© Copyright by Boss Frog Inc.  All Rights Reserved.

Automotive Rollbars and Aftermarket Products for Miatas and MGBs

The Miata Venom V8 Kit

If you are ready for some awesome power out of your NA or NB Miata, then look no further.  Boss Frog has developed some new V8 swap kits that will turn your car from mild to wild!  Boss Frog has been building top quality rollbars and accessories for Miatas for over 5 years, and is far from being a garage startup or a one-man operation.  You can be assured that these kits will have the outstanding quality and customer support of the other Boss Frog products. Also see the Boss Frog photobucket site for additional pictures.

Boss Frog uses the following key drivetrain components;

Engine - the Miata Venom V8 kits use the LS series GM engines typically used in
  Corvettes, Camaros, Firebirds and GTOs, because they are lighter and more powerful
  than the Ford 5.0 liter V8’s.
Transmission - the transmission selected is a Borg Warner T56 syncromesh 6-speed
  manual transmission that was used in Vipers, Corvettes, Cobras, and F-body cars. These
  transmissions use a hydraulic clutch and have an excellent performance record.
Differential - Boss Frog looked seriously at the Getrag differential used in Cadillac CTS.
  The bottom line is that these transmissions were developed for 260HP V6 engines, and
  they have had serious problems with seals, loss of pre-load, whining, and wheel hop
  which has resulted in recalls and thousands of angry customers (see the Cadillac
  forums). Of course these vehicles were heavier than a Miata, but even the similar size
  Pontiac Solstice was recalled for these same differential problems. The differential that
  Boss Frog uses is the track proven Ford 8.8 out of ’89-’97 Thunderbirds with a custom
  independent rear suspension.  This bulletproof differential has hundreds of gear ratio and
  limited slip options combinations available, all of which are readily available anywhere.
Driveshaft – Boss Frog’s driveshaft has the biggest joints and tube size available in
  these kits – over 15% stronger than a stock driveshaft.
Axles - the Miata CV joints were never intended to handle the 350+ horsepower that
  these LSX engines are capable of, so Boss Frog uses the larger axles.

Boss Frog kits will give you the strongest overall drivetrain available, so that you can put down the most reliable horsepower at the track or the strip. The kits are divided up into various components and systems, so that you can purchase exactly what you want, when you are ready for it.  All shipping is to continental USA only.

Installation Instructions - To View Click Here.

Kit #1 – Engine Subframe (LSx) – this kit includes a professionally designed engine subframe, precision built in a welding fixture for the best possible fit. Our plasma profiled mounting brackets and heavy wall rectangular tubing are much stronger than round tubing subframes, and we reinforce the front crossmember and uprights to gain additional strength where it is needed. Our kit does not require cutting into the transmission tunnel, dashboard removal, oilpan modifications, or additional steering joints. The only changes

• engine subframe
• urethane motor mounts and block adapters
• extended steering joint
• front sway bar relocation parts
• grade 8 hardware and installation instructions

LSx-K1a - Engine Subframe kit for NA


LSx-K1b - Engine Subframe kit for NB

Shipping & Handling $60

Kit #2 – Transmission Mount (T56) – this kit includes heavy 14ga frame rail channels to increase the torsional strength of your chassis. The channels include mounting plates where the transmission crossmember is bolted. The transmission crossmember has wide gussets at the ends, to decrease flexing between the frame rails. The transmission is attached to a urethane mount, to allow some movement while holding it firmly in place.
The kit includes;

• frame rail channels (2)
• transmission crossmember
• urethane transmission mount
• grade 8 hardware and installation instructions

LSx-K2 - Transmission Mount kit

Shipping & Handling $30

Kit #3 – Differential Subframe (Ford 8.8) – this kit fits the Ford 8.8 IRS differential center section from ’89-’97 Thunderbirds, which has two vertical mounting holes on the front, and two horizontal mounting holes on the rear. The kit utilizes four existing mounting brackets that are stock on the NB rear subframe, and are easily added to the NA rear subframe. The unique design of the Boss Frog Differential Mount features a two-piece formed mounting system that is much stronger than tubular mounts. The lower differential mount attaches solidly to the rear subframe in four places, and the rear differential mount attaches to the

• lower differential mount assembly
• rear differential mount assembly
• urethane bushings (front and rear)
• differential braces
• grade 8 hardware and installation instructions

LSx-K3a - Differential Subframe kit for ’90-‘97 (Thunderbird)

LSx-K3b - Differential Subframe kit for 99-‘05 (Thunderbird)

Shipping & Handling $30

There are several options for axles, and we recommend looking at our partner's website for the various choices. We no longer sell these axles through our website.

