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MG TD TF 1500 - Safe engine revs?

I wonder if someone can tell me what is the limit of the engine revs in a standard 1250 XPAG engine before damage is caused?
And I wonder if there is a simple explanation of just what that damage is and why?
Don Jackson says in his book 'Your SU Companion' that the valve timing gives max torque at 3000rpm, but that damage is caused at 4000rpm because at those revs a piston speed of 2500 ft per minute is reached and all piston engines suffer a marked rise in piston and bore wear at this point.
A good running speed for my car appears to be between 3250 and 3500revs and I would like to think that I was safe at those speeds.
Any informed comments please?
Michael

I feel most comfortable with a 3700 cruizing speed...however, it has been mentioned that they will cruize at 4000, 4500 all day. Not sure if that is why they don't have redlines, or if that just wasn't done at the time?
gordon lawson

The “cruising speed” of an engine is a theoretical value, and IMHO unprovable. If anything, it might be a guideline for extracting a good mileage from an engine under ideal conditions.

It postulates that it’s the point at which potential and kinetic energies are in balance, and the point at which the stresses on the reciprocating masses are at their minimum (other than 0 RPM).

It also postulates that engines of the ‘boxer’ design, like the Dyna Panhard, Jowett, BMW bikes, Testa Rosa, Porsche etc, are in perfect balance because the reciprocating masses are equally distributed and must by design, be in balance.

How anybody arrived at the number 2,500 piston fpm has always been a mystery to me. I’ve tried to chase this down several times, and even the response of an old professor of mechanical engineering to this challenge, is not printable on this BB!

According to my calculations, the 90mm stroke of an XPAG motor, would take 3.38668 strokes to travel a foot. So 2,500 feet would be 8,466 strokes, or 8,466 strokes in a minute = 8,466 RPM. Yeah ... Right!! Even old EX179 putting out well over 200 HP at 7,000 RPM on dope and with 27½ # of boost, could not achieve its “theoretical cruising speed” on the Great Salt lake in 1951.

Also I believe that Don Jackson meant camshaft speed and not crank speed, in which case this would translate into 6,000 RPM which seems sensible.

Nope! Cruising speed to me, means what you feel comfortable with, or if the engine sounds like its stressed, or about to be.

Gord Clark
Rockburn, Qué.
Gordon A. Clark

The engine revs to be aware of and try to avoid is sustained rpm around 4700. There is a harmonic torsional vibration at this rpm which occurs at a multiple of the natural frequency of the crankshaft. This contributes to the cracking of many of the XPAG crankshafts. This should also be a concern of some that have changed their rear end gearing, if they now cruise at this rpm.
Dallas Congleton

Gord, your arithmetic is right on, but there's a slight problem --- those strokes have to go both ways before it's an engine revolution. Your calculation of 8466 strokes in a minute equates to 8466/2=4233 RPM. Not a bad value for a decent XPAG. In their 1951 TD road test Road & Track calculated 3,070 fpm at 5200 RPM. That comes out to 4234.5 RPM at 2500 fpm. About 61 mph in a stock TD.

Engine rev limit is not a simple number. The 2500 fpm value is probably a good guide for safe cruising, but it's way below valve clashing. I guess Michael would need to better define the type of limit. I've thought of pasting a US$ on my tachometer at the 4500 rpm mark.

Bud Krueger
Plymouth, Mass.
The Colonies
Bud Krueger

"rpm around 4700. There is a harmonic torsional vibration at this rpm which occurs at a multiple of the natural frequency of the crankshaft"

Have read an interesting article on mga twin cams and this harmonic was mentioned as the point where, when sustained, the twin cam engine let go... something about harmonics and fuel starvation from 'bouncing' fuel in the carbs?

