SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: theflyingfarmer on July 02, 2020, 04:28:19 PM
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Howdy Yall,
My name is Joshua, go by JW or Farmer on the forums. Been lurking and love the info on this forum. Anywhos, just bought my first boat! Call it a mid life covid crisis. My boating experience is with an aluminum outboard semi v fishing boat. This is my first jet boat, first fiberglass hull, and first BBC. We farm but I do mechanics/fabrication. Ive built a high performance auto transmission before, but never an engine. Been doing lots of research and lots of math. I bought it without a motor, did a float test, all is well, only leak is a small drip at the rope packing, but its been dry for a long time, and didnt give it time to waterlog.
Ill attach boat pics. Its a 1979 DeMarco Daycruiser 21.5ft. All original including the interior. Ill need to repair some cracks in the gel coat, and some interior plywood, but overall I feel it is in great condition. Previous owner said had blueprinted and balanced berkely jet drive. Tags on the drive stamped Model 12JG-ASS, BM NO 7338.
I pulled the 454 from a 1989 R3500 rollback truck. Wanted to use the truck, but rust was worse than I feel comfortable with. Gonna build an aluminum rollback gooseneck 8). Its a virgin block. Tested 140-150psi comp across all cylinders. Mark IV block, 4 bolt main, stamped 445 high perf.
I need second opinions on my build sheet. I am shooting for at least 600hp(think this build might do 700ish), and would like to run 87-93 fuel. I think it should be capable of cruising 5400rpm? Rev limit set to 6000rpm? Its a 6 passenger day cruiser, I want to have good marina/no wake characteristics, I want to tow tubes with the family, and be able to punch it and go! I want more acceleration than top end speed. To me 60-70mph probably fast enough top end speed.
Rotating assembly:
scat 4340 forged crank 4.25" for 496
scat 4340 h beam rods 6.385" with arp 2000 bolts
wiseco 2618 alloy forged pistons 0.060" over, 20cc dome
Camshaft
comp xm296hr .566/.566 lift, 242/248 @ 0.050" duration, 112 lobe sep
Heads
skip white bravo 274 aluminum heads, 123cc comb chamber, 335cc runners, 2.250" intake valve, intake 320cfm @ .500, exhaust 228cfm @ .500"
Valvetrain
morel pro sportsman retro solid roller lifters .300 tall
comp ultra pro magnum roller rockers 1.7 lift ratio, 7/16" stud
dart adjustable guide plates
arp 7/16 rocker studs
cam retainer plate
billet double timing chain with torrington bearing
Intake
edelbrock performer rpm air gap intake
Carb
quick fuel brawler 950cfm 4 barrel mech secondaries
Ignition
Not sure, but thinking multispark, adjustable rev limiter, perhaps the pertronix flamethrower III distributor
My math says im
9.87:1 static compression ratio
7.09:1 dynamic compression ratio
Questions
For the rpm range, should I instead run these heads
Skip white large oval 300cc intake runners, 119cc comb chambers.
or this cam
Comp xm284hr with .547/.547 lift, 230/236 duration, 112 lobe sep
I can move around compression ratio a little based on piston dome 20cc, 25cc, or 30cc, and my machine shop can shave the heads to shrink the combustion chamber.
Any thoughts, advice, suggestions welcome!
I have a reasonable budget, but its a huge investment for me. Im looking at $6000ish with parts and machine work.
Joshua
aka TFF
PS filter made me remove all the shortcuts?
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Donor
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Engine
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Boat
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Welcome and congratulations.
I would really recommend against taking a good standard bore block .060 over, that takes all the life out of it. The cylinder walls will tend to flex more and ring seal will suffer. You don't have enough material for another overbore shoud you need. All that for 6.8 cubic inches, which is about 8 horsepower at your power level?
If your boat doesn't already have a AA impeller, I would strongly recommend one and an inducer.
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Also ring gaps need to be within marine specs.
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Welcome and congratulations.
I would really recommend against taking a good standard bore block .060 over, that takes all the life out of it. The cylinder walls will tend to flex more and ring seal will suffer. You don't have enough material for another overbore shoud you need. All that for 6.8 cubic inches, which is about 8 horsepower at your power level?
If your boat doesn't already have a AA impeller, I would strongly recommend one and an inducer.
Thanks Flusher! I'll look into the impeller and inducer. So just .030 over? Or hone and .000 dome pistons? Sorry, I'm new to engine building.
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Also ring gaps need to be within marine specs.
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Ok, will do, thanks! I'll call wiseco, but do you have recommended gap per inch for marine? .006 x bore? What about piston to cylinder clearance?
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You’ll need to calculate your compression ratio to nail down piston choice.
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The piston manufacturer you choose will list marine gap recommendation
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Here’s a compression calculator for ya...
https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator
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Here’s a compression calculator for ya...
https://www.summitracing.com/newsandevents/calcsandtools/compression-calculator
Sent from my iPhone using SoCal Jet Boats
Thanks. I have my static and dynamic cr listed above. I'm just not sure they're what I need for my goals and uses listed above.
