free revving on park/neutral
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Originally Posted By: KrisZ
Let's hear some technical reasons why it's "never" a good idea to rev the engine without load, and no made up, feel good nonsense please.
I can think of few undesirable effects such an event can produce, but nothing that would warrant a "never" advice.
That's a really good question, and I await the determinative results...because I can't see it either.
The closest that I've heard, and it was somewhat borne out by my early car buying days was that if a previous owner was comparatively mild as a driver, my flogging it as the new owner would have the top ring make contact with any ring ridge that had formed, and possibly damage the top ring/land.
Let's hear some technical reasons why it's "never" a good idea to rev the engine without load, and no made up, feel good nonsense please.
I can think of few undesirable effects such an event can produce, but nothing that would warrant a "never" advice.
That's a really good question, and I await the determinative results...because I can't see it either.
The closest that I've heard, and it was somewhat borne out by my early car buying days was that if a previous owner was comparatively mild as a driver, my flogging it as the new owner would have the top ring make contact with any ring ridge that had formed, and possibly damage the top ring/land.
WOT on the rev limiter was standard proceedure at the bike shop. I don't see a problem...without a rev limiter it's a bit harder to guess, and you could over rev and valve bounce.
A couple of quick unloaded revs to redline make no difference.
Holding unloaded WOT to redline for any duration of time might not be such a good idea.
Holding unloaded WOT to redline for any duration of time might not be such a good idea.
Some cars rev their own engines to redline automatically upon engine start.
Originally Posted By: DoubleWasp
Some cars rev their own engines to redline automatically upon engine start.
Which cars actually do that? I've seen it, but the car sure didn't sound like it went anywhere near red line. I thought it was a function of the electronics, because the speedometer pegged itself to the max speed on it along with the tach hitting red line, but the engine sounded like any other engine when it started up. It was in Park so the speedometer wasn't registering speed either.
Some cars rev their own engines to redline automatically upon engine start.
Which cars actually do that? I've seen it, but the car sure didn't sound like it went anywhere near red line. I thought it was a function of the electronics, because the speedometer pegged itself to the max speed on it along with the tach hitting red line, but the engine sounded like any other engine when it started up. It was in Park so the speedometer wasn't registering speed either.
Yeah, I'd like to know which ones redline on start too. I've seen a lot that rev more than I would personally do, but nowhere near redline. Give us some examples.
Originally Posted By: Shannow
The closest that I've heard, and it was somewhat borne out by my early car buying days was that if a previous owner was comparatively mild as a driver, my flogging it as the new owner would have the top ring make contact with any ring ridge that had formed, and possibly damage the top ring/land.
Yup, heard that one as well. The other thing I can think of, and it's purely speculative on my part, is that without the load the internals may develop some sort of harmonic frequency at certain RPM range and if done enough times may cause problems.
The closest that I've heard, and it was somewhat borne out by my early car buying days was that if a previous owner was comparatively mild as a driver, my flogging it as the new owner would have the top ring make contact with any ring ridge that had formed, and possibly damage the top ring/land.
Yup, heard that one as well. The other thing I can think of, and it's purely speculative on my part, is that without the load the internals may develop some sort of harmonic frequency at certain RPM range and if done enough times may cause problems.
Any concern about free revving a properly warmed up engine within the safe operating zone (below redline) is COMPLETE GARBAGE!!!
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
Originally Posted By: DriveHard
Any concern about free revving a properly warmed up engine within the safe operating zone (below redline) is COMPLETE GARBAGE!!!
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
Actually your engine IS loaded in every one of your scenarios. Well except the blip to match rpm to down shift. But thats only for a second.
When you decelerate, your crankshaft, rods, and pistons are loaded. Though a diesel is less loaded, unless you have a engine brake.
Any concern about free revving a properly warmed up engine within the safe operating zone (below redline) is COMPLETE GARBAGE!!!
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
Actually your engine IS loaded in every one of your scenarios. Well except the blip to match rpm to down shift. But thats only for a second.
When you decelerate, your crankshaft, rods, and pistons are loaded. Though a diesel is less loaded, unless you have a engine brake.
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$50 site donor 2026
What is compression braking ?
4wd
$50 site donor 2026
I'm saying it applies a load IMO ...
HERE IS THE REASON.
under load, there is compression and ignition at the top end of the piston travel. That gaseous pressure pushes the piston towards the rod and back towards the crank. If you do not have the pressure density and ignition, the rod is "yanked" on by the piston, so that it is not only pushing the piston outwards, it is also now snatching it back. Each material has an ability to handle compression, and a different ability to handle tension. Think concrete- great at compression, bad at tension, which is why we add rebar. Rods are primarily designed to handle compression. Aluminum is not as sturdy in handling tension. Not to say that rods are not able to handle some tension, but they fatigue more under tension.
Some vehicles actually have lower rev limiter settings for P and N. We had an chrysler IIRC that had a 4500 limit in P/N.
