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Old 10-19-2009, 05:02 PM
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A unique BBC application

This is my first post on Hotrodders- so I should first say thanks to all the helpful info that Ive been able to gather from searching these past few months. Ive been doing a lot of reading since my father and I bought an inboard ski boat with a 454 in July. For those familiar, its a '79 Barefoot Nautique, which was the first year that Correct Craft made this barefoot-specific model. I realize this is a pretty unique application compared to most here, so I figured Id throw a few questions out there that I couldnt find answers to.



First of all, here's what I know. Most Barefoot Nautiques came with 330 hp 454's, which were typical marine longblocks with peanut port heads. These boats typically topped out around 48-49mph. We first suspected that this one had been previously worked on when some carb and ignition tuning produced a much stronger 56mph on the gps. We started pulling casting numbers and here's what we've got (from Mortec):

Block: 14015445, 454 2 or 4 bolt 75 87
Intake: 3933163, fits 68-69 396 375 hp, 427 425 hp
heads: 6272990, 70-up...rect...OPEN...454 service replacement, used on some MKIV crate engines, 118cc chamber

The engine is running great, but we will be pulling it over the winter since we are doing some structural repairs to the boat. We're considering tinkering with the engine to some degree if there are easy ways to add hp. I do not know anything about the pistons (I assume the block has been apart at some point), is there any way to determine my CR without pulling the heads? I also dont know anything about the cam- though I plan to measure at least the lift before doing anything. I suspect that it may be a stock-ish cam (peak hp on the 330hp engine occurs at ~4400), for 2 reasons. 1) We did not pick up any extra speed when we changed props that gave us 300 extra RPM at wide open throttle (5200rpm vs. 4900rpm). 2)This is a reverse rotation motor, so the availability of reverse rotation BBC blanks is and was an area that no one seems to know much about- including the Correct Craft forum, which has a lot of reverse rotation knowledge.

Based on the boats performance, my guess is that the engine is putting out somewhere in the neighborhood of 400hp, right around 5000 RPM. We would like more, obviously, without any adverse affects- namely low end power. We get an instant 2500 RPM or so from a standstill now, and while the holeshot is OK, its not great. Without propping down, we'd like to at least keep what we've got. I know the rectangle port heads are likely to be called overkill for our application, but it does run strong once the boat gets on plane, so Im hesitant to change them (though if building from a stock 330hp engine in the future, Id probably pick 781's or 049's as are commonly suggested).

I am not sure if a stroker is in the cards, but if I can convince Dad, whats the optimal size we should consider going up to? Ideally, Id like to see a powerband of 2500-5500 or so with the 990 heads, with a max RPM of 5500 or so (we can play with the prop to dial this in). Reliability is also key. What should we be looking at for a compression ratio- ~9:1? We run 93 octane now and dont mind doing so. What about cam? How big can we go on 454ci and not harm the bottom end power? Comp tells me they do reverse rotation BBC cams, though Im not super confident in the info I was given.

We do a fair amount of barefooting (39-45mph), so we need useable power, but we also would love to have the bragging rights that a 60mph ski boat would afford! I think another 50hp would get us there. If it matters, we're running a Performance Distributors DUI (HEI) with 24 degrees of advance (initial is set to ~12 deg) and a 750cfm 4160/4150 marine carb with the choke horn milled off. It starts and idles great right now.

Thanks in advance for any input!

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Last edited by TRBenj; 10-19-2009 at 05:11 PM.
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Old 10-19-2009, 06:51 PM
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56 MPH on a 454 BBC? how heavy is that boat?? my faimily used to have a 21' 1973 Starcraft Chieftan cabin cruiser with a 181 CI Chevy 4cyl with a Mercruiser I/O outdrive and in calm water, would turn out 53 MPH on the GPS.

Diffrent pitch props can effectivly change your powerband sort of how a torque converter works
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Old 10-20-2009, 07:23 AM
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Originally Posted by matt167
56 MPH on a 454 BBC? how heavy is that boat?? my faimily used to have a 21' 1973 Starcraft Chieftan cabin cruiser with a 181 CI Chevy 4cyl with a Mercruiser I/O outdrive and in calm water, would turn out 53 MPH on the GPS.

