Pete you are right.....but there's a catch. With a smaller engine, 272 or 292, you'll want a carb with small primary venturii's for better throttle response and mileage at the lower RPM's and Throttle openings  you use on the street. the 390's and 465's have those. you'll only need the 600 cfm part at full throttle and higher revs. Maybe someday Ted or John will get a chance to do back to back carb tests on a mild 292/312. It would be an interesting duno run.  

Frank/Rebop

Bristol, In ( by Elkhart) 

I seem to remember there's a rough calculation to determine the CFM requirement for a carb. I believe it's cubic inch X max revs/3456 but I'm sure there's others out there who can confirm this. So a 312ci @ (say)4500rpm divided by 3456 = a requirement of about 400 CFM. I guess this is pretty rough but should be a fair guide for a basically stock engine.

Cheers

Richard

I’ve a low opinion for those carb calcs as internal circuitry on a particular carb makes all the difference.  I’m currently in the process of dynoing a 312 that has unported G heads with a multitude of carbs on a variety of intakes including a dead stock intake with the original ½” spacer.  Fifteen different carbs have so far been tested.  The best overall players so far are a pair of 600 Holleys and a L1848 465cfm Holley.  The original L1273 (ECZ) Holley (410cfm?) and the original ECZ-AA Carter (400cfm) were the worst of the lot.  The star of the show on the stock intake is the 1.08 Ford (480cfm) and while it was down on upper end horsepower, the lower end torque numbers were out of sight.  An 830cfm vacuum secondary Holley was also tried on the stock intake and was found to perform the same as the 600’s on this particular engine while idling characteristics and low end reponse were just fine.  The problem with all this testing is it’s primarily associated to this particuar engine combination and as head porting and better intake manifolds come into play later, much of what’s learned here may not be very applicable.

 

What makes vacuum secondary carbs very nice to work with is that engine demand will only permit the secondaries to open by the amount that the engine wants if the carb is properly tuned.  It's difficult to overcarb an engine with a correctly tuned vacuum secondary carb but selecting the correct size double pumper carb is difficult in this regard.

Lorena, Texas (South of Waco)

Ted, have you had a chance to see how one of the Demons stacks up? 

Frank/Rebop

Bristol, In ( by Elkhart) 

I have to agree with Ted on this one, although now on my rebuilt 309 Y Block I am considering the 525 Jr. The Old original 4100 1.08 Autolite ran the best and to me had the most power on my Original 292, She never pulled big numbers but as far as drivability and power the Autolite performed better than the Holleys I played with, 390 and 500 CFM models. I also ran a 1.12 off the FE 352 from our Ranchero performed well on the FE, but the 1.08 performed better than the 1.12 4100. They must still have a cult following coz they still pull good money for the 4100 carbys.

1957 ________________ Ford

\___((_______________))___/

(@)________V__________(@)

[________I_____I_________]

__[__]__ o_______o___[__]___



Y Blocks, The New Flathead!

Have not tried any Demon carbs yet.  My 525 cfm Road Demon Jr. is on my ’55 right now which is sitting at the paint shop.  Although the ‘55 was suppose to be ready last week, I’m still waiting for it.  As soon as I get the car back, I’ll run that carb also along with what I’m considering the baseline carbs for the next series of testing.  The 650 vacuum secondary Speed Demon that's sitting here will get run this week though while the stock intake is still on the engine.

Lorena, Texas (South of Waco)

Ted:

What was the stamping numbers on the Autolite 1.08 you were testing.

It will be on the the drivers side front foot. It should be C###-#



I have found the C6PF 1.08 carbs the best of the 1.08 size. These were the over the counter replacement carb and last models built.

marv

I just bought a Holley 465 for my 56 and was wondering if there was any reason why I couldn't hook the electric choke wire to the positive wire for the coil.

Ol'ford nutCentral Iowa

56 Vic w/292 & 4 spd.

Hi Ol'ford,  I'm running a Carter AFB with electric choke on my '57.  I ran the wire for the electric choke off of the input (12V) side of the ignition coil resistor.  It has worked fine for the past three years or so.  It is recommended that you put a fuse in the wire to the choke, on mine I guessed and went with a 3 amp fuse.  So far so good.  Nick Brann, K.C., MO

So after reading all the reply's, & searching the site, I decided on the Demon Jr 525. Lots of good comments & I don't want to regret going too small with the 390 later. I will attempt to resize the B intake ports & hope I don't screw that one up.

In searching anything to do with this carb, I don't see much mentioned with having to change jetting or secondary springs on the Demon Jr. Anyone with personal experience with the need for tuning out of the box Demon Jr?

Brian - 56 T-Bird

Woodinville WA