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MG TD TF 1500 - Safe RPM

Just a question, what does everyone think is a safe constant engine speed (RPM) for a TD on the open road? I can't seem to bring myself cruising more than 4,000 rpm's.

Bill Brown

I cruise at 3200 or 3700 ( I have a shimmy between) and feel comfortable keeping it there all day.... I know many drive 4500 or even 5000 , but I keep thinking of the money i don't have....
gblawson(gordon- TD27667)

If you have the stock 5.125 ratio rearend, you will probably be doing 4000 rpms at around 56 mph. 60 is going to be about 4300. Several posters have, through the years, said that this is okay with them. Check out the archives and you will see what I mean. The XPAG is a very free revving engine and seems to have no trouble maintaining these speeds and I run mine out to 4500 often before shifts. I was not comfortable, however, with the constant noise and busyness of the motor. I found myself either avoiding highways or sitting in the right lane at 52 mph, being contantly passed. Carl Cederstrand, who wrote the book (literally) about the ring and pinion interchanges, argues that these speeds really shorten the life of the bottom end of the engine.
My feeling is that you won't know how badly this is abusing (or not) your engine until you have it apart. If your engine is old, who knows what the clearances are presently.
If you intend to take long trips with your TD, I would recommend thinking about the 4.3 conversion. It will save many miles of piston travel and wear in the bearings and will increase your enjoyment and give you peace of mind. Since doing mine, about 10 years ago, I have found that I can keep up with traffic doing 60 mph, while my engine turns a nice relaxed and quiet 3500 rpms.
Many folks on this board have done the 5 speed conversion, which can cost 3 times as much and a few have done both! I think that the rear conversion is the best deal for a couple of reasons...first, you now have a 1st gear that you can actually use, which is a good part of the bonus that the 5 speed gives you, and second, it looks totally origional-only a practiced eye, using a lift, can spot it.
Steven Tobias

I've had my TD since 1964 and remember once of going down a long hill and getting her up to 5,500 and took out a rod bearing. After that repair, I settled in a a max of 4,500 rpms. Also my oil pressure does a funny thing. Hot at around 3,200 rpms, it will register around 45psi drop to a around 38psi at 3,500 rpms and back up to 45psi at about 3,900 rpms. Engine has a fresh rebuild but it has always had this nature. The one thing that was not rebuilt were the rocker arm bushings and I suspect that.

Bill Brown

The XPAG/XPEG engines have a VERY long stroke. The pistons have a lot if ground to cover in every revolution, so the piston speed is very fast. Also remember the pistons have to stop, and reverse direction, twice every revolution. A well machined, well built ballanced engine with good lower end hardware will fair better than others, but physics can only be pushed so far. A lot of these engines have grenaded after being pushed too far. Spun bearings, broken rods and broken crankshafts will turn a valuable engine to scrap metal very quickly.
Usuaully an engine with a spun bearing can be repaired, however a broken rod or crank can quickly ruin the block and sump.
D. Sander

4.3 rear on a XPEG w/oversized tyres.
I'm very comfortable at around 3500/3700 RPM going about 65MPH.
The way people around here drive I find the need to merge unto the freeway winding out to at least 4500 RPM to avoid being road-kill.
Once I find a comfortable "slot" I can cruise at a more sane speed. (65/70 MPH)
I am still passed by most SUV's in Ohio as the normal speed "for them" seems to be somewhere between 80/90...unless the phone rings or they see a cop. Then it drops down to about 40MPH!
When I see one behind me texting, eating a sandwich,reading a book, and talking on the phone whilst re-programming the GPS and watching TV I normally seek the nearest exit.
Safety Fast,
David Sheward

Remembering the 1950's, it was understood that after 20K - 30K miles you had to have your engine rebuilt. Now with the changed rear ends and the 5 speed trannies that is no longer true.

I bought my first TF1500 with 45K miles on the odometer, it ran poorly until one of the valves stuck open and had an argument with the piston. Rebuilt the engine and had a very go'in machine. Just wish I still had that car.

