Which IAG Block do I need? A look at the IAG Staged Subaru Short Blocks

IAG has put together a number of staged Subaru Shortblocks so that there is a stage block that is right for almost any application. But the differences between them can be a little bit hard to pin down, so we wanted to take a look at each of them to go through the options that they have to hopefully give you a better idea which block you need for your build.

First, the IAG Staged Block is the IAG Stage 1 Shortblock

This block simply addresses the first weak link of the 2.5 Liter EJ Series engine, the pistons. To build this block, IAG starts with a stock Subaru case, and uses a Subaru OEM crank and rods, but uses 99.5mm IAG spec JE pistons. That is the main aftermarket part used in this stage.  For that reason, it would ideal for 475 bhp, or a max of about 400 wheel horsepower.

Recommend oil pump - 11mm Oil Pump or Roger Clark 11mm Oil Pump.

IAG Stage 2 Shortblock

For the Stage 2 block, IAG assembles the engine, but the cylinders are not bored, so the same 99.5mm IAG spec JE pistons are used as with the Stage 1 block. But for the IAG Stage 2 block, IAG spec H-Beam rods with ARP 2000 rod bolts are used. The stock crank and case bolts are still used in the stage 2 block as well.

With IAG doing the full assembly of the engine, and the use of the IAG spec H-Beam rods, the target hp for the IAG Stage 2 Shortblock is 575 bhp, or a max of about 400 whp.

Recommended oil pump is the Subaru 11mm Oil Pump or Roger Clark 11mm Oil Pump.

IAG Stage 2 Tuff Shortblock

At Stage 2 Tuff, IAG now bores the cylinders of a new Subaru case out to 99.75mm. This allows them to precisely control the piston to wall clearance of the block, and they use a 99.75mm IAG spec JE piston. IAG also moves up to an IAG spec Tuff H-Beam rod with ARP 2000 rod bolts for this block as well.

The IAG Stage 2 Tuff block is the last of the factory semi-closed deck blocks, and the stock crank and case bolts are still used on this block. Stock head gaskets are still recommended for this block as well as 11mm head studs.

Because IAG hones the cylinders of this block to precisely set the piston to wall tolerance, and because they use the IAG spec Tuff-H-Beam rods, the IAG Stage 2 Tuff Shortblock is rated at 625 bhp, or a max of about 440 whp. Because the factory cylinders become a vulnerability at the upper end of that limit, it would be recommend to look at the Stage 2.5 block if your target is over 450 - 500 whp or so.

Recommended oil pump is the Subaru 11mm Oil Pump or Roger Clark 11mm Oil Pump.

IAG Stage 2.5 Shortblock

The Stage 2.5 is the first staged block where IAG installs their closed-deck inserts. IAG also uses 99.75mm IAG Spec JE FSR pistons in the stage 2.5 block. Other than those two changes, the Stage 2.5 closed deck block has the same build components as the Stage 2 Tuff block.

For the Stage 2.5 block 11mm head studs are still used. But because of the addition of the closed deck inserts, JE Pro-Seal head gaskets are now used instead of a stock style head gasket.
With the addition of the closed deck inserts and the up-rated pistons, this block is rated at 675 bhp, or a max of around 470 whp. Because this is a closed deck block it will be more reliable above roughly 500 whp than the Stage 2 Tuff block, and because 11mm head studs are still used, you may have issues if you are running over 22psi – 25 psi of boost pressure.

Recommend Oil pumps would be Subaru 12mm Oil Pump or Roger Clark 12mm Oil Pump.

IAG Trojan Shortblock

The IAG Trojan Shortlbock was developed with reliability in mind for high power road racing application. It is very close to the Stage 2.5 block, but it has some key additions.
IAG spec JE FSR pistons that are coated, and have Electro nickle plating on the ring lands, and thicker wrist pins are used. The Same IAG spec H-Tuff rods are used, but the rod bolts are upgraded to ARP 625 rod bolts.

The biggest addition for the Trojan block is the O-Ringing of the deck, and the block is machined for ½” head studs to help ensure that you have a reliable seal with the head and headgasket. With the o-Ring, JE Pro-Seal Headgaskets are still recommended.

From the addition of higher spec pistons, rod bolts, and the o-ringing of the block with the up-grade to 1/2” head studs, the IAG Trojan block is rated for 750 bhp, or a max of about 525 whp. Because you have much better sealing with the headgaskets on the IAG Trojan block, this is a much better option for running high boost or over about 500 whp for extended periods compared to the Stage 2.5 block.