-m
under load, there is compression and ignition at the top end of the piston travel. That gaseous pressure pushes the piston towards the rod and back towards the crank. If you do not have the pressure density and ignition, the rod is "yanked" on by the piston, so that it is not only pushing the piston outwards, it is also now snatching it back. Each material has an ability to handle compression, and a different ability to handle tension. Think concrete- great at compression, bad at tension, which is why we add rebar. Rods are primarily designed to handle compression. Aluminum is not as sturdy in handling tension. Not to say that rods are not able to handle some tension, but they fatigue more under tension.
Some vehicles actually have lower rev limiter settings for P and N. We had an chrysler IIRC that had a 4500 limit in P/N.
-m
Originally Posted By: Srt20
Originally Posted By: DriveHard
Any concern about free revving a properly warmed up engine within the safe operating zone (below redline) is COMPLETE GARBAGE!!!
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
Actually your engine IS loaded in every one of your scenarios. Well except the blip to match rpm to down shift. But thats only for a second.
When you decelerate, your crankshaft, rods, and pistons are loaded. Though a diesel is less loaded, unless you have a engine brake.
No...no it is not. There is no combustion events taking place (in modern FI engine). The only "load" is friction that is resisting the force that is spinning the engine.
If you are talking about forces inside the engine, the diesel engine has MORE force pushing the piston into the con rod under free spin than a spark ignited engine. The diesel takes a full "breath" of air every single time, whether you add fuel or not...so there is always the same compression force, and you get that energy right back out (which is why it seems that a diesel has less drag than a SI engine). On an SI engine you are drawing a near vacuum when the engine is in free spin...so you don't even get the force of compression. You get a pumping loss...which is why it feels like an SI engine has more drag.
If you throw de-compression braking into the mix you get an even "worse" type of loading. You get the compression forces on the piston as it approaches top dead center, then quickly and violently that force disappears as that compressed air is allowed to be released into the exhaust system. The con-rod and piston is then dragged back down to bottom dead center with no help of the compression force to push it down...by the logic being thrown around here, that means every time you use your "Jake brake" you are destroying your engine!
Originally Posted By: DriveHard
Any concern about free revving a properly warmed up engine within the safe operating zone (below redline) is COMPLETE GARBAGE!!!
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
Actually your engine IS loaded in every one of your scenarios. Well except the blip to match rpm to down shift. But thats only for a second.
When you decelerate, your crankshaft, rods, and pistons are loaded. Though a diesel is less loaded, unless you have a engine brake.
No...no it is not. There is no combustion events taking place (in modern FI engine). The only "load" is friction that is resisting the force that is spinning the engine.
If you are talking about forces inside the engine, the diesel engine has MORE force pushing the piston into the con rod under free spin than a spark ignited engine. The diesel takes a full "breath" of air every single time, whether you add fuel or not...so there is always the same compression force, and you get that energy right back out (which is why it seems that a diesel has less drag than a SI engine). On an SI engine you are drawing a near vacuum when the engine is in free spin...so you don't even get the force of compression. You get a pumping loss...which is why it feels like an SI engine has more drag.
If you throw de-compression braking into the mix you get an even "worse" type of loading. You get the compression forces on the piston as it approaches top dead center, then quickly and violently that force disappears as that compressed air is allowed to be released into the exhaust system. The con-rod and piston is then dragged back down to bottom dead center with no help of the compression force to push it down...by the logic being thrown around here, that means every time you use your "Jake brake" you are destroying your engine!
Originally Posted By: meep
HERE IS THE REASON.
under load, there is compression and ignition at the top end of the piston travel. That gaseous pressure pushes the piston towards the rod and back towards the crank. If you do not have the pressure density and ignition, the rod is "yanked" on by the piston, so that it is not only pushing the piston outwards, it is also now snatching it back. Each material has an ability to handle compression, and a different ability to handle tension. Think concrete- great at compression, bad at tension, which is why we add rebar. Rods are primarily designed to handle compression. Aluminum is not as sturdy in handling tension. Not to say that rods are not able to handle some tension, but they fatigue more under tension.
Some vehicles actually have lower rev limiter settings for P and N. We had an chrysler IIRC that had a 4500 limit in P/N.
-m
see statement above. If this is the case, every time a de-compression brake is used you are hurting your engine. Every time you coast down a mountain in a SI engine, you are hurting your engine. This is simply not true.
HERE IS THE REASON.
under load, there is compression and ignition at the top end of the piston travel. That gaseous pressure pushes the piston towards the rod and back towards the crank. If you do not have the pressure density and ignition, the rod is "yanked" on by the piston, so that it is not only pushing the piston outwards, it is also now snatching it back. Each material has an ability to handle compression, and a different ability to handle tension. Think concrete- great at compression, bad at tension, which is why we add rebar. Rods are primarily designed to handle compression. Aluminum is not as sturdy in handling tension. Not to say that rods are not able to handle some tension, but they fatigue more under tension.
