Slant Six Forum :: dividing plenum

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dividing plenum https://www.slantsix.org/forum/viewtopic.php?t=6416	Page 1 of 1

Author:	Guest [ Fri Aug 01, 2003 9:53 am ]
Post subject:	dividing plenum
I have an '84 D-100 with a freshly rebuilt, stock 225. It needs a carb and the Lean Burn computer is bad (running, but poorly). I do want to make some performance upgrades to the motor, once I fix all the other problems with the truck; but, for now, I'm leaving it stock. Since I need to replace the carb, I'm gonna look through the local junk yards here to try to find either a super six manifold, or one of the 4-bbl manifolds (Clifford, Offenhauser). I've looked through the forum and I understand that the 4-bbl is generally too much carb for an otherwise stock engine. I'm just curious, has anyone tried dividing the plenum in an Offy manifold; and, would that allow my engine to run properly with the 4-bbl?

Author:	Super6 [ Fri Aug 01, 2003 10:12 am ]
Post subject:	Ditch that lean-burn!
I don't have an answer on the plenum question, but IMO if you upgrade the exhaust as well (dutra duals or headers and a single 2.5" or dual 2.25" pipes w/ free-flowing muffler(s) ) you could tune a 4 bll to run well on an otherwise stock slant. To really make use of the 4bbl you would eventually need to do some head work. On the lean-burn, DITCH IT! Use a stock electronic ignition or upgrade to an aftermarket ignition (eg MSD), it will save you headaches down the road. Keeping a lean-burn ignition running well is a real test of patience, and who wants all those vacuum lines running everywhere under the hood? I would guestimate 70% of my factory original wiring diagram (from a chilton's, for an '87 D150) was simply replaced by my MSD box. If I ever get access to a scanner I can post the diagram as a guide to removing the lean-burn setup. BTW-converting to a factory electronic ignition will do the same as above, but you will have the ecu and ballast resistor in place of the MSD box. -S/6

Author:	mnecaise [ Fri Aug 01, 2003 11:22 am ]
Post subject:	divided plenum
BTW, I'm "guest" -- got all registered... I looked at (took a close technical look at -- I'm an electrical engineer and programmer by trade) the lean burn computer and knew it was junk. It will be replaced along with the carb. I'm actually thinking about using the standard electronic ignition with an ignition computer that I should be able to whip together for less than $100 and a few weekends of time. That way I can play with the ignition curve and set it to whatever I want. The truck will need new exhaust before March (inspection will run out by then) and I need to replace it all from the manifold back since it's still got the factory converters (130k + miles) and they've got to be bad... So I was planning on opening up the exhaust to 2 1/4" with the factory manifold, which by the way, is warped... If you run a divided plenum, wouldn't the engine see the carb as two smaller two barrels since each half of the engine can only access half the carb? It seems to me this would be equivalent to, say, Proline's dual Weber setup. Would this avoid the overcarburation problems? It may be a moot point since I haven't found a manifold yet. This truck is going to project me to death...

Author:	DusterIdiot [ Fri Aug 01, 2003 11:54 am ]
Post subject:	Vaccuum Secondaries....
If you use a carb with vacuum secondaries the secondaries will only open as much as needed by the demand of the engine itself. Lots of stock motors out there are running a 390cfm Holley #8007 (soon I will too...then a 600cfm carb that probably won't ever flow more than 450cfm). The divider will reduce plenum volume and increase throttle response suppossedly, along with allowing the 4 barrel carb to get better signal and from there you can better change the jets to handle which side needs what.... Another thing to note is your transmission type, if it's manual no worries, if automatic, you'll want to research a kickdown linkage solution (plenty of guys on here have answers). food for thought, -D.Idiot

Author:	Super6 [ Fri Aug 01, 2003 12:04 pm ]
Post subject:
Welcome to the board I am a civil engineer (EIT) by trade (so I'll build the box to put all your electronics in ) While dividing the plenum would effectively distribute 1 bbl each of a 2 bbl carb to cylinders 1 thru 3 and 4 thru 6, a modified stock manifold would not have the "more equal" length runners thatthe prolinefuel setup has, thus cylinders 1 and 6 would still tend to run lean and 3 and 4 would tend to run a bit rich. Additionally, a Carter BBD 2 bbl or Holley 2 bbl do not have the meticulous tunability nor as much cfm adjustibility as the Weber carbs prolinefuel uses/sells. I do not have personal experience with prolinefuel's setup, but at least one member here is running it and seems pleased (Wes, chime in here). Personally, I think if one were to find a way to baffle the plenum to make for more even fuel distribution (especially to cylinders 1 and 6) without restricting much flow, even a 'stocker' slant would run more smoothly and create more power. Just some mental ramblings.... Do a search here on "intake" and "manifold", IIRC, this topic (or one very similar) has been discussed somewhat recently. -S/6

Author:	Doc [ Fri Aug 01, 2003 2:38 pm ]
Post subject:
I have run divided plenum intakes on a SL6 with good results, the divider does help with off idle and mid-range throttle responce. In the case of a sideways mounted 4 bbl, I find that the divider also helps fuel distribution. If you use a divider in the intake manifold, be sure it is welded or epoxyed-in solid, I have had dividers come loose inside the plenum, good thing the divider piece was to big to get down a runner. DD

Author:	Bruce Johnson [ Fri Aug 01, 2003 11:58 pm ]
Post subject:
welcome mystery guest, About a year ago I was experimenting with a tall 3" spacer, it was four aluminum tubes cut at 45 degrees on my Offy with a holly 390 I diden't like the way the air flow is slammed into the floor of most 6 cyl. manifolds. I was at a slant 6 of america meet here in Oregon when a guy, sorry don't remember the name, suggested that I divide the plenium under the spacer I was working on so I tryed it. Originally the spacer was still bolted on I was able to slide it to and from the engine. I got to the point where I felt that the 390 was to small for my engine. About that time I found an Edlebrock 600 at a flea market . I reworked the spacer and added an additional 1" phenolic spacer reworked with epoxy and aluminum tube from a mini WD 40 spray can. When every thing matched the bores of the carberator I had the spacer and offy manifold welded together. 123 and 456 are totaly divided with a piece of 1/4" aluminum welded in with some additional large fillets to help direct air flow . the thickness of the divider provided a good seal to the spacer with a small amount of epoxy where welding was unaccessable. I use an Edelbrock 14" air cleaner but had to cut out one section of braceing in the stock 78 hood. I've got an Oxegen sensor mounted to prove the mixture is most often right on. I can get 19 mpg on the hyway or I can get 8- 9 mpg tossing my can around town the truck has a A833 4spd 3.09 First gear,3.50 sure grip and 295 50's on 10"rims The distributor is "locked out" for an MSD timing computor yet still very streetable. The tall slanted spacer adds amost 5'' to the runner lengths thus decreasing the ratio between long and short runner lenghts , and it seems to work, easyer starting,great throttle responce and lots more real noticeable low end power. Idle mixture is easyer to ajust . and it has never seamed over carberated though I should say I did allot of head work including oversized valves from Egge. For a Truck with all that hood clearance and with the power brake booster in the way I'd do it again in a heart beat Bruce J.

Author:	mnecaise [ Sat Aug 02, 2003 3:14 pm ]
Post subject:	divided plenum
Thanks guys, I'll keep it all in mind. Now I need to head out and see what I can find...

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