I have a L6 250 Chevy Nova. I plan on replacing the headers and intake manifold, but when it comes to carburetors I am some what not sure. Clifford performance recommends a 390 Holley carb. Would the following carb work, if not could anyone give me a suggestion. thanks.
Well, here’s the thing: you’re gonna have one helluva time finding a four barrel manifold for a street use on a six cylinder engine. Here’s why.
Your 250 cubic inch engine takes in a maximum of 250 cubic inches of air per 2 revolutions. There are 12 times 12 times 12 cubic inches in a cubic foot. You can fairly assume your engine to be about 90 percent efficient, meaning it will really only take in 90% of that 250 cubic inches. That gives you the following formula:
(250 c.i) (1 cubic foot) (x revolutions)
--------------------------------------------------- X (.90) = 500 CFM
(2 rev) (12x12x12 c.i) (1 minute)
that’s
(125 x revs)
------------------ X .90 = 500 CFM . Solving for x gives you
1728
500 x 1728
------------------ = x revolutions = 7680 revolutions.
125 x .90
All this means that you’re going to have to spin that 6 cylinder crankshaft to almost 7700 RPM to effectively need a 500 CFM carburetor. That’ll never happen. All the other times, when that little motor is cruising around 2 or 3 thousand RPM that carburetor will be killing intake manifold vacuum when you hit the throttle because it’s so big.
The bottom line is this. That carburetor is WAY too big for that engine. the 390 is FAR better. Do the math yourself. A little smaller would probably serve you even better, in terms of throttle response.
Look to history for your answer, What carb set-up did Chevrolet use when the Corvette came equipped with a 6 cyl?
Try your best to duplicate this set-up.
I hope that price doesn’t get in the way.
Reasearch what BMW did when it equipped it’s inline sixes with carbs.
Looks too big - why don’t you want to go with Clifford’s recommendation? They are the experts on this, they’ve been doing it for a long time. I’d go with exactly what they recommend, even if it’s a few bucks more.
My answer is the same as before; I still think you’re trying to overcarburate this thing. A 450 CFM carb will happily feed a a mildly modified 300+ cubic inch engine so a 390 is plenty big enough for a stock 6.
Will it work as a bolt-on? Who knows. You’re missing a valuable piece of your scenario here. You mention headers, carburetor, and intake manifold but there’s a key link in all of this to make it work right and that’s the camshaft profile.
There’s no way of knowing what will or will not work because you’re still dealing with a smog era de-tuned 6 cylinder. Ignition timing, carburetor jetting, etc. all plays a part in this too. As with most of these things, some trial and error is to be expected.
thanks for the info. I will be driving the car around 10 miles a day at the most so should i look for a manual choke because of the 3 mile short distance as recommended by someone on here on a previous question or should i not worry about this?
It has to have a choke, either automatic or manual, for sure.
The electric choke just stays closed for a set amount of time (mostly) regardless of the ambient temperature, so in practice it usually stays closed for much longer than it needs to, resulting in a very rich condition in the first few minutes of driving, washing the cylinder walls with extra gas in the worst case scenario. With your 3 mile commute, the choke will probably be just opening as you get there. It’s not going to be good for your engine and you’re definitely not going to like the gas mileage you’ll get!
With a manual choke, you can use only as much choke as you need, including none which may be the case on warmer days. It’ll be a lot easier on your engine and your gas budget. You just have to remember to open the choke yourself, although in my experience, usually the choke is only actually needed for the first minute or so. My routine with my old Dodge truck was I’d hop in, set the choke, start it, put my seatbelt on, tune in the radio station I wanted, adjust the mirrors and then I’d be able to open the choke and drive off and not worry about it.
The other option is some sort of thermal choke, like your stock carb has, but most aftermarket carbs don’t provision for these because it requires either a heat riser or a coolant supply which are hard to provide for in a universal application.
thanks for the help. I wasn’t sure how the electric choke worked, but the carb i have right now has a manual choke because the original owner took the automatic choke off. I’m pretty sure that’s going to stay that way.