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MG TD TF 1500 - HP and other improvements to the MG

I'd like to start a general thread on possible performance improvements to the MGTD.

Two areas, engine and gearing.

I've heard reference to changing the differential and different ratios (4.3) can someone either explain the advantages here or point me to any good web sites covering this issue?

And in regard to engine, I've read about head changes to harden the engine; but also there are other performance improvements that can be made to the head (pocket porting, coatings)... I'd be interested to hear what others have done here.

Geoff Baker

Geoff Beyond what you get when you overbore the engine to correct for wear and taper of the cyl. bores, there is quite a lot to be gained from raising the compression ratio (with todays fuels 9.3 is a good target). Then there is the size of the valves Mark II or TF valves are good here(but you must unshroud them). Then the inlet ports can stand quite a bit of work(professional shops call this flow testing). Also better exhaust manifolding is another way to get some extra urge when you put the pedal-to-the-metal.

Then there is the camshaft, since Crane Cams is out of business that doesn't leave a lot of choices and most of them are in the UK. But Len Fanelli has provided a kit to install a roller tappet cam with several grinds available, he shows up on this BBS from time to time.
All of these things were discussed in Hot Rod magazine back in the late 1940's and since our engine were designed back in the 1930's all that stuff works just dandy.

I'll get down off my soapbox now.
R. K. Jeffers

On my website in the technical section is a PDF of WKF Wood's tuning book on the XPAG. It is a good place to start when trying to understand the potential of the TD's engine.

I think it is all about money eventually... the bottom end work is pretty easy, it is the head work that can run up the dollars. That said it may be a bit like putting silk ears on a pig or whatever that darned expression is. The truth of the matter is the TD is like flying a Piper Cub. No amount of horsepower is going to make that airplane go faster. As the drag increases with the square of the velocity, you simply will run out of power before you overcome the drag.

There is something to be said about supercharging if you have the bottom end to handle it.

The really cool thing about the XPAG is the very long rod and small throw allows a much higher piston speed then generally accepted because the crank angle is much reduced over a squarer engine. In other words, the engine is quite happy at 5000 rpm, all day.

I might mildly disagree with Bob (I can't believe I just wrote that) on the CR that is reasonable to foist on an XPAG. I find the engine very tractable at about 8.5:1 and would be a bit nervous about running it up to 9.3:1, mostly because I like running on any fuel I can easily buy (87 Octane), and I can see the day coming when high octane fuel is NLA.

With regard to the gearbox, a lot of people are choosing to put in a fully synchronized five speed overdrive box, in part to overcome the slow synchronization of the original box (let's face it, the TC box was better) and partly to mitigate the unusually low gear ratio standard in most of our cars (5.125:1). The factory gave a higher ratio to the MkII or TD/C models, and the TF came with a higher ratio as well (4.875:1). Additionally there was a 4.55:1 ratio available on request.

For my money, the original gearbox is a lot of fun to master, although I wish I could have gotten first gear quieter on my rebuild. If you throw the MGA gearset of 4.3:1 in the rear axle you find first gear is much more usable (my acceleration speed actually increased), and the cruising range of 4700-5000 RPM will run you close to 80 mph even with the squish of radial tires. That puts you on par with almost anything on the road, and that's where you run the drag up to such an extent that the ride actually becomes annoying with wind buffeting, etc. My slightly bored out engine and slightly higer CR seems to deal with hills and what-not just fine at the higher rear ratio.

Certainly the 1970 MGB with the 1800 engine and the fully synchromesed six speed (four plus two overdrive) is effortless on a windy hilly road compared to the TD. But it is like comparing scout knives to machetes. I guess I want to drive an old MG, for the looks, the experience, the engine and the gearbox. When I want something different, I drive the B. I guess I compromised a bit when I put the 4.3:1 in, I know it changed the charactoristics of the car. I rationalize it by telling myself that if the British sport motoring public hadn't been so willy-nilly into hillclimbs, they would have put the readily available ratio in as an option from the start, and not wait for a bunch of Americans to rectify the shortcoming 50 years later. But I don't think the factory would have put the Austin engine in their TD, nor would they have slammed in a Japanese gearbox... so I don't either.

I am going to fit a front anti-sway bar in the spring.

Hope this helps,

Dave Braun

Bob, I have never heard the expression "unshroud" the valves. Could you enlighten me? Thanks. Larry
Larry Brown

The XPAG/XPEG engine is one of the most "tunable" engine in existence.
The Factory managed to get up to over 200 HP by extreme measures for the engine used in the speed trials on the Bonnevulle Salt Flats in the 1950's. (EX 179 had over 95 HP at 6000 RPM for the endurance euns)
See John Thornley's Book"Maintaining the Breed".

