Can someone explain a bit to me about heads and head tuning? Basically trying to find out which is the most significant factor on them and what needs changing to get the best from them as i read a lot on here about big power cars having loads of head work for big power levels

Currently have a uk v5, i understand the v5/6 STI run more aggressive cams. Assuming my heads are the same design could i tune them in the same way you would tune STI heads. for example replace the cams and add adjustable pullies.

Is Porting worth the gains?
Whats the deal with changing valve springs for double ones, bronze guides etc, what does this achieve?

cheers in advance.

 

Not only do the sti heads have different cams but also underbucket shims to enable them to work to the raised STI rev limit (8250 IIRC)

 

Talk to A.J. @ Engine Tuner, headwork is needed for (very) high power builds as it allows more air to flow ( > 500 bhp IMHO ! ).

Fancy guides, retainers, over-sized valves and double valve springs are just the "jewellery" nededed to support 8000 rpm antics.

For us normal folk a bigger turbo just crams more air in !

HTH

dunx

 

so basically the jewellery is kinda flash, but not neccessarily essential

 

i think you need your heads done doen on a standard head after about 380 -400bhp as but that just what i have heard to be honest not sure how true it is but sti one can flow a fair bit more

 

yeah later classic sti ones will support 500hp

 

ttt anymore info?

 

No the "jewellery" is essential to support 8000+ rpm and high boost levels, but way beyond 400 bhp IMHO.

A trackcar/TA monster needs all the help it can get, and porting a road car may aid performance, but at what cost ?

It very rapidly spirals into a very large bill, (cams, pullies, valves, springs, guides, etc. )so are you really chasing 500/600 bhp at all costs, or is the overall package adequate for the use you put it to ?

IMHO

dunx

P.S. For comparison my N/A Imp made 115 bhp/litre with all the above, an STI manages 130+ in standard form.

 

as stated above, valve float is your problem at high revs, double springs, retainers etc are required to control the valve, as for portng while you will see small gains it's not as important as on a N/A car, the turbo simply forces the air in regardless, so unless your after ultimate performance I'd not bother.
The best way to getting the engine and the power you what is to plan it very carefully, decide exactly what you want to use it for, any tuning is a compromise bigger turbo more lag, high lift cams move the power up the rev range and can lower torque, the basic rule in engine tuning is to get as much air through as quickly as possible, make the engine strong enough, provide the correct amount of fuel keep the temps in check and you should be OK.
However there are lots of ways to acheive big power gains and it largely depends on how much money you throw at it (how fast can you afford to go?), I'm pretty new to Imprezas so I'm not up to speed on all the various cam, turbo, boost, ECU combinations available, so please that the above with a pinch of salt, but IMHO the best way to realise your aims is in the planning, sit down with a trusted tuner talk through your options based on your goals and ultimately finances. Good luck.

 

im in the stage of planning now, just wanting some info on head differences, if i can tune my current heads il not get STI items. and should save me some money at least as i only want to do 'what i need to' to go as fast as i aim

 

Depends on what you want, for example if its a 2.5 bottom end you won't need the higher rpm limit (unless you're building a v.expensive short block), so then you could just stick some cams in your current heads, or TBH use them as is if aiming for 400ish

 

i want a good 450hp/450ft lb 2.1/2.2. nice and revvy but with headroom for more so capable of 500-550.

 

To answer your first question, it would be cheaper for you buy V5/6 STI heads out right and use them on your build, converting your existing casings will cost more. Version 5/6 STI heads will give you want you want in terms of figures.

 

so there is a physical difference in the non sti and sti 5/6 head castings?

 

Runing gear is different, casings are the same.

 

cheers for clearing that up

 

does anyone know the spec/lift etc of sti cams?

 

V5/6 heads are more often than not cracked too around the plug hole.

 

Inlet opens at 9 deg BTDC closing 53 deg ABDC
Exhaust opens at 58 deg BBDC closing 10 deg ATDC
Base circle 37.0mm
Inlet lift 8.25mm - 8.35mm
Exhaust lift 8.60mm - 8.70mm

 

These look good, but are not cheap:

CNCHEADS - Subaru Impreza

Anders