I am shooting for at least 600hp(think this build might do 700ish), and would like to run 87-93 fuel. I think it should be capable of cruising 5400rpm? Rev limit set to 6000rpm? Its a 6 passenger day cruiser, I want to have good marina/no wake characteristics, I want to tow tubes with the family, and be able to punch it and go! I want more acceleration than top end speed. To me 60-70mph probably fast enough top end speed.........My math says im
9.87:1 static compression ratio
7.09:1 dynamic compression ratio
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I’m not a Chevy guy but I think you’ll have to adjust some things to approach you’re goals. Especially with that size boat. Chevy guys will pipe in sooner or later.
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Thanks Flusher! I'll look into the impeller and inducer. So just .030 over? Or hone and .000 dome pistons? Sorry, I'm new to engine building.
It's YOUR engine, YOUR boat, and YOUR money. You need to do what you feel is best for you.
When we were actively building engines, all there was available was OE blocks. Now there are a lot of aftermarket blocks available. A good OE block that could stay standard was rare and more so now. We looked for standard bore blocks for the rigidity and strength of the cylinder walls.
I thought aftermarket blocks wouldn't have that problem, because the cylinder walls are cast thicker and of a higher tensile strength material. Of course a hot rodder is always going to push the limits of everything.
This is what I (re)learned about my junk. I made the step up to a Dart Big M last year, for my new build. The block I bought was half the price of a new block, but had already been bored/honed to 4.531. Dart lists max overbore at 4.625. At first glance, it doesn't seem like a problem.
Bostick, a well respected engine builder here, recommends not going over 4.600 because head gasket sealing becomes a problem between the the head bolts and the bores. Okay, still doesn't seem like a problem. Seems like I have plenty of life left in my new to me Dart block.
Next, I see that some experts recommend not going over 4.560 because high boost applications flex and could possibly crack the cylinder walls. Okay, still not a problem however, I will need to buy custom pistons so that I don't have to bore my block. That almost doubles the price of the pistons.
I could bore it to the max and run it. BUT, what if I have a little oops? BAM! Now I need another new block and pistons!
I'm not trying to tell you what to do with your build, I'm just sharing my thoughts. As you stated, building an engine is a large investment. I would like to get as much life out of my investment as I can.
First, I would inspect/measure my bores for size, roundness, barrel, and taper as well as for any imperfections that would compromise ring seal. If the block still has the original hone crosshatch pattern, that's a great indication that your block could remain standard. The good news is that forged pistons need more clearance, than cast pistons, and jet boats need even more clearance.
Next, I would look to see what bore sizes are available in an off-the-shelf piston (for yourkit). If a .010 oversize is available, it is possible that your block will clean up with just a hone. If not, .030 would be my third choice. My last choice would be .060 over. That's just my thoughts.
There are a lot of 496s running hard out there. The engine that you build could easily last you the rest of your life in that boat. Again, do what you feel is right.
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Thanks Flusher! I'll look into the impeller and inducer. So just .030 over? Or hone and .000 dome pistons? Sorry, I'm new to engine building.
Don't be sorry! If someone doesn't like it, they don't have to read it.
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Thank you for sharing your experience. I have a machine shop lined up, I'll go ahead and tear her down to inspect. I'll search for your engine build. I really dont have any experience with bbc and cr. My KTM ran 13.5:1 on 100oct, but that was a whole another beast. With 91 or 87 with aluminium heads I'm trying to see what a safe Compression Ratio is in the marine environment.
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Regarding the cylinder heads, that's a tough call. A jet boat is not like a car where bottom end torque is needed. To give you an idea, here are the actual measured horsepower requirements for a AA impeller:
1000 RPM - 4.05 HP
2000 - 32.37
3000 - 109.26
4000 - 259.0
5000 - 505.85
6000 - 874.11
7000 - 1388.05
8000 - 2071.96
You can see that it doesn't take much to spin a AA at low RPM, but the higher the RPM you want to turn it, the more you need to step up your game.
From a jet boat pump perspective, the more bottom end torque you put into it, the more it is going to want to blow a hole in the water.
I think that going with a larger port than what would be in a car would benefit a jet boat. For a race boat application, I think the heads should just be coming into their power curve, just as the impeller is starting to stall the RPM.
For a cruiser, it's harder to theorize. Big boats do not like small impellers. To put it bluntly, your holeshot will totally blow! However, your holeshot and fuel economy with a larger impeller will increase.
It sounds like your particular usage model calls for a AA impeller. Be advised that if your engine is 650 horsepower at 6700 RPM, a AA impeller will never let you see that RPM.
Let's assume that you go .060 overbored for 496 cubic inches. You and your fuel economy would probably be happier with the smaller head, because of the port velocity where you will be operating the engine (part throttle). You might consider looking at the 320s.
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Thank you so much for your help Flusher :thumbup: That was exactly the information and data I was looking for. Have a great weekend! Ill post any questions as they come up. Ill start a build thread on the boat restoration too.
Joshua
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Personally i would go with the smaller heads you listed or similar.
give this a quick watch...
https://www.youtube.com/watch?v=OJRVCumisjU
also, you cant choose compresion fully until you choose cam, cant choose cam fully until you nail down your exhaust choice. 600 hp would be fairly easy with 496 with the right exhaust.
I just built a 468 to 503hp with smaller cam and 9.2 compression with factory rec port iron heads and basset headers 750 carb pulling 1.6" vacuum at top end....