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I bought a set of used griggs coil-overs front and rear with Koni yellow shocks and struts. I got two sets of springs for the front and one set for the rear, the fronts are 450# and #475, and the rears are #250. So I used the info on the maximum motorsports site to figure up the wheel rate for the 450# front springs and then what this would equate to in a normal spring, and what I'm getting is 1620#.

I took 450# x .9 = 405# wheel rate
405# x 4 = 1620#

I'm thinking that the numbers probably don't translate quite this linear, but if this is even close that seems far far to stiff for a street car.

These were on a fox with a 351w, which is what I was intending to install them in, but due a fortuitous turn of events I have a clean sn-95 roller that I was going to try to use them on. Griggs lists their coil over kits as 79-04 otherwise I wouldn't even think of this.

So my question is, is a 450# spring appropriate for a 90% street driven sn-95 with an iron headed 351w in it?
 

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It's a little stiff, but with the right damper it could be tolerable. 300-350 would be more livable for the uses you're describing.
 

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I have 425s in my 3000 lb. notchback with koni yellow SAs and it can be very harsh on mediocre and crappy streets. However, on the highway, it's no big deal unless there is a really bad seam joint. I drove my car from Texas to Boston like that and it was fine up until I got to Boston (which has terrible roads). Still, the car rides better than when I had 1000# conventional front springs in it.

I say start with the 450s once you have the car moving and go from there. If you don't like it, it only takes an hour to swap in different springs. You can sell your 450s and 475s and get some 350s or 400s later.
 
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