Carb question
Yes your right but the less you go you trade top end power for throttle response and driveablity, a serious street strip you probably want around 1"Hg and a full race car low as you can go that still works.
Still learning here :) was I wrong or just off? (I know I was just genralizing) Up to this point I've only been a 4bbl,cam and header kind of guy nothing bigger than a 650 carb/340cam, my 273 gonna be my 1st full build.
cubic inch x 2 . if it don't reach the next 50 go down to the smaller carb. for high rpm HP. go to the next larger. 318 +318 = 636 - 36 = 600 cfm. dual plane/ med. cam. single plane with larger cam use 650 cfm. Always worked for me
273;
In an ideal setting ......... 1Hg. is max and zero is right. So to say your "just off" is OK in a good way. Above, I quoted Rick in his method. I also use this method. It is a general way of getting you there. Some will trash it as being to basic and maybe even barbaric LOL! But, it'll work. For a general purpose.
In the 318's math above, this works. Do note you can get away with a 500 cfm carb. It'll also start to pull a few Hg.'s at WOT. This indicates a carb slightly under sized. This is not a bad thing. But a indicator that a larger carb could/can provide more power/performance.
Setting #1;
IF I was building a 318 for mileage with stock manifolds, a 500 could get the nod, I myself would do a 600. It would get the nod for service.
#2. Replacement carb, slight performance enhancement over stock? A 600 would go to work. This carb/engine combo could still be on a stock engine or one with the basic bolt on parts and a cam. Even increased compression.
#3 When the build starts to get hot, a 650 should be employed. Things like a Hyd. cam above 236 @ 050, ported heads, headers, increase stall and 3.91 gears.
These are general settings. Things like increased cam size, manual tranny and higher gear ratios can have the CFM go up 50 cfm. A drag car, a Duster lets say, can use a 750 to 800 on a 318. You'll have to consider that the engine lives on the top end of the RPM scale and should have a weight under 2800 as a min.
What you'll find in your build is once your done. You can try other size carbs and each will have a slightly different effect on, well, basicly everything! Going slightly smaller will increase throttle response and could help mileage. Going larger will decrease throttle response while allowing more top end power. Mileage may drop if you can not tune it well. The bigger the carb (On your mill) the harder it will be to get mileage and a good running engine. The butterflies are to large, open to fast, even if care is taken with the right foot, it becomes difficult.
The signal to the carb weakens and fuel metering become hard to deal with. This is why most times a smaller carb is the one to choose. Better to error on the small side. And yes that means puling a Hg. or two is not the end of the world. Loosing it all together is bad.
You can also couple up the HP level of the engine to a carb size. This also must be done with the intended use of the engine as well as the cars weight, gear ratio and stall, etc.....
The OE Poster said 400 HP. For a 360 small block, a 650 will do for the street. A 750 would be a better choice since the engine is probably one that is a bit cammed with increase compression with the basic bolt on's. Bowl ported heads are possible for this. As stock heads would require a good bit of cam for 400HP.
He wants a cam to cover both bases. Well, a 650 would do it.
I'll bet blindly that the 650 on the 360 will pull 1.3 or better Hg.'s.
I myself would purchase 2 carbs, 1 for each engine. Then, once done with them, sell them, roll the money over into the next build..