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Turbo advice and other stuff on a 2.5 conversion.


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Hi guys, as the knowledgeable bunch u are wondered what your thoughts and advice would b on the project build I'm doing. Have got to he stage where I need to order my turbo and decide which way to go with the manifold. Quick rundown on what I have first. It's a 2.5 short motor forged internals V5 STI Heads 14mm studs all new and put together. It's going in a V4 classic so not sure on whether to go down the adapter plate route or convert to a V5 manifold and what that would entail. Next is the turbo. Going to go for either a TD05 20g or TD06 20g. Which do you think would b the better option? Or if anybody thinks there might b something better, say a twin scroll conversion. Thanks and look forward to hearing from u.

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I don't really know much about tuning the 2.5 ltr tbh so can't be much help I'm afraid but I think the chambers on the 2ltr heads need to be machined to accompany the bigger 2.5 ltr piston diameter ,unsure what difference that would make on the compression ratio and gasket thickness

I'd imagine it would be easier to fit the original inlet manifold with the conversion plates unless you're fairly good at splicing looms to bridge the v3/v4 phase 1.5 to v5/v6 phase 2 differences with the icv ,tps ect .

Using the cars original inlet would probably mean that you can retain the phase 1.5 fuel rail set up but you'd have to check that with whoever you buy the adapter plates off .

Although it's forged with oversized head studs I'd imagine the main weak link will be the deck type so 450/450 would probably be it's limits.

I think Classic heads do tend to be a bit restrictive above those power levels anyways ,so I'd research what turbo would hit just under those targets on a 2.5 ltr .

I thought about a twin scroll conversion but on a 2ltr I'd really need avecs heads to make the most of it (unsure about on the 2.5 though) .

Sounds like a interesting project and I was considering a 2.5 build when my 2ltr goes pop but I'd probably use v3 sti heads which will bolt straight on to my current v1 reversed inlet.

Keep us posted on your project and feel free to start a build thread ,bud

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Thanks for the help. Agreed on the original manifold I do think it's going to b easier in the long run. I had lined Carl Davy up for the adapter plates but was told he'd stopped doing that sort of stuff now so spoke to a firm called delta engineering opposite from were I work he had a look and said it wasn't a problem to sort so think I'll take him up on it. Decided on the 2.5 because of the low down torque over the 2ltr. Compensates big time for the big turbo lag of a 20g.

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I think alyn at as performance does some of carl daveys stuff along with harvey smith headers and upipes ,give alyn a bell as his website doesn't list much of his stock .

Tidgy yours wouldn't need adapter plates as its a v5 type r so the heads are the same phase as your inlet /loom

the adapter plates are to convert v1 to v4 "inline" inlet pattern heads to v5 onwards "staggered" inlet pattern so you can use early inlets on later heads and visa versa

The 2.5's definitely have less lag and more torque than a 2ltr but i got my v1sti cdb long engine for free so stuck with the revvy 2ltr platform for my self build but would consider a 2.5 or 2.1 with avecs if i did it again

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