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Power limits of RB25det bot end


Shibby
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I've noticed some great power figures popping up lately from some well set up RB25's such as b-mans etc and got to wondering what would be the power limit of a balanced, forged pistoned and shot pleened rods cryo treated etc bot end

For instance would 400rwkw be out of the question?What rpm is a prepped RB25 bot end able to rev to without shortening the life of the motor to much?

Please don't go into detail about how much it will cost etc or 3 litre would be easier to do it with as I have read the 3L thread from back to front and agree it would be easier and probably cheaper, I'm mainly interested in the RB25 limits

What would be the power limit of the stock gear box?

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I've noticed some great power figures popping up lately from some well set up RB25's such as b-mans etc and got to wondering what would be the power limit of a balanced, forged pistoned and shot pleened rods cryo treated etc bot end

For instance would 400rwkw be out of the question?What rpm is a prepped RB25 bot end able to rev to without shortening the life of the motor to much?

Please don't go into detail about how much it will cost etc or 3 litre would be easier to do it with as I have read the 3L thread from back to front and agree it would be easier and probably cheaper, I'm mainly interested in the RB25 limits

What would be the power limit of the stock gear box?

if its all checked and balanced properly they should handle around 350rwkw i think, the only thing that will cause problems is the rods, if you will spend a extr 1500 get new forged rods then it should handle over 400 easily.

The rpm limit is determined by the head, components like lifters, valve springs will need to be changed, std 8k should be doable with a built motor

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ok so what would be the limit of stock prepped rods?

Hi Shibby, if it was only that easy. The tuning and how the engine gets to the power is just as important as the amount of power itself. A 500 bhp engine at 1.3 bar might be OK, but a 500 bhp engine at 1.6 bar is less likely to survive. A 500 bhp engine with an a/f ratio at 11.5 might be OK but a 500 bhp engine at 12.8 is far less likely to avoid detonation.

Look at it another way, a set of prepped rods is only going to handle maybe 50 bhp more than standard. By the time you replace the big end bolts, shot peen the rods, cryo them, replace the litte end bush bearings and balance them, you have almost spent as much as buying a set of forged rods.

There are quite a few RB25's running around the 500 bhp mark (300 rwkw) with totally standard internals, but how long will they last? I have always considered 450 bhp to be a reasonable limit. The pistons being the weakest, then the rod bolts and finally the rods themselves.

Hope that helps

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ok so what would be the limit of stock prepped rods?

I put GTR rods in my rebuild with new rod bolts (thanks SK). Can find them cheap on the forum...300 - 400 a set and 140 for bolts and nuts as opposed to the retail price.

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I put GTR rods in my rebuild with new rod bolts (thanks SK). Can find them cheap on the forum...300 - 400 a set and 140 for bolts and nuts as opposed to the retail price.

How much stronger are the GTR rods and bolts over the rb25 ones?

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Denham, the GTR rods are more heavily tapered out toward the big end giving a greater load area for the forces of higher power/rpm to be withstood. They are also thicker in the web while the 25t rods have more pronounced end plates on the I beam area.

http://www.skylinesaustralia.com/forums/at...achmentid=10064

http://www.skylinesaustralia.com/forums/at...achmentid=10063

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The rpm limit is determined by the head, components like lifters, valve springs will need to be changed, std 8k should be doable with a built motor

Rod length plays a very, very large part as well.

4500 feet-per-min is a safe limit for stock rods (6000 fpm for nice ones)

RB25 rods = 71.7mm

Piston speed = (Stroke * RPM)/152

=(71.7*9000)/152

=4245fpm

A stock RB25 bottom end is limited to a little more than 9000RPMs

If we shoot for the 6000 FPM number...

=(71.7*12750)/152

=6014fpm

A fully done up bottom end should safely withstand 12750RPMs routenly.

That is just the lower end - valvetrain and fuel mods would be necessary too.

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Yes but you still have to take into account the second moment of area for flexure/torque and integrated momentum force over cross sectional area to ensure the stress limit is not exceeded or even more importantly the fatigue limit so that discounting wear they will have an infinite life.

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Good topic! :(

Has anyone done any testing on strength of the RB30 rods?

From what I've seen - it's not generally the amount of power that'll kill rods, it's torque... Hence why a single cam RB30 won't be able to make as much top end power as a twin cam...

Cheers,

matt

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Well my previous engine which i thought was full house (built by rigoli) had standard internals (and standard head and cams) except for arias pistons. I had it up to 340rwkw @ 1.9bar but dropped it down to 320rwkw. Lasted a month before it dropped a rod. Max rpm was 7200rpm aswell.

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Ouch! 1 month? Mine's been running for coming up to 2 years now with around the 320-340rwkw mark (depending on which dyno you put it on)...

I'm only running around 16psi and it's rev limited to 7500rpm.

I was going to start upping the boost to 20-22psi, but after hearing that, I think I won't get greedy!

Cheers,

matt

Well my previous engine which i thought was full house (built by rigoli) had standard internals (and standard head and cams) except for arias pistons. I had it up to 340rwkw @ 1.9bar but dropped it down to 320rwkw. Lasted a month before it dropped a rod. Max rpm was 7200rpm aswell.
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Nah mate, I've got a RB30 bottom end... The static CR would be too low with GTR rods!

I've just got shot peened, nitried rods... They are series 3 VL rods though - apparently they were slightly thicker at the big end than the series 2.... Doubt that's gonna make it much stronger though...

If I decide to hang onto the car, I might pull it down in 6-8 months time and stick in a set of aftermarket rods and bump up the rev limit to 8000...

Cheers,

matt

matt

u r running gtr rods 2 aint u?

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so geoff how much rwkw power u recon u can get with your gtr rods before they start to bend?
I think it will be track safe on the ZR road tyres up to 250 rwkw, any more and it will overcome the rears and I'll lose time, depending on the tune. If I get some decent sponsorship for track rubber I'll go hunting for 300+ and a few extra rpm.

After that it will be head off and solid lifters and new springs.

I doubt the RB25 crank will allow the big rpm and power numbers GTRs enjoy on stock internals + forged slugs so the GTR rods will not the the limiting factor. If some bastard at work will give me back my laptop I'll be able to model and FEA the rods for limitations

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Thats a good question. Depends if the turbine was replaced with steel. It certainly doesn't respond like a stocker, more lag until 4K then the switch goes, but I hope to tune that out. If it is a steel turbine I would guess it's not ball bearing. Also produces power differently...more power at 11psi than a stocker at 15psi so it isn't as restrictive in the exhaust.

I could replace it and be more certain, or I could drive it until it's dead or I'm surprised. Just not sure yet.

I did the hicas replacement bracket today and got the alignment, including the caster, and it is on rails now with some fresh front (30% used) rubber.

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