I hear you about making life simpler when it comes to tuning fuel curves but the funny thing is.. that 1.75" throttle bore sizing is right along the same lines as a 750-850 cfm Holley 4bbl. Make no mistake about it that the booster design of that unit is still akin to a good old fashioned carb booster and they can be made big enough to hurt throttle response and torque spreads on a smaller lower powered motor. Sooo.. here we've come full circle to the major importance of getting the carbs calibration(fuel curve) closer to right vs worrying only about venturi/throttle bore sizing alone. It's much more about the overall fuel curve vs load vs engine acceleration rate. So IMO, all you really accomplished there after spending $2,500 bucks on that particular system was to get rid of the hand tools and move to the computer/handheld controller interface style tuning. Of course the laptop interface can help offset some of that initial investment cost when it comes to dyno.. or even street and driveability.. tuning simply because it's faster to make changes and you can hit your tune quicker with fewer pulls to get there. Dyno time is money. Self learning features are nice too if you know what AFR makes peak torque and peak power for any given combo. Using cookie cutter numbers of "what's usually the best AFR" will only get you close to the bullseye.
You're hilarious, assuming that I don't know how to "tune" a carb, and essentially claiming that the vacuum secondaries are the only factor involving a carb's CFM rating. If it was simply a matter of vacuum secondaries, then a 750 CFM carb would not have larger primary jets and venturis than a 600 CFM carb (but it does). It's the PRIMARIES that aren't going to run worth a damn if the carb is too big for the engine, and there is zero value in bolting on a carb that's too big for the engine and then essentially disabling the secondaries so it runs like a 2-barrel with primary venturis that are too big. It would run worse than the stock 2-barrel carb that these 302s came with from the factory. They will be sluggish, and consume more fuel, so you're making less power and burning more fuel to do it. I am very familiar with Holley carbs, and I know each and every major component that they consist of. If your idea of "tuning" is basically redesigning the entire carb, then 1) you have a misconception of what the definition of "tuning" is, and 2) doing so would be a huge waste of time and money versus getting the correct size carb in the first place. You can try to spin it all you want, but there is no way an engine that can't flow more than 535 cfm is going to make efficient use of a 750 or larger carb. (The O.P. was talking about Holey and Demon, not Q-jets which have small primaries and would run & drive fine but foul the plugs and run slow with the secondaries open). The O.P. was also not talking about a 7500 or 8000 RPM race-prepped SBF. Let's compare apples-to-apples and quit with the "bigger is better" mentality. A 600 CFM carb is already bigger than what a stock or mildly modified street SBF can flow, but not big enough to be a detriment. Under 65oo RPM, a bigger carb will hurt performance more than help. The bigger carb makes more HP at W.O.T. on a dyno from 6500 to 8000 (if the valves don't float first). That does no one any good on the typical 302 in a street-driven Maverick/Comet. With engines that spend most of their time at W.O.T. (race cars) driveability is not a concern, but for the rest of the automotive population, it's not practical.
The OP said "Street Demon" which has the same tiny primaries as a Q-jet and even has the triple booster setup just like a Q-jet. And no, the 800 cfm Q-jet on my 289 did not foul plugs and it definitely wasn't slow. You keep arguing your point but you don't give any examples of personal experience, just repeating what you've read or heard. I've given examples of my own and others personal experiences and you still want to argue. Maybe you have tried a "too large" carb and it didn't work for you. That doesn't mean someone else isn't capable of doing what you couldn't.
I thought I gave an example but maybe I didn't. I had a 670 Holley on my car and the motor did not like it at all. I've had a Holley 600, Holley 570, and Edelbrock 600 on it and it liked all those. This is on a 9.5:1 302 with a performance cam. A stocker would have been even worse.
I have a 650 hp double pumper on my sbf and it runs great also had one on my Comet and also ran great my engine guy said it would handle a 750 but I already had a 650 at home.I've never had problems?
A 650 is not huge, but "runs great" compared to what?? I'm not saying that to be derogatory, I'm genuinely curious. When someone (not necessarily you) says that the big carb on their mild smallblock "runs great", how much better would it run with a proper sized carb? And now that I think of it, I should also add that my carb experience was at high altitude where there's "no air". Perhaps that was the reason my motor likes a 570-600 better than the 670, as a 670 is certainly not a huge carb. Perhaps you guys near sea-level can gt away with running bigger carbs. Just a thought -- I admit I don't have all the facts on that one.
I can believe that. So many people have problems with the 670s not running right. A friend of mine is dealing with one right now. Had it on a dyno two weeks ago and the AFRs are basically backwards, very rich idle and cruise and power are lean. He just picked up a Quick Fuel metering block with replaceable IFR and PVCR jets to try and sort it out. One pro carb builder has stated that those carbs are so screwed up he thinks Holley should give everyone who bought one a free replacement primary metering block. Could be the problem you encountered was just a bad carb and not that it was too big.
I don't know what the heck has happened to Holley but there carbs are not the same take it out of the box and run it piece they once were. I bought two new carbs this summer and both carbs needed a rebuild and retune before they would work! One Carb was a 670 the other was a basic cheepo 1850 600 cfm! the 1850 needed a rebuild to make the engine run! I ended up using a metering block from old 3310 750 I had to fix the 670!
