Ey8s

I've been a long time lurker on this site gathering notes and ideas for my '85 GSL build. One idea I really liked was the FC Subframe swap so I've searched the web high and low for information and have basically found a handful (maybe less) of people who have actually performed it and have been kind enough to share information on how they did it. Two of those threads are in this forum. I did find some minor discrepencies between them in relation to locating the subframe and spacing from the FB frame, but they provided a good starting point. My thanks go out to those guys for providing that information.

I purchased a 1988 vert parts car since it had the good brakes and the quick ratio ps rack. I'm not swapping the rear because, honestly, I'm performing a LQ4/4L80E/8.8 Explorer drivetrain swap. I already had a 2001 2500HD parts truck and a friend gave me the '85 GSL after he removed the engine/trans for another project.

I wanted to create this thread to share my findings and provide some pictures illustrating some aspects of this swap that will give people perspective. I've seen a few pictures of the actual subrame swap and modifications but none of how things line up in the wheel well.

Up front I will state that I'm not an engineer and I didn't perform meticulous, instrumental measuring for this report but I have been building/tinkering with vehicles all of my life including plenty of design, fabrication, engine building, welding, etc. on street and offroad vehicles that were far from stock.

OBSERVATIONS:

1) The spindle to strut angles appear to be the same on both the FB and FC assemblies. I measured an 11 degree cant(or 101, or 89 depending how you're looking at it)
2) The spindle centerline to bottom spring perch appear to be similar at an observed 8 3/4" - FC and 8 5/8" - FB.
3) The spindle centerline to the pillowblock mounting surface on an uninstalled strut/spring/spindle assembly appear to be similar at 22" - FC and 21 7/8" - FB.
4) The front subframe mounting bolt location appears to be the same between the FC and FB in relation to the front subframe mounting bolts and the pillowblock mount centerline in the strut tower. I measure 4" of offset but the distance from the mounting flange to the front bolt under the frame rail is 1" greater on the FC.
5) The FC springs (uninstalled) are an 1" or so taller than the FB springs.
6) The front subframe mounting bolt distance is approximately 31 1/4" for both subframes - although I had to "massage" the FC subframe holes a little to mount the subframe on the FB for my initial mockup. The rear FC subframe mounting holes are approximately 13" back from the front ones and have a little more spread at 32".
7) The pillowblock mounting flange center to center distance between flanges (right and left sides) appears to be the same between the FC and FB at approximately 41 1/2". I'm thinking the track width difference of .6" on each side is due to spindle offset opposed to a difference in strut camber between the FC and FB.
8) I determined that the correct rear spacing of the FC subframe to the FB frame is 3/4". I've seen one thread stating 1", another using 1/2" - although he thought 5/8" was ideal. Both may have very well been correct for their particular vehicle and subframes used.

MOCKUP:

My spindle/strut/spring assemblies consist of FC spindles, struts and pillowblock with FB springs and spring "hats". It all lined up and worked for this purpose. I still has the 22" length from spindle centerline to pillowblock mounting surface distance. Using the FB spring hat with the FC pillowblock puts the hat 1 1/4" closer to the spindle which works for this setup because the FB spring is about an 1" or so shorter anyway. The FC Spring and Hat won't fit in the FB strut tower as-is. The FC pillowblock has a bearing in it that allows the FC spring hat to rotate during turns. The FB spring hat has a bronze bushing brazed on it for the same purpose but it matches up with the FC pillowblock bearing. I'm not sure if this ultimately a viable assembly or not for street use - if I go this route I'll be happy to share my findings.






INITIAL CONCLUSIONS:

1) Using the stock FB front subframe mounting bolts to locate the FC subframe will give you proper caster geometry for the spindles.
2) If you want to maintain proper (factory) caster you have to maintain to same mounting relationship between the subframe and the pillowbock/strut tower mount. For my initial mockup I used the factory located front bolt as a starting point because it properly locates the FC subframe as far as geometry is concerned.
3) Doing this moves the spindle centerline about 1 1/2" forward from center of the wheel well which would not appear to work with the stock 'vert wheel and 205/60/15 tire.
4) To properly center the wheel in the wheel well and maintain caster angle the subframe AND pillowblock mount have to be moved back at least an 1". I'm going to remove the spring from one of my spindle/strut assemblies to explore the required distance. I've read where somebody used the same mounting location as my mockup - but as you can see in the picture - I don't know how this worked for him unless he's using aftermarket fenders/flares with modified wheel openings.
5) Relocating the pillowblock mount would be an involved affair. I'm going to look at the possibility of grafting in the FC strut towers. Doing this would properly locate everything AND allow the use of a complete FC spindle/strut/spring/hat/pillowblock assembly. There would still be adequate room for the master cylinder/booster assembly IMO.

