Well I got my little suspension experiment completed this weekend, and it was close enough to a success that I'm pretty happy with it.
Started out by analyzing the stock 745 springs in my car: Fairly puny, as you'd expect for a grampa series. 125 lb-in front spring rate, 100 lb-in rear spring rate (with a 1.2:1 motion ratio from the trailing arm, making the wheel rate about 120 lb-in) Stock ride height in my car might have been increased a little bit with the Bilstiens, but the front end sat about 3.75" off the bump stops. Since the front of the car weighs about 1700 lbs, it would only take a 0.55 g hit to bottom out the suspension.
Then I tried something I've wanted to try for a long time, and got a set of rear springs off an '83-91 Ford LTD Crown Victoria sedan. These are plentiful in the JY and require no tools to pull, and are the right major diameter to fit the 740 spring perch. There is also a pigtail at either end of this spring, similar to (but a little smaller than) the pigtail on the top of the 740 spring. The wire diameter is .595", up from the .540" of the stock 740 spring, and since the spring rate is variable to the fourth power of the wire diameter, that suggested an approximate 50% increase right there.
For the rear spring, I looked a little closer to home: 245 rear springs. FYI, if you're looking for a stiffer "overload" spring for your 740, 240 springs are close enough to work just fine. Wire diameter of the 245 springs I had laying around was .515", up from the .475" of the 745 spring, suggesting an approximate rate increase of 40% increase. Most other dimensions are close enough to be identical - major diameter & pigtail specifically - but the height of the 245 spring is about 1" greater than the 745 spring.
I modified the Ford front spring by cutting off the pigtail from the bottom and trimming the pigtail at the top so that it would fit into the stock 740 upper perch. The lower perch on the strut tube is shaped so that the spring doesn't need a "finish coil" on the bottom - indeed, the stock 740 spring is unfinished from the factory. I bolted the spring into place just like that and installed the 245 rear spring untouched, and measured the ride height again. The car sat like a 4x4, about 1.25" higher than my already tall stock car sat.
Measuring up the now-compressed coils, it looked like if I took 1 full wrap off the bottom of the front spring, I'd get a 2.25" drop from it's "current" position, about a 1" net drop. With that one coil removed, the spring sits firmly between the spring seats, but not so firmly that a spring compressor is required to install them. Another quarter coil is about as far as I'd go before I got worried that the spring might fall out when you jack up the car. Moving to the rear, I calculated that about 2.5 coils (including the finish coil) would drop the rear suspension back in line with the front, and I got it just about perfect first try. However, now that the rear spring is so short and stiff, it's in danger of falling out with the axle at full droop and it clunks around a little bit. I've got plans to fix both of those problems, but for now a hose clamp is holding the bottom coil to the trailing arm, keeping things from going sproing if I take too big of a jump.
As the car sits, with the coils that I've removed, the front spring rate is about 260 lb-in, (110% stiffer than it was before) and the rear spring rate is about 185 lb-in (85% stiffer than stock). The car is sitting somewhere about 1-1.25" lower than stock, giving me about 2.5" compression travel in the front. Since the springs are now so stiff, it'll take a 0.75 g hit to bottom out the suspension, a definite increase from the 0.55 g load stock.
Ian's going to measure up his IPD coils for me, but judging by various statements he's made, his early IPD springs are only about 40% stiffer in the front and only a little over stock in the rear, and about the same loaded height as mine are. IPD coils are designed to work well with stock shocks and lower the car a bit without killing the ride quality, and beyond that their performance gains are understandably moderate.
My ride is stiff, but not punishing. I've got the Bilstien HD shocks, and on stock springs they were too firm, giving me a fairly choppy ride. With IPD springs they're probably an excellent match, controlling the stiffer springs properly, but with my new springs I feel that I'm somewhat underdamped. It's not crazy, and I don't think I've got the "ricer bounce" that you see when kids install 2.5" drop springs and use stock shocks, but it's definitely a little bouncy. After driving around with them Saturday and Sunday, I think I can live with the shocks until I trip across a set of Koni yellows or a set of Bilstiens that have been revalved for springs like mine. There's no way I'd install these springs on anything less than Bilstiens - stock shocks will be completely outmuscled here.
I haven't really pushed the car yet, being that I'm still on snow tires, but between these springs, shocks, the full poly front end, and the IPD bars that I've installed in the past 2 weekends, I think it will handle very well for a baby hauler. I can't wait to get the brakes & powertrain up to the level that the suspension is at!
Stock Ford spring:
745 rear spring (left) and 245 rear spring (right), both wearing the upper 700 series retainer/cushion:
Shot of my car sitting with the springs at max height - let's go rallying!!!
Old nasty forged steel control arms versus new shiny forged aluminium arms, plus poly bushings. Note the difference in sway bar end link mounting.
New stance, sorry about the exposure:
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