Stromberg 97 vs. Rochester 2G on a 3x2

Engine is a healthy 350 Chevy which currently has an Edlebrock Torker II manifold with single 600cfm Edelbrock carb and some other performance goodies.Want to replace the manifold combo with a better looking 3 Deuce set-up,maybe 4x2.Engine pulls small Coupe pretty hard,scary fast,definitely don't want to lose any power.

Offenhauser offers 3x2 manifolds for 3 bolt carbs ( Strombergs etc.) and 4 bolt carbs ( Rochester 2G's etc.).The Stromberg is the better looking carb but I'm concerned the carbs won't flow enough and engine will lose power.

Anybody on here who successfully runs three 97's on a SBC ? I need your input please.4x2 Stromberg owners too.

The Rochester seems to be the better performing carb on a SBC.Don't just want looks,although it's high on the list too,but again,don't want to lose horsepower.And which of the 2G's is the "better" carb,small or large base ?

Hot Rod Racers,what do you suggest ?

 

I am by no means an expert but I went through the same dilemma you did. In the end I had Dick York build me a 3x2 setup using the high performance #3903 Holley 94s. This was the perfect compromise in my mind because I got carbs that are more appealing to look at than the boxy Rochesters but the Big 94s are rated at 310CFM. Three 2bbls at 310 CFM ought to equate roughly to the same amount of flow as your 600cfm 4bbl so you shouldn't lose power.

I highly recommend working with Dick - he was awesome - really built me a killer set-up and was extremely helpful.

Good luck!

 

I think the Rochester's ( small base) are a more traditional look for a small block. Here's a rough formula to help you decide flow: engine size (cid ) X maximum rpm. divided by 3,456 = cfm @ 100 % efficiency ( nothing runs 100% and yeah I know - this doesn't take in a lot of variables but it's a starting point). Now having said that I'm running 4 - 94's on a stroked 348 where 3 Rochester's used to sit. Feeding the pump is always a matter of experimenting...

Terry

 

Interesting.Your input is really appreciated,thanks guys ! Headhunter,how can I get in contact with Dick York ?

Btw,do all Offenhausr manifolds come with this "old-style" breather hole ? Engine has Offenhauser valve covers without breather holes and don't want to put nreathers on top of covers to keep looks clean.Plan was to mount some rectangular Offenhauser 90 degree breathers to side of valve covers but I won't need that type of breather at all if all manifolds come with the front breather hole...do I understand this right ?

 

He's Dickster27 on the HAMB - just shoot him a PM.

As for your question about the breathers... that's not an easy one to answer. The hole you are referring to is the oil fill tube that, when properly equipped, can breath *some* of the crankcase gasses out, but it you have it as your only breather, you'll likely get a lot of oil leaking out of the top of the tube. I had originally wanted to have smooth valve covers so I had a PCV circuit put into my intake with the output and PCV valve coming from the valley in the back of the intake and the input below the base of the rear carb. But that just kept pulling in too much oil and dumping it down in the intake - I'll eventually try building a deflector plate for it but that will require pulling the intake which I don't want to do right now. For now, I put some Moon right angle breathers on my valve covers and I think they look pretty good. I still don't have a proper PCV circuit, though, so it's not ideal. I was thinking of connecting a hard line with the PCV valve from one of the breathers to my inlet under the rear carb so I'd have it all breathing properly.

 

Hello..Here is an idea..A custom built 4x2 with Strtomberg 97s
Duane.

 

Headhunter,thanks for the contact info and I value the given nfo !

Yes,it's actually the oil filler hole.I run two 97's on a Merc Flathead and the oil filler tube with breather cap is all I have to let the motor breath.It was wishful thinking the Chevy would do the same...any chance to post pics of your set-up ? Sounds cool.

Duane,that's great craftmanship ! Do you sell manifolds and carbs ? Just curious.

