After spending many hours searching here, I found that rods are the weak point in our engines. Doing even more research, I found that rod stress goes up by a square of RPM, so stress on the rods at 6K rpm is 4x as much as the stress at 3K rpm.

My plan is to boost up to 14 PSI at 3K rpm, tapering off to 5PSI at 6K rpm, with complete fuel and ignition mods of course.

So, if you broke a 2200 with stock internals under boost...

How much PSI were you running when it broke?
About what RPM was the engine when it broke?
What exactly broke?
How many miles did the motor have on it?
Synthetic or dino oil?

Hopefully we can draw some correlations.


2002 Cavalier 2200 5spd

Before I built my motor, the thing that "blew it" was a seized/failed oil pump. Lost oil pressure completely and threw a bearing. Was running 6psi, spiking at 8psi at the time.

If you drop the compression ratio and watch your timing and fuel, you can really push that motor upwards of 300+hp or more from all the threads I have seen in the forums.

If you were to stay all stock internals and near stock everything else I would do two things minimum: new aftermarket head - sand casted, thicker head gasket. These two things would help you out. Another good thing would be ARP head studs and main studs.

Engine speed alone won't blow your engine - poorly tuned certainly will. Just remember that when you're boosted you added additional stresses to nearly all your engine internals, cooling sys, etc etc. If you protect your con-rods you may just spin a crank bearing in the end.....

FYI the more power you make down low, the harder is on your crank - bearings, trans, & drive-line. Remember HP = T * RPM. So if you're making 250hp at 2500 rpm you are hammering the the engine a lot more than making 250 hp at 4500 rpm. Sure, the stresses are always higher on your con-rods at high rpm, but the high torque also causes high stresses and high amplitude vibration (impact forces) that can be as detrimental on everything.

Yes, rotating parts experience stress from torque, but the stress to power relationship is pretty linear. With reciprocating parts, it's exponential.

Rod stress with 14 PSI at 3K rpm should be about the same as 0 PSI at 6K rpm.

You're right about all the rotating assemblies though.

Retarding the ignition does a lot to save the pistons, crank and bearings, by limiting the increase in peak pressure.

Aside from freak oil pump failures, has anyone with a flat boost/rpm curve (standard wastegate actuator) busted anything except the rods? Has anyone shredded gears with a 2200 that survived? I'm sure there's a torque limit where gears shred, and clutches slip (though mine's good to 260lb/ft).

The consensus seems to be that 7 psi intercooled on a 2200 is OK up to redline with the rods being the weak point.

When it comes to pure rod stress, 7 psi @ redline is roughly equivalent to:

11 psi @ 5500 rpm
16 psi @ 5000 rpm
24 psi @ 4500 rpm

Obviously, there are other roadblocks before you can hit 24 psi, but the rods should hold up.

If a good amount of ignition retard is used and peak pressures don't get too high, what problems do you anticipate with 260lb/ft of engine torque? Our F23s are rated for engines upto 155lb/ft, but lots of boosted ecotec guys have been running it no problem.








2002 Cavalier 2200 5spd

Quote:

Rod stress with 14 PSI at 3K rpm should be about the same as 0 PSI at 6K rpm.

Think beyond simple rotational forces, and add in forces due to combustion. 14 psi at 3k rpm and 25 degrees spark advance produces huge compressive force as crankpin rotates past TDC compared to 14psi at 3k rpm and 6 degrees advance. Most often, rod failures are more likely attributed to excess cylinder pressure in a boosted engine.

-->Slow

"Think beyond simple rotational forces, and add in forces due to combustion. 14 psi at 3k rpm and 25 degrees spark advance produces huge compressive force as crankpin rotates past TDC compared to 14psi at 3k rpm and 6 degrees advance. Most often, rod failures are more likely attributed to excess cylinder pressure in a boosted engine."

Obviously boost is what pushes it over the edge. I don't think many N/A guys lose a rod under the stock rev limiter. My point was the rods can handle far more cylinder pressure at lower RPMs than near redline where they have to change the piston's direction 100 times per second.



2002 Cavalier 2200 5spd

Quote:

Obviously boost is what pushes it over the edge. I don't think many N/A guys lose a rod under the stock rev limiter. My point was the rods can handle far more cylinder pressure at lower RPMs than near redline where they have to change the piston's direction 100 times per second.

