Carb Question?
#2742203
02/12/20 11:11 AM
02/12/20 11:11 AM
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Joined: Oct 2015
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UK
rb446
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Is this statement correct? Holley rates their carbs at 1.5 inches of manifold vacuum, if the vacuum is high than that at WOT the carb will flow more than it is rated for, if it's lower than that it will flow less than what it is rated at. The main thing you want is the least restriction possible as that will create the most HP. If we go to wallace calcs and use their...... http://www.wallaceracing.com/carb-cfm-vacuum.php for our carb which is a 950HP, but has a different base plate only, we input our parameters and the 2.0vac@6000 from the dyno reading it suggests the carb is flowing 1097cfm, possible? So which is correct, less vacuum@WOT close to 0 for best HP or not....what do you guys say......Intake is a 440-2.
Last edited by rb446; 02/12/20 11:16 AM.
1969 'Cuda 446ci, best 9.96@133.9 in 1990
1971 340 'Cuda, best 11.01@122.8 in 1987
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Re: Carb Question?
[Re: rb446]
#2742230
02/12/20 12:16 PM
02/12/20 12:16 PM
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Joined: Jan 2003
Posts: 14,492
So. Burlington, Vt.
fast68plymouth
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If you’re referring to that Indy headed motor making 700hp...... it’s “extremely unlikely” that motor was using almost 1100cfm. On a more serious effort 572” build here....... the power output while using 1091cfm was 893hp. The 572 street build that was similar to yours was using 959cfm@6100 while making 703hp with a Mighty Demon 850 carb. Any air flow rating “number” is useless without knowing the test pressure. Just like flowing a head at 28” h2o test pressure. That doesn’t mean that’s all the air that can go through the head. It just means that’s what it flows....... at that pressure. If you flow it at a higher pressure....... then more air goes through it. Same with a carb....... pull on it harder than 1.5” Hg...... it’ll pass more air. My experience is that in most cases, unless there is a fairly high pressure drop across the carb...... there won’t be a real big difference in power from only reducing the pressure drop. Example....... swapping from the Demon 850 to a Dominator 1150 on that street 572 was only worth 7hp...... some of which was from the 2” adapter on the intake instead of a 1/2” spacer. However, on an 18* SBC Busch motor running the mandated 390cfm carb, pulling nearly 8” of vacuum at 8300rpm.......swapping to an HP950 would pick up around 100hp. The air flow through the carb/motor goes up as the rpm goes up. More rpm pulls on the carb harder, and the pressure differential goes up. There is way less vacuum at WOT at 2000rpm than there is at 7000rpm....... yet the flow keeps going up(as the vacuum goes up). The other thing to keep in mind is, it matters where you’re taking the reading. If it’s near the throttle bores of the carb where the velocity is high, there can be some Venturi/siphon effect going on near the vacuum port...... inflating the numbers(over what the true manifold vacuum is). As an example of bigger isn’t always better...... and that the quality of the mix is a big player in power output....... on a 452” bracket build I tested, where multiple carbs were tried, the biggest one didn’t make the most power, and the smallest one was arguably the best overall, including making more power than the biggest one. If we are talking about that big Indy headed motor....... I thought during one of the tests a 1050/4500 carb was tried with no big changes in power. One last thing....... if you do some digging you’ll see that the generally acknowledged “real world” flow rating for an original Holley HP950(80496) is 830cfm....... not 950. https://blp.com/cart/index.php?main_page=index&cPath=72http://www.candsspecialties.com/ratings.html
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
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Re: Carb Question?
[Re: rb446]
#2742234
02/12/20 12:20 PM
02/12/20 12:20 PM
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Joined: Oct 2007
Posts: 4,205
New York
polyspheric
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if the vacuum is high than that at WOT the carb will flow more than it is rated for, if it's lower than that it will flow less than what it is rated at
Correct
To estimate engine vacuum: CFM = D × peak power RPM × VE (use 100% here for well developed heads and good cam choice) ÷ 3,456
572" × 7,000 × 100% ÷ 3,456 = 1,159 CFM
It will draw more than 1.5" Hg vacuum* from an 850 (and CFM will increase), and less from 2 × 750 (and CFM will drop off). Size is somewhat self-correcting.
* a carburetor much too small will not only increase WOT vacuum, if will do weird things to the mixture curve (too rich on top, but not necessarily linear), and may also trigger vacuum spark or close the secondaries.
Boffin Emeritus
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Re: Carb Question?
[Re: fast68plymouth]
#2742274
02/12/20 02:03 PM
02/12/20 02:03 PM
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Joined: Jan 2003
Posts: 14,492
So. Burlington, Vt.
fast68plymouth
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I went and looked back at the dyno sheet I was shown for the 589.
