Carburator Advice

[Hallett19]

coming from working on FI 5.0 mustangs, now having a carbed 460 ford is a whole new world. I got pushed into buying a pile of crap 650 edelbrock automotive carb for my medium modified 460, weiand stealth intake, decked block, slightly bigger cam,anyways, it turns 4400-4600 depending on altitude and I have not checked the mph, but its fast up top, keeps up with alot of offshore I/Os, but I want to go bigger on the carb, I heard from ***boat mag that the holley marine 830 is the best application for boats, if so, does anyone out there think it will get me anymore top end or just suck up a bunch more gas ?

[058]

Holleys will generally make more HP than the Edelbrock [AFB] at WOT. Another Holley that you might want to check out for your application is the list #4781 [850 D.P.] which would work good on the Stealth manifold.

[flat broke]

The ***boat mag also says that Jets like b and c cut impellers. While this is true for race applications where they are spinning the shit out of the engine, I don't think the average lake/river racer would agree. I currently have an 830 DP on my 21'er and the I am very suspicious of its viability for my purposes. My mill should spin 5500 with the A-B that is in the pump right now, but it can't. I may very well be overcarbing the hell out of it. If you run the volumetric efficiency equation for your 460, it would go as follows
(460 ci)(4600 max rpm)/3456=cfm@%100 VE
So at %100 VE your 460 should require about 612.27 CFM. Want even better news, your motor doesn't even reach 100% VE, so assuming you're more in the %90 VE category giving your mill the benefit of the doubt, you would only require 551.04 cfm.
From this you could plug in your desired max RPM (providing the cam could actualy hit it) and come up with a new CFM requirement for your mill. The only problem is that there are other factors to consider when using a larger carb. For instance the mixture could be too rich in the secondary circuit causing a bog thus inhibiting you from reving as high as you would like. Of course you could jet around it, but down jetting an 830 below that of a 650 on the secondary circuit would definitely defeat the purpose of going to a bigger carb. The bottom line... unless your talking (*)(*), bigger isn't always better.
good luck,
Chris

[Cas42]

This subject came up a couple of weeks ago on the Real Jet Boat message board.
here's the original question
At some point soon I want to change from my current 750 CFM 3310 Vacuum secondary to a double pumper. Is there any performance difference between and 800 and an 850 on a BBC that at its best may see around 5000 (currently at 4700 on an berk AA impeller). I keep seeing recs to go with an 850 but when I do the math it would seem an 800 would be plenty. What am I missing
and the answer
Best carb size is based on getting enough flow through the carb to meter and mix fuel correctly with the least amount of restriction. So sizing is based on engine displacement, rpm and volumetric efficiency.
5k on a Berk AA is 450 Hp according to their chart. So let's call that a well-built BBC engine with a volumetric efficiency of 85% Then required CFM is:
454/2 x 5000/1728 x .85 = 558 cfm.
Rounding up, your best bet is to either stick with what you have or go down to a 650 or possibly 600. This is another case of bigger is not always better. The guys at the track running big carbs are usually turning high (7000+) rpm not 5000 rpm.
If you do go with the 850 you may be over-carbed and get poor throttle response, fuel economy, and bog in the low to middle rpm range...and probably no more top speed compared to your current carb (UNLESS your current carb has problems and is not running properly...in which case a good running but oversized carb may in fact improve things)
In any case, at 850 cfm, I wouldn't recommend mechanicals as this will force the secondaries open when the engine isn't ready for more fuel.
based on what you posted, the 650 is probably your best bet. You just might need to find the right jetting and metering rods.

[flat broke]

Cas knows his tacos on the AFB so pick his brains if you decide to stick with your current carb. Sorry Cas, couldn't resist.
Chris

[lostlake]

O.k. Flatbroke, and Cas.. Answer this one:
502, Edelbrock alum 315 runners,ligthing headers,victor jr.manifold w/2"spacer,280 x 570 cam. ( curently a 750) oh yea turning 5200 rpm w/A impeler legend set back, droop, diverter.. Should I be running a Angular 830 or what????? THanks Carb guys.

[phillyray]

Must agree,468... at 5200-5400 rpm.Tried a 850 DP. did nothing but use more fuel,and was flat in the midrange. Went back to my 715 vac. sec.

