911SC cross member.

[bighead]

Has anyone upgraded their Gbody Porsche with one of these billet subframe cross members?

[Hinamotortalks]

I haven't personally run one, but a few guys in our local PCA group swear by the billet cross members for stiffening up the chassis. The stock piece is solid, but if you're tracking the car or just want that extra precision, it might be worth the upgrade.

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[bighead]

What companies make them?

[silverlock]

Are you referring to something like the PMO aluminum front cross-member? https://pmbperformance.com/products/aluminum-front-suspension-crossmember-for-porsche-911-912-914-and-930-models-1969-89

Or something else? (A quick Google turned that up...)

[bighead]

Originally Posted by silverlock

Are you referring to something like the PMO aluminum front cross-member? https://pmbperformance.com/products/aluminum-front-suspension-crossmember-for-porsche-911-912-914-and-930-models-1969-89

Or something else? (A quick Google turned that up...)

Is that as cast piece or billet? I’m looking for a billet aluminum cross member.

[mepstein]

I’ve never seen a billet aluminum cross member but perhaps someone is making them. Empi is reproducing the cast aluminum ones.
RSR’s had a forged cross member. I know my buddy paid over $7k to get one for his RSR - and that was years ago.

[75 911s]

Maybe you're thinking of billet x member that come on 935 front end kits. Check out RSR products, Elephant Racing, or Tarret Engineering for a 935 kit. They use coil overs as they replace the lower control arm and torsion bar aren't used.

[bighead]

Saw this one being sold by Aircooled Exclusive on IG. Anyone have personal experience with one?

[Jonny042]

Cool! Price?

[PeteKz]

Jeez, the stuff people will spend money on...

Looking at that "billet" piece as an engineer, I can't see how it "stiffens up the chassis." As a separate part, it's obviously stronger and stiffer (and heavier), but it still bolts to the body at only two points--at the ends. It holds the torsion bar ends and resists their torque with a 3-ft. long I-beam section. The amount of deflection of that cross member is minuscule relative to the body. The billet piece just makes it more minuscule.

[bighead]

Originally Posted by PeteKz

Jeez, the stuff people will spend money on...

Looking at that "billet" piece as an engineer, I can't see how it "stiffens up the chassis." As a separate part, it's obviously stronger and stiffer (and heavier), but it still bolts to the body at only two points--at the ends. It holds the torsion bar ends and resists their torque with a 3-ft. long I-beam section. The amount of deflection of that cross member is minuscule relative to the body. The billet piece just makes it more minuscule.

I was looking at it more from the standpoint of the strength of the crossmember not really looking to “stiffen the chassis.” I have seen guys get cracks in the factory aluminum crossmember so I think this might be a better option to avoid that.

Thoughts?

[PeteKz]

It depends on where the cracks are. I haven't seen cracked aluminum cross members (or steel ones either). Where do they crack?

[bighead]

Originally Posted by PeteKz

It depends on where the cracks are. I haven't seen cracked aluminum cross members (or steel ones either). Where do they crack?

Another member posted about his cracking. There are some pictures on his thread.

https://forums.pelicanparts.com/porsche-911-technical-forum/989493-my-low-budget-dream-car-build-211.html

[dannobee]

What is the opposite of good? "Better."

I've never seen the old steel ones crack. And for a car with a light front end anyway, reducing weight at the lowest part of the car didn't sound like a good idea, even when the aluminum ones first came out.

[bighead]

Originally Posted by dannobee

What is the opposite of good? "Better."

I've never seen the old steel ones crack. And for a car with a light front end anyway, reducing weight at the lowest part of the car didn't sound like a good idea, even when the aluminum ones first came out.

Did the steel ones come on or the 911, or is this a swap from the 914?

[dannobee]

Some 911's had steel up until 77. Carrera's and RS's had aluminum since 73.

[PeteKz]

My 1973.5T has the aluminum cross member. I read somewhere (I think in Johnson's book) that they phased out the steel crossmembers in 1973.

Bigheahd: I looked at the pictures of the cracked cross member. Odd. If I managed to crack an OE one, I might consider the billet as a replacement. Or the steel version.

[mepstein]

Maybe it’s better for the aluminum crossmember to take the impact and crack rather than the chassis. I imagine it happens when hitting speed bumps, etc.
I would say about a third of the cars we work on have a piece broken off the crossmember. Many of them are slightly bent if you put a straightedge against them.

Aircooled executive seems to be effram’s /blackbird’s site. Is he selling billet cross members or just documenting the difference?

[dannobee]

Originally Posted by PeteKz

I read somewhere (I think in Johnson's book) that they phased out the steel crossmembers in 1973.

This is wrong. You can find factory steel ones up to 77. I saw enough of the cracked ones when I was working at the stealership. My bone stock 75 had a steel one.

[mepstein]

Originally Posted by dannobee

This is wrong. You can find factory steel ones up to 77. I saw enough of the cracked ones when I was working at the stealership. My bone stock 75 had a steel one.

I thought it started on SC’s.