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| Is my carb to powerful for my stock engine? https://www.slantsix.org/forum/viewtopic.php?t=7823 |
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| Author: | 19Dart70 [ Wed Dec 10, 2003 5:35 pm ] |
| Post subject: | Is my carb to powerful for my stock engine? |
I have a 1970 dodge dart and i bought a holley 2 barrel 500 cfm carb for it. i got a bigger manifold so it will fit, but i don't know if 500 cfm is too much for my engine. some people say i need to get a different cam shaft.. and i need to know what kind would be right. |
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| Author: | steponmebbbboom [ Wed Dec 10, 2003 6:30 pm ] |
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Oh.... I'm sure I read this somewhere on a previous board, can't remember which but I think it was the Doc that said the engine can only handle like 360 cfm if the engine was operating at 100% volumetric efficiency, and in real world it's closer to 80%? I'm sure you could get close to or even beyond that figure with some heavy mods but I doubt you will ever use the full 500 cfm, especially if it's a 2BBL carb. I think the stock carb off a 318 motor is just about right. |
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| Author: | 83Ram150dude [ Wed Dec 10, 2003 8:40 pm ] |
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Ya gotta remember one thing about carbs.......2bbl cfm ratings are different from 4bbl ratings. One is rated at a different pressure rating than the other, so a 500cfm 2bbl is not the same cfm as a 500cfm 4bbl. The Holley 500cfm 2bbl might be rather large for your application, but a 500cfm 4bbl carb would be closer, not perfect, but closer. A stock bbd for 2bbl apps. would be fine, where as a 390cfm 4bbl is best for 4bbl apps. Triple Weber sidedraughts anyone?? 
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| Author: | 87Slant_sickness [ Wed Dec 10, 2003 10:41 pm ] |
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actually at 6500 rpm the slant would need 420cfm  the 500 holley wont be too big, it should run fine however you'll need to tune it in, this means changing jets (ask me so far i've run a 600 Holley, 625 Carter AFB, 500eddy, and a Holley 390) i gotta say the 390 was me least fav.Justin |
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| Author: | Charrlie_S [ Thu Dec 11, 2003 5:11 am ] |
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I don't think you will be happy with the 500 Holly 2bbl. The barrels are too big, for good throttle response on a stock street slant. I use that carb on my Cuda, but the engine is not stock, and I have 3.91 gears, and a high stall converter. The carter 2bbl from a 318 or super six, would be a better choice. If you really want a Holly, use the 350 cfm 2bbl, or a 390 4 bbl. You will have much better drivability. |
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| Author: | XECUTE [ Sat Dec 13, 2003 3:22 am ] |
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| Author: | Charrlie_S [ Sat Dec 13, 2003 5:41 am ] |
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WOW |
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| Author: | steponmebbbboom [ Sat Dec 13, 2003 2:20 pm ] |
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Quote: Ya gotta remember one thing about carbs.......2bbl cfm ratings are different from 4bbl ratings. One is rated at a different pressure rating than the other, so a 500cfm 2bbl is not the same cfm as a 500cfm 4bbl.
I was completely unaware of that.The Holley 500cfm 2bbl might be rather large for your application, but a 500cfm 4bbl carb would be closer, not perfect, but closer. A stock bbd for 2bbl apps. would be fine, where as a 390cfm 4bbl is best for 4bbl apps. Triple Weber sidedraughts anyone?? :P Could you explain the process? I never thought twice about it until now! |
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| Author: | 83Ram150dude [ Sat Dec 13, 2003 2:36 pm ] |
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Quote: Quote: Ya gotta remember one thing about carbs.......2bbl cfm ratings are different from 4bbl ratings. One is rated at a different pressure rating than the other, so a 500cfm 2bbl is not the same cfm as a 500cfm 4bbl.
I was completely unaware of that.The Holley 500cfm 2bbl might be rather large for your application, but a 500cfm 4bbl carb would be closer, not perfect, but closer. A stock bbd for 2bbl apps. would be fine, where as a 390cfm 4bbl is best for 4bbl apps. Triple Weber sidedraughts anyone?? Could you explain the process? I never thought twice about it until now! |
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| Author: | XECUTE [ Wed Dec 17, 2003 3:09 am ] |
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Yep, 83Ram150dude, that's it. The pressure drop is sort of like the total amount of 'suck' the testing apparatus imparts on the carby. A stock 2-bbl is assumed to be under more 'suck' than a 4-bbl, seeing as most 4-bbls have twice the carb area. In practice you can argue over this, but the fact remains a 500 cfm 2-bbl is really a 354 cfm carb, while a 390 cfm 4-bbl is really only about 10% bigger than a 500 2-bbl. As the 83Ram150dude says, the 2-bbl is rated under a steeper pressure than a 4-bbl. 3" of mercury(hg) can be sucked up a tube placed in the port of a 2-bbl carb, and only 1.5" or mercury in a 4-bbl carb. (Don't try this...mercury is a nasty poision) To convert, take the 2-bbl presure drop (mostly 3") the 4-bbl presure drop (mostly 1.5") This gives 2. Then you take the square root of that, which gives 1.414. Any time you see a 2-bbl rating, just divide it by 1.414 to get a 4-bbl baseline cfm. Eg, 350 cfm 2-bbl Holley is really 350/1.414, or only 248 cfm. A 320 cfm @ 3.0" hg Holley/Weber is really 226 cfm. Hope that helps some. |
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| Author: | Pyper6970 [ Thu Dec 18, 2003 12:27 am ] |
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So would my Mod Slant work better with a 500 eddy 4 bbl? I have a Holley 390 on it now, I have rebuilt the head, , milled 60 of the bottom of the head, 1.70/1.44 Si Valves, .436 lift cam, 3.23 peg leg, 904 tranny. The motor is about 9.3:1 CR, running an MSD 6a box, I have the 4 bbl Offy aluminum on it now. I took it to the dyno about a month ago and the secondaries werent opening up, ( i opened em manually and the car put 102 rwhp. The guy told me im way under carbed but I didnt believe him as to thinking my secondaries were the culprit, I think I fixed her now but Im still wondering about the 500 Eddy, what do you guys think for my setup? 
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| Author: | Dart270 [ Thu Dec 18, 2003 7:23 am ] |
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XECUTE, Thanks for all the carb info. I may be in NZ next fall, so maybe we can hook up for a beer. Pyper, You are running basically a stock cam. Is is degreed? Has the head been ported? How much have you messed with ignition timing? There are a lot of factors that could make that HP number low, and the 390 should not be holding you back if it's working right. You would get by fine with a 500 Edel, and I have run it on a street car with a similar buildup to yours with good success. Lou |
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| Author: | Pyper6970 [ Thu Dec 18, 2003 9:27 pm ] |
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>>>Pyper, You are running basically a stock cam. Is is degreed? Has the head been ported? How much have you messed with ignition timing? There are a lot of factors that could make that HP number low, and the 390 should not be holding you back if it's working right. <<< The cam was not degreed, its a street car, didnt think it needed to be, I have a stage 1 Port and polish, 5 angle valve job. I need to recheck my timing, she is starting to ping under WOT but i think thats due to bad gas. I have never detonated this motor. |
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| Author: | Dart270 [ Fri Dec 19, 2003 7:11 am ] |
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FYI, you will lose a LOT of power if it is too far advanced. I've picked up as much as a whole second at the strip just by retarding the timing. Have you checked cylinder pressure? If you are much below 140 psi, then the cam may need to be degreed in. Timing set or cam may be off. Lou |
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| Author: | GunPilot [ Fri Dec 19, 2003 7:33 am ] |
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Great info in this thread! |
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