Jump to content
SAU Community

Recommended Posts

Hi,

Need some help here I just got back and had installed a new forged RB25 looked all good until put on the dyno to find out that it makes less power then before (made 240rwkw which wasn't even final tune before an assembly fault saw it go back to the builder to come back with 218rwkw).

Tuner did a comp test to find out it was 100psi across the board (seems low as I was expecting 150 and the tuner expected 140-150) did a leak down test as well everything is sweet and sealing up perfect, checked, re-checked and then another two times checked timing and everything is sweet.

JE forged pistons

Forged eagle rods

1.3mm head gasket

hks 264 cams

Anyone with some suggestions that could possibly result in not removing the head please let me know!

Regards,

blinksta

Link to comment
https://www.sau.com.au/forums/topic/354081-forged-rb25-w-low-comp/
Share on other sites

they seem a bit low but as long as they are even then that is the main thing. If you are really worried do a leak down test and that'll tell you the full story.

You can't really use the factory compression figures when you played with the head especially when you have changed the cams. The opening and closing times of the cams are different and therefore you will lose more compression before the valves close, this is part of the reason why bigger cams are more efficient at higher revs.

Edited by D_Stirls

yea done a leak down test all comes up perfect and managed to speak to the builder unfortunately he wasn't much help either and said that the cam timing must be off, but that has been checked I guess will have to check again.

The pistons were the exact same as before when making the 240rwkw, im thinking it might be the cams that are dodgy and have to go back to stock :S

I probably should have explained that better the tuner was speaking to the builder about the engine as to why it's making low comp and they were on the phone for roughly an hour and the end result was the builder suggesting that the cams could be the problem.

Sorry I don't know the actual comp ratio I will find that out in the next hour when I go have a look, but I would think that having standard size pistons or even a half mil over combined with a 1.3mil HG and drop in cams shouldn't reduce the comp by that much, I would expect maybe 10-20psi drop?!?

ok so I find out that the comp was built to 8.9:1 but they did yet another comp test and have some interesting numbers of 140, 115, 120, 125, 125, 105 or close to that which as you can tell is just completely rooted so to check they are going to measure (another r33 in the yard which is 100% healthy) from the seat of the spark plug to the piston (or something along those lines I will still angry from the readings) to confirm 150% that the engine wasn't built properly.

If the rings haven't bedded in correctly then it isn't the builder fault. there seems to be something wrong with the compression tester since the first test was all even and now they are all over the show. Also that much variation should have shown up in the leak down test and it would have been far from perfect.

As i said before you can't compare the results for a standard rb25 to one with bigger cams the one with bigger cams is guaranteed to have lower compression.

Leak down test was <=2% so everything was sealing, another comp test was done today and those numbers came up there is no blow by at all after checking the blow by hose and catch can, it was all measured with snap on tools as well so they are very realiable (granted not impossible to break or fail).

Putting bigger cams on a stock block will reduce compression but not that much they would have to be some very aggressive cams for that to happen and these are only 264.

no the cams have a large affect on the compression. When i played with the cam timing on my CA the compression went from 165 before the timing adjustments to 140 after so that is 25psi drop with just the timing being adjusted on stock cams. You have also got longer duration cams so your valves are open for longer and you will lose compression because of that, how much depend on the timing.

At the end of the day if you are getting good leak down results and no blow-by then the cylinders are sealing.

Create an account or sign in to comment

You need to be a member in order to leave a comment

Create an account

Sign up for a new account in our community. It's easy!

Register a new account

Sign in

Already have an account? Sign in here.

Sign In Now


  • Similar Content

  • Latest Posts

    • It is a kunfine Android screen . Does anyone know the wirering diagram of the fuga ??
    • just an update to this, poor man pays twice  Tried sanding down the pulleys but it didnt do the trick. Chucked another second hand alternator in the na car which I got for free off my mate and its fixed the squelling. Must have been unlucky with the bearings.    As for my turbo car, I managed to pick up a cwc rb alternator conversion bracket + LS alternator for 250 off marketplace, looked to be in really good nick. Installed it , started the car and its not charging the battery.... ( Im not good with auto elec stuff so im not sure if this was all I needed to do but I verified such by using a multimeter on the battery when the engine was running and I was only getting 12.2v )   I had to modify the earth strap for the new LS alternator , factory earth strap was a 10mm bolt which did not fit the bolt on the LS alternator which was double the size so I cut it off , went to repco bought some ring terminals that fit, crimped it onto the old earth strap and bolted it up to the alternator , started the car and same issue. Ran like shit and was reading 12.2 at the battery.  For a "plug and play" advertised kit thats not very plug and play but alas.  My question is , am I missing something ? Ive been reading that some people recommend upgrading the stock 80 amp alternator fuse to a 140 amp but I dont see how that would stop the alternator charging especially at idle not under load.  Regardless ive pulled it out and am going to get it bench tested by an auto elec tomorrow but it would be handy to know if ive missed something silly or have done something wrong.   
    • My wild guess is that you have popped off an intake pipe....check all of the hoses between the turbo and the throttle for splits or loose clamps.
    • Awesome, thanks for sharing!
    • To provide more specific help, more information is needed. What Android screen? What is its wiring diagram? Does the car's wiring have power at any required BAT and ACC wires, and is the loom's earth good?
×
×
  • Create New...