Diffrent pitch props can effectivly change your powerband sort of how a torque converter works
This is a direct drive inboard- so it has a 1:1 transmission (BW Velvet Drive) and a fixed shaft angle. Getting boats to run fast is 20% engine and 80% hull- the more you can get out of the water, the less drag, so the more speed. Outboards and I/O's can trim up the engine/outdrive in order to minimize the running surface... theres no such variable with an inboard... youve gotta make all your gains by adding more power.

Inboard tournament ski boats are built to pull- the term "water tractor" comes to mind. Theyre not built for top speed though- most small block V8 direct drives (not v-drives) top out in the mid 40's. Typical wakeboard speeds are in the 20's, slalom skiing speeds in the 30-36 range, and barefooting in the 38-45 range, so theres little functional reason for them to go faster... but there are some of us who think building up classic inboards with big V8's is a lot of fun. Naturally there are some bragging rights involved, hence my question.

To answer your question, the boat is about 18', and dry weight should be around 2700 lbs with the big block. My guess is that its a few lbs heavier than that due to a soaking wet structure underneath the floor, so its getting rebuilt this year.
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Old 10-20-2009, 07:49 AM
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With those heads you may want to shoot for 6500 rpm to meet your goal.I dont know anything about reverse rotation cams,but you could call UDHarold Brookshire for a custon grind,662-562-4933,or 662-301-1245.You may also want to consider going with a solid roller cam.There are better intake manifolds available,the one you have is decent,but its an old design from the 60's.A Weiand Stealth would be a better choice.Your carb is a bit on the small side,an 800 might be better suited,and your timing is too low,big blocks like to have 36-38 degrees BTDC total timing in by 3-3500 rpm.If I were you,I would work on this combo before going stroker.
Guy
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Old 10-20-2009, 08:31 AM
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Ski Boat

Hey TRB, Nice ski boat. I'm a HotBoat guy too, 20ft Eliminator, hotrod sbc, mercruiser. I've always wanted to get my hands on a v-drive ski boat and modify it for high speed rather than its low speed pulling power. Alot can be done, gear ratio changes, prop size and pitch, engine hop ups, etc. But all these modifications are a trade off, reducing pulling power for higher top speed. In your case, you can have your cake and eat it too. There are several torque boosting modifications that will increase your top speed without losing low end grunt to get skiers up on the water.
About your engine, its likely 8.5:1 CR so it will run on any junk marina gas available. Most had 224/224@.050 .500/.500 lift cams. It likely has forged pistons. As far as reverse rotation thats a reverse ground camshaft & distributor gear. This is normally reserved for twin engine boats, I'm not so sure your engine is reverse rotation, got a feeling its just a reverse rotation propeller because of the v-drive gear box. The rectangle port heads are generally too large for operation in the 5000rpm range. The marine manufacturers had to use what GM had available in the Gen 4 era. There were the peanut port truck heads, the medium port pass car heads(781,049) and the big rect. port heads. When GM quit producing the medium port heads, marine manufacturers chose the still available rect. port heads vs. the peanut port heads that were weak on the upper end.
I think that stroking your engine is way overkill. You can probably get what you want with some torque boosting. Aftermarket high performance exhaust manifolds, change the rectangle port heads out to medium port with big valves, maybe a cam change, but in that order. Likely the exhaust manifolds will get you what you want.
You should get this book; Big Block Chevy Marine Performance by Dennis Moore. It is loaded with information specific to marine performance engines, tons of part numbers, specific camshaft recommendations, etc. Ok, there's food for thought, hope it helps. olnolan

EDIT- I had to leave the computer for a while, now I see it's a direct drive, not a v-drive. The torque boosting approach is still valid though.olnolan

Last edited by OLNOLAN; 10-20-2009 at 08:36 AM. Reason: a little late
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Old 10-20-2009, 08:36 AM
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Quote:
Originally Posted by Guy Hiltz
With those heads you may want to shoot for 6500 rpm to meet your goal.I dont know anything about reverse rotation cams,but you could call UDHarold Brookshire for a custon grind,662-562-4933,or 662-301-1245.You may also want to consider going with a solid roller cam.There are better intake manifolds available,the one you have is decent,but its an old design from the 60's.A Weiand Stealth would be a better choice.Your carb is a bit on the small side,an 800 might be better suited,and your timing is too low,big blocks like to have 36-38 degrees BTDC total timing in by 3-3500 rpm.If I were you,I would work on this combo before going stroker.
Guy
Guy, thanks for the response!

