MG-Cars.info

Welcome to our Site for MG, Triumph and Austin-Healey Car Information.

Parts

MG parts spares and accessories are available for MG T Series (TA, MG TB, MG TC, MG TD, MG TF), Magnette, MGA, Twin cam, MGB, MGBGT, MGC, MGC GT, MG Midget, Sprite and other MG models from British car spares company LBCarCo.

MG TD TF 1500 - Cruising RPMs

What do you consider a safe cruising rpm for our engines without straining it? Not racing, just cruising. PJ
Paul161

Paul, I seem to remember from decades ago (not reliable) that there is a certain critical piston speed that equates to about 4300 RPM. Above that, when the piston changes direction from top to bottom, there is a force applied to the piston that in time, causes excess wear or fatigue, and I assume that relates to failure. I dunno but maybe the experience and knowledge of our group will chime in and set the record straight. I have used that number as a reference point for all my TD driving from start to cruise.
Jim Merz

Paul,

This is something I've been concerned about as well. I went to the archives under "safe rpm" and found a very good right up. You may want to check it out.

Dan
Dan Nordstrom

Jim, I my memory serves me wel : the critical RPM where the engine develops a harmonic unbalance ( standard engine ) is indeed around 4300 rpm,

Gerard
Gerard Hengeveld

I have been using a corrected (true) cruise RPM of from 3,000- 4,000 RPM and up to 5,000 rpm to get away from trucks on the interstate ( Later crankshaft ) OEM otherwise except for a roller camshaft. 4.3 axle ratio OEM gearbox, 85 MPH when needed, for as above to get away from trucks.
These are sports cars with peak HP @ above 5,000RPM.
Len Fanelli

I find 3800 very comfortable, up to 4500 ok, above that will cause accelerated wear. I'm typically around 42-4300 on the highway at 60-65 MPH.
Steve Simmons

Oooops, make that "write up". Beersunday.
Dan Nordstrom

According to the Blowers Manual, the specification you are looking for is the RPM at which the pistons are traveling at 2500 RPM the engine is not being stressed. For a TF with a standard rear axle ratio of 4.875:1, the RPN for 2500 FPM of piston speed that wikd be 4230 RPM. Cheers - Dave
DW DuBois

Dave what is the max non stressed RPM with a 4.3? Cheers
Peter
P Hehir

Thanks fellas, that is what I have been looking for, the critical rpm. At 4000 rpm my engine runs as smooth as silk and with the 4.1 rear 72 mph is fast enough for me in this car. Most of the country roads around here have a 65 mph limit, the turnpike is 75. Most of my driving is around 60 mph. PJ
Paul161

Peter - The RPM remains the same regardless of the rear axle ratio. I probably didn't word my explanation very well. The specification in the Blowers manual is "Road speed at 2500 ft. per minute piston speed - 64.3 MPH" I calculated the RPM based on (what I believe is) the standard rear axle ratio of 4.875 for a TF. I will let you do the math for the MPH with a 4.3 rear axle.

By the way, the TF specification in the Blowers Manual is for the early TFs with the XPAG engines. That said, it is probably the same for the XPEG engines, I believe that the stroke is the same for both engines (although I won't swear to it). Cheers - Dave
DW DuBois

My understanding is that there is a significant resonance frequency for the crank, piston and three bearing set-up around 4,300 rpm.

Since iron and steel have stress fatigue limit, this can be tested by long periods of running at these rpm and cause the crank to crack.

Best to avoid when you can.
Ian Bowers

Ian - I have heard of that caution, but I have been running our TD at 4300 just about any time we take it out, especially when going long distances since 1974, when we purchased the car and have not had such problem with the crank. That said, ours is a later car with the stronger crank, so that may be the difference. Cheers - Dave
DW DuBois

The following seems relevant to this discussion & was written by Carl Cederstrand some years ago. It seems just a few hundred revs makes one hell of a difference in terms of engine wear.

"I will categorically state that operation of any T series engine at a steady 4500 rpm (this is 64.8 mph with the original 5.125/1 rear end) amounts to little more than an exercise in engine destruction.

At about 10,000 miles a newly installed set of rod bearings will have developed fatigue cracks, a newly installed timing chain will have become noticeably slack, and the seating of new exhaust valves will have generally deteriorated to the point where a gentle "pist" is audible from the exhaust pipe. I have seen this occur over 20 years without exception to every T series that has crossed the southwestern United States at normal slow lane highway speeds with its engine flogging 5.125/1 rear end. When they were available, a change to the alternate 4.555/1 ring and pinion made it just possible to drive a TD or TF across country without also destroying the engine. You were still a slow car in the slow lane but at least you didn't feel you ought to ship an engine ahead just to drive across country and back. This ratio provides 65 mph at 4000 rpm and is the axle ratio have run for the past 15 years in my TD. It is also that ratio that gives the TD and TF the same overall gearing as was supplied with the TC. My TD has run on freeways and back roads from Canada to Florida and crossed the United States many times at a steady 4000 rpm. With this gearing, fatigue cracks generally appear in the rod bearings at about 30,000 miles, while the main bearings (center main first) begin to crack at about 45,000 miles -- a considerable improvement."

Cheers
Peter TD 5801
P Hehir

Paul
I had my TC engine rebuilt in 1992 (1380cc, 9.1:1CR, TF1500 Cam, fully balanced etc) by Crankshaft Rebuilders in Melbourne. Ian Shugg (proprietor) told me that I could "run all day at 5000RPM with bursts to 5500RPM". I have indeed followed that advice on long runs but now sit on around 4500RPM as I've fitted a 4.625:1 CW&P from Roger Furneaux. The crank is factory.
I would suggest that other Aussie T Series owners would do likewise (Bob Schapel?).
Peter (TC9356)
Peter Malkin

With out madification (which is I believe where this thread started) I was told by the infamous T TYPE Master Dave Raymond, that these motors will happily cruise all day at 3800.


Steve
Steve Wincze

Madification :-) I might fit that bill. I'll run at 4500 all day long. I can say that a stock engine does have some bad vibes slightly above 4000 rpm and yes if yours is one of those then stay away from it. If you did a proper job on your rebuild and had everything balanced, you won't have that vibration.


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

This thread was discussed between 26/06/2016 and 28/06/2016

MG TD TF 1500 index

This thread is from the archives. Join the live MG TD TF 1500 BBS now