The Factory "Special Tuning Manual" shows some of the things that can be done to increase HP. ( Also included in Thornley's book
(Changing the rear end to a 4.3 (or 4.1 also helps)

My TF 1500 is Stage 3 tuned (9.5 to 1 CR) and was measured on the dynomometer at 74 HP at 5000 RPM (82 HP at 6000 RPM projected) and has a 4.1 rear end.
Because of it's lower weight I can outrun any standard MGB in the mountains.
It has done something over 90 MPh, which is faster than you need to go in such a light car, but it cruses at 70 MPH on the expressways just fine.

I recently drove home from a GOF on a nearly empty expressway and found myself driving at 75 MPH without realizing it ( I dropped back to the 65 MPH Speed Limit_
Don Harmer


I'm running +30 thou pistons [gives 1280cc, I think] a Judson supercharger, MGA 4.88 rear end with a Ford Sierra 5-speed with an effective final drive ratio of 4.10 in 5th. The head's been flowed, we're running oversize valves, and I've got a 4-into-2-into-1 hand-built ceramic-coated header Running a set of tall 185/16 tires should put my revs at about 3400 @ 70 mph, I think. I've got a set of cycle fenders on the front end for less drag and a pair of small Brooklands screens for when the weather is good and I can fold the windshield flat. I've installed an electric fan so there's no drag on the crankshaft [just from the supercharger, which is I know is a lot] and a pertronix electronic ignition for what I hope will be a more consistent spark. MGB swaybar and higher-rate [550 lb] MGB springs on the front for more neutral handling, tube shocks at the back for longer life and tuneable jounce/rebound. The alternator isn't a performance part, but it will help to run all the extra electrical stuff like quartz headlights and big driving lights because I want to see and be seen. Inertia-reel 3-point shoulder belts for [a little bit of] safety and 12-inch hand-machined alfin drums by Bob Grunau for as much stopping power as I could get short of some MGB discs.

I know I won't be able to outrun my daughter's 1.7 Acura EL/Honda Civic, but I hope I'll at least be able to keep her in sight. I'm pretty confident I'll be able to race Smart Cars for pink slips, though. We've got long, long stretches of roads in Western Canada, and my goal for the TD is to have a car that'll cruise at 80 mph all day without overheating or getting cranky.

But it's been a five-year project, and I want to drive it now rather than build it - don't take on a hot-rodding project unless you're really patient.


Dave Jorgensen

Don, I think there is a picture in the Thornley book of the mechanics doing a dyno run of the 200HP motor standing shielded behind a concrete wall! George
George Butz

There are many books and websites that list the 'Stage Tuning' methods... think its been done so many times and for so long that they cover just about everything....

Larry Thats an expression I picked up from some book I read about the XPAG engine. The problem with the TF valves is that they become too close to the chamber walls in the 1250 head. Hence they are shrouded. To unshroud them the walls of the compression chamber are moved a bit away from the valves. I think if you have the larger valves flow tested the people doing it will see the problem and recommend a little reshaping of the chamber walls.

Sorry to have used a word that caused confusion.
R. K. Jeffers

The amount is very, very small, but does put a tiny 'indent' in the wall of the combustion chamber...


On page 148 of Thornley's book he described:

"The XPEG opened out still further to 1517 cc in 1953.... gave 219 HP at 6900 RPM, And to crown it all....., the addition of 15 percent Nitro Methane was tried and 239 HP appeared. But this frightened even the case hardened hibitues of the test bed and the experiment was not pursued."

As I understand it, it was thought that if the engine broke at full power it would have killed even those behind the barricade!

Don H.
Don Harmer

Well, I'm not nearly as ambitious as you guys :)

Sounds like the engine is capable of quite a lot!

I'm thinking of just starting with the head so I'll start reading up on it...

Thanks for the pointers!
Geoff Baker

To follow up on Bob's discussion of *unshrouding* the valves see:
Gene Gillam

If you ever have the opportunity to examine the combustion chamber of a Laystall head, you will see an example of "unshrouded" valves. Only the wall of the chamber is indented around the valves. The shape of the chamber at the head surface is not changed. As Gordon L. has commented, the amount of material removed is very small, to improve the flow.

George Raham
TD 4224
G. L. Raham

This thread was discussed between 25/11/2009 and 26/11/2009

MG TD TF 1500 index

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