Hey everyone,

I've got a 20' boat, hull dry and without engine is 900 lb, engine is a 550hp (Dyno) big block Chevy with aluminium heads...all in wet weight likely 2000 lbs with me.

SD309 with the performance intake (shoe and plate), AA impeller. Have trim.

Last year when I took the boat out, the ride plate was not installed. The boat ran very happy, but would start to be a bit unstable at very high speeds if I trimmed out. So, I added the ride plate to try and prevent that. Verified angle of 2.7 degrees, which is how it wants to be based on the machined mount.

Now it's not nearly as happy and has a tendency to porpoise at lower speeds, and has a much narrower window of "happy trim" at higher speeds. What gives? I used to be able to hit 60 with confidence, but above that got hairy. Now, 60 is hairy, and even 40 can start smacking the bow if I trim up.

Any help much appreciated!

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!

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.

Hi guys,

So right now timing is 10 degrees initial until 1000 RPM, then I add in 24 mechanical all in by 3000 rpm for 34 total. Runs great, but pretty sure I could have more timing and bring it in faster.

So I swapped springs to the lightest pair (was on second lightest already). No change to the 34 final, but now it's in by 2k, and starts coming in at 500 - which meant my "initial" (timing at idle) is actually closer to ~13-14.

Well, it ran a little better at a slow cruise, but couldn't idle to save it's life. Would hunt like crazy, stall frequently, all sorts of crap. I swapped back to the medium spring and all is well again. Here's my questions:

- I've done a lot of research on timing and I cannot believe that an extra ~3-4 degrees of idle timing, when it's already so low, is causing me to idle so badly. I know a lot of people lock out their timing at ~32-34 right at idle and are mostly fine, lots of people have 15-25 initial, etc. So what gives...why do I need such a ridiculously low initial timing?
- My idle speed is 800 rpm. From everything I've seen on this site, that's *really* low. I love it because it's quiet, makes it possible to dock somewhat easily, etc. Could this be why I need such retarded timing at idle? Is there any downside to idling so low?

Let me know if you need any engine specs (492 chevy, cam is a hydraulic roller ~230 @0.050, 0.600", 112 LSA, installed 4 degrees retarded) or if you've got any thoughts...thanks!

Hey guys, thanks for the quick answers!

#1 It's an aluminium impeller, idle is ~800 right now. I only really notice it when pulling away after the first start of the day when the engine is still steadying into its groove, so I'm pretty sure you guys are right...just kind of weird that there's enough slop to do that on a brand new everything.

# 2 Easier said than done for sure - don't know if it would be possible to look at the rear main or pump seal while underway. I have gone to hold my hand on the thrust housing of the pump immediately after a longer, harder run, and it was cool to the touch. My front main leaks a couple of drops while running but I've replaced it twice already so I've left it for now. Very interesting point on the air intake - I actually thought it might be that too. Due to the *stupid* coast guard regulations, I've got a tiny spark arrestor hidden inside my air cleaner...and there's no gasket between them, or between the cleaner and the throttle body, so I'm thinking that might be it.

# 3 I'll give the valvetrain a close look. I was incredibly paranoid / meticulous about cleaning everything when I built the valve train, and it's a full roller setup running a good break-in oil...but worth checking for sure. I did notice when I put the boat away for the week yesterday that my china-special timing pointer has rubbed on my ATI balancer...so I'm hoping that's what this is. Fingers crossed!

Hey all,

Sorry for the delay - brutal week or two at work. Thank you all for the comments - and the wealth of knowledge from Joe!

I'll definitely be talking with the grinder before I commit to anything but I'm trying to cover all the bases so I can go into the conversation as knowledgeable as possible. I would have thought that installing it a few degrees advanced would have been the ticket since "jet boats love torque", but the more I learn the more I realize that they don't love torque in the traditional sense that it makes the car spin it's tires - they *need* torque to spin a certain RPM, and if you don't have it you'll never hit that. In otherwords building a high RPM screamer with little torque will never hit it's peak power - whereas a low RPM torque monster will waste it all as the pump won't need all it has to offer at such a low RPM. I'm probably doing a shit job explaining but it makes sense in my head - I'm an electrical engineer and it's just like feeding something with the right amount of current, for the required voltage. You can't give it "too much current" (torque) - it's only going to draw what it's going to draw, like a pump. But if you don't have enough current...it won't be able to actually hit the stated voltage (rpm).

Improving the low-lift valve flow determines WHEN the flow motion starts and stops.  If valve seat geometry A flows more @.025" valve lift than valve seat geometry B, the cam timing events need to be altered.

Why?  Because, with the same exact camshaft, but different valve seat geometry, the engine with valve seat geometry A will act like it has more duration AND overlap than the same engine with valve seat geometry B.

I'm also mostly wrapping my head around reversion / overlap, power potential, and how they relate too - but are not *directly* dictated by - LSA, duration, etc. This comment definitely made a lightbulb go off for me - it makes a lot of sense why the numbers alone can't dictate things, because what *actually* causes reversion (or power potential, or the RPM that peak power occurs, etc) is flow at a given point of time in the crank's rotation. And things like the speed of the intake charge based on head geometery, exhaust back pressure,  and of course cam timing events all influence that flow value at that point in time. It makes my head spin a bit, but at least it's spinning for the right reasons!

Overly simplified:  Overlap relative to Cubic Inch Displacement (CID) influences the RPM that the engine will produce peak power.  Lift influences how much power will be made.  There are a lot more factors that influence this, especially valve seat geometry and low-lift flow.  These four cams have a pretty broad range.

