IC Engines continous power Vs. Instantaneous power

Thread Starter


Joined Oct 3, 2010
It has always been my understanding that the vehicle HP ratings provided by automotive MFGs and the peak HP ratings obtained by dyno testing are instantaneous maximum output figures and the engine of the vehicle should not be relied upon to provide that level of power continuously. Similarly it has been my understanding that industrial engines such as those found on pumps, generators, tractors, etc. are rated on continuous power output. For example a sports car rated 500hp can indeed deliver 500hp for a few seconds on the drag strip, but if you were to hitch a loaded 18-wheeler trailer to it and expect it to deliver 500hp for a long period of time, the engine would burn up. Conversely a 500hp rated 18wheeler would not have this problem (exclude gearing and such; focus is on power output). And I feel this is evident in the design of the 500hp 18wheeler engine (huge thermal mass & giant radiator) vs the 500hp sports car engine (light weight, small radiator).

I tried to find a rule of thumb for determining the reasonable expectation of continuous HP of an automotive engine based on the peak HP rating of the MFG. In doing so, I was surprised to find absolutely nothing, and further, I cannot even find any hint of confirmation of my original assumptions. Am I wrong? If a sports car is rated 500hp can we expect it to deliver 500hp continuously if used in a different application than getting through a quarter mile in under 12s? If I used that engine in an agricultural application pumping massive volumes of water for weeks on end, would there be a problem? If not, then why are the agricultural pumps in use, using massive diesel engines which weight 3x-4x as much?


Joined Sep 7, 2010
Engineering engines is always a trade off ... generally the lower the revs the longer the life ... there are still steam engines working over a hundred years used for pumping water , they have a few RPM and weigh many tonnes ....

the work horse of the the farming world is the diesel engine , big and heavy are rather low RPM but they last decades ...

Petrol engines are much lighter , higher RPM more prone to breakages and shorter life ...

Race cars push the petrol engine to the limit , maximum RPM , as light as possible , at the cost of a short life.


Joined Mar 10, 2018
High performance engines that have to run over extended time
periods are designed to pull heat out of sensitive areas. So water
flow, dry sump oiling systems, precise timing to insure no detonation,
friction reduction in cylinders, bearings, and valve trains, exhaust valve
material specs........

Regards, Dana.


Joined Jan 18, 2008
A top fuel dragster puts out well in excess of 10,000 HP for less than 4 seconds. Then it needs a rebuild (partial). I don't think that would do well cross-country at any HP. In other words, as I understand your question, I don't think there is a conversion. Some engines/motors are just not built for continuous duty.

For a continuous duty motor, there may be peak ratings, however. Aircraft engines are one example and may have a T.O. rating and max. cruise rating.


Joined Aug 21, 2017
At most poor times here many peasants was creating their tractors on basis of light-car engine. The life span was not many seasons. First is the radiator. Next is the clutch, the third are the gearbox. In early 90ies I run a company trailing every lovely day the wagon of 2-3 tons by 0,7 ton soviet Zygulyi (Lada). In the year it ate 2 to 3 shifts of gearboxes (20 USD each) and 4 to 5 spare clutches (3 USD each). It was so far away, now my WV-B-5.5 clutch cost 400 and gearbox near 1000. However last year I pull those wagon out again when in hands fell 100 m3 free timbers, just need to take em out of wood before stolen or rotten. I adapted to put the 4 m3 of 3m long logs in each load, but I risked with rather harsh penalty if catch me, as the wagon is rated for 800 kg (however strengthened accordingly after registration). Not catched even when I crushed the springin the middle of highway. Car survived all, however week later in far away of home in Germany it broke timing belt. Still havent sure is it or is it not connected to that 25 super-heavy loads. Then I bought an identical car for 1KEur and trailer for 1,5 and got all my belongings 3000 km afar home. Adventure to remeber for years. Now my neighbour is pulling with it the bricks to his farm every weekend, normally 1 m3 silicates=2,5 tons. Of course, yesterday he laid a woue, the screws of one wheel was smashed so the wheel in mid-way was fell off. Needed to help him with good domcrat and spares.
P.S. Its important to clarify for enthusiasts to violate the physics - if wagon is heavier than some 1,5-2 tons, the only way to not damage the car springs and amortisators is to apply a connector elongator. Normal wagon stays about meter from rear to front walls. For my, 4 meters off, thus the any waving/swinging is unnoticeable.
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Joined Aug 21, 2017
On basic question: may hope that at rated nominal speed and nominal FC it will have the nominal work-span. All max parameters are for temporary jumps exclusively. So, if car have a 190 on speedometer, means 120 is must-to-be speed, and if there FC is say some 4....6 liters per 100 km, then power may be easily calculated of the fuel calorific value 46 MJ/kg.


Joined Sep 30, 2009
A top fuel dragster puts out well in excess of 10,000 HP for less than 4 seconds. Then it needs a rebuild (partial).
That rebuild is do more to the extreme corrosiveness of the fuel than wear. There is a ton of blowby in those engines and it eats the bearings. If you watch them at the line, you can see raw fuel coming out of the pipes they are so rich in fuel.


Joined Jan 18, 2008
Oh yeah, I have seen it. I needed some nitromethane for a pulse jet engine when I first got to Cleveland. A place near Thompson's ( Thompson's Raceway Park) was the nearest place.


Joined Aug 21, 2017
At the younf age I boosted a moped engines rather much. Normally it was 1 hp at 5000 rpm, but at special fuel and changing the phases and apertures plus polishing it was able to get as well 11 000 rpm as well even 33 000 rpm. But if normally this engine gave at least some 5000 motorhours, then at 11K it gave 1 hour and at 33K gave about minute long lifespan before crankshaft ball-bearings was destroyed. At such turns none oil is not working anymore.