Hey, thanks, Jim, that really is interesting! This discussion is making me antsy about finishiing, installing, and firing up my engine, just to see how good or bad the cylinder mis-matches are. I might end up wishing for a Dual Port that had that all worked out at the factory. I wouldn't wish for the Offy C for this heavy truck, even though they evidently got the flows evened up, because each cylinder still has to draw vaccuum on a big volume. But for street performance it might be the deal sheneille. My Clifford manifold had no such flow diverter; maybe it worked okay with its somewhat different shape, or maybe it really looks better than it works, at least through low and mid-range. Don't know, probably won't test it. I could test it because my "splitter" is just bolted in place, but I don't have a suitable carb.

I wish I could justify buying three sidedraft Webers! The Smith clan, this branch, has always been frugal, okay, CHEAP. Actually, for the few old guys here who still care about Webers, I did find a 40DCNF, which is a smaller, cheaper, and simpler but still sophisticated "performance" carb, a downdraft simultaneous-opening 2bbl, in great shape and I got it CHEAP. One of the projects that made me set my 300-six aside is a 1987 Dodge Colt Vista, a 4cyl, 5-speed, 2/4wd micro-wagon that Vista enthusiasts like to call, "The Swiss Army Knife of Cars". Built by Mitsubishi, and modified to suit Chrysler, it has a 2.0L 2-valve hemi engine, and I had to rebuild mine after buying the car for $200 and towing it home. Being an old 2-stroke racer, I had to get out my porting tools and clean up the ports and bowls. Cammed it up just a hair, mostly just a little extra lift. Mirror-polished the piston tops to reject heat, and did a do-it-yourself heat-cured molycoating of the skirts. ARP rod bolts for when I race it.

Anyway, the stock "smart" carb, a emissions special continuously adjusted by the computer, was terminally corroded and no replacement was available. I won't have to get this car sniffed anymore, so my first choice for an "easy" replacement carb was a Weber 32/36 DGAV, I think that's right, anyway a little progressive-opening 2bbl that the Formula Ford crowd uses. Adapting that carb to the Mitsu manifold and throttle cable, and especially, to the Mitsu air cleaner, was more of a hassle than expected, but I hope to have it done this weekend. But after that, as we have been discussing, I have to sort out the cylinder-to-cylinder mixture evenness. And I have much less, what, emotional attachment?, to this engine than to a Ford Big Six, so this tuning process is more of a chore than fun. Silly, ain't it.

Soooo, as I think of the hours I've put into adapting this "economy" Weber to the Mitsu manifold and aircleaner, I wish that instead I had bought a second 40DCNF, welded up some nice, simple individual runners, and had a better induction system than what I will have with the current scheme. I might do it yet.

 

For what it's worth, my 300 with an Offy C constantly and consistently has higher vacuum on the vacuum gauge than my 300 with an Offy DP. Both are 19-20hg at idle. The Offy C is around 15 - 18hg cruising through town, whereas the DP is around 12 - 15.
Under a good, healthy acceleration, the Offy C drops to around 5 - 10hg (unless I really floor it), where under normal acceleration, the Offy DP drops to 0 - 5.

 

"For what it's worth"?? Real-world results are always worth reporting, even if other reporters get other results. Extensive, careful dyno-testing, isolating to a single variable, and testing multiple combinations, would be more conclusive, but there isn't a lot of that done. A big successful magazine like HOT ROD could maybe do it, but they use publishing deadlines as an excuse to do half-baked, non-scientific dyno tests that never seem to settle questions. Maybe they don't want any of their advertisers to look bad, or maybe they write articles on a "just in time" basis, and can't put anything off until a later issue. The Engine Masters contests are really interesting, but again, they are comparing apples to oranges in the sense of "proving" anything. Some of the best opinions come from guys like David Vizard, who has done many hundreds of dyno pulls on all kinds of things over the years, and have a vast frame of reference.

Interesting, AB. I take it you use the same carb with either manifold; did you find you needed to re-jet or bump the timing around? Have you rebuilt the motor? What head is on it?

 

The Offy C actually has a 600 cfm Holley on it (stock jetting), stock distributor curve. Timing is timed by vacuum so I'm not sure what its numeric setting is.
The Offy DP has a 390 cfm Holley on it. It's been rejetted to lean it out a bit since it was too rich. Recurved distributor.
Both motors have great compression but've never been rebuild. Stock heads, no porting, etc.

 

I had to re-visit this thread and get another look at Abandoned's info.

One thing that would make it slightly apples-to-oranges is that the D-P engine has to pull against a taller rear-end ratio. Still, very interesting, especially the big carb on the Offy C engine. That is a LOT more carburetion than most V-8s have, considered on a per-cylinder basis. Don't know what Offy recommends (AB?), but the carb companies recommendations were normally pretty conservative. David Vizard used to say they were considerably too conservative . . . but then he'd show you how to maximize the vacuum signal at the venturi and venturi booster to make a big carb work. Offy makes both manifolds; wonder what they'd say about this (AB?).