Years ago I was taught to shift the MGA coupe I had at 3200 rpm and not let it go below 2000 That has always seemed like a comfortable range with some highway 3700 to 4000 bursts (well, the xpag doesn't exactly 'burst', but you know what i mean).
gordon lawson - TD 27667

While I am running 4:55 rear end in my car I run around 4200-4600 RPM consistantly. Others in our club with the original rear axle run 5000+ with no apparent problems. The Service manual for the MGTD says maximum BHP for the standard engine is 47 BHP at 6000 RPM which is also where the book says the valves crash. The book says that the "safe maximum RPM's is 5700. I have .060 oversize pistons and run 4200-4600 all the time all the time and have never had a problem with the engine. I feel very comfortable at highway speeds with those rev's and have even pushed it up to about 5200 or so. THat fast and you do it with lots of prayer. At 3000-3500 the engine is barely working. With the standard rear end you would be running about 55 MPH
Tom

2500 fps of piston speed is the theroetical point above excessive cylinder wear occurs. This is a figure that is tossed around a lot and interestingly it occurs at the same RPM in a MGB B series engines as it does for the XPAG engines. I am with Gordon, in that I don't have the slightest idea of where this figure was derived or what is "normal" wear vs "escessive" wear is. We run out TD at 4200 RPM all of the time and I got 80 some thousand miles on the engine before it needed a rebuild, which is not bad for an engine that is supposed to be in need of an overhaul at around 50,000 miles. Red line is normally at the point of maximum BHP but below the point where vlave crash or float occurs. There is another limiting factor with the XPAG engines as far as maximum safe RPM is concerned and that is the split little end of the con rods. Even with strengthend valve springs to raise the point of valve crash, the RPM of hte XPAG should not exceed around 6500 - 7000 RPM due to the weakness of the split little end design of the con rods. I have a graphic reminde in our garage of what happens when the gudgeon pin pinch bolt fails at speed, in the form of a block minus the bottom two inches of the right side crankcase (this was not the result of excessive RPM, but rather excessive torque on the pinch bolt). Cheers - Dave
David DuBois

Bud and others,

I'm getting too long in the tooth to do this any more, but I couldn't resiste the challenge.

You're right, of course.

GAC
Gordon A. Clark

Tom,

With the standard rear end you would be running about 14.42 mph per 1000 RPM. 55 mph requires 3814 rpm if you something close to the original tire size. 60 would require 4161 rpm. In a standard car you would go 75 mph at 5200 and with your rear axle ratio (16.24 mph per 1000 rpm) you would see 84.5 mph at 5200! I can see why you are praying!

The horsepower curve peaks at 54 hp between 5000 and 5200 rpm, and drops to 47 hp at 4000 and 6000. Of course, the drag on cars goes up by the square of the velocity so the drag is almost 2.33 times bigger at 85 than 55. It takes both rpms and hp to acquire speed, rpm to drive the gears and hp to overcome drag. With the .060 over and the slightly decked heads from machining the rough spots on our heads and blocks, you and I are probably in the range of a CR of 8 to 8.5, which will give us about 58 to 59 hp max if we didn't polish our ports.

Your car's engine is indeed in good shape since you can do 5200 rpm in fourth gear with the gears you are running. With my 4.3:1 (17.19 mph per 1000 rpm), I know that my speed at 5200 rpm will be about 89.4 mph, something the hp may not allow. In fact I hope I don't ever see that as much as I trust my mechanical skills, I don't think I want to trust them and the ancient engineering at that speed!

One of the things I'm planning shortly is to do a turns per mile calculation of my speedometer cable, and then have the proper calibration applied to my speedometer. I'm looking forward to running a GPS check of my speed and comparing it to the rpms and the speed shown on my instruments. Of course, I should probably finish my car first...

warmly,
dave
Dave Braun

Thank you for all your comments and advice, I am most grateful.
I am also amazed at how fast you guys in the US run your engines! I do a lot of miles a year in my TD, about 10,000, and have always thought that to take the revs above 4000 was very risky. In the light of your posts I’ll have to re-think that, (and drive a bit faster).

Michael
Michael

Watched the F1 Sunday. New V-8's still turning close to 20,000 RPM. Anyone care to figure the fpm for those babies?

LaVerne
LaVerne

With their V10's they had less then 2" of stroke... assume only a bit more for the V8's... You can get that much out of an engine not throwing heavy metal around and around.
gordon lawson - TD 27667

This thread was discussed between 12/03/2006 and 13/03/2006

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