FE Power Forums
FE Power Forums => FE Technical Forum => Topic started by: john a on December 10, 2015, 01:44:18 PM
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I notice on most 445 builds they have 750 carbs. When I originally built mine in 2006 I also used a750 dp. It ran solid but seemed to me it could use more. I borrowed an 850 dp from a friend and it pulled much better mid to upper rpms with little to no change in the lower rpms. Its a run of the mill 445, 10.-1. Edd heads,rpm intake, no porting, comp 282s, 1 7/8" headers. Both carbs were tuned by the seat of the pants. I would appreciate you're thoughts, opinions, experiences please.
Happy holidays.
John
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I have had a 750, a 1000 vac sec and soon to try a worked 850 DP on mine and it's in a 4400 lb 4x4 and very similar to your build
I personally think that, within reason, you cannot overcarb, only undercarb. The limit being the point where when you cannot tune the main circuit to respond quick enough due to low airflow due to too large of a venturi, the other circuits do not care as much. 445 inches moves a lot of air, if someone said they were running an 850 or bigger on a stout 427 it wouldn't be surprising.
That being said, the gains would only be at WOT and only at higher load and/or RPM, so depending on use the bigger carb may not be needed.
My hunch, your engine probably likes it, 445 cid and decent overlap pulling on a big intake port, makes sense to me!
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I have always been able to make a carb that was a little big run better then one that was to small.
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750DPs are pretty common and cheap. Lots of tuning info around. 850s are considered kinda lazy on the bottom end because of the large venturi. Big CID under one tends to mask that issue. Lots of people have had good luck with the Holley 950s, which seems to be a good compromise. What I like to use here is a ProForm body and baseplate, with Quick Fuel blocks. The ProForm body has been tested and seems to flow around 840 CFM at the normal rating criteria. One thing to keep in mind is that 4bbls are rated at 1.5 in/hg pressure drop. If at WOT your selected carb reduces that to .5 in/hg, your effective flow rate is much less than the name plate. The lower pressure drop also means that more atmospheric pressure is being applied to the induction system, which can result in more power generation.
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the quick rule is double the engine size for carburetor ..... 445 +445 = 890 ..... so the 850 falls within that rule of thumb ....myself , I have been running a Holley 1000 HP series double pumper with annular boosters for 15 years on a 416 and have no complaints
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I would suggest a simple vacuum gauge test at WOT to see how much vacuum you are still pulling. If it is more than 1.5" vacuum at peak RPMs/load, then it can use a larger carburetor, or better intake manifold, or both. If the vacuum is less than 1.00, then the carb may be too big. Joe-JDC
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I run an 850 d.p. on my 428. I ran it on the dyno and I remember it pulled right at 1.5" on the top end. This was on a 517 hp 428 turning about 6200 rpm.
I now run the same carb on the same motor with more cam and some other mods, turning about 7000 rpm, and 540-545hp or so. It runs great. One change I made was to install a Proform main body. That made a big difference. Maybe it improved the booster/venturi relationship compared to a stock 850 d.p.
Another thing I would add is a carb spacer can make a significant difference. A 4 hole or tapered 4 hole spacer can improve the signal to the carb a lot. This is especially noticeable if the carb is on the edge of being too big. On the other hand, an open spacer can help a carb that is on the edge of being too small, act like it's bigger. On a stock 4781 Holley I'd be inclined to run a 1/2" or 1" 4 hole spacer if hood clearance allows.
I ran a 4 hole spacer on my milder 428 with the stock Holley 850. On my wilder 428 with the Proform main body I run a clover leaf spacer. It's kind of a compromise between the 4 hole and open spacer.
Also, optimum jetting can change when you go from a 4 hole to an open spacer or vice versa. This is because the signal to the carburteror's boosters is affected. 4 hole = less jet. Open = more jet. It could be as much as 4 jet sizes between the two extremes. I would expect the hybrid spacers to be in the middle with regard to jetting.
Another note. I tried to get a Holley HP main body to upgrade my 850, but they told me to go suck rocks, unless I would buy the entire new carb. That's why I got the Proform main body. That was a long time ago, though. So maybe Holley will sell you the main body by itself now. But, I have to admit the Proform main body was well worth it. Responsiveness and power were improved significantly. It was noticeable by the seat of the pants which to me means 15-20 hp.
So, to sum up, on your engine, I would consider trying an open or clover leaf spacer with the 750. With the 850, I'd lean toward a 4 hole or tapered 4 hole spacer.
JMO,
paulie
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Another thought: The number 4780, 800 Holley double pumper, is kind of a "forgotten carb". It is often overlooked. But it offers more flow than a 750 without, I think, the possibly undesirable booster/venturi relationship of the 850. I believe the secondaries are slightly larger than the primaries on the 800. I also think it's an underrated carb.
