Rebuild Q1 - Compression Ratios

[SeriesIIGTST]

Hi all,

After dropping a cylinder (stupid cyl 6 with it's foobarred ring lands....) it's time for me to start some serious research on rebuild options.

Let's start at the top. Not aiming for high horsepower (read 250rwkw MAX) so while I got this thing apart, I may as well build it to reliably run that sort of power. It's a street driven car and my daily runner for work so it sees traffic duties.

Question 1 (there's bound to be more) - Head gasket.

Should I just get a replacement stock item or get a metal one? If I get a metal one (from HKS or the like) should I be considering lowering the compression? Seems the thinnest metal head gasket will result in 9.0 compression ratio. Higher? Lower?

Feedback would be appreciated.

Cheers

[meggala]

I would fit rb26 pistons they arent dear but drop compression from 9 to 1 to 8.6 to 1 and use the standard head gasket these are proven to at least 280 rwkw in gtr's and fit straight in they are over $100 check with your nissan dealer but much cheape than forgies

[Warpspeed]

Have to agree completely with Meggala, a set of rb26 pistons is definitely the best way to go, combined with a stock head gasket.

[Bl4cK32]

read the post "r33 rb30 conversion"

[Warpspeed]

There is nothing in the first post to suggest he is building an RB30 ? So why would any of that be relevant ??

[Bl4cK32]

To give him a number of chioces on what he can spend money on. How much would it cost to rebiuld his r33 with rb26 pistons? A rebuilt rb30 bottom end is 1k, rb25de head is $750 ish - is this close to what he's gunna spend?

[Guest RedLineGTR]

good idea with the rb26 pistons..

[SeriesIIGTST]

Guys, thanks heaps for the replies. Will an 8.6 compression ratio drastically affect off-boost performance? I've also been offered a super deal on some Arias forged pistons and rings (read less than $1300 delivered) through a friend who has contacts over east (no, they're not hot). Is it worth the extra expense given the fact that by the time I buy RB26 pistons and rings to match i'll probably be looking at that sort of dollars anyway?

Bl4cK32: I'm not really interested in an RB30 conversion. Thanks for the idea though.

[Cake]

SeriesIIGTST

how did you manage to cook your cylinder? any tips to prevent doing this to my own car? hehe

also have you managed to find an intercooler solution using the stock piping?

cheers,

Chris.

[SeriesIIGTST]

Chris, cyl 6 has been a little dodgy ever since I've had the car and I guess the problem has just got worse. It has also been running quite lean. In terms of prevention, I'd simply say make sure your car is in a good state of tune. Something I neglected to do upon purchase of my car. Get the A/F ratios analysed to make sure it's not leaning out. As for intercooler, I was considering the Trust cooler as people suggested in my other thread however, $$ now have to go to get it rebuilt.

[SeriesIIGTST]

All, particularly, Meggala, I have just been informed by one of the performance shops over east, who have it on good word from a reputable performance shop (yeah i know, second hand knowledge) that putting RB26 pistons into an RB25 causes detonation on low boost and will cronically wreck your engine.

Thoughts?

[Steve]

Only one thought, did they say why?

[Sydneykid]

SeriesIIGTST, for 250 rwkw stick with as close to 9 to 1 compression ratio as you can. Off boost performance and fuel economy drop off dramatically with lower compression ratio, as does throttle response and boost build.

To help the #6 problem, get the injectors flow tested and put the highest flowing one in #6, the next in #5 etc.

Hope that helps

[Cubes]

Depends what you are after.. I'm only after 250rwkw's.

Check out Raists dyno run of the before and after with the RB30..

Here's the dyno run.... Look at the RPM compare.

http://www.skylinesaustralia.com/forums/at...=&postid=444741

For a street car you really can't beat the RB30 conversion for its response and considering the price is very similiar to rebuilding the RB25DET.

Warpspeed.. You sound like you are beginning to get annoyed with the RB30 suggestion.. heheh

[Col-GTSX]

Go a Rb30 bottom end .... very cheap and new pistons from toda are as cheap as too.... rings and bearings from AVO and youll just need a new after market ecu...

done....

Oh and a new turbo HKS 2540

Not serious power 400+kw's.......HEHEHE

[SeriesIIGTST]

Steve: I don't know a lot about engines at all and the guy started to explain something about them being too high or something. This info has come from the people who are supplying me with forgies if i decide to get them. Anyone care to comment on Arias pistons and rings?

[Steve]

Too high? I was under the impression that the gudgeon pin in a RB26 is closer to the crown of the piston than on an RB25. As megalla pointed out, it would result in a lower CR

If someone is trying to sell me something, generally I believe 2/5s to fark all of what they say. I will listen to what they say, but alot of time people who are trying to sell you stuff do it because they want to make some money. If I were you I would as somewhere else, as GTR pistons will probably cost half or 1/3 of the cost of forgies (therefore 1/2 - 1/3 the profit for the salesman)

I am not knocking what you have said, just find it hard to believe that there is a problem as the design of the head for RB26 and RB25 is very similar, ie same casting as far as I know. But if this is genuinely the case, that would be good info.

I am a bit of a sceptic at heart:)

As far as I know, Arias, Wiesco, JE - all good pistons, ie I have never heard any bad about them and it wouldnt make alot of difference which one you chose if you are chasing 250rwkw.

[Cubes]

Arias apparently have better thermal expansion properties that reduce the typical forged piston knock.