Yep Because only a very few are aware of that trick.

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Charrlie_S
65 Valiant 100 2dr post 170 turbo
66 Valiant Signet 170 nitrous
64 Valiant Signet
64 Valiant 4dr 170
64 Valiant 4dr 225

If you study the SL6 engine's oiling passage ways, you will see that main bearings #2 & #3 feed oil into two connecting rods, where the front and back main bearings only have to supply oil to one con rod.
I feel that the way the middle main bearing oil feed "cross-holes" intersect ( " T " into) the main oil gallery, limits oil flow into those passageways. It is easier for the oil to go past these interscetions and flow to the end of the gallery, instead of making the turn midway down. This "poor flow" effect happens when oil demands are high... and the pump / passageways can not deliver enough oil pressure / flow. When this happens, the middle bearings will get "shorted" first.

So why don't con rods #2 & 3 fail as often?
If you look at how the pressurized oil comes out of the SL6 oil pump and enters into the main gallery, there is a pretty straight shot to the front section of the main gallery but the oil has to take a 180 degree "turn around" in order to enter the rear half of the gallery.
Racers who have seen some of my high RPM engine set-ups have spotted a hard line, oil "by-pass" tube, that delivers oil flow directly from the rear of the oil pump and into the rear of the main oil gallery... used to get more oil flow into the back half of the main gallery.

So why rod #5 and not #4?
I can say that when I tear down an engine with a failed #5 con rod bearing, number 4 bearing is also pretty "distressed" because it usually has received some of the displaced bearing material from it's oiling system "room mate".
I also know that cylinders number 2 & 5 tend to run the leanest so detonation or other lean condition "side effects" may contribute to early bearing failures. Crank journal stability may increase the amount of oil needed to keep a bearing from failing and #5 may-be less stable then #4.

To reduce the chance of bearing failure, I spend a lot of time on the cylinder block's oil passageways, drilling them out and "blending" all the corners, to help get the oil to where it is needed most... to the bearings.
I also do the "by-pass tube" addition to high RPM engines.
Most important... good oil pump selection and reworking. If the oil pump can not keep-up with the oil demands of the engine, the con rod bearings are the first to go.

Please "chime-in" if you have other ideas or observations... I am always thinking about ways to get great oil flow through the SL6.
DD

What do you mean? I'm very interested.Wes

It means that there is a lot of extra weight on a SL6 crankshaft and some of that weight can be removed, in order to help the engine spin-up the RPMs faster.

As with most things in life, there are "trade-offs".
- It is a lot of work to remove weight off a big, forged steel crankshaft.
- The crank will have to be rebalanced after you do the weight reduction work.
- The engine will spin-up faster but has less internal inertia, in a stick shift car, that makes it harder to get moving and easy to stall.
- Small counter weight cranks have more harmonic stress, you need a good damper and even then, light cranks tend to crack more often.
DD

A freind built a 440 with an extremely cut down Hemi crank balanced it within a gnat's butt hair and is not running a balancer.

Majorly cut it down! Auto car. Wants to drop it in a 1900lb Art Morrison chassis.

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick

So basically the stock oiling system is only good to about 6k? I had heard that it was good to 7500.

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Rob M
'80 D100 SWB stepside, S6 4-spd OD

Best of luck to your friend! A damper would be a very good idea. Cranks last longer with dampers and testing has shown small block Chevys to be faster down the 1/4 mile with the large factory damper as opposed to the smaller ones.

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Joshua

It has a lot to do with how you set-up the oil pump.
Many racers will shim the pump's releif valve spring so it pumps at a higher pressure and that allows them to run at higher RPMs.

Pressure is not the same as good oil flow. You can "cover-up" some flow problems with increased pressure but doing that eats more power and runs the risk of shredding the oil pump's drive gear.
DD

The rest of the motor isn't good to 7500 anyway.

Gratuitous engine pic:

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Official Cookie and Mater Tormentor.

Does that price include shipping?

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Yeah....Im the one who destroyed this rare, vintage automobile.....

Shipping to your location would be $39,995.98 USD. Sorry, no checks.

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Official Cookie and Mater Tormentor.

What would it take to make the bottom end hold 7500-8k? Aftermarket rods and bolts? I know it'd be pushing it still on a 225, but maybe a 170 or 198 could handle it. Of course, you'd need some major head porting and a big ol' cam to get it there

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Rob M
'80 D100 SWB stepside, S6 4-spd OD

Aftermarket rods. Stock rods will not last at 7500-8000.

Oh, and some new kind of head that will flow enough to support that kind of RPM and make power.

And a block that will keep the main web in at the RPM you speak of.

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Official Cookie and Mater Tormentor.

Hopefully I will have an answer for you in the next year or so. I got a m90 over the summer and in the process of acquiring a 170 from Joshie. The plan is to start at 10lbs of boost, and possible work my way up but will look at some dyno tuning from there. The plan is to run around 7200 rpm and see if I can't creep her up to 8000.
I got some older H-beam rods from a small block that come in at 6.123 so I will have a "long" rod set-up which Doug has proven to work well. I hope to mimick the Mahle setup piston that weighs in like hockey puck. The crank will have some lightening to it, but nothing to exotic. Also, the longer rod can allow me to move the pin up higher, and since the 170 is a flat deck piston so moving the piston down a little bit shouldn't kill the compression ratio.
Planning on a roller cam and roller rockers, though the cam doesn't look to be in the cards. I'm worried about valvetrain destruction. Everytime you solve one problem another one pops up huh?

I wonder why top alcohol/fuel motors don't run balancers?

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'72 Duster 198 stock cam, 3:23's Hookers on jack stands for 8 years in the driveway
'79 Maxivan 360 Offy Qjet Comp RV cam/rusting in the driveway.
93 D350 160HP Cummins Auto Dually Clubcab needs a injector pump
2005 Golden Couch Buick