That is a big carb.

What model is that super charger, AMR500? Japan made.

Nice little units. Mini-charger?

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).

While much of what Johny has said is quite true, I'm going to disagree with his comment about the benifit if a dashpot type carb on a drawthrough setup. There are doubtlessly some benifits to be had on a drawthrough turbo set up, (I.E. keeping the off-boost manifold vacuum high to maintain a strong signal to the venturi, and sustain crisp trottle response) but these benifits aren't so critical on a supercharged engine. When you open the throttle on a blown engine, the blower instantly begins pulling air. And can reach maximum boost in the time it takes for the pedal to reach the floor. This gives superb throttle response, and means that greater gains can be acheived by having as little restriction to the blower intake as possible when the throttle blades of the carb are at WTO. On a N/A motor, there will ussually be very little or no manifold vacuum at WTO. But the space between the blower and the throttle blades on a supercharged engine can register enough vacuum to prevent the powervalve from opening. (On a holley) Hence this is why externally referencing the powervalve is a standard modifiaction to a holley when fitting to a blower. (I did that to mine)
Also, another pitfall with the holley's on an in-line engine is accelerator pump discharge nozzle dribble. The holley's use a small needle as a checkvalve on their accelerator pump. On an inline engine the motion of the engine rocking, (Especially with a lumpy cam) causes this small needle (which is only held in place by it's own weight, 1-2 grams) to unseat. This leads to the airspeed through the throats of the carbs, to syphon fuel into the barrles though the accelerator pump discharge nozzles. This problem even exists when the choke housing has been milled off. I emailed Holley to ask for advise, and they put my onot a bloke named Scotty Mclendon, of Mclendon Carburator. he sold me a kit consisting of a new check needle, and a spring including a few shims to set the amount of preload on the needle. Since the accelerator pump actuator is spring loaded, if the sping presssure on the check needle is too great, the spring on the actuator arm can't over come it, and a hole in the fuel delivery results. To be 100% honest, I still have a whisker too much prelaod on my check needle, and there is sometimes a flat spot. (if you look at my dyno sheet in my album, you'll see the dip in the curve right at the start.)

fwiw the externally referenced powervalve was a necessity on drawthrough turbo setups with holleys - at least those I played with. They were an interesting scenario - the powervalve would open when the acclerator was floored, and it'd 'work' for a moment, then as the turbo spooled up and the onset/increase in boost occured, the vac between carb and turbo would increase and pull the darn thing shut again :)

With respect to the acc pump dribble, there are alternate acc pump squirter 'block' designs that can assist there simply by creating a 'dead' or low airflow area just outside the squirter discharge point. The phenomenon also occurs when a 2 barrel is put onto an engine that is way to large for it (which is the case for certain race categories in the US to restrict output, not that I know which categories they specifically are!!) - they setup a bleed on the back of the metering block in the acc pump circuit to help prevent it. That means (although it's not cheap) you can actually source an off the shelf 2 barrel that is prepped to prevent that, or alternately you could modify an existing metering block (though to be honest it's not actually something I've had need to do).

I can absolutely see where you are coming from with respect to the fact that superchargers are instant flow/boost and the rise of a piston on an SU type carb isn't (and shouldn't be, as it will actually dull throttle response, as it leans out momentarily if it opens too fast) instantaneous. No argument there, but I would add that at part throttle, once you are actually operating on the main metering circuit of a fixed venturi carb, it's venturi cross section and booster size/design that determine signal strength and mixture delivery/consistency, and having enough carb to supply the peak rpm/throttle needs will still result in (arguably) less than ideal signal strength at part throttle. Without a doubt the throttle response and output will be good for full power/competition stuff, but it can (and of course it depends on just how big the carb is relative to the application) be less ideal for part throttle.

Having said all that, it's pretty obvious that there's plenty of drawthrough (and blowthrough) setups out there that use either of the types of carbs being discussed, and the result is great either way provided the carb is set up to suit the application!