Increasing CR. Pin Height or Deck.

[Cubes]

Problem is i've heard of and seen to many RB25DET's not last to long with their stock pistons when up over 250rwkw.

Maybe its the tuners I'm not sure.

If there was plenty of RB25DET's clocking up 20-25 thou a year putting out 250rwkw with stock internals day in day out then I would call stock pistons reliable at that power level.

[Bl4cK32]

my thoughs exactly

[Warpspeed]

The biggest problem with stock RB25DET pistons is the compression ratio they give, not the piston material. People tweak up the power to detonation and then say the stock pistons are crap, the ring lands are weak.

Stock GTR pistons do not give nearly as much trouble because the compression ratio is lower, they are actually no stronger.

But in the end it all comes down to money and engine life. If you want high power it is going to cost you, and that powerful engine is not going to last 100,000Km to the next rebuild.

So what is reasonable ? Would you spend fifty grand building a racing engine that was worn out junk after only 10,000Km on the street? A lot of people would.

Been there, done that. I think we all need to do it at least once in our lives to get it out of our system. So go ahead, have fun.........

[Steve]

Joel, I think alot of people try and do shortcuts, which is understandable. They get away with running xxx power on a dyno, even though injectors may be on the edge, and detonation is only just under control.

Then the real world of the street comes along. Look at GTR engines, alot of people have 300rwkw no probs. Bigger injectors, and lower CR.

The japanese have been extracting 300rwkw reliably with RB25 stock internals - what do they do? lower CR and use larger injectors.

There are no shortcuts when it comes to building a reliable engine, as it will become the weakest link.

Do injectors, drop the CR and you will find the engine will last - there are a few places around that have massaged 300rwkw from rb25 reliably in australia too, but not many - perhaps its because people think they can reinvent the wheel and do it better?

Oh, and do go for around 8.5:1 CR, I gave my car a bit of a flog the other day 40 degrees, 1.3 bar and knock only went to 26 on the power fc - before I would have had trouble running 0.8 bar on a day like that. And it is still putting enough power down that when 1bar hits its like driving on wet grass (dyno early Jan)

[Cubes]

From fiddling with Dyno2003 it looks like you require another .2 of a bar to reach the same power level as previous if you drop .5 of a CR.

Once again I have no idea how accurate it is or how it applies in real life but it would be interesting.

So why is it every single engine builder that I've spoken to, said for the power level (300rwkw) & boost (1.4bar) I am after I am better off going for a set of forged pistons.

When I ask why, they say that the cast pistons won't handle the heat for a long period of time.

I'll go have a chat to Tim @ RPM which from what I know has never had a reliability issue due to a bad tune. He's currently got Freebaggins R33 running ~1.1bar with a 9:1CR, bigger injectors and is making 253rwkw. He's using an internal gate .61/.63 exhaust a/r GT30. Thats been a daily driver for some time now with no problems.

[Warpspeed]

Lower compression will reduce torque pretty much evenly throughout the whole range at any given boost level. But that is the price you have to pay to stay out of detonation.

Forgies are stronger, no argument, but there are also some disadvantages that go with it.

Engine builders are only interested in two things, making money, and getting results. Results are measured on the dyno, not by overall engine life. So forgies and an agressive high lift cam, with super strong valve springs will get the job done.

A year later you have massive ring blow-by, a rooted valve train, and the engine clatters at any speed, and uses rather a lot of oil. But the engine builder did his job. It made the power (when it was fresh) and it has not blow up.

A lot of top professional drag racers dismantle and inspect their engine after only minutes of full throttle operation down the strip. To expect a similar engine to last months or years on the street is silly.

So you read in the magazines how Joe Bloggs is running 35 psi and making 1000+ BHP, and he probably is. The guy reading the magazine thinks he can put an identical engine in his Commodore to drive to work in it every day.

Some things are just not practical on a street engine, even if they are possible.

But it all depends on your point of view I suppose. I expect many will not agree with me. It is not right or wrong, just an opinion.

[Cubes]

ok,

I'm going to look in to this engine life with forged pistons a little more.

So on that note would you:

1. Use the already used pistons in the RB30E (NA)

2. Buy new Nissan cast pistons (~$700-800)

3. Buy Hypertutic what ever they are with the heat coating on their top.

[Warpspeed]

I would use the n/a RB30 pistons. I believe that this will give you about 8.5:1 cr which is not a bad compromise. If you have any trouble at all it will be broken ring lands through detonation. This is not going to be the fault of the piston though.

On an RB25 I would prefer stock GTR pistons (plus crank, rods, and oil pump).

There is the belief that high power requires stronger pistons. I have never been able to understand this. High power comes more from high BMEP, which is AVERAGE gas pressure on the piston. Peak combustion pressures are going to be limited by detonation anyway, so you cannot go much higher there. So what breaks the pistons ? I doubt very much it is just gas pressure.