 

I think there may be variables...differences between the vehicles that are not be taken into consideration. JMO, no two vehicles even if they are identical in every way and only one SN apart will necessarily run the same. If AB were so inclined...it would be interesting to see the results from a swap of the intakes and carbs between the engines.

IIRC, Offy recommends up to 500 cfm with the DP and 600 cfm with the C. But, keep in mind that the big end of driving is on the primary side of the carb making the big end of the running on two barrels. The difference in venturi and throttle bores of the primaries between two carbs of different cfm ratings might not be all that much different.

 

I'm not sure on the C, since it was on the engine when I bought it, but I know the paperwork with the DP specifically said to use a Holley 390.

I've been thinking about doing some mix and matching between the two to see how they respond. If I get the drive this weekend, I may put the 390 onto the Offy C and see how differently it responds with the 3.55 gearing.

 

IF the spirit moves me, I might try my Holley 4160/1850 600 cfm this summer on my DP...after I convert it to electric choke. The only difference that will make a difference is the larger primaries since it gets run very little with the secondaries open.

 

[quote=Harte3;9850245]But, keep in mind that the big end of driving is on the primary side of the carb making the big end of the running on two barrels.


Okay. But again, on any cylinder of a V-8 most of the driving is done on only ONE of those two primaries, and when you're really pushing it you add ONE secondary barrel.

 

What you said, smitty, yes on the dual plane type manifold or like what 80broncoman did with his manifold. On the C or Clifford all 6 cylinders are drawing on the whole open plenum fed by the whole carb. On the DP the divide is between the primaries and secondaries with each system having it's own plenum/runner system.

 

(sigh)

AB's first (Dual-Port) engine, with a progressive-opening 390cfm Holley 4bbl, draws from one little primary throat, and sends its vacuum signal down a runner with roughly half the usual volume. When secondaries open, it draws on only one of those throats. At WOT it draws on a maximum airflow of 195cfm (half of 390).

My '67 El Camino 4-speed, a lightweight car (basically a Chevelle) had a factory 327. I replaced the original Q-jet with a progressive-opening 600cfm Holley like AB has on his second six. That easy-winding, lightly-loaded V-8, with a factory dual-plane manifold like every other factory V-8, drew from one little primary throat, and sent its vacuum signal down a runner with roughly half the volume (of a single-plane manifold). When the secondaries opened it drew on only one of them. At WOT it drew on a maximum airflow of 300cfm.

But AB's second six, with that same 600cfm Holley on the Offy C, never draws from less than TWO primaries, then TWO secondaries, must draw a vacuum on TWICE THE RUNNER VOLUME as the others, and at WOT must draw against the full maximum 4bbl flow of 600cfm.

I want to know why what the factories believed was not desireable for their V-8s should be desireable for lower-winding, usually more heavily-loaded sixes.

A Ford 400M V-8, a big, slow-turning torquer and Ford's base motor for trucks deemed too much for the 300-six, has the same bore and stroke as the 300 (4" X 4") and runs in a similar rpm range. Ford put a dual-plane manifold on the 400M. They mostly used a 2bbl carb, of which (am I beating you to death with this?) each cylinder ever saw only one throat. If you asked Holley what a good upgrade carb would be for a 400M, they would point you to that 600cfm unit that AB is using. And Holley would do that knowing that the 400M would treat it as a progressive-opening 2bbl with maximum 300cfm at WOT.

I don't get it.

 

The Offy DP doesn't quite work the same way that a "standard" dual plane works.
On the DP, each cylinder still sees all of the carburetor at WOT, not half. Think of it as two manifolds on top of each other. Both primaries feed all six cylinders through one manifold and the secondaries feed all six cylinders through another manifold.

Something like this:
(red = secondaries, blue = primaries)



The bonus is that they are smaller passages instead of one large one, which brings up velocity.

 

Dang, got in a hurry and forgot that; duuuhh!! But my question remains.

 

What size are the venturis and throttle bores on the Holley 390 vs. 600?

Edelbrock 500 and 600 cfm carbs are the same other than the jets/rods/pump squirter.

I've got the Edelbrock 500 on an Offy C now, but I picked up a couple of 2-bbls - a 2100 and a 2150 -that I'm going to swap on just for funsies. At least until I can find and order some parts for the 4100 I've got. The Edelbrock runs just fine, but I want to give the 2-bbl a go for some of the reasons Smitty mentioned above. The 2150 is a 351 cfm off a 351C Torino and the 2100 is a 300 cfm off a 289 Mustang. Maybe there's some mpg to be found.

Anyone ever made a 1-bbl adapter for an Offy C or Clifford intake? A guy on Craigslist had an old, unused Edelbrock, maybe, or Offy 1-bbl manifold for $300 but it must have been gone since he never replied. It had a lot less plenum volume, very narrow runners. Pretty trick looking. I should have saved the picture.

 

I'm not sure of the exact sizes, but I do know from experience that the throttle bores on the 390 and 600 are different. I think there's nearly a 1/8" - 1/4" diameter difference between the two.

When I ordered a new baseplate for my 390, they sent me one for a 600 cfm, and when I put the plate on (which fit), the baseplate bores on the new vs. old baseplate were significantly different.