View Full Version : Spread Bore or square bore????
09-09-2007, 10:36 PM
Well I'm setting up my 51 with a 700r and I got a great deal on a Holley 650 Spread bore. I also have a nice line on a 650 square bore. Let me here your opinions and experiences and any tec. on these carbs and what the main difference is.
09-09-2007, 10:43 PM
Spread bore = mileage
Square bore = performance
09-09-2007, 10:51 PM
Are they mechanical or vacuum secondarys? Yeah, spreadbores are generally for better milage. What sort of intake do you have?
09-09-2007, 10:52 PM
You saying the spread bore is weak? I'm building a custom not a rod but I when I want to go I want to go. This is going on a 307 Chevy small block a mild cam and head work latter.
09-09-2007, 10:52 PM
Spread bore is not weak, but in a performance application at wide open throttle, it distributes fuel more evenly.
09-09-2007, 10:55 PM
Vacuum secondaries Edelbrock intake
09-09-2007, 10:59 PM
The reason I asked was I ran a mechanical secondary spreadbore Holley years ago. Anything but weak.
09-09-2007, 11:01 PM
I need to run the vacuum secondaries for the 700r or change the valve bodies. From what I've been told.
09-09-2007, 11:17 PM
09-10-2007, 01:00 AM
I had a built 350 with a quadrajet in high school. I thought a holley 600 vac. secondary was the way to go (holleys are cooler than a stock carb right?)... I lost 400 rpm off my redline. car wasnt as fast.
17 yrs later I have a mild built chev 350 in my daily driver trans am. 350 4 bolt 305 hipo heads edelbrock performer intake, edelbrock square bore 600 carb. It is not fast. , and it drinks-drinks premium gas. I am going to find a good qudrajet and try that., I think I will get better mileage, and perform better.
I am starting to think qudrajets are the way to go until you are ready to pony up big bucks for a proven, matched performance package.
09-10-2007, 01:47 AM
Spread bore Holley is a piece of junk...go with the regular Holley...you will not be sorry. And if you still aren't convinced...call me, I will give you one...have a couple of spreadbores hanging around...free if you Just have to have the pain.
09-10-2007, 07:04 AM
Well I'm setting up my 51 with a 700r and I got a great deal on a Holley 650 Spread bore. I also have a nice line on a 650 square bore. Let me here your opinions and experiences and any tec. on these carbs and what the main difference is.I think the 650 is a little too much carb ,hell I've got a 600 Edelbrock on a HO305 with a 278 Crane came , Hooker supercomp headers and I've detuned the carb as much as I can and it still too rich.I used Holley for years and liked them until I tried the Edelbrock and found they are mechanical, very easy to set up with the step up spring kits and respond very well.They have small bowls but I've never had it run out of gas going through all 4 gears hard.Regardless of what carb you choose I wouldn't go bigger than a 600. My .02
09-10-2007, 06:08 PM
So your saying all those years GM ran spread bores, They were crap years. NO cheap shots ford boys.
09-10-2007, 10:21 PM
Quadrajet. Better design than Holley. They are cheap 20-25 used and easy to rebuild.
Here are a couple of links:
09-10-2007, 10:26 PM
no, the spread bore quad was the best and the 4 jet was good too. i think that the people who call them junk really don't know how to work on them.
09-10-2007, 11:36 PM
now old timer you sound like some one that works on there own cars thank you but at this point I've locked my self in to the holley because of the TV cable I bought.
09-11-2007, 04:50 AM
Please don't confuse the Quadrajet with the Holley "spread-bore"...the quadrajet can be made to work dam fine with a little work...the Holley will NEVER work..........go with a standard square bore Holley.........and if you can't find one...call me...plenty of those hanging around also.........
09-11-2007, 12:20 PM
I was just reading an article on the spreadbore Holley because I'm running one of these trouble makers on my t. The article said the Holley lacks the better parts of the Holley or Quadrajet and recomends replacing it with a standard Quadrajet if you have a spreadbore intake.
In the couple weeks that I've had the car on the road I have retuned this carb at least 3 times using a vacume guage. It will run great one day, next like a piece of crap, I've never seen anything like it. I would run a Quadrajet over this carb any day but not over a standard square port Holley.
09-11-2007, 12:24 PM
Check your intake first................see if it will take a spreadbore, squarebore or both. Edelbrock Performer will take either.
As for my choice..............a spredbore Holley wont get any better mileage than a squarebore Holley. A Quadrajet will get better mileage than both............and has the potential for power as well.
Von Rigg Fink
09-11-2007, 01:01 PM
love my quadrajet..
09-11-2007, 01:06 PM
It all depends on which Holley spreadbore you're talking about. If it is a vacuum secondary spreadbore Holley...throw it away (or give it to your worst enemy). On the other hand, the spreadbore Holley 650 mechanical secondary double-pumper with reo accellerator pumps is just about the best performance carb ever made. I've run one since 1979 on a 429CJ with 4-spd. Made all the difference in the world over the factory Q-jet (yes, a Ford with a factory Q-jet!). I've experimented with running squarebore Holleys on it, too. They either made no difference or ran worse. Anybody that says differently about this particular spreadbore Holley either does not know what they're talking about or they had one that was screwed up by somebody that did not know what they were doing when they monkeyed with it.
As to gas mileage, over the many years I've had the car, I've only checked gas mileage once and it was with the spreadbore Holley. When I checked the mileage, I was on the highway for 120 miles, did four hours of cruising the strip, and ran two drag races. It got 13 mpg. Not too shabby.
My otherwise (other than the carb) stock car is approximately 1/2 second quicker in the quarter mile than other 429CJ cars that are running the factory Q-Jet (my 13.9-14.0 vs. their 14.4-14.5). Even though I have the CJ hydraulic cam, my times are down there with the 429SCJ cars (which have a hotter solid lifter cam and a squarebore Holley carb).
09-11-2007, 01:39 PM
I have continued to run the Q jet on my cars and I believe they are the better carb. I purchased a book on the Qjets by Doug Roe and it was well written and provided me with the instruction and tips on rebuilding them. Everyone can rebuild a holley but the Qjet takes a bit of time to learn but its a far more sophisticated carb in general. The Qjet does give you very good gas economy and performance when you need to get on it.
09-12-2007, 03:36 PM
It all depends on which Holley spreadbore you're talking about. the spreadbore Holley 650 mechanical secondary double-pumper with reo accellerator pumps is just about the best performance carb ever made. ).
That's the one I got, like I said one day great, the next it'll try and stall out as soon as you put it in gear. One day great, next day it seams to hang up and idle high. I can turn the idle down and it'll run fine the rest of the day and then the next it won't run unless I turn the idle back up. Some times I can run it hard then it will hardly idle at the next stop light other times it just keeps running fine. It's weird I tell ya !
I have no vacume leaks and the fuel pressure is good.
09-12-2007, 07:54 PM
OK so the vacuum advance is not your peoples favorite How about a Spread bore 650 "double pumper?" with #s of 6499 3465 does this make sense to any one. That is a mechanical secondary.