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Author: Race Engine Technology

Source: Race Engine Technology

Publication Date: 7^th January 2008

For some while now the option has existed for an entrant to run a naturally aspirated 4.0 litre production-based engine against the established 2.0 litre turbo-supercharged and 3.4 litre naturally aspirated engines. Sodemo unveiled the first such challenger at the 2007 Professional Motorsport World Expo in Cologne, Germany early November, a V8 based upon a BMW production block.

The empirical evidence has it that, given natural aspiration sheer displacement is a factor at Le Mans, in spite of the fact that horsepower is pegged by a mandatory intake air restrictor. Even better for the Sodemo V8, the ACO, which governs the Le Mans 24 Hour race, has given the 4.0 litre production-based engine a larger air restrictor than its 3.4 litre rival: 43.6 mm (1495 mm2 intake area) versus 42.9 mm (1445 mm2). With a 3.4 litre V8 good for around 540 bhp, on paper that puts the 4.0 litre newcomer at 560 bhp.

But to what extent might its production base compromise its performance?

If we look at current LM GT cars we find the naturally aspirated, 6.0 litre Aston Martin DBR9 V12 making 570 bhp from 1490 mm2 in spite of an uncomfortably wide valve angle head (40 degrees included). That is 38.25 bhp per 100 mm2 intake area: the comparable figure for a 2007 5.5 litre LMP1 naturally aspirated gasoline engine making 635 bhp is 38.48 bhp per 100 mm2.

A decent production base need not significantly compromise engine performance under air restricted ACO conditions. That follows from the operation of the pre-plenum, venturi-shaped restrictor itself and from the fact that the ACO permits substantial internal modification of GT engines. In fact, the GT rules require retention of little other than the block and head castings and the production firing order. The same applies to 4.0 litre production-based LMP2 engines, which the ACO insists must be: "non-stressed engines homologated into LM GT and complying with LM GT technical rules".

That brings us to the other major consideration: the overall package. Even though the Le Mans track is a power circuit, engine mounting system, size and weight and centre of gravity height come firmly into the performance equation. In fact, under most conditions packaging considerations easily outweigh any marginal performance gain. A key question, then, is: must use of a non-stressed engine based upon production castings compromise the overall LMP2 car package?

The Sodemo V8

As with the question of performance, the question of packaging clearly revolves around the particular choice of production engine. Can a base engine be found that is potentially good in both respects?

Sodemo has based its V8 upon the new BMW M3 E92, which is particularly promising in a number of respects. A 3.4 litre LMP2 V8 makes its 540 bhp at around 10,500 rpm. If the new Sodemo contender is to make 540-560 bhp, to do so it will need to run to around 9250 rpm. As standard the E92 makes 420 bhp from 4.0 litres and runs to 8400 rpm. On top of that it has a 90-degree bank angle, linerless aluminium alloy block from the same foundry as the BMW Formula One V8. This block has 98 mm bore spacing and the production dimensions are 92 x 75.2 mm. Thus inherently it is more compact than the Formula One engine that inspired its design (that has 106.5 mm bore spacing and a 98 mm bore).

The 98 mm bore spacing makes 92 mm about the practical limit for the bore size but that is not far from the 94 mm of the rival Judd V8, for example. As stock the compression ratio is 12:1, the four valve per cylinder heads carry 35 mm intake, 30.5 mm exhaust valves and the valve angle is a very attractive 24 degrees included. In many respects the E92 is clearly a close relation of the current BMW Formula One engine. As stock it weighs in at 202 kg fully dressed and Sodemo claims 140 kg for its LMP2 version. That is not so far from the 120 kg of the rival Zytek V8.

In fact, so suited is the E92 to this LMP2 application that Sodemo has started with extremely conservative internal modifications. Initially the philosophy is to produce a cost effective engine for privateer use, above the quest for outright performance. Even so, following conversion from wet to dry sump the crank height is a respectable 98mm - only 6 mm higher than that of a Judd V8. It is the con rod sweep profile that ultimately determines the depth of the engine below the crank centreline. That being the case, Sodemo could consider bolting down into the rod caps since it is the bolt heads that normally tend to be at the extreme edge of the rod sweep profile. That may gain a few millimeters advantage for the cost of a set of conrods. But this sort of development is not yet on the agenda.

Of course this compact engine will have to be fully supported by a chassis frame and due to its stock-based heads centre of gravity height will not match that of the best purpose designed rivals. However, over the last couple of years the LMP2 minimum weight limit has been increased from 725 kg to 825 kg, playing into the hands of this newcomer. It does represent a packaging disadvantage but not one that is likely to be serious, at least at the Le Mans circuit. The Sodemo V8 project only commenced in October 2007 and track testing was anticipated for February or March 2008, with a view to contesting selected Le Mans Series races prior to a full season in 2009.

At the time of writing it was too early for Sodemo to have struck a deal with a team to run the engine. In fact, it had still to be homologated by the ACO. Assuming it is duly approved, the questions then are, will it be campaigned by a fully competitive team and will further development be funded, as necessary to show its true potential? On paper this engine has the potential to be a very attractive alternative to a current LMP2 race engine albeit perhaps then no longer an inexpensive alternative!

Moreover, if the full potential is realized and if that should provide an edge over existing 3.4 litre race engines, how will the ACO react? Will it cut restrictor size to provide parity?

On the other side of the coin, there exists the possibility that however this engine performs, the ACO will give its’ like an even more generous helping of restrictor size, simply to encourage a switch from race to production-based engines. Who knows the long-term philosophy of the often outwardly inscrutable albeit highly successful French organisation?

Added to the database on 7^th January 2008