There is a VE number on the sheet, so there was an air turbine being used, but no column for air flow on the sheet I have....... so that data is just on another page.
The VE shown for peak hp(5500rpm) is 102.9%.
The VE shown at the end of the pull(6000rpm) is 96.8%.
Since we know the displacement, the rpm, and the VE..... we can calculate the air flow.
102.8% @5500 for 589” is 964.5cfm
96.8% @6000 for 589” is 989.8cfm
This probably won’t agree 100% with the air flow numbers on the sheet, since there are some calculations in the software to convert to standard cfm........ but it should be pretty close.
My recollection is, on the sheet from SF, the “VE” is a corrected number, and the air flow is uncorrected(raw).
On the street 572 here, the cfm column shows 959cfm, but the calculated cfm based off the VE number is 976cfm.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
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Re: Carb Question?
[Re: fast68plymouth]
#2742290
02/12/20 02:24 PM
02/12/20 02:24 PM
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Joined: Oct 2015
Posts: 570
UK
rb446
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If you’re referring to that Indy headed motor making 700hp...... it’s “extremely unlikely” that motor was using almost 1100cfm. On a more serious effort 572” build here....... the power output while using 1091cfm was 893hp. The 572 street build that was similar to yours was using 959cfm@6100 while making 703hp with a Mighty Demon 850 carb. Any air flow rating “number” is useless without knowing the test pressure. Just like flowing a head at 28” h2o test pressure. That doesn’t mean that’s all the air that can go through the head. It just means that’s what it flows....... at that pressure. If you flow it at a higher pressure....... then more air goes through it. Same with a carb....... pull on it harder than 1.5” Hg...... it’ll pass more air. My experience is that in most cases, unless there is a fairly high pressure drop across the carb...... there won’t be a real big difference in power from only reducing the pressure drop. Example....... swapping from the Demon 850 to a Dominator 1150 on that street 572 was only worth 7hp...... some of which was from the 2” adapter on the intake instead of a 1/2” spacer. However, on an 18* SBC Busch motor running the mandated 390cfm carb, pulling nearly 8” of vacuum at 8300rpm.......swapping to an HP950 would pick up around 100hp. The air flow through the carb/motor goes up as the rpm goes up. More rpm pulls on the carb harder, and the pressure differential goes up. There is way less vacuum at WOT at 2000rpm than there is at 7000rpm....... yet the flow keeps going up(as the vacuum goes up). The other thing to keep in mind is, it matters where you’re taking the reading. If it’s near the throttle bores of the carb where the velocity is high, there can be some Venturi/siphon effect going on near the vacuum port...... inflating the numbers(over what the true manifold vacuum is). As an example of bigger isn’t always better...... and that the quality of the mix is a big player in power output....... on a 452” bracket build I tested, where multiple carbs were tried, the biggest one didn’t make the most power, and the smallest one was arguably the best overall, including making more power than the biggest one. If we are talking about that big Indy headed motor....... I thought during one of the tests a 1050/4500 carb was tried with no big changes in power. One last thing....... if you do some digging you’ll see that the generally acknowledged “real world” flow rating for an original Holley HP950(80496) is 830cfm....... not 950. https://blp.com/cart/index.php?main_page=index&cPath=72http://www.candsspecialties.com/ratings.html Yes Dwayne, only another 3hp with a 1050 Dom/adapter which@only 6000 max is possibly understandable?.......I was not there for dyno day so cannot say what/how things were done but at 5500 vac was 1.7, VE was 103%, at 5900 it was down 1hp pulling 1.9 vac and 99.7 VE, I'm not sure what that tells me apart from the carb is too small? ....and yes 950HP is an 830 carb. I have been looking at big 4150's with claims from vendors of 25>35hp inceases giving them all the specs, as its not my money I don't want to go into it unless I can believe or be convinced that a 1050 4150 will actually flow or make the hp. Unfortunately as we haven't run the car yet (in May) we don't have any worthwhile data apart from being told the engine is very snappy with the 950 on fire up for a 4.500 stroke. So perhaps we will wait till the car has some passes on it and then decide the best way forward. Really I don't seem to have got an answer to my question but I guess the only way is to try things, perhaps I'm trying to ask questions that cannot be answered for a specific combo, wouldn't suprise me if it went faster with an 850DP which we probably could borrow from 1 of the other racers At the end of the day this is a brkt car so max hp is not really the concern, (apart for me) its repeatable 60's and ET, losing some@the top end will hurt mph but its the former that matters most, perhaps we can get both from a bigger carb.
