Engine braking = bad?
02-24-2007, 06:30 AM
#1
Engine braking = bad?
Do you guys engine brake regularly? I do. I was bored today and looked at the manual for my 91 MR2. It says when you engine brake (well, it actually says "downshift too early") you create high negative pressure in the cylinders which may cause motor oil to get sucked up into the combustion chamber, which increases oil consumption because you burn oil.
Is this right? I thought the combustion chamber was supposed to be really air tight! Can motor oil really go inside (from engine braking)? I engine braked on my other cars I never noticed any increase in oil burning/consumption.
Is this right? I thought the combustion chamber was supposed to be really air tight! Can motor oil really go inside (from engine braking)? I engine braked on my other cars I never noticed any increase in oil burning/consumption.
02-24-2007, 07:33 AM
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engine braking does cause damage to the motor...it may not cause problems now...but if you constantly do it you're going to be at risk...if you want to downshift...i recommend the heel and toe technique...i first learned it when i was 13...my father taught it to me in an empty parking lot...i took me many sessions to get it...im 15 now and do it no problem...i warn you in advance it is frustrating...i hope this helped you out a little....
02-24-2007, 09:23 AM
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First of all, what the hell does heel and toeing have to do with engine braking?
Second, oil is inside the cylinders already, its what lubricates the moving parts.
Third, engine braking is as simple as letting off the gas at high enough rpms that the engine friction will slow the car. Drive your S2000 and cruise at 6k and see what happens. That drag feel when you let off the gas is engine braking. No down shifting required, and certainly no heel and toeing required.
This may be an oversimplification, but there's nothing wrong with engine braking.
Second, oil is inside the cylinders already, its what lubricates the moving parts.
Third, engine braking is as simple as letting off the gas at high enough rpms that the engine friction will slow the car. Drive your S2000 and cruise at 6k and see what happens. That drag feel when you let off the gas is engine braking. No down shifting required, and certainly no heel and toeing required.
This may be an oversimplification, but there's nothing wrong with engine braking.
02-24-2007, 10:10 AM
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Originally Posted by s2kantstopme,Feb 24 2007, 08:33 AM
engine braking does cause damage to the motor...it may not cause problems now...but if you constantly do it you're going to be at risk...if you want to downshift...i recommend the heel and toe technique...i first learned it when i was 13...my father taught it to me in an empty parking lot...i took me many sessions to get it...im 15 now and do it no problem...i warn you in advance it is frustrating...i hope this helped you out a little....
IBTL
15???........
02-24-2007, 10:36 AM
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Enigne braking doesn't cause engine damage. It uses the action of the engine to slow the car. When the throttle is closed, the engine is not ingesting enough air and gas to generate more power than it uses filling the combustion chamber. Any gasoline engine has an idle air circuit that allows enough air past the closed throttle blades to keep the engine running at a low rpm, to keep it from stalling every time you take your foot off the gas.
Any time you lift off the gas, you are "engine braking." When you down shift coming down an exit ramp, for example, you are using the compression and friction of the engine to slow the car. The car's momentum is pushing the engine, while the engine is pushing back. Unless you are at wide open throttle, there is always vacuum in the intake manifold, so the idea that coasting the engine with the throttle closed is "bad," because of "negative pressure" is silly.
The engine will have a natural tendency to slow to a lower rpm if the throttle is closed, whether in gear or not. If you use this tendency to slow the car, you merely prolong the time it takes for the engine to reach idle rpm. You aren't increasing any force on the engine at all. Any time you accelerate to high rpm in second gear, then have to let off for any reason, you are exposing the engine to the very same conditions as leaving the car in second gear while you descend a hill or approach a stop sign.
Diesel trucks have no throttle, so they can't engine brake in the normal manner. To address this, the Jake brake adds an exhaust valve that only opens at the top of the compression stroke to vent compressed air. It closes as the piston descends, using the resulting vacuum in the cylinder to slow the truck. Over the road diesel tractors run well over a million miles. In a gasoline engine, the vacuum is generated by the closed throttle body, which as stated before, has a bypass to let enough air through for low rpm running. You aren't generating nearly the vacuum under engine braking that a diesel engine does under Jake braking.
In addition, your pistons have rings which prevent combustion chamber forces from affecting the oiling system. There is no reason to assume these rings only work in one direction on a production street engine. If combustion chamber vacuum drew oil into the cylinder, that would happen every time the piston descended on the intake stroke. It is the vacuum in the combustion chamber that draws air and fuel in, so the argument that there is "negative pressure" during engine braking is just a red herring. There is "negative pressure" every intake stroke, whether the throttle is open or not. If there weren't, the chamber wouldn't fill with air/fuel, and your engine wouldn't make any power.
cliff notes: engine braking is fine.
YMMV.
Any time you lift off the gas, you are "engine braking." When you down shift coming down an exit ramp, for example, you are using the compression and friction of the engine to slow the car. The car's momentum is pushing the engine, while the engine is pushing back. Unless you are at wide open throttle, there is always vacuum in the intake manifold, so the idea that coasting the engine with the throttle closed is "bad," because of "negative pressure" is silly.
The engine will have a natural tendency to slow to a lower rpm if the throttle is closed, whether in gear or not. If you use this tendency to slow the car, you merely prolong the time it takes for the engine to reach idle rpm. You aren't increasing any force on the engine at all. Any time you accelerate to high rpm in second gear, then have to let off for any reason, you are exposing the engine to the very same conditions as leaving the car in second gear while you descend a hill or approach a stop sign.
