was just thinking today, if you turbo a n/a motor where weak rods are the limiting factor to how much hp can be made....
would lowering the redline and increasing the boost pressure allow you in to make more power without increasing the loads on the rods?
It really depends on the engine and the tune. As far as boost pressure goes, I would think the in cylinder pressures (especially during combustion) would create the greatest loads on the rods.
Yeah I agree. I busted rods at a 34mm restrictor induced limit of 5500 rpm by giving them a tad too much boost. I think if you're gonna go through the effort of sticking a snail on a non-snail-equipped engine, it's false economy not to put forged slugs and hbeams in it.
is there a quick way to estimate cylinder pressure? the thought i had was with loads being exponential reducing redline from 7000 to 6000 would equal something like a 25% reduction in load on the rod, but i don't know if that 25% is really significant in the overall load the rod is seeing when you factor in cylinder pressure.
I'm a software guy, not a mechanical engineer, but...
Reducing the redline will reduce the loads that the crank places onto the rods on the exhaust, intake, and compression strokes, where it's moving them around. On the power stroke, the "bang" is pushing the piston down, and RPM isn't going to have much of an effect on that (if anything, higher RPM is probably less load on the rod, because the crank is moving away faster).
Tensile and compressive loads on a con rod due to RPM is measured by the square of RPM, meaning doubling RPM = 4 times great stress, compressive loads on a rod due to cylinder pressure is linear, it is also worthy to note that rods have much more fudge room with compressive loads than tensile loads. so in theory if the heads allow and turbo can provide, you could lower redline to reduce some stress. But the stress your alleviating may not be opposite the stress your creating, the point being making power at lower RPMS will tend to be easier on the rods.
Ignition timing with a boosted engine is critical. The boost itself and even the RPM load is minimal compared to the combustion pressures, so you need to make sure you aren't running too much ignition advance or you will be hammering the rods to death.
Short cut method to determine what your peak hp should be on that engine: what's the safe working load for the rod bolts? (ARP can be helpful to a point on that one.) It really all boils down to cylinder pressure on the piston at the time of combustion. 20lbs of boost at 2500rpm with weak rods, and boom. ventiated block.
Dr. Hess is right, watch out for the timing in a big way.
Hopefully you've got one of the more popular, current engines, or one that had a higher hp option. Because aftermarket rods can be costly. For example, Carillo rods for a 1500cc Spitfire were in the $300 each range, last I shopped around. Not exactly a rare engine, but out of production, low demand, etc...
OTOH, I do know of one guy who welded rebar to his stock rods. One per side. He said it worked. I've since lost track of him, so I don't know the final outcome. but he was running some high boost pressures. Of coure there's always the wrist piston and piston strength to consider, too.
bravenrace wrote: Ignition timing with a boosted engine is critical. The boost itself and even the RPM load is minimal compared to the combustion pressures,
Something about the phrasing of this sits wrong with me, but your point about ignition timing is a very good one. I think you cannot ignore RPM's in a discussion of con rods and their limits. But assuming your not going to raise redline and your not using a CF SC your prolly fine.
This of it this way, on a 200HP Civic SI motor, which is more likely on stock internals, acheiving 400HP with stock redline and a turbo, or increasing revs 2K rpm and doing an N/A build?
In short, no.
Cylinder pressure is what kills rods.
In the Honda World, D-Series rods were "known" to be good for over 200HP. I bent all 4 of mine @ 188HP. Why? Cylinder pressure. I was making 50-80 ft-lbs more torque down low than those other guys.
What I meant is that you can more easily kill the rods under boost with improper ignition timing than with boost or RPM, but of course there are logical limits to all three. Say you run 10 psi boost. That will raise your compression pressures something less than 10x's normal. But its combustion pressures that kill the rods, and that pressure can be 3-4kpsi or more. The key to making the rods last is to retard the timing so that the cylinder fires at or just after TDC. If it fires before TDC, the rods get hammered and bend.
For example, I ran 14 psi boost on a stock block D16A6, which have pretty soft rods, for 8 years without a failure, and that was a car that saw a lot of open track time. This engine made a lot more than 200hp. I've seen others bend rods with only 5 psi boost because they had the ignition timing advanced too far. That was my only point.
bravenrace I agree in full 
The reason i was asking was i was thinking of a low budget boosted civic for my next project. Seems the common opinion is around 200hp is the max for the stock motor. That won't quite give the performance i'm looking for. If getting more requires a built motor i think i might be better off using the money to buy something with a stronger stock motor such as an integra with a b-series or something entirely different. I was thinking of the civic due to the huge amount of cheap parts available, light weight, and i like the styling of the EG civics. The integra would still have the cheap parts available, but is heavier.
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