I realize the rectangle port heads like lots of RPMs to make power, but Im very leary of running 6500 on this application. Thats a ton of RPM's for a ski boat thats always under heavy load (unlike a car). We'd be pushing 5k RPM's at barefooting speeds with a prop that allowed us to turn that much at WOT, which we hold for minutes at a time. Reliability is a major concern! The factory HO 454's from the late 70's through the mid 80's were rated between 390 and 425 hp and were propped the same as the 330hp motors- they just spun more RPM's (5500 vs. 4400), hence my desire to keep RPM's in the mid-5k range. This is why I mentioned stroker- a few more cubes would help me take better advantage of my existing heads while keeping the RPM's down, right? Ive built engines before, but Im still a BBC newbie.

Ill give Harold a call and see what he can tell me- thanks for the reference! Ive been looking for a BBC cam expert, I dont have a lot of faith in the techs at Comp when it comes to this oddball application. (Ive found Cam Research to be a great resource for my Ford boats.) From what I understand though, a roller cam is going to be big bucks- apparently those blanks would need to be custom made and run $2k+, which isnt in the cards. Id just like to stick with a warmed up hydraulic flat tappet that can make better use of my top end flow without killing my bottom end power... hopefully theres a happy medium here.

As far as the Stealth goes, I assume you mean something like the 8018 (dual plane, rectangle ports)? Ive got one of these on a 351w and absolutely love it. Any idea how the height compares to the vintage piece Ive got? Ive got workarounds if there are motorbox clearance issues- just curious.

As far as timing goes, Ive got 24 degrees of advance built into the DUI, so with 12 initial, we're seeing about 36 total and its all in by 3000 RPM's. The ignition upgrade really woke up the holeshot of the boat, so it was a huge upgrade from the antique Mallory that was on there before.

The factory carb on the 330hp 454 was a 650cfm Holley. For whatever reason, marine engines seem to run pretty well with the smaller carbs- my 302's run 450cfm, my 351w a 600cfm. I figure unless I want to run this 454 past 6500rpm (which would require 768cfm at 90% VE) or stroke it to 496 and run it past 5800rpm (which requires 750cfm at 90% VE) then what Ive got should work, right? Or are BBC's notorious for running better with huge carbs?
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Old 10-20-2009, 08:56 AM
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Quote:
Originally Posted by OLNOLAN
Hey TRB, Nice ski boat. I'm a HotBoat guy too, 20ft Eliminator, hotrod sbc, mercruiser. I've always wanted to get my hands on a v-drive ski boat and modify it for high speed rather than its low speed pulling power. Alot can be done, gear ratio changes, prop size and pitch, engine hop ups, etc. But all these modifications are a trade off, reducing pulling power for higher top speed. In your case, you can have your cake and eat it too. There are several torque boosting modifications that will increase your top speed without losing low end grunt to get skiers up on the water.
About your engine, its likely 8.5:1 CR so it will run on any junk marina gas available. Most had 224/224@.050 .500/.500 lift cams. It likely has forged pistons. As far as reverse rotation thats a reverse ground camshaft & distributor gear. This is normally reserved for twin engine boats, I'm not so sure your engine is reverse rotation, got a feeling its just a reverse rotation propeller because of the v-drive gear box. The rectangle port heads are generally too large for operation in the 5000rpm range. The marine manufacturers had to use what GM had available in the Gen 4 era. There were the peanut port truck heads, the medium port pass car heads(781,049) and the big rect. port heads. When GM quit producing the medium port heads, marine manufacturers chose the still available rect. port heads vs. the peanut port heads that were weak on the upper end.
I think that stroking your engine is way overkill. You can probably get what you want with some torque boosting. Aftermarket high performance exhaust manifolds, change the rectangle port heads out to medium port with big valves, maybe a cam change, but in that order. Likely the exhaust manifolds will get you what you want.
You should get this book; Big Block Chevy Marine Performance by Dennis Moore. It is loaded with information specific to marine performance engines, tons of part numbers, specific camshaft recommendations, etc. Ok, there's food for thought, hope it helps. olnolan

