Rods... and Torque

[Joe Perez]

Ok, I'll grant you that if I were in our position reading this, my first inclination would probably be to tell me to use the damn search function. Unfortunately, I'm rather lazy.

I'm looking to gather some statistical data on what kind of torque figures people around here have been able to achieve on the stock rods without breaking anything.

I don't want to hear about how I'd be an idiot not to pick up a set of the M-tuned rods for only $299 during the group buy, or hear about how you're making 950 ft/lbs at idle on your Carrillos.

I also don't want to hear about how you broke your stock rods because you're an idiot and you tried to 18 PSI on a powercard at 15:1 AFR and cracked three pistons in half. On the other hand, if you did kill a rod on an otherwise well-tuned engine that wasn't detonating, that'd be good info so long as you've got a dyno chart to show how much it took to do.

[RotorNutFD3S]

Joe, sorry, I can't help you with a dyno sheet unless Jared (leatherface) has one that I don't know about, but I bent an OEM rod (cyl. 1) while starting the car oddly enough. The actual cause is unknown, the cylinder was not hydrolocked or anything out of the ordinary, I think the rod had begun stressing long before, and that particular start was it's last hurrah. I can tell you at least that the setup was a stock '95 motor, older FMII kit with GT2560R, BEGi DGDP, FM exhaust, at 15psi with a solid tune. No detonation to note either. Not really sure what the average HP output of that would be, but hopefully it will give you an idea.

Pics because why not?

[y8s]

226 torque. so just barely a D-

these are 2001 stock bottom end rods and pistons. runs like a clock. lost count of miles.

[pdexta]

240ft/lbs on a stock 95 motor and still holding on, but only ~3000 miles on it so far.

[Savington]

Joe, much like 5-speed tranny data or 1.6 rear end data, it really depends on how you drive it. If you daily drive it, run up the occasional onramp, don't overrev, and don't overboost, I think 230-240tq is doable on a long-term basis. If you overrev the motor, track it, and generally beat the shit out of it, you can toast a stock rod at 200tq.

Short, lazy-fuck answer: If you want reliability over 220whp, you should probably have forged rods in the motor.

[y8s]

but i drive like a jackass! I bark the tires in 2nd or higher almost every time I drive it.

[neogenesis2004]

Sav- I think it has more to do with accurate, well tuned, ignition timing

[Savington]

Detonation plays a big part, but so do revs. If you spend a lot of time at 6500+rpm, the rods won't last as long.

[neogenesis2004]

Revs will kill rod bolts before it bends a rod.

[Savington]

Spec Miatas overrevved to the mid 7k range have been found to have stretched rods. Stretching a rod is the same as coldworking it, which drops its tensile strength and weakens it.

[Braineack]

Rods are cheap now...

[neogenesis2004]

Well if we are going to get to nitty gritty. Were these spec miata rods measured before hand and then afterward, or are these stretch measurements based on the book value of length? How much was the supposed stretch?

[y8s]

overrevving during a missed shift (>redline) is another potential failure path. puts more strain on the rod than compression.

[JasonC SBB]

Originally Posted by Savington

Spec Miatas overrevved to the mid 7k range have been found to have stretched rods. Stretching a rod is the same as coldworking it, which drops its tensile strength and weakens it.

I over-revved my motor to >8000 once, 2 weeks later one rod popped. 205 ft-lbs.

[Savington]

Originally Posted by neogenesis2004

Well if we are going to get to nitty gritty. Were these spec miata rods measured before hand and then afterward, or are these stretch measurements based on the book value of length? How much was the supposed stretch?

I have no idea. Bent rods aren't the only failure mode. Mine just straight up broke during a 3800rpm cruise.

[Joe Perez]

Originally Posted by Braineack

Rods are cheap now...

Yeah, thanks for that reminder. I had no idea.

Trouble is, I know how things work when I start tearing into engines. If I do have to pull the motor and open it up, it'll go something like this:

Well, while I'm in here I might as well toss in some lower CR forged pistons. So I'll just drop the block off at the machine shop to have it overbored.

Hmm. The machinist said it'd be a few weeks before he could get to my block, so there'd be plenty of time for me to send the head out to Replika Maschinen. Guess while it's there I'd be an idiot not to splurge for oversize valves and the fancy springs.

And you know, I've really been drooling over that absurdflow manifold lately. Of course, I'll need a new turbo to sit on it. And a new downpipe. And I might as well ditch this 2.5" exhaust system for something a bit bigger.

Ooh, can't forget the billet OPG setup. Hey, look! A whole custom-machined high-flow oil pump. What the hell- it's only money.

Wow, glad that finally all done. Motor's back together and in the car, time for a test drive. It'd be a waste not to crank this baby up to 20 PSI or so. (loud noise, followed by quietness.) Huh... What the hell are all these vaguely transmission-shaped bits of metal doing by the side of the road?

I hear the ZF S6-40 is supposed to be pretty strong. We'll just machine off the bellhousing here...

[neogenesis2004]

Thats what happened with the motor I sold to Tom. Started out as rods and pistons. All said and done I spent almost 5k. The only stock parts on it are the block, oil pan, and the head casting.

[JasonC SBB]

See this about ignition timing
Actual data, advance vs. torque (MBT)

[Marc@M-Tuned.com]

Originally Posted by Braineack

Rods are cheap now...

Yup.. $299 with ARP 2000 bolts

[wes65]

How much are you making on a 100% STOCK MOTOR?

Not sure if this is of any help but here it is.