Finally, the "cause" of engine noise at cold start up explained
10-15-2007, 10:15 PM
#1
Racer
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Finally, the "cause" of engine noise at cold start up explained
I spoked to a tuner from ptuning.com regarding engine knock at first startup. What he told me makes sense:
The GT3 pistons are forged. A typical cast piston has a skirt clearance of approximately .0007 to .0009. A forged piston has a skirt clearance of .005 to .007. The forged piston has 10 times more slop in the cylinder (because of the dramatic expansion of cast vs. forged).
So when you start up the car, the piston slop is the sound you hear. When the pistons get to operating temperature, the pistons expand and the tolerances between the skirt and the wall is significantly reduced to normal clearances. Hence the sound goes away after a minute after start.
Of course, other mechanical parts also have a play in the cold startup noise (i.e., chain tensioner, lack of oil on the cams, etc...) but those can be explained.
The GT3 pistons are forged. A typical cast piston has a skirt clearance of approximately .0007 to .0009. A forged piston has a skirt clearance of .005 to .007. The forged piston has 10 times more slop in the cylinder (because of the dramatic expansion of cast vs. forged).
So when you start up the car, the piston slop is the sound you hear. When the pistons get to operating temperature, the pistons expand and the tolerances between the skirt and the wall is significantly reduced to normal clearances. Hence the sound goes away after a minute after start.
Of course, other mechanical parts also have a play in the cold startup noise (i.e., chain tensioner, lack of oil on the cams, etc...) but those can be explained.
Last edited by NSXTC; 10-16-2007 at 12:40 AM.
10-16-2007, 06:35 AM
#5
Racer
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Piston slap was a common problem on some older Imprezas (also boxer engine), but also on Ford Cosworth engines (with forged pistons).
I don´t think I got piston slap on my engine yet (5500 km so far), so can´t comment on this.
I do have, however, another type of clicking sound at idle rpm (engine hot, not after start). It is better heard inside the car than outside and has a frequency of about 2 sounds per second. I can best compare it to a sound that would be made by a lose piston pin. It disappears once revs get above 1000 rpm.
Does anybody know what this could be?
I don´t think I got piston slap on my engine yet (5500 km so far), so can´t comment on this.
I do have, however, another type of clicking sound at idle rpm (engine hot, not after start). It is better heard inside the car than outside and has a frequency of about 2 sounds per second. I can best compare it to a sound that would be made by a lose piston pin. It disappears once revs get above 1000 rpm.
Does anybody know what this could be?
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10-16-2007, 12:53 PM
#9
Racer
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I do have, however, another type of clicking sound at idle rpm (engine hot, not after start). It is better heard inside the car than outside and has a frequency of about 2 sounds per second. I can best compare it to a sound that would be made by a lose piston pin. It disappears once revs get above 1000 rpm.
10-16-2007, 08:54 PM
#11
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I have the ticking too. You guys made me feel better.
I dont buy the piston slap though. The noised goes away too quick for a come up to
temp solution. In my car and my 996 GT3 it goes away in seconds. Also if I start the car
the next day the sound is not there. The cooling would be complete. I still think its a function of
oil distribution. The oil slowly drains from the motor. over several days it completely vacates the upper portions of the motor. THen on restart all the oil quickly gets pumped to all parts in seconds and the noise goes away. The oil takes days to completely drain so if you start it the next day the noise will not
reoccur. if You wait 2 days or more the noise comes back until the oil is redistributed within seconds.
thats my story and I am sticking to it.
BTW I also have oil smoke on start up after a few days of not running
I dont buy the piston slap though. The noised goes away too quick for a come up to
temp solution. In my car and my 996 GT3 it goes away in seconds. Also if I start the car
the next day the sound is not there. The cooling would be complete. I still think its a function of
oil distribution. The oil slowly drains from the motor. over several days it completely vacates the upper portions of the motor. THen on restart all the oil quickly gets pumped to all parts in seconds and the noise goes away. The oil takes days to completely drain so if you start it the next day the noise will not
reoccur. if You wait 2 days or more the noise comes back until the oil is redistributed within seconds.
thats my story and I am sticking to it.
BTW I also have oil smoke on start up after a few days of not running
10-16-2007, 09:37 PM
#12
mine varies on start up smoke... sometimes its a humongous puff of smoke and other times very slight amount of smoke... its an adventure at times and others a down right embarrassment...
Smoke on start up depends on the positioning of the oil ring on the piston... so i have been told.
Smoke on start up depends on the positioning of the oil ring on the piston... so i have been told.
10-16-2007, 10:18 PM
#13
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Don't know the cause, but if you just idle for half a minute or so before turning the engine off (as the OM recommends after a spirited drive), it won't happen as much.
btw, it's a good time to routinely check the oil during those few seconds, since there's no friggin' dipstick!
btw, it's a good time to routinely check the oil during those few seconds, since there's no friggin' dipstick!
10-17-2007, 04:03 AM
#15
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I also have smoke on start-up, sometimes less, sometimes more. There is also a half-second drill-bit-in-concrete sound on start up, probably because oil is missing somewhere. Both things differ, however, depending on whether the car has been standing for one day, or two days.
There are lots of other sounds as well. The gearbox whine when idling along, the clutch bearing or gears with clutch engaged in neutral, the vibration from the inside rear-view mirror housing when the temperature and humidity is just right, the vibration from the driver´s side door opener, rumbling of brakes at a certain temperature...
In my mind, all part of the whole experience.
There are lots of other sounds as well. The gearbox whine when idling along, the clutch bearing or gears with clutch engaged in neutral, the vibration from the inside rear-view mirror housing when the temperature and humidity is just right, the vibration from the driver´s side door opener, rumbling of brakes at a certain temperature...
In my mind, all part of the whole experience.