Chuck's RX7 / CMC w/IRS challenge
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Author:  rx7locost [ July 21, 2008, 6:16 pm ]
Post subject:  Chuck's RX7 / CMC w/IRS challenge

Hello, and let me introduce myself. I am Chuck, a member of LLA (LOCOSTUSA Lurkers Anonymus) I've been thinking about the Locost for several years and been planning my build. This is my second automotive challenge. I have completed a nut and bolt resurection/restoration of a 1956 MGA and needed something a bit more challenging.

I live in the western suburbs of Chicago. I planned on building from scratch and was in the planning stage including driuvetrain possibilities when a friend found a sick RX7. He wanted the body and offered me the running gear and electrics. So that choice was sort of made for me. We stripped the car and split the parts.

With a pile 'o car parts sitting in my garage, it was then that a partially finished project jumped in front of me and pleaded for salvation. It was a CMC chassis kit (cut pieces sent to the customer for them to weld up) originally set up for a Pinto drivetrain.

Now what to do? Do I run a solid rear axle or build up a deDion from the RX7 parts? I decided on a full IRS rear using upper and lower wishbones. That was last year. I will try to fill in with a photo update of the last year's work.

I will be posting my build as I go and asking advice from all. Where I think I can add to a discussion, I will do so.

Please forgive any poor spelling. It is not so much of poor spelling, but of poor typing. My eyes are on the keyboard and not the screen when I type; and I have fat fingers. :?


Author:  rx7locost [ July 21, 2008, 7:00 pm ]
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Here are a couple of photos of the donor pre and after the teardown.

Author:  mookie [ July 21, 2008, 7:22 pm ]
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Wouldn't happen to work for either Signode or Gerrard, would you? I noticed the coils of strapping in the background.

Welcome to the board. The RX-7 was my first choice for a donor but they're so bloody expensive up here still.

Author:  rx7locost [ July 21, 2008, 8:58 pm ]
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I am, how they say, currently in between work. The strapping you see is from the shop of a friend. We stripped the car in his shop on a couple of weekends over 3 years ago. My how time flys. We ended up chopping up the body and putting it in the dumpster. You can do so much more in an industrial area than you can do in a residential area!

Author:  rx7locost [ July 21, 2008, 9:14 pm ]
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The CMC chassis is pretty much by the book. The RX7 engine does not fit well. The problem is that the intake manifold is serpentinr shaped and adds mucho height to an otherwise very small engine. In order to keep the looks "proper", I decided to remove the intake manifold and spin my own. That will be down the road some. The real reason for building the Locost is to streatch myself and learn as much as I can. The rotary engine has intrigued me for some time. I have rebuilt a handfull of engines over the years, all of them 4 clyinders. I will learn a lot.

Author:  rx7locost [ July 21, 2008, 9:21 pm ]
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Sorry, still getting used to posting here.

Author:  rx7locost [ July 21, 2008, 10:05 pm ]
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Still getting the build updated with the work done over the last year.

The unfinished kit that I bought, came complete with a partially built 2nd frame which was stripped down and used as donor steel. The package came complete with the CMC nose, 4 fenders, and 4 GAZ coilovers. The fenders are too narrow as they are and 2 of the 4 coilovers are giving me some issues fitting in the new design. We'll see how things work out.

The engine fits so much better with out all the intake stuff. In Illinois, the car can be registerred as a "Custom Vehicle" As such, it is not subject to Emissions testing or inspection. This makes the build so much easier. I don't have to keep anything emissions related and I have freedom to do what I want.

Once I decided that the car was to have IRS, I tried fitting up parts to see what had to go and what could stay. The front half of the rear section has to be cut out and rebuilt for the IRS. Once that realization sets in, the next step is clear.

The half shafts will have to be shortened by about an inch or so to allow for some suspension movement.

Author:  rx7locost [ July 21, 2008, 10:43 pm ]
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The a-arms were all designed in Autocad. I had chosen on rod ends for adjustability reasons. the dImensions were taken from Autocad and transferred to a scrap piece of drywall. Bolts were inserted at the appropriate locations. Then it is a matter of adjusting and placing the tubes on the bolts and tack welding in place. All tie rod ends were designed for a nominal of 5 turns from seated so that +- adjustability can be done. Full toe-in and camber adjustment can be achieved.

Chassis height is designed for 4.5" clearance with the engine oil pan at 3.5 inches. May have to protect that in some manner when the time comes.

This is the culmination of last years work, having a chassis on its rear wheels.

jig loaded

Author:  SportsCarDesigner [ July 21, 2008, 11:03 pm ]
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rx7locost wrote:
The half shafts will have to be shortened by about an inch or so to allow for some suspension movement.

Why's that? To end up with a specific track width? Fender fit? :?

Author:  a.moore [ July 21, 2008, 11:36 pm ]
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Looks like you're off to a great start.

