Building my first jet boat engine. (Looking for some expertise or knowledge)

« **on:** September 24, 2021, 09:43:02 am »

Good morning,

So this is my first time building a jet boat engine. I’ve built engines for street vehicles, diesels and semi truck Diesel engines. I was told that these engines need to be almost sloppy loose with clearances? Can any help with and specifics or knowledge of this I know with street engines cylinder piston clearance but was told to go a little over the 4.290 bore to 4.291?

Here’s what I’m working with
Berkeley jc with a loader grate, b+ imperllar that’s all brand new I pulled the bowl off to find a never ran pump when I bought my hull.
It’s a 1976 Miller bubble deal weights about 400-500lbs max empty with pump in it.
I’m running fiberglass bench and buckets and carpeted floors nothing else.

- I have a 454 4BOLT main with arp studs.
- .040 over 29cc dome pistons
- hbeam rods with arp 2000 hardware
- I have a forged 7416 ls6 (from what I was told) crank shaft that’s polished std size.
- Aluminum pro comp heads 119cc combustion chamber, 320cc intake
(yes they are going to be gone through and have all new guides valves and springs installed)
- single plane manifold
-Ot dry headers
.030 compressed mls head gasket

with the calculator it puts me at 10:76:1

Looking for help with cam selection. I want to do a roller camshaft, either hydro or solid.

Any help I’d appreciate thanks in advance
Evan. S

« **Reply #1 on:** September 24, 2021, 10:31:58 am »

Clearances and cam *really* depend on your goals. If you're running no thermostat and just straight water, cold, then you'll need bigger clearances. If you put a Tstat in it needs to be done right, but then you can tighten things up a bit better, and ultimately the engine will be happier (massive can of worms on this site - but I promise it will. Just might not be "worth it" for you).

Similarly cam selection depends on what kind of exhaust you're running, because if there's going to be any water in it, you have to worry about reversion.

Otherwise a jet boat is a "one RPM" engine. The faster you want to turn the impeller, the more torque you need...meaning that the HP requirement is exponential. So you pretty much want to build *only* for top end...nothing else really matters, because the pump won't use any of the energy at that point. Happy to share the specs of my build / some more info if you'd like, just ask.

« **Reply #2 on:** September 24, 2021, 02:35:55 pm »

Denon,

Thanks for the reply, the goal is 600 +\- hp and put a small squeeze of nitrous 100max shot. I won’t be squeezing on a regular basis just a put the boots to some buddys on the water lol

I’m just trying to do it the right way I’ve spend some good coin getting most of my parts in Line so not trying to cheap out just want to have a solid hot rod river racer but not trying to do 100mph lol I’d be with a 70-80mph float

« **Reply #3 on:** September 24, 2021, 02:59:25 pm »

im on my 3rd jet boat build , including motor. your cam wants to match your intake and and your pump impeller. as far as clearances, id do standard clearances, just on the loose side. and a straight weight oil nice and thick, 40 or 50 racing , which ever brand you like.

Sent from my iPhone using SoCal Jet Boats

« **Reply #4 on:** September 24, 2021, 07:46:59 pm »

Those goals sound very attainable, and very similar to what I'm doing. My dyno sheet says I'm making 555 hp @ 5300 RPM, which is right in line with where I'll "hit the wall" with my massive dinner plate sized AA impeller. I didn't pull higher than that. So far in the boat, there's either something wrong with the pump, or there was something wrong with the dyno...because at less than full throttle I'm safely into the 5000s of RPM (EFI logging has me "learning" at 5250). So I'm likely somewhere in the 550-600hp range. What I'm a *big* fan of is my volumetric efficiency - 120% according to the holley EFI system

- and the top speed, which is safely into the 70s on GPS and may well hit the 80s if I ever get the stones to floor it. My hull was around 900 lb dry, 20 foot custom wooden build (see attached pic).

