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aselby

Sohc heads on a wrx engine

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For starters I’ll say I did search and didn’t find any definitive answers, I found similar questions and the only answers were go search. Back story: I have a 2004 Saab 92-x turbo with a bad engine (I will call it a wrx from now on). I plan on putting an engine in the car for a few months and then transplanting it into my 1966 vw bug. The Saab will just be used to dial in the car and make sure it runs good. I have an ej255 short block and I want to put sohc  heads in it and run it with the stock wrx harness and pcm. Ideally I’d like to use 251 heads because I have a rebuilt set. I know the heads will physically fit and the turbo will have some clearance issues that can be worked out. I know I will need to plumb oil and coolant to the turbo. What I’m not sure about is ignition coils and camshaft gears. The wrx uses Coil on plugs and the 251/253 heads use conventional wires. Does subaru make a coil on plug coil from a later model sohc engine that I can fit on a 251 head and be compatible with the wrx harness? I can tig a bung to bolt down the coil if needed. Will the camshaft sensor read the sohc cam gear properly? Also if anyone has done a similar conversion I’d love some feedback.

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Won't run on stock management.  IIRC,  Cam sensors are all different setup.  Turbo EJ255 is AVCS, with sensors at the back of each intake Cam, not on the gear like EJ251/3 setup.

And also the EJ255 intake won't bolt onto the SOHC heads.  Maybe with an adpater/spacer?  Or you could try to swap wiring over to a 251/3 manifold?  

The DOHC heads really aren't much larger, especially width wise........Why not just run a full EJ255 rather than some hybrid?

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The short block is a 255, the heads would be 251, the pcm harness and manifold are stock 205 from an 05 wrx. No avcs to deal with. The dohc in my bug means extensive body modifications. Do you know of the cam gears will work or a cop that will fit? 

Edited by aselby

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The crucial question (which I do not know the answer to), is whether the EJ205 ECU can read the EJ251 cam sensor. If you have them, take a look at that sprocket and compare the tone wheels.

 

The spark plug tubes in the later EJ253s (which had COP) are deeper than the older 251/253s. So no, those coils are not a direct fit. But the extra length on the boot could probably be dealt with with some sort of spacer/adapter/mounting bracket.

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As I think about it, I might have it backwards. I think we were trying to use COP heads on a non-COP car, and the tubes on the COP heads were shorter, as the regular plug wires didn't reach the plugs, and the older valve covers didn't seal on the newer tubes (I don't remember what was different that required the use of the older valve covers...).

So unless you can find a longer boot for the stock plugs. But since the valve covers are different, you'll probably be making custom mounts anyway, so a different coil might be a better option anyway. The Audi R8 Coils (used on many other things, but that's the easy way to look them up) are powerful and reasonably priced, I see those being used a lot on 1UZ COP conversions. Otherwise it might make more sense to use 4 standalone coils (Chevy LS style), with a short plug wire. Lots of options.

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You'll need to use the 05+ sohc ej25 cam and crank timing gears to get the correct pattern, the 205 intake manifold will bolt to phase 2 sohc heads. You may be better off going with GM LS coil near plug setup and doing custom wires. You'd need to adjust the ignition dwell settings in the 205 ecu to keep from frying coils though. 

Edited by 86BRATMAN
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Great information thank you. You mentioned the camshaft and crankshaft gears need to be from a 05+ sohc engine. Do you know if the crankshaft sprocket from the stock wrx engine is the same. I’d like to keep the coil on plug if possible. Does anyone have a listing of the different lengths of boots that will fit on the coils? Thanks again for all the help

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The wrx crank gear is the same. You'll most likely have to fab up a mount for the coil and then see how long the boot needs to be. The 2010-2012 ej25 uses COP, the boots from them may be compatible with the wrx coils. 

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