I ran one a long time ago on a 450 hp 440 Chrysler with a TM7 single plane intake, and it ran great.
JMO,
paulie
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That HP 950 is a really nice compromise piece. It was the first HP series carb they released, and I had one of the first ones on my car for quite a while. Its essentially a 750 carb with an 850 base - a combination that was really popular with the racing community for years before they made it "for real".
Real flow was closer to 830. They used the 950 designation because by then many of the carb modifiers had started using fabricated flow numbers for advertising and they felt they need to do the same. A bad, bad thing to do because it was a slippery slope...
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There has been a few rumblings of fabricated flow numbers over the last several years, I'm wondering then Barry, are the quick fuel carbs also useing fabricated numbers or are they closer to reality ?
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So the 850 flows more than a 950?
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So the 850 flows more than a 950?
Yes, if it uses the small venturi and bigger plates.
Like Barry said though, comparing advertised cfm is tough. An HP 750 with contoured entry and no choke is advertised to be the same cfm as a 3310 with a big choke tower which is claimed to be the same as an annular booster 750 with a giant banjo in the air stream. The cfm really indicates a basic plate and venturi combo now more than a real number, at least with Holleys. Good thing is, does 750 cfm vs 780 cfm really make a difference, unlikely,and generally if it did, go one size bigger
The point I'd like to make again is that with some thinking, even with the 850 with less venturi taper (or hourglass, whatever you want to call it), can be tuned to do what you want to an extent, changing the HSABs, raising the float level, even air cleaner design changes how quick the mains can respond. Even booster changes are easy if you have the tool and the ability to tune it afterwards
It's all about getting the airflow around the boosters to make the boosters change what they provide. Idle circuit doesn't matter, it's tunable and just a hole below the plates, transition doesn't care about the venturi shape or size, it just needs airflow around a plates as they open, it all comes down to if engine demand changes, how quick can the booster "feel" something different, and it's affected by many things, the booster, the air bleeds, the emulsion circuits, availability of fuel (float level)
I personally really like a good 4781 850, stock out of the box usually works well, but I tend to prefer a with a pair of 30cc pumps instead of the big 50cc rear. I have also had good luck with the 1000 HP series, but only after a lot of thinking.
*** On edit, truth in advertising, I have never actually used or tuned a 950 Holley, but have had my hands on pretty much any other Holley you could think of. They are interesting, I assumed they originally resulted from a creative solution to a class racing restriction, but smart guys sure like them. So I am certainly not knocking them, mostly saying that the 850 isn't the problem child that the 950 made it sound like.
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So the 850 flows more than a 950?
Just to be clear, if you buy a box stock Holley 850 and a box stock Holley 950, the 850 flows more air than the 950. The Holley 1000 carbs are basically the same story, just 850 cfm carbs with some minor flow improvements. They do not flow 1000 cfm. As Barry mentioned, as soon as the aftermarket companies started fabricating their flow numbers, Holley felt they had to follow suit, and now you can't trust anybody's flow numbers.
Also, not to change the subject up too much, but for years hot rodders used a formula to calculate carb cfm requirements based on engine size, RPM, and volumetric efficiency. For example, take a 428 cubic inch engine, multiply 428 by 6500 RPM (peak engine speed), then divide by 2 (only half the revolutions are used to pull air into any given cylinder), and divide by 1728 to turn cubic inches into cubic feet. Then multiply by a VE of 92% (0.92). You end up with an airflow requirement of 740 cfm, so it sounds like a 750 cfm carb will work great. But the four barrel carbs are rated at a 1.5" pressure drop, which means that on this engine at 6500 RPM, you will have 1.5" of manifold vacuum. You'd really like to have much closer to zero manifold vacuum in order to make peak horsepower; on the dyno I don't think the carbs are sized correctly unless they are giving 0.5" vacuum or less. So, provided you can tune a larger carb and make it work acceptably in the lower speed ranges, bigger is better with carburetors, for power production.
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Thank you guys. This is a cool discussion.
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Thanks for all The replies, very informative. Shame on the crab manufactures, makes things very confusing to the uninformed. As I said my 850 runs nice. I wonder, has anybody comparred the two different carbs on the dyno on a mild built 428-445?
John
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>Shame on the crab manufactures
God made the crabs, Holley makes the carbs ;D OK, slap me. 8)
FWIW - Somebody on one of these lists (some Ford related forum somewhere) in the way back put a ProForm "750DP" bodied 4779 Holley on a Superflow and ran it at the 1.5 in/hg carb rating, netting as I recall 840 CFM.
