i'm building a 400(i think- have't measured the stroke yet) for my friends 1978 F250.. it has headers ad i'm gonna slap on a performer intake with a 780 holley with vacuum secondaries.. my question is this, what's the best torque cam for this motor without going in and changing compression via the heads and pistons? I figure I may as well stab a cam if i'm halfway there for the intake anyways, i just don't know how much this motor likes on stock compression... thanks

 

With stock CR, a Comp 255DEH or XE256 should give you the torque that you want. Use a new double roller timing set, and set the cam timing 'straight up'.

 

great.. thanx!!

 

You will not be happy with that 780 though. Too much for a 400 in your trim. Mine turns 5500 easy with a 600 cfm. Your going to have some over carburetion issues with that.

 

i second the carb being to big i would go with the 600-700cfm range. 600 being more of a mild build and 700 being more of a street strip

 

you guys running vacuum seondaries or mechanical? i run a 600 on my 390 and went to a 780 ad it loved it... they were vacuum though...

 

Vac. 750 on 351m had to jet down 2 sizes, runs good with 9.5to1 CR and 265DEH. You can't stomp it untill it real warm.

 

Soapbox warning

I am firmly of the belief that carburetor tuning especially Holleys is more an art than a science. Now I like Holley carbs and when tuned corredtly they are among the best out there. However I also think there are a whole lot of way over carbed engines running around on the street. Selecting carb size should be a simple matter of plugging your engine size into the magic formula, picking out the carb with the closest CFM and slapping it on the engine, in which case 99% of the 400 engines out there wouldn't need anything over a 650.

But throw in some "emissions tuned" carbs with lean jets and dinky power valves and things start to get a bit dicey. This is true on all engines, not just Fords. Back in my younger days I succumbed to the dark side and raced a Bow tie (gasp). It had a highly modified small block engine which put almost 380 HP to the rear wheels. Now even at 100% VE it shouldn't have needed more than a 650 CFM carb at 6 grand but I can tell you it ran best with a Holley 780 double pumper with mechanical secondaries.

If you go to the BG website they recommend their 725 cfm carb for any street engine over 400 cubes and having just installed a BG Road Demon on my stock '70 429 with 11:1 compression I have to say this is definitely the right size carb for this engine. If you throw that engine into the calculator with an 85% VE and 5500 redline it says all it needs is 580 CFM. Go figure!

So what's it all mean? Will a mildly modified 400 run on a 600 CFM carb? Sure it will and if that carb is properly tuned you'll get every bit of power that engine has to offer. Do most of us take the time to really dial in a carb? No. Does this mean that simply putting on a carb with over 100 CFM more than the engine needs work better? Probably. Will most of us sacrifice idle quality, off idle performance and fuel mileage for an engine that cruises fine and runs like a winged rodent out of hades when you romp on it? Hell yeah!