8ba 275 carb options
 

Hey guys, I recently picked up a rebuilt a rebuilt 8ba, the guy I got it from didn't know much about (he had it listed as a 239) from what I’ve found it was done in the 80’s and it’s been sitting since, I found it has the 4” crank, bored .030 over, came with the offenhauser tri power intake and old offenhauser heads stamped 325, according to an old offenhauser chart that I found it’s now a 275 and with those heads it’s 11.3:1 compression, I also picked up a super dual intake as an option
My question is can I run the single carb intake on this engine since the stock 255 was ran with the the single 94, and can I run 87 octane fuel with these head or do I need 91?, I also have a set of the aluminum Ford 8ba heads I was considering running.
I do drive the car quite frequently pretty much year round so reliability and economy is more of a concern to me over speed(the first year I ran the car with a worn out 239 and this past year I’ve been running with another engine with incorrectly adjusted valves and a misfire)

Re: 8ba 275 carb options
 

For a daily driver, I would use the 94 and the Ford heads.

Re: 8ba 275 carb options
 

In fact the 8cm or EAC Merc heads will help lower compression slightly over 8ba's or EAB heads. All late '49+ heads will ship by pair in a large Flat rate USPS box, reinforced inside, 45lbs on a limit of 70lbs. Postal rate increase about Jan 22 2023. Newc

Re: 8ba 275 carb options
 

That piston reads 3 5/16 and .030. Would that be .030 over 3 5/16?

Re: 8ba 275 carb options
 

I hate to sound like a broken record, but the best carburetion for the engine you have and the way you drive is a small base Rochester 2G from a 265 or 283 SBC on a bored out Mercury 4-bolt manifold. The small base 2G flows about as much as 2 94's or 97's and is a lot easier to tune. You will have to change the distributor to use one though.

Also, I wouldn't put a lot of faith in the CR numbers from the manufacturers charts. There are too many factors affecting the final CR. The heads should also be checked and modified for optimum efficiency (increasing both performance and economy). The camshaft can also effect the dynamic compression ratio; do you know what's in your engine?

Re: 8ba 275 carb options
 

It sure looks like it!

Re: 8ba 275 carb options
 

2Xon the 2G, works great with chev dist

Re: 8ba 275 carb options
 

With that bore (3.3425") and the 4" crank you have 280.8 cubic inches. As for the heads, no way is it even close to 11:1 compression...maybe between 8 and 9:1. I also agree with the 2G Rochester setup, but to answer your question... yes the single 94 will work and will have great power down low but limit top end.

Re: 8ba 275 carb options
 

I’ve got a bunch of stock cast iron heads, if these offenhauser heads don’t have too high compression then I’ll run them, I wanted aftermarket ones for better cooling in the summer anyway, my other concern about the higher compression is the starter being able to keep up
The aluminum 8ba heads I have are from early 49 from what I’ve researched, same dimensions as the regular 8ba heads just made from aluminum( in Canada) due to an iron shortage supposedly?

Re: 8ba 275 carb options
 

So there shouldn’t be any fueling issues or having it run too lean with the single 94? That’s what some people have told me, as a daily could those offenhauser heads still work?

Re: 8ba 275 carb options
 

Because of the very design of the transfer area in a flathead engine, it just about impossible to end-up with TOO HIGH of a compression ratio.

Sounds like you need to steer clear of all the flathead experts you have been talking to up until now.

Coop

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Re: 8ba 275 carb options
 

Yeah, ideally I’d run the 4bbl but I haven’t been able to find one yet, there is one on rock auto last time we looked for out 55 bel air but then it’s another 800 or so and I have a surplus of 94’s especially since the tri power came all set up with three other than the fuel lines, the distributer is planned, 12 volt is first though and I needed to buy a 12v solenoid for the overdrive, how do you modify the offenhauser heads for efficiency? I have a hunch it’s a stock cam due to the high compression heads so there can’t be much lift, it’s another thing I need to check, I can just see how much a valve moves off it’s seat? The guy who built the engine just stuck a stock distributer in with the tri power which seamed a little odd unless he changed out the springs?

Re: 8ba 275 carb options
 

The guy who said it would be too lean with the single carb doesn’t know flatheads… he’s a newer Chevy guy, the one flathead guy that i spoke to whole builds them said it might be a little tough to get enough fuel and air into it with a single
I also considered relieving and porting but that would involve completely disassembling and not sure it would be worth the gain

Re: 8ba 275 carb options
 

So it should cause any damage running a single 94 for now? Part of why I was going to keep it on a single with this engine was for break in and get everything set up to run multiple or 4bbl
And then my question with the heads is having to run higher octane or not

Re: 8ba 275 carb options
 

Just so you know, a 2G is a two-barrel carburetor used on the early SBC's and other GM V8's. They come in two basic varieties; small base and large base. The mounting bolt patterns are different. The small base variety will bolt to a stock '49-'53 Mercury manifold which is a a bolt on for a Ford. They are quite a bit cheaper and less complicated than a 4 barrel. The throttle bores are bigger than the Mercury manifold and it should be bored out to match the carburetor

Any heads you use. whether stock or aftermarket, should be checked for fit. Basically you want to get a uniform clearance of .045" to 050" over the piston crown. There are several threads on here and the H.A.M.B. that detail the process. If you have the manifold off. check for the presence of adjustable lifters. If present, chances are you have an after market cam.

The 8BA series of flatheads used a different advance system than other cars. It is called a Load-a-Matic system. It is vacuum advance only with no mechanical (centrifugal) advance. It uses a vacuum signal taken of of a port on the middle back of the carb. Using regular manifold vacuum (or even ported vacuum) will cause the distributor's advance system to function improperly. The solution is to use either an old Mallory dual point distributor or have a SBC distributor converted to fit your engine. Since the springs inside the distributor only function to control the special vacuum signal, changing them won't hep.

Re: 8ba 275 carb options
 

No, it won't cause any damage. Actually it's a good plan to break it in and make sure all is well before modifying the fuel system. My bet is that you'll be fine on regular fuel. The compression is just not going to be that high.

More important will be ensuring that the radiator is absolutely clean, correct and in good condition.

Re: 8ba 275 carb options
 

Right, I should have payed more attention, I thought you were referring to the small 4bbl that came on the Chevy 265, as for lifters yes it has adjustables, is there a way to see what cam is has without pulling it out?
There a bigger 2bbl carb mounted on an intake in ns, it looks like it’s a 4 volt base

Re: 8ba 275 carb options
 

Fine on regular with the 325 heads?
I’ve flushed the rad out several times the last time was with vinegar, it is the correct one for a shoebox

Re: 8ba 275 carb options
 

FYI, I have a 67 327 300 hp SBC with a wcfb that the primaries are 15/16 the same size as a 94

Re: 8ba 275 carb options
 

Pretty good chance it'll be fine with the 325 heads. Without actually checking the cc's of the chambers you can't know exactly how they compare with stock heads, but it's VERY difficult to get even moderately high compression on a flathead without a lot of work and modification.


UPDATE! I found the chart listing the heads. VERY small chambers that may well do just as you said...too much compression for pump gas. I apologize for the misinformation...I didn't know those heads existed.
Hopefully Pete may offer an opinion...he is a good source of info on this type of situation.


Terry