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On my 452 comet with Edelbrock heads c6 237@.050 569 lift lunati cam I had a 750 .It is a proform main body quick fuel metering blocks .It is really responsive idles good and whenever you kick it naturally it hauls ass!I had a left over 950 proform main body with annular boosters .I had a pair of metering blocks from an 830 bought a new base plate from quick fuel a rebuild kit and some more parts I needed and assembled my carb.It ran circles around the 750 .It idles better going down the highway it just runs better .And when I first kicked it it was like waking the dead .So much more power .And from just a driving perspective it doesn't use any more gas that the 750.I would say bigger is better if you have a larger than normal engine that can flow more air .One thing about quick fuel base plates on their billet aluminum the throttle shafts are milled to increase flow and they use button head screws as well every little bit helps .On the 1000 cfm carbs.do they really flow 1000 cfm? Seems to me a 1050 dominator is a lot bigger than a 4150 series that is supposed to flow 1000cfm .Doesn't a 1050 dominator have 2" throttle plates ? How does that compare to 13/4 throttle plates for the 4150 carbs. I haven't figured that one out yet.Merry Christmas Doug
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Its been a while - but I did a test with a 750 & 850 swap on a 445 and got something like 17 HP in 10 minutes. Also did one with four different carbs and saw significant differences between a 750, a modified HP900, 850 vacuum, and a 950 3 barrel. The 3 barrel "won" that contest - it was a simple matter of airflow at WOT. Bigger is better. You just have to tune around it.
And if you can work around the intake and air cleaner stuff a Dominator is hard to beat.
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Its been a while - but I did a test with a 750 & 850 swap on a 445 and got something like 17 HP in 10 minutes. Also did one with four different carbs and saw significant differences between a 750, a modified HP900, 850 vacuum, and a 950 3 barrel. The 3 barrel "won" that contest - it was a simple matter of airflow at WOT. Bigger is better. You just have to tune around it.
And if you can work around the intake and air cleaner stuff a Dominator is hard to beat.
Or just put two carbs on. Looks stock also. 8)
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I have a belief that most all these issues stem from a plenum being under sized. Plenums should be the same size in cubic inches as the engine it sits on or even larger. There has to be a reserve capacity sitting in the plenum to fill the requirements of a given engine. So in the end there is only one cylinder filling and firing at a time so having to mount large carbs to gain more power makes me suspect.
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The opposite could be true about that though. Too large of a plenum and you will hurt the performance of the engine, and waste fuel. If the carb can easily handle the demand of the engine, chances are the plenum volume won't be the restricting variable. If the carb is too small, a larger plenum will only delay starving the engine of air a moment longer. You can hurt an engine by having too large of a plenum, you can't hurt the engine (within reason, paying attention to AFR) from having too large of a carb.
Eliminating the plenum all together is actually the best solution. Once fuel has left the carb, it is no longer being controlled. A plenum is a large area where air can slow down. If the plenum is too big, the air will slow down just enough to cause fuel separation. This is why individual carbs like Webers develop better power, there isn't a plenum for the air to slow down at any point in the RPM band. So the best intakes would be ones that decrease the plenum size and opt in favor of individual runners that go almost from the head to the carb without sharing fuel. An example of this would be the 2x4 sidewinder intake for the 427 SOHC.
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Never put to much thought into fuel separation. I could see cold low speed issues until an engine warmed upped. But shouldn't goal of the intake manifold be that the ports draw off a semi calm clean air supply though? Yea I get that the IR is the ultimate way to go but I also believe that the inlet of the IR has a radius that air travels across all 360* but in an shared intake manifold it physically can't. So many diff port entries in there causing turbulence or dirty air causing the lack of good flow to the combustion chambers.
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I think in most, maybe all intake manifolds with a central plenum, the flow is much more chaotic than one would expect. It's not like a linear "here to there" type deal. Pressures are constantly changing from place to place, flow changing directions, etc. I could be wrong, but that's my belief.
For instance, I have seen in oil "communicated" from a leak on one side of an intake manifold to the opposite side.
JMO,
paulie
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Phooey!! Too much carb can look great on a dyno and not accelerate near as well in a car.........especially a heavy car. Carb spacers are the same way. Sometimes you can keep stacking spacer on one and keep seeing dyno power go up.....only to hurt the ET slip later. Rather than say the biggest carb is always better, I would say that SOMETIMES a bigger carb is better, depending on the carbs in question. I seem to remember a certain 4150 carb that tested better than a Dominator on Jay's test engine...........but got excluded from the final decision, LOL.
