mill porsche 911T heads

slakjaw · 03-02-2002, 05:41 PM

has anyone here ever had the heads milled on there 911
i have a 1971 targa and i would like to raise the compression
so i was thinking about pulling the heads off to port and polish them and also have them shaved some. but i am not shure how much i could safely take off has anyone here ever done this
thanks for any help

Kyle
71 911 T
83 yoda land crusier

late

911tmaster · 03-02-2002, 06:19 PM

if you want to keep it running on pump gas then you should not do it. if your building a race motor milling it good but i assume its a street motor so i wouldn't mess with the heads. you'll need probably over 100 octane gas and that gets pricey. you can get horsepower cheaper and easier by doing exhuast stuff.

slakjaw · 03-02-2002, 06:39 PM

it seems to me that i should be able to get the thing above 8.6:1
thats where its at now

john walker's workshop · 03-02-2002, 06:41 PM

when you move the heads/cam towers inwards, the cam thrust plate won't go in it's bore in the chain housing. the housings can be milled an equal amount to fix that, but then what about the sloppy chains now? can-o-worms.

Clark Griswald · Clark Griswald's Garage

John,

Wouldn't the tensioners take up this slack? What are we talking 1-2 mm or so?

J P Stein · 09:21 PM

To a point, yes, the tensioner will take up the slack.
The problem here is :
As the heads/cams move closer to the crank, (say 1 mm or .04 thou) the chain get twice as short (.08).....

That .08 has to go somewhere....the tens. push it to the side.
The hypotenuse of the triangle remains the same length. the long leg is shorter, the short leg must lengthen geometrically.

Using some rough numbers, I figure it at about .250 more offset required on the short leg. That's about 1/3rd of the travel of a tensioner.

Prolly why Anderson reluctantly advises 1 mm max and new chains.

Just one nonexpert WAG.

JPIII

911pcars · 03-02-2002, 09:52 PM

Tolerances can stack up a lot during after machine work; the cylinder head and cylinder mating surfaces as well as the cylinder spigots in the case.

The results in a reduction in distance between the case and the cam housing. To maintain the proper relationship between the tensioner plunger and the idler arm you can replace the timing chain idler gear with a slightly larger one. This takes up the extra length of timing chain caused by the above factors.

A knowledgeable machine shop can supply you a gear if needed . Ted Robinson from German Precision does this. He can make smaller ones too.

Sherwood Lee
http://members.rennlist.org/911pcars
www.seinesystems.com

911pcars · 03-02-2002, 10:03 PM

I forgot to mention. Although the tensioner may take up any increased timing chain slack, this will change the geometry of idler arm assy and place it very close to the edge of the chain box. With tolerances stacked to one end, the idler arm could start banging against the chain box. This is more pronounced on the driver's side due to the location of the idler arm assy.

Sherwood Lee

Miniac · 03-03-2002, 07:58 PM

Don't mean to side track, (so maybe a new thread should be started), but I bet I'm not the only one who'd like to get a "bang for the buck" HP increase list on a 72 911T in a ranked order from best to least if anyone has any ideas.
Thanks.