[CandiMan]

All the research I've done only list them as sport or non-sport. But doesn't go into details as mono or twin. I've even called a couple dealership and their computer is saying the same thing, sport or non-sport.

 

[darkman]

Monotube.
https://www.rockauto.com/en/moreinfo.php?pk=1308689&cc=1433648&jsn=452

 

[CandiMan]

Okay, so this past weekend I tried installing my H&R springs and luckily I started with the front first. The inner dia of the front bottom H&R spring was too narrow and did not fit onto the strut perch.

The ID of the bottom of the stock front spring measures 2.75"
The ID of the bottom of the front H&R spring measures 2.50"

It's like the H&R springs were design for the small dia twin tube struts

 

[darkman]

Okay, so this past weekend I tried installing my H&R springs and luckily I started with the front first. The inner dia of the front bottom H&R spring was too narrow and did not fit onto the strut perch.

The ID of the bottom of the stock front spring measures 2.75"
The ID of the bottom of the front H&R spring measures 2.50"

It's like the H&R springs were design for the small dia twin tube struts

The current H&R catalog lists lowering springs for the 2004-2007 CTS-Vs, but not for the CTS Base models with either 6 cylinder engine for those same model years. Additionally, the GMPARTSDIRECT catalog list difference part numbers for 2004-2007 CTS-Vs vs. 2004-2007 CTS Base model front coil spring. It also indicates they are NOT interchangeable. Based on that evidence, it looks like you may have a lowering kit for CTS-Vs that was not designed to work on a 2007 CTS. I would contact H&R to see if that is the case.

 

[CandiMan]

That's possible, I have the FE5 suspension on my V6 CTS which I thought was the same suspension for the V