Some advice from the EXPERIENCED engine builders out there...

[Leighos]

I\'ve got a BD5a Legacy, original EJ20H converted to single turbo running a 6S ECU, 440cc injectors etc (all standard v3 STi stuff), with a TD05H-18G.

It goes reasonably well running on 1bar (on boost approx. 3200k), but I\'m not convinced the v3 STi tune is well suited to the old EJ20H, plus its done almost 200k so I\'m looking at a rebuild.

Not looking for huge power here, just a reliable street engine producing close to (or maybe a bit more) what the v3 STi tune is good for.

Now I\'ve collected a few parts over the years and I\'m thinking of putting them to good use finally, rebuilding a spare engine while I keep my daily driver operational.

There\'s a v2 STi EJ20G that\'s run a bigend on #2, crank and bores are good but one of the pistons (factory forged) has some score marks on the top land, from possibly eating a compressor blade at some stage in its life.

Next I have a set of EJ20R cylinder heads that are in good condition, apparently \'reconditioned\' before the engine they were attached to met its demise.

I\'m hoping these will be a better match the v3 STi tune than the 20G/H heads I\'m running now, would that be fair to say?

Now my plan here is to rebuild the short block with some new forged pistons (Wiseco?), new H beam rods (Eagle?), ACL bearings, Cometic head gaskets etc.

My question is regarding selecting the correct combination of piston for compression ratio and head gasket thickness.

I\'ve done a bit of internet research but I can\'t get a conclusive answer on the original CR of the v3 STi, was it 8:1 or 8.5:1? what about taking into account the differences of the EJ20K and EJ20R heads when working out CR?

Not wanting to go to all the trouble of rebuilding an engine only to find out is all out of whack because I didn\'t get the CR right, I\'m asking for some advice from guys who may have done this combination before.

[marc]

First off if you are going with aftermarket pistons u need to decide what and look at their comp ratio options. Cant go far without looking at/knowing that lol.

Is the V2 block you have closed deck? If you are putting rods in it itd be a waste if its open deck (some are).

Also if its run a bearing most likly the crank wont be good. (unless you have another crank and im mis reading what you wrote).

I would have to look in my notebook but from memory EJ20K and EJ20R heads are the same cc volume and wont affect your comp ratio. Ither way cant suggest gaskets or anything till your pistons are chosen. Best way is to calculate your comp ratio properly using your selected components volumes but obviously you dont want to/trying to get around that hence why your asking here..

will let the rest of the EXPERENCED builders chime in now

[Leighos]

Cheers Marc,

Good call on looking at the piston options, I was looking at Wiseco at first (just the first one I came across in a search), but they don\'t do a standard bore for the pre 98, only 92.5 and 93.

What brand of pistons do you use/can recommend?

Block I intended using is open deck, although I have a closed deck sitting around also, but that is a bit suspect as it may have been overheated badly, haven\'t checked it out yet.

I didn\'t go down the route of using a closed deck block as from what I\'ve read (internet of course) they\'re really only useful when it comes to big HP, also other folks have recommended to not use the factory rods in a rebuild.

I guess that is the problem with gaining knowledge from various forums, there are so many conflicting opinions.

Yes crank is off another engine, no scoring on the journals and will mic them when I get a chance to ensure they\'re still in spec.

Didn\'t mean no disrespect with the \'experienced\' in the title, you obviously know what you are talking about, just wanted to get the attention of guys like yourself that had actually rebuilt engines before.

[rex-leggy]

just my opnion. but for a little extra (machine costs) id go for .5 oversized pistons.

I built a ej20 with factory sized je forged pistons and it is quite slappy when cold. atleast with the oversized you can get the tolerances much closer.

from when I was researching head volumes I found that ej20 r and k heads both are the same being 46cc\'s

only thing I can say is future proof. im doing my 3rd engine now and wish I just did it all from the start.

but that all depends if you have future plans

[gotasuby]

If it was me I would use closed deck block, acl bearings race, eagle rods and cp pistons. I have used this combination 10 or more times and have all been 100 percent reliable up to 300wkw. With genuine head gaskets and factory ej20r heads

[Leighos]

Thanks guys that\'s all good advice, and gets me going in the right direction.

As far as expectations go, I\'m really after just maximising the factory tune of the 6S ECU and to build something that will see me thru for another 10 years ;D (yes has the old RS for over 10 years now and not planning on getting rid of her anytime soon).

I understand what you saying about future proofing there rex-leggy, but I have a shed full of other projects too so have set myself this realistic limit on the daily driver for budget/time constraints.

