mechanical vs vacum

Mike,
For your application, I think I would stick with vacuum secondaries. Your engine isn't much different then what several have done to theirs and they're all running vacuums.
I do know that Plan B in Tampa put a mech. 650 QF on his bone stock 351 and he said it ran good last summer. I reached out to him a month ago to see how it was running after putting some hours on it but I never heard anything back.

That's the way I went too another vote mechanical.

lakedog55 wrote: Had to wait for the wife to fall asleep before ordering parts. mike

Make sure you have the parts shipped to work or a friend! That's the trick I use!

hi,
got a little conflict


also sent the bars to adjust mechanical but it is set up for vacum

Eddie, I have been eyeing the QF M600 for a while. I believe it only comes in a Vac set up. You can tell by the secondary bell crank in the picture that is a Vac carb. It uses that rod to assist closing the secondaries if the carb is slapped shut suddenly.

This QF carbs sure look awesome. We are going to be experimenting with one on Mark's footer this spring.

If my boat made more power than it does it would be a pain at foot speed. Right about 39-41 with a footer in tow the secondaries will open up. It is running out of steam pretty good with a footer behind it so it doesn't take off like a rocket or anything but you can feel it slightly and hear it. If it made 400 hp I am sure I would go by the way of mechanical secondaries.

Curious, in general with these Quick Fuel carbs (both vac and mech), do you have to plump new fuel line, or can you use adapters to set it up just like the Holley?

I found a nice soft braided line with metal fittings on either end that works well when switching to the M 600 Series.

Eddie, I am really interested in these carbs because of all the updated features the offer over a Holley and holy crap are they cheaper than Marine Holleys.

I am guessing they just had a hung over guy typing labels up that day haha.

Woah, where did you get that line Zacky boy? That appears to be the one that SkiDIM used to sell (which we have on the BFN), but they no longer list it. Whats the price on it?

TRBenj wrote: Joe runs a 650 mech secondary on his 408... and I'm planning to go from the 750 vac to a 750 mechanical on our BFN. I dont care if I burn more gas, Im just tired of fighting the with the secondaries coming in and out at footing speeds.

I'm leaning more toward your thinking Tim. I've been reading where running a dual plane intake can benefit from a bit bigger carb due to the split plenum in the intake. Not so much in stock applications where you just aren't spinning it that high but middle to upper RPM can certainly benefit in Hi-Po or BB applications. Again, all the data represented is based on auto/truck applications and not marine. The data results also specifically state a larger vacuum carb will often perform better. Manual secondary carbs will more often than not lose performance.
You and Joe have certainly proven that theory wrong but that's also on a couple of pretty hot motors. I'm still interested in trying a 750 vac and manual sec. on my engine but I need to find one to borrow first. I'm certainly not going to buy two or three carbs to see which one works better.

As for the secondaries coming in at footing speed. My 650 carb has never given me any problems in that regard. The secondaries come in so smooth you never feel them at all and you can hold any speed called without a problem. I've always thought that it was because of it being a 650 vs. the standard 600 in that CID range but if that's the case, I would think a 650 on a stock 350/351 would be more the norm then. I've also got the lightest spring Holley makes in the secondaries. Maybe Indmar did some tuning of it or I just got a good one. Who knows.
I also know that I had to tune John's 600 a bit because we were running into exactly that same scenario. I ended up putting the heaviest secondary spring Holley makes in his. We lost a couple MPH at WOT and it takes a bit longer to get there but the skiability between 40 and 44MPH is rock solid now.

It is hard to pull out all the variables in terms of comparing performance of different carbs and then figure out which is the “best”. All cfm numbers are not only rounded and vary between manufacturers but also represent the flow through the carb at a specific pressure drop, one that is unlikely to match your engine exactly. Bigger duration cams, higher flowing heads, etc etc are going to influence your plenum pressure at wot and that is what drives how much air flows through what carb.

Because I was putting a relatively aggressive intake manifold/heads/cam on my 408 I went with what I considered a smallish 650cfm carb and an MSD ignition because I was worried about idle/off idle performance and throttle response. It worked out pretty well the engine has no throttle response issues and the idle speed is determined by damper rattle limitations.   I bought the 750 BG after they came out with one with annular boosters cause I thought I could they would allow me to go a little bigger and not lose any day to day functionality. I put it on and my boat went 2 mph slower, pulled it off and put it on the self for future 383 usage.

but as seen here even between two carbs of the same size from the same manufacturer even the type of booster can have an effect.

