Forged Rb25 /w Low Comp

[blinksta]

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

[r33_racer]

When you say timing do you mean cam timing?

[blinksta]

Yea all timing has been checked and cams dialled in

[D_Stirls]

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 February 15, 2011 by D_Stirls

[hypergear]

might have been a 8:1 compression pistons. You can use it to run lots of boost if that is the case

[R31Nismoid]

Asked the person that built the motor what's going on?

[blinksta]

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

[R31Nismoid]

What do you mean he wasnt much help?

He built the motor, he should know what compression ratio it is running.

[blinksta]

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.

[R31Nismoid]

but you have still not answered the question about the comp ratio.

until you do so, how do you expect anyone to help?

its required info. without it, nothing can be ruled in or out as an issue.

[blinksta]

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?!?

[blinksta]

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.

[D_Stirls]

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.

[blinksta]

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.

[D_Stirls]

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.

[DVS JEZ]

Wat went wrong the 1st time? And wat did he replace? Was the head gasket changed?

[blinksta]

oil squirters snapped off (wrong clearence) and the HG was changed

[DVS JEZ]

Was it the same thickness headgasket?

[blinksta]

yea same size HG 1.3

[blinksta]

well it seems like the height from the seat of the spark plug to the top of the piston is fine, actually better so the theory is the piston rings were installed upside down which provided everything in the head is fine makes total sense