JDWade

I am running a normally aspirated 2l Blacktop (Focus) which had the head ported by CNC, fitted with their cams etc but currently standard CR (10:1). They recommend 11:1 and this will require 1mm skimmed off the head.

Can anyone advise what is the maximum allowable to skim off the Zetec head? ie is 1mm OK

Thanks

John

crazycage

imo you could take 1mm off a blacktop head with no prob's

tabetha

1mm will be fine, but won't increase from 10:1 to 11:1.
tabetha

JDWade

OK, that's great. The sums I did indicated a 1mm skim would change a 10:1 to ~11:1

Here are the figures for my head/engine (measured with oil):
combustion chamber 45ccs
head gasket 0.63mm thick, volume 3.6ccs
piston crown volume 9ccs

Hence the total chamber volume at TDC 57.6ccs
From bore and stroke (not checked) the swept volume is 497.1ccs
Hence total volume at BDC 554.7ccs
CR=554.7:57.6 or 9.63:1 so doesn't quite make the factory 10:1

Remove 1mm is 5.1ccs ( due to the small 'crescents' on the head you get ~90% of the full bore area)

New CR 549.6:52.5 or 10.5:1

If the engine started at 10:1 the total chamber volume at TDC would be 55.2ccs (55ccs is what seems to be quoted for the Zetec head)

A 1mm skim would then give 547.2:50.1 or 10.9:1

I was a bit concerned about going for more than 1mm in one go and with stock pistons I think 11:1 must be absolute maximum, probably 10.5:1 is a 'safe' level.

Anyone got views on knocking, is it an issue at ~11:1?

tabetha

I don't think you will have a knocking problem, if using a decent fuel like V power, higher cr ones on track without knock, how your ecu would handle it is another matter.
tabetha

JDWade

If my sums are correct I will only be getting about 10.5:1 so it will still be pretty modest.

I'll use V power or similar. I don't have a knock sensor or ECU control of knock so would need to adjust the map if I do get a problem, that's easily done (MBE ECU).

I've just checked valve clearance and that should still be just over 2mm at TDC.

pdfbt40

Pity, if you were planning a CR increase (without changing pistons) you could have started with a 1.8 head. That will give a 11.0:1 on a 2.0 without any other machining.

If you've found 55cc quoted as a head volume, its most likely an American source as that's the combustion chamber volume on their (lower compression) engines IIRC.

Martin

crazycage

1.8 and 2.0 blacktop heads are the same mate and they are stamped 1.8/2.0 in the casting ,the silvertop 1.8 and 2.0 are different from what i have been told .

pdfbt40

I'm sorry, but for a Blacktop, from both personal experience, and that of others, you are incorrect.

The blacktop heads have a part number cast on them,which can be used to identify the different year types and engine capacity/combustion chamber volume. There is no stamped mark except on FRS and ST170 (obviously different). Only the blocks have the capacity 1.8/2.0 clearly cast on the outside.

The combustion chamber of a 1.8 head is 4cc smaller than a 2.0.

A friend of mine sourced a head and had it ported for a supercharged engine. It was only after it burnt a piston that he checked the CR and discovered it was a 1.8 head.

Concerning US heads, again anyone who has buit an engine using American rods and pistons has found that the CR is not to the quoted spec in a European engine. For example a 8.0:1 piston will give 8.5:1 with European heads.

Martin

crazycage

what ever . its is cast on on like i said i will post a pic

pdfbt40

As you say, what ever. Surely what's important is not the marking, though if it did have 1.8 or 2.0 it would be useful, but that they're not the same combustion chamber volume.

Martin

hineho87

did you end up getting your head skimmed? If so did u notice much difference performance wise?Matt