I can tell you categorically it's not too big - it's adequate for a blowthrough (I've run a 350 on a high boost 3k corolla, and they don't breathe as well, and frankly it had more low-mid range with the holley 350 (with wastegate actuator disconnected so the flap was open and it got no boost, for preliminary carb setup) than with the stock carb. I'd _seriously_ go as far as saying it could do with a 500 on a 1.5 in blowthrough.

when you blow through the carb, you basically need around the same size carb as would feed the engine non turbo'd at the same general rpm range. The volumetric efficiency 'technically' stays about the same (it does actually improve but that is of lesser importance carb size wise), it's just that the air being breathed in is far denser.

You'll often find that there's 'guidelines' for carb sizing, and most of them are deliberately _very_ conservative. The thinking is (presumably) that anyone really needing the chart/guideline is a beginner (and there's nothing wrong with that, everyone starts at that point) and so it's better to have a carb that is smaller than ideal, it'll give good signal strength and good fuel metering and not require anywhere near the same attention to get a decent (albeit not 100% optimal) output. It's also because mid range torque actually means more than peak power for 90% of the options.


The thing is , however, even if we work out the airflow of the engine based on rpm and volumetric efficiency, we haven't got 'apples to apples' - you see carbs are flow tested at a particular pressure drop (vacuum level, call it what you will) - the tradition for the 'big guns' is 1.5"Hg for 4 barrel carbs, and 3.0"Hg for 2 barrel carbs. Due to the way flow testing works, although it's double the pressure drop for 2 barrels, it won't produce twice the cfm.

If you use relevant (and it's still only a close approximation, actual on bench testing always has idiosynchrasies which mean they are never completely even stevens with the math) conversion factors - well the 500 holley 2 barrel only actually flows around 350cfm on the 4 barrel scale. And the 350 holley flows around 250 cfm on the 4 barrel scale (I've rounded them out to the nearest 10cfm)

SO the 500 holley is only really a 350, and the 350 only a 250. So now if we do the math based on engine size, rpm and volumetric efficiency we have apples to apples right? Unfortunately wrong :) . It gets even harder. 250cfm is what the 350 flows at around 1.5"Hg. Now if you actually had that much vacuum in the manifold below the carb when around peak torque and peak hp rpms, the carb itself would be a restriction, and costing you potentially about 5% potential output. For optimal power, you need teh carb to be as little of a restriction as possible. The problem is that in order to get fuel flowing, you need _some_ pressure drop across the venturi section of the carb - manipulating air speed/pressure to develop the necessary incentive for fuel to be pushed through the circuits (by atmospheric pressure acting on it in the fuel bowl) and into the carb. For a race engine, that's probably as low as 0.5-0.6"Hg . On a streeter, you'd still want a little more wiggle room and it'd be hard to get below 1.0"Hg without a fuel metering issue somewhere (likely just beyond idle, as the main circuits are trying to come on line, which is roughly suburban street cruising speed, or 'legal' highway cruising.

All well and good, but there is still more. What should become apparent is the 350 holley (250cfm on 4 barrel scale) will only flow that much at those pressure drops.. When actually on the engine, and sized to suit it properly, the vac in the inlet at full throttle is lower. In very broad terms, an 800 holley might only 'actually' flow 650cfm when put on an actual engine that can utilise it. So if you go by that, the humble 350 cfm 2 barrel holley probably only flows 200cfm whilst actually 'on' an engine. If you plug in the various formula and they indicate a need for 350cfm, you're actually technically not giving it what it needs by fitting a 350 holley!!! You'd actually be still a touch shy of optimal with a 500cfm 2 barrel.

Although yours is drawthrough, YET another problem with fitting a too small carb to a blowthrough, is that if the carb is acting as an unnecessary restriction, to get the same amount of boost actually through it and into the intake manifold, takes more force. Meaning you get losses from exhaust pumping (turbo) or extra hp to spin the blower) - and EVEN WORSE, aside from the power being robbed, to push it through that restriction means it creates more heat, so the intake charge (by the time it's in the intake manifold) is hotter and closer to detonation. So less power AND closer to detonation.

The same thing applies with a drawthrough, the turbo or blower has to work harder to try and get the air in through the carb, but it's technically a little less severe as far as heating the charge, since if it can't get the air through the carb, it hasn't got to try and compress it against the restriction. Still an issue, but not as bad as it sounds.

I could probably have just said 'look I've done engines this size, and in general have a lot of drawthrough and blowthrough carby experience' but the 'whys' behind it all are at least as important as the end decision all by itself.

Without a doubt you'll get a decent result with a 350 holley, boost is a wonderful thing, but in a drawthrough, I'd make a strong case for the 500 holley. I'm not sure about the specs for that particular blower, but I'd hazard a guess it can be spun faster than the somewhat popular toyota blowers (even if it's the same manufacturer, and I don't know that it is) but it still is likely on the small side itself. Useful for sure, but you might come to the flow/limits of the blower earlier, which means that a slightly smaller carb won't be restricting the potential output as much because the blower itself couldn't pump a hell of a lot more.

Now having said all that, there is still always going to be such a thing as 'too big' and going to that size won't give you more power, it'll just kill off mid range, and so forth.

Last of all - it's also one of the big reasons SU type carbs were so popular in drawthrough turbo setups. you need a 'big' carb to cater to full rpm/boost (which effectively is equivalent to putting a much larger non turbo engine underneath this su carb) but needs ok idle, and low-mid part throttle drivability. The SU has a piston internally, that is spring loaded and lifts out of the way of the airflow, as airflow increases, but drops back down lower at low flow, this effectively means it is an infinitely variable sized carb, always changing to meet the required airflow, which means it always has decent signal strength and fuel metering. Of course arriving at the right spring/needle combination for an SU takes time (but you can diy it with a file - more details on request).