Driveshaftshop offers axle options including axle kits that fit the 8.8 differential and stock Miata hubs, and kits that use a Miata hub which has been broached for a larger spline (for more HP). Please see their website for the kits that are currently available.

Kit #5 – Driveshaft – Boss Frog has decided to supply a heavy duty 1310 series driveshaft with a 3” tube, for additional strength and durability.  The  driveshaft assembly includes the rear flange yoke and a front slip yoke for the T56.The kit includes;

• custom length 1310 series driveshaft
• front and rear yokes
• grade 8 mounting hardware

LSx-K5 - Driveshaft kit

Shipping & Handling $30
• new clutch master cylinder, modified to fit
  the Miata clutch pedal
• master cylinder adapter plate
• 36” stainless braided hose
• slave & master cylinder adapter fittings
• 36” remote bleeder assembly included with “b” kit
• mounting hardware and instructions

LSx-K6a - Clutch Hydraulic kit

Shipping & Handling $15

Kit #7 - Headers - Needless to say, the space between the LSx engine and the chassis is rather tight, and a stock set of headers will not fit without modification. You can make the modifications yourself, or buy this kit pre-made. Even though mid-tube or long-tube headers are nice, they make fitting into the tight space a nightmare, so we use the shortys.
The kit includes;

• custom short-tube headers with 3” collector
• 2-1/2” down pipes
• gaskets and hardware

LSx-K7 - Header kit

Shipping & Handling $30


1. How much cutting and welding is required? The gussets at the rear of the engine compartment must be removed, but this can easily be done with a reciprocating saw. No other cutting of the tunnel is required, so the dashboard will not need to be removed. Stock control arms, shocks and springs (depending on axle choice) hood, etc can all be reused without modification. There are several holes that get drilled for frame rails and other mount points. The NA’s require 4 small tabs to be welded onto the rear subframe assembly (to match the NB setup). However, the NA Miata’s rear subframe can be removed with about 6 bolts, and the welding can be done separate from the car (or at a welding shop).

2. What steering changes are required? The lower steering knuckle is replaced with a slightly longer version, which moves the steering rack forward about an inch. The stock steering cylinder is behind the center of the front wheels, so the true effect of moving the cylinder forward should make absolutely no difference to steering feel or bump steer, and does not add any additional joints or increase the joint angles.

3. Why did BF choose the 8.8 differential? BF considered both the 7.5 and 8.8 Ford differentials, and a variety of other IRS setups including the Getrag/Cadillac and Supra. The Getrag was eliminated from consideration because of the lower torque rating, limited choices of gears and posi combinations, and it’s recall and failure history. The Supra rear ends are too hard to find. The 7.5 differential would be a good choice, but the supply of these is also limited. BF chose to use the bulletproof 8.8 center section and developed a unique mounting system to maximize both the strength and ground clearance. This rear end arrangement offers a higher overall torque capacity than either a Getrag or a Ford 7.5 differential system can deliver.

4. Why are new rear hubs needed? New OE Mazda hubs are custom broached to fit the larger outer CV’s for the Level 3 axles.

5. Is weight and balance affected? Yes, but not very much. The engine subframe is about the same weight as stock, and the engine is only a little heavier than the 4-cylinder. The transmission and rear differential are both heavier than stock, so some of the extra weight is shifted rearward again.  Our ’99 with a ¾ tank of gas and no driver weighs 1380 (52.3%) front and 1260 (47.7%) rear for a curb weight of 2640

6. Is power steering available? BF recommends eliminating the power steering system, but it is possible to keep it.  Some re-plumbing will be required to clear the engine subframe.  The required adapters and hoses are available from Boss Frog.

7. Is air conditioning available? BF does not currently offer a kit to enable the AC system, but it is fairly easy to add, and there are other sources that do offer a retrofit kit.

8. Do your kits fit both NA/NB steering racks? Yes – but the mounting and plumbing of the NA and NB steering racks are slightly different, so the type must be specified when ordering the engine subframe.

9. What about front sway bar clearance? The stock front sway bar would run through the balancer pulley, so it must be shifted forward a couple inches. The engine subframe kits include relocation brackets to move the sway bar forward. The stock sway bar can be used, but the arms should be lengthened about 2” on each side. BF supplies brackets to extend the Racing Beat aftermarket sway bars to the stock (or aftermarket) end links.

10. What about shock, spring, brake, and wheel clearances? Stock or fixed height shocks and springs can be used with the Level 2 and 3 Axles.   Big brake kits work fine.  Wheels and tires up to 16x8 and 245/35-16 or 15x9 275 A6’s will fit without modification.