EDIT- I had to leave the computer for a while, now I see it's a direct drive, not a v-drive. The torque boosting approach is still valid though.olnolan
Olnolan, thanks for the reply!

Concerning the reverse rotation engine, it definitely is. Correct Craft primarily installed RH engines in all of their direct drives from the early 60's through 1989, at which point they introduced a transmission that reversed the direction of engine rotation. This allowed them to keep the RH prop that offsets the weight of the driver and keeps the boat running level with just one person aboard (important for the ski wake). I havent torn it down yet, but everything Ive read indicates that the reverse rotation 454's (and possibly all RR Chevies) run gear to gear timing sets that have the cam spinning the same as a LH engine, albeit with a reverse firing order. This keeps the dist gear the same as a LH engine. (My RR Fords do it as you mentioned with reverse cut dist gears and chain drive timing sets.) If a RH BBC chain drive cam exists, Id love to know about it!

With all the reading Ive done, I know the 781/049's are the way Id go if I was starting with a regular peanut port headed engine... but this thing already runs so strong once its on plane that I am very hesitant to swap out the 990's. It is easily among the fastest boats in our nationwide community of Correct Craft nuts, and Id like to keep it that way! Id like to keep the 990's and either boost the cubic inches if necessary, or just bump the CR and cam if that would be considered adequate. I dont plan on pulling the engine out of the boat more than once- so now is the time to get it right (thats why Im here). Like I said, the holeshot isnt bad as it sits, and the top end is amazing.

New exhaust manifolds are certainly in order, but I was hoping to go with a classic set of aluminum logs and do some port matching, rather than go with a high dollar set of new-tech shorty header type manifolds. I was already planning on increasing the exhaust size from dual 3" to either 3.5" or 4" (these are wet exhaust pipes, my 351w spinning to 5200rpm loves the 3.5" Ive got on it). Maybe I need to rethink this if the exhaust is a major BBC bottleneck.

Like you caught, its a direct drive with a 1:1 gearbox. I like the speed vs. RPM's its pulling now, so I dont see major prop changes in our future. It runs pretty good with both the 13x14 and 12.5x15 we've tried, I can play with the cup to get the WOT RPM's in check if necessary.
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Old 10-20-2009, 09:03 AM
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Its funny...ten years ago when I pulled the 454 out of my 79 Camaro, the guy I sold it too said he was putting it into a boat. Ironically, it had that same block and heads. Wish I had the numbers off it. It would be a hell of a coincidence if it was the same mill.

