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RB30/25DET

the-31

By the-31,
in Engines & Forced Induction

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the-31 Apprentice

the-31

Posted
Posted

Hello,

 

I looking at cylinder compression on my RB30/25DET, Its an older build in a R31.

 

The block was built around 2008 with 40thou over size forged pistons (can't remember brand, maybe acl) they were only 8.5compression ratio (can't remember why) Ran it as SOHC with around 200rwkw for a few years. Had Boost works in Adelaide complete the 25det head conversion and it was putting out 297rwkw for a year or so and then I had Jaustech give it another tune after I installed cams and they had it running just over 335rwkw which is has stayed around that since.

That was late 2013 

they did a compression test and it recorded:

1 - 125PSI

2 - 125PSI

3 - 120PSI

4- 125PSI

5- 130PSI

6 - 130PSI

the car has only been drivin 3 times in the past 3 years, only to the local drag strip. Shes due for a new clutch.(looking for recommendations on this also, looking at NPC 10" single plate?)

Last Easter at the drags it had clutch issues and over rev'd a few times, ive only just done a compression test (12 months later) and the results are:

1 - 100PSI

2 - 100PSI

3 - 96PSI

4 - 100PSI

5 - 105PSI

6 - 110PSI

as you can see its nearly exactly 20PSI drop over all 6 cylinders, there is also 7 years between these figures. 

I tested with all plugs out and throttle open, tested each cylinder twice with a brand new TOLEDO compression tester.

I did a search for info but maybe searched the wrong area. 

 

Is it too low? What other tests should I do? 

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Ben C34 Grand Master

Ben C34

Posted
Posted
4 minutes ago, the-31 said:

Hello,

 

I looking at cylinder compression on my RB30/25DET, Its an older build in a R31.

 

The block was built around 2008 with 40thou over size forged pistons (can't remember brand, maybe acl) they were only 8.5compression ratio (can't remember why) Ran it as SOHC with around 200rwkw for a few years. Had Boost works in Adelaide complete the 25det head conversion and it was putting out 297rwkw for a year or so and then I had Jaustech give it another tune after I installed cams and they had it running just over 335rwkw which is has stayed around that since.

That was late 2013 

they did a compression test and it recorded:

1 - 125PSI

2 - 125PSI

3 - 120PSI

4- 125PSI

5- 130PSI

6 - 130PSI

the car has only been drivin 3 times in the past 3 years, only to the local drag strip. Shes due for a new clutch.(looking for recommendations on this also, looking at NPC 10" single plate?)

Last Easter at the drags it had clutch issues and over rev'd a few times, ive only just done a compression test (12 months later) and the results are:

1 - 100PSI

2 - 100PSI

3 - 96PSI

4 - 100PSI

5 - 105PSI

6 - 110PSI

as you can see its nearly exactly 20PSI drop over all 6 cylinders, there is also 7 years between these figures. 

I tested with all plugs out and throttle open, tested each cylinder twice with a brand new TOLEDO compression tester.

I did a search for info but maybe searched the wrong area. 

 

Is it too low? What other tests should I do? 

Was the first test done after the cams went in or before?

Why did you do a comp test? Does something seem wrong?

 

Personally I would just ignore results and carry on.

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the-31 Apprentice

the-31

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Posted
1 hour ago, Ben C34 said:

Was the first test done after the cams went in or before?

Why did you do a comp test? Does something seem wrong?

 

Personally I would just ignore results and carry on.

After Cams were installed. No nothing majorly wrong at all just running abit rougher than usual, short window for testing as I had clutch issues so just parked it up. 

 

Going to start pulling the box to replace clutch but thinking I should pull engine and install rear head oil drain as I still have issues even with oil restrictor installed in gallery. I dont think I have enough room behind the head for the drain, 20mm between firewall and head.

20210218_194835.jpg

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