11. What about ground clearance? Ground clearance depends on many things, including exhaust height, shocks and springs, etc. The BF system will keep everything at or above the scrub line – the rest is up to you.

12. Any wheel hop issues? Wheel hop seems to be unique to each car’s setup.  BF supplies new Energy Suspension bushings in the nose of the differential and heavy urethane bushings in the rear to help reduce any tendency to wheel hop.  Differential braces are also supplied to tie the rear subframe into the floor and main frame.  Urethane bushing kits will help to reduce any tendency to hop.

13. What about a radiator? The stock radiator is too small to cool the big V8’s, so a new one is needed. A wide variety of choices exist, including Miata racing radiators and custom made radiators. BF does not current offer a “bolt in” radiator…yet.

14. Shifter location? The T56 shifter will fit right in the Miata’s body hole, slightly forward of center, but without any modification to the body or boot.  Most people will cut a stock shifter lever to be shorter throw.

15. Exhaust? Trying to furnish a bolt-on exhaust kit is a big challenge, because of the variety of sizes and types of exhaust systems available.  In addition, differences in hose routing, muffler type, and axle choices will affect the exhaust system design.  Boss Frog offers headers kits, but the best choice for the rest of the exhaust is really to custom build it.

Kit #6 – Clutch Hydraulics – the Miata clutch hydraulics are too small for the LSx clutches, so a new hydraulic system is required. A stock LSx or universal clutch master cylinder will not fit, so a special adapter is needed. In addition, the GM slave cylinder includes a special quick disconnect fitting that needs to be replaced. Bleeding the slave cylinder is a huge PITA, so we also offer a 36” remote bleeder line and fittings to make this job easier.
The kit includes;

from stock are some trimming of the engine bay gussets, swapping the lower steering knuckle for a slightly longer version, and relocating the sway bar slightly forward.
The kit includes;

vertical member of the rear subframe, using existing holes for alignment. Because of this unique mounting system, the differential can be mounted very high in the rear subframe to maximize ground clearance.

Boss Frog also uses a pair of differential braces that bolt between the lower differential plate and the floor/frame, to stiffen the rear subframe and help eliminate wheel hop. The kit also uses urethane bushings both the front and rear of the differential, to dampen the shock load. Stock rear sway bars, shocks and springs can be retained. This kit can also be used to replace the stock differential for high horsepower turbo or blown Miatas.
The kit includes;

Kit #7 - Headers

Kit #6 - Clutch Hydraulics

Kit #5 - Driveshaft

Kit #4 - Axles

Kit #3 - Differential Subframe (Ford 8.8)

Kit #2 - Transmission Mount (T56)

Kit #1 - Engine Subframe (LSx)

Kit #1 - Engine Subframe
Kit #2 - Transmission Subframe
Kit #3 - Differential Mount (Rear)
Kit #3 - Differential Mount & Braces
Kit #3 - Differential Mount (Side)
Kit #1 - Swaybar Mount
Kit #4a - Level 2 Axles
Zelda’s baby pics
Click Images to View Larger

To view the installation instructions, please click here.


LSx-K6b - Clutch Hydraulic kit w/ remote bleeder

LSx-K8 – Power Steering – the stock plumbing on the steering rack will not fit in the tight space around the oil pan.  This kit replaces the two hard lines from the rack’s gearhead to each end of the steering cylinder so that your NA or NB rack will fit.  We also provide two metric adapters for the pressure and return lines at the gearhead.  Note - all additional plumbing hookups to the pump, reservoir and cooler are up to you.
The kit includes;

• metric adapters and stainless braided hoses for cylinder operation
• metric to AN –6 male adapters for pressure and return lines

LSx-K8 - Power Steering

LSx-K8a – NA Power Steering Rack kit

Shipping & Handling $10

LSx-K8b – NB Power Steering Rack kit


** Free Shipping Offer **
If you order a complete set of V8 kits (at least kits #1 – 5) you get free shipping on everything ordered at the same time (a $180+ savings!).  Contact us for a final total.

Inner CV Tripods – 8.8, 7.5
Clutch Bleeder

Note – Note – these kits are designed for Thunderbird/Cougar and Cobra 8.8 differentials ONLY.  Other 8.8 differentials out of Explorers, Lincolns and other vehicles will NOT fit.

lose the tail, grow some legs

LSx-K3c – Differential Subframe kit for ’90-‘97 (Cobra)

LSx-K3d – Differential Subframe kit for ’99-‘05 (Cobra)

Auto Parts Warehouse

To find used parts and tires
for your V8 swap project,
try these websites