just a theoretical question. but why don't i ever hear about people milling the heads on these 4.0's to bump up the comp a bit?

i have a gasp, 94 civic with the d16z6 motor in it, and when i redid the head gasket. i had the head milled down and bumped the comp to about 10.5:1, yes i have to run mid grade gas, but i felt a noticeable bit of power increase.

i was thinking of doing the same to my xj if i ever have to pull the head off.

any reason why not to?

 

We did this on our race jeep.milled the head and bumped the compression and had the motor set up to run on half race gas and half premium.but on a street application im not sure the how realiable this would be.

 

Well I plan on decking my block .010 but that's cuz I'm doing a stroker, that's the only real reason id see it being worth the trouble IMO

 

Cause most people just buy kits or are content with its power.

"The Crane camshaft produces a 12hp/15lbft gain over the stock Jeep camshaft. The CompCams #68-231-4 camshaft produces a 13hp/14lbft gain over stock. Both are midrange camshafts that work optimally with a 9.5:1 compression ratio (CR). This is achieved by milling 0.020" from the block deck, 0.010" from the head, and installing the 0.045" thick Victor Reinz (Dana) head gasket to produce a quench height of 0.047".

You can reuse the old head bolts but it's better to get new ones. They're cheap anyway.

 

Sweet. Im gonna have the machine shop mill 10 to 20 thousandths off my 99s head once Im done porting and polishin her.

 

I would assume doing a piston swap would be much easier than milling/decking to raise the compression. With the milling/decking, I'd imagine that getting the cam timing correct would be a major PITA.

Milling the head will change the valve to piston clearance, as does changing the cam timing, so it's important to check the valve to piston clearance.

 

Go to jeepstroker.com use the compression caclulator you can play around with the numbers see what get it also uses cam specs to give the dynamic compression which is what the engine really sees

 

Hello,
i'm Alex from Italy,
i was also thinking about give more compression ratio to my 4.0 because i'm running LPG not Gasoline, and it have 105-110 octan so i can run safe for example 11:1 compression ratio.

I need to know from someone expert wich is the easyest way to give my engine this compression ratio: change pistons, change headgasket with one more thin, drop cylinder head by working on it?

If i drop cylinder head i have also to do something for the cam timing?

Regards

 

Sound the least complicated, and has the least variables to mess with.

 

So the easiest way it's change pistons with other more compressed?

Why drop the cylinder head it's the least complicated?

 

shaving a lot off a cyl head also will create valve train geometry change probs taller pistions will have no effect ,but a lot of research must be done with this also so not to stick a vale thru the pistion, 100 shot of laugh gas will give you what you want when you want itand run on low octaine gas

 

I don't think that the valve train will be affect to a big geometry change...but i'm here to ask, to someone that already did this operation, for example to run racing gas...

 

Probably if your trying to achieve that much compression you better do pistons and machine work. Im no expert but I bet if you mill the head way down and create a lot more compression you probably will have issues with your rings, especially if they are older with high mileage. Its much safer to do a little machine work with a high comp. piston than trying to get it all from millwork or all from pistons. I think you would achieve a strong engine that is more reliable by doing both. I once simply changed a head gasket on an old international engine, about a week later I noticed I was burning oil from the rings that were worn and not ready for the sudden restored compression. I would love to hear a 11.1 4.0 motor run. I bet it would sound killer. Let us know what kind of power it makes when your done.

 

Maybe the easiest way it's get on a good supercharger, with my LPG system i've only to adapt the supercharger and i'll put more lpg into engine with a simply screw regulation, so it should be really easy...

And maybe cheaper

 

this is probobly off topic, but in changing out the cam, does anyone know how you adjust your rockers in accordance with a performance cam? i cant find a chart for it anywhere.