Hey Monty:

Considering what you drive on the street, I admire your restraint on the 540s.

Yeah, LOL. The reason for my restraint is mostly due to my wife. For some reason she has a real aversion to being stranded out in the middle of Lake Michigan on a boat, so durability and reliability were my primary considerations. It's self-serving in the long run.

Monty, this what we use and why. The cam you have chosen is more than what is in the engine that the dyno sheet is from and AFR heads should flow more than stock VE numbers.

You obviously have a wealth of knowledge and I'm not trying to start an argument, just share a little of what has been working for us for the last few years.

CID x RPM x V.E. / 3456 = CFM

That formula is still being quoted by magazines and companies etc...but times have changed. 10 years ago a carburetor would require 10 inches of water to pull signal and shear fuel. Now they can can pull and shear fuel at only 3. Remember 20.4 inches of water (wet) is the cfm rating guide with reputable designers.

20.4 = 1.5 hg.

CID x RPM x V.E. / 2820 = CFM
540 x 5500 x .9 / 2820 = 948 CFM

The Society of Automotive Engineers (S.A.E.) established a standard test vacuum of 1.5 inches of mercury for a 4 barrel carburetor as a test standard. Most carburetor manufacturers and custom carburetor modifying companies measure their carburetors using dry flow CFM (cubic feet per minute) and some companies will only flow 1 barrel, then multiply times 4 to arrive at their overall CFM air flow numbers.

A carburetor in actual use on an engine has to supply fuel for the engine to run, and this is called wet flow. For example: A standard carburetor rated at 750 CFM actually tested on a wet flow bench will flow 690 CFM to 700 CFM. This is why industry professionals have always suggested that after you calculate your engine air flow supply CFM requirements, you always buy a larger CFM rated carburetor.

After all that, I agree that the 850 will probably be more responsive up to the point that the secondaries open. After that I think the dominator will walk all over it as well as be more fuel efficient. If you need a dominator to try, I'd be happy to borrow you one.

Rod

Rod,

You make some very good points, and I'm not disagreeing with you either. You're entirely correct about accounting for fuel displacement in the cfm calculation when discussing wet flow vs. dry flow. To be honest, this is the mildest engine I've ever built. Actually, it's the first carburated engine I've ever built, so everything has always been dry flow.

I've always used FAST programmable EFI before with wide band O2's. I feel alot more comfortable with my laptop than a screwdriver and a jet kit, but I've already got so much going on this winter with the boat, converting to EFI would have put me in jeopardy of not being ready come spring time. Next winter I'll add the EFI, that's one reason I'm trying not to invest too much in the induction system at this point. Part of my engine combination decision was based on the current situation, as well as what I'm planning for next year. Plus I'd rather wait for the new FAST or Big Stuff III systems which should be out by then.

Additionally, I've already got engine hatch clearance restraints that are limiting my intake selection, which in turn affects what carbs I can use. Most intakes with a 4500 bolt pattern are much taller, such as the Super Victor, and would never fit under my hatches without adding scoops or raising the hatches several inches. Even a 1" spacer is most likely not going to fit, which is probably worth a cheap 20 hp. I'm willing to raise them an inch or so, but I don't wnat to get more involved than that, and scoops are out of the question - so says the admiral, lol. When I go to an EFI manifold next year with a forward mounted throttle body, the hatch clearance won't be as much of an issue.

In any event, I think that the 850's will still support my target 600hp at 5000-5500 rpm. I'm sure the Dominators would give me some additional power above 5000 rpm or so, but I don't see what good it's going to do me if I'm already pushing the limits of the TRS drives as is. Maybe I'm underestimating what I can get away with without sacrificing too much drive reliability. It's just that I've been told be a few people (BAM, etc) that anything over 500hp or so is asking for trouble with them.

Monty, no problem here at all and please let me know if I can help in any way. Thanks

Rod

Thanks Rod. I'm new to high-performance boating, my previous boats were cruisers, and my previous engine expereince has been high-performance automotive. While they are mostly similar, there are some unique characteristics that need to be addressed. I have heard many good things about you and your shop, and I appreciate the time you took to look at my combination and offer suggestions. It is always beneficial and valued to have experts offer opinions and suggestions.

Here's something to consider....
 

This is only what I have based on my experience, but when I originally built my 540's with a mid-grade porting job on the Dart Pro-1 310cc heads, hyd roller cam 236*/244* on 114* lobes with I an HP830cfm Holley carb. With that carb on Tom Earhart's dyno it produced 630hp @5400rpm. Tom commented that in the 20+ years he has been dynoing engines he never saw an engine follow as well as mine did that day with the carb/engine combination/set-up. I asked him to translate what he meant by "follow" and he said it had very crisp throttle response and the power curve was "right there".

The next year I had the heads FULLY ported and increased the comp ratio by about 1/2 a point. Only this time I added a HP950cfm Holley carb and we headed for Tom Earhart's dyno again----however we still took one of my 830 Holley's with us too for comparison. The only time the 830 out performed the 950 was at 3000rpm and below otherwise the 950 beat the 830 by 16hp at best. If I remember correctly the 830 began to fall off at between 5200-5400rpm and that's where the 950 continued to make power up to as high of RPM as we wanted to run it which was 5700rpm. At best the 950 made 16 more HP on top over the 830 and varied between 1hp/tq and 16hp/tq from 3000rpm up to 5700rpm. The 950 still gave me good idling charctoristics on the dyno and in the boat it will idle just as well as the 830 did-----@775-825rpm in neutral and @550-600rpm in gear with 32" Bravo One props. I don't have the dyno sheets in front of me so I cannot be as precise as I would like.

I'm sure the Dominator would give you a little more yet as Rod has mentioned. He has given some very good info here and it's really nice and interesting to see all the differing comparisons. However, if 600hp is your goal, I am quite confident that you'll get all of that and MORE with the AFR heads, 741 cam and an 850 Holley. ;)

Kaama,
I'm not disputing what Rod said. He was kind enough to offer me and others some very good advice and suggestions.

If it weren't for the engine hatch issues and the limitations imposed by the TRS drives, I'd be running Dominators with Victors/Super Victors. I know I would make more power with Dominators and the taller single planes. However, don't discount the performance benefits of increasing your compression when comparing your dyno results. Obviously, the bigger carb accounted for some of that. You didn't state what your static compression ratio was and is, but in the 9.0 to 11.0 range, increases in compression generally have the most benefit in terms of percentage.

But unfortunately, I don't even know if I can fit a Performer RPM or Airgap RPM under the hatches without rising them some. That limits me to a 850 cfm 4150 carb , which I beleive is the largest Holley Marine carb available.

Assuming a Dominator and a single plane would give me 30-40 more hp around 5500 rpm, what's that (x2) going to translate into in terms of boat speed? 1-2 mph? Is reworking the engine hatches worth it?

You guys are a bad influence, lol! Here I am trying to be good and conservative, and you're putting thoughts of "more power" in my head.

By the way, anyone have any idea what the mechanical efficiency of a built Velvet Drive and TRS combination is? i.e. if I have 600 flywheel hp, what would I typically estimate my "prop" hp to be? The reason I ask, is that the boat currently has 15 x 21 props on it, which I will need to replace. Just trying to get an idea of what props I should start out with.

The bravo uses about 30 hp at the 500hp range and the TRS with trans about 50 or 10%.

I totally agree---Rod always offers great advice/help! :)

In my previous post I was actually trying to discourage you from going out and spending more $$$ on a carb when I am quite sure the 850 will net you the 600hp you are looking for. :)