Carburetor talk.
 

Seems like there's been lots of carb questions, and discussions lately. Thought I would start a thread, about carb's, their functions, applications, etc. I am certainly no carb expert, so please feel free to chime in on any of this. There are soo many carb myths out there, that one must try to forget about what was the trend in 1985, and what your brothers, friends, uncles, cousin put on his car. First thing I would like to talk about, is the most obvious, and most common question. Carb size.

Lets first realize, this is offshoreonly. Just about all of us, own high performance boats, with big block chevy engines. Today, if you have a 454, its considered an infant, in the marine engine world. 30 years ago, it was king of the street. Many guys here, are running 502, 540, 572, 598, and even larger ci engines. These are BIG engines, and in most circumstances, need a good size carb.

One thing many assume, is that a larger CFM carb, will simply use more fuel. Another assumption, is two carbs, must use twice as much fuel as one carb. Forget all that. There is no changing, that it takes X amount of fuel, to make X amount of HP. Whether you're making 600HP with a 750cfm, or 600HP with a 1050 CFM, if both engines are tuned PROPERLY, there is no measureable different in fuel consumption. Remember, CFM is Cubic feet per minute of AIRFLOW, not fuel flow.

There is the common myth, that too large of a carb, can hurt low end power, be hard to tune, sluggish, etc. This is true. However, going back to my statement, that this is offshoreonly, and we must put into perspective, what we are running for engines here. This is NOT 1985, and we are not, running a 300HP 454, in our saturday night cruiser Monte Carlo. Sure, back then, we slapped a dominator on it, hoping it would be super awesome. Problem was, that was in fact, way more carb then needed, and, that old dominator, is a dinosaur, compared to whats on the market today.

The biggest hurdle with that scenerio, was at low speed, the airflow signal thru the venturis of the carb, was simply so weak, that getting good atomization, or fuel delivery from the boosters, was simply hard. Especially on a car, that needs to be able to cruise around town in top gear, lugging at 1500rpm. However, today, carb companies have developed different booster options , venturi sizes, etc, that help with that scenerio.

Back to where we are today, engine wise. Many guys here, are purchasing very nice high flowing cylinder heads. Very nice roller camshafts with higher lift. High flow intake manifolds, high flow exhaust, etc. But why is flow so important with that stuff, but yet so many want the bare minimum from their carb? Why? Because of myths .

I see many times here, that guys make statements like "my engine made xxx HP with XXX carb". Lets say he makes 650HP with an 800CFM holley on the dyno, with an air fuel ratio of 12.5:1. Does that mean, that everyone else should bolt an 800CFM holley on their 540, if they are in a quest for the best carb possible? No.

I've also seen where guys have said, "I swapped my 800CFM out, for a dominator on the dyno, and only gained 10hp". Well, lets talk about that. Were each of these carbs tuned properly for the engine? Or were either one slapped on, did a pull, saw no big difference, so called it a day.

We need to think of a carb, like we think of EFI throttle bodies. While obviously a carb is way more complex, as its job is to deliver fuel, its other half of the job, is to deliver air. I think many focus on the fuel part only. Modern aftermarket BBC throttle body kits, normally come with some pretty good size throttle bodies. Rightfully so, as their alot more forgiving when erroring to the "big" side of things, than a carb. But, if a guy was building a 650HP big block, people would laugh if he said , "im going with a 750cfm throttle body". No, guys would be telling him, "you need a 1200CFM throttle body. But, if he said he was going to put a 1200CFM carb on, they'd blast the poor guy, and tell him he's a moron, and his engine is gonna run like crap.

Making big power, with a smaller than textbook carb, is very possible. happens everyday. The question you need the answer to, is will you make even MORE power, with a larger carb. Maybe, maybe not. Just something's to ponder, when shopping for carb's. Here is a carb CFM calculation I like, from Prosystems.

CID x RPM x V.E. / 2820 = CFM
350 x 6600 x .9 / 2820 = 737 CFM

Using that equation, here's a couple common examples of a modern high perf marine engine, turning max 6000RPM, using 90% volumetric efficiency.

454ci = 869CFM

502ci =961 CFM

540ci = 1034 CFM

572ci = 1095 CFM

598ci = 1145 CFM

Now, lets look at a basic merc engine.