Diesel trucks have no throttle, so they can't engine brake in the normal manner. To address this, the Jake brake adds an exhaust valve that only opens at the top of the compression stroke to vent compressed air. It closes as the piston descends, using the resulting vacuum in the cylinder to slow the truck. Over the road diesel tractors run well over a million miles. In a gasoline engine, the vacuum is generated by the closed throttle body, which as stated before, has a bypass to let enough air through for low rpm running. You aren't generating nearly the vacuum under engine braking that a diesel engine does under Jake braking.
In addition, your pistons have rings which prevent combustion chamber forces from affecting the oiling system. There is no reason to assume these rings only work in one direction on a production street engine. If combustion chamber vacuum drew oil into the cylinder, that would happen every time the piston descended on the intake stroke. It is the vacuum in the combustion chamber that draws air and fuel in, so the argument that there is "negative pressure" during engine braking is just a red herring. There is "negative pressure" every intake stroke, whether the throttle is open or not. If there weren't, the chamber wouldn't fill with air/fuel, and your engine wouldn't make any power.
cliff notes: engine braking is fine.
YMMV.
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02-24-2007, 11:01 AM
#9
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I think some here are talking about lift throttle engine braking and some are talking about downshift w/o rev-match - some people also refer to that as engine braking.
02-24-2007, 12:23 PM
#10
Registered User
Originally Posted by GT_2003,Feb 24 2007, 11:36 AM
Enigne braking doesn't cause enigne damage. It uses the action of the engine to slow the car. When the throttle is closed, the engine is not ingesting enough air and gas to generate more power than it uses filling the combustion chamber. Any gasoline engine has an idle air circuit that allows enough air past the closed throttle blades to keep the engine running at a low rpm, to keep it from stalling every time you take your foot off the gas.
Any time you lift off the gas, you are "engine braking." When you down shift coming down an exit ramp, for example, you are using the compression and friction of the engine to slow the car. The car's momentum is pushing the engine, while the engine is pushing back. Unless you are at wide open throttle, there is always vacuum in the intake manifold, so the idea that coasting the engine with the throttle closed is "bad," because of "negative pressure" is silly.
The engine will have a natural tendency to slow to a lower rpm if the throttle is closed, whether in gear or not. If you use this tendency to slow the car, you merely prolong the time it takes for the engine to reach idle rpm. You aren't increasing any force on the engine at all. Any time you accelerate to high rpm in second gear, then have to let off for any reason, you are exposing the engine to the very same conditions as leaving the car in second gear while you descend a hill or approach a stop sign.
Diesel trucks have no throttle, so they can't engine brake in the normal manner. To address this, the Jake brake adds an exhaust valve that only opens at the top of the compression stroke to vent compressed air. It closes as the piston descends, using the resulting vacuum in the cylinder to slow the truck. Over the road diesel tractors run well over a million miles. In a gasoline engine, the vacuum is generated by the closed throttle body, which as stated before, has a bypass to let enough air through for low rpm running. You aren't generating nearly the vacuum under engine braking that a diesel engine does under Jake braking.
In addition, your pistons have rings which prevent combustion chamber forces from affecting the oiling system. There is no reason to assume these rings only work in one direction on a production street engine. If combustion chamber vacuum drew oil into the cylinder, that would happen every time the piston descended on the intake stroke. It is the vacuum in the combustion chamber that draws air and fuel in, so the argument that there is "negative pressure" during engine braking is just a red herring. There is "negative pressure" every intake stroke, whether the throttle is open or not. If there weren't, the chamber wouldn't fill with air/fuel, and your engine wouldn't make any power.
cliff notes: engine braking is fine. YMMV.
Any time you lift off the gas, you are "engine braking." When you down shift coming down an exit ramp, for example, you are using the compression and friction of the engine to slow the car. The car's momentum is pushing the engine, while the engine is pushing back. Unless you are at wide open throttle, there is always vacuum in the intake manifold, so the idea that coasting the engine with the throttle closed is "bad," because of "negative pressure" is silly.
The engine will have a natural tendency to slow to a lower rpm if the throttle is closed, whether in gear or not. If you use this tendency to slow the car, you merely prolong the time it takes for the engine to reach idle rpm. You aren't increasing any force on the engine at all. Any time you accelerate to high rpm in second gear, then have to let off for any reason, you are exposing the engine to the very same conditions as leaving the car in second gear while you descend a hill or approach a stop sign.
Diesel trucks have no throttle, so they can't engine brake in the normal manner. To address this, the Jake brake adds an exhaust valve that only opens at the top of the compression stroke to vent compressed air. It closes as the piston descends, using the resulting vacuum in the cylinder to slow the truck. Over the road diesel tractors run well over a million miles. In a gasoline engine, the vacuum is generated by the closed throttle body, which as stated before, has a bypass to let enough air through for low rpm running. You aren't generating nearly the vacuum under engine braking that a diesel engine does under Jake braking.
In addition, your pistons have rings which prevent combustion chamber forces from affecting the oiling system. There is no reason to assume these rings only work in one direction on a production street engine. If combustion chamber vacuum drew oil into the cylinder, that would happen every time the piston descended on the intake stroke. It is the vacuum in the combustion chamber that draws air and fuel in, so the argument that there is "negative pressure" during engine braking is just a red herring. There is "negative pressure" every intake stroke, whether the throttle is open or not. If there weren't, the chamber wouldn't fill with air/fuel, and your engine wouldn't make any power.
cliff notes: engine braking is fine.
YMMV.