Stock Rod limits discussion

[pizipe]

I posted this inside another thread but didn't get much discussion, so I'll start it's own thread:
I keep hearing guys talking about rods bending from sheer power levels, I would think a rod bending would have alot more to do with peak cylinder pressures. Example: 2 cars, 1 with gt28's, 1 with RS6's. Both tuned to make the same WHP. I would think the RS6 car would be more likely to bend the rods because of the way the turbos spool. The RS6 car would hit more suddenly than the GT28 car causing higher peak cylinder pressures. Isn't this why EPL is tuning their RS6 kits to try and get the boost to come on more gradually on engines with stock rods? Comments?

[BastardS4]

500whp

[The_Jerbel]

Peak torque is what kills rods. By ramping up the boost, you're limiting the peak torque (which is made around 4000 rpms)

[pizipe]

Peak cylinder pressure would equate to peak torque, no?

[The_Jerbel]

Yup.

[sCeRaXn]

Not always. On our cars with the limited amount of tuning available, peak pressure will usually be around peak tq, but this isnt always the case. Lets say someone is running a ton of boost on a big turbo or twin gt's, unless peak boost is realized before 5250rpms then peak cylinder pressure wont be at or near peak tq. Or if someone ramps boost to hit its peak later in the powerband, it would be impossible for peak cylinder pressure to coincide with peak tq.

With that being said, youre(theoretically) right about the RS6's bending rods because of quicker spool. The higher in the powerband you are the more momentum(engine speed) youve got, so cylinder pressures will have less affect on the rods structural integrity. In lower rpm's the rods are still struggling to gain that momentum so the force trying to generate the power is closer to the force trying to resist it. This, in turn, means there is more pressure on the rod. Theres alot more to it than this, but its a good starting point.

[The_Jerbel]

I think you're confusing the torque that the engine creates with a hypothetical formula used to calculate hp, which is: tq x rpm/5252

That formula has nothing to do with where in the power band peak torque is made. Physically speaking, pressure, torque, and force will be directly proportional.

[generationjdm]

Theres NO magic number on rods breaking, just because 2-3 rods broke everyone thinks thats the limits...

Tuning plays a larger roll in rods breaking, as Vast proved this on there GT car.. Spool has nothing to do with it, rs6 spool later then gt2554s, so are gt2554s prone more to snapping a rod, I dont think so...

The limits have always been in the tune, thats what i think...

The rods aren't indestructible, but I dont think theres a set number...

The rod bolts on the stock rods are a complete joke, I would almost bet thats the weakest link in the rod that alone would make them more prone to causing rods to bend or snap...

[The_Jerbel]

^^agreed. There's no magic number, but there is a point at which the chances of the rods failing go up significantly.

VAST has recently built up the motor on the GT car. They were running stock motor just to see how far they could push it before a failure...

[generationjdm]

And it never failed... I can tell you this, the dyno they made the 509whp on was very very accurate and they would of made 550+ on some other dynos for sure....

[The_Jerbel]

Yeah, they were probably pushing the upper limits of the stock rods' integrity though...

I think it's safe to say that you may be able to get away with more power than 550whp on stock rods, but anything above that, and you're playing with fire...

[generationjdm]

for years the honda shops where telling people that the stock b series rods where good to 300-325whp after better tuning came out there where many people making 475+whp on the stock short block....Now it's common to make large power with the stock short block...

[pizipe]

This brings me back to my "theory" Hondas that are making 300 to 325 are generally using smaller turbos that spool quicker. Once you get em up to 400 or 425 they are generally running bigger that don't spool as quick. And yes, if Gt2554's spool quicker than RS6's then I would think they be more prone to bending stock rods if they were pushed to the max than RS6's

[Skydive@AI]

This brings me back to my "theory" Hondas that are making 300 to 325 are generally using smaller turbos that spool quicker. Once you get em up to 400 or 425 they are generally running bigger that don't spool as quick. And yes, if Gt2554's spool quicker than RS6's then I would think they be more prone to bending stock rods if they were pushed to the max than RS6's

My TiALs spool a lot quicker, and make more boost than do RS6s. Fortunately, I've got Cunningham rods as well.

-Michael.

[Reich]

Guy's at vag said to me that 470-480 whp is the safest, reliable point and it's getting dangerous going over this on stock rods. That's the route i choosed for this year.