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I think when it comes to an intake manifold plenum, a lot of thought needs to be given to fuel separation and pooling. If you really wanted to get an engine dialed in, I'd run a wet flow simulation on a completed engine and then remove the intake and heads. Look at the dye traces to see how the engine is performing at your critical / desired RPMs. This is an extreme analysis though, that would require a hell of a setup. It would be the most realistic though, short of actually running the engine on a dyno and making various changes.
An intake plenum is very chaotic, and chaos is probably better than calm when you need that air to keep fuel in suspension. If you go for an EFI conversion where the fuel is injected into each individual port, then slowing down the air is better. Laminar flow is the best for performance but if that carries the risk of fuel separation, then it's better to have turbulent air. This is why I say it's better to not rely in the volume of the plenum in the first place. A small plenum and a properly sized carb that has no issues on feeding the engine what it needs. If you can keep a steady and fast supply of air/fuel to the plenum, you can keep it pressurized that way and not risk fuel separation. So the air/fuel is delivered more on a just-in-time basis rather than relying on a reserve capacity.
Remember, I did say paying attention to the AFR. Too much of anything can hurt an engine. A bigger carb will be better if it's dialed in to control the AFR better. That's my point in my previous post. It's better to deliver the air/fuel when it's needed rather than have it waiting around. Also, when an intake manifold is created, it needs to be able to manage the needs of a wide range of displacements reasonably well. So unless you are fabricating your own intake manifold, chances are the intake you are using does not have a plenum volume that suits your engine the best. If you really want to use an off the shelf intake and make sure it has the right volume for your engine, you would need to find out the volume of the intake manifold and then use this volume to dictate displacement.
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Phooey!! Too much carb can look great on a dyno and not accelerate near as well in a car.........especially a heavy car. Carb spacers are the same way. Sometimes you can keep stacking spacer on one and keep seeing dyno power go up.....only to hurt the ET slip later. Rather than say the biggest carb is always better, I would say that SOMETIMES a bigger carb is better, depending on the carbs in question. I seem to remember a certain 4150 carb that tested better than a Dominator on Jay's test engine...........but got excluded from the final decision, LOL.
Is it possible to vary the load on a dyno during a run, to better simulate the actual conditions in a car doing a quarter mile. Instead of a certain amount of rpm per second, could you vary the load based on rpm and calculated speed? Maybe with some "swoopy" electronic controls?
I'm not a big fan of chassis dynos in general, but it might be easier to do there.
paulie
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I seem to remember a certain 4150 carb that tested better than a Dominator on Jay's test engine...........but got excluded from the final decision, LOL.
I dont' remember it that way Blair, I thought that your 4150 carb and my box stock 1150 Dominator were pretty much neck and neck. I'll see if I can find that data...
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I gotta have a little fun Mr. Jay. ;D
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It's all bench racing until it's proven on the track or street. We've all seen plenty of "That won't work as good" examples, only to be proven wrong. In the end, the engine always tells you what it wants, it just takes lots of arguing with it before you listen. It's almost like having a wife ::)
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It's all bench racing until it's proven on the track or street. We've all seen plenty of "That won't work as good" examples, only to be proven wrong. In the end, the engine always tells you what it wants, it just takes lots of arguing with it before you listen. It's almost like having a wife ::)
No wonder I have so many FEs ;D
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That carb comparison (Blair's special 4150 vs. the 1150 Dominator) did actually make my book; graph on page 226. The carbs were very close, but the Dominator came out a little ahead. This was on a port matched Victor with the Dominator bolt pattern, so an adapter was used to fit the 4150 carbs (Blair's carb and a Barry Grant 1000 carb) on the manifold. I suppose that may have colored the results a little, but in any case the carbs were very close. Also of note is that on the Victor that Joe Craine ported, which had the 4150 bolt pattern, Blair's carb beat all the others that were tested, which included the Barry Grant carb, a 1050 Holley 3 barrel, and a Holley 1000 with annular discharge boosters. That carb/intake combination also made more power than the stockish Victor, attesting to the value of Joe's porting job.
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The old Holley three BBLs came in two sizes, 950 and 1030 CFM. The smaller (@950 CFM lol) used the standard booster venturi on the secondary side while the larger just used two bass tubes.
I loved them as their vacuum secondary was forgiving if you went to wide open throttle too early on the street where a double pumper would stall.
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It definetlly is relative to desired RPM use, I have run an 840 CFM carb on a 380 cube engine with big heads and it ran great. I have also run a 650 CFM carb on a 460 and it pulled great due to the tiny head ports on the stock 385. I would size the carb to the rated CFM calculated by your intake and cylinder head CFM, that is going to get you the best idea of where you want to be, but if you want to drive in mid range RPM most of the time go somewhere in the middle.