Is reving an engine to redline without a load (in neutral) bad?
Is reving an engine to redline without a load (in neutral) bad?
(OP)
Hi,
I'm new to the forum. I'm not in the engineering field, but I race and track BMW's as a hobby. I'm wondering if anyone here can clear up something for me. I've heard that it is bad to rev an engine to red line if there is no load on the engine (i.e. in neutral). Is this true and if so, why?
Thanks!
I'm new to the forum. I'm not in the engineering field, but I race and track BMW's as a hobby. I'm wondering if anyone here can clear up something for me. I've heard that it is bad to rev an engine to red line if there is no load on the engine (i.e. in neutral). Is this true and if so, why?
Thanks!





RE: Is reving an engine to redline without a load (in neutral) bad?
RE: Is reving an engine to redline without a load (in neutral) bad?
- Steve
RE: Is reving an engine to redline without a load (in neutral) bad?
RE: Is reving an engine to redline without a load (in neutral) bad?
The idea being that there was little compressed gas in the combustion chamber to "cushion" the piston at the top of its compression stroke.
Popular legend has that the very long and partly downhill Conrod Straight on the Bathurst circuit (in Oz) was so named because drivers would worry about holding the engine at peak revs for so long and lift off the throttle to give the engine a rest - resulting in a conrod immediately breaking.
Just how dangerous it is to the engine running unloaded at peak revs is I don't know - but is probably wise to avoid doing this if possible.
RE: Is reving an engine to redline without a load (in neutral) bad?
Also high manifold vacuum at high rpm does reduce the pressure over the piston as it passes over TDC is reduced so the stretch on the rods is higher at least on the power strokes and probably also on the exhaust strokes as there is little exhaust gas to displace.
If the engine is tuned so there is no high speed lean out and increased spark advance, then the engine should be safe enough, but the question is why rev to red line for no real reason as any engine will have increased stress and wear and will use up a portion of the engine life for no benefit.
Regards
Pat
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RE: Is reving an engine to redline without a load (in neutral) bad?
I don't think the valve train loads are much different under power, except maybe the force required to crack open the exhaust valve.
A lot, if not most rod breakage (and metallic part failure in general) is the final stage of progressive cracking from fatigue. In the days when lots of passenger cars were still using forged steel rods I used to magnaflux rods at a hot rod engine shop. It was common to find small "indications" in the area brutalized by factory machining to accept the rod bolt head. That was true of abused engines as well as used passenger engines. It was much less common to find indications along the I-beam or around the wrist pin eye.
One of the differences between specialty race rods and poor pitiful passenger rods was the bolt seat detail.
Studebaker sixes were actually FSB'd shortly after they adopted a sharp corner in the bolt seat and bod breakage began.
For years Chevy V8s had a gruesome torn finish on a fairly generous radius.
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High performance Small block Fords had a 3D Curve to accept football shaped bolt heads. Generelly speaking having the rod material wrap around the bolt is "better" (lower stress concentration) than a straight across notch.
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In addition to gorgeous materials and superb preparation Carrillo rods used a cap screw, and the area around the tapped hole was sculptured.
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Powdered metal con rods are popular today, and often include low stress details in bolt area of the rod.
(Factory PM rod is the upper rod in this picture.)
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Even good details are not always enough.
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RE: Is reving an engine to redline without a load (in neutral) bad?
From a practical point I do not do it on my engines. However, I have a couple friends, non engineers/non engine builders that rev their race engines (Fiats) to red line and hold them their while "adjusting" whatever. Scares me just to watch.
Rod
RE: Is reving an engine to redline without a load (in neutral) bad?
- Steve
RE: Is reving an engine to redline without a load (in neutral) bad?
RE: Is reving an engine to redline without a load (in neutral) bad?
Can't be good - might be bad..... Why push the envelope if there is no good reason??
RE: Is reving an engine to redline without a load (in neutral) bad?
