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Unread 10-01-2012, 06:27 PM
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Fair! Fair! is offline
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Default Re: Vorshlag Miata LS1 Alpha Project

Project Update for October 1, 2012: Well we've had a busy few weeks since the last update, after the blur that was August (Pikes Peak hill climb event), spending September racing at the SCCA Solo Nationals, a Global Time Attack event, and some other autocrosses. But the LS1 Miata project hammered along with a few test fits, some changes, new parts arriving, and key decisions being made. We acquired several donor parts, weighed and tested each of them, and whittled down our options quite a bit. Let's get caught up:

Three Transmissions, One Winner

The transmission to use in this chassis was always an unknown. The tunnel is tight in some areas and we didn't want to require our end users to hack up the tunnel in their cars to fit the transmission. A little engine bay trimming isn't so bad, but slicing and widening the tunnel is major work that most people do not want to tackle - us included!

The first manual transmission candidate we tested with was the Tremec T56 6-speed transmission. This one is very well known, but with increasing demand and a finite supply from the 1998-2002 Camaros and Firebirds, the used prices have been creeping up towards $1500. The T56 "Magnum" (really just an aftermarket version of the 2010+ Camaro's TR-6060) is around $2800 new, but has increased torque capacity to 700 ft-lbs among other needed improvements. We used one of these in our E36 "Alpha" car behind a 490 whp 7.0L LS2 and it was a great trans. The Miata fitment testing we did on the LS1 + T56 fit "pretty well", but still had some width limitations inside the stock NB chassis' tunnel. We really do not want our LS1 swap kit to require tunnel surgery and we want the engine as far back as possible, so this gave us reason to think twice.

1998-2002 GM F-Body T56 weighed in at 125.8 lbs, dry.

The T56 also didn't have an easy way to attach the Power Plant Frame ("PPF" or Torque Arm) to the rear end. Also, this transmission is relatively heavy at 126 pounds. It is a great transmission, capable of handling some good torque (450 ft-lbs), shifts well enough, but has two overdrives... which is overkill for this chassis.

Next up, we purchased and tested with the ZF S6-40 6-speed manual from a 1989-1996 Chevy Corvette. This "ZF6" is a freagin' TANK of a transmission, used in several BMW models (with a different tail assembly missing the PPF mount) with either 650 or 1000 ft-lbs of torque capacity in stock form. It also has a provision for a very similar torque arm as the Miata, as used in the C4 Corvette. That was my favorite choice up front, but as some of you pointed out, it has a few flaws.

1989-1996 Chevy Corvette ZF S6-40 weighed at 153 lbs, dry.

First is that weight! Good grief this thing is heavy, even using a factory magnesium C4 bellhousing. That big torque rating comes with a lot more steel on the internal gears. It also has a divorced shifter that mounts to the body, unlike the T56 which has an internal rail shifter (which makes for better shift feel and easier swapping). We found a good specimen with a magnesium bellhousing made for the LT1/LT4 engine, but it was going to take an expensive adapter plate to mate to the T56 bellhousing made for the LS1 engine - and it would only gain weight.

The shifter location was not right and the clearance to the tunnel was non-existent.

Finding the ZF S6-40 new is impossible, as they have been out of production for over a decade. Finding them used is difficult because they only came in certain C4 Corvettes and rebuild costs were also high. Physically the remote mounted C4 shifter location was way too far forward and the transmission itself was WIDE. There was no way this big beast was going to fit the Miata tunnel without a major fight. The only feature it added was the potential to use a modified C4 PPF, but that was not enough to overcome the many other obstacles. So the ZF6 was a dead end. Hey, you sometimes have to try something to find out what works and what doesn't. We had seen weights listed all over the place, and now we know.

Yes, some of you figured out this was the transmission I eluded to in my first post, and yes, you were right that it is not a good swap candidate. I am not always correct.

Our third choice was the Tremec 3550/TKO series 5-spd, and I think we have a winner. This 5-speed has an internal rail shifter and only weighs 95 pounds, but the two aftermarket TKO versions have 500 and 600 ft-lbs of torque capacity. They are hard to find used, but only cost $2200-2300 brand new. Of course it has no provision for the PPF, but I have pretty much given up on this as an impractical goal. Sure, we could hobble together something that attached to the back of the TKO, but we already have enough custom fabricated parts in store for this swap to make all of our other "wish list" items work.

We have seen these TKO's in all manner of race cars and kit cars, including a Cobra with a Ford big block that was in our shop for some work for about a week. We got to measure the trans, drive it, and this trans is pretty slick. It is so compact, yet so strong! The only question left was which aftermarket version to use?

