Rb25 fresh rebuild low compression

[dee242]

Hey I have a rb25 the short block was built and assembled by a shop. The main mods are cp 9.01 pistons 1.3mm head gasket arp head studs I took it to my tuner and what he said was he  Did a compression test 
90-90-100-85-80-80
Cold.

cross hatch don’t look too good on cylinder walls and there was Oil trace  all cylinders and 1st cylinder is at worst

Alot of detonation mark on spark plug 4 to 6

motor only has 7000k on it

it doesnt burn oil never overheated kinda don’t know where to go from here

Edited March 25, 2019 by dee242

[iruvyouskyrine]

Its f**ked. Compression should never be that low unless its done 1,000,000km.

Rebuild time

[tridentt150v]

Do it hot first, but not looking good.

[GTSBoy]

Rings never got bedded is most likely reason.

[dee242]

9 minutes ago, GTSBoy said:

Rings never got bedded is most likely reason.

How would that happen

[Dose Pipe Sutututu]

Strap it to a dyno, load hold it at 4k rpm for a good 30 mins

Dump the oil and pop in fresh oil see how it goes, if not rebuild.

I laugh if the "engine" builder didn't clock the rings LOL

[GTSBoy]

39 minutes ago, dee242 said:

How would that happen

How would the rings not get bedded?  It's easy to glaze the bores if you don't know what you're doing.  You have to get an engine up and running very quickly, then have at least a rough enough tune in it to be able to run it at varying revs and load for the first XX minutes.  If you let it sit and idle, like a lot of people have done, then you will glaze the bores and never get the rings to bed in.  From there it is low comp and oil consumption to eternity.

The fix back in the days of rebuilding red motors was a spoonful of Bon-Ami sprinkled down the carby.  Ahh.  The good old days.

[dee242]

Excuse me while I kick myself in the a**

[tridentt150v]

The Bon Ami/Ajax...heard of it and people swear by it, but I've never needed to try it.  They reckoned you used it then flogged the crap out of the motor on a good straight road then watch the smoke out the exhaust until it cleared.

You could try a more commercially available product these days:

https://costeffective.com.au/product/ftc-decarbonizer/

There are others, this is only one of them.....and again, I only heard of them, no idea if they work.

[dee242]

Before I pull this thing apart would the timing affect this when the motor was built I put in cp 9.0.1 pistons and used a .051” Cometic headgasket  and had the head decked then set the timing back to factory spec

[Ben C34]

cam timing does impact compression test readings. i would definitley be doing the test hot to see how it goes.

just thrash it, unless there is an actual problem. if the comp test is low but all seems well whats the problem?
 

[dee242]

I will try a hot Test what type of numbers should I see 

[dee242]

Hot test came up with 

1 145

2 145

3 145

4 140

5 135

6 140

[Ben C34]

11 minutes ago, dee242 said:

Hot test came up with 

1 145

2 145

3 145

4 140

5 135

6 140

That's good Enough for me. 

[dee242]

This was hot no oil in cylinders throttle plate closed and no charger hooked up to car my starter cranks pretty slow

Edited March 28, 2019 by dee242

[tridentt150v]

Nothing wrong with those numbers....doing a comp test cold is always 'a trap for young players' as the saying goes.  Forgies rattle a bit when cold anyway. 

Enjoy your Skyline mate!!!!

[dee242]

Only other issue now is I get a vibration in neutral when I rev the car after installing a poly transmission mount im not sure if it’s there under load cause the car is waiting to be tuned. But I have changed the harmonic balencer and I don’t think it’s the clutch/ flywheel being loose as  the engine mounts are stock but in good shape

[Lucky Phil]

On 3/26/2019 at 9:39 AM, tridentt150v said:

Do it hot first, but not looking good.

agreed - do the comp test once warmed up. like, proper warm. go for a decent drive first. and take all the plugs out and full throttle for each cylinder run. a rb25 cold comp test throttle closed might give you 130, 135 etc when warm and open throttle it will be more like 155 each pot. but yeah those low numbers look pretty baaaddd....

[Lucky Phil]

On 3/29/2019 at 8:12 AM, dee242 said:

Hot test came up with 

1 145

2 145

3 145

4 140

5 135

6 140

argh i piped up too soon... apologies

the factory manual specifies that lowest acceptable comp is 130 psi each cylinder, with no more than 12% difference from highest to lowest reported value.... so you are within that. happy days  ?