/c my road TC?)

A 5-speed gearbox with an overdrive 5th would cover all gearing bases. It is not original but seems the most logical cure-all. I have not done it but (apart from general "hotting-up" mods) would be the first major change I would consider for my old MGs.

On the point of TCs at speed ..... I think that a lot of the instability is caused by the lack of front axle location and/or wheel/tyre dimensions. My road TC goes well but doesn't feel great at high speed compared to my race TC. The only difference in Steering/front suspension is the radius rods and 15" wheels/Hoosier race tyres on the race car. The race car has a TC steering gearbox, TC shockers, TC steering ball joints but doesn't feel too bad at 125mph! I get 6,500 rpm with a 3.727 diff in the race car at Phillip Island (with 15" Hoosiers). It is a heap of fun! One day I must try radius rods on the road TC.

Good luck with the ratio choice.
Bob Schapel
R L Schapel

Bob, 125??? Surely an April fools joke. :-)

Tim
TD12524
TW Burchfield

I think he meant kilometers per hour. (77 mph). At least I hope he did! Grin. PJ
Paul Jennings

Bob

The quickest and most affective way to improve a TC is to fit a front Panhard Rod. Sold in the UK.
Rod Brayshaw

Bushings in the forward eyes of the front springs is a really big improvement. . Helps locate the front axle by getting rid of a lot of play. Probably accomplishes the same as a Panhard rod.
Charlie
c mac quarrie

I have seen Bob race. He means mph.

Michael
M R Calvert

A photo of Bob's car packed up after a meeting at Winton Motor Raceway in 2016.

Michael


M R Calvert

Charlie and Rod,

Yes, I agree. Location of the front axle is VERY important. If the axle moves sideways, there is a steering effect because the steering gearbox is located on the chassis, not the front axle. If there is sideways movement in the front spring eyes, the car will "wander". A panhard rod will fix this. (I have panhard rods front and rear on my road TC.) I believe the panhard rod should be chassis mounted on the RHS and axle mounted on the LHS so the arc of the rod movement matches the arc of movement of the draglink.
.... shouldn't this be on a TC thread? ....

Tim and Paul, I DID mean MILES per hour. Revs and maths suggest higher speed but I was corrected by a GPS device. Perhaps I should post some video on YouTube. I am always looking for it from other T-Type racers but feel reluctant to post my own.

Cheers,
Bob Schapel
R L Schapel

Thread is hijacked...
LC Laurent31

Yes I agree, Sorry Laurent.

Have you made a decision on your ratio? I have a spare 4.875 TF CW&Pinion in the shed but that is in South Australia! I run a 4.55 ratio in my road TC, which has quite a few engine mods (including 1366cc) to improve power. It has a 4.555 diff and 16" wheels. I think it is an ideal ratio.

Re the move to TC steering discussion, what is protocol on this site? I think that if someone transferred the TC discussion to a new TC thread, that would make sense.

Good luck with your project,
Bob Schapel

R L Schapel

Hi Laurent,

My maths and physics are getting rusty with the passing of time, and so I would be very interested in seeing your calculations for arriving at the slopes your excess hp will allow with the different diff ratios.

My calculations, for what it’s worth, tell me that at 55 mph you would need about 31 extra hp to climb a 5.8 degree slope and 46 hp extra to climb a 8.7 degree slope.

Thank you for getting things back on track. It is a most interesting thread.

Michael
M R Calvert

Hi Bob. Dunno about the protocol but you could open a TC thread then copy & paste the relevant content from here onto that new thread in the TC section. Cheers
Peter TD 5801
P Hehir

Hi Michael. Here is the rational to find the slope at a given speed with the surplus power :
P = mgv
P = power (Watt). 1HP = 736 W
m = car mass (kg) : 960 kg
v = vertical speed (m/s)
g = 9.81 m/s/s

v = V x slope
V = Horizontal speed. 55 mph = 24.6 m/s

slope = P/mgV

All this is probably not exact because of many things that are aproximated or neglected.
But what is demonstrated here is that the 4.3 is still able to climb important slopes at 55 mph. Just have to depress a little more the pedal.

But why do I feel I have more power than the calculations ?
Is it possible that today's fuel allows more power than the autocar curves ?

Laurent.


LC Laurent31

Michael,
I can see the confusion :
I'm talking about slope percentage and you are talking about angle in °.

So you may also be right.
Laurent
LC Laurent31

Hi Laurant,

It is very gracious of you to put it that way. Yes, I did read your post incorrectly and “saw” degrees where you had written percentage. It is interesting how once read the brain can keep seeing it the same way.

I was also surprised at the slopes you mentioned as important for freeways in France as I thought it sounded very steep. That should have been a hint.

I used the formula:

Power = mass x g x V x sin(alpha)

Where:
Mass estimated at 950 kg (TF unladen + 72 kg)
g = 9.81 m/s/s
V = speed up hill 55mph = 24.6 m/sec
alpha = angle of incline (degrees!)
Conversion 1 hp (imperial) = 746 watts
Alternative is 1 hp (metric) = 735.5 watts

Of course I still may have something wrong.

The diagram in your earlier post says it is from Maintaining the Breed by John Thornley which was published in 1956. A number of references (including New Scientist Jan 1977) support the generally held view that radial tyres have a lower rolling resistance than the cross plys that would have been used for the tests used to derive those curves. I cannot find any reference that quantifies that improvement.

