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700hp+ Impreza Time Attack engine building tips?

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I'm in the process of planing a big Impreza Time Attack project. Because I only built EJ engines with up to 500hp until today, I would be happy to hear some tipps and suggestions from people with first hand experience with very high powered EJ engines.

First, the EJ257 blocks are very good known to fail, because the cylinders are wandering which can lead to head gasket failures or cracks in the block. Regardless to that, I've seen 1000hp EJ25 in dragracing. Really wondering how they hold the block together at this powerlevel?

The most robust Subaru block is the closed deck one out of the Legacy turbo. Does anybody know how much power they can handle without and with sleeving them?


Which heads will flow best?

- GC8 non AVCS

- EJ207 with AVCS

I'm know for sure that the EJ257 have worse flow than the EJ207. What I'm know Roger Clark run no AVCS on his high reving EJ20 (up to 10'000rpm!) are there any benefit to delete it or do simply the AVCS struggle with such high RPMS?

Would like to hear some first hand experience!

Hi Adrian,

The Drag racing guys quite often grout fill the blocks to help stop the bores from wandering, they can get away with doing this as the engines do not run under load for long periods of time. This will not work with a TA car. the best option for the EJ257 would be to have the blocked dowelled and sleeved. This will help prevent the bores from moving, as well as strengthening them. You do suffer with the 2.5 block having thinner bores compared to the 2.0. You could also get a EJ207 and stroke it out to around 2.3l, thereby getting a semi closed deck block with thicker bores walls that will still rev.

Regarding the heads, the AVCS Spec C 207 heads I believe have the largest port and valve sizes from the factory, with the 257 heads not that far behind. There are also a couple of different combustion chamber shapes and sizes, so you have to be careful when matching the piston squish areas to the heads, as well as the compression ratio.

AVCS is useful in off boost situations, but you typically pull most of the advance out of the cams high up in the rev range, so a set of cams that is optimised for the operating rev range of the engine would be the better solution, you just may suffer some drivability issues if the engine falls outside that range. This can be compensated for in the tune to a large extent,

From what I have seen the outfront motorsports closed deck ej257 and the iag performance closed deck blocks are most popular and both are very popular here in the U.S. I have a personal friend with a outfront closed deck ej257 making over 600awhp, but even he wants to go with an iag block i'd look them up if you have a chance.

I built a 2.6 litre engine for drag use a few years back that ran on methanol. I sleeved the block using Darton sleeves and then went further by solid filling the block and just running water through the heads. As BlackRex mentions, this won't be any use for circuit use though.

Sleeving the block is a little harder than it sounds from my own experience and I'd strongly advise using a machinist who is very experienced in sleeving. In many instances sleeving the block actually creates a new set of issues and it's quite common to hear of a sleeve dropping and resulting in a head gasket leak.

If you're aiming for 700 hp I'd also suggest moving to 14 mm head studs as head gasket integrity is an issue at the sort of boost level you would need to run to achieve that sort of power level. This in itself is a specialist operation as the heads need to be re-dowelled to the block.

Basically my advice would be to find a specialist machine shop who is experienced in Subaru engine machining at your target power levels. Also realise that you're going to be pouring quite a lot of cash into the machining part of the build.

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