The block was from a 75 Monte Carlo and the heads were from a 1970 Ambulance. The Monte had the peanut heads so I dropped the big rectangles from the ambulance on the motor. No way to tell if it is the same mill, but wouldn't that be something!
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Old 10-20-2009, 10:08 AM
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Hey Bud, The rectangle port heads are highly regarded for automotive applications,will flow @7000+rpms. Like Guy Hiltz suggested, you may have to up the revs to take full advantage of them. If you want to maintain current rpm levels, the medium ports will boost torque right in the rpm range that you need and also boost low end grunt to jerk double skiers out of the water. You might find a automotive machine shop that would be willing to trade you a set of completely rebuilt medium port heads for your rect. ports(highly desirable for drag cars).
As far as the exhaust, you need to go full 4". A lower priced alternative in marine exhaust manifolds with individual runner design is the Kodiac brand. I used them on my 406 sbc with 3.5" and did a 468 bbc w/4" for a friend in his boat. I was well pleased with the end result. They are half the price of Gil, Imco and others. The aluminum casting and powdercoating is great, remove a buttload of weight. As far as the classic aluminum logs, I dig um for looks, especially polished, but a log is a log, the individual runner manifolds perform like short tube headers and a log performs like a LOG.
You may need to jet the carb up a bit after the HP manifolds resulting in more top end power. The GM manifold responds well to staggered jetting, the design runs some cylinders a little lean. Gotta read the plugs. Sounds like you got the timing pretty good.
Again the book I referred to is a good read, got mine off the shelf at Books A Million. I have been building hotrod engines for cars for a while and think I've got pretty good insight on do's and dont's, but when I started playing with boats too some years back, I had to start thinking on different terms, for instance, targeting power improvements in a specific rpm range versus just bolting together the biggest power makers you can afford on a car.
More food for thought. Keep us updated regardless of what you decide, I love HotBoats, especially the classics. olnolan
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Old 10-20-2009, 11:01 AM
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Originally Posted by OLNOLAN
Hey Bud, The rectangle port heads are highly regarded for automotive applications,will flow @7000+rpms. Like Guy Hiltz suggested, you may have to up the revs to take full advantage of them. If you want to maintain current rpm levels, the medium ports will boost torque right in the rpm range that you need and also boost low end grunt to jerk double skiers out of the water. You might find a automotive machine shop that would be willing to trade you a set of completely rebuilt medium port heads for your rect. ports(highly desirable for drag cars).
As far as the exhaust, you need to go full 4". A lower priced alternative in marine exhaust manifolds with individual runner design is the Kodiac brand. I used them on my 406 sbc with 3.5" and did a 468 bbc w/4" for a friend in his boat. I was well pleased with the end result. They are half the price of Gil, Imco and others. The aluminum casting and powdercoating is great, remove a buttload of weight. As far as the classic aluminum logs, I dig um for looks, especially polished, but a log is a log, the individual runner manifolds perform like short tube headers and a log performs like a LOG.
You may need to jet the carb up a bit after the HP manifolds resulting in more top end power. The GM manifold responds well to staggered jetting, the design runs some cylinders a little lean. Gotta read the plugs. Sounds like you got the timing pretty good.
Again the book I referred to is a good read, got mine off the shelf at Books A Million. I have been building hotrod engines for cars for a while and think I've got pretty good insight on do's and dont's, but when I started playing with boats too some years back, I had to start thinking on different terms, for instance, targeting power improvements in a specific rpm range versus just bolting together the biggest power makers you can afford on a car.
More food for thought. Keep us updated regardless of what you decide, I love HotBoats, especially the classics. olnolan
Another boat guy, cool!

Either increase the revs or increase the cubes, right? I would think that either would help take better advantage of the 990's. I dont want to prop down, but Id be OK turning in the 5500-5800 range- but Id need the hp to get me there (probably 50-80hp since Im turning about 5200 now and it takes 10-15hp to pick up 100 rpm).

Stock compression ratio on the marine 330hp 454 would have been about 8.12:1. The HO version (390/425hp) would have been around 8.6:1. Again, I dont know what Ive got now as who knows what it was rebuilt with. I guess we'll have to pull the heads to find out, right?

Assuming Ive got a pretty tame cam in it now, would I stand to make any gains with a piston (to bump CR to ~9), cam, intake manifold and exhaust change? Again, I dont want to do anything that will hurt holeshot.

Ill look into the Kodiaks, as well as other (used) header options. Ill go with 4" if at all possible... though I REALLY like the look of the logs. Ive seen minimal gains with high $$$ headers on a BBC (330hp, otherwise minimal mods) and a flow test which proved that some ported aluminum logs can outflow ported cast iron center riser manifolds on a Ford, so I had hopes I could make a little bit more power with them (and save some weight at the same time), while adding a cool look for not a lot of money.

Heres a shot of the boat in action, just for fun.

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Old 10-20-2009, 01:13 PM
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The Weiand Stealth is approximately 1 inch taller than the factory # 3933163.
Guy
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Old 10-20-2009, 08:35 PM
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NO & YES

Quote:
Originally Posted by TRBenj
Another boat guy, cool!

