Converting an OEM style strut to a coilover is an old trick.... but not a very good one.
A long time ago we had a lot of customers asking us to make one of these kits, and we produced a short run of the silver "over collars" above. This would be used with an OEM style strut where you cut the lower perches off, slipped the collars on, and then switched to the appropriate diameter coilover spring (60mm or 2.5", depending on the diameter of the strut and collar). The problem was, the results are almost always terrible. Why? Because the struts are still always too long for lowered ride heights and the car bottoms out a LOT. The second problem was the shocks would never be valved properly for the SIGNIFICANTLY stiffer spring that you would want to use. Re-valving a Bilstein OEM style strut is TRICKY, too.
In 2010 we built a BMW E30 LSx for a budget restricted event where we had to build the car for $2010, and compete in a 3 event magazine sponsored challenge: drag race, autocross and concours. Its called the Grassroots Motorsports $20XX Challenge.
Since we couldn't use proper coilovers, we bought some old Koni OEM style struts for $25. Then we cut the lower perches off and bought some cheap threaded circle track collars.
With some used 2.5" ID coilover springs, we had a stiffer spring rate and "homemade coilovers" on the car. How bad could they bad, we figured? Oh they were awful... they rode like a jackhammer and were in the front bumpstops ALL THE TIME.
But for this extremely tight budget build, they worked well enough in competition (against other crap can cars), but I would
never recommend this for the street.
We did "OK" that first year, but the car failed miserably in the drag race portion (355 whp + T5 transmission = BOOM!). We came back in 2011, after just doing some suspension set-up tuning, and won the event.
But when we weren't doing this ONE specialized, weirdly low budget magazine event, we swapped on AST 4100 coilovers and some CCW 18x11" wheels (in place of the 15x10" $50/each circle track wheels). Those AST struts had 2" shorter strut housings and that made for
proper bump travel at the low ride heights we ran. They also had the
proper valving for the much stiffer than stock spring rates we ran. We tracked it in NASA TTU and it was FAST! It rode 100 times better on the street, too.
So that's one of many horror stories I can share about "home brew coilovers."
Just like we have explained in the "OEM vs Coilover" discussions here, lowering a McPherson strut car requires both a shorter spring
AND a shorter strut housing. Just putting lowering springs on an OEM style strut leads to a direct loss of bump travel. Doing the same thing with a coilover adapter sleeve and short coilover spring on the same OEM length housing does the exact same thing - no bump travel, and you are in the bumpstops after every bump.
As for other "low cost coilovers" designs.... you always get what you pay for. Some lower cost S197 coilovers use a Koni twin tube insert, and we've discussed the limitations of those in the past. A properly built, quality monotube shock is better in every performance metric, and in ride quality as well. The AST 4150 monotube single adjustable used to be the lowest cost proper monotube single adjustable coilover for the S197, but those are still in limbo for several more months as the new distributor gets set-up and inventory rolls in. But we did find out that the price of these 4150s has risen from $2200/set to $2650/set.
Which is the same price as the proven, reliable, and strong MCS TT1 coilovers. And yes, with springs (+200) and camber plates (+479) it approaches $3300. What can I say, it is what it is... but the only alternatives are Chinese made junk or twin tubes. We are working on some things to reduce the price of these kits, trying to get them closer to $3000.
We know that $3300 price tag is a bit steep for folks, and we already offer the shorter strut housing Bilstein
Street Pro monotube inverted strut/shock + spring + camber plate combo for $1500. This works for good street ride, proper bump travel at a lowered ride height, and is strong as a tank. They are good for some autocross and track use, but aren't what we'd use on a serious competition autox or track car.
Vorshlag is working on another solution, using Bilstein motorsports parts, that could
potentially get closer to the $2500 mark. We should have something this summer, if we can get time to develop and build this kit.