Hitting the rev limiter?
02-20-2003, 08:31 AM
#4
Registered User
iTrader: (17)
Join Date: Nov 2002
Location: Sterling Heights, MI
Posts: 993
Car Info: 2002 WRX OAKOS Edition
PimpWagon,
There are pros and cons to using spark cut or fuel cut to limit RPMs. First, either method can create harmonics within the engine that can be harmful, that is why it is not recommended to be bouncing off the rev limiter for exteded periods of time.
In theory, if you cut all the fuel going to a cylinder it would be ok because there isn't any fuel to mix with the O2 and combust , this wouldn't be a lean mixture it would be no mixture. On the other hand, cutting just the spark to a cylinder would leave a mixture of fuel and air to to be compressed on the compression stroke in the cylinder. If the right conditions occured this mixture could detonate even without the spark plug firing and cause engine damage.
-Dave
There are pros and cons to using spark cut or fuel cut to limit RPMs. First, either method can create harmonics within the engine that can be harmful, that is why it is not recommended to be bouncing off the rev limiter for exteded periods of time.
In theory, if you cut all the fuel going to a cylinder it would be ok because there isn't any fuel to mix with the O2 and combust , this wouldn't be a lean mixture it would be no mixture. On the other hand, cutting just the spark to a cylinder would leave a mixture of fuel and air to to be compressed on the compression stroke in the cylinder. If the right conditions occured this mixture could detonate even without the spark plug firing and cause engine damage.
-Dave
02-25-2003, 06:18 AM
#6
Guest
Posts: n/a
think of it like this........if you were driving down the road and you lost spark totally your car would roll to the side of the road, no problem would occur other than having no spark.....if there is no spark the cylinder will not fire....and as far as i know, there is no simply not enough compression for that cylinder to EVER fire on its own. it just wouldn't happen. let me know if i'm wrong, but i just don't get that.
rev limiters are meant as a safety to keep the engine from rolling into a bad rev zone......you could sit there all day and hammer on the rev limiter and it won't hurt anything!!!!
rev limiters are meant as a safety to keep the engine from rolling into a bad rev zone......you could sit there all day and hammer on the rev limiter and it won't hurt anything!!!!
02-25-2003, 08:33 AM
#7
In regards to detonation and pre-ignition:
If one isnt using the proper temp range spark plug, the electrode itself could cause pre-ignition... not detonation.
Pre-ignition and detonation are completely different things that occur under different yet similair circumstances and are commonly confused.
Detonation is when DURING ignition, the flame front burns quickly and wildly out of control in a quick "poof", if you will, rather than a slow controlled even burn. This quick "poof" is what can cause damage.... and the non-uniform nature of the flame front can cause some pretty severe hot spots... melting through pistons and rings in the process.
Pre-ignition occurs after fuel is injected, however before the plug is fired... so, in the compression stroke this is SEVERELY damaging phenomenon somewhat like hydralock.
Pre-ignition can also be caused from something as simple as carbon build up on the crown of the piston... this build up can absorb large amounts of heat reaching temp = to the a/f mixtures flash point. Poof! Bent or broken rod or blown head gasket... or both!
Anyway... with out the prescence of spark... pre-ignition can occur to the a/f mixture if something in the combustion chamber is hot enough to ignite it.
def
If one isnt using the proper temp range spark plug, the electrode itself could cause pre-ignition... not detonation.
Pre-ignition and detonation are completely different things that occur under different yet similair circumstances and are commonly confused.
Detonation is when DURING ignition, the flame front burns quickly and wildly out of control in a quick "poof", if you will, rather than a slow controlled even burn. This quick "poof" is what can cause damage.... and the non-uniform nature of the flame front can cause some pretty severe hot spots... melting through pistons and rings in the process.
Pre-ignition occurs after fuel is injected, however before the plug is fired... so, in the compression stroke this is SEVERELY damaging phenomenon somewhat like hydralock.
Pre-ignition can also be caused from something as simple as carbon build up on the crown of the piston... this build up can absorb large amounts of heat reaching temp = to the a/f mixtures flash point. Poof! Bent or broken rod or blown head gasket... or both!
Anyway... with out the prescence of spark... pre-ignition can occur to the a/f mixture if something in the combustion chamber is hot enough to ignite it.
def
02-25-2003, 08:40 AM
#8
rev limiters are meant as a safety to keep the engine from rolling into a bad rev zone......you could sit there all day and hammer on the rev limiter and it won't hurt anything!!!!
remember... you must hold it @ fuel cut ALL day, full throttle... (that is if you survive the 1st 10 minutes...)
Rev limiters are there to prevent over revving... mostly to protect the valvetrain and piston crowns... (ie- valves can and will slap the pistons if over-revving occurs... resulting in bent valves)
def
02-25-2003, 09:24 AM
#9
Registered User
iTrader: (17)
Join Date: Nov 2002
Location: Sterling Heights, MI
Posts: 993
Car Info: 2002 WRX OAKOS Edition
An engine doesn't have to be a diesel to experience detonation.
Just because the fuel is cut doesn't mean that no fuel will enter the combustion chamber. Fuel can be scanvenged from the walls of the intake manifold. If ignitition takes place this lean mixture can ignite and cause a "lean burn" or like I said earlier, under extreme circumstances this lean mixture could lead to detonation.
