Low compression after rebuild

[S13NismoStyle]

So, 2 months after my sr20det spun a bearing a decided to rebuild, bigger and better.

Long story short, I assembled my new motor(after having it rehoned due to slight rust after machining) with stock rods, CP 86.5mm 8.5:1 pistons and rings gapped to the manufactures spec for a turbo car, 1.1mm cosworth hg, arp studs, and the head had .003 decked off to true it up, valve job, all new oem seals, studs, bolts, timing set, oil pump...you get the point.

I got the motor running, had a slight issue with too much fuel to begin with. Got that corrected, and ran her at 1500-2000 rpm's for 20 mins with conventional oil.

Changed the oil, put 500mi on it, taking it easy, changed the oil again. I then put 250 more easy miles(taking it up to 4500 or so but no boost). After 750 miles started boosting it, but nothing crazy.

The motor has 1200miles on it now and compression is 140psi in all 4 cylinders. Plus tons of blow by. A couple of wot runs at 15psi and i will fill half my catchcan.

I am thinking that my rings have not seated. I figured that they should have seated by now and there is probably a something wrong. What's the chances that they will still be able to seat?

If the motor is garbage the car is getting parted out and I'm getting a 350z. lol

http://i201.photobucket.com/albums/aa305/christopher3426/DSCF9238.jpg

[Killa_D]

wow that sucks man and yeah it really does sound like the rings haven't seated yet. Also who assembled your motor and was it done right and by the way your motor its very clean. If i was you i would just replace the rings again and see if the second time around will do it. If not sell it and buy a different car lol.

[nismolvr]

do a wet compression check, put a small amount of oil in each cyl.throtle tons of blow by followed by low compression ,sign of bad rings, or no oil to oil rings there fore not making seal.

[S13NismoStyle]

Ill try the wet compression check. If it is in fact the rings, I will try to put some new rings in it and have it re-honed.

Should i get a new cosworth head gasket? How about new main studs and rod studs? I just replaced those. My arp studs should be ok to use one more time, right?

[240sx123$]

Measure them. Some bolts are designed to stretch when torqued. There should be a maximum length specification- if they are under the max length, they're ok to use again. Do not reuse the head gasket. Head gaskets are good for one torque down only.

[S13NismoStyle]

Yeah thats kind of what i was thinking about the head gasket. I wasnt totally sure with it being a MLS headgasket though.

Thanks. Hopefully the second time around will go much better than the first.

[nismoman]

140psi sounds like good compression to me for 8.5:1 pistons I don't the blow by is from the rings.

[S13NismoStyle]

140psi sounds like good compression to me for 8.5:1 pistons I don't the blow by is from the rings.

140 is low. I am running stock compression pistons. My motor before it spun the bearing had 155psi in all 4 cylinders with over 100k miles on it.

I've read that when rings seat you can pick up anywhere from 10-20psi per cylinder which would make sense.

I spoke with a local guy that has built a couple of high power sr's and he said that he had a very similar problem with his cp rings. He told me that his hone was not 'aggressive' enough (for lack of better word) to properly wear the chromoly rings.

Has anyone heard of this before? Cp was pretty specific about their honing procedure. Unfortunately for me, the machine shop I took my block to did less than a stellar job with everything. So i wouldn't be surprised if that was part of the problem.

I want to run a leakdown test and use a boroscope to check out the cylinders, before i rip it all apart.

[let5l1de]

I'm sure ring gap clocking, ring installation order, installed ring gap specs and piston to wall clearance specs were all reviewed prior to final assembly? Proper break in procedures with a fresh engine is always a plus.

Proper Break-In > Hard Break-In
(unless you have unlimited funds and downtime)

grp
~mario

[240sx123$]

If youve tried a proper break in procedure and youre afraid the rings arent seating, as a LAST RESORT before pulling the motor apart, try a hard break in. Lug the car up a hill, out of boost, in as high of a gear as possible without the car shaking. I used to race motocross, and we used to do this to speed up the break in procedure on our bikes after putting in a new piston. Obviously heat cycling the engine is the best way to seat the rings, but if theyre washing out, a hard break in may be in order. If it doesnt work, youre no worse off than you are now.

[S13NismoStyle]

I'm sure ring gap clocking, ring installation order, installed ring gap specs and piston to wall clearance specs were all reviewed prior to final assembly? Proper break in procedures with a fresh engine is always a plus.

Proper Break-In > Hard Break-In
(unless you have unlimited funds and downtime)

grp
~mario

Yes, all of those were verified prior to assembly, by me, wit the exception to piston to wall clearances. I just had to take my machine shop's word for it. As far as I know, I broke the engine in properly.

