Carburetor "sizing" for a SBF
04-21-2009, 12:42 PM
#11
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Haha, the trick is Pro Systems carb. Those guys definitely know what they're doing with carb technology. It'd prolly go slower with some out of the box 950 from Holley or something though.
In the end as long as you can shear and atomize fuel, the size becomes less relevant. That used to be the EFI advantage, you could run unrestricted induction and the injectors did the atomizing for you, rather than needing a bunch of carb signal to do it. Modern booster and metering technology means that you can run much larger carbs than in days gone by and still get good atomization, or even better atomization than EFI.
Pro Systems "carb sizing calculator" is now CIDxRPM/2820 x VE. They say the 2820 rather than 3456 accounts for modern technology in a carb, and will size it larger than traditional and get you closer to where you need to be. And with annular boosters and better metering systems you can go even larger and be fine.
In the end as long as you can shear and atomize fuel, the size becomes less relevant. That used to be the EFI advantage, you could run unrestricted induction and the injectors did the atomizing for you, rather than needing a bunch of carb signal to do it. Modern booster and metering technology means that you can run much larger carbs than in days gone by and still get good atomization, or even better atomization than EFI.
Pro Systems "carb sizing calculator" is now CIDxRPM/2820 x VE. They say the 2820 rather than 3456 accounts for modern technology in a carb, and will size it larger than traditional and get you closer to where you need to be. And with annular boosters and better metering systems you can go even larger and be fine.
... yep, he paid to get the correct carburetor.And still, for a 331 revving to 7,000 RPM's and 100% VE, their calculator shows 821 CFM (closer up would be = 850 CFM... isn't it?).......
... why would a 950 CFM be customized then? Why was I able to use (due to budget limitations then, believe it or not) an 850 CFM DP on a mild 289 .040" over with 289 HiPo heads and a small HFT camshaft back in 1978-79....... a 650 DP on a 12B wankel with some bridge port work, and a 700 DP on my junk since 1991?...... all of them daily drivers.I don't mean (before this is taken to the extremes) that a mild/streetable setup should be topped with an 850+ CFM carburetor all the time....... a 750 will work well, and a 650, and a 700..... but, the "over-carbed" description of a 302 when a 700-750 carburetor is used, is not true 100% of the time.
04-21-2009, 01:18 PM
#12
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Not all of them are over carb'd, that's true. But it seems like 99% of the time when they are over carb'd, someone just slapped some large out of the box carb on it and it's just not setup right for the application. It seems like most people do little more than maybe change jets, and even then they only get it kinda close.
04-21-2009, 02:18 PM
#13
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Not all of them are over carb'd, that's true. But it seems like 99% of the time when they are over carb'd, someone just slapped some large out of the box carb on it and it's just not setup right for the application. It seems like most people do little more than maybe change jets, and even then they only get it kinda close.
04-26-2009, 07:08 PM
#16
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Most of the out of the box Holleys aren't really that great. Many of them still run archaic strait leg booster technology, and little has been done to change the metering systems in the last 20 years, though that's not true for all of them. That's why when you look at carbs from companies like Pro Systems, or Quick Fuel, or *****'s they seem to be light years ahead of a lot of Holleys. Box stock Holleys work pretty well, but they have some definite limitations compared to more modern carburetors. Holley does have some newer carbs out that are better than most of their line, but they're pricey.
07-14-2009, 10:17 PM
#17
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for bringing this back up, but some of the information/facts to continue this discussion has been made available. Imagine.....no, picture the following setup:
Car is a '65 Mustang fastback with simple Shelby style (more road course) suspension. 100% street car. T5z transmission, 9" rear with Detroit Locker and 4.30 gears, M/T ET Drags 26x8.5, and skinnies in the front.
Engine is a very street friendly (spends most of it's time driving around in nice weather and the streets and highway) .060 over 289, 10:1 comp to be pump gas friendly, basic Probe SRS piston, stock block, stock crank, stock rods with entry level ARP bolts, Canfield 195 heads (through CI), TLSR cam from CI, Parker Funnel Web intake (bought used), 950 Holley HP based Pro Systems carb, cheap Hooker Comp 1 5/8" to 3" collector headers, 3" exhaust with Magnaflows and no H or X pipe, and no power adders.
Engine is a very street friendly (spends most of it's time driving around in nice weather and the streets and highway) .060 over 289, 10:1 comp to be pump gas friendly, basic Probe SRS piston, stock block, stock crank, stock rods with entry level ARP bolts, Canfield 195 heads (through CI), TLSR cam from CI, Parker Funnel Web intake (bought used), 950 Holley HP based Pro Systems carb, cheap Hooker Comp 1 5/8" to 3" collector headers, 3" exhaust with Magnaflows and no H or X pipe, and no power adders.
2. 289 .060" overbore = 297 CID
3. Canfield 195 cc heads unported = HUGE for such a small engine.
4. Parker Funnel Web SP intake = a TDL (Torque Down Low) killer.
5. A 950 CFM Holley HP based Pro Systems carb = monstrous carburetor for a "street" setup on a smaller than a 5.0L engine.
6. 3" collector headers through a straight 3" full exhaust = exhaust velocity killer messing with scavenging.
7. The camshaft "required" needs to be huge for it to work "killing" driveability..... right?...... WRONG!....
Int./Exh. Duration @.050": 225.7 intake, 244.8 exhaust, Int./Exh. Lift: .649 intake, .637 exhaust, Lobe Separation Angle: 111.65..... 11.95° overlap @.050"
Track results?
60' - 1.636
330' - 4.710
1/8 - 7.281
mph - 96.32
1000 - 9.505
1/4 - 11.382
mph - 120.35
Isn't the combination as a whole what's important? Not a generalization based on historical misconceptions, product of faulty/defective evaluations? BTW... this is not a "made up" combo.... this is is real no BS stuff however, and just in case..... take a look.
Where did that "CFM" formula go? .... 297 CID @100% VE and 7500 RPM = 645 CFM ....
07-15-2009, 04:59 PM
#18
Join Date: May 2009
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That's my combo and what it ran first time to the track with ZERO track tuning. I believe there is a fair amount left in it. Also the picture of the engine compartment are from when I had the Victor Jr on it. The times are with the PFW intake which is on it now. The Vic Jr will end up back on to see which runs better at the track. It's a simple 100% street car with a very street friendly and FUN pump gas 289.
07-15-2009, 07:07 PM
#19
Join Date: May 2009
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Just thought I would add that the combo has excellent throttle response, plenty of low end power, does not bog, and screams up top. The carburetor on this combo is very streetable and can cruise around under 1,500 rpm in 5th gear and accelerate clean and well from there. No need for downshifting unless I'm slowing down. As long as the carburetor can properly meter the fuel there is no such thing as too big.
07-15-2009, 09:43 PM
#20
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And that's the key, as long as it can properly meter fuel. As long as you can get it to do that then the bigger it is the less restriction it creates. The problem most people get into that I see, is they're running a mediocre at best carburetor that can't meter fuel with slow air velocity. It's a lot easier to over-carb in a case like that.