Wow, so maybe it's a Holley thing. I can tell you that my 570 never gave me any problems. I just had to change the #31 pump nozzle to a #28 to make it more smooth & crispy. The 670 had a bent kickdown bracket and bent accelerator pump arm right out of the box. This was a brand-new carb from Summit. Rather than return it, I just straightened the kick-down bracket and put a new pump arm on it. It also benefited from a #28 pump nozzle. Despite all that, the motor ran sluggish and consumed a lot more gas. The exhaust would stink something awful at W.O.T. (an acquaintance who was driving behind me informed me of this). I still maintain that a 600 (to maybe 650) is just about ideal for a stock to mildly modified 289/302. But if others are having good lick with bigger carbs, no sense in fixing what isn't broken. Run what works for you.
I have to agree with you Larry a 600 cfm carb is plenty carb for a mild small block engine It may be possible to see a power increase at wide open throttle but a larger carb will hurt drivability over all! If you were going drag racing it may help but on the street it will just burn more gas!
"tuning" a carb means you just adjust the idle speed and mixture, maybe change the jets or the power valve or squirters and that's it. There's way more involved than that. Once you start playing with idle and high speed air bleeds, idle fuel restrictors, power valve channel restrictors, emulsion tubes, etc. you can make a pretty wide range of carbs run on even a small engine. instead of learning how to do all this, why not buy the right size carb. to start with...
This is entirely untrue. Saying that using any carb on a 302 cubic inch engine having larger than the 600's primary venturi size of a MEASLY 1.25" is "too big and hurts driveability and mileage" shows how misguided some people reading this thread really are when it comes to using cfm ratings alone for choosing the size carb for their application. I can absolutely GAURANTEEE you that I, and countless thousands of others who can actually do more than just bolt something on and twist mixture screws can.. and have.. tuned, tailored, or modified.. or whatever nomenclature you want to use for the required fuel curve tuning mod's required.. to use a 2300 series 2bbl 500 cfm carb on these little engines. And that carb is half of an 850 cfm Holley when you compare the venturi and throttle bore sizing. Hell.. some guys cheating rule restrictions actually bore them out and fiddle with acid dips to cheat rule restrictions in some forms of racing. As has been said repeatedly, when you have the proper compression ratio, camshaft duration, and gear ratio in a lightweight car?.. you can go MUCH larger than these cookie cutter recommendations. Did you guys not watch the video of that "little 289 with the 950 carb" I posted earlier in this thread? I see similar overachiever combo's running around the streets and car shows every single year. Not all are big inch strokers yet they push well past the typical boundaries for what is considered to be optimum sizing. They are not turds and cruise around with only a barely cracked throttle to get more than decent gas mileage. Because a carburetor is far more than just the main body sizing. So, here it is again.. http://www.lbfun.com/warehouse/tech_info/fuel & intake/CarbList.pdf Now look through that sheer volume of part # listings and tell me that ANY 600 bolts onto ANY 302 and acts exactly the same as ANY other 600. And that my friend is why there is an entire industry aimed at guys who don't want to or simply can't understand why their car doesn't run perfectly and make the exact same power as another similar combo using the same carb sizing. There are literally thousands of potential combos that can be made out of the same casting/main body when various circuits, metering blocks(plates)emulsions, orifices, bleeds, jets, and about 10 other variables that will affect the overall fuel curve of any carb regardless of its main body sizing. Knowledge is free IF you're open to learning, AFR gauges have dropped dramatically in pricing, and reading spark plugs, although technical as it may seem, isn't exactly rocket science. Just tossing money at EFI doesn't get rid of the same basic requiremental fueling issues in correctly tailoring an engines optimum fuel curve for any given combo. It only changes the toolset required to get you to the same exact place in the end. So, while it might not seem very tech savvy to be using epoxies/brazing and pin vices to adjust fuel curves when compared to a laptop of fancy handheld tuner.. it's been getting the job done and trouncing high dollar EFI setups for many years now with no end in sight. Well, at least until someone comes up with newly revamped cylinder heads and EFI setups which can retrofit direct injection onto these old engines.
I would also like to add that saying a 750 vac sec carb is entirely too big and won't work for these motors.. would be like saying that a 650 double pumper is massively too large a carb size for these motors too. Especially when you consider that the ONLY airflow modulation available on a double pumper is the drivers right foot. But do we constantly hear that a 650 double pumper is an absolute turd on a Ford 302 and delivers horrid fuel mileage when it's being driven more civilly? The reality is that a 650 double pumper can work just fine on these motors when the compression, cam, gear, and vehicle weight is well matched. Stomp the gas and beautiful things can suddenly happen without it feeling like a big ole' turd off the line. Same goes for a big "humongous" 750 cfm vac sec as it will flow substantially less air than the above mentioned 650 DP until the rpm rises and the secondaries begin to add to the equation. So, by the naysayers logic of the 750 VS having way too big a venturi and throttle bore?.. the 650DP is even larger when it comes to initial throttle tip in airflow capabilities.
Yes a 650 double pumper works great on sbf and I also get great gas mileage, you can drive by just slightly opening the throttle and it gets up to speed with ease and is very responsive.