Either increase the revs or increase the cubes, right? I would think that either would help take better advantage of the 990's. I dont want to prop down, but Id be OK turning in the 5500-5800 range- but Id need the hp to get me there (probably 50-80hp since Im turning about 5200 now and it takes 10-15hp to pick up 100 rpm).

Stock compression ratio on the marine 330hp 454 would have been about 8.12:1. The HO version (390/425hp) would have been around 8.6:1. Again, I dont know what Ive got now as who knows what it was rebuilt with. I guess we'll have to pull the heads to find out, right?

Assuming Ive got a pretty tame cam in it now, would I stand to make any gains with a piston (to bump CR to ~9), cam, intake manifold and exhaust change? Again, I dont want to do anything that will hurt holeshot.

Ill look into the Kodiaks, as well as other (used) header options. Ill go with 4" if at all possible... though I REALLY like the look of the logs. Ive seen minimal gains with high $$$ headers on a BBC (330hp, otherwise minimal mods) and a flow test which proved that some ported aluminum logs can outflow ported cast iron center riser manifolds on a Ford, so I had hopes I could make a little bit more power with them (and save some weight at the same time), while adding a cool look for not a lot of money.

Heres a shot of the boat in action, just for fun.


Hey TRB, I think you're a pretty sharp dude but, maybe you need to do the math on prop changes. Okay, first I agree there is no substitute for cubic inches, the bigger the better. I also agree that more CI will help your rect. port heads act smaller in respect to boosting torque. You need to think in terms of torque in respect to boat engines rather than HP. A boat does not overcome it's mass like something rolling on wheels. Once you get a car moving it takes less torque to keep it moving faster and faster till you run out of gear ratio or HP. A boat on the other hand really never overcomes its mass.
Getting up on plane is just the action of rolling over the barrel of water that forms under the boat when you're trying to get it moving. Once you get the boat on top of the water, the drag is more constant. If you trim up, you get less boat it the water and less drag. Do you have the full size trim plate on the transom, is it fixed, adjustable or what?
As far as more RPM are concerned, it does not do you any good to prop for more RPM if you're decreasing pitch to get there. You did say a smaller prop,
which leads to the math on prop changes. You said you had a 13x14 and a 12.5x15, these are super pulling props w/1:1 drive but if you want more speed you gotta go up in pitch as in 16",17",18" to gain speed. Every inch you go up in pitch is gonna bring the revs down but what you're hunting for is the sweet spot in the torque that can still turn the prop at a speed desired. In your case, you can probably still go up in prop pitch, gain some speed and not give up too much low end pulling grunt.
If you still want to give up some cash and build a stroker motor, cool. But you can likely gain a little speed with a prop change, still pull skiers and only be out of the cost of a new prop.
Examples; 14" @ 5000 w/20% slip=53mph
15" @ same=56.8
16 @ same=60.6
19 @ same=72.0mph
You gotta see what it will spin up to with different props.
example; 19"@4600 w/20% slip=66.2
Food for thought, speaking of that, its supper time, later.olnolan
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Old 10-21-2009, 11:28 AM
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Originally Posted by OLNOLAN
Hey TRB, I think you're a pretty sharp dude but, maybe you need to do the math on prop changes. Okay, first I agree there is no substitute for cubic inches, the bigger the better. I also agree that more CI will help your rect. port heads act smaller in respect to boosting torque. You need to think in terms of torque in respect to boat engines rather than HP. A boat does not overcome it's mass like something rolling on wheels. Once you get a car moving it takes less torque to keep it moving faster and faster till you run out of gear ratio or HP. A boat on the other hand really never overcomes its mass.
Getting up on plane is just the action of rolling over the barrel of water that forms under the boat when you're trying to get it moving. Once you get the boat on top of the water, the drag is more constant. If you trim up, you get less boat it the water and less drag. Do you have the full size trim plate on the transom, is it fixed, adjustable or what?
As far as more RPM are concerned, it does not do you any good to prop for more RPM if you're decreasing pitch to get there. You did say a smaller prop,
which leads to the math on prop changes. You said you had a 13x14 and a 12.5x15, these are super pulling props w/1:1 drive but if you want more speed you gotta go up in pitch as in 16",17",18" to gain speed. Every inch you go up in pitch is gonna bring the revs down but what you're hunting for is the sweet spot in the torque that can still turn the prop at a speed desired. In your case, you can probably still go up in prop pitch, gain some speed and not give up too much low end pulling grunt.
If you still want to give up some cash and build a stroker motor, cool. But you can likely gain a little speed with a prop change, still pull skiers and only be out of the cost of a new prop.
Examples; 14" @ 5000 w/20% slip=53mph
15" @ same=56.8
16 @ same=60.6
19 @ same=72.0mph
You gotta see what it will spin up to with different props.
example; 19"@4600 w/20% slip=66.2
Food for thought, speaking of that, its supper time, later.olnolan
Olnolan, I appreciate the input. Maybe I should have prefaced this thread by saying that I have a pretty good understanding of how to make these boats fast with respect to engine powerbands and prop sizes. Ive done a LOT of experimenting with props on my other (similar) hulls so I know what works and what doesnt. What I lack is the BBC knowledge and how to end up with an engine that will give me the desired power in the 5500-5800 range. Just trying to figure out the best way to get there.



