Increasing CR. Pin Height or Deck.

[Cubes]

Is there a reason why you shouldn't deck the block to get your required CR instead changing the pin height?

[Steve]

Have you got any more info Joel? Would it effect the squish zone characteristics? definately a new one for me (not that thats very rare:))

[Cubes]

Yer thats what I'm trying to find out.

[BIGBLOCKR31COUPE]

What CR are you trying to achieve Joel?

[Cubes]

8.5:1

Wiseco have told me they do a set for the RB30DET (R32 RB25DE Head) that will achieve a 7.8-8:1 CR.

He said I can get the pin height changed or deck the block to achieve the required compression ratio i'm after.

Reading how combustion works it looks to me that those RB25DET's that have been cracking ring lands with no signs of detonation may be failing due to the cast pistons not being able to handle the heat (Run it richer). Forged pistons are able to handle higher temperatures so they shouldn't have a problem at 9:1 providing the piston design is good and the combustion chamber has been polished and cleaned up.

I'd say this is why Freebaggins R33 has hung in there with no problems at 250od rwkw. (plenty of fuel, Tim's Tune(RPM)) Freebaggins R33 does blow a little bit of black smoke so I'd say its a conservative 'safe' tune to keep things cool.

What CR did u go for mr. Bigblock?

[BIGBLOCKR31COUPE]

On 'Rowans Cobra Site' he claims to have put forged Wiseco pistons into his RB30det giving him 8.5:1 CR. You may want to ask him what the story is. This is a link to his page http://home.iprimus.com.au/cobra30/twincam.htm

Geoff.

[Cubes]

I like that torque curve..

Thanx Bigblock.

I'll find out what the volume of the hedas combustion chamber is (its had valves deshrouded) then let wiseco know so they can sort something out. If you change the pin height doesn't it also change the geometry of the piston/rod? That is really what I was concerned about as I have heard of people fiddling with the pin height to prevent piston slap etc. I don't want to change it and get piston slap as generally the wiseco's are pretty good with no piston slap.

[BIGBLOCKR31COUPE]

I remember when I built my RB30det that the pistons came right to the top of the cylinder bore. If you decked the block they would then come too high.

[BIGBLOCKR31COUPE]

Yeah, very nice torque curve. VERY nice!

[Cubes]

I just checked the NA pistons in the RB30 block and yer they do come right to the top.

[Cubes]

I just realised that thing is putting out 618NM of torque.

Thats a hell of a lot for 348kw. Whats the LS1 300kw engine put out 500NM?!?

With mine putting out some where near 400kw torque would be getting close to the 700nm mark..

[Warpspeed]

Joel, the consensus seems to be that you should aim for about thirty thou squish clearance with steel conrods. Any less and you might be in trouble, any more and you quickly lose squish effectiveness.

So you juggle deck height and head gasket thickness. How you do this is up to you, but decking the block after a trial assembly is the most usual way. Also there is a pretty good choice of head gaskets if you wish to go aftermarket.

If you are after 8.5:1, just order replacement (forged) GTR pistons, how about some factory N1 pistons? These will have the internal oil passage around behind the rings for the squirters, other aftermarket forgies most probably will not have this feature.

[Bl4cK32]

what if there are no oil squirters?? Would u go for the other aftermarket pistons Warpspeed? ie Wiesco.

[Steve]

What is the cost of N1 pistons? Also, is there much advantage over stock GTR pistons?

then again, why even bother with forgies. I know alot of people say you need to use them, but I am yet to be convinced except maybe for full on drag type application. Forged pistons,although they may handle more heat, are more brittle than stock pistons and cost alot more. Stock pistons, with a conservative CR and a good tune will do the job very well.

Sorry, I am straying off track a bit:)

[T0nyGTSt]

I have GT-R N1 pistons in my RB25DET.

I'm unsure what CR it is even but they seem to work ok and there's no doubt that the tolerances of Nissan pistons, rings, gaskets etc. etc. make for a very tight, smooth running motor.

T.

[Bl4cK32]

I think some else wrote it betterthan i could say it :

Why use forged aluminum pistons? It’s simple: because forged pistons are lighter than most cast or hypereutectic pistons, they enable the engine to rev quicker with less stress and ultimately provide an immediate gain in performance. And, as heat retention is minimal, the engine operates at cooler temperatures. The piston also can withstand increased loads, even with higher compression ratios because the denser, forged material is formed into shape under 2,000 tons of pressure.

[Warpspeed]

I agree with Tony 100%

If you are building a long life street motor, factory pistons are going to give you a good strong tight well sealed quiet running engine. If you simply must run forgies N1s would be my first choice.

Aftermarket forgies are HEAVY, and they usually expand more depending what they are made from (silica content). So you must often run loose pistons. This is not good from the aspect of blow-by, oil control and piston rattles.

Some of the short skirt slipper pistons are reasonably light, but piston rock adds to the loose piston syndrome to make ring sealing an even bigger problem.

Pistons rarely break except through detonation. If you have detonation it will break anything you can put in there, and so will a dropped valve..

Oil spray is very good at keeping piston temperature within bounds, but it depends on how long you plan to hold full throttle. The aluminium alloys all lose strength at elevated temperatures to a greater or lesser degree. I would rather run sprays and stock pistons than no spray and rattly old forgies on the street.

The last forgies I used were a set of Arias blower pistons and I was not at all happy with them.

Forgies would be o/k in an all out drag motor though, but not on the street.

[Steve]

and then there is the added benefit of doing a rebuild/freshen up every 10-20,000kms.

Tony, what did the N1 pistons cost, and what CR do they have when used with stock RB25 rods?

[Bl4cK32]

*thread hijack* - sorry -Joel-

what is the factory clearance recommended for their std. pistons? Wiseco recomment as little as 0.003

[Warpspeed]

Don't know exactly how much N1s cost, but I recall someone here mentioned that new from Nissan in Japan they are not much more than standard GTR pistons. Try doing a search ?

Not sure what factory clearance is on standard GTR pistons, but I have a set of six here that were removed from a low mileage R34 GTR donk. Four are .001 under correct standard bore size, one is .0015 under, and one is .0005 under.

The stock pistons are a snug fit. I run about .0015 clearance with cast pistons on my Laser turbo. When I ran the Arias forgies those were set to the recommended .007 under, and they were pretty loose and rattly when cold.