Opinions on Offenhauser Dual Port manifolds

[Imo Apita]

Kinda came across these Offenhauser manifolds.
Did a search on this board and came up with very little
One popped up on Ebay for $40 + shipping ($27).
Did a google search and opinions (as usual) seem to be mixed.
Some rave about 'em some hate' em.
Read somewhere that they work best with a q-jet setup for this manifold.
Any experiences?
I am just intrigued by the design.
I know that if they were "wunderbar" , everybody would be running them.
Looking for something else then Edelbrock.
Since I'll be shopping for a new cam might as well look for something else too.
Nothing fancy or extreme, maybe 300-350 HP out of a SBC with decent heads and cam.
Not interested in high revving max horsepower;daily driver that will see the occasional stomp at the traffic light.
More looking for stump pulling torque between (let's say) 2000-4000 rpm.
Will probably take it to a 1/4 mile once, just for the fun of it but definitely not a street/strip kind of car.
What I'm really looking for is something else then Edelbrock, don't ask me why.
I just don't like the "all-in-one" packages.Not my thing. I am not into the " I got a $5 million crate motor and it dropped right in" crowd.
I would rather puzzle and put together a combo of parts that few have ever heard of.


...flack jacket and hiding.....

 

[gkz]

Haven't seen one of those in awhile; my friend had one on his '68 GTO. Everytime he stomped on the gas, some part of the drivetrain broke; so you may be right about the tq:rofl:

 

[Imo Apita]

I posted the same question on two other (non Corvette) forums.
Most of the replies are that it's a horrible manifold, couldn't get it to run, stumble off idle yadiyadiyadida...:skeptic:
Still two replies of people that knew people that used Offenhauser and rave about them.
So somewhere out there is some knowledge on how to match up a cam and a q-jet to one of these.
I've more or less convinced myself that I need one of these manifolds.
Just want to do it right so I still need some help on this.
Cam choices and q-jet tuning tricks would be appreciated.
I have read through Lars's papers and bought the Q-jet book by Cliff but a rebuild is something that I'd rather leave to the pro's. At least for now.
.

 

[mrvette]

IMO, cam by itself will not make any hotrod manifold stuble,,,....but I found out from a Qjet expert many years ago...he rebuilt my Q and it ran like a raped ape on a Pontiac 455 I had for a while....what the deal was, according to HIM,....is the trick of the OFF IDLE Transfer circuit....to do with those little slots on the front edge of the pirmary throttle blades.....they feed into two of the brass tubes in the top plate....so slightly enlarge those tubes by drilling them with a small # drill....trick is, I don't know the size anymore....

the reasons, apparently, are that many guys are putting the new mani's in place of a EGR mani....so EGR feeds inert gasses, so instead of those gasses, it's all fresh air, so the off idle stumble is due to too much leanness in the mix....hense drill the brass tubes....

I don't pretend to totally understand WTF is that all about, I can't see the NEED for those slots, but it's what he told me on the phone near 20? years ago....

:surrender::thumbs:

 

[Imo Apita]

Thanks MrVette, I read about that trick in Cliff's q-jet book.
Have to check but the drill sizes, should be in there.
Looking at a budget top end without any play-money, as in money to make mistakes.
This is the plan so far:

• Dart heads from Ebay $300-$600
• Lunati Voodoo cam ( need more info on the Offenhasuer for specs) $150
• Schneider cam ( need more info on the Offenhauser for specs) $150
• Offenhauser Dual Port (if I can get more info) $50-$250
• Shorty headers from Summit $90
• Q-jet rebuild by Cliff

Trying to save up $1000 for parts and another $1000 for labor as this will take me 3 weeks on my driveway so I'm going to have the man do it.