 

The math doesn't do much unless the factors are included.
The tri-power (shown) is a 180° dual-plane type - only 1 barrel of each carb feeds 4 cylinders (center pair of 1 bank + end pair of the other bank), if they're 310 CFM each 1 cylinder gets 1/2 × 310 × 3, or 465.
In addition, if the 310 is rated for a 2 bbl. @ 3" Hg, it's only 219 if rated @ 1.5" Hg (as are 4 bbls.). Total for each cylinder: 329 CFM.
A single-plane manifold with a 600 4 bbl. feeds each cylinder with the entire 600 CFM.
The 4 × 2 log manifold (shown) feeds each bank of 4 cylinders with 2 carbs. Each cylinders gets all 4 bbls. (plus slightly more if the balance tube is big).
If you use 4 97s, each cylinder gets 2 × 155 CFM (@ 1.5") or 310.
By comparison, the big Rochester 2G (1-3/8" venturi), is 435 CFM @ 3", 308 CFM @ 1.5". A 2 × 4 would give each cylinder 616.

In short, you need to know the manifold design before you can tell how much carb you need.

 

Offenhauser manifolds can be ordered with or without the oil fill tube. All the manifolds I stock have the oil fill tube hole. I also have the oil fill tubes and breather caps in stock. Also, the Offenhauser 3x2 manifolds are a dual plane (180 degree) design. The Offenhauser 4x2 manifold is more of a log design with equalizer passages between the two plenums.

Dan Marvin, Owner
Exeter Auto Supply

 

Here's my set-up:

http://www.jalopyjournal.com/forum/album.php?albumid=126

Let me know if you have any more questions!

 

TheFrenZ if you decide on Rochesters, get the small ones. 3 of those goes like heck, its like a 750 CFM mechanical secondary triple pumper.

 

The Frenz..Yes, I sell custom intakes, Stromberg 97s and custom built linkage..PM me with you email address and I'll send you some pictures of my work..
Duane..

 

Thanks for the input everybody,we appreciate it ! Wanna bring this back to top one more time and maybe hear some more opinions.Thanks !

 

I am fighting this same issue. I have a few 97's and a few 94's, looking at buying the intake next. I have a 283 which could live on a lot less cfm's than your 350, but still.....

 

i have a x-1 intake / six 97's how much cfm is that?
Im running a cam -int-224 dur exst dur-234
int465 lift-exst488 lift the motor was a 290 hp gm crate ,put some vortec heads , will the motor handle it ? is this going to be to rich

 

How are you mating the x-1 to the vortec heads? adapters?

 

This is a very interesting thread. It would seem to me that many of the multi-carbed engines would have too much cfm for their size (ie 6x2 on a 283 or 327). What do you do to avoid over doing it with a multi carbed motor?

 

Kinda pricey but Barry Grant offers a very nice intake and carb setup for 3 - 2 barrel carbs that can be bought complete called the six shooter. There is no substitute for the nostalgic look but if you are concerned with performance of a hot small block , this will feed it. We use the mighty series on almost all engines we build. -Amos

 

With multi carb set-ups you can't just add the cfm of each carb and get a grand cfm total. With the pressure drop from adding each additional carb you loose some flow.

The simplest thing is to use what is tested and proven to work. Three bolt carbs for mild smaller displacement motors (265 & 283) and small base Rochesters for larger medium performance small blocks (327, 350). Big base Rochesters for larger motors (IE Pontiac 389). Holley 2bbls on 440 Mopar, 390 Ford, 427 Vette.

Just like 4 barrels they have to be jetted right, too big of jets = too much fuel, if jetted correctly they will run great. Thats why if you don't know how the multi carb set-ups work getting one of those kits from Speedway, Vintage Speed, etc. for a 3x2 set-up for small Roch on a SBC is a good idea.

Marty McF.

 

McFly, I hear ya but I am a poor white gov't worker. Someday I will be able to buy one of those kits but not until my kids move out.

 

Ok, I'm going to hyjack this for a Q of my own! LOL
Factory Iorn 348 intake with 3 swap meet 2 jet carbs and progressive linkage. The jet sizes are 49, 51 & 55's in the carbs. Where should they go on the intake?
Will this setup work better than the factory 4 bbl carb and intake?
Also, how big (and small) are the different base sizes?
Thanks!

