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MG TD TF 1500 - 3.9 Gears behind a 1500 engine

I know I'm not the sharpest tack in the box, but comparing the 55TF with a 1980 MGB in horse power, torque and overall weight, the 55 TF with 3.9 gears should perform very well. At 1900 lbs, the TF with a rating of 63 bhp, according to my figures,=.033 hp per pound. The 1980 MGB at 2070lbs with a rating of 67 bhp =.032 hp per pound. Aside from a non syncro first gear, why would the 1500 TF not perform as well as a 1980 MGB? Forget the aerodynamic comparison. Some 80s even pull an OD. PJ
P. Jennings

Because they are blowing smoke with that 63 Hp figure Paul. It's not BHP but rather measured at the flywheel...big difference. Heres the graph of actual brake horese power on my freshly rebuilt 1250 and even with a blower it doesn't come close to 63 BHP.

The 69 BHP for the B is probably correct.


...of course the 1975 MGB with full emission control and single carb developed 62 horsepower.... (the beginning of the end)....

Judging by my 4.3 rear end, I think with the larger valves I could squeak by with a 4.1 .... and maybe to try it?
gblawson(gordon- TD27667)

My TF 1500 with stage 3 tuning tested 83 real HP on a dyno.
I have a 4.1 rear end and I have blown out a 1980 MGB in the mountains,
I am doing 75MPH at 4200 RPM where my HP is about 67
it is 75 HP at 5000 RPM and 83 at 6000 RPM

It has been driven over 90 MPH in spite of the aerodynamics,
but the front end gets a little floatey
Don Harmer

Lesson in terminology:
A <brake> is what you measure hp with, so any "hp" measurement is BHP. More info must be specified, as follows.
All measurements are done to a standard. In the US & UK (mostly), this means one of several SAE Standards.
These are normally measured at the flywheel, hence "Flywheel Bhp" = FwBhp, but usually just "Bhp".
Numbers taken on a "Rolling Road" or chassis dyno, are Bhp <at the driven wheels> = WBhp. These may be converted to FwBhp by various approximating formulae, but they are still guesses of varying accuracy. Typically, "Wheel Bhp = WBhp" is 15-20% less than FwBhp.

Charts such as LaVerne's drive me nuts. It should be specified whether it is WBhp or FwBhp, and if FwBhp is calculated, what the conversion factor is. I prefer to see the <measured> data in all cases, though it is nice to have the conversion too.

Following is a compendium of MGB data from various sources. WBhp figures are from Peter Burgess or derived therefrom.

Before 1971, engines were advertised as "SAE Gross". That is the maximum the engine produces with no parasitic losses, which includes engine driven accessories like generators, and things normally on the engine which cause losses, like exhaust systems, air filters, water pumps etc. They also seemed to use a "lying coefficient", judging by some numbers used in the old days!
Under this system, the best US MGB were 18/G-GB at 96-98BHP SAE Gross, WBhp = 64@5000. 68-71 are 92BHP, WBhp WBhp = 60 est.

After 1971, the standard is "SAE Net". That is the developed BHP with the engine in the configuration "as installed", which means all the stuff left off in the "Gross" standard are in place and operational. However, the exhaust will include the manifolds, but may not include the actual system on the car, and you can be certain the alternator is not working to charge a dead battery. The measurement is still taken at the flywheel.
Under this system, the US MGB were 72-74 at 79 BHP SAE Net, WBhp = 60 est. The difference between 68-71 and 72-74 is primarily a compression drop, which might account for 3-4 hp, so the measuring difference between the two standards is about 10 Bhp at this level.

US 75-77 drop further to 62 Bhp Net, a loss of 17Bhp, so WBhp = 43. WBhp is probably slightly better, as the losses in the driveline are less at these pitiful levels, but it is probably still well under 50Bhp. This drop is the result of the ZS carb, cat, and its horrid manifolding. For reasons I've never seen explained, 79-80 cars are said to have regained 4-5 Bhp, but it is still sad. And now you know why those of us who grew up with the early cars hate the piggish performance of the later ones!

Additional factor bearing on PJ's Q is that MGB weights went up a lot. From 2030/2310GT kerbside for early cars, to 2303/2401 by 74 to somewhere around 2500 for the 75-on cars (tourer). (Don't know where you got "1980 MGB at 2070lbs", but it ain't true!)

