Topic: 2.5L Rebuild using a commercial 3.0L stroker?

[jwrape]

Well ok......I guess i will still stick with the bore .60 over and shave the head, polish and port and balance the bottom end and be done with it.
After that and my Inertia ring I should have enough power to hold highway speeds no problem.

[Jeffy]

Only if you're geared right.  :wink:

[jwrape]

Only if you're geared right.  :wink:

Yea, I am not using my 5th gear right now except over 75mph. LOL!!
I hope to go to 4.88's eventually but will most likely rebuild the motor first...

On another note, I think my AX15 is getting ready to take a dump cause I am getting A LOT of noise between in nuetral and when I push in the clutch around 4th and 5th gears. Sounds like gears clacking together. Gonna need to rebuild or find a nother AX15 for cheap. $250 or less. Don't have a lot of money left after doing everything else to the Jeep I have lately. Not happy about that since I have only had that tranny in for about 4 months. Still drives just fine BUT make too much noise for my comfort level. Gonna add another quart of Redline and see if that helps.

What's weird is it got this loud after my SOA. Almost seems as if the TC lowering kit might have made the fluid sit at a different level that's another reason for adding the extra quart. The other is it just sounds better in general and I know it will hold it.

[RNandKT]

::::Revival:::::

I did some reading and would be possible to have a 4.2 lt cranckshaft cut down the 2 cyl and then have it setup for the 4?

[Jeffy]

I don't think so.  I believe they use a different profile and I think 1 & 6 are removed to make a 2.5L.  You'd also have to be an outstanding welder to put those pieces back together and balance it. back up.  At that point it would probably be cheaper to get a custom crank.

[RNandKT]

well a custom crank would be cutting welding (and balanceing) every cyl's so would'nt it be worse than cutting an welding and balenceing the ends? I am no mahcinist or pro welder or a good engine guy for that matter, just thinking if you were having a shop custom make your crank that that might be the way to do it. (more of a base to start with.)

[Jeffy]

It would probably be cheaper to swap in a larger engine.

[RNandKT]

thats a good point.

[CaliGold]

Hey guys, just wanna chime in on some additional info regarding a stroker crank.  There was a guy who races a 2.5 XJ in the Jeepspeed competitions that was working on it, but I haven't heard any further developments.  Didn't sound cheap though.  He could be just welding and offset grinding the crank like you guys are mentioning. No too much info but, here's a link to read up:

http://www.jeepspeed.com/forum/YaBB.pl?board=tech_1;action=display;num=1127536495


Also I'm not sure what you guys think of this, but I've been cautioned by my machinist that the max amount I should bore out is .030 with our motors.  I'm told there are two reasons for this.  Our cylinder wall thickness is about 3/16 of solid useable casting.  Given the tolerance in core shift (the bore being concentric to the cylinder casting during manufacturing), there could be a risk to making the walls too weak if boring over .030.   I never thought the 2.5 block was too different from other motors from a manufacturing standpoint, and know that boring .060 is common on other motors, so I'm wondering what is fact.  

The second reason I'm told is that the no.1 cylinder is relieved (shaved) to clear the water pump vanes.  Although this relieved area is not the at the thrust surface of the cylinder, and not the heaviest loaded part of the cylinder, it should be considered.  This seems to be true when I inspected my block.



My machinist recommended sleeving the cylinders if I want to significantly increase the bore (4.00 or 4.125 inch chevy pistons) and shaving the vanes on the water pump if clearance becomes a problem, but at 100 dollars per jug, I don't know if it's the best use of my money.  Chevy pistons seem to be a feasible alternative in that they have a .020 lower compression height, and will work with the stock AMC rods if the small ends of the rods are machined just a bit...given that you got the wallet to cover the sleeving.

My spare motor is completely apart with the bare block on the engine stand.  I'm still trying to gather info and decide what I want to do about increasing the displacement, and been reading your posts to find out what seems to work.  Thanks for all the info guys.  I'm glad us 4 bangers have a place like this place to swap ideas.  Although it's another one of my long term projects, I'll try to keep you guys posted.

[CaliGold]

Oh BTW, Chevy SB guys have their cranks offset ground without welding and use rods with smaller journals.  The machining price is reasonable.  at $200 for a V8 crankshaft.  

Adding material by welding then offset grinding is between $425 to $725 for a SB Chevy depending on how much you stroke you want.  

http://www.flatlanderracing.com/crankshaft_services.html

The AMC crank shouldn't cost any more.  Probably less.  Then use the 4.2 pistons and rods...

