Thread: RPM Range for 4cyl Series III SWB

I think a 2.25 would spin to over 4000 rpm provided it was reasonably healthy. I have found keeping it between 2 and a half and 3 gives reasonable economy.

With standard gearing and no overdrive, etc a Series sits comfortably at 85 to 90KPH and will do 100 if you can put up with the noise and fuel consumption. Any tacho you can fit somewhere will do - If probably go a smaller
50mm unit and stick it somewhere. You’ll know what’s good and what’s not once you’re on the road, the whole vehicle will tell you when it’s happy or not.

Originally Posted by pop058

I think a 2.25 would spin to over 4000 rpm provided it was reasonably healthy.

And the rest - mine would go to near 5000 but while it sounded Ok I wasn't all that keen.

Originally Posted by brianharrison2

Greetings all,one of the posts asking about RPM's triggered a couple of questions from me.1. What would be an appropriate working range of RPM in a 4cyl Series III SWB for normal driving on the road?2. What is the max safe RPM to take the same engine too?Being somewhat paranoid about overworking a far from new motor, I'd like to fit a Tachometer at some point. Any suggestions as to what sort?ThanksBrian

I've clamped a 2" VDO rev counter on to the steering column. On long flat outback roads I can get up to a 100km more out of a tank by keeping the revs at about 2600pm with a 7:1 Cr engine.
.W.

The four cylinder petrol engine produces maximum power at 4250rpm, and maximum torque at 2500rpm. The engine is designed to operate at high rpm, and the cooling system is "oversize" to provide adequate cooling in very hot climates, working hard at low road speeds. There is large oil capacity compared to most comparable engines, providing good oil cooling, and the lubrication system is good.

These features mean that provided everything is in good condition, and adjustments such as ignition timing correct, there is no problem with running this engine at high rpm, with the proviso that sustained high rpm with low power is not a good idea, but not really harmful. At the other end of the scale, the engine will not be harmed by using full throttle at low rpm, although you need to be aware that repeatedly opening and closing the throttle as is likely in this driving mode pumps excess fuel into the engine that wastes fuel and, especially at low rpm (same amount of fuel is pumped in over fewer intake cycles) can reduce cylinder wall lubrication.

Operating the engine over the maximum power rpm is something you are unlikely to do as power reduces rapidly.

The other factor is engine efficiency - as a general rule, N/A petrol engines are at their most efficient at approximately maximum torque rpm so as far as the engine is concerned this rpm is most efficient. But if talking about driving on road in a square box, this effect is overwhelmed by aerodynamic drag, so that you have steadily dropping fuel economy well before getting to that speed in fourth.

The other consideration sometimes raised is engine noise - certainly in my 2a, engine noise is never an issue (although it was definitely worth making sure every hole in the firewall was blocked), being drowned out by wind noise, gear noise rattles etc.

Summary - don't worry about engine rpm.

Don't waste your money on a tacho, you might look at in once and then wonder why you bothered .

you can use Ashcrofts ratio calculator , it will tell you your exact RPM at whatever speed your doing, not that it even matters.,

Ashcroft Transmissions - Ratio Calculator

Guys,
thanks for the responses. I acknowledge that road/engine noise will most likely dictate driving style more than a gauge, but being a nerd I'll still fit a small tacho. Probably more for information, than for defining how I drive.

Once again, thanks

Brian

I have fitted a small tacho - but its main use is to check road speed - being on a 2a, it is in front of the driver and the speedo is not - I have memorised the rpm for 50/60/80/100. Not an issue with a S3