I'm hoping to wet compression test it soon and check it out with a borescope, but I have been pretty busy. I honestly have no clue as to why the rings wouldn't seat.

If everything looks ok, I plan on putting another 1000 miles on it before I make a decision on whether or not to keep it. Maybe I'll get lucky lol.


If youve tried a proper break in procedure and youre afraid the rings arent seating, as a LAST RESORT before pulling the motor apart, try a hard break in. Lug the car up a hill, out of boost, in as high of a gear as possible without the car shaking. I used to race motocross, and we used to do this to speed up the break in procedure on our bikes after putting in a new piston. Obviously heat cycling the engine is the best way to seat the rings, but if theyre washing out, a hard break in may be in order. If it doesnt work, youre no worse off than you are now.

I will probably try that and cross my fingers. I just spent way too much money to have to go through and do this again.

[jspaeth]

Measure them. Some bolts are designed to stretch when torqued. There should be a maximum length specification- if they are under the max length, they're ok to use again. Do not reuse the head gasket. Head gaskets are good for one torque down only.

Yeah thats kind of what i was thinking about the head gasket. I wasnt totally sure with it being a MLS headgasket though.

Thanks. Hopefully the second time around will go much better than the first.

I was under the impression that quality MLS headgaskets like Apexi or Cosworth could be reused....

Someone else should chime on this and save you the cost of a new headgasket.

[240sxvaj]

hummm.......... my compression is reading 140 across too and im running 8:8:1 on my kade. arias pistions. i have about 30k on my motor rite now and it feels good to me. no lost of power.

[let5l1de]

I just thought of another possibility. Improper compression check procedure? I know its like 101 but it happens.....

disable injectors...
pull all spark plugs...
well charged battery....
throttle body in full open position.....
blahblahblah......

~m
~m

[S13NismoStyle]

I just thought of another possibility. Improper compression check procedure? I know its like 101 but it happens.....

disable injectors...
pull all spark plugs...
well charged battery....
throttle body in full open position.....
blahblahblah......

~m
~m

It was performed correctly with wot, engine brought up to temp, etc. Verified with 2 different gauges.

Aside from the low numbers, the blow-by is very significant. I don't know what else would be causing the huge amounts of blow-by besides the rings.

Any know any other way to tell if the rings have seated? It seems like that is something that does not happen very often.

I had thought of the possibility of an issue with the headgasket, but it's too wierd that all the cylinders have the exact same compression. Who knows, thanks though.

[j87w]

I had a similar problem, heres a link to my thread if you would like to read through it. If not short story is leakdown test showed car had issues, put another set of rings with a different hone and joe gibbs break in 30 oil and its ran great since. I would also do a leakdown test if you have not yet, they will help pinpoint an issue.

http://zilvia.net/f/engine-tech/268166-need-some-help-engine-diagnosis.html

[importdude]

did u load the motor properly and let it decel in vac thoese 1200 miles?

hopefully your gaps are correct and placement is correct

[240sx123$]

I was under the impression that quality MLS headgaskets like Apexi or Cosworth could be reused....

Someone else should chime on this and save you the cost of a new headgasket.

You may be right- but Ive definitely seen people reuse them only to later find out that the head gasket is leaking. Ive seen it here on zilvia and ive seen it firsthand. I wouldnt reuse any head gasket ever, even if the manufacturer says its ok. If the $100 saved is worth the extra 4 hours of work when it does leak, by all means, reuse it. My time is more valuable than that, and I dont like to do things twice. But thats just me. :)

[S13NismoStyle]

Yeah, it was broke in properly. It saw a lot of vacuum.

I will probably re-use the headgasket, just to make sure that re-ring/honing fixes the problem. I would hate to spend another $150 on headgasket and have problems again with the rings. I have no issue changing it out later.

Its been way too cold here to mess with it at all. Im just going to drive it a little longer and check it out when it warms up a bit.

I will be performing a leakdown test as well. I have heard from a couple people that they have had issues with their cp piston rings seating. So atleast I am not the only one.

[Davidna2fi]

400 grit stone hone used, or a ball hone???? It does affect ring seal BTW. Post up what hone you used. Did a refresh on a factory SR20 using Stone Hone with FSM guidelines, and got 165+ Psi in all cylinders stock cams, head and block had been resurfaced so Psi would be slightly higher.

[tbssgto]

I know this thread is a little bit old but I was wondering what your results ever ended up being? I have almost the identical build and am having similar problems after just putting the engine back in. Any info would be greatly appreciated!