Last edited by rb446; 02/12/20 02:38 PM.
1969 'Cuda 446ci, best 9.96@133.9 in 1990
1971 340 'Cuda, best 11.01@122.8 in 1987
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Re: Carb Question?
[Re: rb446]
#2742301
02/12/20 02:44 PM
02/12/20 02:44 PM
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Joined: Jan 2003
Posts: 14,492
So. Burlington, Vt.
fast68plymouth
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Really I don't seem to have got an answer to my question but I guess the only way is to try things That’s always going to be the case.........Unless you’re dupicating an existing combo where someone else has already done all the testing prior to arriving at the “known good” package of parts to use. The street 572 here picked up only a couple hp going from the 850 Demon to a QFT Q-1050 4150 carb. As said previously, the 4500/1150 was worth about 7hp. Frankly, I don’t see how there would be 25-35hp if swapping only the carb on that motor....... unless the current one kinda sucks. I don’t know how much stuff you guys tested, the street 572 here was tested with 3 manifolds, different spacers and no spacers, and 3 different carbs. Over 30 pulls over 2 days on the dyno. As you’d expect..... the biggest carb on the biggest intake using the tallest spacer made the most power. But the smallest carb on a smaller intake with a smaller spacer was down only 21hp. Worst to best was about 40hp. That’s the smallest carb, no spacer, dual plane intake .......to the biggest carb, biggest spacer, 440-2. Seems like a stretch that just a carb swap would get you another 20+hp when youve already got the big manifold on the motor.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
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Re: Carb Question?
[Re: fast68plymouth]
#2742320
02/12/20 03:15 PM
02/12/20 03:15 PM
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Joined: Oct 2015
Posts: 570
UK
rb446
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Is there anything here in these numbers that shouts out anything?......5500 (max hp) vac was 1.7, VE was 103%, at 5900 it was down 1hp pulling 1.9 vac and 99.7 VE?..........I can't see 20+hp from a 1050 4150 either, just vendors making claims.....Of course if this motor was 12.5:1, .750 roller it would be easier to know what carb to go with.
Last edited by rb446; 02/12/20 03:36 PM.
1969 'Cuda 446ci, best 9.96@133.9 in 1990
1971 340 'Cuda, best 11.01@122.8 in 1987
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Re: Carb Question?
[Re: rb446]
#2742332
02/12/20 03:42 PM
02/12/20 03:42 PM
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Joined: Jan 2003
Posts: 14,492
So. Burlington, Vt.
fast68plymouth
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Well, here’s some numbers for perspective......
572”, 14:1, .825” cam, 440-3x, 1150 carb, cnc365’s.
918 Peak hp at 6800, VE @ 6800 was still 107.5%.
Peak tq is occurring 100rpm higher than that 589 is making peak hp.
It’s the sum of all the parts.
If you want to make significantly more power, you either need way more efficiency and/or cubes...... and move the tq peak up the power band...... and have it fall off slower.
You’ve got low CR and a small cam..... with heads designed for high compression and a big cam.
The sum of that pile of parts is apparently about 700hp.
“If” I bought a carb for it, it would be a 4500........ with the understanding it might not do much of anything to the ET with the motor in its current configuration.
I’d only be making that purchase if making upgrades to the motor combo that could really benefit from a 4500 carb(cam & compression) were part of the long term plans.
However........ I wouldn’t buy anything til the car has been to the track a few times.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
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Re: Carb Question?
[Re: fast68plymouth]
#2742371
02/12/20 05:40 PM
02/12/20 05:40 PM
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Joined: Oct 2015
Posts: 570
UK
rb446
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Yes thats how I see it as well, not enough comp, race heads, too small a cam etc., was just trying to understand the 0@WOT vac -vs-1.5+vac. ONLY way is try it and see. Got another racer here with same/size spec motor but @12.5:1, .765 roller who now runs 2 x4 Doms which make virtually no more power than a single 1250 did, was expecting around +30hp
thanks again for the comments.
Last edited by rb446; 02/12/20 05:47 PM.
1969 'Cuda 446ci, best 9.96@133.9 in 1990
1971 340 'Cuda, best 11.01@122.8 in 1987
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Re: Carb Question?
[Re: rb446]
#2742388
02/12/20 06:16 PM
02/12/20 06:16 PM
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Joined: Jan 2003
Posts: 14,492
So. Burlington, Vt.
fast68plymouth
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I’ve done the Indy TR swap on the dyno twice......30+hp both times.
68 Satellite, 383 with stock 906’s, 3550lbs, 11.18@123
Dealer for Comp Cams/Indy Heads
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