All in all, the TF 1500 should pull the 3.9 with no problem, especially if you don't bog it by being in the wrong gear/wrong rpm range. FI, I find that my Mazdas will pull a hill at 70 in 5th, with no speed drop, but if I try the same hill at 60 it will slow by the top, and starting at 55 I will have to drop to 4th midhill or sooner. Also, I used to drive a MG ZB(2500 lb approx)with a tired 1500, MGA 4.3, and seriously oversized tires, which came out about the same as a 3.9 would have; W PA mountains no problem.

FR Millmore

Put yer glasses on it's a pretty clear graph Fletcher

DJ= dyno Jet

Wh = wheel

Pw= Power

Tq= Torque

Stock rebuild of 32 hp, not the advertised 54 hp, says a 3.9 is gonna be a real slug even at lower elevations and flat land driving.

Bottom line is your never comparing apples to apples unless your doing the testing on the same machine in the same conditions.

Mine shows it's peaked at 48 hp.

Don says he's got he's got 83 hp.

Site unseen I'd say it's a real strong bet that my blown 1250 will walk away from Don's Stage 3 1500.

Stock 4.875 with the Datsun 5 speed

At 12000 ft pulling severe mountain grades it's down to 3 gear. If she had a 3.9 I'm sure she would be down to second and you can ask those that have ridden in her... she pulls real strong. And for those that give a crap I've had her over a true GPS observed 100 mph...Not for the faint of heart. I've put about 10000 miles on her this year and run her all day long at 70 to 75 mph.

4.3 I'd like one. 4.1 maybe. 3.9 No way in hell
for me in the mountains.

The mid 70's RB B's were a real performance pig and I'm sure my TF will out perfom one. On the other hand my slightly modfied 69 B will leave the TF. Your call I suppose but my 2 cents says bad choice.


The TF 1250 Scott and I are restoring has a supercharger and a 4.3 rear end. It measured similar BHP curve on the same dyno as my TF 1500 but the 1250 is not so radically tuned ( the 1500 has a 9.5 to 1 compression ratio, etc,(
Both were measured at the flywheel with fan and generator on.
I agree with you on the 3.9, gearing down too much leaves you running at lower RPM AND at lower HP due to the curve at any speed

By the way, Scott has had my TF 1500 at much higher RPM; (and speed) than I have the courage to do, both on the street and on the track at Seabring for a brief time. We will see how our supercharged 1250 compares when it is finished.

Maybe we can schedule a race as we have for Bob, But you won that!
Don Harmer

Yep! headlamps need adjustment.
Funny, I looked at the top and the bottom, where I expect such info, and the peaks, but evidently not in the low end middle.

Not fair comparing Colorado Mts to Pa ones, or Iowa ones!
Altitude alone causes you a one gear drop or better, from what I've experienced there in various cars.
Were those figures as at Standard Conditions? 32 bhp from an advertised 54 indicates more than usual overstatement by MG, or bad tuning; of course, if that is at Standard Conditions, then you only have 25 or something available at altitude, which doesn't bear thinking about.

Your 4.875 x an approximate .8 for the Datsun 5th is - guess what = 3.9.

Must say that I don't mind downshifting, so if the car can pull my desired cruise on a levelish road with less than a 30 mph headwind I'm happy, but for people who expect to put it in 4th at 25mph and leave it there, a 3.9 wouldn't do.

Still think PJ would be OK with a 1500 in corn country.

FR Millmore

We have an MGB 3.9 rear in our supercharged 1250 TD. Cruising, great, like only 3500rpm @ 70mph. (that is 70mph on GPS and the tach is really close to digital readout around 3500 range). Our Magnacharger is huffing around 4 psi, never up to 5psi (but that is on a cheap boost gauge).

Performance, not bad. But, like he said, "I agree with you on the 3.9, gearing down too much leaves you running at lower RPM AND at lower HP due to the curve at any speed."

I am posting another plot of horsepowers for XPAG- normally aspirated/6 psi/10psi boost.

Fortunately for us, the "correct" engine going back in someday, is already punched out to TF bore, so we have something to look forward to. AND, in Michigan, we don't have LaVerne's Colorado moguls.

Match race at Michgan International Speedway next year, anyone? The catch, they order us to stay under 70mph!

Jim Northrup

This is getting very interesting! (Grin).
P. Jennings

This thread was discussed between 29/10/2011 and 30/10/2011

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