There are luxury sedans, there are muscle cars, and then there are the rare creations that somehow manage to be both. Jaguar XJ8 L owner Karl T. Muth built exactly that kind of machine — a refined British luxury sedan hiding the heart of an American supercar beneath its timeless sheet metal.
Teacher, author, entrepreneur, and recognized TED speaker Karl T. Muth has become well known in enthusiast circles for his heavily customized LS3-powered 1999 Jaguar XJ8 L, a car that reportedly produces an astonishing 575 horsepower. That figure places it among the most powerful documented Jaguar X308 sedans ever built, and it completely transforms what was already one of Jaguar’s most elegant modern classics into something genuinely extraordinary.
The X308-generation Jaguar XJ8 arrived at the turn of the millennium as one of the last truly old-school British luxury sedans. With its sweeping body lines, low roofline, wood-and-leather interior, and unmistakable Jaguar presence, it carried the spirit of classic Coventry-built saloons into the modern era. Unlike many luxury cars of the period that chased futuristic styling trends, the XJ8 stayed faithful to Jaguar tradition. It looked dignified, understated, and expensive without trying too hard.
But beneath that polished exterior was a surprisingly capable performance platform. The XJ8 came standard with Jaguar’s silky-smooth 4.0-liter V8 producing around 290 horsepower, along with suspension and chassis improvements that made the big sedan feel far more athletic than its size suggested. Rear-wheel drive, balanced proportions, and a low center of gravity gave the car excellent composure at speed, impressive cornering stability, and a ride quality that remained comfortable even on rough pavement. In long-wheelbase XJ8 L form, it also offered limousine-like rear legroom, making it as comfortable for passengers as it was enjoyable for drivers.
Karl had admired this generation of Jaguar for years. To him, the X308 represented the last truly “British” XJ before later generations lost some of the charm and nostalgia that made the older cars special. He specifically wanted the final steel-bodied XJ platform, and after searching for the right candidate, he found one on eBay for just $1,201.
The bargain Jaguar came from its original elderly owner and had only 72,000 miles on the odometer. The seller even apologized because the driver’s seat adjustment no longer worked. But Karl already knew the car’s future had little to do with preserving factory originality. The engine had poor compression on cylinder number three and was due for a rebuild anyway, making it the perfect starting point for something far more ambitious.
Initially, Karl planned a relatively straightforward LS swap using a stock GM Performance CT525 6.2-liter LS3 crate engine. Even in factory form, the naturally aspirated CT525 is a serious performer, delivering 525 horsepower at 6,600 rpm with the kind of reliability and simplicity that made the LS platform legendary. For many builders, that would have been more than enough.
Karl had other ideas.
Drawing inspiration from the supercharged LS9 used in the Chevrolet Corvette ZR1, he began modifying the LS3 with a collection of carefully selected high-performance components. Everything above the head gasket was upgraded using re-machined LS9 hardware optimized for naturally aspirated performance. The engine received LS9 cylinder heads with a custom three-angle valve job, along with a Lingenfelter Performance Engineering GT1-1 camshaft, titanium intake valves, sodium-filled exhaust valves, and numerous additional performance upgrades.
The result was a naturally aspirated 6.2-liter LS3 producing a staggering 575 horsepower on the bench.
To handle the increased output, Karl paired the engine with a Tremec T-56 Magnum six-speed manual transmission using shorter “road race” gearing. The heavy-duty Magnum version of the legendary T-56 has been trusted in high-performance machines ranging from the Corvette ZR1 to the Dodge Viper ACR, thanks to its strengthened gears, shafts, synchros, and bearings. In Karl’s Jaguar, the transmission featured an exposed shift gate that added a mechanical, motorsport-inspired feel completely unlike anything Jaguar ever offered from the factory.
Surprisingly, the swap didn’t just increase power — it improved the car’s overall balance as well.
According to Karl, the original Jaguar engine and ZF automatic transmission were actually heavier than the LS3 and Tremec combination replacing them. The LS engine also sits nearly three inches farther back in the chassis and substantially lower in the engine bay. Because pushrod V8s carry more of their mass low in the block compared to overhead-cam designs, the swap improved both weight distribution and center of gravity. In a large luxury sedan, every pound saved and every inch of improved balance matters.
The transformation extended far beyond the drivetrain. Karl fitted the car with an XJR Arden suspension system designed to preserve the Jaguar’s luxurious ride while adding the sharper responsiveness needed for a 575-horsepower sports sedan. The result is a car that feels planted, composed, and stable even at high speeds, with handling characteristics that defy its size and vintage luxury roots.