Wiki says that “Approximately 5–15% of the fuel consumed by a typical car may be used to overcome rolling resistance.” For our cars with their very poor aerodynamics I would assume the percentage of power used to overcome rolling resistance at speed would be at the lower end of the range.

It would seem reasonable that, as you suggest, improvements in tyres may account for a little of what you describe, but not more than a few hp. At the moment I have no additional ideas.

Michael
M R Calvert

OK, Michael, I agree with your formula for angle in degrees.

I also agree for the rolling resistance about 5%. So it is not the major part of global resistance.

There is also the yield of gearbox and axle and propshaft. Could the oil do the difference ? No, I don't believe it.

Most probable reason is that it is just a feeling as I don't dare to rev the engine beyond 5500.

I'll try to drive along a long freeway slope which is between 5 and 8%. Maybe I could reach a top speed without revving.

Laurent.



LC Laurent31

hello,
First test drive with the 4.3 axle.
First I could easily get out of my underground garage with quit a steepy slope. That was my first fear !
Then first is long and pulling hard. Second is also pulling hard. Then the third is pulling quite hard till 40 mph (maybe like top with the 5.25) And top is still accelerating, sluggish but I reached a top speed of 77 mph on a flat highway. Maybe I could get a little bit faster but I choosed to stop this first try.

One concern is the axle temperature. I can lay a finger on it but no more than one second. I would say 60 °C.

Is it going to be colder when it gets breaked in ?


Laurent.
LC Laurent31

We have a TD that is supercharged running an MGB 3.92:1 rear. Besides the ratio, it was set up for wire wheels. It cruises on the highway at low engine speed just fine, but it is running way below where it makes horsepower. (3,000rpm @ 60mph, 4,000 rpm @ 80mph) It's a compromise. Without a supercharged, I'm sure it would be very disappointing as far as performance. The 4.3 is likely the wisest decision with a stock gearbox.

I'm just finishing up a S.Co.T supercharged XPAG punched out to 1466 and selected Len's modest street/supercharged roller lifter cam for high midrange power rather than peak horses at high rpms.

Bear in mind, the 5 speeds with overdrive coupled to the 4.3 give a ratio pretty close to our 3.92, so 4th gear would seem to be a good overall driving gear.

You might even do the math considering a 5 speed with the original gearing. You'll still have a low first gear for great acceleration and 4 closer spaced gears to shift through and cut your rpms with the overdrive.
JIM N

Laurent,
I had to reread your last post to understand you've already made the upgrade.

I would not expect any "breaking in" is going to remedy a
ring & pinion issue.

You need to jack the rear wheels off the ground and roll the driveshaft by hand to see if it is in a bind. It should take little effort to rotate it with one hand.

I'm assuming you're checking the gearing in the middle and not having a brake drag generating heat. You can check your brake resistance at the same time.
JIM N

two points,

1: Laurent,
Jim's comments make sense to me. His check for "binding" sounds good and I doubt that "breaking-in" will help. Maybe there is no problem? I don't find 60c an alarming figure for temperature, especially if the car was driven fairly hard on a warm day. Can you measure the temp accurately using one of those modern infra-red trigger pull things (I can't remember their proper name). Do make sure you are running a HYPOID oil. I have seen a hypoid diff destroyed within a few kilometres by the use of an oil designed for a spiral-bevel diff (like original TC).

2: Rod, LaVerne, David, Tim, Paul and others,
I have uploaded a video to youtube which is relevant to the banter above from last April (about TCs at speeds over 70mph). I have put details on the TA, TB, TC thread for those interested, so this thread stays on track.

Bob Schapel
R L Schapel

Thanks to all.
Yes the heat is from the axle mechanism, not the brake drums.
I jacked up the rear wheels and I can easyly turn the shaft with one hand. Either cold or warm.
Also I can turn it easyly with one rear wheel stopped with the other hand. No binding at all.

I reckon that it was very hot yesterday and today to : 30 to 31 °C

Yes the oil is Hypoid 80W90 API GL4.
Since there is no bronze in the axle, Maybe I could swicth to API GL5.

Another test today and top speed is 82 Mph. What a car !


Laurent.
LC Laurent31

And last result today after a 500 Miles trip on small country roads, mix flat and mountain. max speed limit : 50 mph.
Fuel consumption is 7.8 l/100 km. 36.2 mpg. Plugs are light brown.
Great improvement, before, with the 5.125 it was around 10 l/100 km.
But other things were modified (ignition, needles, jets)

Anyway, I'm happy with this fuel consumption and I think that most of this improvment comes from the 4.3 rear end.

Laurent.
LC Laurent31

Here is TORQUE!Huffaker Engineering Dyno of a normally aspirated XPEG with my street performance roller cam and a Laystall head, otherwise a completely STOCK engine! A ported OEM head will flow almost as good as a Laystall head.


Len Fanelli

And another one, XPAG 120" overbore Tom Lange S/C Manley Ford stroker crank, Peter edney extractor manifold, OEM cast Iron cylinder head ported & my street performance S/C Roller camshaft kit.102 pound / foot torque at 2600 and 99 at 5400 RPM.


Len Fanelli

This thread was discussed between 16/03/2017 and 28/09/2018

MG TD TF 1500 index

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