another carb question

-
trudysduster

trudysduster

FABO Gold Member
FABO Gold Member
Joined
Aug 24, 2009
Messages
5,033
Reaction score
224
Location
Corinth, Ky.
I have been doing some reading on different things getting ready to change the cam in Tylers car and was reading about carbs and what would be best on this application. I think according to what I have read the 750 Eddy that is on there is way too big. I am installing the comp 268 cam in it. what do you think.


Here is the math to figure out the size of carb you need. Max rpms you will turn with the engine times its cubic inches (is it a stock bore or oversized, exact number is needed), divided by 3456, times 0.85 for street carb. So for example if you're working with a 273 and its max revs are 6,000 rpm

I figure on a street application he would stay under 5800 RPM's. It is a 360 engine. according to this formula, he requires a 514 CFM carb. should I drop down from a 750 to a 600 for this application. what are your thoughts on this. Thanks,Bill
 
that's not going to be too much. I figured a 600 but if that isn't going to be too much I am game with that. I have a 650 Thunder series on the 440. just want to make sure that all the work I am getting ready to do isn't going to be affected by the wrong carb. thanks
 
What that math does NOT calculate is the flow. Using the math you provided, head flow or cam flow or exhaust flow is not calculated. In other words, ported X heads with headers will flow much more than a stock LA 360 head with manifolds, however, if using a roller street cam, the RPM's may not be much more than what you would shift a stock LA 360. But the cfm's would be much more, so a bigger carb would be required. I have taken different size carbs to the strip, and I've only gone slower with less cfm's. Bigger carbs may not have always helped, but usually never hurt.
 
We have used 750 cfm carbs on hot 273's and Ford 289's and they loved them. AVS and TQ's. I don't do the math any more.
 
Think of your engine as a huge air pump,, by putting on headers and dual exhaust, you've changed the outlet size from 2 inch to 3 inch,..

With the camshaft change, you're changing the door style from an open/close, to a revolving door..

Now you've increased all your flow capacities,, what size inlet do you think you should put on,, reduce it to a 2 inch inlet, (650), or a 3 inch inlet (750).. ????

hope it helps
 
it sure does. I will stay with the 750. wonder why they put out these formula's when they don't mean much. Thanks guys.
 
The formula works in a general sense for the average working car designed to get groceries for grandma. Once you introduce higher performing parts, the rating rises.

Then, It is a choice between W.O.T. Performance or nice street manors. Everything else is in the tune as so long the carb application isn't outside of the box.
 
That formula is for sizing carbs on mild applications. It also doesn't take into account the different flow testing methods and that will mess you up too. Basically, short stroke, lower rpm, like smaller carbs. If the stroke gets to 3.58 (360) or more and/or the engine must rev, bigger will almost always do better WHEN PROPERLY TUNED. A smaller carb can be way far off and still be "good". A larger carb cannot be too far off without showing it's displeasure...lol.
 
well we played around with the carbs about a year ago. I originally had the 650 on his and the 750 on my 440. he didn't like the way it ran so we swapped carbs. for some reason he liked the 750 better on his 360 engine and I think my 440 runs better with the 650 Thunder Series on it. so I had planned on getting a 650 thunder series and put on his. I still don't know which would be best. It seems like the consensus is that either one would work if it is tuned in properly.
 
I like smaller Carter type carbs for things I drive a lot. If it's going to be raced, I like the tunability of the Holleys. Size wise - my truck's mild 360 has a 318 intake and Edelbrock 600. It never revs past 4200 and lives at 2200. In a lighter car, with a mild cam, I'd be looking around a 750 Carter-type carb. Again - it needs to be tuned, and not just set the idle mixture and speed and you're good. Nothing should be just bolted on.
 
Something more to think about.............The formula does not take into account of the QUALITY of the air flow. Decades ago reading my new issue of (at the time) "Hot Rod" (I think, might have been "Car Craft") there was an article on this very subject. The gist was that while the formula may spec carb"A" which will have good response, the velocity of the air will carry enough inertia to make it not want to turn down a port before it crashes into the floor of the plenum causing some fuel to fall out of the air stream. They went to a larger carb "B", it slowed the velocity down, response remained good, and they made more HP. Foggy memory makes me think they compared a 600 cfm to a 750 cfm, single plane intake, SBC (what else).

