MR2 SC GruntBox


The GruntBox is a simple solution to the dreaded “lean condition” that has plaugued MR2SC owners since its introduction. This condition occurs when running higher boost levels along with the stock ECU. The Gruntbox was designed to be inexpensive, easy to install, invisible to normal driving operation and requires no tuning whatsoever. The bottom line is, if you have added a pulley to your 4agze, you need a Gruntbox to correct your Air/Fuel ratios.

1 in stock (can be backordered)



Q. What is the Grunt Box?
A. The Grunt Box is a system designed to cure the lean condition experienced below 3500 rpm on 4AGZE engines equipped with larger than stock supercharger pulleys.

Q. Do you still make these??
A. Yes indeed! They are built on a per order basis, so turnaround can take a few weeks, but they are still available. I assume there will be a point in time when I stop entirely, but I haven’t reached that point yet.

Q. How does it work?
A. The Grunt Box uses the factory installed cold start injector to provide extra fueling under the load/rpm conditions where an engine would run too lean without it. This is at high load and under 3500 rpm.

Q. Does the Grunt Box affect cold starting?
A. No. The normal function of the cold start injector is preserved.

Q. Does it work?
A. Yes. Dyno tests showed a 10 ft-lbs increase in torque at and around 2000-2500 rpm when the Grunt Box was added to a 4AGZE engine equipped with a Cusco pulley. Measured air/fuel ratio improved from ~16:1 to better than 13:1 at low rpm and high load. Engine response below 3500 rpm was much improved. Measured performance testing with the pre-prototype on a more highly modified engine showed gains that indicated an even larger improvement than that indicated by the dyno. We think that gains may vary depending on how modified the engine is.

Q. Who designed the Grunt Box?
A. Initially Rob Files and Mike Gruber both of whom have EE degrees and owned Supercharged MR2s, designed, tested, and built the Grunt Box. After a few years, Mike moved on and sold his MR2. Rob is now the one man show.

Q. Does the Grunt Box work on imported 4AGZE motors (JDM)?
A. It should, but that isn’t supported yet. We need to find somebody with one of these motors local to us (Puget Sound area in Washington State), and do testing to establish that it does work.