Join Date: Mar 2002
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Carbureted 4.0 Head: finally there and happy THNKS
Six months after getting the new motor in my '87 YJ, I can finally say that I am happy with the results. For a review (and if you're too lazy to check my prof)...
Components<ul>[*]'79 258 short block - bored .040 over, crank .010 under, stock rebuild kit[*]'95 4.0 head (new) - water jackets drilled, tapped, fitted with pipe plugs, resurfaced[*]Crane 260 cam[*]Borla header for '95 4.0 - top half of primary tubes wrapped with header tape[*]2-1/8" pipe straight to muffler (no idea what kind), no cat[*]Clifford water-heated 4bbl intake for '79 258[*]Holley 390 - jets changed to 54[*]Stock Motorcraft distributor base - spacer, Conrad brass cap & button, 1 stock super-stiff spring and 1 med/stiff spring[*]MSD 6A[*]Taylor 8mm plug wires[*]Accel SuperStack coil (premium stock coil in a yellow dress, but hey it was free)[*]Champion truck plugs - gapped to .045[/list]
Problems Encountered<ul>[*]Pilot bushing hole is a different size from '79 to '87 (POS Puegot tranny). The '79 is bigger OD and ID, and actually the '87 bearing almost fit inside the '79 bushing --> ding, ding, ding. Fortunately I have a friend with a small lathe [img]images/icons/smile.gif[/img] so he took a little more out of the inside of the bushing and I was golden. [img]images/icons/laugh.gif[/img][*]Tried to go with Pacesetter headers and they interfered with the pan. They are made to wrap in front of a full rear sump. My short block has a 3/4 rear sump. In retrospect, I am happy I had to make this switch. More $$, but that Borla header is NICE!!! [img]images/icons/cool.gif[/img][*]Clifford intake caused me some headaches. Although the ports are a perfect match, the bolt tabs do not line up and are 3/8" thick. Borla header tabs are 1/2" thick. If I had the mount job to do over I would have cut some 1/2" thick aluminum blocks, shaped them with a Drimel, and welded them on. Grinding down the exhaust header tabs would also have worked, but I didn't care for the risk of getting them uneven, too thin, or the grinder grabbing at the wrong time and really causing problems. Grinding stainless is not my favorite passtime either. I also had to put a small dent in the top of the #5 header tube and grind a little off the bottom of the intake to get the ports to line up and intake to fit flush.[*]Tuning<ul>[*]I believe that most of my tuning issues were more a result of the cam, intake, and headers than the 4.0 head itself. But put all those variables together and I had some chasing to do. My biggest problem was detonation at 1600-1800 with part throttle. The problem turned out to be a combination of lean mix and too much advance at that RPM.[*]The mix was resolved by changing the Holley 390's jets from the stock 51's to 54's and a lot of tinkering with the idle mixture screws. Most sources told me to tune it by vacuum and I did, but this didn't get it close enough for me. I put a digital volt meter on the O2 sensor and tuned to a balanced mix (.5 volts) for desired idle RPM. It may not sound like much difference, but it had a surprisingly large impact on the off-idle mix.[*]1600 is where I was reaching full mechanical advance (as evidenced by a timing light and an isolated vacuum advance), so I replaced the lighter mechanical advance spring with a med/stiff one (thanks, TR [img]images/icons/wink.gif[/img]) and gave it a little preload by bending the spring hook tab ever so slightly. [img]images/icons/blush.gif[/img] I now reach full mechanical advance at about 2200.[/list][/list]
Results<ul>[*]Power: Pulls smooth from 500 RPM all the way up. 500-1500 is comparable to a stock 258. At about 1500 it starts to wake up a little more and by 2500 it is pulling like a V8. [img]images/icons/cool.gif[/img][*]Mileage: Just got back from a trip over the mountains and got 19 mpg. YEAH!!!![*]Gas: Still running 93 octane right now. Experimenting with lower octane is my next step.[/list]
What I Would Do Differently
I would have stuck with my stock intake, my Weber 32/36 (which I sold - DAMN!), and used an RV cam. I was really wanting more of a stump puller than a hot-rod. Of course I could still go that way, but replacing the cam means pulling the head and then I'm starting this tuning journey all over again. Guess I'll just have to just have to live with my hot-rod six for a few years... shucks. [img]images/icons/smile.gif[/img]
Thanks for all the help to everyone on this board. This community is awesome! Thanks especially to Jaffer for advice and access to priceless information in your web pages, and to TR for schooling me through this, a hard-to-find spring, and a wealth of knowledge that I can only hope I gleaned something from.
The search function can be frustrating at times, but holy crap there is three billion tons of knowledge in that archive!
I be one happy Jeeper these days!