I have a 750 cfm summit racing street strip carb on a 318 it has swirl port heads weiand stealth dual plane intake pmp performance cam headers hot ignition clutch fan 3500 stall converter shift kit and 355 gears and 9.5 to 1 compression and the carb is a perfect match for the motor it doesn't bog or have a flat spot idles great and performs as good as it gets sometimes bigger is better if everything is matched to work together
If the carb is a vacuum secondary a 750 will work just fine. Simply because the secondaries won't ever fully open and you'd be running off the 375cfm primary side. Sure a 600 or 650 might work marginal better but a 750 will work fine. And just an FYI reducing the jet size will not reduce the cfm. So, it may not even be necessary.
I've run several carbs that guys here are calling "too big" on 289's and 302's. They run just fine and nobody ever called them "boggy or slow" either. Guys that have those types of issues are just looking for simple bolt on parts and don't understand how to tune carbs. Don't mean to be rude or demeaning here.. just what I've learned after dealing with many motors and gearheads most of my life. Sometimes it's not as easy as just changing jets and secondary springs. Ignition tuning also goes hand in hand with fuel curve tuning as well.
'Sorry, but those primary venturis are so big, the motor will be a sluggish dog and will burn tons of fuel. it may also idle rich as well.
We're talking about an engine that cannot flow more than about 535 cfm. I've already had my fun with over-size carburetion. My 302 with a performance cam and 9.5:1 compression is much happier with a 570 to 600 cfm carb. Keep in mind that this was at high altitude t the time, so a 650 to 670 may work well closer to sea-level, but a 750 is just much too big for a street 289/302. Sure, a race-prepped version with big valves that spins 7000+ RPM can probably make good use of a 750.
No race prep.. street racing only. This is a vacuum secondary carb and is very easily calibrated to suit the demands of any size engine. Look up tge venturi size differences betwee. A 600 and 750 holley.. not like comparing a 450 holley to a 1150 dom... boosters pull just fine and throttle can be crisp if you tune everything right. And as I tried to say before.. dual planes are an entirely different animal as well.
so why do they make carbs smaller than a 750? there must be more of a reason than...understanding how to tune carbs.
ignition timing means a lot just watch power nation when they build motors they start with a baseline then start adding timing changing jets etc they usually end up with a lot more torque and horse power than when they started power nation is great on the internet they have a ton of awesome two minute tech tips my 750 carb is vacuum secondaries and it fully opens that's when my wife grabs the dash and starts yelling at me
you really need to do your homework and match parts that are meant to work together talk to experts at each company Summit racing Holley crane cams etc all have tech support they are gear heads too tell them what you are using the car for gear ratio torque converter intake carb headers ignition tire size and diameter fuel pressure timing jet size transmission weight of car all of these factors matter and parts should match for the intended purpose drag racing cruising street strip etc matched parts mean everything it is better to buy what you need sometimes and pay a little more up front than have to buy it later when something doesn't work as well as you thought it would it is easy to over cam over carb if you just throw parts on or in a motor
over cammed over carbed been there done that expensive and disappointing time to quit ranting and go to the garage i hope you have the info you need later
Frank, this is a very confusing subject and highly technical for most that do not even fully understand the basic metering layout of a carb.. not to mention how to even modify the fuel curves created by them. Maybe I was not very clear in my responses but I too would recommend a smaller venturi/bore for a milder application that intends to stay that way over the long term. BUT.. as has been said repeatedly, it's far more involved than just basic airflow requirements via online calculators. There are TONS and TONS of guys running 750's on 302/5.0 combo's and many are street cars. It's a mainstream staple because not everyone uses the calculator and bases their requirement on dry airflow ratings with 1.5" of depression. The more serious guys(NOT just "all-out race cars" either) will often use a well tuned carb that never sees above .5" of depression at WOT/peak power. Gets tough to compare apples with bowling balls sometimes. We quickly get into how many mod's(now AND in the future), what drivetrain combo, driving style(punk kid attitude types bunny hopping their way from every