Cold compression test after rebuild... should I be worried?

[MidnightR32]

Welp, this is where my compression lands after my rebuild. Thoughts? I have ~6 hours on the motor. 

[MidnightR32]

This is for an RB20DET. Sorry for not including that. 

[Duncan]

How have you run it in / what has been done in those 6 hours? Also, you need to do the test hot.

You have to load the new motor pretty hard to bed the rings in quickly and properly, doing it on the dyno is best and only takes an hour-ish

The other question is.....what were the measured clearances post machining and do they match the specs for the piston and rings

[Murray_Calavera]

1 hour ago, Duncan said:

what were the measured clearances post machining

$50 says no measurements where done post machine shop. 

[MidnightR32]

1 hour ago, Murray_Calavera said:

$50 says no measurements where done post machine shop. 

Nope and I only have myself to blame here. I hired the shop and didn't do my due diligence.

[MidnightR32]

3 hours ago, Duncan said:

How have you run it in / what has been done in those 6 hours? Also, you need to do the test hot.

You have to load the new motor pretty hard to bed the rings in quickly and properly, doing it on the dyno is best and only takes an hour-ish

The other question is.....what were the measured clearances post machining and do they match the specs for the piston and rings

 

The first hour or so was not on a dyno, but 2 of them were on a dyno and it was pushed hard enough to get tuned. 

[Duncan]

My suggestion would be, even with 6 hours on the engine already, take it back to a dyno and give it a good high load run in, then test again when warm.

I've learned through experience that fuel glazing bores is very easy on a new engine but both times it was saved by pushing it harder. You just can't get that sort of good load on the road

[MidnightR32]

21 hours ago, Duncan said:

My suggestion would be, even with 6 hours on the engine already, take it back to a dyno and give it a good high load run in, then test again when warm.

I've learned through experience that fuel glazing bores is very easy on a new engine but both times it was saved by pushing it harder. You just can't get that sort of good load on the road

Thanks for the suggestion of being on it. As the car has a break in tune, I went and hit 10psi a few times while driving, drove it kind of like a jack ass for ~1.5 hours, did another test, and all 6 cylinders were at/just below 125PSI. I took photos of it this time as I only my memory to remember seeing 150psi.

As for the ones that were low, my battery is kind of shot and dies pretty quickly if not charged, so I'm going to assume I just didn't let them build up enough compression or the battery didn't have the power.

The 150psi for #4 is definitely odd, but having no photo proof, I'm just going to make note of it. I'll retest it again when the car is cold to see where things are, but everything is insync now so 🤷‍♀️

[GTSBoy]

5 minutes ago, MidnightR32 said:

I'll retest it again when the car is cold to see where things are, but everything is insync now so

No. Test it warm. No-one cares about cold compression results.

Have another battery handy that you can jumper on to make sure you can complete the test.

[MidnightR32]

Sorry if it wasn't clear, I retested right after getting home and the car was still hot. 

[Duncan]

100% there is no point doing a compression test without an excellent battery.

Per what GTSBoy said just hook up a good battery with big jumper leads, running the starter for the compression test takes a heap of juice and is likely your main problem in the first test.

[GTSBoy]

44 minutes ago, MidnightR32 said:

Sorry if it wasn't clear, I retested right after getting home and the car was still hot. 

Yeah, I was responding to this....

56 minutes ago, MidnightR32 said:

I'll retest it again when the car is cold

 

[joshuaho96]

On 22/06/2025 at 3:50 PM, MidnightR32 said:

Sorry if it wasn't clear, I retested right after getting home and the car was still hot. 

Not only should you be testing compression with a strong battery, you should be putting a charger on it. These cars have pretty small batteries and it's possible to discharge them enough that by the last cylinder the starter isn't turning as fast. 

[GTSBoy]

24 minutes ago, joshuaho96 said:

These cars have pretty small batteries

Yeah, nah. Anyone not putting the absolutely largest battery that will physically fit (ie, NS70 X) is not thinking straight. the NS70 X is close on double the capacity of the "stock" unit.

[MBS206]

47 minutes ago, joshuaho96 said:

Not only should you be testing compression with a strong battery, you should be putting a charger on it. These cars have pretty small batteries and it's possible to discharge them enough that by the last cylinder the starter isn't turning as fast. 

And most chargers that most DIY people own would put bugger all power back into the battery between cylinders.
I've always done it with jumper packs/second battery connected. And that's on any car.

 

22 minutes ago, GTSBoy said:

Yeah, nah. Anyone not putting the absolutely largest battery that will physically fit (ie, NS70 X) is not thinking straight. the NS70 X is close on double the capacity of the "stock" unit.

Because race car, and wanting the lightness.  At least in an R33, putting a bigger battery in can at least help shift the weight balance backwards