resurfacing heads bigtime for compression bump

6SENSE

Member
Ive got a set of 8445 heads that have been ported/polished w/ larger valves, etc, etc. Checked them and they are now at 42cc.
Forged pistons for my 4.1 that I grabbed off this board werent exactly what I expected and as a result after short block assembly/balancing, etc., compression isnt what I expected (7.8:1 w/ .039 head gasket).
Sooo, to get compression to 8.6:1, for instance (with a .039 gasket) I would need to get heads down to 30cc! Do any of you great buick engine gurus here know if this is possible? Or is that going too far (42cc to 30cc). Im guessing 80 to 100 thousandths off to do get there!?
Ive got to also do a little more research to see how thin of a head gasket I can find (hoping for a steel shim gasket .015 or so thick). Im figuring I will still need to take some off the heads.

I originally wanted 9:1, but at least 8.5:1 ?

Anyone?
 

86gn

Glenn is slow!!!!!!!!!!!!
Cometic will make a gasket(MLS) that thickness for ya.. I think that is to much to try and take off the heads.Have to watch out for piston to valve clearance
 

John Wilde

I'm getting too old for this $#!%
Staff member
IMO run the heads as is and run a little more boost.
Keep in mind you have have a little more ci so you will probably
have similar response to a stock compression 3.8.
I would run that set up no problem.
 

6SENSE

Member
Thanks for the replies.
I thought the thinnest the cometics came in were .026-.027? I will have to call them.
The piston and deck came out to 54cc by the way. I dont know if these pistons were meant to be used with custom length rods or what?
Thinking about maybe finding a common ground and shaving about .040 + find some thin gaskets?
 

Scott Atk

Senior Member
I wouldn't cut that much off the heads. If you do, you will have to cut the intake to match and the stock intake will get thin on the front corners. Was the block decked to zero or .05 in the hole?
Weren't the original 84-85 steel shims only .015" thick? That might be an option. I know Jason (RJC) makes them in two separate thicknesses.
 

VadersV6

Active Member
The old school trick is to angle mill the heads. Mount the heads in the surfacer with the deck on the exhaust side, .030" higher than the intake side and mill until the entire face cleans up. That way you're removing the most material from the side where combustion chamber volume is the greatest, and every amount you remove will have a much greater impact on the cc's than if it were flat. But you'll have to have the intake manifold's mating surfaces milled at the same angle so everything mates up. Another good thing about angle milling is that it raises the angle of the intake runners, which has a big effect on flow across the board. But at the same time it drops the angle of the exhaust, so you should raise the exhaust's up a bit .030" is just a number. Ive done .050" before. It just requires more work on the intake to make everything line up. But given the improvements in flow, its worth the extra work. And doing it this way will provide less valve to piston clearance headaches. Another good thing is that you wont weaken the deck as much as you would if you hacked off so much the normal way.
 

turbofabricator

Well-Known Member
Angle milling a Buick head isn't worth the hassle. The rocker geometry is messed up, manifold alignment WILL be an issue, and the added compression doesn't need to be there with a 4.1. Just turn the boost up. Low compression/high boost is better than high compression/low boost. Thermal dynamics are at stake here. You can cool the added air via a good intercooler, vs. no way to cool the increased temp with added compression. (This is talking the same cylinder pressure.) Run what you have. If you factor in quench to the equasion, angle milling may, in some cases, improve performance, BUT with a turbo pushing air into the chamber, the need for better quench is not as important. Our combustion chambers are not the best in the world, but they are FAR from the worst.
Some of the engines I have built in the past had 6:1 compression, but also ran 130 in./Hg (approx. 60 psi) boost. They made GREAT power. (over 5,000 hp.;) )
 

VadersV6

Active Member
Angle milling a Buick head isn't worth the hassle. The rocker geometry is messed up, manifold alignment WILL be an issue, and the added compression doesn't need to be there with a 4.1. Just turn the boost up. Low compression/high boost is better than high compression/low boost. Thermal dynamics are at stake here. You can cool the added air via a good intercooler, vs. no way to cool the increased temp with added compression. (This is talking the same cylinder pressure.) Run what you have. If you factor in quench to the equasion, angle milling may, in some cases, improve performance, BUT with a turbo pushing air into the chamber, the need for better quench is not as important. Our combustion chambers are not the best in the world, but they are FAR from the worst.
Some of the engines I have built in the past had 6:1 compression, but also ran 130 in./Hg (approx. 60 psi) boost. They made GREAT power. (over 5,000 hp.;) )

