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Fly cutting 964 heads vs compression ratio
Hi List,
I'm in the middle of two arguments. On the one hand I'm being advised that I should have my heads fly cut approx. 0.1mm. On the other I'm advised that this should never be done as it changes the compression ratio and the cylinder and heads should be lapped together. (whatever that entails). I've attached a photo of one of the heads. I'd be interested to have your input, please. TIA John PS. I've posted on Rennlist as well. 
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03-28-2018, 01:09 AM
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Join Date: Oct 2002
Location: Phoenix, AZ
Posts: 484
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FWIW, flycutting the head surface by .1mm will not significantly change the compression ratio. Anybody telling you differently has his mechanical knowledge from books with no practical experience. If anything you might find that your heads need to be cut a bit more to have the cutting tool touch every part of the surface due to slight warpage. Lapping after cutting the mating surface is not a bad idea and will not harm anything.
BTW, cutting the head surface by .1mm will increase your compression ratio about .05 points. |
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03-28-2018, 02:44 AM
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I should add that this is a 1992 gasketed engine
Thanks Ollies930. |
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03-28-2018, 03:01 AM
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Chain fence eating turbo
Join Date: Dec 2008
Location: Austin, TX
Posts: 9,125
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Looks like your heads when media blasted, the sealing surfaces where blasted too, leaving a rough finish.
It should be smooth in those areas. Not sure how you'd lap the sealing ring groove.
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Cory - turbo'd '87 C3.2 Guards/Blk, 3.4, 7.5:1 CR P & C's, 993SS cams, Borg-Warner S366 turbo @ 1.2-1.5 bar, depending on mood  , Treadstone full bay IC, 70mm TB, TiAL F46 WG, HKS 1 1/2" BOV, twin 044 pumps, MicroSquirt AMP'd w/GM smart coilpack, Bilstein coilovers, Tramont wheels (285's rr, 225's frt), Big Reds frt, 993 rr., tower brace, MOMO wheel
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03-28-2018, 04:38 AM
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Location: Phoenix, AZ
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Sorry, brainfart at 3:30am this morning. I was thinking turbo compression, not normally aspirated. Compression increase will be more like 0.1 point increase instead of 0.05(not that it really matters).
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03-28-2018, 09:12 AM
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Join Date: Aug 2004
Location: The Brink
Posts: 2,838
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I hope I am not hijacking - sorry if I am.
Ollies930, is that the Ollies that does the machine work for cylinder heads? Does the compression ratio change when the fly cut is done for the early 964 heads and the cylinders are machined for the ring? Thanks, I can start another thread, however it is a very similar question. |
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03-28-2018, 01:45 PM
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Join Date: Oct 2002
Location: Phoenix, AZ
Posts: 484
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Sorry, different Ollie. But to answer your question, anytime you machine the sealing surfaces, you will increase the compression ratio unless you put appropriately thicker base gaskets under the cylinders. However, the compression ratio change is minimal when surfacing heads. Not worth mentioning, unless you intentionally cut a substantial amount off.
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03-28-2018, 08:30 PM
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Join Date: Jul 2013
Location: Brisbane, Australia.
Posts: 2,604
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John, which shop is saying machine and which is saying lap? I went through a similar thing.
I'm in Aus too.. feel free to PM. Mike |
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03-29-2018, 12:45 AM
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Join Date: Oct 2003
Location: Central Fla
Posts: 1,864
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seal surface
Just a cleanup cut will take care of that surface, you will not lap that surface in the picture, with the heads apart and cleaned it around a 200.00 to 300.00 job and done
Mike Bruns
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The two most useless things to a driver are the braking distance behind you and nine-tenths of a second ago. |
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03-29-2018, 03:27 AM
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1990 Porsche 964 C2
