Turning the Exige into an Exocet!
One of the more interesting projects over the last few months has been performance modifications on a normally aspirated 2006 Lotus Exige.
Unfortunately, the 2ZZ hasn’t possessed the reliability for which Toyota engines are generally renown. Its high revving character has seen many spectacular failures including inlet valves, oil pumps, rockers and the occasional connecting rod. The use of an aluminium block with no liners, while great for weight reduction (albeit with a ceramic matrix), has also seen aggressive bore wear when the engines are used under competition conditions. Nevertheless the adoption of Toyota engines has been a successful and clever move by Lotus enabling the 2ZZ equipped Elise and Exige models to achieve sensational performance.
The brilliant and innovative Lotus aluminium chassis has always been able to handle significantly more power as evidenced by the proliferation of Honda K20 powered conversions for the earlier Rover engined cars. Even the supercharged Exige S can do with more oomph for more excitement. One of our lucky customers wanted his Exige modified to emulate the performance of his previous GT3 Porsche. Winning at track days was everything and Meridian Motorsport was up for the challenge.
A two stage plan was configured. This included reliability and performance modifications for the normally aspirated mill with Stage 2 including the possible installation of a high spec supercharger kit for maximum performance. We dismissed turbo charging at the outset for fear of upsetting the mid corner balance of what is a great handling car on the limit.
Stage 1
Engine
The standard 2ZZ aluminium block was bored and fitted with nodular cast iron sleeves to suit forged pistons (11.5:1 CR) complete with high performance rings and wrist pins. We could go higher with the compression ratio but we erred on the side of safety should we go down the supercharger route.
The rotating assembly was swapped out to include billet CrMo rods together with a brand new stroker crankshaft residing in heavy duty main and big end bearings. ARP studs were also used to fasten the main bearing girdle and to clamp the cylinder head to block. Billet steel oil pump gears were fitted to prevent oil pump failure under high RPM/load conditions. The increase in stroke (91.5mm new versus 85mm old) raised capacity from 1798 to 1933 CC with expected nominal horsepower and torque increases of 7.5% respectively.
The cylinder head in standard form is very efficient but was lightly ported to a flow bench tested design and surface milled to ensure flatness. Valve seats were profile machined to suit new one piece stainless steel valves with higher rate valve springs being fitted to ensure reliable high RPM operation. The camshafts were replaced with intermediate bespoke items from Piper in the UK. These increased lift and duration for both the low and high speed profiles with the objective of increasing mid range torque (one of the weaknesses of the 2ZZ) and high speed power. The standard exhaust manifold was replaced with a larger diameter 44.45mm, 4:2:1 design with optimised runner lengths to further increase mid range torque. The head was also fitted with a large diameter drain back to the sump, to prevent accumulation of oil under high lateral G’s in the cam cover. A fabricated 5.5 litre baffled aluminium sump was fitted in addition to an oil pressure accumulator to prevent oil surge and improve cooling.
All other standard parts were cleaned and checked and the motor blueprinted prior to assembly.
Transmission
The standard 6 speed gearbox fitted is a compromise providing ease of use around town and quiet high speed cruising with good economy; not ideal for a high revving track car. First gear is too low for performance starts, with sixth gear being significantly overdriven. The spacing between gear ratios does not ensure that the engine stays on the high lift cam between gear changes. The “gear change gap”, also means that synchronisers have to work harder during high speed, up or down changes. The gearbox also lacks a limited slip differential (LSD) ensuring continued traction on greasy surfaces or when a wheel is lifted during spirited cornering. Drive out of corners is very important for lap times.
A KAAZ Close Ratio 6 speed gearbox kit and plate type LSD were fitted to solve these issues. These transmissions are typically worth 2 seconds a lap at tracks such as Winton or Eastern Creek and will provide the best improvements in lap times for performance dollars spent. The 1.5 way LSD provides improved traction with less push on trailing throttle. The only downside is that theoretical top speed is lowered to about 149 mph; the same speed as a stock transmission at 5th gear redline.
Gearbox Ratios | |||||||
|
Gear No. |
1 |
2 |
3 |
4 |
5 |
6 |
Final Drive |
|
Std. ratios |
3.166 |
2.050 |
1.481 |
1.166 |
0.916 |
0.725 |
4.529 |
|
KAAZ. ratios |
2.929 |
2.176 |
1.70 |
1.364 |
1.167 |
0.916 |
4.529 |
Clutch and Flywheel
The proposed increase in engine torque required a meatier clutch. We have good experience with sprung paddles (for driveability) which includes 6 blades fitted with ceramic faced buttons. This was combined with a 900kg pressure plate which itself is fitted with a CrMo friction surface. All of this is mated up to a lightweight CrMo flywheel (3.8kg) with integral ring gear. The stroker crankshaft uses a smaller clamping PCD for the flywheel mounting and so the bolts were also upgraded to ARP items, torqued to a higher pre-load. This combination has been well proven with the Exige race cars that we have built.
Engine Management
All Toyota powered Exiges up to 2006 were fitted with a Lotus T4 ECU with a standard mechanical throttle body. To our surprise, our car turned up with an electronic throttle driven by a very sophisticated EFI ECU. This ECU has so far proven un-crackable with Lotus jealously guarding the engine strategy and unlock codes. Our customer was keen to retain the existing digital dash and engine immobiliser so we installed a MoTeC M800 for engine management in tandem (fuel, ignition, valve lift and timing), with the EFI ECU being left to control CAN bus, electronic throttle, dash, security and all other subsystems.
Stage 2
Stage 2 included the installation of a recently released supercharger kit developed by TurboXS out of Sydney. This is a beautifully developed installation using the same M62 supercharger as per the Exige S. Unlike Lotus however, the kit uses a large intercooler mounted in the existing air duct in the left hand side of the engine bay. The pipe runs are fractionally longer than the factory installation but cooling is better and rear vision is retained which is completely absent with the S version because of the intercooler mounted on top of the engine. The quality of the kit is better than OEM with all components being professionally designed and manufactured for ease of installation; not something easily achieved in the restricted confines of the Exige engine bay.
Results
Normally Aspirated
Needless to say, results have proven impressive. The close ratio box and LSD transform the car as expected. The ability to stay on cam during gear changes makes a significant contribution and the action of the LSD maximises drive out of tight corners with higher speeds being held for longer on straightaways; a real improvement in lap times.
The stroker engine in naturally aspirated form combined with Piper cams gains significant mid range torque which itself is amplified by the new gearing of the close ratio box. This makes the car very punchy in any gear feeling much stronger than its 1933CC may suggest. The car is 2kW shy of the Exige S and maximum torque is nearly the same. The torque curve is very linear and gently rising through the rev range making the car very easy to drive. It doesn’t however have the low speed punch of the Exige S. We measured performance of the engine with and without the 4:2:1 extractors. Compared to the standard Toyota headers, they appear to add around 3-4Kw at the top end without any significant mid range improvement. But our client needs to go faster ………………..!
Supercharged
Installation of the supercharger was a straight forward process. The only major issue was the supply of 550cc injectors which are a lot larger than the standard Toyota items. Once we had corrected our fuel map the car was started and duly tuned. Peak power was now a spectacular 187Kw at the wheels at 8300RPM (we have a Dynapack dynamometer).
The torque curve is almost flat (just starting to decline over 7000 RPM) and the engine continues to make power the harder you rev it. After a comprehensive tune, detonation even with the 11.5:1 compression wasn’t a problem at full load, low speed sites. Boost pressure was a maximum 6PSI and we never saw intake temperatures more than 60deg C, although we were tuning on one of Melbourne’s cold winter nights.
So how does it drive ………………………….?
Without doubt, this is one of the fastest street cars we have driven. The close ratio box is almost redundant as one struggles to change gear quickly enough to keep up with the revs. We have yet to install some GPS based data logging but we will bring you performance figures when available. The customers comment after an initial drive was “!@#$ this is now quick enough! If only I can keep it in a straight line”.