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351w carb tuning

E

Edub

Active member
Trying to get my DD build finished up. I just feel like something isn’t right. Setup is a 13ft rivermaster with poly, 79’ 351w DD with summit 4400 cam (idle-4500), edelbrock performer intake, 302 heads, HEI distributer with curve kit, shorty headers into 3 chamber thrush mufflers. It’s spinning a 66” 5 blade warp drive 3000 rpm set at 6 degrees which I feel isn’t too strong. I’ve set the timing at 34 total, and I’m now looking at the carb.its a Holley 4 barrel 450 cfm. When should the vacuum secondaries be opening? And is there anything else I should look into tweaking to get the most out of this setup?
 
Just a thought but perhaps at 3,000 RPM, there's not enough vacuum to open the secondaries. Perhaps a 2-barrel might be the way to go?
 
Ya I was hoping to hear from someone else who is running a 4 barrel on a direct drive and see how they have it setup. I see lots of people running the Holley 500 2 barrel but also have seen people say the 450 works good and offers great economy (I’m definitely not getting good mileage with it)
 
Yeah I'm not gonna be much help here, I wandered into the 21st century with fuel injection years ago! Other than tuning on a dyno or on side job hot rods, I don't mess with carbs much anymore. To easy to get it wrong, believe me after seeing dyno numbers on carbs that were supposed to be "TUNED", without some type of measuring device like wide band or dyno, it's like throwing darts in the dark. I'm with diggin about the vacuum signal, but not as not enough vacuum. Vacuum secondaries open when the vacuum signal drops off allowing the spring in the vacuum pod to override the diaphragm thus opening the vacuum doors. So if your constantly opening the primaries to the point where the vacuum is dropping enough for the spring to force open the blades then a lighter spring in the pod would be in order. This should hold the secondaries closed until later in the throttle sweep. Alot of this direct drive stuff has to do with manifold airspeed. Here's what I've learned pn that subject. https://southernairboat.com/phpBB3/viewtopic.php?f=48&t=72272
Same engine, fitech 2 barrel vs. 4 barrel.

That's a decent sized boat to keep moving with a warp drive imo, what rpms does it take to keep it moving? Also a stock 351w at 3000 maybe makes 200-250 hp.
 
After looking at it today, the secondaries are opening at the right time. Pretty much right at WOT they open all the way up. Does turning a 66in warp drive at 6 degrees seem right? Or should I be getting more out of a prop that size. The engine runs flawless, idles great and has great throttle response.
 
Personally, I think you have to much carb. A 520 cubic inch Continental only needs 485 cfm @ 2900 rpms. I would be willing to bet that a 500 holley with a 50cc accelerator pump would be much more crisp and deliver better performance than that 450. It is as much about fuel/air velocity and atomization as cfm. The 2bbl would deliver better on both of those. Also, I have a Lyc. 290 (135 hp) that will swing a 5 blade 66'' warp drive (set on 5) at 2800rpms. Also check your timing, that motor does not need more that 20 deg of timing at 3000 rpm or less. If you pay attention to any of the shows where they dyno motors, timing = horsepower, at the expense of torque in a low rpm engine. You are in it for the torque! 003.JPGI'm a ford guy JMO!
 
I will play with the timing and also try and get my hands on a 500 Holley. Aside from that is there anything else that would hinder performance that I could check. I feel like I should be able to put much more pitch in this prop for with this motor.
 
Edub said:
After looking at it today, the secondaries are opening at the right time. Pretty much right at WOT they open all the way up. Does turning a 66in warp drive at 6 degrees seem right? Or should I be getting more out of a prop that size. The engine runs flawless, idles great and has great throttle response.
Click to expand...
I think it should turn more prop honestly. I know it's a 383, but mine turned a 72" 5-blade warp at 6-7 degrees. Compression check?
 
Where are you located? I have a great holley 500 and an adapter plate you could try. Mine was set up for a circle track car, I got it from a friend of mine that races. I used it to run my 521 c.i. BBF to break it in. I didn't go back and look, what cam are you running? What kind of compression ratio? Big cubes and a small cam are the way to go on a dd motor. Stock cam even! Of course, a dual plane intake or even an old Port O Sonic intake that has really small runners which creates more velocity. There is an old man here locally that has a stock Ford 400M on a 15 ft freedom craft that goes like stink. Im talking stock 2 bbl intake and manifolds too. He is slinging a 72'' NGQ. It is a beauty to behold. Now, it wont run dry ground, but its a tank of a fishing boat, the damn motor must weight 850lbs by its self!
 
Running a summit cam for idle - 4500 rpm. It was the lowest power range I could find for a 351. And the intake is also made for low torque and is a dual plane. I was also thinking I may need to check compression. I have someone locally that wants to trade a holly 500 for my 450. It sounds like this is the right move from what I’m gathering from everyone’s input. It’s not that the boat doesn’t run good. It actually is pretty impressive for a direct drive. itll dry load and run dry pretty easy with 2 people. I just see posts about what props other people are turning and it just doesn’t seem right.
 
Probably not much help, but it should run fine on that carb. We run a Holley 670 vacuum secondary carb on my Son's DD. His timing is set at 34*, 70" Q series prop, turns 2800 - 2900, and it runs great.
 
Probably just the limitations of the rig and power plant at this point. Stroke that mofo! Night and day guaranteed.
 
I totally concur that in the direct drive car motor catagory of airboat power the displacement gain by lengthening of stroke is where torque can be gained.

What is a stock 351W stroke and what is the stroke in you custom engine now HD ?

Think about this for a minute. When Cadillac introduced the new 472 in 1968 it had a 4.06 stroke. The engines that Mr. Branch builds today exceed 4.5 inch stroke and have a displacement that is 15% larger with 12% of that gain being made from the longer stroke.
That my friends is the blueprint for torque.
 
4.00" stroke in my Windsor. Zero clearance issues. 16% larger displacement. 14% by way of stroke. 4.030 on the bore. Makes over 500 lb./ft. of torque at 3000 rpm. Here soon hope to put it back on the dyno with the 2 barrel injection. Should make more power!
 
I am running a 455 Buick with a edlebrock 600cfm and swinging a 72 super snapper pretty well and I have timing set to 32* and an idle 11* and it’s more responsive and better on fuel than my 650cfm Holley vacuum secondary I had on it and I tuned it to maintain 480* exhaust temperature at wide open throttle on a ten second pull

Just performed the wot pull for ten seconds and hit the headers at the head with a temp gun before releasing throttle and plugs are damn near perfect just a little moist at that point and it runs pretty good for what it is


Sent from my iPhone using Southern Airboat
 
Went ahead and rebuilt my carb and and reset the timing. Also ditched the warp drive for a water walker and man it’s like a different boat. I know the prop makes a huge difference but I believe I was dumping fuel from somewhere in the carb. Went for a long ride and ran a good bit of dry and barely used any gas(1/4 tank) where before I would have easily gone through 3/4 of a tank.
 
This is with 3 people in the boat. Short video but we ran for another couple hundred yards before returning to water with no issues.


[youtube-video]https://youtu.be/azeSdMEvOPA[/youtube-video]
 
390 Holley with the secondary metering plate kit, start with 68 primary and 73 secondary jets as a baseline, tune from there, you have way too much cfm for vacuum secondaries to open on a DD application.
 
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