454 420HP Merc, max RPM 5200 = 753 CFM (came with 800CFM holley)

502 HP500 max RPM 5200= 833 CFM (came with 830CFM holley)

Recently, an marine engine builder friend, built a mild 509 for a customer. Started out as a basic HP500 engine, then was treated to a set of brodix heads, cam upgrade, couple other little things. Nothing fancy at all. Low compression 87 octane build. I can tell you, dynoing back to back, with the stock carb, against a 1050 CFM modern 4150 carb, the power gains were nothing short of impressive. Both him and I were thinking maybe, 20hp max or so on a mild build like that. We were wrong. It was substantially more than that. I don't recall the exact number, but it was definitely enough to make a new carb purchase, very worthwhile. :ernaehrung004:

Another thing I read here often, is carb "reviews". I see many guys saying how well their particular carb works, whether it be from a custom carb shop, off the shelf from holley, QFT, or whomever it may be. We need to understand, that just because a carb doesnt backfire, idles good, and all that, isn't necessarily the end of the line.

I have been around a fair share of carbs in my time, and worked on tuning a bunch with buddys. Up until recently, I had a very hard time distinguishing from what I thought was a "good" carb, to a bad carb. Today, with portable widebands, it really helps being able to look at whats really going on. Whether its idling, cruising, or wide open. I've seen carbs that ran very well, or what thought was well, be so far off from a textbook fuel curve, it was sad. I really recommend purchasing a wide band, and installing it. Under many circumstances, the fuel savings alone, can pay for the cost of the wideband setup in short time. Not to mention, the whole "burning" pistons part of things.

I've been slammed, or criticized, for recommending quick fuel carb's here on oso . I would like to explain WHY, i have recommended them. First, I have 0 interest in their company, I drive a friggin truck for a living. So lets get that out of the way.

The reason I recommend them, is not because its bolt on and go, its because of the hardware you get, for the money you spend. They are very tuneable.

I see it time and time again here, that guys say things like "my guy can set my carb up so its perfect, bolt on and go". All I can say is that, to do that, it is extremely hard. Why? Because simply, there is no way to be able to determine the load on ANY engine, in ANY given boat. How does the carb guy know what percent of throttle input, your boat will cruise at? Oh, you tell him 3500RPM? Is this in a cat, single engine vee, twin engine vee, stepped or straight bottom, is this tabs down drives down, or tabs up drives out? Is this propped for 5000RPM max, or 6000RPM max? What I am getting at here, is there are an extreme amount of variables. Another reason, a dyno sweep pull, isn't the end all for dialing in a carb either. Again, varying loads and conditions.

Perfect example. Past two weekends I was out doing some readings on my AFR's. I have roots blown engines. Water was rough. This required me to drop my tabs down, and keep drives tucked. At 3200RPM, I had looked at my meter. AFR was 11.7ish. I thought to myself, "damn, I tuned this thing to cruise there around 12.5 last summer". Then it dawned on me. The engines were loaded down quite a bit at 3200RPM. So, I ran in some calm flat water. Raised tabs, trimmed drives nuetral, and brought to 3200RPM again. Bingo. AFR was now back in the mid 12's. The reason this took place, is because with the tabs and drives tucked, and engines loaded down, it required more throttle opening, to maintain 3200RPM. What happens is, the vacuum drops. What happens when vacuum drops, power valve opens. So, my "Ideal" tune, was ONLY ideal, in calm water. Is there much I can do about that, not really. If I change to a later opening Power Valve, I will then go too lean at calm water cruise. But, I only bring that up to make a point. Now, how on earth, can some guy, sitting in his carb shop, possibly know, how to set MY carbs up, based on solely my engine specs??? If I removed my 28P props, and put some 31's on, guess what, I'd be rich in calm water now, because the props are loading my engines harder, and again, opening that power valve. I'm sorry, but I don't know any carb guys who are that good.

Hence, why I strongly suggest, investing in a wideband. Tuning a carb is not that hard, unless of course, you have a carb that isn't very tuneable.

Alot of misconception on powervalves, and their operation. First one, is that if a powervalve "opens" at idle, it will flood the engine. This is not true. When a powervalve opens, it simply add's fuel to the main well, just as main jets do. If you took out 82 jets, and installed 92 jets, the engine will not idle any different. A power valve opening, simply add's more fuel, hence why if you block a PV, you must increase jet size. Now, a BLOWN powervalve, can flood an engine. Why? because when the vacuum diaphram is torn, it will simply suck fuel from the bowl, right thru the valve, into the intake, thru the power valves vacuum source.

