External Combustion Rocket Anti Lag System + JDM Spec C Impreza STI

anti lag rocket beyond the dynoExternal Combustion Rocket Anti Lag System + JDM Spec C Impreza STI

By Eric Hsu

The external combustion anti-lag “rocket” combustor was used on the factory Prodrive built WRC STi Subaru cars to improve the EJ20 engine's inherent mediocre torque and engine response. The rocket is so good that a version of it is still in use today in the 2012 Japanese Super GT GT300 STi BRZ which also uses an EJ20. Last time I wrote a teaser article, but this time we'll take a look at some of the details of the rocket combustor anti-lag system as it applies to my buddy Ben's JDM Impreza STi Spec C street car over in the UK. 

If you missed the last article and video of the rocket at work, check it out here: External Combustion Rocket Zero-lag Teaser

When I met Ben back in 2008, he had just left joined Cosworth Electronics and had recently left Prodrive due to the cancellation of the Subaru WRC program. It was almost perfect timing with Roland, our other buddy/colleague and the Pectel Program Manager at the time, needing somebody intimately familiar with production cars and Pectel ECUs and Cosworth USA wanting to order an entire product line of plug in Pectel ECUs. Being extremely good at engine calibration, problem solving, testing and all that good stuff, Ben was sent to Cosworth USA for the express purpose of developing the ECPro ECU [Cosworth's plug in version of the Pectel SQ6] for the USDM GR Subaru hatchback.  During the work week we were working together most of the time. Ben sat at the desk directly across from me in the office and when we were out of the office, we were always either in a GR Impreza STi hatchback or EVO X developing, calibrating, testing what would become the ECPro. In fact, Ben got to experience most of Southern California in STis and EVO Xs which wasn't a bad deal for him. We got along pretty good because even though his roots were in motorsports, his passion was high end street cars. High end doesn't always mean Ferrari, Lambo, etc. In this particular case it means a properly built production car using high end parts and craftsmanship. At the time Ben's toy of choice was a built R33 Skyline GTS-t that he drifted all over the country roads near Banbury and beyond. The difference between Ben's R33 and the others was the SQ6M ECU [$7k by itself] with GPS, traction control, Omega dash, bang bang anti-lag, launch control, four switchable maps, sensors galore, and datalogging speed and capacity that probably exceeded Nismo's own LeMans R33 GT-Rs.

With Ben sitting across from me, he would sift though his images, data, notes, etc. looking for information and everytime he pulled up something really cool (which was quite often), he would be like, “hey check this out.” And for me it was like car porn because how often did you get to see the behind the scenes WRC stuff?  Whether pictures of failed WRC components (car or engine), Prodrive's facilities and dyno cells, WRC engine components, or pictures of test components that never went into production (e.g. high speed DC servo controlled wastegate), it was all ultra cool stuff. It was a good thing for me that Ben liked to take pictures of everything. The one thing that always caught my attention though was the combustor; especially after he told me about 5 bar of boost off the line being possible. Ben always affectionately called it the “rocket”. Even back then he said, “I'm gonna sell the Datsun and build a demo car with the rocket. It'll be 'bad ass dude' [in a faux American accent].” The pictures in the article are the pictures of that demo car.

stock spec c engine

The Spec C's bone stock engine before the project start. The Spec C is the homologation special that Subaru built for FIA Group N rally. It comes complete with 12 liter intercooler spray bottle (even the factory knows the short comings of a top mount), transmission and engine oil coolers, hole in the roof with an air duct, high flow water pump and radiator, roller bearing turbo, STi strut tower bars front and rear, better flowing cylinder heads, and slightly more aggressive camshafts. What it doesn't come with is a stereo or any sound deadening. Air conditioning is an option.

Roland, Ben and I have all since moved on from Cosworth, but Ben's rocket demo car is an ultra clean, tastefully built, GD Impreza STi Spec C. It was purchased from Iain Litchfield at Litchfield Imports located in Gloucestershire, England. BTW, you Americans should try pronouncing Gloucestershire and not getting laughed at in the UK. Even if you can say it right, it doesn't sound right without the British accent. Anyhow, when Ben first got the car, he called me from his mobile and said, “HEY DUDE, I BOUGHT A NEW CAR!!  IAIN CALLED ME AND SAID HE JUST GOT THIS LOW MILEAGE SPEC C IN!! WHEN I SAW IT, I HAD TO HAVE IT!! FUCK DUDE IT'S TOO LOUD!! LET ME PULL OVER!! THERE'S WAY TOO MUCH ROAD NOISE!!” As Ben slowed the car, you could literally hear the car come to a stop. The loud and sticky S compound tires coupled with the lack of sound deadening in the Spec C was pretty damn serious. You could even hear through the phone the road gravel being picked up from the soft tires hitting the sheet metal that make up the wheel wells. Some of the noise has been remedied with Pirelli P-Zero Neros for daily street driving, but it's still not exactly a Cadillac.

Enough of me reminiscing of the recent past. Here are some of the details of the Spec C and the rocket. Ben isn't afraid of sharing because he knows that it cannot be successfully duplicated from the pictures themselves. STi and Prodrive spent tons of time and money perfecting it. Lucky for Ben, he handled a lot of that development; especially from the controls standpoint. The controls are very complex and most commercially affordable standalone ECUs have no real chance of controlling the rocket either from a software or hardware standpoint.



  1. Is it possible to get a hold of drawings of the rocket? I would like to know how how the inner tube is done and there isn’t much info anywhere.

  2. Hi i want external combustion rocket anti lag system for subaru impreza sti which use twin scroll turbo, upper pipe and equal headers exaust.

    Can i get that up pipe which can fit my car


  3. Hi i want external combustion rocket anti lag system for subaru impreza sti which use twin scroll turbo, upper pipe and equal headers exaust.
    Can i get that up pipe which can fit my car

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