Forged pistons on stock rods?

mitsubishi

#1

I have a car that just had an engine rebuild, I was told aftermarket rods wouldn’t be necessary so I went with aftermarket stock eqivelant forged pistons and used stock rods . The cars stock power levels are 293hp and 300lbs/ft of torque, the car has mild modifications, stock turbo, and makes 308hp and about 316lbs/ft of torque now and I dont plan on going higher. The car will see the occasional wot pull and spirited driving but its just a daily driver and it will never see the track. Do I have to worry about the rods bending or breaking?


#2

Only if you run the engine beyond the point at which you can damage the stock rods. I would have gone with forged connecting rods because used stock rods are now the weak point in the engine. Stay away from that engine builder in the future. They sound like amateurs.


#3

What car is it? Some stock parts are better than others. Are u saying ur stock rods are not forged?


#4

I’ve heard they are forged from other owners but im not certain.


#5

Just don’t rev beyond the stock redline.


#6

Have you asked on an Evo forum? This is very vehicle-specific.


#7

It sounds like the engine is stock configuration and you’ve just lowered the reciprocating masses a bit with the new pistons. You should have no problem whatsoever with the rods. Rods become endangered when you create greater stresses, like increasing the compression pressures via an increase physical compression ratio or a supercharger, or a longer stroke. From the sound of it, you haven’t done any of that. I might have also gone with forged rods to reduce the reciprocating masses a bit more, but that doesn’t mean the stock rods are insufficient. I’ve never heard of a stock configuration engine popping a rod except due to oil starvation at the bearings. The stresses are pretty much well within their abilities.

Happy motoring.


#8

TSMB’s right - you’ve increased power maybe 5%, I’d be amazed if there was any problem.


#9

Thanks for the clarification and you’re right, the forged pistons are a few grams lighter.


#10

That’s why they’re used. A piston needs to be stopped, turned in the other direction, and started moving again thousands of times a minute. This eats up some of the energy that could have otherwise been converted to torque. The less mass you have to do this with, the more of the cylinder’s explosion you can convert to torque at the crankshaft rather than it getting eaten up stopping and starting masses like pistons and connecting rods.

Actually, since the forged pistons are lighter, you have LESS stress on the rods than you would have with the stock pistons. Sorry I didn’t think of that before.


#11

Boost adds stress linearly to the reciprocating parts. I.e. double the horsepower at the same RPM by adding boost from a supercharger or turbo, doubles the stress on the rods.

RPM adds stress with the square of the rpm. I.e. Add 50% more horsepower with rpm (hot cams, better heads) rev only about 21% higher, to double the load on the rods and pistons.

But @texases is right, it really depends on whose engine. Some designs have great factory rods and some are awfully weak. Same for the crankshaft. Some you can rev to the moon and some will break with 500 rpm more.


#12

The OP did not add any boost. Or any additional compression mechanically.
RPM adds less stress on the rods with lighter pistons all other things being the same… which in this case they are. Mass matters.

Lighter pistons with the same stroke will change the power curve, and might create the ability to rev higher, but without knowing more about the engine it’s hard to say. With the stroke and the rod weights the same, breathing the same, and all other factors the same, I can’t imagine much of a change in the ability to redline just from the change in piston weight. I suppose that is debatable, but that’s my feeling.


#13

I would suspect the biggest issue with rods and their potential failure could be due to excessive pinging; more so on a forced induction engine.
Excessive boost pressure at WOT on a track could also be a factor.

Odds are you will be just fine.


#14

It’s not that bad.


#15

Did that come from an engine with the new curved cylinders?? :lol:


#16

OK4450, everything being the same except the weight of the pistons, I wouldn’t expect any pinging that wasn’t there with the stock pistons.


#17

The answer is NO…you dont need to worry about it. IF the vehicle came Turbo Charged from the factory…you will 99.9 % of the time find Forged internals…Hell a lot of NA Toyota engines are fully forged internals for a long time now… If you are using the stock rods…they are most likely forged…and the info is out there so look it up. I dont know of any Turbo engines that would have used anything less than a fully forged internal setup to be honest.

Blackbird


#18

Yeah my engine feels like stock honestly, doesn’t rev faster or higher. The car is on the stock turbo plus theres been only a 5% increase in power over stock (293 vs 308 hp) with the intake and EBC I have installed so judging by the consensus, I can rest easy. Thanks guys.


#19

You won’t need stronger connecting rods to accommodate an aftermarket air cleaner, if there is any change in performance it is insignificant.