Low r.p.m. urban myth

[StewartD]

Jordan,

Thanks for the compliment.

I disagree when you say 'that despite all common sense about low revs and high load being bad for an engine'. I don't think this is common sense at all; I agree that real data is elusive to support that view though.

However, there is sound engineering theory, if there is no hard data to support what I still think is a myth, (with exception to a motor that is suffering detonation), then this is what we must turn to. Engineering has been firmly underpinned by mathematics and physics since Brunel's groundbreaking Broad gauge railway (1837 ?) and the S.S. Great Eastern, (launched 1858, it was exactly twice the length of the Himalaya, the largest merchant ship at the time). There is not much mystery to a simple big end bearing.

Tell me how that engine burst to life, I'd love to get rid of that pesky Mach 1 kickstart!

Harvey,

Sorry for the error. The 350 Sebring's Torque curve has the plot at 2500rpm wrong. I divided 2 hp by 3591, (the Mk 3's rpm), instead of 2500 rpm. The Torque should be 5.7 N.m instead of 4 N.m. The rest of the curve is ok I think. My figure of 16% of peak torque should now read 24% of peak torque.

That's a good point about the Air/fuel ratio below 3400 rpm. It doesn't get to the optimum ratio (Dashed horizontal red line), until 3750 rpm. It is highly probable that if someone did a dyno run, taking readings from low revs making sure throttle opening was optimum at all points, that there would be not be as much drop off below 3400 rpm as our graph shows. I maintain there would still be a drop off, and that lower torque equals less load on bigend.

[Jordan]

I bumped into I.K. Brunel in '04.

[graeme]

There's a gent worthy of praise,,,,

[Jordan]

Gee, thanks Graeme.
Oh, you mean Brunel.

[StewartD]

They should have anchor chains hanging on that wall; that would be a great photo opportunity! Was it at the S.S. Great Britain museum?

Cheers,

Stewart D

[Jordan]

Madame Tussauds, London.

[Harvey]

by StewartD

Harvey,

Sorry for the error. The 350 Sebring's Torque curve has the plot at 2500rpm wrong. I divided 2 hp by 3591, (the Mk 3's rpm), instead of 2500 rpm. The Torque should be 5.7 N.m instead of 4 N.m. The rest of the curve is ok I think. My figure of 16% of peak torque should now read 24% of peak torque.

That's a good point about the Air/fuel ratio below 3400 rpm. It doesn't get to the optimum ratio (Dashed horizontal red line), until 3750 rpm. It is highly probable that if someone did a dyno run, taking readings from low revs making sure throttle opening was optimum at all points, that there would be not be as much drop off below 3400 rpm as our graph shows. I maintain there would still be a drop off, and that lower torque equals less load on bigend.
However, there is sound engineering theory, if there is no hard data to support what I still think is a myth, (with exception to a motor that is suffering detonation), then this is what we must turn to. Engineering has been firmly underpinned by mathematics and physics since Brunel's groundbreaking Broad gauge railway (1837 ?) and the S.S. Great Eastern, (launched 1858, it was exactly twice the length of the Himalaya, the largest merchant ship at the time). There is not much mystery to a simple big end bearing.

. Engineering has been firmly underpinned by mathematics and physics since the Titanic was built.

It is highly probable that if someone did a dyno run, taking readings from low revs making sure throttle opening was optimum at all points,

We don’t take dyno readings at part throttle. Its got to be WFO.

Oh Stewart you disappoint me. I was absolutely convinced that my brillante use of Mathematics would have convinced you, oh well, guess we will have to agree, to disagree.

But, I think you are ignoring the facts that are shown at 1000rpm, in the table that I posted. A torque reading of 42.48, a cylinder filled to 79.6%, producing a peak combustion pressure of 52.3Bar/764psi, on the short area of track that is used at low rpm, will produce a very high load for the roller contact area.
In my 456cc at 11:1, the rod has an inserted track, harden pin, and smaller rollers, and I would not apply full throttle below 3500.
Your big end, your choice.

[LaceyDucati]

Stewart
I don't have a chart for a standard Single (well apart from a 175 that never even made 10hp at the back wheel!) If this Dyno chart of a pepped up Sebring road bike helps, here it is.

Happy pontificating

Nigel

Kevins 350 dynojet.jpg

[Harvey]

Nigel, as you and Eldert have probably split more cranks to check for wear that most have, I was wondering if you have noticed where the wear first appears. Which component shows the first signs of fail.?