Quickfuel Q-Series 750 CFM Drag in a street car. No choke, mechanical secondaries. Started and drove like a dream. Soon to be for sale.
We're just trying to save you the embarrassment of saying that in public, then getting called on it. But congrats on choosing a "better" carb in my opinion over a double pumper for street use. Most double pumpers I've seen are too big for the engine they're sitting on top of.
oh ok that's cool just come off nightshift fuzzy thinking I agree you really can't beat the summit carb for the price 269.00 flow tested right out of the box all the best features of a holley I bolted it on and have never touched it it runs great and everyone needs to save a buck nowadays
Maybe I'm off base here and this forum wouldn't necessarily be the best place for a hardcore thread of this type.. but me thinks it would be very cool to have a "hardcore carb mod's" tech thread. Maybe include slabbed shafts, choke-horn removals, polished bores, booster mod's, air bleed mod's, etc, etc. While it seems that most posting in this thread so far have fairly well sized carbs.. I do know from experience that most guys have absolutely no idea how little airflow 302 cube's really actually needs. Even moderately cammed engines running aftermarket heads lose very little power when completely shutting off the vac secondary's(often less than 5%, and even NONE with 750 cfm carb's). Mechanical secondary setups can typically use adjustable rate secondary linkage's to do similar. Cockpit mounted vacuum gauges will easily tell you where the motor is beginning to starve for air with minor depression buildup(vacuum) at WOT. Which is not as bad as it sounds since carb's will actually flow more air under higher depressions. Which is also why 2bbl class racing can make so much horsepower with 500 cfm rated carbs. They only really flow 500 cfm when the depression actually reaches 3 inches(which is how they were rated from Holley) and many 2bbl racing combo's reach upwards of 9 inches at WOT. 600 horsepower from a 500 Holley is not unheard of. So, in essence.. that Holley 750 that we're often so proud of displaying only actually flows 750 cfm when the engine is producing 1.5 inches of vacuum at WOT(because they're rated at 1.5 inches by Holley). And a 302 CID engine with gigantically sized/full race intake ports that allow VE's to reach 100% or more would only reach that airflow requirement when spinning around 8,600 rpm. Short of full race.. and annular booster setups requiring larger bores.. I almost always prefer to stay with smaller venturi's and work the carbs main body to improve the flow curve while also extending potential airflow into the higher rpm ranges. This is because it more evenly leverages all 4 barrels(reduces variation in depression between them) for improved mixture distribution, especially at WOT, and provides broader torque curves as an added bonus. Oops.. sorry I ran this so long. I was multitasking too much as usual and kept adding more info to it as time went along. Just an idea I thought I'd throw out there and got carried away on the "what for's?"
MY 750 is not on a 302 it is on a stroked 318 on my 302 I am going to run a 600 cfm carb bigger is not always better and hey a hardcore carb thread would be cool I would like to hear all the modifications every one is doing to there carbs
Most 302's would be fine with a 600cfm carb. That is actually on the big side except at the upper ranges of RPM. Vacuum secondaries should be on the street, not double pumpers. When buying a carb, you need to be TOTALLY HONEST with it's intended usage. A drag strip is a very different place from a street. We changed our dirt late model carbs depending on the track and surface. Anything from a 390 to 980 cfm carb on those engines depending on amount of bite, track shape and track condition. One size does not fit all. To properly suggest a carb, other than it's physical appearance, is impossible without knowing it's true intended use. You also need to remember that a 2bbl is rated at more vacuum than a 4bbl is so that 2bbl is actually not what you think it is by just reading the cfm rating. SPark
Well, the OP didn't ask for a bunch of carb theory, just asked what people were running. Most of the carb size formulas I've seen are very conservative and the depressions that carbs are tested at have almost nothing to do with what is encountered in a real, running engine. It's always funny how different carb manufacturers can have the same flow rating but in the real world they're often different. As far as cfm needed, more information would be needed about the engine and vehicle. As an example, a dual-plane intake manifold will allow you to run a much larger carb than a single-plane since in a dual-plane each cylinder only "sees" half of the carb.
Groberts touched on this too. What any carb actually flows in operation depends on the engine it's sitting on top of. The advertised cfm rating is only to be used as a guide in picking a carb. And for those who have no experience with carbs, there is one rule that should be followed in picking a carb: bigger is not always better.
Bigger is better when smaller doesn't work for that engine. I started smaller, and the engine told me it wanted bigger. Trust me, the last thing I wanted to do was fork out $600 on a new carb. You all know how cheap I am. But short of pulling out drill bits and gouging out the other carb(s) which I am not comfortable with, and have only a dangerous amount of knowledge about, newer and bigger was what I needed at the time.
that's what I am running weiand dual plane pretty radical cam with a lot of exhaust late model swirl port heads headers high flow mechanical fuel pump matched to carb hot ignition clutch fan 3500 rpm stall shift kit and 355 gears everything is matched for each other starting with the cam and if its in a heavy vehicle like my truck vacuum secondaries are recommended plus you should take tire size into consideration too I should really have 410 gears with my setup but I like to go places on the interstate
Actually this one question has taught me a thing or two! I was pretty much trying to find out what would be the best carb/intake combo to use if I was to replace mine.
Street carbs w/ vac secondaries for small mtrs should probably stay under 700cfm.Drag mtrs 351 & up,750cfm or larger depending on comp ratio & desired rpm.The 2 best carbs I see at the trk right now are the Mighty Demon,& the Pro Systems SV1.If you can afford the SV1,it makes BIG HP.I`ve seen several 400ci windsors using it turning 8 sec 1/4 mi times.
I was primarily setting up the reasoning behind my specific question.. and I simply took the OP's question and ran in the direction it seemed to be pointing towards in the first place. Why else would anyone ask such questions, right? And it appears that I may have been right for a change. As for the formula's being conservative? Sure.. if they aren't really taking calculated or specific VE into the equation. But the more advanced versions used in engine simulators are much more accurate since the various engine parameters are entered by the software operator. Typically, I find them very close to reality on most occasions. The most accurate estimations will use the cars actual ET's.. or horsepower figures from chassis/engine dyno's to verify any combo's true airflow requirement. It takes air to make horsepower and one can simply measure the air required to reach a specific number. Or just measure the air going in.. and the rest falls into place from a carb sizing requirement standpoint. And just because a dual-plane "will allow you to run a much larger carb than a single-plane".. certainly doesn't mean that it's correct for the application. Just that a dual-plane buffers and tolerates larger mistakes in optimized carb sizing, is all. In the end.. the carb is simply a fuel metering device that should be designed around and matched to the specific airflow requirement of each parts combination. Unfortunately.. most think of it in terms of absolute airflow potential(CFM) rather than the reality of it being more about correct fuel metering over a given airflow range/rpm. Fortunately for most who go slightly.. or moderately overboard.. they're very forgiving since they are "on demand" type devices. In fact.. an overly large Dominator can be made to run fairly decent on a very small motor if correctly recalibrated to allow for the appropriate fuel curve that matches much lower airflow through the engine. It'll just have smaller sweet spot and be somewhat tougher to calibrate over wider rpm ranges as you must band-aid tune it(secondary opening rate, jets, emulsions, air bleeds, etc) to compensate for the poor signaling when installed on such a small motor.