I mention power valve channel restrictions often here. Reason I do, is because the power enrichment circuit of a carb, is highly tuneable, or should I say can be. Typical old holleys, all we could do it change "when" the PV opens, with a different PV. With a modern carb, like a quick fuel with billet metering plates, they have screw in power valve restrictions. Which, are nothing more than mini jets. Many years ago, stock holley's had certain size pcvr's. A 750 carb, would have much smaller pcvrs', than a dominator. Therefore, when that pv opens , the dominator can move maybe 12 jet sizes of fuel, while a 750 may only move 6 jet sizes of fuel when that happens.

But, here's the cool part. Lets say you go and buy a quick fuel 850 or whatever. You go out in your boat. Your air fuel ratio is awesome, from 2500-4500RPM. But, as soon as you hit 4600RPM, the PV opens, and afr dips into the "rich" . Now, traditionally, you may say, "well, I'll just back down my secondary jets". But, at 4500RPM, you are already into your secondaries. By removing secondary jet, now your "perfect" afr at 4500, isnt so perfect anymore. Now its lean. So then you might say, "well, I'll add some primary jet". Then, again, your cruise, and 4500RPM afr's, are now rich, in a quest for the optimum wide open AFR number. However, by being able to tune your PCVRs, you can now tailor, the amount of fuel introduced, WHEN, the power valve opens, without touching any other area of the fuel curve. Again, this is something you simply cannot do, on a dyno . The power valve circuit, can be looked at as a 3rd circuit, or a wide open throttle circuit. Being able to tune that circuit, is simply a nice feature.

Doing a sweep pull on the dyno, is great for getting a wide open throttle baseline. It gives you an idea, of how MUCH fuel, your engine needs. However, it does NOT tell you, where it needs the fuel. We can add fuel from the primary, secondary, and power valve circuits, to get a good fuel curve. On the dyno, its simply taking fuel from ALL circuits, as the carb is wide open. This is NOT , real world scenerio. Its alot better for a drag car, then a pleasure boat.

Good info.

Here's another big thing. Many carb cfm #'s are WAG's. Wild Ass Guesses.

Some are flowbenched dry. Some flowbenched wet.

When measured on flowbench (wet or dry) many don't use the same flowbench air psi depression, which leads to many different #'s.

Here's an example I did with a throttle body I massaged and had flowed at different depressions. Many flowbenches use water column height.(H20 or WC) instead of vacuum or psi.

587cfm at 10" H20
716 at 15" H20
827 at 20" H20
927cfm at 25" H20

Rob, how about you run one of my new Gen 3 Holley Dominators in your boat and see if you like the result? This weekend will be our last run in 2015. I could ship it to you next week.

Great info Mild, I have been playing with carbs forever it seems, and now in a world of EFI seem to get looked down upon, was told by many would never get a blowthru to work properly...well that just ain't so...after flogging a demon for months and working with holley tech..thats a crap shoot, was finally told that a 850 was not going to work after they had told me to run it. So then went to quick fuel, because I liked the way Zack talked on their tech line, and within 3 weeks had it dialed in and the AFR's were great and easily adjustable, as Mild said the key now is the power valve tuning with the changable channel restrictions as well as they restrictors in the meetering block...so Mild, great info and yes these "old" carbs can work very well...

At what RPM do the boosters start flowing? If they aren't flowing power valves are not in operation. You would be on the transfer slot.

All depends on booster design (annulars will start sooner), throttle bore + venturi size, air bleed size, and throttle opening. vs the airflow which is determined obviously by engine cid, camshaft, cyl head, intake manifold, throttle opening , exhaust and etc.

On the same engine, A smaller throttle bore/venturi carb with annular boosters and smaller air bleed will start flowing fuel a ton quicker than a huge bore/venturi with larger bleeds and say a dog leg booster.

Correct on the power valve statement.

Following with interest

great reading mild.one thing to remember.carbs have been around for a very long time.the holley carb is one of the simplest one s to work on and reliable.i feel good about being able to fix my carb when it breaks or gets out of tune.the efi guys can look down on me all they want.i got a product that works and has been time proven.

Going rich at high RPM might have nothing to do with jetting but could be acc. pump pull over.

is extremely easy starting an indicator of anything on a carbed motor? Does it reflect the tuning at all?

Idle circuit tuning.

Heated intake helps. Most of us don't have heated intakes though.

Initial ignition advance setting is just as important as the carb tuning itself.

Ooops. Double post.

I like to "Lock Out" the timing..meaning ZERO mechanical advance. Put an MSD Start retard on if you do this! Smooth out the idle a bunch!

Yes, Really helps when you are running a long duration camshaft. Thats one of the nice thing about some of the new programmable ignitions from Daytona sensors, msd, etc. You can have lots of timing at idle, bring it back once off idle, then bring it back in gradually, or quickly, or based on a map sensor value, etc. Locked timing is cool for idling, but on certain setups, can really cause issues.

