Carb questions?, Will idf 40's keep up? Options

[914_classic]

I'm starting a 2.0 build and I have set of idf 40's but I don't know if they'll keep up. It's going to have the biggest cylinders and pistons I can buy in it with a knife edged and balanced crank, port and polished heads, big cam... Blah blah blah. I have a set of IDF 40s and I am wondering if they will be able to keep up. Any help would be appreciated.

[Jake Raby]

If you are looking for hi RPM then high volume will be necessary. The 28 vented 40mm IDF is barely good for a 2 liter engine. Going up to a 32mm vent makes it good for a 2056 that will turn 6.250K RPM. Going to a 34mm will give you a solid 300 more RPM. After the 34mm venturi the carb loses it's ability to atomize and the circuitry gets all screwey and it won't tune at the transition points.

I have built race engines that HAD to use a 40mm carb and could use up to a 38mm venturi, BUT I found that the engine made more power, tuned easier and was faster with a 36mm venturi. One of these engines would turn 9K, but was only 1832ccs.

There may be some formulas for calculating this; however I have yet to find one that was worth wasting the time on. In general, if you are looking for Hi RPM performance just start with a 44 IDF and the carb flows 290 CFM/ barrel right out of the box Vs. the 40 IDF @ 212 CFM. The 44 will tune easier, run better and can retain a smaller venturi/throat ratio to net better mixture quality and progression between the circuits. Making the smaller carb flow more requires enlarging the venturi thus reducing the air speed that breeds atomization..
If you have to run 40s for some reason, I have a set of race prepped Solex 40 P11s that are already maximized for hi revs and they are for sale.

Through my development I have found that a pair of 40s are maxxed out at 2143cc, the 44s take us from a 2165 clear to a 2.7 liter engine, then we use 48s from that point on up. My record for a 40mm carbed engine is 207HP and 8,200 RPM. Record for a 44 IDF is 2666cc @ 221 HP and for a 48 is 284 HP from a 2.9 liter twin plug daily driver. All but the 2 liter was running pump gas, not race gas.

Bigger is not better, BUT if you plan on heads flowing more than 190 CFM, a cam with more than 290 duration@ .020 and more lift than .480 on a displacement of greater than 2100ccs you will be better suited with a 44 IDF.