This block has been discontinued by IAG.  Because the Magnum block was close in price, but offered more features, that is what the majority of customers purchased, so IAG feels that the Magnum block is the best fit for this segment of their range.

Recommend Oil pumps would be Subaru 12mm Oil Pump or Roger Clark 12mm Oil Pump.  A Dry Sump Oiling system would be recommend for a dedicated track car.

IAG Magnum Shortblock

The IAG Magnum block is a further refinement of the IAG Trojan block. The IAG Magnum Block is still closed deck, and running 99.75mm pistons. The Magnum block gets the same grade pistons as the Trojan, but IAG uses their spec Tuff H-Beam rods that are 2mm longer than stock. This extra length gives some interesting benefits such as less side-loading on the piston, and slightly longer dwell time at top-dead-center and bottom-dead-center.

The Stage 2.7 block also gets the IAG Fire-Lock system and it is also machined for 14mm head studs which are a step up from the 1/2" head studs.

With all of these features, this block is rated at 850 bhp, or roughly 600 whp. The addition of the Fire-Lock gaskets, 14mm head studs, and the longer rods make this block even more ideal for running high boost pressure, and sustained high power use, such as road racing.

Recommend Oil pumps would be Subaru 12mm Oil Pump or Roger Clark 12mm Oil Pump.  A Dry Sump Oiling system would be recommend for a dedicated track car.

IAG Stage 3 Extreme Shortblock

The IAG Stage 3 Extreme Shortblock is the original step-up from the Stage 2.5 block. This block uses the 99.75mm IAG Spec JE FSR pistons, and the IAG spec Tri-Beam rods with ARP 2000 rod bolds. It goes back down to being O-ringed, and to 1/2 inch head studs.

This block is rated at 800bhp, or about 560 whp. This block is a step up from the stage 2.5 block, and it would be ideal for short high power use. If you want to use the block for long duration high boost/high power use, the IAG Magnum or a higher grade block would be more ideal.

This block has been discontinued by IAG.  Because the Stage 4 Tuff block was close in price, but offered more features, that is what the majority of customers purchased, so IAG feels that the Stage 4 Tuff block is the best fit for this segment of their range.

Recommend Oil pumps would be Subaru 12mm Oil Pump or Roger Clark 12mm Oil Pump.  A Dry Sump Oiling system would be recommend for a dedicated track car.

IAG Stage 4 Tuff Block

The IAG Stage 4 Tuff block uses IAG's stage 4 closed deck with deck plugs which is a slightly different process than the earlier closed deck blocks. It is also machined for the Fire-Lock head gaskets as the Stage 2.7 Magnum, and it is also machined for 14mm head studs like the 2.7 Magnum block. The IAG Stage 4 Tuff Block uses the 99.75mm IAG spec JE FSR Electro Nickle Plated pistons with Perfect Skirt Coating, and it uses IAG spec Tri-Beam rods that are 2mm longer than stock with ARP 625 rod bolts. This will give the same benefits as the +2mm rods in the Stage 2.7 Magnum block, but using a more durable rod.

It is worth noting that this is the last IAG Stage block that uses the stock STI crank, and OEM case bolts.

All of the improvements of the Stage 4 Tuff block over the Magnum or Stage 3 Tuff block give it a rating of 1000 bhp or roughly 700 whp. This block has basically all of the components needed to hold lots of power, and lots of boost pressure.

Recommend Oil pumps would be Subaru 12mm Oil Pump or Roger Clark 12mm Oil Pump.  A Dry Sump Oiling system would be recommend for a dedicated track car.

IAG Stage 4 Extreme Block

The IAG Stage 4 Extreme block is a further refinement of the Stage 4 Tuff block, and this shortblock represents the maximum potential that IAG can build into a stock EJ257 case, as the next block up from here is the Stage X billet block.
The key additions here from the Stage 4 Tuff block are the use of a Manley Billed Crankshaft, Pinned Mains, and the use of ARP case bolts. All of the other aspects of the build of the block are the same as the Stage 4 Tuff block.

With those key additions, this block is rated at 1100 bhp, or roughly 770 whp. This block is the pinnacle of what you can do with a 2.5 liter Subaru block, and with the pistons, rods, crank, fasteners, etc. used, this engine should hole up to anything that you throw at it.

Recommend Oil pumps would be Subaru 12mm Oil Pump or Roger Clark 12mm Oil Pump.  A Dry Sump Oiling system would be recommend for a dedicated track car.

Thanks very much for reading, and if you still have any questions at all, please don't hesitate to reach out to us.