higher compression with milling?

[SHOtime2511]

im taking the heads off my 91 this weekend,due to failing
valve stem seals.and im going to have the heads sent out to
a shop to be hot tanked,checked for any other problems,and re-assembled
with new seals.the question i have is that id like to raise the compression a little bit via milling.ive done some searching and see that members are saying they dont recommend it,and im wondering why? i understand it could cause an issue with the timing belt,but im talking .020 nothing big.the tensioner can take up that with no problem.right?


also how would this affect plug selection,with N2O? 75shot

 

[SASHO91]

IIRC, the you cant go very far on the sho's heads... also, if gone too far it could cause problems in the valve-hitting-piston area.... IIRC....

 

[AREA 91]

I don't have my book in front of me, but I thought the max you could take of the heads was 3 thousandths. After that you have timing issues. Someone else may chime in.

 

[Off Road SHO]

As you take material off of a v type motor with overhead camshafts, the cam gears or sprockets get closer to the crank sprocket AND the other head. Just think about that for a second or two. It might help to visualize the problem if you exagerate what you are wanting to do. Shave enough of the material off to move the heads one half of a timing belt notch closer together. Now try and time the heads to the crank. Think about it.

Tom

 

[SHOtime2511]

As you take material off of a v type motor with overhead camshafts, the cam gears or sprockets get closer to the crank sprocket AND the other head. Just think about that for a second or two. It might help to visualize the problem if you exagerate what you are wanting to do. Shave enough of the material off to move the heads one half of a timing belt notch closer together. Now try and time the heads to the crank. Think about it.

Tom

i understand the problems that this could cause when over milled,however there must be some tollerance,what if someone over heated there car and blew a head gasket,there head cant be milled if it has warped some?

 

[Minnesho]

i understand the problems that this could cause when over milled,however there must be some tollerance,what if someone over heated there car and blew a head gasket,there head cant be milled if it has warped some?

.....

I don't have my book in front of me, but I thought the max you could take of the heads was 3 thousandths. After that you have timing issues.

 

[SASHO91]

the book says maximum allowed warpage is .008(.2mm)

 

[drivinhard]

.008 is the max warpage, but it's basically throw away as the factory manuals say the head is not to be milled.

if you do it, i wouldn't do it much at all

 

[Black '93]

My roommate is a machinist and race motor builder. I was talking to him about it like a couple moths ago.. He said there are several factors to deal with including timing, valves, slapping etc. Replacing the cam & timing belt sprockets with others of a different teeth ratio could be a possibility for timing, but a custom belt would be needed. I think it definiley is possible, but you need to know serious stuff to make the timing work properly. If you know what your doing or have help from a fabricator/machinist/performance expert - I see no reason it is not possible. The thing is, a new crank, rods valves might be nessacary to do it so it's really not worth it for an experiment in case it fails. I'm sure someone somewhere has gone pretty high on compression with an SHO motor.

 

[Off Road SHO]

I'm sure someone somewhere has gone pretty high on compression with an SHO motor.

Sure they have, but they do it the correct way with different pistons.

Tom

 

[Black '93]

Sure they have, but they do it the correct way with different pistons.

Tom

That's what I'm saying... It would have to be done right to see good results.
That's some major money to do that type of stuff. Some lightweight wisecos or special application pistons would definitley be important. I mean it's already 10:1 right? If I did it, I would do it right. On the other hand, I cant even afford a tranny rebuild let alone a $10k high-comp project build....lol

 

[1stSHO]

My god my old baby is going to be in the junk yard soon, i can see it already!!!!:cry:

 

[SHOtime2511]

My god my old baby is going to be in the junk yard soon, i can see it already!!!!:cry:

LOL!!!
if i dident fix the things that break she would already be there!
have no fear! next time you see it....i think you'll be suprised

 

[SHOtime2511]

.008 is the max warpage, but it's basically throw away as the factory manuals say the head is not to be milled.

if you do it, i wouldn't do it much at all

thank you,thats what i was looking for.
well....i think that ill pass on the milling,as the risks are
greater than the small possible gains,i think ill just have them
cleaned up real nice,and clean up some of the casting marks
as well as do a gasket match.and leave it at that!

 

[windsormw]

milling is a bad idea ...........

unless the dia. of the flycutter is larger than the width of the head ..........

most auto machine SHOps use a broach .....for what its worth , like mentioned above the gains would be min. and the problems abundent

 

[rangerj]

Neither the heads nor the block should be milled unless they are warped. 020 is NOT SMALL, it is HUGE. A warped head can usually be corrected with .003 to .005 milling, and then a thicker gasket is usually used to make up the difference. There is NOTHING to be gained by simply milling the heads, and there are many potential problems as pointed out above. These engines are not 1960s vintage chevy engines.

You can modify these engines but you must address EVERY engine component, and the machine work and custom parts would cost a small fortune.

 

[SHOtime2511]

Neither the heads nor the block should be milled unless they are warped. 020 is NOT SMALL, it is HUGE. A warped head can usually be corrected with .003 to .005 milling, and then a thicker gasket is usually used to make up the difference. There is NOTHING to be gained by simply milling the heads, and there are many potential problems as pointed out above. These engines are not 1960s vintage chevy engines.

You can modify these engines but you must address EVERY engine component, and the machine work and custom parts would cost a small fortune.

i know my original post i dident have enough 0's in front....my bad
however im sure just milling the head would provide some added compression
i dont care how "special" the SHO engine is,its still just an engine.

*anyone know how much the compression would rise with a .003-.005 milling?

 

[rangerj]

What is it you think a little more compression will do for you? What are you trying to accomplish?

 

[Rockledge]

As you take material off of a v type motor with overhead camshafts, the cam gears or sprockets get closer to the crank sprocket AND the other head. Just think about that for a second or two. It might help to visualize the problem if you exagerate what you are wanting to do. Shave enough of the material off to move the heads one half of a timing belt notch closer together. Now try and time the heads to the crank. Think about it.

Tom

The SHO being my first OHCammer, I had never considered that.

Good post Tom.

 

[SeanMc]

I believe they are 9.8:1. I'm running high compression at 10.5-6:1.