Yeah, I kinda like the idea of the AD Boosters too! Looks like Holley doesn't offer an Annular Discharge Booster for their Ultra XP series carbs.....bummer. We're about to order my Holley carbs. We plan on using the Holley Ultra XP carbs (950cfm).

Anyway, I enjoyed the read guys.

I still need to get in and closer tune mine, waiting for Mild to make a trip to Ohio and help... lol I been just running and enjoying the boat he last 4-5 weeks, after Labor Day I will start tuning again and beating it more...

So what wideband do you use?

I bought a F.A.S.T. system for mine and monitor all the time, not sure what Joe uses.

I have a portable NGK powerdex unit. It is very basic, with an easy to read large display, a calibration mode, and uses a NTK oxygen sensor. Most use bosch sensors. I used it on several engines, including my own, and have good luck, still on the original sensor. Downside is, NGK is no longer making it. There is another company making it now.

http://www.ebay.com/itm/Ballenger-Mo...a3bd58&vxp=mtr

Lots of options though out there. AEM, Innovate, Fast, etc. The Fast meter, is available with dyno cell cables, which are longer, and work well for boats with long run of 22ft cables to the dash. #170602 These guys seem to have the best price on it from what I have seen on the web.

http://www.dfperformance.com/airfuel_meter.html

First off, great read! I am planning to buy new carbs this fall as I absolutely hate the demons. I will also be adding bungs for O2 sensors.

Silly question - do you run 2 AFR screens at once or move them back and forth between engines? I see they have a 2 sensor option but I assume that's two sensors per engine (both exhausts)?

Thanks
Jim

Hey Jim. I would run both sensors on one engine. Find out which side runs leaner. Tune off the leaner bank. Then do other engine. Once you know lets say your odd banks are the leaner ones, you can then install your sensors in each engines leaner banks for further tuning.

Carbs are certainly not as good as port efi as far as optimizing fuel mixtures , and cyl to cyl distribution. But they can be made to work well, and be user friendly, and reliable. Later i will touch base on supercharger carbs.

Can you lock out the timing with the DUI distributors?

Cool thanks, who installed your bungs, and do you have pics?

Joe, what bank on your motors is running leaner, if at all?

On mine, they are actually almost dead nuts. But, I think that is due to the straight lobe blowers. When I ran the Fast dual channel better, the banks were just about .1 to .2 max different from each other. However, some engines, can be off quite a bit from bank to bank. Plus, cyl to cyl. Since the AFR is taking an average of the 4 cylinders on that bank, we gotta remember, that while the meter might show your target AFR number, but certainly can have a cylinder running leaner or richer than that number.

Its been said, a gasoline engine makes it best power at 12.5 AFR. Thats on pure gas. Throw ethanol blended fuel in, and now that number goes down, or "richer". Icdedppl's blown 540's were a good example. We always dyno tuned our stuff, with our own fuel. We go to the local shell or citgo, and that is what goes in the dyno's fuel cell. His engine, simply made the best power, with an AFR of 11.5. We tried leaner, we tried richer, and power wise, 11.5ish was the number. So the old theory that "lean is mean", etc, doesnt hold true for all engines.

A port injected, precisely tuned efi system, can certainly get away with leaner afr readings at the collector. With a carb'd engine, or throttle body EFI, I'd be careful chasing internet textbook afr numbers. Currently, my engines are 11.1-11.3 at wot. I tried leaner, gained nothing in power or speed. I mainly run ethanol blended fuel, as thats all we can get around here.

To touch base a little on supercharger carbs...

This is an application, where you really don't want to cut yourself short on carb CFM. One thing superchargers like a roots, don't like, is a restriction on top of the blower. For example.

Your buddy builds a 540 with an 8-71 blower setup. He makes 850HP, with 7lbs of boost, 12% overdriven, and a pair of 750 carbs. Your other buddy, builds the exact same engine, but using a pair of 1050 carbs. He's making 7lbs of boost as well, but blower is only 3% overdriven and made 875hp.. What happened here, was buddy #1, was undercarbed, and to get the boost he wanted, he had to drive the blower harder. We all know what happens, when you spin the blower faster, more heat.

Using this BDS blower calculation figuring 7lbs of boost and 6000 rpm {(CID x RPM) ÷ 3456} x {Boost ÷ 14.7) + 1} = CFM required. Most blower shops, say going as much as 30% larger than "required" is acceptable.