[Cas42]

Lost,
If you use that formula with your motor and estimating it to have a volumetric efficiency of 90%, this is what you'd get
502/2 x 5200/1728 x .90 = 679.7 cfm
641.9 at 85% VE
717.49 at 95% VE
755 at 100% VE
that is all your engine really needs. The thing is, in order to get the optimum performance for your particular application, you will need to do quite a bit of testing. People, including myself, expect to buy something off the shelf, bolt it on and have your motor spin like a top. It's not going to happen. All carb manufacturers put together generic carbs that will run "good" on many application.
I recently contacted Edelbrock about jetting and metering for my carb with my particular motor. They gave me a set up that will be a real good starting point and said I shouldn't have to change the jets any more than one size larger or smaller.
The numbers they gave me were quite a bit different than the stock jets.
does anyone out there have that website for rebuilding Holley's? I seem to have lost my link. It has a lot of good info on it along with the above formula.

[77charger]

I use a holley 3310 750 vac secondary.Motor is a med modified like yours 70 429 11.5 550 lift cam.dove heads.What i did though was remove the metering block for the secondarys and put the 4150 kit on as it allows you to change the jets.(similar to double pumper but with vac secondarys)I spin an a impeller at 5000 with a pump in good cond.

[squirt]

reciently spent some quality time on my 715 vac sec holley put it on the boat 19' 454 bbc 330hp headers. Results were no bog 65.1 mph for the 1st time out. I'm sold this is the right carb for me even when I add more hp this winter maybe 75-100 still think 715 is good. But I'm just a jet rookie, your results will vary.

[spectras only]

I had my boat for 25 years ,originally with a 460 with 780 Holley vac secondary [practically all BB came with that size or bigger],it's now a 429 cu with a moderate SVO cam,4750 RPM with "AA" impeller, 20 foot Spectra,never bogs on acceleration, this is the original carb.Have the 850 Holley marine vac sec on a 460 19 foot spectra ,turning 5400 with supertrapps, 5600 without. "A" impeller.That one never bogs either.Tried 600's and 650's with all kinds of jetting ,those went flat at 4600RPM with the BB engines.The CFM ratings by Holley are ment for cars rather than jetboats . IMHO http://free.***boat.net/ubb/smile.gif

[bajaruner]

Personally I feel that the volumetric efficiency formula stacks up much like a static mathamatical compression ratio figure, while it will give you a ball park number each engine varies on actual readout depending on cam size and type of heads. As for the application most jet boats run between 3000 & 5300 rpm moving the normal operating range way up the ladder as compared to a street car, with this in mind, and through what most of us have experenced a slightly larger carb comes into play usually 750 vacuum secondary or a 850cfm mechanical secondary depending on the build of the engine and the size and lenth of the boat. heavier boats need a little more low end torque to get out of the whole a smaller carb with vacuum secondary's will do this by creating higher air speed volocity at lower rpm, lighter boats just the opposite.

[Hallett19]

my boat does bog at low RPM's, I have to feather it sometimes to keep it from backfiring when I first mash it.I want to move up but obviously not too much, I too am a rookie with carbs, so I am going to go with this 780 holley and ditch the edelbrock, and go from there, if you have any other advice, speak now or forever hold your peace. thanx guys, you are all my new friends, how can we post pics of our boats so we can see what we are talking about here ?

[Cas42]

didn't work
[This message has been edited by Cas42 (edited September 06, 2001).]
[This message has been edited by Cas42 (edited September 07, 2001).]

[Hallett19]

not sure, would you like to buy it ? I don't know if I will sell it untill I found out that this new one will work real good and if it does, you got first dibs on it. oh, I could not get the link to come up, I am fairly computer iliterate.

[Cas42]

this is from our Tower Park get together a couple of weeks ago. My boat is the one in the front right of the pic. I hope it works
Hallet, whatcha gonna do with that 650 edelbrock?
http://mrmbers.aol.com/_ht_a/ctc100m/big.jpg

[Cas42]

here, try this link and look at the threads with Tower Park in the headings. There are a couple of them on the first 2 or 3 pages of the message board. Each thread has a few different pictures on them.
http://disc.server.com/Indices/152642.html

[Squirtcha?]

Link for Holley rebuild and tuning. http://www.fordmuscle.com/archives/1999/12/holley/index.shtml

[jetboatjake]

MY MATH SUCKS Ok guys run my cfm# Idont get where the 2 after cu comes from and the number after the max rpm SOOOOOO mine is a 496cu in turning an Agressor A that I am told equaites to a Berkley AA @ 5100 Heres the Kicker 1050 Dominator.This is in a 21 foot marlin Cobra, Read HEAVY boat.Was spinning 5250 with A Alum Berkley impeller Running 67mph, now at 59mph with agressor Bronze (Plan to cut Agressor down this winter)Setup is very economical at 3900 just before secondaries come in, and when you open secondaries it GOES. Also no bog on low end. Would seem to me i am overcarbed.But runs good. You guys tell me?
[This message has been edited by jetboatjake (edited September 07, 2001).]
[This message has been edited by jetboatjake (edited September 07, 2001).]