Now with a 3.9 rear end, my present TF1500 sails along at highway speeds without working up a sweat and is a real joy to drive in todays traffic. Lot's of thumbs-up from cars passing (going well over the posted speed limit).
Bob Jeffers

While it is true that the XPAG has a long stroke (compared to the bore) the length of the stroke has little impact on the safe engine speed without considering other factors.

Piston speed is a function of RPMs and is measured in feet per minute. Every piston in every engine has to reverse course four times per cycle, or twice per crankshaft revolution. The reversing is done over many degrees of crank travel compared to the linear motion during the transit time between reverses (i.e. the piston does not suddenly stop and reverse course, rather the conversion between the rotational motion of the crank and the translational motion of the piston provides a gradual slowing down and speeding up of the piston).

Finally, while the stroke is long (and is set by the crankshaft) the rods are even longer. This means that the angle experienced by the rod at each reversal is considerably less than the angle experienced by a more modern engine where the stroke to rod length ratio is closer to unity.

Modern engines are typically figured to have a safe piston speed of 2500 feet per minute. The XPAG, because of the relatively long rod as compared to the stroke, has a safe piston speed of about 3500 feet per minute. With a stroke of 3.5433 inches, each piston is traveling 7.0866 inches per revolution of the crankshaft, or 0.59055 feet. At 2500 fpm for a safe piston speed, you can set 4233 RPM as a safe limit. At 3500 fpm for a safe piston speed, you can set 5927 RPM at which point you are well into valve float and other issues. But the piston and crank are fine.

That said, there is also a resonance at about 4400 RPM that should be avoided, and yes, the harder you drive your car the more wear you will place on the crank and rod bearings and journals, but it isn't in the catastrophic category.

I will also note that I burn more oil cruising at 4600 RPM for sustanied periods (a quart every 1000 miles) than I will at 4200 RPM (a quart every 3000 miles). For the record with my 4.3:1 rear end, and the tires and inflations I run, 4200 RPM is 68 mph and 4600 RPM is about 75 mph. I consider 4200 RPM (corrected) my normal sustained cruising speed.

I hope this helps,
Dave Braun

A new higher speed rear end is definitely in my future plans. Hate going down the road holding up traffic.

Bill Brown

My Choice is much like Mr. Tobias. I tend to go on the freeway only when I have to.
I still run a standard 5:11 gear. The car is geared fine for most of my driving. Engine is ballanced and only 20 over.
Head is milled: Crank is 60 years old.
Carries arround 50+ pounds of oil presure when hot. Tach is so far off that I never look at it. It will run the hills in top gear.

Safety Fast


T. L. Manion Thomas

Also have the crank magna-fluxed, the rods resized, & magan-fluxed,new rod bolts, pistons, flywheel & all of the above balanced. 3,600 RPM all day long @ 70 MPH W/ a 4.30 gearset. The magnetic tach / speedo are not all that accurate, use a dvom / & gps to determine true RPM & MPH. Len (what year is your car, which instruments does it have?)
Len Fanelli

With my 4.3 rear end and tires I get about 17 mph/krpm.
4,000 is a very comfortable 68 mph on the freeways. Lazarus has a thing of his own where he creeps up to about 4200 and wants to stay there, ~70 mph.

Len, you must either have a 5-speed or some monster tires to get 70 @ 3600. (~19/1000) Bud
Bud Krueger (TD10855)

With a 4:1 as in my car. 4464 RPM equals 80 MPH with a 4:3 you are doing about 4600 rpms. Both are well within safe limits for the XPAG . At 70 MPH with a 4:55 you are only doing 4300 and that is very safe and really do not stretch the engine. I run usually about 3800 to 4200 all the time. I have the harmonic problem at about 3200-3500 RPM so I usually run over that or under that. If anyone wants the ratios, e mail me and I will send the data to you via e mail,
t maine51 at yahoo dot com
Tom Maine (TD8105)

The "harmonic problem", someone please explain? What am I looking for?

Bill Brown

The harmonic problem is a dreadful noise/vibration. It is as if the whole car turns into a tuning fork that just got hit hard. Some cars hit it at a particular speed/RPM. In the TC It is about 54 MPH. I don't really remember hitting it in the TD or the TF.
D. Sander

Thanks, I don't ever remember experiencing that.

