Pics of my broken 3900 lz9

[Shaun41178(2)]

Stock bottom end. I'd say stock rods are good for about 600 tq. Keep it under 500 wheel tq and you shouldn't have issues. I pushed this thing hard.

Dynod at 500 wheel tq on 15lbs but I had it up to 20 lbs several times on the street tuning it. I'd guess that's around 600 wheel tq. That's prob where the damage occured. One broken rod which detonated the piston, and one severely bent rod. The others passed a quick visual but I prob wouldn't use them again.

Stock pistons held up to the abuse. Cast pistons seem just fine with a nice safe time on the and e85.

IMG_20230204_215123786_HDR.jpg (852.74 KiB) Viewed 1937 times

IMG_20230109_192513527_HDR.jpg (793.21 KiB) Viewed 1937 times

IMG_20230109_192314120_HDR.jpg (792.08 KiB) Viewed 1937 times

[Shaun41178(2)]

IMG_20230107_203030611.jpg (3.9 MiB) Viewed 1936 times

[ericjon262]

ouch, I bet that made a hell of a noise.

[pmbrunelle]

Yeah, I agree with your idea that the connecting rod broke and then cracked the piston, versus the piston breaking and causing a problem with the connecting rod. I think this largely because of the bent rod in the other cylinder, which shows you were running the rods at the limit.

I know a guy who designed a connecting rod for a high-performance production engine, but it's hard for me to pick his brain. He doesn't speak French, and his English isn't so strong

[Shaun41178(2)]

I'm contemplating going with the Pauter rods that are available. Maybe $1600ish but it seems I have reached the limit of factory rods.

Either that or just turn it down to about 450 whp and call it good but that's no fun.

Spec 5 with modified pressure plate looks good and never slipped. It's a unit

[ericjon262]

These rods have the right journal diameter for an LX9, I haven't looked to see if they would possibly fit the later VVT engines. They are a touch narrower than stock LX9 rods, but could probably work.

https://www.summitracing.com/parts/esp- ... make/buick

[Shaun41178(2)]

In a vvt you would need custom pistons as those rods from summit will be too long for the 3.9 crank, or too short for the 3.5 crank.

[ericjon262]

In a vvt you would need custom pistons as those rods from summit will be too long for the 3.9 crank, or too short for the 3.5 crank.

they make a longer 6.3" as well, I agree it would need custom pistons too. I just posted them as an option worth investigating.

[The Dark Side of Will]

These rods have the right journal diameter for an LX9, I haven't looked to see if they would possibly fit the later VVT engines. They are a touch narrower than stock LX9 rods, but could probably work.

https://www.summitracing.com/parts/esp- ... make/buick

Didn't Upson use 4.3 Chevy rods in his 3900? They also have 2.250 rod journals, right?

[Shaun41178(2)]

He turned down his crank to use the 2 inch rod journals.

The lengths for the rod need to be around 5.830 or 6.030 or you have to use custom pistons

[ericjon262]

These rods have the right journal diameter for an LX9, I haven't looked to see if they would possibly fit the later VVT engines. They are a touch narrower than stock LX9 rods, but could probably work.

https://www.summitracing.com/parts/esp- ... make/buick

Didn't Upson use 4.3 Chevy rods in his 3900? They also have 2.250 rod journals, right?

I think some 4.3's have 2.25" journals, but I don't remember for sure.