stop light/sign and losing some rubber around every corner?.. or the smug old guy who likes to keep it safe with lower insurance rates but have it on tap when they rarely like to use it?.. and every kind of in between), all play a huge role in what is optimum for any given combo. Not many guys want to, or can afford to buy different carbs just to fully optimize each specific combo of parts through 3 or 4 engine build/"bolt on parts" iterations. Plus, each combo STILL needs to be fully tuned and optimized for each various combo of parts. Luckily a vac sec carb is EXTREMELY forgiving compared to a "too large" double pumper. And larger cfm carbs are even more forgiving on dual plane manifolds.. IF you understand why that occurs. Turns the motor into two 4 cylinders pulling on 1 primary each/only and allows a MUCH larger carb to be run without severe penalty like it would on a big single plane with too big a cam and not enough compression. And don;t just think static compression ratio.. think running or "dynamic compression ratio" as this is what the carb actually sees to create the depression needed to keep the boosters alive. Small venturi/large/r throttle bore creates a stronger booster signal and usually makes for better throttle response, as the venturi gets bigger on a given size throttle bore the booster signal begins to diminish but if you've got enough motor.. and/or gear/rpm.. it's not really an issue. I'll ask a rhetorical question to get the old juices flowing. Anyone here ever run a 2300 series 500 cfm 2 barrel on a little motor like ours? Because if you tuned it correctly, and therefore "the motor liked it".. then you just compared you combo to the OP's 750 cfm carb on the primary side. Same same. For the most part, the larger the difference/relationship in venturi to bore diameter measurement will allow a stronger and more distinct "signal" to be seen by the booster venturi. The less the difference, the total mass airflow will be increased on a dry bench but not necessarily equate to more average power if the carb was not deemed as a restriction in the first place. SO, it is always a balancing act to get it all correct for the application being used and one size does not fit all. Also differences in ENTRY designs(chokes/towers/smoother HP style entries) can account for a large difference in rated airflow and needs to remain the same in order to accurately compare the overall affect seen from any hole size differences. Like I said.. highly technical.. racers or those "1 percenter type guys" looking for that last bit of power/throttle response/edge type stuff. Not trying to come across as holier than thou or a "no-it-all" because I too still have much to learn and too little time left to do it. Just sharing what I've learned so far and hoping to add more perspective, is all. Pretty much everyone in this thread has made valid and useful points that just need to be put into proper perspective.
Like Groberts said, lots a good advise, insight, and experiences to pull from. Thank yall for the feed back. I ordered the intake a couple days ago, fed ex will deliver here shortly. I havent ruled out the Street Demon yet, and a good priced Speed Demon 750 just popped up as well. I will keep the both as options, but right now a coworker offered me a Holley 650 something for basically the fuel money to go get it. I'm sure it needs a rebuild, but a good carb for under $100...too much to pass up. I'm sure if the Holley works out, I'll never look back. I'll still follow the conversation if it continues. This is a topic I'm less experienced with, so I try to pay attention to you guys smarter than me. Thanks again to everyone's inputs. Northern
Apparently you dont understand how a vacuum secondary works. The primary side is roughly the same as the 2v carb. You have to understand ifrom it's to big it won't open the secondaries.
Vacuum secondary has nothing to do with it. A carb that is too big is a carb that is too big, even with a dual plane intake manifold. And a 350cfm 2-barrel will run far differently than a 750 4-barrel with the secondaries not opened. It's not a legitimate comparison at all. The 750 has larger jets and larger discharge boosters. With a 302 run with a 750? Of course, and as I stated earlier, will be sluggish, not "crispy" at all, and a gas-hog. And simply installing smaller jets won't fix the problem. I know how to "tune" a Holley and the 670 I had on there was a pig. This is on a street engine. Above 6000 RPM, the 750 would make more power, but from idle to 6000, a 600cfm carb will run far better. Same thing with a vacuum advance on the distributor. You want that on the street, versus a race motor that is either at idle or at W.O.T. on the track. No in between. Bottom line: component selection should be based on what the intended use of the car is, not what you can great a great price on.
