318 proper CFM carburator size?

[The_Handier_Man1]

From: PhÃ¥rÃ¥oh  (Original Message)
Sent: 7/5/2003 10:10 PM

     After reviewing some figures, I concluded that my gas mileage to the Jam and back was around 5 mpg.  I need a new carb anyway so I might as well get the correct size for the job.  I am running the 318-3.  My question is what CFM should I be looking for?  I know where there is a Holley 390 CFM carb off of a Ford.  Would this work?  I have a Qudrajet now, but it belongs to a friend of mine and he needs it back.  The qudrajet that came on the beast is worthless, I had it rebuilt and it still leaks fuel like a sive.  So I have a bit of a delima.  If anyone can tell me what size I should be looking for, I'd appreciate it.



From: Jupp318
Sent: 7/6/2003 5:38 AM

  If you look at the "2bbl to 4bbl" thread you'll get plenty of information there.  I would recommend looking at ebay and maybe you'll find you can do the Edelbrock performer mod (Carb and Manifold) which is supposed to do the trick.  I still have a pile of bits next to me waiting to be installed, which include Heads, Manifold, Cam and Carb all bought on ebay.  I think it will be Autumn before I install them though.

Cheers Ian...



From: dave76Chieftain
Sent: 7/7/2003 10:25 AM

For a 318, I would not go any higher than 600 CFM.  Even that might be a bit high unles you have headers and an after market intake.  I would look for something in the 400 - 600 CFM range.  Stock 318 engine just doesn't flow enough air to go beyound that range.  72-75 chassis book indicates all 318 engines were originally supplied with a 2bbl carb which tells you it needs a lower CFM rate.

Dave



From: Jupp318
Sent: 7/7/2003 1:19 PM

  I didn't answer your question really, the Carb I have waiting to fit is a 600cfm Edelbrock.  If you think that I was turning in 12mpg (UK) on the Lemans trip I reckon that would be about 10mpg US.  I have heard that Holley carbs are no longer in favour as they have a bunch of gaskets holding the petrol in, and tend to leak Whereas the Edelbrock has no gaskets below the fuel level.  Also the Edelbrock is a relatively new design and you would hope that it has got to be better, especially as it has been designed since all the smog regs have been brought in.

Cheers Ian...



Sent: 7/7/2003 5:45 PM

Hmmmm,
    Lets see.  If I were to find a carb in lets say, a magic toilet.  And that carb was lets say, a newer Edelbrock.  And off of a big block.  What CFM would you think it is?  If it is around 700 CFM, could you jet it down a little lower to maybe 600 CFM?  On this magic carb where would you look for the CFM rating?  If not, would you think that a 390 CFM is too small?  Wait that is a Holley, I really would like to steer clear of Holleys. (I've heard horror stories)  I guess what I'm trying to ask is:  Can I make Sob's magic carb work on my MH.  We know this about it.

Edelbrock
4 BBL
Shiny and relitively new
Off of a late 70's early 80's Pace Arrow
    (if I'm not mistaken P.A. used the GM chassis, and I would presume that it is probably a 454.)

The Q-jet that is on my MH now was off of a Chevy 350 LT-1 motor, so I assume that if the carb off of it worked on my intake that the carb off of the P.A. would also work on my intake, right?  All info so far is helpful, but more would be great.



From: denison
Sent: 7/7/2003 6:27 PM

I would be mildy surprised if you had to change the jets on a carburetor solely to use it on a different displacement engine. Unless it had been set up for a higher altitude. And as far as Holleys go - my 413 has a Holley on it, that I thought was original, but nope - it is a replacement. And right now, after 8 or 9 years of use alternating with neglect, I believe it needs its float bowl gaskets replaced again. I had replaced them about in 1993 I think.
By the way - if you want to see what your best mileage is, I think you need to run through a tank of gas someplace flatter than the terrain between you and Raystown Lake - where you dont have to use the brakes very often. Crossing Ohio for instance.
denison