- Steve
RE: Is reving an engine to redline without a load (in neutral) bad?
RE: Is reving an engine to redline without a load (in neutral) bad?
RE: Is reving an engine to redline without a load (in neutral) bad?
The 1966 movie "Grand Prix" comes to mind .
Dan T
RE: Is reving an engine to redline without a load (in neutral) bad?
Still my two Fiat/Abarth racers, both old dudes like me, have peaked their engines since, forever with no failures. I don't have a dime invested in their race cars but I still don't like to watch them do it.
Just one more little tid bit. I was "tuning" the carb on my sons old 318 Dodge pickup leaning over the RF fender when with just a "slight" rev (???) one of the connecting rod poked it's little end out the block just a few inches below my CHIN!!!!! Gets your attention.
Rod
RE: Is reving an engine to redline without a load (in neutral) bad?
RE: Is reving an engine to redline without a load (in neutral) bad?
Regards
Pat
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RE: Is reving an engine to redline without a load (in neutral) bad?
One phrase that some of our petrol-heads here use is "bore polishing". Don't really know enough about the trib of pistons/rings/liners to comment, but they think it's an issue with unloaded running at high speeds.
- Steve
RE: Is reving an engine to redline without a load (in neutral) bad?
RE: Is reving an engine to redline without a load (in neutral) bad?
I borrowed a '57 Ford V8 from the lot where I was working back in '58 and, like any teen I "went through the gears" several times obviously and, equally unknown to me, exceeded the "red line" by a bunch ending with a bent pushrod. Hey, couldn't take it back like that so I stopped and removed the rocker cover, pulled the pushrod out and beat it straight with a big rock. No one the wiser...Isn't it amazing that I have managed to live this long?
Rod
RE: Is reving an engine to redline without a load (in neutral) bad?
1) There's no reason to do it.
2) Others will think you a fool [who needs that].
RE: Is reving an engine to redline without a load (in neutral) bad?
A lot of people have pointed out that there is no point in actually holding an engine at high RPM for long periods. But there is one "legitimate" excuse for doing this - and that is durability testing of camshaft and valve gear systems. If you can't afford a Spintron machine it is a simple and cheap (even if you destroy an engine or two it still would be a lot cheaper than a Spintron) and a good real-life way to test.
I have personally been involved in tests like this lasting half an hour or so. One well-known cam grinder I know says that if a cam lasts even ten minutes in a test like this without any visible wear or damage it will last all day at Bathurst.
I have heard that one well-known car maker did 24 hour continuous tests in this fashion.
RE: Is reving an engine to redline without a load (in neutral) bad?
- Steve
RE: Is reving an engine to redline without a load (in neutral) bad?
That being said, if you go to a drag race, any car that has a clutch has the rev limiter set somewhere north of 8K so that the driver can simply put the pedal to the metal after staging and be ready to drop the hammer as opposed to the old days when one had to carefully raise the RPMs to the ideal point which required looking carefully at the Tach rather than concentrating on the starting light. I've never seen anybody lose an engine while staging.
RE: Is reving an engine to redline without a load (in neutral) bad?
Additionally, in the late '70s, I ran a car with a destroked small block that didn't make any power until it reached about 8K. I had a set of (very rare) 7.14 gears in the rear that at 142 mph put me through the traps at soming around 12K. I launched this car at 10K and never boke anything on the starting line.
RE: Is reving an engine to redline without a load (in neutral) bad?
Motoring engines and skip firing them will testing is normal procedure in all of the places I've worked and I've never once come across an issue in terms of catastrophic failure.
IMO it's one of those old wives tales- up there with " If you lean out your mixture too much at idle- you'll labour the engine"- you may reach lean combustion stability limits and go into misfire but you WON'T "labour the engine"!
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RE: Is reving an engine to redline without a load (in neutral) bad?
RE: Is reving an engine to redline without a load (in neutral) bad?
RE: Is reving an engine to redline without a load (in neutral) bad?