On the flip side, cutting the spark and not the fuel can lead to "fuel wash" or raw fuel building in the combustion chamber. This fuel can remove the oil film from the cylinder walls and increase cylinder and ring wear. The unburnt fuel can also get pushed past the rings and wind up in the crankcase and contaminate the oil. This unburnt fuel will also travel out the exhaust and can cause damage to the catalytic convertors.
Also, Crane indicates in the instructions for their DEC9000 Rev Limiter that the method used to control max revs "minimizes harmonics and vibrations that can stress engine and drivetrain parts". Not eliminates but minimizes these stressors.
All I'm saying is that running an engine at max boost past redline and intentionally introducing a misfire isn't the best path to maximum engine longevity & reliability.
-Dave
Just because the fuel is cut doesn't mean that no fuel will enter the combustion chamber. Fuel can be scanvenged from the walls of the intake manifold. If ignitition takes place this lean mixture can ignite and cause a "lean burn" or like I said earlier, under extreme circumstances this lean mixture could lead to detonation.
On the flip side, cutting the spark and not the fuel can lead to "fuel wash" or raw fuel building in the combustion chamber. This fuel can remove the oil film from the cylinder walls and increase cylinder and ring wear. The unburnt fuel can also get pushed past the rings and wind up in the crankcase and contaminate the oil. This unburnt fuel will also travel out the exhaust and can cause damage to the catalytic convertors.
Also, Crane indicates in the instructions for their DEC9000 Rev Limiter that the method used to control max revs "minimizes harmonics and vibrations that can stress engine and drivetrain parts". Not eliminates but minimizes these stressors.
All I'm saying is that running an engine at max boost past redline and intentionally introducing a misfire isn't the best path to maximum engine longevity & reliability.
-Dave
02-25-2003, 11:06 AM
#10
Guest
Posts: n/a
ok def, i got your point.....smartazz i'm just making sure everyone understands it's not the end of the world if you tap the rev limiter. for us mere mortals that once and a while run our shifts long by accident.....it'll save us from blowing our crap up.....HAHAHA
i'm just making sure everyone understands it's not the end of the world if you tap the rev limiter. for us mere mortals that once and a while run our shifts long by accident.....it'll save us from blowing our crap up.....HAHAHA
02-25-2003, 11:25 AM
#11
Registered User
iTrader: (17)
Join Date: Nov 2002
Location: Sterling Heights, MI
Posts: 993
Car Info: 2002 WRX OAKOS Edition
I never said I haven't or don't hit the rev limiter from time to time. 
It happens, but it should be avoided if at all possible.
This reminds me of the time one of the car mags (SCC?) built a car and let Paul Tracy do the track work. Tracy apparently thought the fastest way through one of the turns was to bounce the car off the rev limiter. Needless to say the engine didn't last long before it expired.
Take care!
-Dave
It happens, but it should be avoided if at all possible.This reminds me of the time one of the car mags (SCC?) built a car and let Paul Tracy do the track work. Tracy apparently thought the fastest way through one of the turns was to bounce the car off the rev limiter. Needless to say the engine didn't last long before it expired.
Take care!
-Dave
02-26-2003, 03:59 AM
#12
ok def, i got your point.....smartazz i'm just making sure everyone understands it's not the end of the world if you tap the rev limiter. for us mere mortals that once and a while run our shifts long by accident.....it'll save us from blowing our crap up.....HAHAHA
def
02-28-2003, 10:03 AM
#14
Registered User
iTrader: (7)
Join Date: Dec 2002
Location: Sacramento CA
Posts: 10,029
Car Info: 02 Impreza WRX sedan
Originally posted by spclagenth
...does anybody know how the rev limit of an engine is determined?
I'll take a guess. Engineers know the rated design limits of drivetrain parts, taking into account production line tolerances (a blueprinted engine can safely rev higher and produce slightly more power than a production engine). They tune an engine for a given output within the design limits of the engine (and driveline) components then set the rev limit at a point just beyond the power peak. That's why we don't see redlines of 10,000 r.p.m. in a factory EJ20 rated 227 h.p. at 6000 r.p.m.
--
^ww^
"…axles of evil…" - George W. Bush
...does anybody know how the rev limit of an engine is determined?
I'll take a guess. Engineers know the rated design limits of drivetrain parts, taking into account production line tolerances (a blueprinted engine can safely rev higher and produce slightly more power than a production engine). They tune an engine for a given output within the design limits of the engine (and driveline) components then set the rev limit at a point just beyond the power peak. That's why we don't see redlines of 10,000 r.p.m. in a factory EJ20 rated 227 h.p. at 6000 r.p.m.
--
^ww^
"…axles of evil…" - George W. Bush
03-01-2003, 09:05 PM
#15
Guest
Posts: n/a
Not trying to steal your thread, but I have a related question.
Redline in the WRX is 7,000, but my rev limiter doesn't cut in until 7,500. This usually only occurs when at the track, and I am unable to keep an eye on the tach. But it has happened a few times......
Should I be worried? I expect to install some type of shift light in my peripheral vision before track season starts. It's only 500 RPM, but it can't be good... Your thoughts/opinions?
FYI, no engine/ECU mods.
MSG
Redline in the WRX is 7,000, but my rev limiter doesn't cut in until 7,500. This usually only occurs when at the track, and I am unable to keep an eye on the tach. But it has happened a few times......
Should I be worried? I expect to install some type of shift light in my peripheral vision before track season starts. It's only 500 RPM, but it can't be good... Your thoughts/opinions?
FYI, no engine/ECU mods.
MSG