Compression for RB25Det

[maxim]

Hey guys,

What is a good compression in psi for an RB25det?

Reason I just had mine tested and it ranged from 130-140psi. Seems low?

Thanks

[Chris32]

A mate of mine had his tested, the motor was a good shape and it was 148psi all round

I would say its ok

Anyone else?

[maxim]

Hey chris, thanks for your reply. yeah I think i remember you saying that in another thread. Just wanted to be 100% sure though. cheers

[Chris32]

I'd say as long as there all even, should be fine

Could be the gauge is a little old or somthing, never know really

But they are all even, so I'd give it a thumbs up

if it was 140,138,65,142,70,140 or something like that I'd be starting to worry, but they aren't, so nothing to worry about I reckon

[Sydneykid]

Hi maxim, there are number of factors other than condition of the internals that affect the compression test readings. The most obvious is battery condition, if the engine turns over sowly you will get a lower reading. Did you do it with 5 plugs out and the gauge fitting in the other spark plug hole? That will give a higher reading than 5 plugs in as the engine will turn over slower. Another is cam shaft timing, if you have a set of cams, or adjusted the base timing with adj pulleys, you can get a lower reading as some compression gets out past the open valves.

This is why I prefer a leak down test, much more accurate and repeatable.

Hope that helps

[KamikazeR33]

how is a leak down test done?

[maxim]

  Sydneykid said:

Hi maxim, there are number of factors other than condition of the internals that affect the compression test readings.  The most obvious is battery condition, if the engine turns over  sowly you will get a lower reading.  Did you do it with 5 plugs out and the gauge fitting in the other spark plug hole?  That will give a higher reading than 5 plugs in as the engine will turn over slower.  Another is cam shaft timing, if you have a set of cams, or adjusted the base timing with adj pulleys, you can get a lower reading as some compression gets out past the open valves.

This is why I prefer a leak down test, much more accurate and repeatable.

Hope that helps

Hey Sydneykid, thanks for your input.

A mechanic friend of mine conducted the test while I was not present today. The engine is otherwise stock, with a new battery (Bosch unit) installed only a week ago.

Just one more quick question if thats cool, so far the car (91 180sx with rb25 conversion) has been performing pretty poorly. Only mods are a fmic (flow roughly 400hp size) 3" system from back of dump (dump standard) and ~600hp Bosch intake pump. New plantinum sparks plugs gapped to .8mm were added last week also. The engine also has spitfire coils which came with it.

The best 0-100 v stopwatch (g-tech is coming) is 6.5s which is pretty average from about 10 runs but was consistant. Thats with a variety of launch techniques and I don't think im a shit driver.

The fuel comsumption is terrible. Last top up of 20l lasted around 100km's. My mechanic was trying to determine if the knock sensor was faulty, thus retarding timing and increasing AFR when he found the std computer did not have the LED (?) lights (not 100% sure on this) to read / extract codes from.

Just wondering if you simply name a couple of things to look at next. The car's power delivery seems very linear. I raced a friend in a CA 180sx (his std bar exhaust, std boost) and beat him by only 2 lenghts to ~140 so any help would be great!

Appreciate any comments, thanks.

[Sydneykid]

Hi maxim, I have never seen an RB25DET computer with LED's, they use the dashboard warning light (engine) to display error codes as far as I know. I suggest you get a wiring diagram (I don't have one) and look up the dashboard warning light circuit and install a globe to read the errors. Better still, borrow a Power FC and Commander and read it off the display. A few quick things I would do follow;

1. If the engine revs over 2,500 rpm then it isn't in limp home mode, but it still may pay to do an ECU reset. There are instructions on the forum if you don't know how do this.

2. Check the AFM voltage with a multimeter. Clean the AFM if necessary, there are instructions on the forum if you don't know how do this.

3. What's the boost look like? Have you fitted a boost cut defeater?

4. Check the engine temp sensor.

5. Tried advancing the ignition?

If all else fails stick it on the dyno and do a quick power run (cost $40), have a look at the resulting boost curve and A/F ratios. That should tell the story.

Hope that helps

[DJ_L3ThAL]

i had mine tested a couple weeks ago, the motor itself is factory and mine were all about 170 except for cylinder 4 which was 160, i was told this is perfectly healthy

[Sydneykid]

  KamikazeR33 said:

how is a leak down test done?

A leak down test is like a compression test, except instead of cranking the engine over to get compression, you use a compressed air fed from outside the engine. You connect a pressure gauge and an air feed up to each cylinder (via the spark plug hole), one at a time. Build up pressure and read off how much each cylinder leaks over a fixed period of time. You should get consistent numbers over 90%. There are two types of leak down tests, dry and wet.

I do a "dry" a leak down test first, this tells me if there is any problem. If that cylinder leaks down more than I expect, then I do a "wet" test. With a bit of oil dribbled down the spark plug hole and sealing the rings, I can tell if it is the valves that are causing the leakage. If not then I know it's rings. All good mechanical shops should have a leak down tester, usually a Snap On brand.

This is the inexpensive one I use....

Hope that helps

[maxim]

  Sydneykid said:

Hi maxim, I have never seen an RB25DET computer with LED's, they use the dashboard warning light (engine) to display error codes as far as I know.  I suggest you get a wiring diagram (I don't have one) and look up the dashboard warning light circuit and install a globe to read the errors.  Better still, borrow a Power FC and Commander and read it off the display.  A few quick things I would do follow;

1. If the engine revs over 2,500 rpm then it isn't in limp home mode, but it still may pay to do an ECU reset.  There are instructions on the forum if you don't know how do this.

2. Check the AFM voltage with a multimeter. Clean the AFM if necessary, there are instructions on the forum if you don't know how do this.

3. What's the boost look like? Have you fitted a boost cut defeater?

4. Check the engine temp sensor.

5. Tried advancing the ignition?

If all else fails stick it on the dyno and do a quick power run (cost $40), have a look at the resulting boost curve and A/F ratios.  That should tell the story.

Hope that helps

Hey Sydneykid,

I'll go through all your suggestions. Appreciate your advice, your a great asset to the forum.