I'm going on a holiday, with my 2008 BMW 116i (122HP). The redline is at about 6500rpm, but i don't know exactly. Driving at highway speeds, the engine revs are around 3500rpm. Is this safe for the engine for a longer time?
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Welcome to Motor Vehicle Maintenance & Repair!– Pᴀᴜʟsᴛᴇʀ2 ♦Commented Jul 17, 2018 at 8:37
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Is that in top 5th or 6th? seems high - my car is 2100 at about 80mph– Solar MikeCommented Jul 17, 2018 at 8:55
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@SolarMike then you probably are not driving a BMW 116i– MadMarkyCommented Jul 17, 2018 at 11:08
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Top of sixth for me at 130-140 km/h (80-86mph) is betveen 3000-3500rpm. 122hp petrol engine.– Bakos DominikCommented Jul 17, 2018 at 11:10
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@MadMarky most cars I have driven tend to cruise at about 2000 to 2200rpm at 75 mph give or take, but I have not owned a bmw and am not likely to....– Solar MikeCommented Jul 17, 2018 at 11:11
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4 Answers
This is a normal rpm range for a gasoline engine. Especially the smaller non-turbo engines need to run at higher rpm's in order to have adequate performance for highway speeds. You do not have to worry about the safety of the engine. These modern engines are thoroughly tested by the manufacturer in far more extreme conditions than you will encounter. They can endure this all day every day.
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That´s actually not quite accurate. Most turbos only provide boost at higher rpm´s. Low end performance is mostly a strength of large displacement engines. RPM is a function of gear ratio and vehicle speed, on any engine!– DanielCommented Jul 17, 2018 at 13:44
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@Daniel that depends on the engine configuration. Nowadays there are variable geometry vanes or stacked setups, allowing for boost at 1500rpm or less. It's not the 80's anymore where turbo's where mainly used for sportscars.– MadMarkyCommented Jul 18, 2018 at 9:32
Let's start from the physics.
According to physics, force is mass times acceleration.
Acceleration, on the other hand, is velocity divided by time.
If the engine is spinning faster, velocity is greater, and the time to change it is shorter. This means acceleration is a quadratic function of the rotational speed.
Thus, force is also a quadratic function of the rotational speed.
Your engine can withstand the required forces at 6500 RPM.
(3500 / 6500)^2 = approximately 0.29
This means you are experiencing forces of only 29% what the engine is capable of. (There might be additional elastic deformation of material at high speeds, causing additional forces, so the 29% is an upper limit.)
I would say based on this your engine will have a very healthy and long life at 3500 RPM.
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Actually the limiting component for any engine is piston speed. If it get´s too high, lubrication of your piston-rings fails and your engine is damaged the 6500 RPM are just some safe-limit from the manufacturer.– DanielCommented Jul 17, 2018 at 13:42
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@Daniel That seems interesting, I thought the limiting factor would be maximum force, not lubrication... Anyway, on the 2011 Toyota Yaris with its 1.33 litre engine I used to have, the factory service manual suggested that over-revving failures would be seen from valvetrain failure, not from pistons.– juhistCommented Jul 17, 2018 at 14:15
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Yes there are other limits. It´s hard the vales move fast enough without increasing wear too much but motorcycles and formula 1 cars where the real limits are (note short stroke makes for lesser piston speed so high rpm engines are always short stroke). Also other components (bearings, rods, etc.) can become critical and unbalanced weights play a bigger role at higher rpm.– DanielCommented Jul 17, 2018 at 14:22
Normally your Engine is fine as long as you don´t rev it as long as it is cold. Always make sure to drive in low rpm´s for the first 5-10 minutes!
Other than that, the manufacturer has limited the rpm´s to a safe range that is well before any problems arise. There is an electronic rev-limiter so you can´t really over-rev.
That said, every revolution will amount to a certain wear. So from a longevity-perspective it is advisable to always drive in the highest gear possible if you are just cruising. Also from a fuel economy standpoint that is usually the best option.
Exception is when you want to accelerate fast. There it is advisable to use almost your whole rpm-band, because power=torque*rpm. You will feel what I men if you accelerate till the end of the rpm-band. Compared to the engines life, those acceleration don´t amount to much - at least if you are not a racing driver.
Being the most under powered BMW this engine revs at 4000 RPM for max torque at 85 MPH .
3000-4250 rpm is the sweet spot but like any engine use the best synthetic oil and drive conservatively for longest life.
