4.0L HO Jeep Engine Build
Very good write up. Please (if you can) explain on how you figure out the compression ratio, how you knew you wanted the block shaved etc. I am in the need of doing my 4.0 and need to know all the math involved. thanks.
Thank you Moparfreak77,
THe easiest way to figure the compression ratio is to use the KB calculator here:
http://www.kb-silvolite.com/calc.php?action=comp You just enter the bore, stroke, cylinder head cc, head gasket thickness, head gasket diameter, piston to deck clearance and piston cc. Piston cc is entered as positive number for a dish and a negative number for a pop-up. I cc'd the head with a plexiglass plate, some mineral spirits colored with trans fluid and a 10cc (ml) oral syringe that I got from the local drug store for free for the asking. I also got a real world piston cc using the same method as I'm using dishes with quench pads.
Piston cc with domes can also be calculated by turning the crank until the dome just hits a straight edge over that cylinder, then measure how far the piston is down in the bore, calculate the volume of a cylinder, fill with fluid while measuring and subtract that amount of fluid from what a cylinder would be and presto, you have your piston dome cc. For me, it's easiest to calculate it metrically. So for example, say the piston is 10mm down in the bore and your bore is 92mm. First calculate total cylinder volume... diameter /2=r so r=46, radius squared is 2116. The formula goes like this Pir2h, (hard to show without math symbols) so 3.141x2116x10= 66463.56 Now we need to convert this cc's by dividing by 1000 so 66463.56=66.46456. We add fluid to the cylinder and find that we can get 54.5 cc's of fluid in. Now we subtract this from the cylinder volume and get 11.96 cc's of dome volume. :) Easy! I dropped the last digits as it's really not necessary to know your comp ratio to a hundred thousandth. :)
Now I could go through the raw math behind CR calculation but it's a ***** without a white board and having to do it on this format. The above was tough enough without mathmatical symbols. Here's link:
http://en.wikipedia.org/wiki/Compression_ratio
In regards to quench, you want it and I'll tell you why. When you have quench, whn the piston comes up it will squeeze the mixture out from between the quench areas on the piston and head at a very, very high rate of speed. This not only helps quell detonation but adds torque as well as it homoginizes the air/fuel mixture within the cylinder and there are no dead spots that will tend to detonate under compression as the AF is kept moving at a high rate of speed. In general, much higher compression ratios can be run with lower octane fuel and more power produced in similiar engines. :read2:
For quench to have any effect, there must be quench pads on both the piston and head. If you take a look at my previous pictures of my pistons and head, you'll see the pads on both. The piston to head clearance should also be less than .060" and smaller is better although a builder has to allow for piston expansion and rod stretch on any given build. I like to shoot for .040" but I know many people who get away with .025" but that is way pushing it.
After first having my head and block decked for straightness, I mocked everything up and found that the piston was still .033" in the bore. So here we have this lovely compact head chamber with a generous quench area and pistons with quench pads but it's not being utilized by the factory. This is mostly due to manufacturing time limitations and generous casting tolerances.
For me to achieve quench on this build with a .040" gasket I had to have the block milled an additional .030". Why not .033"? Because the rod length tolerance is .002" and that must be brought into the equation.
I hope this answered your questions. :read2:
Before I found that site I thought the variety of aftermarket 4.0 cams was really small but apparently Comp, Lunati, Crane, and Crower as well as the more specialty off-road companies like 505 and Hesco. The Lunati Voodoo line really blew me away though (again), their smallest cam has .484" lift!! The 256/262 in my Duster really rips and to have a cam like that in my Jeep would be bad-@$$.
Stock and aftermarket cam specs:
http://www.angelfire.com/my/fan/Jeep4.0Camshafts.htm
Great info Mopekid! I wanted to use the stock rockers as adjustable roller rockers from MP are almost $900 a set so that dictated cam choices and I'll still have to shim the rockers with the milling that was done. That first Lunati cam is very, very tempting though but I still have to worry about emissions. If I had more money to burn and emissions were not an issue, I'd go with that Lunati and Yella Terra adjustable rockers (about 600 a set). An adustable MAP sensor or possibly a Megasquirt system might be neccesary to take full advantage of that cam though and I haven't researched how that would affect the rest of the computer controlled items like speed sensors, tach, ABS, Speedometer, engine warning lights, etc....
