going to get the 4bbl (thinkin the holley 390, offenhauser manifold). the 1bbl is starting to take a crap and I rebuilt it about a year ago, so i've been lookin and I think I want to get everything from summit, there the cheapest i've found, do I want the spred bore or the square bore? whats the difference, nothings set in stone, so what should I do?

 

I recently put a Holley 390 on my 300 with a Clifford intake and Clifford shorty headers. I love it! I probably added 50 to 80 HP with just bolting those on. I've been tuning the carb for a couple weeks now, and have changed the pump cam and nozzle so far. Once I get those where I want them, I'll tackle the main jets. Man, this old truck will pull, now!

 

Hey, spill the beans on what cam and nozzle you used. I have been trying to figure how to get rid of my off idle stumble.

 

Sure thing! I used the orange cam in position 1, and a 25 nozzle with tubes.

 

whats the difference between the 2 offenhauser manifolds?

 

Okay, this is based off of my experience with an AMC 258 in an 83 CJ7, and with my 83 Olds 307. the olds has the stock spreadbore 800 cfm quadrajet, and the jeep has a holley 390 4bbl on an offy intake. with a V block, spreadbores work well, because then both sides will get the same amount of gas. The problem with spreadbores of any kind is that the back cylinders will be richer than the fronts. This problem isn't seen as much on V engines, especially smaller ones (I.E. less pistons, like a V6 is better with a spread than a V8, which is better than a V10, which is better than a V12...).

Now, on an inline motor, your spreadbore is only feeding one side of the manifold (instead of both), so the big honkin' bore on the outboard side isn't doing a lot. Also, you have a much bigger piston spread than you do in the V configuration typically, so #5 and 6 will be relatively very rich, #1 and 2 will be relatively lean, and 3 and 4 will be just about perfect.

Now, if you get a squarebore, the carb bores are all the same size, meaning that every cylinder gets fed the same amount equally...well, at least, in a perfect world. With squarebores, you'll get a bit of leanness in the end cylinders, and it'll be okay in the middle, so you will want to run it a bit richer, and get a relatively good ignition system, if yours may not be up to par. That will eliminate much of the problem.

When you look at the hi-po slant six intakes, what do you see? they're REALLY long, and the runners are about the same length, which helps get rid of the lean and rich problem that I talked about on the stock manifolds. This is also why SEFI is good on these motors, because you have the exact same amount of fuel at each cylinder, which helps the engine make more power by evening out the intake charge over all 6 holes.


sorry it's so long, but that's my experience with six poppers.

 

I`m sure no expert,but I agree very hard to get equal fuel distribution to all cylinders.....however I believe the correct way to install spreadbore is 90 degrees to normal v-8 orientation.The offy dual port manifold is especially good in this instance because QJET small primaries feed the lower ports for very hi velocity airflow ...then when larger secondaries kick in they feed upper ports and "collide"with primary hi velocity air at head manifold juncture.So I believe in this application primaries go outboard and secondaries go inboard.{90 degrees} to normal v8 installation.I talked to "inliners club" 300 guru and he didn`t seem to have any problems with QJET for this application.My experience with QJET has been ...much better milage than similar sized HOLLEY {small primaries} plus great throttle response and very good in off camber situations.Please note that experience is on v8,but soon hope to have installation as described on 300 six.

 

on the one spreadbore intake I saw (wasn't an offy, I have no idea wtf it was), it was oriented like a V8 carb...anyway, I agree that quadrajets are probably the ideal fuel mixer, in that it has teeny primaries for better mileage, huge secondaries for ***** out performance, and it's relatively easy to tune if you know what you're doing. I'm running a qjet on the delta and get 26 mpg with a 3800 lb car, and I'm running rochesters on everything else (except the ford) and I love them all. Really, great carbs.