43 replies to this topic

[siztenboots]

won't this bugger up the chain too? ie. 4mm in total

[speedster]

won't this bugger up the chain too? ie. 4mm in total

Hmmm interesting.... Shouldn't the tensioner be able to handle the slack?

[Nev]

2mm is a hell of a lot to skim and will massively increase your CR.

 

From memory, the way to calculate it is to:

 

1. Measure the volume remaining in the cylinder.

2. Measure the volume in the head. Then subract 2mm * x-sectional area.

3. X = sum of 2 results above 

4. Y = sum of 2 results above + the swept volume.

5. CR = X/Y as ratio/faction. 

 

If you look on my website (link here) there is a section describing how I did this for my own head CR calcs with some pics of how to lube the pistons and use the perspex + syringe to get accurate measurements. The method is surprisingly accurate.

 

Remember to try and consider the difference between SCR and DCR, mainly affected by your cam profile. On my engine I get a fair bit of reversion in the cylinders until around 2000 RPM which means my idle DCR is really low which causes quite a bad and spluttery idle.

 

HTH.

Edited by Nev, 19 September 2013 - 10:07 AM.

[speedster]

2mm is a hell of a lot to skim and will massively increase your CR.

 

From memory, the way to calculate it is to:

 

1. Measure the volume remaining in the cylinder.

2. Measure the volume in the head. Then subract 2mm * x-sectional area.

3. X = sum of 2 results above 

4. Y = sum of 2 results above + the swept volume.

5. CR = X/Y as ratio/faction. 

 

If you look on my website (link here) there is a section describing how I did this for my own head CR calcs with some pics of how to lube the pistons and use the perspex + syringe to get accurate measurements. The method is surprisingly accurate.

 

Remember to try and consider the difference between SCR and DCR, mainly affected by your cam profile. On my engine I get a fair bit of reversion in the cylinders until around 2000 RPM which means my idle DCR is really low which causes quite a bad and spluttery idle.

 

HTH.

 

Thanks Nev.

 

I'm not saying I gonna take 2mm off, just trying to determine what (parameters) I can use to get to the highest usable CR with standard CR (forged) pistons (coz getting custom high CR pistons manufactured is gonna be expensive).  

[oakmere]

I skimed .75mm of my head but I do get KR so have had to reduce ignition advance. Though I think this my also be cam related and if I had adjustable verniers it could be stopped.

[speedster]

I skimed .75mm of my head but I do get KR so have had to reduce ignition advance. Though I think this my also be cam related and if I had adjustable verniers it could be stopped.

what cams are you using? I'm planning to used some wild race cams, (I need to talk to webcon about this, unless the new pro5 is user programmable).

[Nev]

I have 276 duration on the inlet, and something like 18 degrees of overlap.

 

I also think verniers would help me set the car engine up more efficeiently, but can't be arsed really as I have an excess of power.

[techieboy]

Maybe you'd better get hold of those later direct injection pistons after all. 2mm skim + stupid cams is surely going to mean you need valve pockets in the piston crowns.

[oakmere]

I have Schrick cams. I have very good low rpm power but it flattens after 6500 rpm. This is partly due to the inlet restriction but I think also due to cam timing. It is just that adjustable cam gears are about $350.00. So probably not worth the expense.

[vocky]

A certain engine had a 2mm head skim and the resultant threads are on this site, last one said he was selling the car with no engine as he was fed up throwing money down the drain

 

To save you searching it appears the chain was too loose and thus it jumped a tooth killing a few engines along the way

[slindborg]

 

I skimed .75mm of my head but I do get KR so have had to reduce ignition advance. Though I think this my also be cam related and if I had adjustable verniers it could be stopped.

what cams are you using? I'm planning to used some wild race cams, (I need to talk to webcon about this, unless the new pro5 is user programmable).

 

 

 

If they make it user calibratable, I will be nice for a whole year!

[vocky]

I believe PRO5 is user calibratable

[speedster]

A certain engine had a 2mm head skim and the resultant threads are on this site, last one said he was selling the car with no engine as he was fed up throwing money down the drain

 

To save you searching it appears the chain was too loose and thus it jumped a tooth killing a few engines along the way

 

Any idea what the safe zone is vocky? 

[vocky]

1mm is tried and tested, there are several running well with that amount skimmed off 

[oakmere]

The oversized flat non dished valves also give a little extra static compression as the originals are heavily dished. 1.4mm has been done but I would stick at 1mm just in case another surface skim was required at a later date.

[badgerade]

I believe PRO5 is user calibratable

 

That was deffo the plan when I was talking to them about it a year or so ago.  I think they've accepted that they need to take this route now rather than locking the ECU down.

[oakmere]

Double post

Edited by oakmere, 19 September 2013 - 05:03 PM.

[slindborg]

I believe PRO5 is user calibratable

  That was deffo the plan when I was talking to them about it a year or so ago.  I think they've accepted that they need to take this route now rather than locking the ECU down.

Progress at last. I'm impressed they managed to stay in business so long with the locked approach

[siztenboots]

on a similar note, when CS put in a decomp plate for the early SC kit , presumably they would have had the issue with chain length?

[techieboy]

The decompression plate probably just relieved the sh!tty Z22SE tensioner of some of it's load and gave it a bit of relief.