Unrelated news;
Had been tinkering with the spring setting on the secondary airdoor.
Last week set it way to tight, found the sweet spot yesterday.
Holy macaroni, that makes for a difference; even with a bad cam it ran

like a raped ape

yesterday. even got the Po-Po to tail me for about a mile...:yahoo::yahoo:


Look what I found (still need to read it):
http://www.exeterautosupply.com/Docs/Offenhauser catalog.pdf

 

[lars]

I can't see the NEED for those slots, but it's what he told me on the phone near 20? years ago....

The slots are the transition slots, and every carb has them. At idle, a carb gets its fuel from the idle fuel discharge orifaces. These orifaces are located below the throttle plates, and they are exposed to manifold vacuum. The fuel is discharged at idle due to the manifold vacuum, and not from the airflow going through the carb venturi (there is not enough airflow at idle to make the venturis "work" for fuel discharge). As you step on the gas, there needs to be a transition from the fuel discharging out the idle hole and the main discharge circuit in the venturi - there is some "lag" before fuel starts flowing out the main discharge orifaces. The accel pump tries to compensate for this somewhat, but the transition slot does most of the work: The slot is connected to the idle fuel circuit, which is already flowing fuel. By exposing more of the slot as the throttle is cracked open, more idle fuel is pulled from the slots in addition to the fuel flowing through the holes. This handles the fuel flow transition from idle to main metering. If not enough fuel flows from the transition slots, you will get a stumble or sag at part throttle acceleration from a momentary lean condition.

All of the fuel flowing to the idle fuel discharge holes AND the transition slots flows through the Idle Fuel Restrictor Tubes (IFR). The tubes are sized to provide adequate fuel flow to both the idle circuit and the transition circuit. Fuel flowing through the idle discharge holes can be adjusted through use of the idle mixture screws, but the additional fuel available to the transition slots is simply what is then "left over" and remaining available from the IFR sizing. Thus, the richness of the transition circuit is fixed, and not adjustable. If you find that the carb is running lean in this transition area, but your idle mixture is correct and your accel pump is working right, you can richen up the transition circuit by slightly enlarging the holes in the IFRs. Sizing is relative to the size they are originally, just like making jet changes. I.e., you wouldn't make a blanket statement that a "size 74 jet is the best jet for all applications." Rather, you would state that, "for a lean condition, increase jet sizing 2 jet sizes at a time until proper tuning is achieved." Same thing with the IFR: Determine the existing size by using gauge pins or drills. Once determined, increase IFR hole size .002" at a time until desired results are achieved.

Keep in mind that the idle fuel discharge circuit, through the idle discharge hole and the transitions slot, flows fuel any time there is manifold vacuum, so fuel flows through this circuit even when the main metering circuit is operational (i.e. at cruise). By richening the transition circuit, you will also get a richening of the main metering circuit at cruise. Once you get the transition circuit rich enough, you may find that you have to lean out the cruise circuit through use of fatter primary rods or through a change in the APT setting.

Lars

 

[mrvette]

Lars, I O U a beer....

for all the world, it's about the same thing he told me on the phone, I can't remember that after 20+ years....no way....

too much into electronics and the obvious cousin for cars....

GOOD information for anyone into Qjets/carb though...

:bounce::thumbs:

 

[lars]

Was that Cliff that you had that discussion with? Cliff and I stay in touch from time to time - I think we're 2 of only a handful of Q-Jet & carb guys who still remember this stuff... Could have been Ken Crocie from H-O Racing, too... I saw Ken out in the LA area not long ago. Funny how this circle of people has become very small since the 60's and 70's...
Lars

 

[BRUTAL64]

Was that Cliff that you had that discussion with? Cliff and I stay in touch from time to time - I think we're 2 of only a handful of Q-Jet & carb guys who still remember this stuff... Could have been Ken Crocie from H-O Racing, too... I saw Ken out in the LA area not long ago. Funny how this circle of people has become very small since the 60's and 70's...
Lars

Yea, a small circle. Most of the people that tought me engines in the 60's are long gone dead. I learned from some of best at the time.:beer: