2.5L "performance"

[wolfpackjeeper]

I have already built a stroker, I know I can do that.

 

I am in it more to see what I can do with the 4 cylinder

[project88mj]

try to make that 4banger in to a stroker

[bigd44889]

now thats a good idea

[wolfpackjeeper]

try to make that 4banger in to a stroker

 

I am considering it, I need to find someone to cast a crank with the same throw out as the 4.2 crank. After that you can just use the 4.2 rods and oversized pistons. Should be able to get a 3.0L or so 4 cylinder out of it.

[Eagle]

try to make that 4banger in to a stroker

 

I am considering it, I need to find someone to cast a crank with the same throw out as the 4.2 crank. After that you can just use the 4.2 rods and oversized pistons. Should be able to get a 3.0L or so 4 cylinder out of it.

Nope. It won't be that easy. I believe the deck height of the 2.5L is slightly lower than either the 4.0L or the 4.2L (and those two are not the same, which is one of the issues to be confronted when doing an I-6 stroker build). I suspect the preferred solution (and maybe the only solution) will be to use either 2.5L or 4.0L rods and custom cast pistons, with a special pin height.

[wolfpackjeeper]

Nope. It won't be that easy. I believe the deck height of the 2.5L is slightly lower than either the 4.0L or the 4.2L (and those two are not the same, which is one of the issues to be confronted when doing an I-6 stroker build). I suspect the preferred solution (and maybe the only solution) will be to use either 2.5L or 4.0L rods and custom cast pistons, with a special pin height.

 

Hmm, it was my understanding that most of the 2.5 specs are pretty darn close to the 4.0. They use the same pistons and rods, or at least the last time I looked it up they did. Could have made a mistake, it has happened once before. I will have to get up with 5-90 and pick his brain. On a thread over at naxja we were talking about building a stroker 4banger, and pretty much decided you just needed to fond or make a crank with the 4.2l throwout lengths.

 

For my 4.7 stroker I built, I used a 4.2 crank, with 4.2 rods and .o6 oversized pistons. I actually decked the block and head about .01 or so each. Ported and polished the head, 3 angle valvejob, then seated and shimmed the springs back to stock specs. I was kinda planning on doing the same to the 2.5, with a solid bottom end and some boost you are talking about some serious horsepower in a lighter package.

[dasbulliwagen]

Ive been into air cooled VW's for 20 years, and one of the ways they do a stroker is to regrind an original cranks rod throws off center a slightly smaller diameter, thus moving the crank center diameter outward then using new rods machined to fit that size crank. I'm not sure if that would work on the 2.5 or not, but I'm sure with some research with you local machine or race shop the proper parts could be found or machined to order from a preset casting. I'm not sure the idea of using different rods and pistons with altered pin heights would work, as the overall stroke is still the same. Give me some stuff to think about for the future though!

[wolfpackjeeper]

Ive been into air cooled VW's for 20 years, and one of the ways they do a stroker is to regrind an original cranks rod throws off center a slightly smaller diameter, thus moving the crank center diameter outward then using new rods machined to fit that size crank. I'm not sure if that would work on the 2.5 or not, but I'm sure with some research with you local machine or race shop the proper parts could be found or machined to order from a preset casting. I'm not sure the idea of using different rods and pistons with altered pin heights would work, as the overall stroke is still the same. Give me some stuff to think about for the future though!

 

my way of a stroker is not the only way. There are 2 guys finishing builds right now, that used a 4.2 crank and 4.0 rods with custom pistons.

[tjbliley]

The proper way to build the 2.5 is to change the thermostat, change to the Mopar purple Cam, add a header, a catback exhaust, and go to the jet chip. All this can be done on the MPI engine. For the TBI engine, you can swap to a dry manifold, add a header and catback, then swap the cam. You could try to change the injector, but the renix hates big changes. Other than stroking it which would mean a total rebuild, there are no other options that I know of.

 

I am doing the first setup on my TJ, cause somehow I got a cam and lifter kit on ebay for $30 never installed. Guy just did not know he was holding the unicorn. I am going to try all the other changes for my TBI MJ, when I get some time.

[Eagle]

Hmm, it was my understanding that most of the 2.5 specs are pretty darn close to the 4.0. They use the same pistons and rods, or at least the last time I looked it up they did. Could have made a mistake, it has happened once before. I will have to get up with 5-90 and pick his brain. On a thread over at naxja we were talking about building a stroker 4banger, and pretty much decided you just needed to fond or make a crank with the 4.2l throwout lengths.

When you next communicate with 5-90, tell him I said howdy.

 

Jon should know (and perhaps you have already discussed) the reasons against using the 4.2L rods and stock pistons. First, the 4.2L rods are smaller and not as strong as the 4.0L rods. Second, due to minor dimensional differences between the 4.2L block and the 4.0L block, when you use the 4.2L rods and stock replacement pistons, you increase the compression significantly. This brings you to the need to consider quench height, compression, and cam overlap in order to control pre-ignition. You need a cam with enough overlap to bleed off some of the compression pressure to keep the engine from eating itself alive.

 

Using the 4.0L rods, you can spec the pistons to give you the same deck height clearance as a stock 4.0L engine, and that gives you a stock compression ratio ... which avoids the pre-ignition problem.

 

Getting back to the 2.5L engine, my source for the deck height information is the book Jeep Engines, which is put out by Chrysler. If you're going to build a Jeep engine, get this book. It has a Mopar part number: P5007161.

 

Deck height for the 4.0L is 9.429 - 9.435 inches.

 

Deck height for the 2.5L is 9.320 inches.

 

However, the 2.5L is specified to run at 0 deck clearance (top of piston is flush with top of block). The 4.0L is specified to run with the top of the pistons at 0.0215" below the surface of the block. The connecting rods are, as you mentioned, the same length (and I believe the same part number). Although I have read that aftermarket pistons for both engines are the same part number, the specification is different. According to the book, the dimension from the centerline of the pin to the top of the piston is 1.601" for the 2.5L, and 1.651 - 1.655 for the 4.0L.

 

Back to the drawing board ...

[wolfpackjeeper]

yea, I ended up heat treating and destressing my 4.2 rods just for that reason. Also had them shot peened and re-machined. I ended up with between a 9.5-10 compression ratio. Good info on the 2.5, either way you look at it, it still comes down to needing a custom grind on the crank, just not the same specs as a 4.2 crank.

[Eagle]

That's true, if you're having a stroker crank custom machined, you can make the specs anything you want. Just be sure to verify everything and don't assume because the 2.5L and 4.0L are an essentially common design that all dimensions will be interchangeable.

 

However, if you're looking at using stock connecting rods and stock pistons, that will set a few parameters and tend to preestablish where the top of the piston will have to be. Which may demonstrate that the crank dimensions have to fall within a narrow range -- or that "you can't get there from here."