Safe cruising revs

[Blitz]

I have a Spit 4 without overdrive. Iam wondering what revs the engine should safely cruise at. At around 3000-3500 I’m only doing around 50mph using accurate gps not my wobbly Speedo. 

Experience and advise welcome. It’s a standard 1150cc engine. 

Thanks 

Grant 

[Pete Lewis]

back when new this was probably the average cruising speed , but they are a revy unit with no real high rev.  traits

you could push up to 4500 ...just makes more speed and ....noise 

if youre comfortable and the cars comfortable  then stick with the 50-60 cruise , 

pete

 

[Blitz]

Thanks Pete. Happy to keep around 60mph not trying for 70. Nice to know she can handle say 4000 to 4500 without any issues. All engines are different and we all know the Spit V Rev issues, so wanted to check. 

Thanks 

Grant 

[Pete Lewis]

think we all get a feel for when shes happy  there's always somewhere between tootle and stole it .

pete

[Casper]

My 1200 (lookalike Spitfire) engine is happy at 65 - 70 all day.  Gets a bit wearing on the ears though, so I choose A roads over motorways for journeys.  JD might have some comment about harmonics or something at 4000.

C.

[Blitz]

Thanks Casper. Harmonics sounds interesting. I know the top speed was say 90mph so you would of thought 70mph was doable but I’m not used to using that many revs ( diesel driver ) or motorbikes which you can rev the tits off anyway. 

Big difference in doing 60mph and staying safe ish with HGVs than 50 mph and playing Chicken with them. Yes A roads are good but sometimes due to times you need to sit on a motorway for a bit. 

 

[thescrapman]

My Mk3 Spitfire would cruise at 4500 to 5000rpm before I fitted the OD to save my eardrums,

I have lower profile tyres which didn't help either.

[JohnD]

An IC engine (or an electric motor for that matter) is full of vibrations.   Primary, Secondary, torsional, etc. etc.    Harmonics occur when the crank, the camshaft or some other part resonates at the same frequency, like a wine glass can be broken by singing at it, at the right note!

You can't remove the vibrations, best is not to cruise at a constant engine speed, but vary it up and down, a few hundred revs each way.   Then if there is some harmonic, you won't be there long.

John

[Clive]

The 1200, and indeed 1300 engines are pretty happy to rev, it is the 1500 which doesn't

Cruising at 4000+ rpm should be OK as long as the engine is in good fettle. So 70mph should be fine, but overdrive will help with revs and enable a higher cruising speed.

[Herald948]

3 hours ago, clive said:

The 1200, and indeed 1300 engines are pretty happy to rev, it is the 1500 which doesn't

Cruising at 4000+ rpm should be OK as long as the engine is in good fettle. So 70mph should be fine, but overdrive will help with revs and enable a higher cruising speed.

As someone who has indeed "cruised at 4000+rpm" on many long trips (not the least of which was a 2500+-mile round trip awhile back, almost all on US Interstate Highways at around 65-70 mph, in a Herald 1200), I agree completely!

[Pete Lewis]

If its too many  revs  we have  a local car ,a std. 1200 herald has been fitted with a 3.63 diff,. And he is more than happy with the performance and reduced motorway cruise as a great improvement.

Just an idea 

Pete

[NonMember]

21 hours ago, JohnD said:

Harmonics occur when the crank, the camshaft or some other part resonates at the same frequency,

On a point of pedantry...

Harmonics occur when a sound does not have a pure sine wave and represent contributions at multiples of the frequency. Sympathetic resonance occurs when some other object shares one of those frequencies as its "natural" frequency.

[Blitz]

6 minutes ago, NonMember said:

On a point of pedantry...

Harmonics occur when a sound does not have a pure sine wave and represent contributions at multiples of the frequency. Sympathetic resonance occurs when some other object shares one of those frequencies as its "natural" frequency.

What he said. 🤯

[JohnD]

On the contrary, a pure sine wave can stimulate resonance at many harmonics, which are multiples of the wine waves frequency.    In general the higher the harmonic, the greater the number that multiplies the fundamental, the less well will it resonate.     Fourier Analysis can bre akdown the components of a complex waveform that isn't a  sine into multiple pure sine waves.  Those that have frequencies that can resonate w I'll do so.

John

[Nick Jones]

20 hours ago, Herald948 said:

As someone who has indeed "cruised at 4000+rpm" on many long trips (not the least of which was a 2500+-mile round trip awhile back, almost all on US Interstate Highways at around 65-70 mph, in a Herald 1200), I agree completely!

I Also.  As Clive said, the 1200 and 1300 (esp. small-crank 1300) seem quite happy to hold 4000+ for extended periods, which is enough for 70mph IIRC.  Try that with a 1500 though and at best your ears will bleed, at worst there'll be metal on the road..... btdt.....

Nick