Bill Brown

Oops, yes Bud,4,000 @ 70.
Len Fanelli

Bill, the "harmonic problem" referred to is not the entire car, but the resonance of the crankshaft itself. At this rpm the vibration in the shaft is resonant and the vibration leads to cracking and failure of the crankshaft. You can not detect this yourself over the vibration of the engine, just be aware and try to avoid this speed range.

Dallas Congleton

What rpm should I avoid? Never realized this before, just fat dumb and happy I guess.

Bill Brown

Happy is good! Not really a "set" RPM as much as a "feel" ...some cars have it , some don't.
If it is a problem you will notice noise / vibration at a certain speed (either engine or road).
I have had several cars / motorcycles that have had this. Hard to descripe but you will "feel" it if it is a problem.
I had a 68 Firebird that ran like a scalled ape except @ 2300 RPM was like that foe 280K miles!
David Sheward

Thanks David, that makes sense. I've never felt anything as long as I've had my TD (45yrs). The only thing I"ve noticed is around 3,700 rpms, the oil pressure drops around 7 psi and goes right back up if the rpms either increase or decrease. Could that be what I'm seeing? It has always done that.

Bill Brown

With an MGA 4.56 rear end, a Ford Sierra Type Nine 5-speed, the Judson blower, and 185/16 Michelin X's, I'm getting exactly 3500 rpm at a gps-trued 70 mph [the speedo is out by exactly 10% all the way up the scale.] 80 miles per hour is comfortable with the windshield down, the brooklands screens on, a set of earplugs, and wrap-around sunglasses.

But Gord's right, about the loot it takes to get there.


Dave Jorgensen

In 2005, I built my last XPAG (too old to fuss any more). I'm a firm believer that balance is the key to reliability.

I first took all rods and had them crack-tested and checked for equal length, and no twist. I then polished each rod, 'till I could see my face on the sides and the small end. I then balanced the bare pistons, hand-grinding judiciously.

I then personally balanced the rod/piston sets using two pan balances, including playing the shell game with the rings, gudgeon pins, bushings, circlip, bolts and bearing shells. I then had the rods shot peened.

I zero-ed all assemblies to within .1 gram

I had the rotating mass professionally balanced including the crank, front pulley, flywheel and the assembled clutch, but not the friction plate.

This engine is a GREAT engine - unbelievably smooth, and I feel quite comfortable keeping up with the MGBs on our club runs. My run to Gatlinburg in 2006, was proof - high temps, high revs! I am comfortable with 5,000 but try to keep to about 4,000. CWP = 4.3. Compression is still in the 150 range.

My only worry is oil getting to all the right places.

I have been considering dropping the sump to see if the bearings are OK. But I now find it impossible to locate a set of std rod bearings in case I have to replace them. So I guess I'll go to my grave never knowing!

Gord Clark
Rockburn, Qué.
Gordon A Clark

Dave J,
I like your car more every time I see it!
Wife does too ...she refers to it as "That TD with "attitude".
Big Tyre David
David Sheward

Was asking myself this question this weekend. My TD has a 4.5 rear end and I get 55 at 3600 and 60 at about 4000.

I try to stay between those two. Engine seems to run nice there.
Bruce Cunha

I must be the most conservative owner on the board as I run 3000 RPM at 50 MPH using MGA 4.55 gears. Oil pressure stays at 55 lbs water temperature between 70 and 80C and have thankfully not had to do any internal engine work in the 34 years of my ownership.

John Quilter

Thoralf. Norway TD 4490 3500RPM (top)
Thoralf Sorensen (TD4490)

Power is rated at 5500 for XPAG, and 5000 for XPEG. You guys are making racehorses live at a slow hobble walk, shame!

Only thing to worry about is the crank harmonic, which you can pass through safely, just don't keep it there. I do not have a TD, but if I did I would find out exactly where the harmonic is and avoid it. (This is why more advanced designs have "harmonic balancers", which are really dampers, and they are tuned to the precise resonance you are trying to avoid.) I think Dave is right that it is near 4400, and the band is very narrow - like +/- 50 rpm - and will not be felt in the car. Any other harmonic is due to some other balance or mounting error, and is not nearly as serious to engine life.