I attached my build sheet - it should have literally all of the info you need to see how I hit my goals. Some notable points -

I wanted to be able to safely run 87 octane gas, since that's all that I can rely on getting on my lake depending on what marina I run out of gas close to. Hence the pretty low CR. I always put premium in it, but I know I can cruise home safely on 87 if I get stuck. At 10.7:1, depending on your timing and cam, you're right on the edge for pump gas - and might be over it. a 30 head gasket might be a bit tight for quench / squish too...I think the target is 45-55, and like clearances, you need to run looser if your block temp is colder. I'm right in the middle.
Your impeller is (a lot) smaller than mine, which will favour much higher RPM operation. I wanted mine to be more of a "low and slow" style cruiser that will live forever but still scare the shit out of anyone I take out, and mission accomplished. With a "B" cut, you'll be at ~6k very roughly WOT, which will change your build plans slightly. Same thing with the 100 shot - that will influence stuff a bit.
I have a thermostat for both water and oil. So the engine runs all day long at 170 water, and 200-240 oil temps depending on how hard I'm hammering it. That's why my clearances are relatively tight, for a boat. 50 weight is too thick for me - I peg the gauge at 80+ PSI until I'm properly warm, even with a 55 PSI bypass spring, and never see below 65. I'm swapping to 40 weight after break in, and that should help. If you DON'T run a thermostat for either, your engine will be a lot cooler...or at least the block will be. The pistons, crank, etc will still be ragingly hot...this is why the extra clearance is needed. So think hard about what operating temps you're going to have, and clearance to suit.
I run wet exhaust - the pipes are dry / jacketed the whole way around the curved part you see in the pics, and then the water meets the flow once it hits the straight run back through the transom. My cam is about extreme as you can get in this case, and with your 6k RPM target on that B impeller, you might need something a bit more aggressive. Your heads are a LOT bigger than mine, so that will definitely help even with a smaller cam, but still something to consider.

Hopefully that's a good place to start, and happy to answer any other questions. I am by no means an expert, but did a *lot* of research (and not just the internet kind

) before I did my build...and it nailed all my targets near perfectly, so I can't complain about that!

« **Reply #5 on:** September 24, 2021, 09:20:21 pm »

Quote from: Denon Osterman on September 24, 2021, 07:46:59 pm

Those goals sound very attainable, and very similar to what I'm doing. My dyno sheet says I'm making 555 hp @ 5300 RPM, which is right in line with where I'll "hit the wall" with my massive dinner plate sized AA impeller. I didn't pull higher than that. So far in the boat, there's either something wrong with the pump, or there was something wrong with the dyno...because at less than full throttle I'm safely into the 5000s of RPM (EFI logging has me "learning" at 5250). So I'm likely somewhere in the 550-600hp range. What I'm a *big* fan of is my volumetric efficiency - 120% according to the holley EFI system - and the top speed, which is safely into the 70s on GPS and may well hit the 80s if I ever get the stones to floor it. My hull was around 900 lb dry, 20 foot custom wooden build (see attached pic).

I attached my build sheet - it should have literally all of the info you need to see how I hit my goals. Some notable points -

I wanted to be able to safely run 87 octane gas, since that's all that I can rely on getting on my lake depending on what marina I run out of gas close to. Hence the pretty low CR. I always put premium in it, but I know I can cruise home safely on 87 if I get stuck. At 10.7:1, depending on your timing and cam, you're right on the edge for pump gas - and might be over it. a 30 head gasket might be a bit tight for quench / squish too...I think the target is 45-55, and like clearances, you need to run looser if your block temp is colder. I'm right in the middle.
Your impeller is (a lot) smaller than mine, which will favour much higher RPM operation. I wanted mine to be more of a "low and slow" style cruiser that will live forever but still scare the shit out of anyone I take out, and mission accomplished. With a "B" cut, you'll be at ~6k very roughly WOT, which will change your build plans slightly. Same thing with the 100 shot - that will influence stuff a bit.
I have a thermostat for both water and oil. So the engine runs all day long at 170 water, and 200-240 oil temps depending on how hard I'm hammering it. That's why my clearances are relatively tight, for a boat. 50 weight is too thick for me - I peg the gauge at 80+ PSI until I'm properly warm, even with a 55 PSI bypass spring, and never see below 65. I'm swapping to 40 weight after break in, and that should help. If you DON'T run a thermostat for either, your engine will be a lot cooler...or at least the block will be. The pistons, crank, etc will still be ragingly hot...this is why the extra clearance is needed. So think hard about what operating temps you're going to have, and clearance to suit.
I run wet exhaust - the pipes are dry / jacketed the whole way around the curved part you see in the pics, and then the water meets the flow once it hits the straight run back through the transom. My cam is about extreme as you can get in this case, and with your 6k RPM target on that B impeller, you might need something a bit more aggressive. Your heads are a LOT bigger than mine, so that will definitely help even with a smaller cam, but still something to consider.

