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Crank Strength And Upgrading Options.

LedZepLen

By LedZepLen,
in Engines & Forced Induction

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N1GTR Grand Master

N1GTR

Posted
Posted
They are as relevant as a 7000rpm limit on the std crank.

100% agree on the point you're making, but I'm trying to make a different one :)

my standard rb30 crank goes to 9000rpm no probs so yeah I really do understand what you're saying :(

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nismomiles33
nismomiles33

I used a grub screwed, machined and balanced, nitrated standard crankshaft (Nizpro spec??) .. should hold up with most, I was told that should be good for 800+HP.

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GTRNUR Grand Master

GTRNUR

Posted
Posted

Nice to see the 3.4lt project is coming along still.

Im just guessing, but if the compression heights are kept to a lowish 28mm and the custom rod length is about 152mm to suit the RB30 block, the rod ratio works out to 1.6:1.

I've just run a few numbers on my engine calculator and for a street application with a degree of reliability 8000 rpm would be the absolute upper limit (including a missed gear shift). 8500 rpm would be pushing the limit of what the rod bolts and pistons will tolerate and 9000 RPM will break a conventional designed forged pistons as piston speeds exceed 46m/s.

Also at 9000 RPM the acceleration away from TDC/BDC is 54.5 m/s ^2. Numbers like that are only seen in all out drag engines. ie... engines built entirely from top shelf custom parts the likes of carillo/pauter with wmc5 bolts and special piston designs. Titanium rods would go a long way to improving high rpm reliability as the rotating mass and rod big end loads are considerably less due to less inertia during the acceleration/deceleration phases of the engines rotation.

post-26553-1261660079_thumb.jpg

shanef Grand Master

shanef

Posted
Posted
Nice to see the 3.4lt project is coming along still.

Im just guessing, but if the compression heights are kept to a lowish 28mm and the custom rod length is about 152mm to suit the RB30 block, the rod ratio works out to 1.6:1.

I've just run a few numbers on my engine calculator and for a street application with a degree of reliability 8000 rpm would be the absolute upper limit (including a missed gear shift). 8500 rpm would be pushing the limit of what the rod bolts and pistons will tolerate and 9000 RPM will break a conventional designed forged pistons as piston speeds exceed 46m/s.

Also at 9000 RPM the acceleration away from TDC/BDC is 54.5 m/s ^2. Numbers like that are only seen in all out drag engines. ie... engines built entirely from top shelf custom parts the likes of carillo/pauter with wmc5 bolts and special piston designs. Titanium rods would go a long way to improving high rpm reliability as the rotating mass and rod big end loads are considerably less due to less inertia during the acceleration/deceleration phases of the engines rotation.

Do you feel like running the numbers for a std stroke rb30 for a comparison?

Marko R1 Grand Master

Marko R1

Posted
Posted
They are as relevant as a 7000rpm limit on the std crank.

Well rob @ rips revs his engines past 11500rpm and this is with the factory rb30 crank and making over 1400hp. Need i say anymore?

N1GTR Grand Master

N1GTR

Posted
Posted
Well rob @ rips revs his engines past 11500rpm and this is with the factory rb30 crank and making over 1400hp. Need i say anymore?

yeah I think you've missed the point aswel mate

R31Nismoid Grand Master

R31Nismoid

Posted
Posted
my standard rb30 crank goes to 9000rpm no probs so yeah I really do understand what you're saying :)

For a few dyno runs, with lazy driving for 10k-km's or are we talking a SOLID 30,000km's with the ass wringed out of it for streeter duties/drag days? :(

There is a... sizeable lets say... difference between the two.

BTW - i know the point you are making, not covered in the above i know :P

GTRNUR Grand Master

GTRNUR

Posted
Posted

Sorry but Im not so good at making graphs. Ive got one other table that shows piston speeds and acceleration through 360 degrees crank rotation, and the comparison of the tomei to the HKS kit is really very little when comparing the two engine setups. Really the 119.5 rod was only made as an option to allow the use of a thicker crown piston that uses a conventional RB26 30mm compression height. Its well known that the 2.8 kits are safe for 10k so I didnt bother calculating the 8500rpm values.

Ive attached the rest of the table. You might recall the Racepace 2.9 spaced block engine that used a 84mm crank and SR20 rods as well.

At 9000 RPM the piston acceleration for a RB30 is 47.5m/s^2.

The most important thing to keep in mind is the whole point of having a stroker engine is that you dont have to rev the hell out of the engine to make reliable power. That extra 400+cc of capacity will do for the RB30 what the RB30's displacement does over the RB26, if that makes sense.

The beauty of the larger displacement AND the shorter rod ratio is that while the engine doesnt need to (and shouldnt) rev as much as another type of engine, the faster piston accelerations from BDC and TDC mean better cylinder filling making for a more torquie engine. The cylinder pulls into a vacuum faster on induction cycles than a higher ratio motor will, so the engine breathes more efficently.

I agree with Marko's comment that there is nothing wrong with the strength of an RB30 crank. Rob @ RIPS certainly has proven that time and time again. I do wonder what kind of rods and pistons are in his 11,500RPM engine though, im guessing aluminium/titanium and some very fancy pistons. The inertia loading of chromoly rods and pistons at that engine speed would be increadably high. Acceleration factor of an RB30 crankshaft at 11500 rpm is 77.5m/s^2. That is a 1/2 again as much as the RB30 at 9000 RPM.

post-26553-1261665231_thumb.jpg

GTRNUR Grand Master

GTRNUR

Posted
Posted (edited)
Ash I think it's just not coloured correctly?

Yes thats it. I always was better at maths than I was at art, especially past 12am.

New table attached.

post-26553-1261700716_thumb.jpg

Edited by GTRNUR
Spoolup Mentor

Spoolup

Posted
Posted

My point was that this setup was never designed to be a big rever.

It was designed to ( compared to the equivalent setup in an normal stroke RB30 ) to make considerably more torque, bring boost on earlier and make same power with less boost and less revs.

So if it makes the same peak power as my current RB30/25 with less boost and less revs, and more torque and comes on boost earlier - Ill be happy.

If it revs to 7500 rpm then it realisticly should make more peak power than my current setup as that is what Im revving it to now, due to the increase in capacity.

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