compression ratio and cams

1. webby Established Member Joined: Jul 2007 manchester Posts: 2456

   #1 16th December 2009, 09:51

   right oh lads i over heard a conversation the other day which basically went like this "you cant advance ph3's fully to get the best from them due to piston/valve clearences becoming to risky."
   
   1st question: is that true or have does it have any truth to it?
   
   2nd question (head hurter): if i were to get my standard pistons machined on the valve cut-outs say by 2mm it will lower the C/R not by much but it will then to compensate i would have to get my head skimmed.
   how much would i need to get my head skimmed by to get to the standard C/R back. or would the difference not make any differences.
2. Toad Saxperience Post Whore Joined: Mar 2007 Reading Posts: 8980 French tin can

   #2 16th December 2009, 15:51

   I don't believe the 1st statement is true. The cams are designed to run on stock valve / piston clearances too.
   
   You can only skim the head so much, as you'll literally have no combusiton chamber left.
   
   Better off getting larger hi-comp pistons and a re-bore, then fitting some proper cams.
3. blackie_2k5 Saxperience Hardcore! Joined: Oct 2008 Birtley Posts: 22253 VTR Turbo, 106 xsi track slag, Transit recovery, B

   #3 16th December 2009, 20:24

   ph4 has issues i belive but ph3 iirc is fine? and with that small a machining it wouldnt make much diff tbh, but could compensate by getting a slighlty larger h/g, machining the head right down will be basically defeating the purpose of pocketed pistons.....
4. webby Established Member Joined: Jul 2007 manchester Posts: 2456

   #4 16th December 2009, 21:12

   what thickness is a standard head gasket?
5. user_10176 Saxperience Post Whore Joined: Jun 2006 Ramsey st marys, peterborough Posts: 8288 Saxo's, Corsa (Daily)

   #5 17th December 2009, 00:16

   the standard is 1.5, but can get headgasket smaller or bigger no problem,
   
   andy
6. AlexBp West Midlands Joined: Dec 2007 West mids Posts: 23149 Supra,civic GL

   #6 17th December 2009, 09:29

   iirc its in the jp4 if you run j4 ph3s (i heard it for 708s) at high revs it can be close like
7. user_10176 Saxperience Post Whore Joined: Jun 2006 Ramsey st marys, peterborough Posts: 8288 Saxo's, Corsa (Daily)

   #7 17th December 2009, 09:53

   yes higher the rev to closer it will be, but if everything still standard i.e. the head hasnt be skimmed and the headgasket is the same size you shouldnt have a problem, but the clearance get closer if you skim the head,
   
   but if should be ok.
   
   andy