Inside, the Jaguar received equally thoughtful upgrades. The factory analog gauges were replaced with a Racepak digital instrumentation system seamlessly integrated into the traditional walnut dashboard. Audi A4 front seats improved support and comfort, while the interior was retrimmed by Alea Leather with diamond-quilted Alcantara inserts that blended modern craftsmanship with classic Jaguar elegance.
But it’s the subtle details that Karl seems to appreciate most.
Carbon-fiber bullet mirrors sourced from a Japanese-market Mazda MX-5 race car, Aston Martin-style flush door handles, and a motorcycle-inspired fuel filler cap all give the sedan a unique personality without overwhelming its original design. Nothing about the car feels flashy or forced. At a glance, it still appears to be a tasteful luxury sedan for executives and diplomats. Only enthusiasts notice the clues hinting that this Jaguar is something entirely different.
And that contrast is exactly what makes the car so compelling.
Karl didn’t build the Jaguar as a dedicated track toy or weekend-only showpiece. Its primary purpose is surprisingly practical. He uses it to pick up clients from airports, transport colleagues to meetings, and enjoy long-distance driving in comfort. For many passengers, the experience is unforgettable. Even people who know nothing about cars instantly recognize that there is something special about the big Jaguar.
All told, Karl estimates the project cost slightly more than $38,000 including the purchase price of the car itself. Considering the performance, craftsmanship, and uniqueness of the finished product, that figure almost sounds unbelievable.
The finished Jaguar perfectly captures what makes the restomod world so fascinating. It combines classic British luxury and timeless styling with modern American V8 reliability and brutal performance. It is elegant without being fragile, fast without being obnoxious, and customized without losing the character that made the original XJ8 special in the first place.
In an era when many modern luxury sedans feel isolated and over-digitized, Karl T. Muth’s 575-horsepower Jaguar XJ8 L stands as a reminder that great cars are often the ones that blend completely different worlds together — and somehow make it all feel perfectly natural.
Vehicle Specifications
Vehicle
- 1999 Jaguar XJ8 L (X308)
- Long-wheelbase luxury sports sedan
- Purchased for $1,201 with 72,000 original miles
Engine
- Customized 6.2-liter LS3 aluminum V8
- Approximately 575 horsepower (naturally aspirated)
- Lingenfelter LS9 L150026109 ported cylinder heads
- Lingenfelter 3-angle valve job
- Lingenfelter GT1-1 camshaft
- GM Performance valve locks
- ZR1 valve seals
- ZR1 lash caps
- Titanium valve spring retainers
- Comp Cams 26926-16 dual valve spring kit
- ARP 234-4317 adapter head studs
- Sodium-filled 1.59-inch exhaust valves
- Titanium intake valves
- Comp Cams Ultra Gold 1.65-inch rocker arms
- Manley Performance single-piece chromoly pushrods
- Pankl 12624231 titanium connecting rods
- Wiseco forged 11.8:1 pistons
- Lingenfelter L300025297 oil cooler adapter
- External 13-row single-pass oil cooler
- Custom exhaust system with 4-2-1 headers and X-pipe layout
Driveline
- Tremec T-56 Magnum 6-speed manual transmission
- Close-ratio “road race” gearing
- Exposed shift gate
- Custom driveshaft
- Lightweight GM Performance flywheel
- SL63 AMG intermediate crossmember & driveshaft loop
- Salisbury HU 8.8-inch rear differential with finned cooling cover
Suspension
- XJR Arden suspension system
- Modified lower control arms with deep perches
Wheels & Tires
- Fifteen52 Tarmac R40 multi-piece forged wheels
- 18×8-inch front
- 18×9.5-inch rear
- Yokohama Advan performance tires
Exterior
- BMW X6 side marker lamps
- Motorcycle-style locking fuel filler door
- Kindig smooth door handles
- Bullet-style carbon fiber mirrors
- Lexan clear wheelwell flaps
- Chrome trim repainted in body color
Interior
- Audi A4 front seats
- Factory Jaguar rear seat retrimmed by Alea Leather
- Diamond-quilted Alcantara inserts
- Custom headliner
- Custom trim panels matched to factory walnut dashboard
- Racepak digital instrumentation integrated into factory dash
Additional Highlights
- Improved weight distribution compared to stock
- LS3 and Tremec drivetrain lighter than original Jaguar powertrain
- Engine mounted nearly three inches farther back and lower in chassis
- Rear-wheel drive configuration for improved traction and handling
- Built as a refined high-performance luxury sedan for daily use and client transportation
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