Just to clear things up, this is a 1:1 fixed inboard, so no trim. No trim plate either. This is actually a pretty decent sized v-hull, and despite the tank-like handling and relatively heavy weight with the big block, seems to be a pretty fast hull. The lifting strakes get a decent amount of boat out of the water. Regardless, theres no adjustment here.

I do disagree with your theories on props- whether that works on the types of boats youre familiar with I have no idea, but it doesnt work on ski boats. In my experience, maximizing top speed is a matching game between the hp curve of the engine and the size of the prop. Whatever size prop allows the engine to turn at or near its hp peak at wide open throttle should maximize top speed- other constants like rake, number of blades, etc held constant.

While the top speeds with the 2 props Ive tried are very similar, the larger pitch (12.5x15) had a significantly worse holeshot. I suspect that propping any larger than this would make the boat unskiable, and would likely hurt top end as well since the maximum RPM's would end up lower than 4900, and everything else is telling me that these heads want to breathe more than that.

Conversely, I could prop down to a smaller pitch (13x12) and pick up RPM at WOT and that would help holeshot. If my cam better matched my heads and allowed the engine to make more power (up to, say, 6500) then I would expect to see an increase in top speed by allowing the engine to turn closer to its hp peak at WOT. I just dont want to turn significantly more RPM's at skiing speeds than we do now (due to increased fuel usage and reliability concerns).

In other words, Ive seen smaller (pitch) props make a boat faster, and larger (pitch) props slow a boat down. There is no set formula on how to make one of these boats faster- its a careful balancing act. Generally speaking, when making engine modifications, sticking with a stock-ish sized prop will give you the best performance when doing top end (breathing) mods that add hp- you just spin that prop more RPM. On the other hand, going up in pitch will improve performance when doing bottom end mods that increase cubic inches (stroker) that increases the engine's torque.

In any case, based on factory settings and numerous other accounts, combined with how we intend to use the boat, I am confident that the props I currently have will be pretty close to what I end up using. I may dial in the cup a bit to tweak WOT RPM's, but for the most part, Im there.

The next step is going to be to determine our cam specs (at least measure the valve lift) and check out the pistons to try to determine what we have for a CR. Unless someone here thinks that we could pick up power by carefully tuning either (or both) of these, then I figure we probably have 2 options. A) go to a 496 stroker and take better advantage of the 990's, or B) Swap out the rectangles for some large ovals. Either will likely be accompanied by an appropriate cam change, possibly a CR change as well. If we choose the latter, maybe Ill hang on to the 990's and start planning a ground up stroker build for the future.
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Old 10-21-2009, 09:39 PM
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My Bad

My Bad Dude, You seemed like a "new boat guy" on your first post. I didn't know you had alot of experience. I don't think I can offer you anything that you don't already know or have proven different from what I know. olnolan
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Old 10-21-2009, 09:53 PM
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I would like to see pics of the motor
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