Maybe could work?

[RNandKT]

How much "stroke" is gained by "offset grinding"? I have heard that term before. There is a significant difference in the length of 4.0 (or 2.5) con rods and 4.2 con rods. So if the offset grinding is not significant enough, you would have to really deck your block to get the pistons to come up high enough in the cyl right?

Any displacement change numbers from these small block chevys? What kind of con rods do they use. Are we talking about like chev 350's and are they using con rods from SB 400's ? This would be nice to know so we could compare it to a whole crankshaft swap as in a 383.

Great info I would like to explore this option since it is an affordable option. I am still on the fence between an I-6 swap and building up my 4.

[RNandKT]

I was just reading the services offered from flatfender racing looks like that really might work, does anyone know the legth difference on from the 4.2 rods to the 2.5's. Flat fender racing can offset grind most of the v8 crankshafts around .2" I think that is close to what would be necessary to use the 4.2 rods?

Theoretically speaking if one were to bore the engine closer to thr recommended .030 (which puts displacement areound 2.6lt) and were able to stroke it with the 4.2 rods that would push displacement to around 2.75 - 2.8 L? That combined with other things (such as a hotter cam 4.0 TB etc etc) could probably make a reliable 180-195hp and similar torque numbers?

[CaliGold]

Just checked some specs and here's what I got:

The rods from the 4.2L will work.  The diameters of the small and large ends are the same (.929" and 2.208" respectively).  The length (center to center) of the 4.2L rod is 5.875" and the length of the 2.5L rod is 6.125".  The difference in rod length is .250".  

The stroke of the 4.2L is 3.895" and the stroke of the 2.5L is 3.190" which is a difference of .705".  Half of this amount is the "theoretical" offset grinding for the rod journals which is about .353".  

There is a deck to piston spec of about .015" for the 4.2L (piston lower than top of block) while the 2.5L is at zero and the compression ratio for the 4.2L is 8:1 while the 2.5L is 9.2:1.

Also a correction to my previous post...you would not be using the 4.2 pistons since they are 3.75" in dia whereas the the 2.5L is at 3.875 std.  

I still need to figure out what the "real" value for offset grinding would be considering that I would want the final compression ration at about 9.5:1.  I would guess I would reduce the theoretical offset figure slightly to get this kind of CR.  I'd probably cc the combustion chambers and the piston tops with a burette to be absolutely sure about the true offset grinding number.

From my calculations this would yield me to about 3 liters using 2.5L pistions (with .030 overbore), the 4.2L con rods, and an offset grind of the rod journals of let's say ...about .345" (which BTW will require welding). I think at the very least you could get an additional 30 hp even with stock everything else.   Cam, TB, head work, ... even more hp.  

Okay now here are some unanswered questions I'm still wondering about:
1.  The 2.5L has a shorter stroke than even the 4.0 where this kind of stroke mod is common.  I need to make sure the cylinder bore is long enough to handle the full stroke of this modified crank.  
2.  Will it clear the rest of the block at the oil pan flange, etc...  The 4.0 block needs to be clearanced at the bottom of the cylinders just a tad.  
3.  How will the MPI system take to the severe increase in displacement?  I need to read up more on the results of people who did the 4.0 stroker mod to find out.  

Still working on the figures, but I'm looking for any guinea pigs who wanna try first  

[RNandKT]

Well as far as the increaseddisplacement, I know a lot of the peopl who make the 4.6 liter stroker that results from a 4.0 and 4.2 together, often upgrade their injectors to the 24lb Ford injectors. Other than that no other major stuff is really required. Some get the larger 62mm throttle body but the stardand works fine. But apparently with the increased displacement and a hotter came the stock injectors are a little lean.

Now considering the 4.2 con rods, that much stroke increase would require one of their welded cranks which on their web site for v8's was about 450 or so. But you mention the 2.5's stroke was shorter than the 4.0's, how much? Would the 4.0's con rods be a better option for the offset ground crank? I think you will find as mentioned before that more than a .2 liter increase in stroke will probably be impossible due to clearence issues. I don't think I would do the stroke increase unless I was able to do it by just the offset grind. But I might be your test dummy if you want to go that route. But I think I want to at least go .040 over piston wise. And I think some major head changes. Thats were a lot of hp is lost anyway.

[RNandKT]

Besides there are other things that can be done to affect hp and torque that don't require displacement increase such as knife edging your crank, and having everything balenced. etc etc