carb CFM

[john a]

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

[My427stang]

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!

[wayne]

I have always been able to make a carb that was a little big run better then one that was to small.

[Falcon67]

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.

[e philpott]

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

[Joe-JDC]

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

[plovett]

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

[plovett]

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

[Barry_R]

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...

[fekbmax]

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 ?

[john a]

So the 850 flows more than a 950?

[My427stang]

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.

[jayb]

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.

[turbohunter]

Thank you guys. This is a cool discussion.

[john a]

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