I haven't seen any long term - or post race reports from anybody who has performed this swap. My initial thoughts lead me to believe that moving the subframe back to make the tires work in the stock FB wheel well WITHOUT moving the pillowblock mounts would produce undesirable handling characteristics from the result of reducing the amount of positive caster.
If you've read this whole report - I hope you found it worthwhile and informative. I appreciate any and all comments/input.

Eric

JSmall

Whoa nice introduction to the forum aye! I've got no knowledge about any of this sort of stuff yet.. But I'm keen as to see what you come up with

Ey8s

After studying it more I'm going to cut the strut towers from the body of the '88 and graft them into the FB. The pillow block mount has to be exactly 1 1/8" higher. I'll remove some sheetmetal from either side of the tower as well when I cut them out of the FC. Hopefully I can set it up so I can use all FC stuff (springs, hats, blocks, struts). It's going to be a day or two until I get a chance to start on it.

KansasCityREPU

iTrader: (9)

If you're going to cut out the strut tower on the body, you might think about extending them upwards. There was a kit some time back that would do this to lower the car.

Check out MustangHammer's race car. Something similar to his.
https://www.rx7club.com/build-thread.../#post11811623

peejay

Ayup, SAI is nearly the same and the bodies are nearly the same...

You WILL have to "massage" the inner fender if you used the existing mounting studs in the front. And trim the fender corner.

WJM ROTARIES

I have done a lot of these conversions and use the front bolt in the gen 1 rail in the front hole of the ser5 k frame and to get the wheel more centralized in the wheel well I change the rear lower control arm bush with a nolathane bush with negetive offset to bring the wheel back so is closer to the center .when fitting this k frame and suspension in the handling is greatly improved .plus I fit adjustable spring platforms to ser 4/5 shockers and use the gen 1 top adjustable hats

Ey8s

Cut out the FC strut towers.
I guess we'll see how this turns out. Once I figure out exactly what I'm going to use of it I will make a "locating" jig referencing the frame rails and forward mounting hole for the subframe to keep the geometry correct.
BTW, thanks for the info/input guys!

peejay

I would not say the handling is improved. I would say the opposite, the handling is made worse compared to 1st-gen suspension. Unless you heavily notch the body rails, the roll center ends up being really low and you have to run a lot stiffer springs just to get up to normal.

That said it IS an improvement from the perspective of having wheel bearings that can go more than 5000mi between repack/readjustment, and 1st-gen front end parts are getting really hard to find. 90% of the reason I did my front end swap was because my power box was worn out and for some reason people scrap them even though they are superior to the manual boxes. So it was just about impossible to find a power box.

Next problem: need to find 4 lug FC hubs because one of mine is bent. NLA from Mazda. $700ea when they were available. 5 lug is not an option without having to get rid of/replace five or six sets of wheels. And replace my other cars, because my other cars share the same bolt pattern

Johnny Kommavongsa

iTrader: (13)

Not sure if this helps or not but this is my wife's SA with a turbo S4 engine. Her dad used a FC subframe for this setup and just completed the subframe swap on a FB. The car is a blast on strait aways but handling could use some improvements. Prob just needs a good alignment.

http://www.flickr.com/photos/johnnyk1029/10646913263/http://www.flickr.com/photos/johnnyk1029/10646913263/ by http://www.flickr.com/people/johnnyk1029/, on Flickr

http://www.flickr.com/photos/johnnyk1029/10646910683/http://www.flickr.com/photos/johnnyk1029/10646910683/ by http://www.flickr.com/people/johnnyk1029/, on Flickr

Ey8s

Nice SA, thanks for posting the pics I haven't seen many with the 'vert wheels.
I was looking carefully at the engine bay pic and it looks like the strut tower is in the FB stock location with FC caps grafted in based on the distance between the MC/booster assembly and the strut tower in comparison to mine. When you move the FC subframe and/or lower control arms back to center the spindle correctly and use the FB pillowblock centerline you will lose caster and that's never a good thing.

here's an interesting article about caster:
The Ultimate Handling Guide Part 8: Understanding Your Caster, King Pin Inclination and Scrub

I used CAD to model the angle change in this scenario using the stock FB pillowblock location and moving the the FC subframe back 1" like some people do and 1.5" which is where I'm thinking it needs to go. At 1" you lose 3 degrees of caster and at 1.5" you lose 5. That's a lot! That article (and many others out there) is worth reading if you're new to exactly what caster is/does - I was.

Eric

Johnny Kommavongsa

iTrader: (13)

Yea i was wondering if he caster setting had something to do with the handling. In one of the pics u can see my old drift car in the background and I ran high caster and the steering wheel steered itself, hard to explain but when I drive her car and make a turn from.a stop, I have to use effort to steer the car strait instead of letting the steering wheel spin in my hand.