In no way would I mill my heads down to get my compression where I wanted it, but since he's more willing to hack it up then pull it apart and do it right, I gave him the better option...angle mill vs. a flat decking job.
6:1 is a pretty reasonable amount of compression....under 4 bars of boost jesus! 7.8 isnt too bad but its far from good because you're not planning on being under 60psi for an hour on the way to grandma's house. Thermal dynamics are at stake here, and low compression on a street motor isnt ideal. You're wasting so much energy its ridiculous.... under cruising/part throttle conditions. I would highly recommend that 6sense tear the motor down and either get the right pistons, or use those pistons, but tear the motor down anyway, zero and square the decks, run a thin head gasket and mill about .020 off the heads. This way the quench will allow you to run high compression and alot of street boost with drastically reduced chances of detonation. 8.5 to 9:1 is where I'd like my street motor to be, for quick spool, more torque and better mileage, but Id do it the right way, by decking the block, the heads, and running a thin gasket. Increasing compression while doing nothing to improve quench is not a good idea. 9:1 with good quench is like 7.5:1 with bad quench from a detonation standpoint.
 

6SENSE

Member
Thanks for all the replies! Im very pleased with the feedback.
I know there is definately a right way to do this, and I wish I did it that way before I had a bunch of $$ into it putting it together how it currently sits. I will get with my machinist and ponder all the ideas/info youve given.

Also, spoke with someone at Cometic today and the thinnest gasket is .027" for my app. I noticed that the gaskets they offered for the stage I & II V6 had the same bore as my 291 cast production block has wound up with (4.020"). Is it possible that these forged pistons were originally for some type of stage block? Like I mentioned I got a heck of a deal for them from someone off the board. I knew it was expensive to get a custom set built so thought I was making out like a bandit not having to worry about pistons. Now look what mess Im in.
 

skylarkstage1

New Member
Offset Bush Connecting Rods

Chuck,

Can you provide any additional information about the offset bush connecting rods process ? I have never heard of it before. How much offset can be obtained on a Buick turbo connecting rod using this offset process?
 

bison

Moderator
Staff member
Chuck,

Can you provide any additional information about the offset bush connecting rods process ? I have never heard of it before. How much offset can be obtained on a Buick turbo connecting rod using this offset process?

He means that you are going to lengthen the center to center distance on the rod so that the piston will be closer to, at, or above the deck surface. This is commonplace when building a high performance race engine. Most hardcore racers that have big $ engines run the pistons so close to the head you can see a mark on the head form light contact. This is very common in roundy round stuff. There is nearly perfect quench on these engines and there is a lot of trial and error to attain it. Once they get close they are looking at .0005-.001 inch location difference to get the best attainable quench. If they go to far they will hit the heads to hard and the engine will fail when revved high. You want to remove as little material possible on the stock blocks since they are too thin for the rigors we are going to put them through. Just enough so that they are flat. Fwiw on the last engine i built the pistons were out of the hole .005in. Compression ratio was over 9.0:1 on a 3.8 bore.
 

Chuck Leeper

Toxic old bastard
Staff member
Small end is bored oversize, and a bronze bushing is installed. The bushing ID is undersize, and is then bored off center. Like Bison said, it makes the C/L to C/L length longer.
As for a stock rod, I don't know how much it can go.
What's the piston to deck measure now?
 

Nick Micale

Tech Advisor
Staff member
Ive got a set of 8445 heads that have been ported/polished w/ larger valves, etc, etc. Checked them and they are now at 42cc.
Forged pistons for my 4.1 that I grabbed off this board werent exactly what I expected and as a result after short block assembly/balancing, etc., compression isnt what I expected (7.8:1 w/ .039 head gasket).
..................Anyone?

There never have been [as far as I know] an off-the-shelf piston for a turbo Buick 4.1?:confused:

The 4.1's we have done are all with pistons made to our specs with specific attention paid to the dish volume.

To use a very thin head gasket would probably give you grief with a 4.1, as the concentric cylinders would have a very narrow fire ring and be subject to blow-through at higher cyl pressures.

Your intake will have to be machined for a proper fit even with heads cut to 42 cc.

I do agree with your goal of 8.6 to 1, as this will give you lots of low-end torque and a kick-ass mid range.:)
 
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