FR Millmore

I agree with Gordon - a well balanced engine is key to allowing higher revs. We use an MG TB for classic trials in the UK - we set the rev limiter at 7000 rpm, but if we feel it needs a bit more for some of the hills we will increase it a bit. A well balanced XPAG will sustain quite high revs, so don,t worry too much. Dave
DM Gibson

The shake I was refering to is the "scuttle shake" that occurs in my car about 3300 rpm. Below and above that there is no cowl shake. Have had crank,clutch, driveshaft and wheels balanced and it does not go away. A lot of others have the same problem. I know that prior to putting the scuttle bar under the dash the early TD's had a problem like this also.
Tom Maine (TD8105)

Tom, I have the same at about 57. steering wheel does not shake, but scuttle and dash do. My tires are due for replacement and I am hoping that will fix it. It runs smooth at all other speeds.
Bruce Cunha

Scuttle shake is common in early Healeys and is generally set off by wheel imbalance, not always tire imbalance, but also brake drum imbalance, or a combination. The older style balancing machines that spun the entire wheel on the car could combat this, but they are also a thing of the past.

Hendrix Wire Wheel in Greensboro,NC has made a good reputation and business balancing the brake drums separately as well as the tires, and truing the roundness of the tires. Owners report very satisfactory results.

Np financial interest, etc.
Dallas Congleton

She seems to be a happy girl

Wow LaVerne, that sounds sweet!!

Happy motoring,
Bill Brown

Bruce I am running older tires, but have been balanced and it is still there. Going to go another year before new tires but even balancing does not seem to take away that shake. like yours, below and above the small range the shake is not there.
Tom Maine (TD8105)

Hey LaVerne, Looks and sounds great. I never would have guessed 4400 rpms by the pitch alone. What is your final drive ratio...4.1? Is the car a 1500? Stock? I've had my TD out to 80 with it's 4.3, but I felt very vulnerable! Maybe now that it has fresh tires, I'll give it another try.
I always got a bit of a "scuttle shake" at around 52 mph and new tires, straightened wheels, and balancing did not make it go away. I did, however, replace the windshield glass last year and now that the glass is actually fastened into the frame and everything is tight, I don't notice the shake nearly as much.
Steven Tobias

Dave gibson contact me, if you want to win more trials!
Len Fanelli
Abingdon Performance
XPAG Roller Camshafts Kits
Period piece correct as done by Chet Herbert in the day.
Len Fanelli

1250 with a new high dollar crank. Key is to balance everything. Blower is also a big help. Smooth right up to 5500. stock 4.875 rear behind a Datsun 5 speed. Normally not in this big of hurry but it was a long way from home to the east coast and then back to Reno. She will do over 100 mph if anybody was wondering.


That does sound great. What crankshaft and cam are you using?


Jim Barry
J Barry

As Dallas commented about Hendrix Wire Wheel, I had them do my wheels and new tires on the B and the improvement was amazing! He tuned the new wheels down to 20 thousands rim wobble and perfect roundness, spun shaved the tires to true them up. You would not believe the difference in ride and handling. Their getting the TF wheels and tires when I'm ready. PJ
Paul J

LaVerne, Bud, Tom, Len, et al, you guys make the rest of us drool and dream!! Safety Fast though!
Randy Biallas

Hi Tom,
And you guys with the scuttle shake.
I used to have it in the TD, took forever to find it. New tires, balanced and re-balanced tires, changed rims; pulled trans. put in new bearings, new clutch assembly etc. Still had it, then one day I happened to be talking with this old school mechanic and he said have you had your drive shaft balanced.

Well spent $85.00

Ed Stanfield
Columbia SC
ECS Stanfield

Been there, done that clutch, pp driveline crank wheels and tires, nada it still is there.
Tom Maine (TD8105)

This thread was discussed between 05/08/2011 and 13/08/2011

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