Like Jim, I prefer a tighter LSA.  A camshaft with lobe profiles with less duration, ground on a tighter LSA will bring the overlap back where it will work best.

Let me check my reasoning - I feel like if I can make sense of those two statements I'll be a lot better off. Overlap seems to be the key figure, but why is a tighter LSA with less duration better than a wider LSA with more duration? If the idea is that a lower duration but with the same lift results in a more aggressive profile - which we want - why wouldn't you run an equally aggressive profile but with more lift *and* duration, and then just widen the LSA to keep the overlap in the sweet spot for your application/target RPM?

Either way I'll definitely be getting some valve work done on the heads, and might get them milled down a little bit to get dynamic compression where I want it based on cam choice. I don't think my power goals are too crazy so I'm hoping the 188 heads are good enough - as you mentioned I'm not even using them to their full potential - but since the right cam and the wrong cam are the same price after all...might as well make as much power as I can with my current equipment!

Thanks again all - this forum has been fantastic for learning about everything related to jet boats. I'll be sure to post lots of pics through the winter / in the spring as I make some more progress on the engine, and the boat itself.

I know this topic has been dealt with about 1000 times, but I'm going to toss up some cam specs and see what everyone thinks. Some background:

-Boat is a 20 ft custom built wooden hull - hull on it's own is estimated around 700 lbs. 12 degree Vee.
-Pump is a (brand new) SD-309. It has an AA impeller in it now, but I'm guessing that will be a little bigger than I'd like. I don't mind cutting it down or getting a smaller one.
-Engine is a (rather old, but in amazing condition) Merc 365 Magnum. 454 4-bolt with forged everything, ~9:1, 188 rectangle heads, currently has a high rise dual plane and a 850 Qjet, but I'm likely going to get a Vic Jr and 850 Double Pumper with the new cam.
-Exhaust is Hardin "Seaward" HP500 manifolds and (water jacketed / wet) 4" tailpipes, which I've been assured won't be causing any reversion issues with any of these cams.

I'd like the boat to run on pump gas - 91 is OK - and obviously perform as well as it possibly can. Reliability is pretty important, probably on par with overall power.

Here are the candidates - if they're all completely off base please let me know, I'm by no means set in stone on any of them. Would love to hear everyone's thoughts and opinions. All are hydraulic rollers.

CAM 1:
Intake, 284 adv / 230 @ 0.050, .547"
Exhaust, 290 adv / 236 @ 0.050, .547"
112 LSA

CAM 2:
Intake, 284 adv / 224 @ 0.050, .566"
Exhaust, 284 adv / 224 @ 0.050, .566"
112 LSA

CAM 3:
Intake, 283 adv / 223 @ 0.050, .566"
Exhaust, 290 adv / 230 @ 0.050, .575"
114 LSA

CAM 4:
Intake, 286 adv / 226 @ 0.050, .591"
Exhaust, 294 adv / 234 @ 0.050, .601"
112 LSA

Thanks everyone! Hoping to get the engine fully ready over the winter (I'm in in Canada) so I can launch in the spring. I'm sure I'll have more questions, but figured this would be a good place to start stirring the pot

Well, a quick update on my end...I decided to completely abandon my plan but am still very happy with things turned out.

In short, I came across a Merc 454 Magnum in *amazing* condition for roughly 3k US. Leakdown and compression were like new, and it's a complete motor. It's a 4 bolt, forged everything with 188 heads, decent intake carb and cam - so with a cam swap, a little TLC on the heads, and potentially a new carb and intake, I should be pushing close to 500 hp (it comes in at roughly 400 "as is", so 500 should be pretty easy with the right combo of parts...and the short block is built like a tank so I'm not worried about longevity at that power level). Not quite the plan I had in mind but given the price, condition, effort, and the *sexiest* valve covers I've ever seen...it was just too good to pass up. Plus, having done some number crunching, the boat is actually a bit lighter than expected, and I honestly don't think I'll notice much of a difference between 500 and 550 hp...whereas I will definitely notice the extra few thousand I have lying around to spend on gas!

I'll continue to post updates every few months as I go through the build in case anyone has recommendations, thoughts, etc. In the meantime...pictures!

Hey all,

Apologies for my extremely late reply - for some reason the e-mail notifications stopped coming through. Having done a decent amount of research of the winter, I'm pretty sure I've got a good handle on dynamic vs static compression, quench, etc. The oly two questions I have left at this point are:

1) Is it better to build from scratch, or rebuild / upgrade an old 454 that's lying around cheap? I'm trying to be as frugal as possible but by the time I've changed out rotating assembly, heads, intake, cam, etc...I feel like I'll barely be saving anything by rebuilding instead of starting from scratch.

2) In terms of exhaust I've narrowed my choices down to the basset through transom water injected headers, and the lightning marine water jacketed headers. The basset look much nicer (chrome instead of flat black, way longer and curvier primaries)...but I'm terrified of reversion, and having hot headers in a wood boat. Thoughts?

Now that the ice is almost gone I should be able to start making some more progress on the boat itself, and will of course update with pics accordingly!

Thanks,

Denon

Some pics...


Overall hull, sanded and waiting for another coat of epoxy / varnish. That's a fake / foam block for mocking things in place.


View from the rear.



The camera does a terrible job of picking it up, but the boat's not black at all - it's just stained mahagony that the camera can't pick up.