Why is the 4.2 so weak? - JeepForum.com
 
LinkBack Thread Tools
post #1 of 8 Old 10-02-2001, 06:46 PM Thread Starter
Brendan
Registered User
 
Join Date: Sep 2001
Location: FL
Posts: 162
Why is the 4.2 so weak?

I have a pre fuel injected YJ. Needless to say I suffer from a severe lack of power. I don't understand how such a large engine (4.2 freakin liters) could have so little power. Is it the head design? the fuel system. I mean if there is such a big combustion chamber why do I have barely 120 horses. It's embaressing to know that the 4 bangers have almost as much power as me. What can I do to get more power?!


Duvie

95 YJ 2.5L 5 Speed

Fightin a war against Florida Rust!
Brendan is offline  
Sponsored Links
Advertisement
 
post #2 of 8 Old 10-06-2001, 12:44 PM
87yj350
Registered User
 
Join Date: Aug 2001
Location: minnesota
Posts: 12
The heads on the 4.2's don't flow well, you can put a 4.0 head on which is much better and get more power. You can also change the intake to put a 4bbl or better yet go efi. You should check out cliffordperformance.com they specilize in 6 cylinders.
87yj350 is offline  
post #3 of 8 Old 10-06-2001, 05:07 PM
FLHT_FXRS_YJ
Registered User
 
FLHT_FXRS_YJ's Avatar
 
Join Date: Mar 2001
Location: Michigan
Posts: 471
If your power has been very poor, or it's really hard to get any rpm's, check the cat. converter. If it gets plugged up, it will really rob the engine power.

89' YJ - 4.2 - NV4500 - NP231w/sye - 1" Body Lift - 2 1/2" Rancho Suspension Lift - 31" Firestone MTR - AR Outlaw II - Tomken Bumpers F & R - Warn 8000 front - Warn 6000 rec. mount rear - FlowFit Seats - Tom Woods Rear & Custom 2" front driveshafts
FLHT_FXRS_YJ is offline  
post #4 of 8 Old 10-14-2001, 06:01 PM Thread Starter
Brendan
Registered User
 
Join Date: Sep 2001
Location: FL
Posts: 162
My cat is a hinderance, but its performing like it should If I swapped in the 4.0 L heads, could I still swap in the holley carb, and intake manifold, or are the bolting patterns different? Also, what are the different kinds of ignition, and what the benifits and drawbacks? Thanks alot.

Duvie

95 YJ 2.5L 5 Speed

Fightin a war against Florida Rust!
Brendan is offline  
post #5 of 8 Old 10-24-2001, 06:18 PM
JL
Registered User
 
Join Date: Oct 2001
Location: Charlotte, NC
Posts: 18
I'm researching for the rebuild and build-up I'm about to do on my 4.2. You'll retain the original intake and carb (or whatever aftermarket 4.2 parts)with the 4.0 head swap, but you'll need an exhaust manifold/header from a 4.0. I've heard estimates of between 25-40 hp gains, as well as a gas mileage increase. I'll probably port the head too before I put it on. Might as well while I'm tweaking...

By the way, the kit for this costs somewhere in the neighborhood of $1000 if I'm not mistaken, but all the parts involved can be had for CONSIDERABLY less. This is the most thorough site I've found.

rehabilitating an 86 cj-7
JL is offline  
post #6 of 8 Old 10-25-2001, 02:04 AM
CJ junky
Registered User
 
Join Date: Sep 2001
Location: Poland,Maine
Posts: 16
Guys,go here. http:/monsterslayer.com/jeep/40Head/40Head.htm This is the most complete site I've found about doing a 4.0 head swap.
CJ junky is offline  
post #7 of 8 Old 10-25-2001, 03:21 PM Thread Starter
Brendan
Registered User
 
Join Date: Sep 2001
Location: FL
Posts: 162
thanks you guys, what does porting the heads mean? Sorry for the dumb *** question. Can I swap any 4.0 head in. If I ask a junk yard for a 4.0 Head, what should I look for, one from a Wrangler, and how will I know if the valves sit well? Thanks for all the help

Duvie

95 YJ 2.5L 5 Speed

Fightin a war against Florida Rust!
Brendan is offline  
post #8 of 8 Old 10-25-2001, 05:59 PM
JL
Registered User
 
Join Date: Oct 2001
Location: Charlotte, NC
Posts: 18
Now Brendan, the only stupid question is the one you don't ask. Seriously, I didn't know any of this stuff either until I started reading about it. Porting the head involves grinding the inside of the head out to let more air flow through. Machine shops do it, and you can buy kits to do it yourself. (I'll probably opt for the latter)

As far as what year, most sources I've seen say '91 and up. These are supposed to be the high output (HO) 4.0 engines. The casting # for these heads is on the site I mentioned.

Don't know about the valves, but let me know what you find out.

rehabilitating an 86 cj-7
JL is offline  
Reply

Quick Reply
Message:
Options

Register Now



In order to be able to post messages on the JeepForum.com forums, you must first register.
Please enter your desired user name, your email address and other required details in the form below.

User Name:
Password
Please enter a password for your user account. Note that passwords are case-sensitive.

Password:


Confirm Password:
Email Address
Please enter a valid e-mail address for yourself.



Email Address:
OR

Log-in









Human Verification

In order to verify that you are a human and not a spam bot, please enter the answer into the following box below based on the instructions contained in the graphic.




Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)
 
Thread Tools
Show Printable Version Show Printable Version
Email this Page Email this Page



Similar Threads
Thread Thread Starter Jeep Forums Replies Last Post
Amp too weak for this sub??? tjchad04 Stereos, CB & Ham Radios, GPS, & Electrical. 16 11-25-2009 12:28 PM
antenna weak lilbluetj Stereos, CB & Ham Radios, GPS, & Electrical. 21 05-25-2009 05:36 AM
Weak Spark!!! 4.0 yasika116 Engines & Drivetrain 1 04-27-2009 08:29 PM
ax5 trans weak???? psycopanther Engines & Drivetrain 1 07-09-2008 01:54 PM
Just how weak are BFG sidewalls Fargo Tires & Wheels 15 10-06-2006 02:56 PM

Posting Rules  
You may post new threads
You may post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are Off
Pingbacks are On
Refbacks are On

 
For the best viewing experience please update your browser to Google Chrome