bad rings? - Zilvia.net Forums | Nissan 240SX (Silvia) and Z (Fairlady) Car Forum

oem aero · 07-15-2009, 10:45 AM

hey guys,
my car runs great, till it gets hot then it loses all its power. it also makes like a whooshing sound.
it fills up the catch can real fast too.

i did compression test, compression is the same on all four and it goes up about 10-15 psi with a wet test on each cylinder.

this is a built motor , did the rings just not seat properly, or could it be something else?

speedfiend · 07-15-2009, 11:40 AM

What exactly was the compression on all 4 cylinders. Even though it is consistent all the way across it doesnt mean you have good compression. Also if it is fill up the catch can really fast then you are experience a lot of blowby which would indicate that the rings arent seating. By built motor do you mean that it is stock bore with forged internals or did you have it bored over? If it was bored over then it is a good possibility that they overbored it and set the clearances way to loose on the cylinder to piston wall. Who did the motor build and how long has it been running?
Cameron

oem aero · 07-15-2009, 01:58 PM

it's been running for a while i bought the car from another guy who built the motor

its 135 on all 4

whats normal psi to gain with a cap full of oil?

speedfiend · 07-15-2009, 02:11 PM

That seems like a little low compression to me. I am not sure of the exact amount your supposed to gain with a cap full of oil. Are you running a really aggressive cam setup? That would cause the compression to be a bit lower then normal. To me it seems that who ever bored the block out set the clearances on the high side and when your motor warms up the clearances get looser and are causing your large amount of blowby. Are you getting any blue smoke out of the exhaust when its warmed up?

oem aero · 07-15-2009, 05:35 PM

it doesnt smoke at all, its just slow and has lots of blow by!

i do have cams which i know will lower the compression but is it normal to gain 15psi with a wet test on aftermarket pistons?

speedfiend · 07-15-2009, 08:43 PM

Like I said earlier I am not 100% sure about how much compression you should gain with a cap full of oil. The piston itself will not dictate how much compression is gained or lost when doing that wet test. It is your ring seal that is going to determine how much compression is gained from adding oil to the cylinder. I would suspect that the less the rings seal when dry the more they would gain if oil is added. If you have perfect ring seal then adding oil should not increase the compression much at all. I hope that you understand what im saying. If not let me know and I will explain it another way.
Cameron

aNskY · 07-16-2009, 08:13 AM

If you have dished pistons, the oil may just sit on top of the piston.. thus decreasing the volume in your combustion chamber effectively raising your compression.

sr's are known for lots of blowby.. or maybe your pcv valve is getting stuck open?

*fliprayzin240sx* · 07-16-2009, 08:35 AM

135 is still within spec...minimum FSM spec is 125.

speedfiend · 07-16-2009, 09:34 AM

Quote:

Originally Posted by aNskY

If you have dished pistons, the oil may just sit on top of the piston.. thus decreasing the volume in your combustion chamber effectively raising your compression.

sr's are known for lots of blowby.. or maybe your pcv valve is getting stuck open?

That is a good point on the piston it completely slipped my mind to think og it that way. As for the blow by yeah they do have a bit but it shouldn't fill up the catch can as quick as his is. I was actually fixing to mention to check your pcv valve. Just pull it off and blow through it. You should only be able to blow through it in one direction. If you can blow through it on both ends then it could possibly be just your pcv causing the problem.
Cameron

07-15-2009, 10:45 AM	#1
oem aero Join Date: Jun 2009 Location: NJ Age: 34 Posts: 5 Trader Rating: (0) Feedback Score: 0 reviews	bad rings? hey guys, my car runs great, till it gets hot then it loses all its power. it also makes like a whooshing sound. it fills up the catch can real fast too. i did compression test, compression is the same on all four and it goes up about 10-15 psi with a wet test on each cylinder. this is a built motor , did the rings just not seat properly, or could it be something else?

07-16-2009, 08:35 AM	#8
fliprayzin240sx Man w/ CTSV & a Car Seat Join Date: Aug 2002 Location: Tampa Posts: 17,998 Trader Rating: (19) Rides Gallery: fliprayzin240sx Feedback Score: 19 reviews	135 is still within spec...minimum FSM spec is 125. __________________ If you think Zilvia has too many assholes... CLICK HERE!!!

07-15-2009, 11:40 AM	#2
speedfiend Zilvia FREAK! Join Date: Feb 2002 Location: Destin, Fl./Houston, Tx. Age: 40 Posts: 1,206 Trader Rating: (68) Feedback Score: 68 reviews	What exactly was the compression on all 4 cylinders. Even though it is consistent all the way across it doesnt mean you have good compression. Also if it is fill up the catch can really fast then you are experience a lot of blowby which would indicate that the rings arent seating. By built motor do you mean that it is stock bore with forged internals or did you have it bored over? If it was bored over then it is a good possibility that they overbored it and set the clearances way to loose on the cylinder to piston wall. Who did the motor build and how long has it been running? Cameron

07-15-2009, 01:58 PM	#3
oem aero Join Date: Jun 2009 Location: NJ Age: 34 Posts: 5 Trader Rating: (0) Feedback Score: 0 reviews	it's been running for a while i bought the car from another guy who built the motor its 135 on all 4 whats normal psi to gain with a cap full of oil?

07-15-2009, 02:11 PM	#4
speedfiend Zilvia FREAK! Join Date: Feb 2002 Location: Destin, Fl./Houston, Tx. Age: 40 Posts: 1,206 Trader Rating: (68) Feedback Score: 68 reviews	That seems like a little low compression to me. I am not sure of the exact amount your supposed to gain with a cap full of oil. Are you running a really aggressive cam setup? That would cause the compression to be a bit lower then normal. To me it seems that who ever bored the block out set the clearances on the high side and when your motor warms up the clearances get looser and are causing your large amount of blowby. Are you getting any blue smoke out of the exhaust when its warmed up?

07-15-2009, 05:35 PM	#5
oem aero Join Date: Jun 2009 Location: NJ Age: 34 Posts: 5 Trader Rating: (0) Feedback Score: 0 reviews	it doesnt smoke at all, its just slow and has lots of blow by! i do have cams which i know will lower the compression but is it normal to gain 15psi with a wet test on aftermarket pistons?

07-15-2009, 08:43 PM	#6
speedfiend Zilvia FREAK! Join Date: Feb 2002 Location: Destin, Fl./Houston, Tx. Age: 40 Posts: 1,206 Trader Rating: (68) Feedback Score: 68 reviews	Like I said earlier I am not 100% sure about how much compression you should gain with a cap full of oil. The piston itself will not dictate how much compression is gained or lost when doing that wet test. It is your ring seal that is going to determine how much compression is gained from adding oil to the cylinder. I would suspect that the less the rings seal when dry the more they would gain if oil is added. If you have perfect ring seal then adding oil should not increase the compression much at all. I hope that you understand what im saying. If not let me know and I will explain it another way. Cameron

07-16-2009, 08:13 AM	#7
aNskY Nissanaholic! Join Date: Apr 2005 Location: NY Age: 41 Posts: 1,604 Trader Rating: (10) Feedback Score: 10 reviews	If you have dished pistons, the oil may just sit on top of the piston.. thus decreasing the volume in your combustion chamber effectively raising your compression. sr's are known for lots of blowby.. or maybe your pcv valve is getting stuck open?