 

I have 2g Rochesters on my 327 and thou they need to be rebuilt I am very happy with them other than my damn linkage on the actual carbs for acc pump binds up from time to time. I have the progressive linkage so when I drive it to work 70 miles round trip I like just runnin the single carb. I had Strombergs on my bucket and haded the damn things. I didnt have enough of them on there. If you put 35 of them on your motor they work well. haha..

 

So how many is enough, 3,................6,.........8?

 

I installed three small base rochesters and one of Charlie Price's adaptor plates on my big block Chevy. It is in the truck you see for my avatar. It goes like stink and way out performs the pos Holley spreadbore that use to reside there. I was amased how well the adaptor works. Check it out if you have an engine that 3x2 manifolds aren't available for, or in my case, I wasn't willing to shell out the green for an original factory 3x2 manifold. Too much money plus gotta use leak-o-matic, power valve frail holley 2 barrels.

 

Re: X1


3 × 97 on each side means each cylinder draws from the opposite plenum with 465 CFM.

"is this going to be to rich"

Again, again, again: too much carburetor means too lean. Carburetors supply air, the engine demands the fuel. Not enough demand = lean.

 

Too much carburetor is determined when your wallet runs dry .

Here is some math that can get you close or at least help you see what you are really working with.

First, use the formula Terry O posted:

engine size (cid ) X maximum rpm (redline)
3,456 = cfm @ 100 % efficiency (nothing runs 100%, this doesn't take in a lot of variables but it's a starting point)


Second, compare the venturi area of a known cfm 4 barrel at the ideal cfm rating to the venturi area of all the carbs (yes added together) in the multiple set-up (if you are not sure -the venturi is the narrow throat of the carb, the most restrictive section the air must pass through). To do this:

1) Look up (goolge search) or measure the venturi diameter of the single 4 barrel that has the max cfm rating you calculated for YOUR motor from the above formula.
2) Measure the venturi diameter of each carb in your multiple set up.
3) Calculate the area of the venturis:
venturi diameter/2 = venturi radius
venturi radius x venturi radius x 3.1415 (pi) = venturi area
venturi area x # of barrels (4, 6, 8, etc.) = toal carb venturi area
4) Compare the area of the 4 barrel to the area of the the multiple carbs and see if it is close.

Example: 350 Chevy, medium performance, very high redline for a street 350 of 6000 rpm, going to 3 x 2 barrels

350 x 6000 rpm
3456 = 600.638 cfm

> 600 cfm Holley 1 5/16" venturis (from Google search, square bore -all the same diameter)
1.3125"/2 = .65625"
.65625" x .65625" x 3.1415 = 1.3529 sq in
1.3529 sq in x 4 barrels = 5.411 sq in

> Small base Roch 2 barrels 1 3/32" venturis (283 Chevy carbs, not Olds)
1 3/32" diameter = 1.09375" -> 1.09375"/2 = .5468" radius
.5468 x .5468 x 3.1415 = .93956 sq in
.93956 sq in x 6 = 5.6373 sq in

Is all this written in stone? No...but it is a way to see what you are really looking at.

Marty McF.

 

Yes, seen that formula many times.

Yes, it's still wrong.

 

Frenz.....I'm running 4 97's, progressive, old Weiand manifold on a HOT 350(big cam, hogged out 327 heads, etc)
I had a Performer intake with a Holley 750 before and it ran a little smoother. And I did lose some power swapping to the 4x2......but it still runs very good and definitely has the right look.
If ya go that route, feel free to ask any questions.....I learned a bit setting mine up.

 

Panic, what is the right math? That's what most people have use since.... um...forever.

 

You must know what the manifold design is.
The formula is only remotely accurate for a single-plane (open plenum), but too large in predicting the primary size and too small for the total size.
A divided single plane (also called 360°) feeds 4 cylinders on 1 bank from 1 side of each carburetor - each cylinder does not get the full CFM, but about 1/2.
180° (dual-plane): about 1/2.
Individual runner: only the attached barrel - 1/8 of the total.
Log manifold, 2 × 2: each cylinder gets 1/2.
Tunnel ram: each cylinder gets the full CFM, but the carbs are sensitive to plenum volume.

Read my article (unless you already read it in Mopar Action): http://victorylibrary.com/mopar/intake-tech.htm