Also never forget that Ma Mopar put 850 cfm Thermoquads on 318's with little trouble.
 
I'm confused,, I thought the idea of putting in the cam was for more performance and "lope".. lol

Now,, if he puts in the new cam,, and the smaller carb,, and it doesn't go as good as it did before,, why have you spent all that money for "goodies" ,, just to limit it's potential.?

I think the question I would like to pose is,, were someone come on the board and say ..

I wanna build a performance motor,, BUT,, I will only run a 600 cfm carb...

Would you have him build this motor, with all the goodies..??

What would you recommend he build with a 600 cfm carb..???

jmo

PS: I'm thinking 318..
 
well just before we parked his car this year, he took the car for a drive to the parts house and on the way he called me and told me it was running like crap. he got down to the parts house, shut it off and when he came back out it wouldn't start. I went down and pulled the air cleaner off and when I looked down inside the carb, there was a lot of gas puddled on top of the bottom butterflies. I stuck a rag down in and soaked up the gas and tried it. it started and we brought it back home. hasn't done it since and don't know why it did it then. I have bought a rebuild kit to put in it. so that has me worried. what would cause a lot of gas down on the lower butterflies of the carb.
 
grant, that is exactly the purpose of asking these questions. I don't have the answers. I have read where if you have a bigger carb than needed, it will run like crap sometimes. I have also read where back in the day that some of the Mopars came out with 500 and 600 carbs on them. I have not been into cars like this for 40 years and things changed. I want the best out of this car that I can get without building a race car. what did the RR's come out with back in the day. did I read somewhere a 600 carb. That is why I was asking if the 750 was too much for this application. I will figure it out.Thanks
 
trudysduster said:
well just before we parked his car this year, he took the car for a drive to the parts house and on the way he called me and told me it was running like crap. he got down to the parts house, shut it off and when he came back out it wouldn't start. I went down and pulled the air cleaner off and when I looked down inside the carb, there was a lot of gas puddled on top of the bottom butterflies. I stuck a rag down in and soaked up the gas and tried it. it started and we brought it back home. hasn't done it since and don't know why it did it then. I have bought a rebuild kit to put in it. so that has me worried. what would cause a lot of gas down on the lower butterflies of the carb.
Click to expand...

Either something interfering with the needle and seat, or an improper float setting.

Or an overzealous teenager ?
 
Well,, if it's any help,, the 1969(?) Mopar Hustle Catalog's first suggestions for the 340, was to put on Hooker Headers, Edelbrock LD 340,, and a Holley 780 carb,..

But your already in the position to try both,, put it together with the big carb,, let him run it a while,, then put the smaller carb on,, see for yourself..

I've always plunked a 780 on,, and let the customer go for a drive,, I'm always prepared to put the smaller carb back on,, but never have,,

Of coarse my customer just want the GOOOO and the GRIN ...lol

seasons best .. cheers..
 
trudysduster said:
grant, that is exactly the purpose of asking these questions. I don't have the answers. I have read where if you have a bigger carb than needed, it will run like crap sometimes. I have also read where back in the day that some of the Mopars came out with 500 and 600 carbs on them. I have not been into cars like this for 40 years and things changed. I want the best out of this car that I can get without building a race car. what did the RR's come out with back in the day. did I read somewhere a 600 carb. That is why I was asking if the 750 was too much for this application. I will figure it out.Thanks
Click to expand...

You have a mild small block.
If you want a crisp/clean reliable car run an Ede 600 on it.

The only thing this carb will miss, is on the very top end.
Unless you want your son flying around at full throttle, this would be a wise choice.
 
Since you had trouble with the 750 carb, you should break in the cam with the smaller carb. You don't need any problems at the critical break in time.
 
well Johnny, It wasn't an overzealous teenager because the roads leading to town are back country roads. So I am guessing it is something in the needle or the float. I may just put the rebuild kit in it and sell it and get a smaller one. not made up my mind yet. The carb does not have very many miles on it. I also have a new calibration kit for it from Summit. May just go with a 600 or 650. going to think about all the info I have here and see. Thanks,Bill
 
decided to go with the 600 CFM 1405 Eddy carb. will be here Monday. thanks for all the insight guys.
 
You must log in or register to reply here.
-

Back
Top