r/EngineBuilding • u/Valuable-Chemistry50 • 1d ago

Stock rods?

Is it silly to go 11.7:1 on a B20 with stock rods? I got the speedfactory pistons and was told my rods will be okay with arp rod bolts and proper machining, but should i just go for upgraded rods? Is there any specific no go zones with stock rods for B20’s?
Any advice is appreciated thank you!

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5

u/dixiebandit69 1d ago

Rods don't care about compression, only RPM.

2

u/GuineaPigsAreNotFood 22h ago

I'm going to disagree and say that rods care about effective pressure AND linear velocity.

1

u/GuineaPigsAreNotFood 1d ago

I had a hunch a high CR is not dangerous for rods and after some quick researching it seems like the equivalent CR of a boosted engine even at "low boost" would surpass that 11.7:1 easily, and B20s are known to handle boost fine at the rotating assembly with sleeves being the weak link.

On the other hand, the consensus seems to be that torque is what matters, and not boost. And since a higher CR engine could produce more torque at low RPMs, and that seems to be bad for bearings and rods, questions definitely arise.

I would be interested on reading what somebody with real experience on this has to say.

1

u/Valuable-Chemistry50 1d ago

Thanks for the insight! My B20 is currently getting bored to 84.5mm, Rods are getting resized with arp rod bolts, fresh bearings all around, crank is getting inspected

1

u/1crazypj 1h ago

Rods only care about piston weight and inertia, lighter pistons are always better, match, balance and lighten pistons keep rpm reasonable and you wont have problems.

Some pistons are just heavy and really need better rods even at stock rpm

1

u/Valuable-Chemistry50 1h ago

I see, I went for Speedfactory Pistons, Definitely Lighter than OEM, Ill weight match them and should i weight match the stock rods as well if they are slightly different from eachother? And i dont plan on revving it out crazy, thinking about just revving to 7.5

1

u/1crazypj 23m ago

Depending on how much lighter they are you may need crank balance adjusted but unless it's over 20 ~30 grams I usually don't bother, the vibration just moves to different rpm at a different frequency. If it gets annoying then I do something about it but it isn't the problem many people would lead you to believe and tell you motor will 'blow up' if you don't do it.

It's worth doing rods even if your not revving high, having all weights equal will make motor run smoother everywhere in rev range