I have an early BG "625" on the car(later named a 650). It is mechanical with large accelerator pump CCs. The engine is relatively mundane other than the duration and over lap on the cam. Stock style dual plane. The throttle response when it is in the power band is instantaneous and neck snapping. I could see a Mech secondary carb hurting the off idle and lower rpm performance on a car that doesn't have the correct gear set and torque converter selection to match power production.

It has a crappy ass converter in it currently that Summit sent me wrong twice so I gave up and just ran it for now. It stahls at about 1200 rpms and the engine needs 3200ish to be happy. The thing is a POOCH before it hits about 3200 rpms at which point it bakes the tires. So I can see how the Mechanical set up would hurt something in street trim with lazy gears and a tight converter.

Cars to boats from what I see is somewhat apples to oranges. Most carb companies ask you maximum rpm and CID to determine CFM. If you notice even the guys making big power aren't turning the engines what you would on a drag strip. An engine as healthy as Joe's or Tim's would be cranking close to 7000 rpms going through the traps at the track. My car falls on its face at about 5800 rpms but it runs through the traps at 6100-6200 pending how good the run was.

Tim and Joe's engines are running where they produce peak power generally but they don't fall off and have plenty of rpm band left if they were engines used in a drag application. That being said they require less CFM to produce the power at said rpm and don't need that extra CFM to push through the track at the big end at a much higher RPM than they need to run to produce peak power.

Eddie, he is right with all of the booster etc. talk. All of that stuff is way above any of us guys here carb tuning experience (I assume) you get into booster/secondary air bleeds/primary bleeds/ blah blah. You can seriously mess up a carb which is why I went by way of the speed demon serious vs the race demon series.

I think that you see better off idle performance in say Joe's application because in a boat you don't have the ability to pull the engine up in rpms on a foot break before you stuff it wide open. I am sure a mechanical secondary carb on my car from a standing start with no brake would perform worlds better than a mechanical carb on initial throttle hit until the engine reached the rpm it was happy to take advantage of all that cfm at.

I am just trying to highlight in my mind the differences in a car carb set up vs a boat set up.

When I start talking boat performance with Tim and such I try to completely block out all of my car experience because things like scavenging exhaust systems drastically change proper camshaft selection and such marine vs auto.

Eddie, speaking of only giving the engine what it wants, a friend of mine had a 305 in a Baja sidewinder. We went with a more radical camshaft than I recommended and a set of vortec heads with a mech 600 cfm carb. The carb would never open the secondary's more than 3/4s way. Regardless the boat ran like a raped ape. If we ran with box off and I cranked the secondaries open manually it would lean pop on the big end. If we didn't mess with it was perfect.

We plan on using it on a big block so I am sure it would cover the extra intake on a small block. Tim uses it on his BBC with a higher intake than stock.

I agree on the sounding cooler thing Eddie. One thing that bugs me is the new customers operate on the quieter the better. I if you talk to older guys most of the allure of an inboard ski boat in the 60s-70s was the sound of the American V8 talking through dual exhausts. Now days everyone could care less about the engine as long as it does the job.

Like Eddie mentioned, it's all in the components of the set up working in harmony together. Engines will amaze you and produce way more power/performance than you expect if you make sure every part is perfect for each other in the build.

It always bugged me my friend wanted to over cam that 305. It would have been way happier with my cam selection. Ignition limited it to 5000 rpms but it would have been far more happy around 6200 ish with the cam that was in it.

The guys here are far and above anyone else in the inboard world. Skilled craftsmen and accomplished engine builders. The guys here in the performance aspect are pioneers of this stuff just like Eddie said.(Eddie's boat runs pretty good I have heard )

If I get a wild hair and do cam/heads/intake on the Ford I will throw my BG off the car on it for a shake down run to see how it reacts to a more radical smaller CFM carb. I could bring the BG along to CT or something this fall for some testing. I need to take it off and go through it anyways as it is getting some leaks and such.