Any plans to reinforce the tubes that are drilled for the diff mounting bolts?

Author:  Pete B [ July 21, 2008, 11:45 pm ]
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Hi, Chuck--

I have been contemplating building with an RX7 for 5 years now, so I had a great time looking at your pics. Congrats on a very clean build. I'm particularly interested in more details about:
  • How you modded the rear uprights.
  • If you had a problem with interference between the starter motor and the driver's footwell.
  • Plans for the transmission tunnel.
I'm a bit concerned about the big bolt holes for the diff weakening the tubes the diff is mounted on. My plan has been to copy this:


How about substituting some 1"X2"s?

Re: engine height, it's very encouraging to see how low the engine sits without the stock intake. If you are interested in moving the alternator to the side of the engine, somewhere I have a link to a bracket. I am planning to cut down the oil and coolant fillers; are you aware of any problem with doing that?

And now for some rotary Locost [NWS PORN] :D:


Expensive, but worth it, I think.

Thanks for posting!


Author:  rx7locost [ July 22, 2008, 8:35 am ]
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Sportscardesigner, my Locost rear track width is a bit narrower than the original RX7. Narrow is better for parking lot racing no? Even now I am not aesthetically thrilled with the wide gap between the frame and the tire but I will accept it as is. The halfshaft is fully compressed in its currrent form. The diffy needs to move to install and uninstall the shaft. I would like it to be in the center region of the sliding CV joint for a bit easier serviceability.

a.moore, thanks for the encouragement. I know I need to reinforce those tubes at some point but not sure which method I will use. I am leaning towards some 1" wide 1/8th inch plate steel welded along the sides of the 1 inch tubes. I will be adding some hardened washers on the top and bottom of the tubes for some crush resistance and bridging to the 1/8" steel sounds like a good idea to me. A lot of the design is seat of the pants engineering.

PeteB, again thanks. My suggestion to you is to quit contemplating and start cutting/welding. I have been learning a lot about the rotary engine over at RX7Club.com. The rear uprights are currently standard with no mods. I'll give you a better picture later today. None of my pics show it very well. As far as the starter motor goes, my frame had been modded in this area a bit by the previous builder. It works out that the starter fits in very well and only minimally intrudes in the footwell area. I do not think it will be a problem the way I've got it. I'll shoot a photo of that too. The trans tunnel is currently out and being engineered. As you are aware, the RX7 differential is offset around 3/4-1". This does present a problem on a book chassis. I am thinking that I can widen the RH side of the tunnel by moving it 1" and have enough clearance. It will make the RH passenger width even more narrow than it is now. I'll figure out what to do for seating later. However I can't do any of that until I set the trans mount and diffy nose in place. As soon as I get my welding wire, I am off the the races again. IF I had to do it again, I would build the frame from scratch. a +2 of +4 width frame would have fit everything much better. But I move forward with no regrets.

I believe that most of the force on the diffy is a twisting one, wanting to lift the nose onder hard accelleration. I really don't think that too much strength is needed in the mount area. I know it will twist too, but vertical strength is mostly to support the heavy unit.

The alternator will be moved but I'm not exactly sure where it will end up. I need to clear my steering shaft. Again, later seems to be the answer. That tweakit throttle body is very sexy. I'm not sure how it will work wth my 5th/6th ports. I do want to keep them active for better performance. I plan on keeping the bare block as std as possible. No bridge or street porting for me. I've seen the manifold you show. From what I've read, the optimum intake manifold needs to have a volume close to the engine's volume. That manifiold doesn't have very much internal volume. This may affect the low RPM performance. I think that the manifold should work very well at WOT though. What do I know? Nothing, just what I've read over there. I have no practical experience on modding engines. Under today's fuel prices, and the (poor) efficiency of the rotary, I will be at WOT very seldom. My plans are to find a local TIG welder and cut/modify the existing upper intake to be a straight shot low across the engine and then turn forward. I'll use the existing throttle body (gutted of course) and control it and spark from a Megasquirt system. I did say that I was in this for the learning part of it.

Of course it is all plans and they do change occasionally.


Author:  Pete B [ July 22, 2008, 10:12 am ]
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I have some personal issues to resolve before I can build in good conscience, but I'm hoping it won't be too much longer.

My donor is still together so I haven't gotten a good look at the rear uprights, but they look problematic for the forward lower wishbone attachment, so a pic of yours would be very helpful.

I will have to split the interior room evenly or I will never here the end of it from my principal passenger-to-be. :) I've been thinking of a welded sheet metal tunnel. The chassis will be a +221 for my height (6'6") and her can.

I'm with you on keeping the engine mods mild because I want it to last a good long time, and I will have to pass emissions tests. The focus will be on intake and exhaust. Your approach for the intake sounds very good. My understanding is that individual TBs eliminate the need for a plenum, but I don't know anything, either. :) You are right that the RX7 Club is a treasure trove of info. I have done some reading there, but would want to ask about specific layouts.

