I am getting ready to put my motor in my 19' jet boat, the motor is a 396 with a decent cam ( on the higher scale of "mild" but not radical) the motor is balanced and according to the machine shop will make 500 hp no problem, I think realistically 400 -450 hp, I had a circle track BLP 800 holly that I had a carb shop rebuild with marine components, I ended up selling it because I really just wanted a brand new carb. I am now ready to pull the trigger and buy one, I'm not fixed on brand new, now after I see hollys are 500+ bucks, but I will spend it if I have to. my  question is what kind and what size would best suit my needs? 19' hardin caribbean w/ a berk "A" pump I am looking for a general all around perf and milage set up anybody else have something similar or experience with different carbs that can give me some good pros and cons? or just your opinion is appreciated too thanks, Mike

i too am runnin a 396 in a placecraft tunnel-i have a 750 speed demon on it -mechanical secs-have had excellent luck with these carbs-have swtd w an 850 demon and its been working flawless for a couple of years now

I am getting ready to put my motor in my 19' jet boat, the motor is a 396 with a decent cam ( on the higher scale of "mild" but not radical) the motor is balanced and according to the machine shop will make 500 hp no problem, I think realistically 400 -450 hp, I had a circle track BLP 800 holly that I had a carb shop rebuild with marine components, I ended up selling it because I really just wanted a brand new carb. I am now ready to pull the trigger and buy one, I'm not fixed on brand new, now after I see hollys are 500+ bucks, but I will spend it if I have to. my  question is what kind and what size would best suit my needs? 19' hardin caribbean w/ a berk "A" pump I am looking for a general all around perf and milage set up anybody else have something similar or experience with different carbs that can give me some good pros and cons? or just your opinion is appreciated too thanks, Mike

I don't think the 396 needs an 850 cfm carb.  If you have any plans of upgrading to a a little more engine in the future, the 850 would be nice, but otherwise I think it's overkill.

Some years ago I built a mild 454 for my friend's jet boat, perhaps in the same horsepower range, 049 oval heads.  It had a 715 (that's right, 715, not 750) cfm Holley, ran great, right around 62-63 mph,, heavy 20' open bow boat.  Without asking me, he went and bought a new 850 carb after he had run the boat a year or so, picked up ZERO speed with the larger carb, even after leaning jets down, etc.

I also build dirt track circle race engines, and we have found 850 carbs to be less than ideal on them, preferring 750 cfm.  Not that the 850 didn't run, but in the midrange it was not as good.  We have run 383, 386 (3 9/16" stroke with 4.155" bore), and 406 cu in, higher compression (average 13-1), perhaps 500 plus horsepower, up to 7000 rpm

My original set up was a 454 - 049 heads - 304 soilid flat tappet cam and hauled ass with a 750 vac sec. that was set up to open pretty early.
Didnt notice a huge gain with 850 dp but there was a gain, if you want to call gas milage going down a gain 
Not to mention you'll get better throttle response with the smaller carbs.
There is a formula based off of ci and rpm to get you in the ball park as to what carbto run.
Maybe Barry knows what I'm talking about if not, I'll hit my crew chief up.
 
-beerjet-

Yes, I am aware, that type of calculation gives a theoretical minimum size.    I would agree that around 400 HP is the realistic number, assuming pump gas and the stated "mild" cam.  With the A impeller, it's not going to want to pull too many RPM, so probably even a 600 cfm carb would work.   The  calculator shows 581 CFM for 5000 rpm, 402 cu in (.030" over 396), based on 100% VE (doubt that it's that efficient). 

All that being said, I might still put a 750 on it.

That is good info! But honestly, I don't think any of us squirt gun guys should be running less than a single 750? Am I wrong Barry? Given the real operating range of these things, and the 850 being the normal recommendation?  Understand, and correct me if I'm wrong, CFM rating has NOTHING to do with fuel consumption. It is simply how much air a carb CAN flow at MAX. Yes, way too much CFM carb for an application, a boggy engine, I get that, but that is an extreme.

Just thinking out loud, but at a pretty consistent 4000-5000 rpm, which seems to be where our benchmark and operating range is, all the street calculations go out the window anyway. Fuel economy is only going to be saved by starving the said engine, if it wants more, That will show up sooner or later in the plugs, or later in a blown up engine.

Properly tuned, yeah, therein lies the key where an overly large carb can be detuned somewhat. It's in the tune, and reading the spark plugs!

Barry's the pro, I'm leaving it there, but I agree 750 is where ANY big block Jet Boat should be to start with.

Ray

That is good info! But honestly, I don't think any of us squirt gun guys should be running less than a single 750? Am I wrong Barry? Given the real operating range of these things, and the 850 being the normal recommendation?  Understand, and correct me if I'm wrong, CFM rating has NOTHING to do with fuel consumption. It is simply how much air a carb CAN flow at MAX. Yes, way too much CFM carb for an application, a boggy engine, I get that, but that is an extreme.

Just thinking out loud, but at a pretty consistent 4000-5000 rpm, which seems to be where our benchmark and operating range is, all the street calculations go out the window anyway. Fuel economy is only going to be saved by starving the said engine, if it wants more, That will show up sooner or later in the plugs, or later in a blown up engine.

Properly tuned, yeah, therein lies the key where an overly large carb can be detuned somewhat. It's in the tune, and reading the spark plugs!

Barry's the pro, I'm leaving it there, but I agree 750 is where ANY big block Jet Boat should be to start with.

Ray

I don't ever claim to anyone to be an absolute expert, but I do have some experience and knowledge to base my statements on.  Both of my posts actually indicated that I would use a 750 on a 396.  Larger engines may use more, the last 468 I built got a new Quick Fuel 850 on it (850 was the consensus of both Quick Fuel and Holley techs based on what I was doing with that engine, which made 600 HP) 

It would be interesting to know what RPM that engine will turn, didn't sound like more than a 1 HP per cubic inch deal just by rough guess. 

Hi, New to this site, great info. I would recommend a Holley 830 HP, I believe it is an 80511 # I have this carb on my Hot Rod pickup ( sorry no boat yet ). I love the fact it has drilled throttle plates, it lets you idle a big cam real nice without fouling plugs. The carb is actually designed for Winston cup engines.I used a two inch spacer on a Weiand Stealth for my 472 Ford 50cc pumps 0.040 pump shooter and 3.5 hg power valve.Throttle response is very quick, I purchased it on EBAY new for less than $ 450,It retails for $ 700.

now you guys have me thinking about the actual horsepower of my motor? here is what I know, any thoughts appreciated... 396- .030 over 10.5to1 forged pistons, "truck" rods shot peened and stressed relieved, forged GM crank, rotating assembly balanced, block and heads were decked(just enough to clean them up), oval port small chamber heads, 1.8 ratio roller rockers (miller), edelbrock performer 2.0 manifold, and as far as the cam goes all I know is that it is a 292 with a .450 lift, I know those cam specs aren't to specific but thats what I know, I also told the machine shop that I was running wet exhaust log manifolds so I think that was a factor when they had the cam ground. the guy who built the motor told me "500 hp no problem" I feel that estimate is quite a bit high, anyone else have a similar set up?

now you guys have me thinking about the actual horsepower of my motor? the guy who built the motor told me "500 hp no problem" I feel that estimate is quite a bit high, anyone else have a similar set up?

That would be 1.25 horsepower per cubic inch, which is obtainable, but I would bet not with your combination of parts, cam, heads, exhaust logs.  If I were building something similar for a customer, I would be more inclined to think possibly 1 horsepower per inch or less.