connecting rod strength

[hon87]

I understand that as pistons reach top dead center, their inertia is trying to pull them through the top of the combustion chamber. At that point, the rods and rod bolts are the only things keeping this from happening. At some point, there is a speed at which the inertia of the piston is enough to break either the connecting rod, or the rod bolts. What I want to know is whether or not higher compression ratios (or higher boost levels in forced induction engines) will decrease inertial loads exerted on the rods and rod bolts allowing the engine to turn higher rpms before the rods let go. If so, how much of an increase in engine speed can be expected for each full point that compression is raised?

 

[evelrod]

Just a note here---Almost EVERY time one of my engines has let go(I've blown up more than a few for all sorts of reasons), it was just as I lifted. The sudden deceleration seemed to pull stuff apart in a BIG way. All the boost , the high cr, the high rpm all play a big part in fatigue, but it is that sudden strain on the engine with that CLOSED throttle that did me in every time.



Rod

 

[GregLocock]

Yes, the anecdotal opinion in the engine cells at BL was that the high speed/low load part of the running-in process that usually killed engines.

This cushioning effect might be for real, but remember that at TDC it is mostly combustion pressure, not just the compressed intake charge, so CR is really a secondary consideration. If the original poster wants to claculate this out then he needs to work out the piston acceleration (ivymike has posted on how to do this), and then find out the interaction between cylinder pressure and piston acceleration, and some cylinder pressure diagrams at different CRs.

This doesn't get around evelrod's point - at some time in an engine's life you have to back off the throttle and then this small cushioning effect will evaporate, shortly followed by your engine.

Cheers

Greg Locock

 

[PriamEngineering]

In general the higher the engine speed the higher the tensile inertia force on the rod. Overspeed occurs for a variety of reasons so I design joints in conrods to live at 50% overspeed condition. As for a direct correlation between PCP and engine speed, this will be dependant on the load the engine is seeing.



A description of the type of work I have done can be found here

http://www.fea-uk.co.uk/connecti.htm

 

[mgithens]

Your connecting rods only feel the effects of inertia on the exhaust stroke. The compression stroke has the cancelling effect of inertia one way and compressive forces in the other, so higher compression won't allow an engine to spin faster - when looking at inertial forces.

Inertial forces are a factor of speed and mass, so either lighten the reciprocating mass or slow it down... neither lend themselves to speed, power or reliability.

 

[ivymike]

I'm sorry, I must have missed something... why doesn't slowing down your reciprocating mass "lend itself" to reliability?

 

[mgithens]

ok, I'll restate...

Inertial forces are a factor of speed and mass, so either lighten the reciprocating mass (poor reliability, due to the nature of the weaker design by having less material available to structurally support the load that is being distributed over the surface of the piston) OR slow it down (poor power, due to the design of the internal combustion engines need to have more cycles per unit of time to create power - made evident at the extreme amounts of hp in high revving engines, found typically in motorcycles and the like)

Just poking at ya...

 

[hon87]

There is one way to lighten the mass of the reciprocating assembly without sacrificing reliability. It doesn't lighten the mass by very much, though. Removing the casting flash from the rod beams works really well, but you have to have the rods shot peened to restore the surface tension. I don't know to what performance gains you can expect from that, but I've seen people save a lot of money on performance rods that way. Is there a formula or rule of thumb or something to tell me what kind of increase in rod strength I could expect? The engine I asked the original question about is a 2.0L Honda Prelude engine. It's one of the worst Honda engines to try to build because of the rod/stroke ratio, but since I have several of them laying around, I decided to go ahead and give it a shot. The stock engine red-lined at 6500rpm. I've heard that you could take it up to 7500 without tearing anything up, but I want to go up to 8200, since I'm using a high reving cylinder head. Is that a realistic goal if I remove the casting flash and balance and shot peen the rods?

 

[evelrod]

To start off, the parting lines from the forging operation on connectiing rods (not casting flash) are often ground smooth to eliminate stress risers, or at least limit the area where they are likely to occur. The weight removed is insignificant. Removing the upper and lower balance pads is much more efficient, maintaining proper balance in the process, of course. The last area is the sides of the big end can be cut back a little above the bearing area(you must maintain the rod width at the bearing for side clearance purposes). I have seen the beams polished down, but I don't like it. Also I have drilled out the heads of the mounting bolts (kinda silly). Lighter pins are OK if pin flex doesn't become a problem as are lighter pistons.
With a max effort at reducing reciprocating weight, grind and bevel the crank counterweights and change to the lightest, strongest flywheel clutch assembly you can buy.

Lotta work!!!but your engine will rev faster and decelerate faster. Along with cam and top end work, this is a basic formula for building just about any performance engine. It is often cheaper in the long run to BUY custom made parts for a performance engine. Take your choice.


Rod

 

[carnage]

The best thing you can do to help achieve your goal is to buy a set of strong lighweight pistons. This will greatly decrease the inirtial forces on your connecting rods.