That's not what I'm getting at. A properly tuned engine is more likely to lose a rod NA than under boost. As the crank is pushing the piston up, the rod is in compression. I'm not talking about the compression stroke, but the fact that the piston is "pushing" against the rod at one end and the crank is "pushing" against the rod at the other end, so the rod is being "compressed." This is true on the compression stroke and on the exhaust stroke. During the intake stroke the rod is under tension as piston "pulls" the rod in one direction and crank "pulls" the rod in the other. This is where the rod is most likely to come apart. A boosted engine doesn't produce the same kind of tension. With enough boost pressure, the piston ends up trying to push the rod while the crank tries to pull it. That's pretty close to a free ride for the rod and it reduces or eliminates the chances of rod failure under tension.

What I was getting at is this: At low rpm it takes longer for the crank to clear TDC which means more time for pressure to build in the chamber if the spark is too far advanced. This puts excessive compression pressure on the rods because the crank is pusing up and the piston is pushing down with the force of combustion behind it. Enough pressure and something's going to give. You're more likely to survive a little too much advance at high rpm.

-->Slow

Quote:

My plan is to boost up to 14 PSI at 3K rpm, tapering off to 5PSI at 6K rpm, with complete fuel and ignition mods of course.

so, you think tapering off boost in the upper end is going to help your engine to last longer?

add:

Quote:

Obviously boost is what pushes it over the edge. I don't think many N/A guys lose a rod under the stock rev limiter. My point was the rods can handle far more cylinder pressure at lower RPMs than near redline where they have to change the piston's direction 100 times per second.

boost is not what pushes it over the edge, its the ignition timing and stupid people who don't alter it in relation to the added pressure in the cylinder.
your concept of engine physics and tuning principals are poor are best.


ignition timing for an n/a engine is optimized for n/a... just like cam profiles, it needs to be altered in order to reflect forced induction.
if you don't, your engine will destroy itself, and low RPMs are MORE dangerous than high because spark advance is greater under light throttle inputs.

Quote:

My plan is to boost up to 14 PSI at 3K rpm, tapering off to 5PSI at 6K rpm, with complete fuel and ignition mods of course.

This may sound ideal, but I'll bet money you won't be happy with it in the end. Its going to feel like a diesel - mad torque off the line and then a floppy cock @ high rpm. Plus you;ll need a fancy electronic boost contorler to achieve this.

What i've learned about the 2200:
- Its actually a stoudt engine with respect to internals 250whp + is feasible with careful tuning
- The getrag can handle what you through at it unless you are abusing it, clutch may need upgrading
- the LD9 GMPP s/c tune can be flashed to the 2200 pcm - look into this.

you should make your goals based on horsepower, not PSI. What turbo are you planning to use and what kind of whp/wtrq are you looking for?

oldskool wrote:

Quote:

My plan is to boost up to 14 PSI at 3K rpm, tapering off to 5PSI at 6K rpm, with complete fuel and ignition mods of course.

This may sound ideal, but I'll bet money you won't be happy with it in the end. Its going to feel like a diesel - mad torque off the line and then a floppy cock @ high rpm. Plus you;ll need a fancy electronic boost contorler to achieve this.

What i've learned about the 2200:
- Its actually a stoudt engine with respect to internals 250whp + is feasible with careful tuning
- The getrag can handle what you through at it unless you are abusing it, clutch may need upgrading
- the LD9 GMPP s/c tune can be flashed to the 2200 pcm - look into this.

you should make your goals based on horsepower, not PSI. What turbo are you planning to use and what kind of whp/wtrq are you looking for?

You took the words right out of my mouth... Also, the weakest link (without oil pump failure) are the pistons. Mainly the reason mine finally gave is not because of the HP or boost, it was because of the ring gaps. Too tight of a ring gap (the ends butted) is what caused my ring land to pop....



P&P Tuning
420.5whp / 359.8wtq

The rods are pretty stout, I'm not sure who started the 'weak rod' myth. Crank is solid too.
Tuning, timing set, and oil pump are where to start when boosting.

slowolej wrote:With enough boost pressure and flow across the valve port, the piston ends up trying to push the rod while the crank tries to pull it.

FTFY. A huge pressure differential isn't much help with a poor/sharply contoured orifice or poor valve ramping on the cam..




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