Someone asked me this and I didn't have an answer.So can someone help me with an explanation, theory and or facts? Thanks
Well, I may be missing the point of the question, but more cubic inches equals a larger area that's being mixed with air and fuel which means you need more air and fuel to make things work well. Big engine needs more gas, more gas needs more air for optimum mixture. As I said, I may be missing the point of the question, but if you are talking about compared to say a 300, 350, 400 etc., that's the basics.
Ok, I was having trouble logging in but now I'm back. What I was referring to is the commonly accepted theory that Buicks need more carb than other makes. 455 vs 454 for example. David Frieburger at Hot Rod asked me this and I didn't have an answer so I'd like to be able to get back to him with one. Guess I should email JW directly.
From what I've read, it has to do with the tall intake ports in the 350 and 455heads. Aparentley the engineers designed the heads this way to keep intake velocity up at low RPM thus the reason the 350 and 455 make such torque at low RPM AND still perform well at upper RPM. The 350 and 455 can make the larger CFM carbs work well because of the increased port velocity, whereas other V-8's dont have this design in the heads, so other engines must use smaller CFM carbs to perform well, where the Buick 350 and 455 can use the larger CFM carb for the same displacement. Ever notice the design of the Chevy LS series heads intake ports? Looks familiar huh!! Mark
Thanks for your reply Mark. Sounds reasonable to me. I am surprised that more people haven't weighed in on this. Perhaps like me they don't really know and have just accepted it to be true.
The BBB likes big carbs,I heard it is because of the high volumetric- effeciency of the heads they flow more cfm per cubic inch than other engines,thus resulting in more torque (how else can you get 520 foot pounds at 2800 rmp?)The more flow you have the more torque,the more velocity the more HP. This is just a general rule of thumb but it explaines the need for a bigger carb than normal on a BBB. Just my best guess.
OK, now that I understand the question I can honestly say that I never knew until I read the answer here. Learn something new on this board all the time.
I was always under the assumption that the larger bore and the short stroke of the 455 creates a situation where the motor has less time to pull in a given amount of air. So to remedy this they put a bigger hole on top. A longer stroked motor has more time to pull the same amount of air so a smaller hole is sufficient.
It's not so much the BBB requires more CFM but that it can effectively use more CFM before it negates some low end performance. The large bore to stroke ratio and high velosity ports provides a huge signal at the venture. This differential pressure signal is what makes the jet flow fuel. The differential pressure relates to velosity squared. Rod length also plays a part in this as piston velosity changes is what shapes the impulse. Intake path runner length verses volume drives the signal strength reflection timing. All these effects are adventagous for the BBB when picking maximum effective carb size and booster combos. This is also why we sometimes have trouble tuning a 850 or smaller carbs with annular boosters which amplify the signal providing to much signal thus fuel at maximum signals, (not so for Dominators 2" venturi). Anyway, this is good for us since smaller carbs can limit efficiency and horsepower. This is why buicks make more horsepower with the same build then a pontiac or Olds 455, efficiency at higher RPMs. So I say if you have some compression and a big cam shifting above 6K, throw on a 9375 1050 domi and go racing or cruzin or whatever floats your boat. Your motor can take it! I have 2 750s, 1500 cfm! Instant throttle response from idle to wide open at any motor load with a 4K converter. Mild street - Qjet, holley style 950cfm (dry flow rating) Hot street/strip - 1000cfm (870cfm wet flow) super street/race - 1050-3 quickfuel 4150 or 1050 domi with 3 circuits #9375
This is a great explanation i had a good understanding of it but this makes it much more clear to me thx. Jamie
So if a large bore to stroke ratio (1.105 to 1) is one of the reasons that a BBB can utilize a large carb, how come a SBB can also use a relatively large carb with its small bore to stroke ratio (.987 to 1)?
The 350s have high velosity heads as well but would not use a holley 850. They do nicely with a Qjet spreadbore with tiny 200cfm front with the rear upper air valve not opening until the vacuum overcomes the spring. If you look at the 350 venturi you see that they are double or nested, some early ones may not be. Another difference is the air bypass hole from the big block carb is missing as is the secondary enrichment.
Dave Thanks for the great explanation. Would certainly explain why my mild BBB likes the Holley 950 HP so much more. From idel (4 corner ideling) to ANY blip in the throttle it became a different car
If bore/stroke ratio is a major factor in carb size then a stock 350 sbc should require a larger carb than a sbb, but this is not the case. Sbc ratio is 1.15 to 1 even larger than the bbb.
