Hooked up a vacuum gauge and at wide open and it’s pulling 7 inches at 5500 rpms.

7 inches is a lot a WOT. You want closer to 1.5". That thing must whistle at WOT.

Carb CFM Calculator

Wallace Racing - Carb CFM Requirement



Carb size and WOT manifold vacuum | Yellow Bullet Forums

Intake manifold vacuum "through the lights"? - Don Terrill’s Speed-Talk

 

Intake manifold vacuum "through the lights"? - Don Terrill’s Speed-Talk

That was a good read.

 

Based on you ci, RPM, HP, and carb size 7 inches sound way to high for what you should be seeing

Do you have a dyno sheet you can post?

Single or Dual Plane intake manifold?

Stan

 

Dual plane Holley intake
You can’t hear anything over the open headed and prop just a few feet behind you we have to west ear muffs.

So would a 750 suffice or should I jump to a 850

 

That calculator says 770

I’m pulling my reading from the intake because the Holley ultra xp does not have ports on the base plate

 

Next thing I’ve been considering is going with a annular booster style vs a down leg

 

Annular will mostly help down low, down leg flows more as it's smaller.

 

That’s gonna close the power valves up at wot.

 

You want or have 550 hp.....1.4 x 550 = 770 cfm.....seeing you are pulling 7"....I would step up to 850 and you might make 850/1.4 = 607hp, WAG, lol.

 

 

Something doesn't sound right? 408 inches, 5500 rpm, 650 cfm carb and 7" manifold vacuum? Your vacuum gauge might be off, but not by that much! Try seeing if you can find another place to take your vacuum reading. Maybe put an open spacer under the carb with a vacuum port in it? I am not saying the carb shouldn't be a little bigger, I just don't want you to use that vacuum number in selecting another carb.

 

So I pulled the boat out. And tired it again I was getting the 7 inches of vacumm at 5000 when I hold it wide open for 10 seconds it dropped to 1. I couldn’t hold it wide open before because it was to lean I thought 5000 was close enough to wide open but I guess not.

 

This is where I’m pulling. My vacuum from

 

In my opinion, that's not a very good spot to get a vacuum reading for tunning.

 

So the 7" is not wide open under load........?

 

I wouldn't think, I could buy 3" possibly but not 7" (and I doubt 1" flat is accurate either with a 650).
I dug up some old dyno data (Proper OEM level equipment) from my462" Pontiac:

5600 rpm: 750 carb: 528 HP / 2.2" hg man vac (533 power peak @ 6000 w/ 2.6" hg man vac)
5600 rpm: 850 carb: 535 HP / 1.8" hg man vac (unfortunately no data above 5600)

So, bigger cubes, likely not as good a cyl head, but similar airflow based on power.

 

No when I had put the 7 number it was close to wide open but not fully because I did it want to hold it wide open because my afr was shooting to the 16’s. I finally got that all straight. And. Was able to hold it wide open. Today. Afr in the high 12’s and vacumm was around 1. So the 650 may be feeding it enough after all? 99.9 % of ppl with carbs run 750’s on any thing bigger than a 350. But just because that’s how it’s been don’t make it right. My lil boat with with the 650 quick fuel was running along side of some 632 ci motors. So it’s not a dud by any means but if you on this forum you know or think there is always room for improvement.

 

Carburetors are rated at 1.5"Hg pressure drop. From what you wrote I can't tell if your prop is limiting you to 5000 rpm or 5500 rpm but either way your measured 1.0"Hg manifold pressure is very close to what a 650 cfm carb flows. Depending on what your goal is, drive-ability or bragging rights, a boat can be held at full throttle for a "Long" time so air/fuel distribution is very important. Make sure you know which is your leanest hole before it tells you which one is.

I think your big improvement might come from bumping the timing up a little. Airboats usually run on pump gas so I am assuming you have relatively low compression. (Full throttle, 5000 rpm, 30 minutes) You could bump it up to 30 or more but it's VERY important to remember I am not the one paying for it when it comes apart. Add timing slowly for your application and be cautious about accepting opinions.

 

The prop is the rev limiter. And we run a reduction 2:1 so prop turning half the speed of the motor. The props are carbon fiber and are adjustable so you add pitch or take it away to get what ever you want your rpm to be. And I wanted to get the carb situation dialed in. Before I started playing with timing. All my cylinders are burining pretty close they are all with in 20 degrees of each other.

and if you hold a airboat longer than 5 mins at wide open good chance it’s gonna come apart or over heat.

so the 650 might be enough carb after all. I mean it is running good it’s a 408 ci motor and it keeps up with 632’s.on same size hulls So there’s that.

 

Just for comparison, in an old Chevy High Performance a 402 LS2 made 535hp with a holley 950hp, 608 with a bigger cam and a holley 1000hp and 780 hp on a 200 shot. And a 383 LS1 made 560hp with a 750 street HP with 73/75 jets.