Some vehicles actually have lower rev limiter settings for P and N. We had an chrysler IIRC that had a 4500 limit in P/N.
-m
see statement above. If this is the case, every time a de-compression brake is used you are hurting your engine. Every time you coast down a mountain in a SI engine, you are hurting your engine. This is simply not true.
Originally Posted By: DriveHard
Any concern about free revving a properly warmed up engine within the safe operating zone (below redline) is COMPLETE GARBAGE!!!
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
I'm reminded of something called "the Grapevine" in I think California... long hill climb? apparently it has a long decline after it too. Detroit diesels would need a cooling down at the top, else the long run of compression braking would crack the head.
Completely unrelated to this thread though. Probably just one engine family a bunch of years ago, and long fixed.
Any concern about free revving a properly warmed up engine within the safe operating zone (below redline) is COMPLETE GARBAGE!!!
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
I'm reminded of something called "the Grapevine" in I think California... long hill climb? apparently it has a long decline after it too. Detroit diesels would need a cooling down at the top, else the long run of compression braking would crack the head.
Completely unrelated to this thread though. Probably just one engine family a bunch of years ago, and long fixed.
Material fatigue comes from the change in material structure each time it is cycled and the number of cycles. Internals of car engines are designed for infinite life cycles, meaning the forces they handle are not great enough to change the material structure, therefore no fatigue is taking place under normal operation, no mater if it going up a steep incline or revving in neutral.
Originally Posted By: DriveHard
Originally Posted By: Srt20
Originally Posted By: DriveHard
Any concern about free revving a properly warmed up engine within the safe operating zone (below redline) is COMPLETE GARBAGE!!!
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
Actually your engine IS loaded in every one of your scenarios. Well except the blip to match rpm to down shift. But thats only for a second.
When you decelerate, your crankshaft, rods, and pistons are loaded. Though a diesel is less loaded, unless you have a engine brake.
No...no it is not. There is no combustion events taking place (in modern FI engine). The only "load" is friction that is resisting the force that is spinning the engine.
If you are talking about forces inside the engine, the diesel engine has MORE force pushing the piston into the con rod under free spin than a spark ignited engine. The diesel takes a full "breath" of air every single time, whether you add fuel or not...so there is always the same compression force, and you get that energy right back out (which is why it seems that a diesel has less drag than a SI engine). On an SI engine you are drawing a near vacuum when the engine is in free spin...so you don't even get the force of compression. You get a pumping loss...which is why it feels like an SI engine has more drag.
If you throw de-compression braking into the mix you get an even "worse" type of loading. You get the compression forces on the piston as it approaches top dead center, then quickly and violently that force disappears as that compressed air is allowed to be released into the exhaust system. The con-rod and piston is then dragged back down to bottom dead center with no help of the compression force to push it down...by the logic being thrown around here, that means every time you use your "Jake brake" you are destroying your engine!
https://www.youtube.com/watch?v=o8Cta2cC2Co
Pistons trying to pull air, creating vacuum, IS a load on the rotating assembly.
Originally Posted By: Srt20
Originally Posted By: DriveHard
Any concern about free revving a properly warmed up engine within the safe operating zone (below redline) is COMPLETE GARBAGE!!!
When I drive down a mountain pass with my engine sitting between 4000 and 5000 with ZERO load for minutes on end...
When I blip my throttle every time I downshift to rev-match...
When I downshift to let my engine do the braking...
When I coast down the highway in a lower than normal gear...
Every time I go down a hill when towing in a lower gear...
Your engine is revved with no or little load all the time when operating your vehicle. Simply revving it up there in neutral is no different.
Actually your engine IS loaded in every one of your scenarios. Well except the blip to match rpm to down shift. But thats only for a second.
When you decelerate, your crankshaft, rods, and pistons are loaded. Though a diesel is less loaded, unless you have a engine brake.
No...no it is not. There is no combustion events taking place (in modern FI engine). The only "load" is friction that is resisting the force that is spinning the engine.
If you are talking about forces inside the engine, the diesel engine has MORE force pushing the piston into the con rod under free spin than a spark ignited engine. The diesel takes a full "breath" of air every single time, whether you add fuel or not...so there is always the same compression force, and you get that energy right back out (which is why it seems that a diesel has less drag than a SI engine). On an SI engine you are drawing a near vacuum when the engine is in free spin...so you don't even get the force of compression. You get a pumping loss...which is why it feels like an SI engine has more drag.
If you throw de-compression braking into the mix you get an even "worse" type of loading. You get the compression forces on the piston as it approaches top dead center, then quickly and violently that force disappears as that compressed air is allowed to be released into the exhaust system. The con-rod and piston is then dragged back down to bottom dead center with no help of the compression force to push it down...by the logic being thrown around here, that means every time you use your "Jake brake" you are destroying your engine!
https://www.youtube.com/watch?v=o8Cta2cC2Co
Pistons trying to pull air, creating vacuum, IS a load on the rotating assembly.
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