Hopefully that's a good place to start, and happy to answer any other questions. I am by no means an expert, but did a *lot* of research (and not just the internet kind ) before I did my build...and it nailed all my targets near perfectly, so I can't complain about that!

So what you have is essentially what I plan on doin just add nitrous on occasions. temp wise yes I plan on running some kind of t-stat or a washer to restrict flow it keep temps in the 160+° range, as for like how did you plumb a oil t-stat? I don’t know if you can message on here but I’d love to chat more on your build because your build sounds just like what I plan on doing. As for the impeller I’m not sure how to measure it but it has b+ written on it with paint pen on it. ( don’t know how accurate that is) I don’t want to spin more that 6500 that’s where things start to get hairy without serious bracing and lots of money from what I’ve read lol

Thank you for your replies you’re a great help with this new venture

« **Reply #6 on:** September 25, 2021, 06:45:43 pm »

Definitely happy to help - I think if you click on my name you can send me a PM? But I'm not sure, I haven't PM'd anyone on this forum before.

If you use a water thermostat be sure to use a 4-port / "bypass" one that lets the pressure from the jet exit harmlessly when the thermostat is closed! I got mine from Hardin Marine - https://www.hardin-marine.com/p-73955-polished-stainless-thermostat-kit-for-chevy.aspx (note that CP Performance and a few others are all the same company behind the scenes). The restrictor washers can work, but then your operating temp is going to fluctuate based on a) lake temp b) how hard you're mashing the throttle. For less than $150, it was a no brainer for me.

Oil thermostat is another hardin piece for me - https://www.hardin-marine.com/p-26869-bigsmall-block-chevy-gen-4-thermostatically-controlled-engine-block-adapter.aspx, and I used their oil cooler too (in fact, almost all of my "turn this engine into a *boat* engine stuff, is from them...exhaust, water strainer, coolers, filters, etc). The thermostat bolts to the block where the filter would normally go, and has 4 lines out - two go to a remote mount filter, and the other two go to the oil cooler. It's a very clean install at the end of the day and allows the oil to get to temp pretty much every time I go out (which is good!), without letting it get too hot (which is obviously very bad).

I'm not 100% sure how the berkeley impellers compare to the newer dominator / AT style ones. The first reply by flusher at the post linked below is an AMAZING explanation of impellers, but also includes an impeller chart at the bottom as a pic. A bit hard to see, but spinning a B impeller to ~6500 RPM requires a little over 700 hp - so on your 100 shot you'll be right on the edge. As long as the pump is tight (no wear / slippage), you should be fine. Most of your running will be just over 6k RPM, so tune your cam, etc, to that figure unless you want to swap impellers now.

http://www.socaljetboats.com/jet-pumps/impeller-choices/

evansennett158

Building my first jet boat engine. (Looking for some expertise or knowledge)

Denon Osterman

Re: Building my first jet boat engine. (Looking for some expertise or knowledge)

evansennett158

Re: Building my first jet boat engine. (Looking for some expertise or knowledge)

mobboss

Re: Building my first jet boat engine. (Looking for some expertise or knowledge)

Denon Osterman

Re: Building my first jet boat engine. (Looking for some expertise or knowledge)

evansennett158

Re: Building my first jet boat engine. (Looking for some expertise or knowledge)

Denon Osterman

Re: Building my first jet boat engine. (Looking for some expertise or knowledge)