Titan has made a good point too of fuel - 98 and the occasional limp home on 95.

Economy, a daily work taxi so need to be realistic - basically If I can achieve a \'factory\' type refreshed engine in the guise of a v3 STi, with the parts that I have available, then I\'ll be a happy man.

So just out of interest what \'should\' the CR of the v3 STi be?

[Koom]

If you are just using the factory v3 ecu, I\'d just go with the cheapest aftermarket piston\'s (prob wiseco\'s) and they only have the one cr option for early ej20\'s don\'t they? Then either get any rods that use arp rod bolts and make sure it\'s all built to the right spec. No point in going overboard when it\'s running the same tune/boost that some engines will last many hundred k km\'s on factory internals. If you could find parts were in good nick, I\'d just rebuild it with factory parts (I.e. v3/4 piston\'s and rods or gtb rods).

The ej20r heads have the same part numbers for spare parts i.e. cams so they will be perfect for your 6s ecu.

[Leighos]

Yes the Wiseco\'s I was looking at (P/N K576M925) were 92.5mm and stated 7.8:1 CR.

Like Koom just suggested, I originally had planned to rebuild the v2 block with a factory internals as the bores looked so good, looked like it had just been honed , from memory the car had only done 80 odd thou when removed with BEB failure.

I was going to use the factory v2 forged (or some say semi forged?) pistons, but what put me on the track to use AM forged pistons was this little beaut here...

Shame really.

[thorpy]

This was my combo and worked very well , stock like driveability and made 210wkw on 1bar with the stock ecu,injectors and AFM.

CD Block

Stock crank

ACL Race series bearings

Manley forged H-beam rods

Mahle motorsport 92.5mm forged pistons

ARP head studs

All new genuine gaskets and waterpump

STi MLS head gaskets

I used V4 sti heads with aftermarket valves ( stock sizing )

[thorpy]

Re. pic above , doesn\'t really look like damage as such , kinda looks like a casting flaw.

[Paimei]

Looks like det to me. I\'ve seen the aluminium on the skirts torched from this before. Make sure you have a properly calibrated ECU for the application otherwise you run the risk of the new build having the same fate.

[funkytown]

Yer thats det to me.

[Eight_dollar]

det "pepper" the top of the piston and break the ring land between the compression rings

Thats more like combustion temps way too high and about to gas the side out of the piston

[Leighos]

Yeah I never thought of detonation, I just assumed it was damage from ingesting a foreign object, although the corresponding area of the bore is perfectly clean and free of any damage.

Saying that, after examining the block a fit further - I found a small crack in that same cylinder (approx 8mm) between the machined ring that sits on top of the liner, sorry I don\'t know what that part is called!

[Paimei]

 Eight dollar said:

det "pepper" the top of the piston and break the ring land between the compression rings

Thats more like combustion temps way too high and about to gas the side out of the piston

Remember high combustion temps and det are self perpetuating and it does not always manifest as peppering on the crown or broken ring lands. Combustion heat can reach well over 1000 degrees c from det and aluminium alloy cannot tolerate anything like that of course. It would be Interesting to see a picture of the crack described at the top of cylinder and also the head gasket for that bore.

[Leighos]

Couldn\'t find the old head gasket, but here is a shot of the area on #3 where the crack is.

Its in about a 10 o\'clock position when viewed looking down the bore with the engine level on a stand.

This was supposed to be my \'good\' block in which to rebuild with

[Paimei]

Is the crack at the same position in the bore as the damage on the piston?

[Leighos]

 Paimei said:

Is the crack at the same position in the bore as the damage on the piston?

No the damage to the piston is at the 12 o\'clock position and a few smaller marks either side, viewed as fitted (R mark at the top and arrow pointing fwd) whereas the crack in the bore is at about 10 o\'clock.

[Eight_dollar]

thats not a crack it a casting flaw - many engines have that, only becomes an issue combustion gas causes the sleeve to become loose

I still believe your marking on the piston crown are from combustion temps too high.. As factory pistons break the ring lands with det

It may well have been detting too but the all pistons that have failed that I have seen like this were from lean out

[Leighos]

 Eight dollar said:

thats not a crack it a casting flaw - many engines have that, only becomes an issue combustion gas causes the sleeve to become loose

I still believe your marking on the piston crown are from combustion temps too high.. As factory pistons break the ring lands with det

It may well have been detting too but the all pistons that have failed that I have seen like this were from lean out

Interesting...do you think the block would be useable for an OEM type build with that flaw?

I haven\'t shown my engine builder either the piston or the bore yet as he is away on holiday, he may say \'yep just go ahead and use them\' .