john banks

1. With the difference in lift between the STi and UK, do the STi pistons have deeper recesses for the valves?
2. Are there practical differences in building an engine for STi and UK heads/cams?
3. Is there likely to be any problem in aligning all the cam timing with the STi cams in UK heads? (AFAIK the offset between the models is on the crank position sensor).
4. How can you fit aftermarket cams with extra lift and duration if the standard setup almost leads to collision with the piston crowns?
5. What sort of clearance is normally run and if small how do you raise CR by reduction in gasket thickness or raised piston crown height above the deck?
6. Anyone any experience of Kent or Piper cams on a Subaru engine?

john banks

Thanks MTR.

Silly question alert - how do you reprofile a cam for more lift and duration? Surely you can't add material to the cam lobe?

Pavlo

You reduce the base circle, as lift is the difference between the base and the lobe. The amount ground off the base is compensated for with added shim thickness.

Phase 2 cams can take high lift reprofiling as they have a larger base than the phase 1 cams.

Paul

john banks

Aha. Paul YHM.

Leslie

John

I have no experience as yet with rebuilding a Scooby engine but I used to build my own BDA race engines some time ago. As far as clearance between the pistons and valves is concerned, it used to be quite tight since I ran high compression on a normally aspirated engine. One warning is that at TDC on the exhaust to induction stroke, when all the valves are close to the pistons, as the engine either approaches TDC or leaves it, the valves move more quickly than the distance travelled by the piston. You therefore need to measure the valve/piston clearance a few degrees either side of TDC. I cant remember the number of degrees now but it depends on the stroke of course, you would have to try that for yourself. I used a clock gauge resting on the valve in question and pushed the valve open until it touched the piston. I think for the BDA which was a 1300 cc engine with a 63 mm stroke I used about 8 degrees either side of TDC to measure the clearance. My engines used to rev at 11,500 RPM as long as the conrod bolts would stand it! and I used a minimum valve/piston clearance of 30-40 thou. When the head came off you could see that the valves were running close to the pistons but not touching thank goodness. Hope this will be some kind of guide.

Les

MTR

John,
Note 4.
No cam will be at full lift at TDC.

Full lift is specified at degrees BTDC and ATDC, for exhaust and inlet valves.
The amount of lift is less important as to the valve timing and overlap between the inlet and exhaust cams, for power delivery.
ie more power can be gained from different valve timing than just increasing lift, albeit at different rev ranges and degrees of engine flexibility. Usually.

This is a genereic comment based on a typical 4 stroke engine, not based specifically on the Turbo Impreza engine.

On crossflow Fords I wouldn't drop below 0.050" clerance from piston to valve at closest position.
Check with an old headgasket and plasticine.
They may run Impreza engines closer than this, but I wouldn't have thought so.

Cannot comment on your other points as I have no experience of the subject matter.

Cheers MTR

[Edited by MTR - 3/10/2003 11:54:45 PM]