The Daytona Prototype international class is the premier class of the IMSA WeatherTech SportsCar Championship. It features prototype cars built with international P2 prototype chassis from manufacturers like Oreca and Riley/Multimatic. Prototypes are designed as purpose-built race machines, and although they occasionally feature some bodywork meant to evoke a particular production model, they are proportioned and constructed in a manner that bears no resemblance to what you might find on the street.
Acura fields a pair of ARX-05 prototypes in this class. They’re based upon an Oreca P2 chassis with some Acura-ish body work on top. Although these cars look like they are out of this world, they actually share a bunch of parts with Acura street cars—especially when it comes to the engine. The engine in the ARX-05 is a 3.5-liter twin turbocharged V-6 referred to as the AR35TT. This engine is based upon the venerable J35 that is currently available in everything from the Honda Odyssey to the Acura TLX.
Although many may not think of the J35 engine as a race motor, it has made quite a few appearances in Pirelli World Challenge, beneath the hoods of the GTS-class TSX campaigned by RealTime Racing. There was also a type approval for a TC-class Accord coupe, which was campaigned by a few drivers including our own Jack Baruth. It wasn’t a huge stretch to see the J35 motor tuned for these relatively stock-ish classes but building a variant for 600 horsepower is a much more involved proposition.
Skepticism is often required when hearing the term “production-based” applied to an engine. It often means that the powerplant may just share a bore size with the street variant. Happily, the Acura folks brought me out to Laguna Seca to check out some of their race cars, which gave me the chance to do a little digging in my free time. I also had the chance to chat with the crew from Honda Performance Development in an attempt to find out just how much the prototype motor shares with what one might find in an MDX.
Pulling some part numbers seemed like a good bet and luckily they were open to confirming what was on my list. Blocks for the AR35TT are pulled right off from the standard production line in Anna, Ohio. Block part number 11000-5G0-A11 is used as the basis for the race motor. This block has been in use since around 2013 and can currently be found in most Acura and Honda vehicles that are equipped with a 3.5L V-6 such as the Odyssey and MDX. It is an all-aluminum 60-degree V-6 with a single overhead cam layout and four valves per cylinder.
Once HPD pulls the block from the production line, they do some machining in order to increase strength and insure reliability. Each block receives an individual bore and hone for performance with the use of a deck plate. Once they start assembling the motor, HPD installs a custom set of race-spec pistons and connecting rods which connect to a factory-style crankshaft that is made of stronger material than stock.
Even though the connecting rods are custom, the bearings are right off the production engine and can be found at the parts department of your local Honda dealer for around $12 apiece. The water pump, direct injection fuel pump, oil pump, and oil filter are also right off the street car engine. The oil pan gets converted to a dry sump setup which is pretty standard in race applications to prevent oil starvation. Instead of the oil being permitted to fall into the bottom of the engine where it is then picked up and fed up to the head, it’s transferred to a separate reservoir from where it can be precisely metered and fed regardless of g-force.
Moving up the motor, we find that the factory head bolts are replaced by head studs, for easier maintenance. The heads come directly from the street car engine and are part numbers 12100-R9P-305 and 12300-R9P-810. These heads retail for a little $500 each from your local Honda dealer and can once again be found in a variety of vehicles such as the Odyssey, MDX, and RLX. The head castings are pulled off the production line and are not modified in any manner. The camshafts are customized compared to the street version but only to compensate for the fact that VTEC is deleted from the race engine.
Although there is a concern for efficiency in endurance racing, employing VTEC would not make sense in this scenario as the engines are usually operating at the top of the performance band and would not be able to take advantage of the VTEC cam profile that is most efficient at lower revs. The valves are also directly used as they come from the street car engine along with the rockers and rocker shafts.
The ignition coils don’t appear on any Honda street car engines, but they do appear on the Honda CBR1000RR motorcycle under part number 30700-MFL-003 so even those are kept in house.
The timing system is comprised mostly of production parts with the timing belt, timing pulleys, belt tensioner, and fasteners all sourced from the road-going engine. The alternator is also a standard street car part which probably makes keeping spares around that much simpler. The primary fueling method is accomplished through a direct injection system comprised of injectors taken directly from the Acura MDX that is currently on sale.
The intake manifold is customized for the race application and includes an additional set of port fuel injectors in order to help provide enough fuel for all the air that is being fed by the turbochargers. These upper injectors are OEM Honda parts as they were used on the J-series engine in vehicles like the previous generation MDX and the ZDX. Using dual sets of OEM injectors makes sense as Honda engineers are already familiar with both. HPD can easily get support from the production side and replacements aren’t hard to find as they can be pulled right off the shelf. The simplicity also makes sense as each set of injectors is intended to support around 300 horsepower in the street engine application so combining them works perfectly for the 600-hp goal of the race engine.
The twin turbochargers are Borg Warner units that are closely related to what Honda employs for its IndyCar engines. These turbochargers are from the EFR (Engineered For Racing) series, which includes a lot of modern features such as lightweight Titanium Aluminide wheels and dual-row ceramic ball bearings. They specifically use a pair of EFR 6258 units on the Acura motor with wastegates controlled by BorgWarner electric actuators. The EFR 6258 is a very efficient unit and the pair can support up to 900 horsepower, depending on the housing that is implemented, so they are well within their range running the estimated 600 horsepower needed for the AR35TT in IMSA competition.
All of that air is fed through a single throttle body which is mounted on that custom intake manifold. In order to support the boost, the compression ratio is lowered from 11.5:1 in the naturally aspirated version to 9.5:1 in this boosted version. All of this performance is tuned with custom software developed by HPD that runs on a McLaren TAG-400 engine management system. Many will be familiar with this model number as similar units are used in IndyCar (TAG-400i) and NASCAR (TAG-400N).
Building this engine from a road car unit is an impressive effort that likely required thousands of engineering and testing hours. It shows that it often does not take very much to modify a modern street car engine to produce more power and do it reliably in an endurance racing scenario. In this case, Honda was able to double the power of its motor while still maintaining reliability, which has allowed the ARX-05 prototype to reach the podium multiple times, including a few weekends ago at Laguna Seca.