4.2 vs 4.6 as a DD?

[zachmc2010]

Ive currently got the engine out of my XJ and having trouble deciding just what to build my engine to. I have two '98 complete 4.0s (one is seized up, the other just has a lot of wear) , and a '90 4.2 that ran when I pulled it from my YJ. Got a few options on the table here, freshen up the 4.2 and install a 4.0 head onto it, or donate the crank from the 4.2. My question is, which is more suitable for a DD with a little more go? I plan on freshening up/rebuilding whichever engine combination I go with, and price is somewhat of a factor (looking at the cheap 4.6 build). I would also like to go with an aftermarket cam if possible. Looking at the comp cam Xtreme 4x4 cams with lifter sets. Are there any duration/lifts that don't necessarily need new valve springs? I was looking at the kits with the valve springs, but read in the FAQ that those valve springs will not work with my 4.0 head. Any help is greatly appreciated

Thanks, Zach

[6TIME]

If you're going to go thru the trouble of putting a new motor together.. You might as well do 4.6. A 4.2 will wake up with the 4.0 head on it but it's still a small bore long stroke motor. The 4.6 will definitely be more peppy than the 4.2 due to the larger bore helping the valves breathe better. There's no difference reliability wise. The important part to putting a dependable one together is, paying attention to the details where problems have shown up with these motors...ie Crank thrust, valvetrain setup w/aftermarket cams, piston dish volume/compression ratio etc... Do a search on these topics to enlighten yourself.

[zachmc2010]

Thanks for the quick reply, it sounds like I will go ahead and pull the trigger on the 4.6. How does the transmission take the extra torque and ponies?

[Cheromaniac]

How does the transmission take the extra torque and ponies?

Not a problem.

[sly-jeeper]

4.2 and 4.6 doesn't have much to do with bore size...

[Cheromaniac]

4.2 and 4.6 doesn't have much to do with bore size...

Actually it has everything to do with bore size.
The stroke is the same (3.895") 'cause both engines use the same 258 crank. However the 4.2 only has a 3.750" bore whereas a 4.6 has a bore of 3.905".
The 4.6 can not only deliver better performance due to the extra displacement, but the larger bore size unshrouds the valves and allows the engine to breathe better at higher rpm as 6TIME correctly pointed out.

[6TIME]

Spot on... Putting a little bit of distance between the valves and the bore makes a big difference in creating uniform flow around the valve.

[zachmc2010]

Thanks for the replys fellas, I have decided to build the "poor mans" stroker using my 4.2 crank and rods. I do have a few more questions though: What is a good duration/lift on a cam for these engines using the stock valve springs? I read on here that anything over a .430 lift on a cam isn't good with the stock valve springs and cause problems at high rpms. I have looked into the comp cams High energy cams and found a few that are right around the .430 lift range. What would be the best duration for my particular application? Also is there something I am looking over or another cam that fits the job well? Im not looking for crazy power, but would like to start fresh with a new cam and lifter set to go along with the fresh stroker build. And finally, is there a particular head gasket to use with this build if I do not plan on decking the block/head? Im going for somewhat of a budget build here, but at the same time I do understand to do it right sometimes cost a little extra in the beginning. Im not looking to spool this engine up to super high rpms, really wanting a nice low-mid or an overall power curve gain.

So far the parts list consists of:
.020 677CP pistons
Port the head to match the gaskets
Fresh bore on the lock/reman the head
All bearings and what not
Cam?
Oil pump and all the other goodies for freshening up these engines


Thanks, Zach

[sly-jeeper]

to explain myself i read his original post wrong...i was thinking 4.2 mini stroker and 4.6 stroker...i made an ass of myself again! by assuming he thought the diffidence between a 4.2 mini and 4.6 was the bore size...these 12 + hour days at work are getting to me!

[SilverXJ]

I wouldn't run any performance cam with the stock valve springs as their rate is low. Maybe one of the Mopar Performance cams if they spec a stock valve spring.

[Cheromaniac]

No worries. We all have those days if that makes any diffidence.

Thanks for the replys fellas, I have decided to build the "poor mans" stroker using my 4.2 crank and rods.

If you're going to gasket match the head ports, ONLY do the intakes. The exhaust ports should NOT be enlarged to match the gasket.
I prefer the Victor Reinz head gasket (same as Mopar Performance) though the new stock multilayer steel HG is also the same at 0.043" compressed.

[kkchevy3]

and why should the exhaust not be enlarged?

[SilverXJ]

Loss of low end torque and flow characteristics. Dino's point wasn't that it shouldn't be enlarged. It can be enlarged a bit, but opening it up to the size of the head gasket is way too much.

[zachmc2010]

Thanks again for all the quick information guys, y'all alone have kept me on the right path of this build. I'm still torn about a cam. If I were to go with replacing the valve springs: a) what cam would be a good overall rpm cam duration and lift (pump gas)? b) Is it going to be worth the extra $450 when combined with the rest of my build? I do plan on having the tb bored to 62mm, upgraded air intake of some sort, and running a 2.5" exhaust with a good flowing muffler.

Thanks
Zach

[Radioactivexj]

I just went with an aftermarket stock cam made by Melling just so I could reuse the stock springs. It may not be much of an upgrade but hopefully it will last a lot longer.