454ci=1163 CFM + 30%= 1511 CFM (twin 750s would be my choice)
502ci=1285 CFM + 30%= 1670 CFM (twin 800s would be my choice)
540ci=1383 CFM + 30%= 1797 CFM (twin 850s would be my choice)
572ci=1465 CFM + 30%= 1904 CFM (twin 950s would be my choice)
598ci=1532 CFM + 30%= 1991 CFM (twin 1050s would be my choice)

Thats a mild setup. How about the guys running more boost? Today with intercoolers, guys are running 9-10lbs of boost, and some are turning engines even faster than 6000. So lets look at a 6500RPM, 10lb boost deal.

454ci= 1433 CFM +30% = 1863 CFM (pair of 950s would be my choice)
502ci= 1586 CFM +30% = 2062 CFM (pair of 1050s would be my choice)
540ci= 1706 CFM +30% = 2218 CFM (pair of 1150s would be my choice)
572ci= 1807 CFM +30% = 2349 CFM (pair of 1150's would be my choice)
598ci= 1889 CFM +30% = 2456 CFM (pair of 1250's would be my choice)

Now, some might read these numbers, and call bs. And it they may be. However, when I look at this stuff, I kind like looking at what mercury racing did in their choice of parts. One thing about mercury racing, is cost is not an issue, and their team certainly spent lots of time with R&D on their engine packages. While very outdated, the old 900sc, and 1000sc merc engines, were 540ci, 6000RPM, running 10-12 lbs of boost. I am sure they tried several carb variations. They could have put any set of carbs they wanted on top of those engines. They put a pair of 1050 dominators on them. I would like to believe, that they did it for a reason. It certainly wasnt for cost, it certainly wasnt for fuel economy. Even the 525sc, the little 454 with 177, that turned 5200RPM, got a 1050 dominator. The numbers I posted, would be right in line with that. (it would have called for 915CFM minimum according to that calculation, and 30% larger would have been 1189 CFM). So you can see, its right in that range.

I know of several guys, who have swapped out smaller carbs, for larger carbs, or even throttle bodies when converting to EFI in one case, saw a gain in boost pressure, without changing pulleys. They had to slow the blower down, to get their boost back in line. This is why I cringe, when I see guys running a 250 blower, on a hot 502, or 540, with a single 1050 carb. Now, can you make big power with smaller carbs, of course. And if going to a larger carb nets nothing, than obviously no point in changing. But, don't spin the blower any harder than you have to. Let the carbs move the air in, not the blower sucking it in. A vacuum gauge under the carbs is a nice tool here.

Remember the big argument of needing boost referenced power valves, so the PV doesnt get "sucked" close at high rpm? Well, that is not an issue, if the carbs are properly sized. If you are ever pulling 4, 5, 6 + inches of vacuum under your carbs at wot, you are severely undercarbed. it was a reality back in the day though, when guys were building bigger engines, cranking the boost up, and trying to run a single small carb to feed it.

yes, I run mine locked, seems weird but I removed all the advance weights/springs and used mechanics wire to lock it and twisted tight rotor back on and works fine...

I also use the NGK works great..as long as you keep the 02 dry !

So where can you put the bung and keep things dry?

They go in the risers, I talked to Stainless marine and they will install them, but I called Orlandi performance, and they will do it for $100 plus the cost of the bungs. They are about 20 minutes away, so it saves the shipping hassle.

I am running Stainless exhaust, on 454 with 177."s. I burned a motor last year. But before I put the motor back in I had it dynode tuned. Well now after reading your post, I really think I need the AFR readers in my boat,.

Where should I have the bungs welded on my exhaust.

Thanks Steve

I also had mine dyno tuned. Even the person I had dyno it said I should get them installed. We left it a little rich off the bat, so I am going to drop a size right of the bat on the secondaries. I want to monitor my cruise AF ratio, seems like I use a LOT more fuel than I should compared to when it was stock, my guess is it is is WAY fat part throttle (3000-3300rpm).

This thread also pushed me to finally do it.

dyno tuning will get you in the ballpark but yes you need 02's to get dialed in right, dyno's are not running down the river for long periods...

What size or style fuel/water separator are you guys running. I have the stock one seems really small compared to my fuel lines. Seems like the in and out lets are 1/4 inch.

Running 454/ with 177 Weiands.

Steve

Just the stock one, I just change it every time I change the oils, for $5 I don't know why you wouldn't, it's also super easy to change on mine.

Stock one is 3/8 in/out. Merc ran them with no problems on the 525sc. Above that HP level it propbably needs upgrading.

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