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C/R and Raising it
clinker42 - January 20th, 2011 at 04:05 PM

Hi Guys

I have two heads, one has bigger exhaust valves than the other and one has a lip where the barrells fit against the head. The one without the lip, is it standard like that or has someone fly cut it at some stage.

Now my other question, My cc equipment wont fit in the head, so short of making new stuff, how much do you normally take of the head to boost the C/R up a bit. Do you just remove the lip or do you take some of the head face as well.

Thanks Trev

mnsKmobi - January 21st, 2011 at 11:53 AM

What engine are we talking about here? :spin:

clinker42 - January 21st, 2011 at 01:59 PM

1600 beetle twin port engine

dangerous - January 21st, 2011 at 03:20 PM

Those sound like two totally different part number heads.

You will have to CC them to work out what you need to do,
but likely that you need to find at least one head the same as the other.

In a perfect world you need to aim for as close a piston to head clearance that your application will allow,
and then adjust the chamber volume to attain the ratio needed.

It sounds like a can of worms, but "what compression ratio you need", will be dependant on application.

If you simply remove the step, you may end up with something that is way off right.

Measure and calculate.

Craig Torrens - January 21st, 2011 at 07:44 PM

The one with the lip is probably a 043 part number head, and that is how they are standard (lower comp for emissions). They have smaller exhaust valves and the valve stem is larger, I think 9mm from memory.

clinker42 - January 25th, 2011 at 05:57 PM

Thanks Guys

I bit the bullet, I cc'd them, the one with the lip was 55 ccs and the no lip was 48 ccs. Correct about the valve sizes as well, I bought some bigger inlet and exhs and fitted them, fly cut the heads to the same depth and blended the bowls and cleaned up the ports, both heads now have 48 ccs.

I only guessed the piston deck clearance at 20 thou so if thats correct I will have 8.7:1 without considering overlap. So I should get away with standard unleaded and a mild tune.

Also, geez the short turn raduis on the exhaust ports are a prick to fix.

Has anyone made a high port version of these heads to fix the short turn?

matberry - January 25th, 2011 at 11:03 PM

Well done for the mods mate, measuring is definately the way to go. You want to watch that .020" deck, recommended minimum is .040", and I have seen .020" leave witness marks where the piston kisses the head on an engine that wouldn't have seen over 6000 rpm.