The only internal mod I would do is the auxiliary port sleeves from Pineapple Racing. Pretty cheap, and very sensible. I'm hoping for about 200hp.

Author:  rx7locost [ July 22, 2008, 6:23 pm ]
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If I knew then what I know now, I would have offset the engine, trans and 3rd member to center the driveshaft. With that done, change to RH drive to give more foot room. Then the starter would not be any problem. With a +2 wide, that should give you 18”+ per side in the passenger compartment. Going to a formed sheet metal tunnel will get you more even still. However, I would be worried about the containment of the driveshaft if a u-joint ever fails catastrophically. My MGA solved that problem with a couple of braces placed in the sheet metal tunnel. I know that I have limited the size of the passenger compartment on mine and it will be a problem for, shall we say, family members.

On my car, the 3rd member is centered and the entire drive train is moved right by ¾-1”. My thought was to slightly compensate for the offset weight of a LH driver. The photos show that the RH side of the tunnel, moved 1” will provide about 3/8” clearance from the differential driveshaft flange to the inside of the passenger compartment. This should work for me. The RH side is cut free and only placed in free space at the moment. I plan on not removing the existing framework in this area, just adding on using a 1” square tube inside the existing ¾” tube.

In the other photos, I have shown the rear upright (?”) with my current design. As to the lower leading mount, I have added Kinetic’s adjustable tab to provide for toe-in adjustment. I am not sure that the bracket I built at the hub end is my final design; I’d like to have something a little more substantial. All the stacked washer spacers will be replaced with a single tube spacer to give more rod-end freedom.

Currently, I am in a dilemma as to how to implement my GAZ shocks into the current design. Everything is too tight right now for the shocks to fit. And the existing upper coilover mount on the chassis is 90 degrees from what an IRS would want. I’m sure it will work out in due time as I work on other areas wile contemplating this. Currently there is a tube from the top mount to the chassis supporting the chassis at a ride height of 4-1/2 inches.

Author:  rx7locost [ July 22, 2008, 7:22 pm ]
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OK, here is my last catch-up post on my build. I have only worked on the car for the last couple of weeks since putting it away last Fall. I have completed the front suspension. The upper joint is a 5/8" rod end with a bracket made to fit where the struts originally mounted. I am using the 1st Gen RX7 lower ball joint. It fits perfectly on the 2nd gen spindles. It was not made to be in tension but I have not heard of any problems with others who have done similar mcpherson strut conversions. Time will tell if I am right ot not. The lower mount for the coil over is located directly between the mounting bolts of the lower ball joint. This should provide a very strong connection between the wheel and the chassis. No bent lower tubes here as the tubes are not normally stressed by the coilovers, they only providing a locating function. I also found that the Unistrut c-channel used in commercial construction is ideal for upper mounts. It is about .090” thick and 1-1/2” inside. By using this, I accomplish a few things, 1) the shocks can be mounted more vertically giving a better spring rate with the coil overs I now have; and 2) the upper mount is at the same height as my upper control arms; and 3) it just looks better if all the visible suspension "stuff" out in the air has a visual limit. The upper and lower control arms perform this function very nicely. The coil over body does not have to squeeze between the upper control arm and extend upward to the chassis. It should also provide a cleaner front nosecone. I now see that some others have used a similar construction. Currently I did not have enough material to have one continuous piece across the front, but that is my intention.

The rack is from the donor car, a ’91 RX7. I have depowered it and shortened it by about 8-3/4”. The rack was cut only on the RH side. The rack shaft has a continuous hole down the center. It was easy to drill the hole to 1/2” and tap to 14mm x 1.5mm for the inner tie rod connector. As a side note, I used the cut off rack shaft piece to make the tie rod extenders. I cut to 2-7/8 inches; drilled and tapped both ends to 12mm x 1.25mm. I threaded some rod to join the adapter to the outer tie rod ball joint.

I ran into the same issue that Dave H did concerning bump steer. I can correct for this by moving the rack to the front of the chassis, but I’m not sure if this will be my final location. Maybe his solution of reversing the outer tie rod end raising the rack is best. I’ve got to think that one out.

The transmission mount and the front differential mount were modified from the existing mounts. I thought it wise to provide compliant mounts since the other motor and differential mounts were compliant too. I used some Triumph rubber mounts I picked up at a swap meet. They are nothing more than a rubber cylinder block with an aluminum tube running thru them. It turns out that the steel tube from the shortening of the rack housing was a perfect fit for the new rubber mounts. It is good to be lucky. The chassis side of the mounts will be a couple of tabs welded to some square tubing welded across the existing tunnel framework. A single Grade 8 7/16" bolt will tie them together. A nut will be welded to one side to allow for a single wrench assembly/disassembly.

That brings me up to date with my build. I hope it wasn’t too boring.

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