[jeff-in-ky]

I know that my engine is way over carbed but it is the way I bought it and it runs great.It is a built 454 alum. heads solid cam (.600+ lift) lightning headers etc. in a 21 ft. Edge tunnel. It has a holley dominator tunnel ram w/ 2-750 cfm Barry Grant T/R carbs. It does not have a bog at any rpm. It turns a berk. B imp. 5800 rpm. The fuel economy does not seem to be too bad but I have 60 gal. capacity so you don't really notice. Just shows that it is hard to over carb a jet.

[Oldsquirt]

Hey Cas, spelling counts!
http://members.aol.com/_ht_a/CTC100M/BIG.JPG
[This message has been edited by OLDSQUIRT (edited September 07, 2001).]

[jetboatjake]

hey old squrt nice red X (html challenged)
Rob(Also html challenged LOL)

[Oldsquirt]

Here's another one of Cas' boat.
http://members.aol.com/_ht_a/CTC100M/Cas.jpg
[This message has been edited by OLDSQUIRT (edited September 07, 2001).]

[Cas42]

HEY!!! what happened to my e?
thanks Craig

[Cas42]

OK, just because a motor doesn't bog with a large carb on it doesn't mean it's not over carbed. The point of all this is what is the optimum sized carb for a particular application. An engine only needs so much air and so much gas to run properly, adding more of either doesn't necessarily mean it's going to run better or add more horsepower.
An engine of any size can only take in as much air as the cylinders will hold. You're not going to put 25 gallons of gas in a 24 gal tank. So why spend the extra time and money on an 850 when your engine only needs 600?

[Cas42]

Jake,
multiply the cu by the rpms, divide that by 3456 and you get 100% volumetric efficiency. Since very few engines run with 100% VE and most are around 85%, multiply the 100% figure by .85 to get what you need. If you think you have more efficiency based on what you have, I'd use the 90% figure.
496 x 5100 = 2529600 divided by 3456 = 731.94cfm at 100% VE multiply that times .90 (90% VE) and you get 658.75 cfm. Since nobody makes a 658.75 cfm carb, you're going to end up with a 715 or a 750 properly jetted.
As Chris said, bigger isn't always better when it comes to carburetion.

[058]

No such thing as "too much carb", just "not enough engine".

[smash]

People that have really good engines (intake heads exhuast) might have 85% efficientcy. just because it runs doesn't mean that it runs well. altitude ambient airtemp humidity will all effect how a motor runs. does anybody check their plugs or exhust temps. does anyone notice a difference in how their boat runs with changing weather .
my .02

[Cas42]

good point 058

[Oldsquirt]

If only adding cubic inches was as cheap/easy as changing CFM.

[flat broke]

Smash,
I do pay attention to air temp (not as much to barometer) in regard to performance. I got this from doing a lot of tuning on my Chevelle, and of course carried it over to boats. I will be perfectly honest though and say that I don't do plug checks very often. Mainly because you realistically have to rut at WOT for a while, kill the motor, coast to a stop, let em cool, then pull em. If I was racing the boat, I would definitely do it, but its a little tough to get the girlfriend and passengers to wait around for 20 minutes or so for a reading. So basically I keep track of the air temp, and corelate that with my MPH to get reference points for jetting. If I was running at speed x on a 75 degree day, and on a 90 degree day, I drop 4 mph, I might opt for a jet change. Truth be told I might not see a benefit, but sometimes its fun to play with it.
Chris

[Moomawnster]

Hey Hal , I am going to try a new carb on my boat just cause I can , Ever heard of a PREDATOR ? They meter as much fuel as you need and no more , from 400 thru 900cfm or something like that .

[Squirtcha?]

Hey Moo. Let us know how it goes. I met a guy one day that had a flat bottom v-drive running the predator on a big block Chebby. He said he was going to take it off due to tuning problems. He had it for a few months and could never get the bugs worked out of it. Said he was going back to a Holley. I've yet to see one on a boat that was running well. Personally I don't see why it wouldn't work, but I'm no expert either.