1991 Volvo 780 Coupe; Rear Nivomat IRS Suspension Conversion:

I recently obtained this car down here in the south of Mexico. It has 149,000 miles on it and other than some “issues” with some of the plastic parts and collapsed rear Nivomats it is in excellent condition.
I began my research on the internet seeking information and of course prices & availability for replacement Nivomats; they are prohibitively expensive. From there I located various posts describing converting the rear suspension to “conventional” shocks and springs, as well as a conversion Kit offered by IPD that has mixed reviews.

The most popular describes using (1992) Toyota Previa Springs and Bilstien (or other) shocks for a 960 conversion.
The interesting thing about this “conversion” is that there seems to be 3 different rear Previa springs and the articles I reviewed don’t specify the ones chosen, and that without a 1 ½” spacer on top of the spring the rear of the car is lowered by that amount which causes issues with alignment.

The “MOOG” Specs for 91-95 Previa rear springs are:

MOOG Part # CC251:
End 1: Squared
End 2: Tangental
Inside Diameter: 3.838"
Bar Diameter: 0.593"
Install Height: 11.00"
Load: 456lbs
Spring Rate: 260 lbs/in
Free Height: 12.70"
Type: Variable


“Stock” 780 Nivomant Coil Springs:

19/32 = 0.5938

This Photo is a comparison of the Volvo and Previa springs. This clearly indicates the difference between each and also the significant difference in Previa springs. These Previas measure 13 ½ inches as opposed to the MOOG example of 12.7”.

“Stock” 780 Rear Springs:

End 1: Squared
End 2: Squared
Inside Diameter: ?
Outside Diameter: 5”
Bar Diameter: 0.4742
Install Height: ?
Load: ? lbs
Spring Rate: 75 lbs/in. (About)
Free Height: 17 3/8”"
Type: Constant Rate
Number of Free Coils: 7

Volvo 780 IRS Measurements:

Inner Pivot to Outer Pivot: 24”
To Spring Center: 18”


Conclusion:

I wish to keep the car at a stock ride-height. I would also like to utilize springs from another car or aftermarket.
I have found from my research that the IPD “Kit” uses “Lowering Springs” and they are “too soft”.

Ideally the springs should be: (?)

End 1: Squared
End 2: Squared
Inside Diameter: 3.838"
Outside Diameter: 5”
Bar Diameter: 0.593" (?)
Install Height: (?)
Load: (?) lbs
Spring Rate: 260 lbs/in
Free Height: 13 1/2"
Type: Variable or Constant

Shock Absorbers:
Monroe 31162 (?)

If anyone can add “real life” experience to this I would really appreciate it.