Jeep Strokers

If you have a choice between increasing bore or stroke, bore is a better choice. If you look at the math, an increase in bore will give you more HP than an increase in stroke. That is why a 4.0L puts out way more power than a 4.2

Plechtan wrote:If you have a choice between increasing bore or stroke, bore is a better choice. If you look at the math, an increase in bore will give you more HP than an increase in stroke. That is why a 4.0L puts out way more power than a 4.2

I think it is more related to the superior (to the 258) flowing head and MPFI.

That is true. Increasing stroke generally produces more torque where increasing the bore attributes to HP, to a point.

I think YuppieXJ is correct. There are a number of improvements in the cylinder head, electronic controls, and fuel injection that give the 4.0 such a large advantage over the 4.2.

Caution! do not read this after consuming alcohol

OK, otto cycle engine 101

a long time ago, some smart people mad mathematical models for a internal combustion engine. First the basic formula for Horsepower HP=(torque X RPM)/5252. Now Torque =PLA

In the formula above
P= Brake Mean effective pressure, or the pressure pushing the piston down
L= Length of stroke
A=size of bore

(LA) = D=Displacement

D=4.72 x Stroke x (Bore x Bore) = displacement of a 6 cylinder engine.

So to back up Torque = P x D
Then horsepower is just torque over time.

but if you look at the formula for Displacement, the stroke number is used as is, but the the bore number is squared. (bore X Bore), so a .125" increase in bore should increase hp about 10% a .125 increase in stroke will only yield about half that.

That equation may be effective when adjusting bore and stroke ONLY. But the difference between the 4.2 and the 4.0 is far beyond that of just the bore and stroke....

What you stated might be a good comparison equation for somebody taking a 4.0 6cyl and overboring it, or using custom pistons, 4.0 rods, and the 258 crank to achieve exactly the same deck height.

But i get your point that the squared bore is greater then the single stroke.

For my project (4.3L) i could have used either block, everything else would be the same, Head, ignition, EFI Etc. but the 4.0 block was my choice because of the bigger bore.

Excellent choice I'd have done the same

I have been kicking around some stuff on the Cranks, The 4.0 crank has a 3.44" stroke, the 258 has a 3.9" stroke. I need a 3.6" stroke. The 232 AMC 6 has a 3.5" stroke. So the 232 is the closest. The 232 also has 12 counter weights and weighs 78 LBS. My guess is that it has a long snout, and maybe the wrong size pilot diameter for the trans. Don't know if anybody has experience with this.

The the 232 crank will probably be better with a tall cam, the counterweights will smooth things out a little. The heavy weight of the crank and flywheel will help store energy when you shift. If you has a light crank and aluminum flywheel the engine would bog down when shifted. This is different from drag racing where everyone want to lower their rotating mass.

Plechtan wrote: The heavy weight of the crank and flywheel will help store energy when you shift. If you has a light crank and aluminum flywheel the engine would bog down when shifted. This is different from drag racing where everyone want to lower their rotating mass.

The heavey cranks probly do keep RPMs steadier while shifting, but with a light rotating mass its easier to spin the whole rotating assembly to a certain speed. Mine definitely doesnt bog down with the 4wt and an ax15. It would be interesting to see two indentical engines, one with a 12wt and one with a 4wt, and see how long it takes each to reach redline or a specific RPM, and how it takes each to fall back down to idle. In my head it would be easier to make power faster with the 4wt, because it wouldnt take as long to get to the powerband.

I will be using a Fidanza part# 256401 flywheel, it is SFI approved, but weighs 39 lbs, so i will have about 115 pound of rotating mass just with the flywheel and the crank. You could put one of these flywheels on your truck, but they cost $450.00 plus shipping.

Plechtan wrote:So depending on what block you have the deck height may vary .020 That would make a big difference in the compression ratio. Running the numbers above i end up with the piston .003 out of the block using a 1999 and up block. ( i think). I could probably just shorten the stroke a little to get the CR and Quench height correct.

By the way Peter, what's wrong with running pistons proud of the block??????? Did you ever watch The worlds fastest Indian.
John

Yep assuming a high compression ratio is part of his build plan to make enough power to get a LSR it should only help the cause. The tighter quench will only help things as well...

The plan right now is to run flat top pistons with an .041 head gasket. 0 deck height. the compression ratio comes out a little less than 12:1 This assumes a 57cc combustion chamber. This would set the Quench at .o41. I dont really want to go less than that.

You'll be running race fuel @ 12:1 or that e85. I talked to Fidanza before I finished my build in May. They could make a lightweight flywheel based off the 39# one you're considering. The catch was that 10 had to be ordered at once @$450-500 a pop.

I would make everything as light as you can afford. Heavy and race car should never be in the same sentence.