My reference to building up or down was with regards to the carberator out of the box and changing it with a kit to make it perform better for a higher or lower performance engine. I got the impression the Summit 4100 Replica might be best suited out of the box for a souped up 272 or 292 and not so great for a stock 272. When the thread poster tried it on his stock 272, he thought it was too rich. He contacted Summit to get the parts to tune it down, he was told the only kits available were to tune it upwards for higher performance engines. Hope that makes sense. Of course the made in China was just repeating what I read on another forum but has never been confirmed. 

With no experience in these carbs, I can say I am impressed with the Edelbrock Performance 500 I have for my 292. Problem is I dont have nothing to compare it to.

Darrell Howard Whitefish, MT Outlaw 56 Ford F-100's

The 525 Street Demon JR is technically a Demon carburetor but they are not around as their own company anymore; Holley owns the brand now.  That is a good carb, too.
What I understand about the Summit carb is that it uses Holley parts, so tuning should be a breeze and parts to do so should be easy to get.  I don't really understand the building up and building down thing.  The made in China part would bother me, if it is true.  I've gotten to a point where I try not to buy made In China if at all possible, even if it is a quality product.
I think any one of them will do the job just fine.

When I switch from My Edelbrock 600 to the Holley 570, it was a night and day difference from idle to 6000rpm and it got better mileage.  That was with only a couple hours of tuning after the initial installation vs many, many hours of trying different things with the Edelbrock, before I learned about the air valve "fix."  This isn't a Holley is better than Edelbrock statement.  I think what really is happening is that the Street Avenger series is a modern carb whereas the Edelbrock is not and the new tech in the SA allows it to work better.

With any of these, you should expect to tune it.  They will all probably advertise some version of "guaranteed to work out-of-the-box" and they probably will but working out-of-the-box and properly tuned can be two different things.  It could turn out that out-of-the-box is perfect, but probably not.  Do your best to pick something that is inline with the goal of the engine.  Don't go too big, and don't take a smaller one that you will need to replace with a bigger one in the not too distant future.  With every one of these carbs, you should be able to email the manufacturer to find out what they recommend for your application.  They know their carbs better than anyone else and can tell you if it will be a good fit or not.



Lawrenceville, GA

Carbs are always a source of varying opinions. I have to go with Frank on the tuning part. I've run 1405 Edelbrocks on stock 292's, supercharged 320cid strokers, 265 and 350 scrubs, they all just worked. The tuning manual looks a bit confusing, but once you find the right page and graph, it's pretty simple. Change the rods, pull the top and do the jets, no fuel all over. All the secondary's have stayed stock, even with the blow thru supercharger. Only had to change the rod springs on the blow thru setup. I especially like the low rpm-part throttle performance. I'm told the new Holley's are better there than they used to be. Edelbrocks will work with minor mods no matter what you do to the motor, short of all out racing. And maybe there, too.

I've also had a couple Holley's that drove well, and a couple Q -Jets. The Q's I had done by pro's, I have no clue how they work, and my attempts to tune them proved it.

The only problem I've had, both Holley and Edelbrock, is getting the electric choke to work right in the summer. On a good, cold day, fine. But I never seem to be able to reset them right for summer. The manual choke solves that.

miker
55 bird, 32 cabrio F code
Kent, WA
Tucson, AZ

Two Add my two cents on this subject. On my 312 I was looking thru the Holley Book. And found the number for the replacement Carb. It was for a 289 HIGH PERFORANCE,  Not much of Difference between the two. I have had it on the 312 for three years now, works great. My 312 is built to Run hard,

I can supply the number if needed. Bob

Bob

Whats the difference between a Holley 525 Street Demon Jr and a Holley 570 Avenger? Is one better suited than the other for a stock 272 with a post 57 intake and newer distributer? Keep in mind all I know about carburators is where the mount on the engine. Never rebuilt one or did any "tuning". I have the Edelbrock 500 Performer that is going into a 292 that will be built up towards a 312 if I can collect the parts, otherwise, just all I can get out of the 292. Maybe I should use the Edelbrock on my 272 and get something else when I get the 292 further along?

Darrell Howard Whitefish, MT Outlaw 56 Ford F-100's

I just read a very extensive post on the Summit 4100 Replica over on the FTE Forum. From what I gathered, you can get kits for that carb to build it up, but you cant get kits to build it down. Since I consider a stock 272 on the down side, along with my understanding the Summit 4100 Replica is on the rich side...........Im thinking it might be a little too much carb for my application. My other concern was the terrible gas mileage one member got with serious time and energy put into making that carb work for him.Perhaps the Summit 4100 Replica is a better application for the beefed up 272's, or 292's with option of kits to build up? The other thing that got my attention is the allegation its made in China?

Darrell Howard Whitefish, MT Outlaw 56 Ford F-100's

A 390 to 465 cfm holley is plenty for a 272 even when warmed up a bit with a better cam and intake these two carbs should work fine but the 390 may need to be jetted up a few numbers from stock. These carbs as Charlie said are easy to tune and run great. A 570 avenger should work well but don't won't to go too big and start losing the bottom end torque from take off to 4500 or so RPMs.

-Gary Burnette-

I know I'm new here, but I'd like to add some insight based on experience:

The old formula for carburetor sizing was displacement x RPM x volumetric efficiency / 3456 = design CFM flow rate. Stock motors usually have a volumetric efficiency of .75, and 5000 RPM is a good default value. Using these variables, a stocker 272 will flow about 300 CFM. On the other end of the scale, a hotted up 312 spinning north of 6000 RPM will flow about double that with a VE approaching 100%, or 1.

Now here's where it gets tricky. 2 bbl carbs are flow rated at 3 inches of mercury, and 4 barrel units are rated at 1.5 inches. On top of that, stock carburetors rarely flow at rated value - it's usually less than that. To overcome these variables, I typically take the as-calculated design air flow and add 25%, multiplying by 1.25. Take that number and select the next higher carburetor size and you'll be fine - or you can go down one size and have better streetability.

As Ted said, a vacuum secondary carburetor is the way to go in the selection process. The vac secondaries will maintain a consistent vacuum signal - which is the power source for the analog computer that is a carburetor. A mild 272 will run very nicely on a Holley 390, whereas a 292 would favor a slightly larger carb, especially if it has dual exhaust or a mild cam.

If you go too far in the carburetor sizing process, you run into a characterization issue, which is a fancy way of saying the carburetor loses the ability to control the air fuel ratio as the throttle plates are opened. The vacuum signal drops to near zero along with flow through the venturis. At this point, fuel drops out of suspension and puddles in the manifold - making for very erratic operation. There are work-arounds for this, but they're either band-aids or part of and expensive custom carburetor build. By the time the carb is 75-80% larger than design flow - it's too big, in my opinion.

Hope you guys find this helpful.

Another concern is over carbing my 272. If I remember correctly, Ted said the 600 cfm series would be fine. I will be sure if I get anything off ebay its Holley direct. Will also add the Holley 570 Street Evenger to my list of options. Thanks!

Darrell Howard Whitefish, MT Outlaw 56 Ford F-100's

If you go Holley, 570 Street Avenger,



Lawrenceville, GA