TKO 500 vs TKO 600

There are actually 3 choices when it comes to the GM pattern Tremec TKO transmission. The differences in torque rating and costs all come from some internal gearing differences. But there are key reasons why we want to use the 600:
  • Tremec TKO 500 Chevy/GM - TCET4616
  • Transmission is rated at 500 lbs. ft. Torque
  • 3.27, 1.98, 1.34, 1.00, .68
  • Tremec TKO 600 Chevy / GM - TCET5009
  • Transmission is rated at 600 lbs. ft. torque
  • 2.87, 1.89, 1.28, 1.00, .64
  • Tremec TKO 600 Chevy / GM Road Race overdrive - TCET4618
  • Transmission is rated at 600 lb-ft. of torque
  • 2.87, 1.89, 1.28, 1.00, .82

When you look at the ratios the "normal" TKO600 actually has a much more usable first gear for a car like the Miata. That tall first gear works better when you are using these transmissions in a light, powerful car with not enough tire (we won't ever have "too much tire" on these cars, heh). Way back when I was racing a Mustang 5.0L with the factory T5, it had "3.35" first gear. Later I went to a T5-Zcode, which had a taller "2.95" first gear and the change was very noticeable. The taller 1st gear was much easier to launch with at an autocross and the gap between 1st and 2nd was closer - for a more normal feeling gap. This TKO600 looks like that same upgrade over the TKO500.

So the TKO600 is where we would go, just for the better first gear. It also has a slightly taller 5th gear, for better highway cruising. There is also a TKO 600 "road race" version which is the same box, with the same taller first gear, but a lower 5th gear, for using all 5 gears in anger on a road course (much tighter ratio 4th to 5th). We don't think there's any road course where this would be needed on a Miata with a big thumping V8, and it wouldn't have as much of a "street friendly overdrive" as the .68 or .64. So the TKO600 with the .64 5th is what we think is ideal.

The TKO's synchros are big beefy brass units, which are a bit unusual in construction; we have heard stories that they don't shift well above 6500 rpm, but then other racers using them say they work fine. We talked to a local Tremec rep that came by our shop last week and he said he can make these TKOs with a custom carbon blocker rings and synchro package so that they will shift like butter at high RPMs. For this build the engine we have in store isn't a super high rpm buzz bomb and will likely make peak power at 6000, so this isn't a concern this time.

Other Parts In Store

We are still exploring possibilities with the rest of the swap, but we do know the Ford 8.8" aluminum IRS differential housing from the Ford Mustang Cobra is what we will build the back of the car around.

Left: The BMW E46 188mm diff next to the aluminum 8.8" Ford. Right: RX8 front parts (top) will be used, but the rear uprights are 5-link and wrong (bottom).

Development on the front end is starting with a pair of Mazda RX8 uprights and hubs. These feature 5x114.3 bolt circle hubs, with much stronger hub assemblies than the 4 lug Miata. The uprights are nice aluminum units which we will build control arms to fit, and feature large front disc brakes (12.5" diameter). We still have a lot of work to jig up for these, and we will increase the wheel and tire size to work with the bigger front and rear uprights/hubs/brakes.

Engine Being Built

We have several swaps going on with stock LS1 drivetrains, but the customer for this Miata wanted something more. His 400 whp minimum power level meant that it would be a pretty hotted up 5.7L LS1, or one with some better head work and internals. We contacted HK Racing Engines in Houston and they came up with a potent little combination based around an LS1 5.7L block we sent them. It will have CNC ported heads, a small duration but high lift hydraulic roller camshaft, and extra beefy internals. The machine work is complete and they are assembling the longblock now. We are using one of two oil pans on this engine...

The pans above have a much lower front profile than the Camaro LS1 pan that people tend to (cut up and modify) in these Miata swaps, but are nowhere as expansive as custom fabricated oil pans.

You can see the massive improvement in the front of the sump area, where the steering rack would be. That picture we took is showing showing the factory F-Body LS1 oil pan ghosted over the GM LS1 swap oil pan. Hopefully the Holley or GM swap pan will clear the stock steering rack placement, and more than likely the Holley version will be what we use. We have all three pans here for mock-up test, which we will do as soon as the TKO600 arrives.

OK, that's all we have for now. We have the transmission nailed down, three oil pan options, new front uprights/hubs/brakes to work off of, the rear differential picked, and a few more things in the works. We still have a LOT of work to do, but this swap is being attacked between other LS1 swap jobs underway at the shop right now (five cars). Just wanted to give a few updates on where we are headed with this Miata swap.

More soon,

Last edited by Fair!; 10-01-2012 at 06:33 PM.
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