82transam

iTrader: (18)

Glad to see someone new posting info on the FC subframe swap. I assume my ancient thread (which definitely needs updating) was one of the ones you saw. Hopefully it was of some help to you.

I will be the first to admit that I am not an engineer, nor am I a racer. I just like spirited street driving so it's a perfect upgrade for me - allows easy install of FC engines, rack and pinion, better brakes etc. There are certainly some issues, like what you point out with the caster, that would need addressing to make a proper track car.

Having said that, I have put over 20k miles on the swap in my 83, and another ~5k or so on the swap in my 85. Both handle very well on back road blasts, track straight on the highway into triple digit speeds and have really changed the car's overall feel, for the better. Very few downsides in my opinion

Good luck getting your's all set up!

Just re-read your comments, I agree that 3/4" is the correct spacer size for the rear of the subframe. I actually did that on my second swap (look up my build thread for my 85 "the parts sponge" if you want to see) and it fits much better than my first attempt. i also made tabs to bolt it to the steering box/idler arm mounting points too. Very clean install if I do say so myself

Ey8s

Thanks for responding transam, yes - your thread is one that I came across. Thank you for sharing your information.
You beat me to the punch with the tab setup for the rear of the subframe. I was thinking about that over the weekend and picked up a piece of 2.5"x4"x.125" wall tube yesterday to cut into rear subframe "frame saddles" that will indeed use the FB steering box and idler arm bolt provisions since they are sleeved.
Removed the FB strut towers tonight. My plan is coming together on how I'm going to place/fit the FC strut towers.

Eric

Whizbang

i ran with the front stud located. trimmed the fenders and ran flares.

loved it.

BezRx7

I was also gonna say you could have dialed it in with some adjustable suspension pieces or custom top hats like others have done. But build on!! Love some custom work. Great work so far.

Ey8s

Completed my FC strut tower assembly rough-in.
It probably would've been easier to fab up some new towers/caps but I have the parts car and I was curious how the stock FC stuff lines up. I believe a stock FB strut tower wouldn't work with a new cap setup alone - you would need to modify the firewall side of the towers to work properly, maintaining geometry. I think I can make the factory wheel wells work by slicing them a bit, forming the metal upwards to meet the towers, then filling in the gaps with fitted sheetmetal. I'm deciding whether or not to move forward with that plan or just tack-in the towers so I can install everything and contemplate making room for wider wheels/tires down the road. BTW, the FC top frame rails are 13/16" above and parallel to the FB frame rails for proper placement.

82transam

iTrader: (18)

I'm liking where this is going. Will definitely be watching Glad my old thread was of some use to you at least!

j9fd3s

iTrader: (3)

the higher you have the inboard pivot, the higher your roll center can be, the FD has a higher frame rail than the FC, and then the Rx8 is higher than that.

which is what PJ is saying, with the FC suspension your roll center is going the wrong way.

Ey8s

I don't know how to address a roll center "improvement" at this point - and my only concern was to replicate FC specs for this swap.

As far as raising the strut towers any more (mentioned in an earlier response), they wll be very close to the hood as it is now so there is no room for that.

Eric

Ey8s

I decided to move forward with making the stock wheel wells work. I was able to hammer out the forward portions as-is but the rear portions required a relief cut. After this I positioned and clamped the strut towers in place and made alignment marks and inner strut opening marks. After trimming the strut openings I removed the temporary brace and FC frame rail remnants and welded the strut towers in. I welded the upper and lower portions first, then worked my way down the sides one stitch at a time while hammer "tuning" each spot into its permanent position. I will probably spend some time with a hammer and dolly to smooth them out as much as I can.















You can see that the FC strut tower is a lot closer to the master cylinder than the stock tower. The front inner strut bolt hole is 16 3/16" from the upper firewall on each side - even though it's marked 16 1/8" FWIW.
I still need to weld-in some "fill" strips for the open slots. Then it will be time to address the subframe mounting.

Eric

Stevan

iTrader: (17)

Ey8s, nice work there. Welcome to the forum.

Maybe I'm slow but I'm not getting the need to move the strut tops. If the spindle is inline with the strut on FB and FC and the spindle/wheel is centered in the wheel well as it is with the stock FB strut, how is the castor different with the FC strut? Does a stock FC use/need more castor?

