silent running

As the title says, what's actually wrong with building up a tuned engine using hydraulic valvetrain heads? I'm getting to the stage where I'm trying to choose what will fit my needs and budget best (CDB + phase 1.5 inlet) and found that the V4 heads, whether WRX or STi, dumped hydraulic tappets in favour of shimmed ones, whereas previous to this all WRX seemed to have hydraulic and some STi (whether pre-v4 STi heads had shimmed tappets in particular appears completely unclear).

But in terms of performance, why not use hydraulic heads? Is the valvetrain actually weaker? Or is just that it can't rev as high? And what's with all this overbucket, underbucket, hole in the bucket, top hat etc. etc. Why couldn't Subaru just stick to a design that worked and leave it at that?

And one other question - is it my imagination or are the only differences between STI and WRX/Turbo heads in the actual valvetrain, and not the ports/casting/head cc? My thinking is that ported/polished WRX heads might well be better for a road car than STi heads if you want to make good power but don't need or want to rev it to 8 grand.

dunx

iTrader: (3)

IMHO
All cars are made down to a price, once a cheaper alternative is proven to be reliable, it's used. For service costs a hydraulic follower causes lower bills as there should be no need to re-shim at say 100,000 miles.

I could do an Imp in under an hour, but an Impreza is at least twice as complicated and hard to get access to...

As for castings I dunno !

DunxC

silent running

That's what makes me wonder why they went back to shimmed for WRX heads later on, if hydraulic were adequate. And is it a problem with the amount of power they can make or the revs they can run to?

Welloilbeefhooked

iTrader: (3)

Funny thing is that when I build my next engine I was thinking about using hydraulic lifters due to running LPG and having a slight worry over valve seat recession. The self adjusting type would actually help me out.

I would imagine that the valves on hydraulic lifters would not open and close as precisely as the shimmed type? Also, what happens at high revs with the oil pressure? Do they increase the clearances and not open the valve as far?

Wayne.

dunx

iTrader: (3)

In the old days (cough !) hydraulic followers couldn't live with high revs....

But the Japs will just throw better production methods at it to solve the problem.

IMHO

DunxC

JohnD

iTrader: (5)

Higher reciprocating weight and a tendancy to "pump-up" at high revs - and thus reduce valve clearance - were limiting factors of hydraulic lifters. Modern materials, lubricants and production techniques have made solid, shimmed lifters viable again in terms of production cost and minimal service requirements. Lots of manufacturers have gone away from the hydraulic alternative, although the EVO unit still uses them! (not sure about the X though?) Likewise, many companies are reverting to chain driven cams as opposed to belts.
JohnD

silent running

Interesting. Yes in the 'old days' solid lifters were noisy and needed regular re-shimming. But basically are we saying that going with hydraulic only limits revs, not outright power.