If RPM are limited by piston speed, it is the conrod that breaks under tension, not the piston. A heavier forged piston is probably not the answer if you are snapping rods due to a severe accidental over rev.

If someone tells you to fit forgies because they are stronger, ask why the stock pistons break. What actually fails and why. This is a perfectly valid question.

[Cubes]

I've just been down to RPM but unfortunately Tim wasn't there. I spoke to a couple of other blokes, one that was from another place who builds engines.

I first asked about reliability of forged pistons in a every day driven car. I was told that unless the build was dodgy and the bore was not machined to suit the pistons then you will get knocking and oil using motor. They also said that Porsche, BMW etc all run forged pistons in their performance engines in order for them to last longer as the cast items don't appear to like high temperatures for long periods of time especially as they are governed by the EPA which won't allow a car to run rich to cool things down a little.

They said if I go cast for 300rwkw then I will be rolling a dice, they may last they may not. 250rwkw however with a good tune they said the pistons will last for many years.

They have done daily driven RB30 VL Turbos with forged pistons with no long term reliability or performance degradiation etc and also one that has recently been in there was a 180SX CA18 that was making 280rwkw until recently came in for a few more mods that bumped the power up to 310rwkw. It has been a daily driver for the last 3years. Compression is still spot on, no nasty noises etc. The car not to long ago was stolen by some kids (when it was making 280rwkw) some how the external waste gate broke and it jammed on 30psi where the kids lots control and left the car. The engine was still 100%.

Basically cast pistons break because of the excessive heat caused by more power. A bandaid fix for it is to run the car richer and less timing depending on power. i.e 11.5:1 for a R33 making 250rwkw.

It came down to the forged pistons of today don't slap around and arn't noisy like they used to be. If they are then the build up was wrong and clearances were set incorrectly.

I'm going to go forged pistons. . One thing they did mention though is to definitely get the oil squirters fitted to the RB30 block and then get a set of forged RB26 or RB25 pistons that have the cut out in the side skirt to allow for the oil squirter. This also reduces the weight of the piston dramatically as the pistons doesn't require an oil reservoir or something.. Not sure, i've read it also some where else. I think it was on a 300ZX www site with forged pistons for the VG30DETT.

I just have to find out if there is any real difference between the RB25/26 and RB30 pistons that affect the compression ratio i.e pin to crown measurements.

[whatsisname]

Porsche, BMW etc all run forged pistons in their performance engines

Exactly! If Porsche, BMW, Ferrari & the like can use forged pistons over the long term life of their engines how is it that aftermarket suppliers can't achieve the same results!? One of the reasons I chose to use CP pistons is due to fact that they supply to both Porsche & Ferrari for use in factory engines.

[Clint32]

Joel the hight is different between the GTR and rb25 pistons, as the rods are the same and you can use the gtr crank with the gtr pistons in the rb25 block to make it the same as a rb26.

[Cubes]

Thanx Clint..

All I need to do now is get the specs on the piston.

[JNR24]

Why dont you deck the bottem of the head?

This might be easyier than moving the pin on the pistons & cheaper.

Also could someone tell me the formula to find your compression ratio?

Cheers,

Trev.

[Cubes]

Its not that easy.

You have to find the volume of the head, head gasket bore & thickness, piston down from tdc, volume above piston.

[Warpspeed]

Thanks Joel, I have learned a fair bit from this thread. It has been many years since my own unfortunate adventure with forgies, and things seemed to have changed a lot since then. It is quite possible that the thermal expansion of more modern forging alloys are considerably less, as well as forging technology generally.

I would still be guided by the recommended piston to bore clearance, and try to stay with pistons that have as small a clearance as possible. Sorry if I misled anyone, but I try to pass on what I have learned the hard way over the years, and some things DO change. Most of my engineering reference books, like me, are getting old and they are quite clear about the problems associated with forged pistons.

Compression ratio is the total maximum volume at BDC divided by the total minimum volume at TDC throughout the stroke. So its stroke volume + gasket volume + chamber volume, divided by gasket volume + chamber volume.

A lot of people get confused with this. If the stroke volume is 500cc, and the chamber plus gasket volume is 50cc, then the compression ratio is 11:1 (550/50)

Measuring this on an RB engine is not that easy because of the domed piston, but it must be done.

[Cubes]

No probs Warpspeed.

A good discussion is always worth while as to be honest I was a little iffy about forged pistons and the negatives that come with them.

You made me think and research in to it a little more.

I posted another thread stating how long have you been running forged pistons. Seems to be all positive feedback so lets cross my fingers and hope all is well.

So thanx..