2x4 Oldsmobile 455

[Marx3]

I am planning an Olds 455 build and came across this
http://www.ebay.com/itm/Offenhauser-Big-Block-Oldsmobile-2x4-Dual-Quad-Alum-Intake-Offy-B-B-Olds-/191438784361?pt=Motors_Car_Truck_Parts_Accessories&hash=item2c92a40769&vxp=mtr
on ebay. I would love to have a 2x4 Qjet setup, I know I could tune it to run as good as possible.
The thing is, this intake is not a great performer for real power... Olds builders advice against it for serious powerapplications.
I plan to build my 455 relatively mild for starters, but up the road I would like to enter the 12's, maybe even the 11's in my ´78 Trans Am, so I am going to build it to make pretty serious power.

Question is, would it even be possible to make a 2x4 Qjet setup work flawlessly on a powerful 455 Olds ?
I would use 750 units for the job... that would give me about 500 cfm combined on the primaries as far as I know.
Too much?

[Cliff Ruggles]

Nice looking intake, and it may have enough material to port it for a spread bore carburetor so you don't need an adapter.

I'm not a big fan of 2 x 4bbl set-ups, since a single q-jet can easily support 455 cid to at least 750hp, but they look cool and make for great conversation at car shows, cruises, etc.

I've helped several customers set up q-jets to work on similar set-ups, I would use and prefer the early Chevy carbs for those applications, as they are side inlet and sit nicely on those intakes without crowding.  They are also very common carburetors, zillions of them from the 1971-1974 period floating around on E-bay, swap meets, etc........Cliff

[Marx3]

I am only thinking of doing it because it would be cool to make it work flawlessly. Very few people are running 2x4 setups here in Denmark. I would not be going for the coolfactor as much as the satisfaction in making it work.
I have plenty of cores lying around, I can easily find 2 late 70's 750 unbits with side inlet.

I am undecided as to stroking that 455 to 495, but I have no doubt I could build a 849cfm unit to feed it plentifully wether it stays at 455 or gets bigger.
Well, thanks for your reply, I will keep on thinking about it.

[von]

IMHO the key to making it work well would be to make a progressive linkage so that at below say one third to half throttle it would operate on the primaries of the rear carb only, then at about one third to half throttle the front carb primaries start to open and progressively open faster than the rear carb until both are fully open at WOT. The secondaries of course open as normal on their own in response to air flow requirement.

[Marx3]

I hear ya... I wonder how the factory 2x4 setup is on, say a nailhead with AFB's for instance... A good friend of mine has a true 2x4 Riviera. I gotta check with him.

I agree with the idea of having just one pair of primaries operate at normal light throttle application, but I dont know if the rest would work. I imagine it would be dificult to control the fuel metering at wot. The secondaries would open at a pretty fast rate. Some thought and some trial and error would have to be put into the project.
I am getting more and more hooked on the idea, mostly because it would be cool to make it work as flawlessly as a single unit always do.

[Marx3]

just checked with my buddy. The linkage on a factory 2x4 Nailhead is working like you described Von. That is the way to go.
I guess both carbs would have to be equally jetted somewhat leaner than in a 1x4 setup... ?

[Cliff Ruggles]

No, it doesn't work that way.  They would need the correct jetting and would not need to be leaner simply because there are two of them, as they don't run richer because more than one is being used.

The idle system typically needs attention, as you are doubling up idle bypass air, so we don't use any and use both carbs at idle for dual quad applications.....Cliff

[Marx3]

you dont say... I dont fully understand why this is. The engine does not ingest more air just because there are 2 carbs, but since there are twice as many venturies, the engine would have twice as much fuel available... Why does it not work that way?

I will be fine tuning the afr with my wideband, so I will get a good afr no matter what.
Nice to know about the idle. This makes sense, though :-)

[HeathD]

Im guessing that the air drawn in will only be as much as the engine can pull and because of this the carb will only supply that amount of fuel/air mix.   Just because it could use twice the fuel as a single carb doesnt mean it can use it.

I'm picking the idle set up and transition to secondaries will be the biggest hassle.   Getting them balanced and not fighting each other will be a challenge (or maybe not I'm guessing again).   I guess it will be like driving a 4 barrel carb with 4 massive secondaries so the primary circuits will be restricted a little to get the best out of the pair (and try to get some fuel economy back - yeah, probably not, but the cool factor will make up for it).

Will the secondary springs need to be wound back so they both open at lower vacuum or left std?

I'm watching this thread with interest as it looks to be a real head turner.   Dont forget some pics when you are done.

[Cliff Ruggles]

I really don't have the time or space to go into the "science" of what's going on with 2 x 4 set-ups.  They will not need "leaned up", just set them up correctly as if you were only using one of them, and tune from that point.  They will need more attention to the idle system than the main system, as airflow thru each one at idle speed is very low compared to only using one carburetor......Cliff

[Marx3]

I got ya! :-) it's time for some good old trial and error!

It makes sense why the idle systems will need to be dealt with as the draw of air will be lesser since the "bleed" surface (around each throttle plate ) will be twice that of a singe carb.
I guess this will mean the idle systems will have to be calibrated more like for a big cam...

I will most certainly post updates AND pictures once this gets on the road. The engine it is meant for, I haven't even begun rebuilding it yet, though. saving up some money to buy the 495 cui strokerkit.
Come spring I might be far enough in the whole engine build, that I can start looking into this part of the setup.

[Cliff Ruggles]

Cool, sounds like a fun project, let us know how it turns out?.....Cliff

[Marx3]

sure thing!

[von]

I'm wondering if it would work to adjust the front carb idle so the primary blades are all the way closed and plug the idle air bypass to in effect take its idle circuit away until the throttle is opened. The primaries of the rear carb would be the only idle circuit. This would make it effectively like the Pontiac and BB Chevy 3x2 setups of the 60's where the end carbs had no idle circuits and blades were all the way closed until called upon.

[Marx3]

I think that could work also, but I have an idea it would create a potentially stronger idle if the engine is supplied fuel from both carbs.