ENGINE

This is the standard Esprit V8 Lotus Type 918

3.5 litre, Twin turbo, Quad cam, 32 valve V8
90 degree cylinder bank angle, single-plane crankshaft.
Bore: 83 mm
Stroke: 81mm
Capacity: 3506 cc
Head Chamber Aluminum alloy, four-valve pent-roof
Valvegear: Dual overhead camshafts with one toothed belt per bank, hydraulic tappets
Inlet valves: 33.5 mm
Exhaust valves: 30.0 mm
Block: LM25TF aluminum alloy , cast iron "wet" lines, spheroidal graphite cast iron cross-bolted main bearing caps
Weight: 485 lb with ancillaries





This is the Project M71 engine - the same Type 918 unit with uprated and Keronite coated Mahle forged pistons, gas flowed and ported cylinder heads and the inlet plenum assembly from the GT1 racing program


The "flat plane" crankshaft configuration is used in the 918 V8 (as opposed to the "cross plane" crankshaft used by less exotic V8s (as typified by American versions) - click here for an explanation






Pistons are very important in high boost applications, with the associated heat soak, even when running lower compression ratios

Mahle pistons are about as good as you can get and are ideal for this engine and the Keronite coating is extremely hard and has a very high degree of thermal insulation

Plasma Electrolytic Oxidation (PEO) surface treatment technology is used to deposit the coating




Gas flowing is of less importance on a forced induction engine, as more boost can be used - for a given manifold pressure though, the increased volumetric efficiency will result in power gains

The porting on the engine was tested on a flow bench and was developed in an application where the 918 engine was run normally aspirated




























The standard 918 uses two Garrett T25 water-cooled, oil lubricated turbochargers without intercooling

The cast exhasut manifolds have built up a great deal of heat in this test and this imparts heating into the high pressure air path from the compressor, reducing its density






The Project M71 engine uses Garrett GT28RS turbochargers, which are considerably larger than the T25s, with a much greater potential for power output, whilst maintaining short spool periods with their dual ball bearing construction

Their fitment requires facbrication of specialised tubular exhaust manifolds - click here for more details on the GT28RS and fitting





Click the image for the full details of the
difficulties with using these turbos







This photo on the left shows the "boost piping", high pressure inlet from the turbos into the throttle bodies and standard plenum - the plenum is quite restricive and the throttle bodies, below, are fairly small diameter 






















The GT1 plenum used on the Project M71 engine is grater in volume and much less restrictive, allowing the equalisation of boost pressure across all cylinders

It also used much larger throttle bodies, from the Lotus Carlton

 















With the lack of inercooling, or chargecooling as its called when an air-water intercooler is employed in conjunction with a water radiator in a pumped cooling circuit, many attempts have been made to use twin smaller chargecoolers on the 918 engine, as in the photo below

I was looking to use the twin Lotus chargecoolers on the left, from the Lotus 910 four cylinder turbo engine
















The opportunity then became available to use a Lotus Carlton chargecooler and have it modified to match that used with the GT1, with entries from two turbos

This would line up perfectly with the GT1 plenum throttle bodies, all keeping the path from turbo compressor as short as possible - this helps to keep boost lag to a minimum











The result is here on the engine, with the final touch of the name plate on the chargecooler


A lot the design thought here was from basic turbocharging principle but also inspiration from the Esprit V8 GT1 racing cars and the similarly engined Elise GT1 racer


The works V8s used 1 bar of boost when they used a single large turbo and 1.2 bar of boost with twin turbos - class regulations meant that they had to run with 33mm air restrictors but with mildly fettled heads, the end result was 540 bhp!




Click the image on the left for a full set engine build photos

















This photo shows the chargecooler and plenum on the GT1 Esprit V8 and below shows the comparison with the Project M71 engine







Further developments on the turbo positioning and associated plumbing are in the Progress Section





Click on the images below for enlargements of the V8 engine drawings from the Lotos workshop manual