With increasing the compression and lowering the amount of space in each cylinder you might want to check out boring the cylinders out a little to keep it closer to 3.8Ls and this would give you more effective performance for the higher compression im just not sure what this would cost and if it is in the budget

Boring the cylinders isn't in the plan right now. We would like to do this on a budget, and be able to present the highest NA horsepower for the dollar possible. If this build was strictly oriented toward performance we would definitely consider additional block prepping.

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I figured that if he wanted to increase the compression from 9.4:1 to 9.99:1 the total hight of the cylinder would be decreased .03" changing the total volume from 231.4ci to 229.3ci thats a 2.1ci decrease in volume

Now an over bore of .02" would yeild a gain of 2.4ci so with both the decking of -.03" and the over bore of +.02" the volume would now be come 231.7ci making the total volume .3" larger than stock

This would be more like just raising the compression to ~10:1 and not affecting any thing elce.

The block can be over bored .04" from what I have seen form zzp.

If I have any thing wrong other than spelling please correct me.

I think what Doug was trying to point out is that total displacement is bore x stroke. The stroke of each piston is not affected by milling the heads. Displacement is NOT the bore of each cylinder multiplied by the total height of each cylinder. That'* why you can change displacement by installing a stroker crankshaft. When each piston moves farther up and down the displacement is larger, regardless of any head work that affects the combustion chamber above each piston.

In a nutshell, higher compression does not change displacement. Thus, overboring isn't necessary to restore anything to original specs.

Basically what Ben said in the post following yours. An example. Ford produced the 4.6L v8(281 CI) from 91 till current. Between 91 to 98, the head design was just about identical(minor changes in port sizes and valve guide placement). The one constant was the combustion chamber in those heads(except the 96-98 heads had a little more swirl fin). Referred to by us Ford guys as NPI Heads. 96-98 heads(F5 castings) were superior to the 91 heads(F1 castings), and 92-95 heads(F2 and F4 castings). All of those year heads had a 53CC combustion chamber(I think, I can't remember specifically). All of these had a 9.0:1 CR from the factory


The 99+ heads(PI heads) were different. Bigger valves, ports, cams and combustion chamber. Faster burning, much like the 3800(heart shaped). Those heads had a 43CC combustion chamber. The only difference throughout the entire shortblock(besides plant manufacturing), was the pistons going from 10.5CC(91 to 9 to 16.5cc (99 to current). The 99+ 4.6L had a factory CR of 9.4:1. The engine still measured out to 281 CI or 4.6L


This only accounts for the SOHC 16 valve 4.6L'*, not the SOHC 24 valve and DOHC 32 valve engines.

Again, simply put. The displacement of the engine is measured by the bore and stroke of the short block and does not change unless the stroke/bore is altered. Decking the block down is another idea to raise CR, but not needed as the heads can be milled as far as anyone would ever need them. Even decking the block doesn't change the displacement until you get the top of the piston coming past the deck.

Now if i could only do a .040" overbore on MY engine... I'm limited to .020" max because the headgasket fire ring is so small:(

Well the cam was reground and on it'* way back. The card coming with it will determine the milling on the heads. I spoke with Scott at Delta Camshaft myself (because SOMEONE was holding up the game plan), and we went over the specs via the phone. The lift @ valve with 1.6 rockers should be something like .492" intake and exhaust. Centerline was 114* vs 117* stock. Duration and LSA are two numbers I do not recall, but since the cam is coming to me first, I will be the one to post all those specs.... probably. I believe Scott said that the duration @ .050" stock was somewhere aroun 179* both sides?

I look forward to a certain someone getting his sorry butt home so we can do this swap because I'm getitng excited!(If I have to fund the damn thing I will!)

Looks like a nice mild lift. Very curious as to what duration you ended up with.

Well I did learn that taking 1* off of the lobe center is equal to .020" of base circle(.010" difference for pushrods). I'm wondering if my numbers were a bit skewed on the stock because I loked up the stock specs to double check, 180* intake and 191* exhaust is what was listed at rock auto with a center of 115*. We'll see what happens.

Alright folks, we've got an update.

The cam has been reground. We still don't know the specs, but we've verified with Scott at Delta Cams that we need to remove .040" of material from the heads. That should put compression around 9.8:1, which isn't as much as I had hoped for, but definitely better than stock without being extreme.

The bad news: I think we're going to put the rush on this project because I may have a head gasket acting up on me. I've been losing a few ounces of coolant over the last few weeks, accompanied recently by misfiring on cold start that goes away after a few minutes. I've already verified that my upper intake still appears to be good, and the misfires lead me to believe that coolant is getting into one of the cylinders and then burning off after a little bit of driving. I haven't had the opportunity to check the plugs for coolant fouling, but I will look at them the next time it starts rough.

This all means that there may not be a chance for a preliminary dyno pull. We wanted to get a good baseline run, but I don't want to run high RPMs if it risks making a bad situation worse. We'll see what happens!