Sent: 7/7/2003 6:43 PM

Dave,
     Tell me about it.  Those hills sure did a job on my gas mileage.  I know that I should be getting better mileage, but that was the first time that I had taken her on a trip over 60 miles.  So I figured that I could get a better figure than putting around locally.  The next time that I check it I will have to go somewhere that doesen't have 12% grades.  The original owners records show that it got an average of 7.5 mpg between Seabrook, MD and Florida.  That was with the orig. 2 bbl carb, I assume.  I would like to get a little closer to that number, but if I don't it's no big deal.  I didn't buy her for gas mileage.  Got to pay to play, right?
     I wasn't sure about the jets.  I didn't know if changing the jets would alter the amount of fuel taht the carb was dumping.  I don't really know much about carbs, the winnie is the first vehicle that I've owned that wasn't fuel injected.  I'm dating myself arn't I?  Thanks a lot for all the help.



From: chip
Sent: 7/7/2003 7:50 PM

for what it's worth here's what i recently did;
1. installed a weiand action plus manifold #8007. had to elongate head to manifold bolt holes @ .080" because of milled heads on reman engine, didn't have to mill manifold mounting surface though.
2.installed 1405 edelbrock carb. had to go 1 stage richer to a #1446 metering rod and all the way from a 5" vacuum spring to an 8" spring set. i believe this was necessary because i have dual exhausts with glas packs and a very mild cam.
seems much better than my old 2bbl with worn out throttle shaft bores. have no idea of mileage figure though. it's just nice to be able to accelerate[slightly] going uphill, rather than putting my foot deeper into the carb and and watching the speedo drop. ˇ



From: dave76Chieftain
Sent: 7/8/2003 11:09 AM

Pace Arrow also used the Dodge chassis in the 70's at least.  I looked at a 76 PA with a 75 M500, 440 that had the Thermoquad.  Then PA went to GM P-30 when Dodge got out of the MH chassis business.  Hard to say what CFM is on that Edlebrock.  Do you have a part number you can track back at their web site?  I believe the down side of having a carb that can flow to much CFM is that it can run to rich when you put your foot into it.  Thats why you may have to bring the jet size down so that you get a better fuel/air mix across the mid power range you typically run at.  I would think the 390 CFM ford would work but you may not be able to get the little extra umph your looking for.  Big blocks typically had CFM ratings in the 600 - 700 or so area.  You get into the racing carbs above 700 CFM.  Those higher CFM's are only really needed at high rpm's anyway.  Engine is just not pumping enough air through it until you get into the high rpm range to need high CFM rating.

Dave



From: Jupp318
Sent: 7/8/2003 6:43 PM

  If you get the nice shiny thing that has an Edelbrock badge looking at you, the flange bottom left will have a number on it.  Mine has the number 1405 which is a 600cfm with electric choke.  The reason I am changing the cylinder heads is that they had already had the valve seats changed for unleaded fuel.  The fact that they are "J" heads and have larger valves didn't really come into it :-) . At worst they will improve the breathing still further.  If anyone could recommend some really good tube headers I will keep an eye out for some.

Cheers Ian...



From: Jim83Itasca
Sent: 9/14/2003 12:50 PM

Hi Dave
As the new guy on the block within this group "i own a 83 Itasca 454 p-30 33 ft".
Here is a formula that we use for street/strip carb cfm requirements.
(Carb size)
Cubic inch X rpm devided by 3456=cfm use.
Example
454 cubes X 4000 rpm=1,816,000 devided 3456=525 cfm carb.
For the race folks we take the 525 X 1.23 for a 646 cfm carb. So the Edelbrock 1400,1405 cfm carb @ 600 cfm is a good choice right down the middle.
I use the 4K rpm level as the max rpm for motor homes, Ain't good to spin the big blocks past 4K.
The rochester is one excellant carb because of the smaller primary venturi's so just dont count them out.

Jim



From: leftyizme
Sent: 9/14/2003 7:29 PM

Thanks Jim, I had the formulas for figuring cfm and for compression but I had lost them a long time ago, at least this replaces one of them.--lefty