Before you start mounting the subframe take a look at the thread by username economiser in the build threads. He cut the subframe to raise it where or near where it should be (FC specs).
There is debate about the spacer used for the rear of the subframe (when the the subframe is bolted on without modifying it and also needs to be factored in when modifying it), Some say 5/8 or 3/4" others 1". Years ago I measured the step in the FC frame rails and it was 1". However the subframe doesn't sit perfectly flat against the straight FB frame with a 1" spacer, it does with 3/4". I think this is where the debate comes from. Possibly the FC frame rails have a slight incline, I didn't think to check at the time I measured the step in the rails and the car is long gone. One thing I did check last night was if the control arm is parallel with the frame with 1" or 3/4". It is 1", there is approximately .3" difference in height between the front and rear pivot of the control arm using a 3/4" spacer.
With the 1" measurement in mind, I figured the topside of the subframe needs to be trimmed 1-3/4" off the front and 3/4" rear + whatever thickness metal plate you use weld back on the subframe to reinforce it.
After reading what j9fd3s said about the FD, Rx8 frame rail being higher than FC I am thinking when/if ever I get around to installing the subframe in my project car (car in my garage I stare at) I may attempt to raise it higher.

Ey8s

Thanks for the kind words and welcome Stevan.
I've already modded my subframe and have fabbed/installed my swaybar mounts. I'll post some pics/updates later on. I ended up spacing the rear of the subframe about 5/8" below the FB frame. I believe 3/4" spacing is correct for my application but the frame rail drops 1/8" down where this point is so 5/8" worked best. The problem I see in raising the subframe is that you quickly run out of room to raise the strut mounting plate too. I duplicated the FC strut geometry by sectioning that whole setup, jigging it, and installing it. The strut tower tops are just below the hood as it is. The roll center comment makes sense to me but unless you go with adjustable coilovers - I don't see how you can keep things correct.
As far as strut location in respect to the wheel wells and spindles - The strut towers aren't "centered" in either the FB or FC's, they are canted back to provide caster. You would think that if the caster angles are similar and the wheels are both centered - it should be straightforward but using the FC subframe to strut tower dimensions, it's not.
The roll center discussion has brought up some interesting thoughts but I haven't contemplated it much. I wouldn't want to raise the subframe/steering rack/swaybar anymore than it is now since I want to install my V8 as low as possible. I guess I'll have to consult my chasis engineering book to see where this is coming from but my gut tells me that my roll center shouldn't be any lower/higher than a FC and that the only difference would come from a change in center of gravity due to the drivetrain swap.

Eric

Ey8s

Here's a pic comparison between the FB strut tower location vs the swapped in FC strut tower location. You can see that the struts are now further back and also outboard some as well. The outboard difference is due to the FC's wider track width. Using the FB location you would gain some negative camber (possibly a good thing) but lose some caster (bad thing). I use the dimple in the fender lip as a reference point.

Eric

peejay

If you want the handling to be the same as FB, you will need the front ride height raised about an inch compared to before the conversion.

That's why just bolting the subframe to the bottom of the FB chassis isn't ideal. A hell of a lot easier, yes, but not ideal

I've seen exactly one person do it "correctly" with sectioning the chassis rails to get the subframe higher in the car. The rest of us just get by with having to run stiffer front springs and accepting the downsides that entails. I had been running 175lb springs, with the FC suspension I wnt to 250s and they still felt a little off. I ended up raising the front ride height and going back to 175s.

Stevan

iTrader: (17)

[QUOTE=Ey8s; I ended up spacing the rear of the subframe about 5/8" below the FB frame. I believe 3/4" spacing is correct for my application but the frame rail drops 1/8" down where this point is so 5/8" worked best.[QUOTE]

Yeah, right where the rear mounting bolt hole is in the subframe the FB frame rail starts to curve downward. If I ever do it I think I'd grind the subframe to match the curve.

[QUOTE=Ey8s; The problem I see in raising the subframe is that you quickly run out of room to raise the strut mounting plate too. I duplicated the FC strut geometry by sectioning that whole setup, jigging it, and installing it. The strut tower tops are just below the hood as it is.[QUOTE]

That is true, I thought I had taken measurements from the FC strut tower top, to bottom of frame rail but can't find them. I plan on using some FB strut tower tops cut from a donor car to extend the strut towers upward. The forward outside corner is the highest point on the strut tower top (obviously) but it can be trimmed some and welded, and also a section of the hood bracing can be notched as well. If that's not enough I have an idea to modify the FC pillowlock/spring mount to make it shorter.

[QUOTE=Ey8s;As far as strut location in respect to the wheel wells and spindles - The strut towers aren't "centered" in either the FB or FC's, they are canted back to provide caster. You would think that if the caster angles are similar and the wheels are both centered - it should be straightforward but using the FC subframe to strut tower dimensions, it's not.[QUOTE]

I did not say the strut towers were centered, that doesn't make any sense.
And yes, I know they are canted back for caster
What I said was, if the WHEEL is centered in the FB wheel well, how can the caster be different? The spindle to strut top is a straight line.