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

I have one in between #3 and #4.

Very similar engine combo. 454, 9:1, Victor Jr, logs and snails.

American Turbine AA impeller, 4900-4950 rpm.

I'd go with #4, set at 108° icl.


Dan'l

Hey all,

Thanks for the quick replies! I'm a little weary to run anything much tighter than a 112 LSA for reversion issues, but a 108 ICL makes sense for sure. I'll give Clay Smith Cam a call as well.

I've been leaning towards cam #4 as well but I'm wondering if the lift is a little much - cam #1 has less lift but  a more aggressive ramp (and I'm not quite sure if this makes sense in my head). I know my heads will keep flowing at the .6" of lift cam #4 has but I don't know if the increase in lift will be worth the less aggressive ramp + extra wear and tear on the valve train. I guess I'll add that I'd like to avoid cutting the heads or valve guide so that might limit things a bit...but if I'm forced to do that anyways I guess it won't matter!

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

Hey all,

Thanks for the quick replies! I'm a little weary to run anything much tighter than a 112 LSA for reversion issues, but a 108 ICL makes sense for sure. I'll give Clay Smith Cam a call as well.

I've been leaning towards cam #4 as well but I'm wondering if the lift is a little much - cam #1 has less lift but  a more aggressive ramp (and I'm not quite sure if this makes sense in my head). I know my heads will keep flowing at the .6" of lift cam #4 has but I don't know if the increase in lift will be worth the less aggressive ramp + extra wear and tear on the valve train. I guess I'll add that I'd like to avoid cutting the heads or valve guide so that might limit things a bit...but if I'm forced to do that anyways I guess it won't matter!

Lobe Separation Angle (LSA) is kind of an arbitrary number.  It is not a tangible feature that can be directly measured.  For example, using the 'centerline method,' the lobe centerline is calculated to be half way between the @.050 opening and closing valve events.  A crankshaft rotational angle relative to Top Dead Center (TDC) for the intake lobe and Bottom Dead Center (BDC) for exhaust lobe.  A reading is taken, from a degree wheel, when the lifter has moved .050" off of the base circle on the opening ramp, of the cam lobe, and another reading is taken when the lifter .050 from the base circle on the closing ramp.  The centerline is calculated to be half way between these points.

The centerline is not necessarily the point of peak lobe lift.  Also, most modern cam lobes are not symmetrical on the opening and closing ramps of the same lobe .  In some cases, the exhaust lobe profile is completely different than that of the intake lobe, not just a few degrees of duration and a few thousandths of lift different.

What nobody really talks about is, how much overlap can you run before you experience reversion?

The equation to calculate overlap is:

((Advertised Intake Duration + Advertised Exhaust Duration)/2) - (2xLSA)

Running the numbers for your cam options:

Cam 1:  63-degrees of overlap

Cam 2:  60-degrees of overlap

Cam 3:  58.5-degrees of overlap

Cam 4:  66-degrees of overlap

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.

Your exhaust system is going to play a role in what cam profile will work best.  A log style exhaust will need a little less duration than a good set of headers.

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.

To me, it is all about maximizing output from a given combination.  Don't be afraid of running more lift.  The 188 heads will flow up to .700" lift before the port flow stalls.  Realistically, you are not going to be able to get .700" lift with the duration that you will want to run.  In this case, I would select the most aggressive (modern) lobe profiles available, from your favorite cam grinder, that meet the duration requirements of your application.  I know that CompCams and others have hydrolic roller profiles that can get you into the .650 lift range.

As far as cutting the valve guides, you will need to measure the total clearance between the valve seal and the locks/keepers to see if cutting is required for your specific combination of parts.

Yes, shortening the guide will reduce support of the valve.  You are really talking about small cams here.  Really make sure that you don't have any pushrod interference anywhere!  Optimize your pushrod length you don't have excessive side-loading on your guides.  I really don't see any real reduction in reliability.

Regarding pump gas compatibility, 9:1 will not be an issue.  You might find that to be on the low side with your final cam selection's intake valve closing point (dynamic compression ratio).  Not advancing the cam, per the cam card, to favour top end rather than best average power, will further reduce the octane requirements and might give you better overall performance.

I also think a AA impeller is going to compliment this build nicely in the mid 5000 RPM range.

Joe, when advancing the cam icl, from 112° (as ground) to 108°, doesn't that bring the hp peak lower in the rpm?

Just trying to learn.

Dan'l

It is my understanding that advancing the cam 'rocks' the power curve about peak torque.

This is something that I have been very curious about and looking forward to testing on my own engine.  However, I have put 110% of my energy and resources into the B1 Racing CNC Department and my personal projects have been put on hold.  So, my personal engine development program is also on hold.

Here is my current thinking about jet boat engines:  A jet boat engine is a ONE RPM ENGINE!!!  That is the peak horsepower RPM.  Anything below that RPM is meaningless to me.  For example, it only takes 109 HP to spin a AA (because the OP said that is what he already has) impeller 3000 RPM, 259 HP to spin it 4000, and 505 HP to spin it 5000 RPM.  If my engine can't turn those low RPM numbers, it's time to throw all that junk in the scrap bin.

The last engine I built, for example, made 442 lb-ft and 311 HP @ 3700, but only 418 HP at 5400 RPM.  I don't have access to my paperwork, so I can't quote the other numbers.  The point is that this little cement mixer 355 could spin a AA until it hits the wall around 4200-4300 RPM.

Not only does the pump not require low-RPM power, it can not tolerate it.  All of the literature about camshafts is specifically related to cars, where the engine needs to pull a gear, then recover from the upshift.  A jet boat blows right through these RPM until enough water is fed to the impeller, where the pump can then drag the engine down and reach equilibrium.

My goal is to maximize peak power and let the rest fall where it may.  For a cruiser, it is also desirable to increase efficiency around 80% of peak output RPM, because this is where the engine will speed most of its time, so a slightly broader power band will increase fuel economy.

Advancing the cam optimizes 'average power' and increases cylinder pressure.  A motorcycle mechanic friend of mine showed me how dramatic this is.  While timing dual overhead cam engines, a cranking compression test is performed to verify correct timing.

I have been experimenting with Engine Analyzer Pro, trying to optimize my own combination.  I write camshaft analysis programs to amuse myself.  I have measured out maybe a dozen cams at every 2-degrees of crankshaft rotation, measuring both tappet and valve motion, trying to learn.

Then I went dark side and bought an S197 Mustang.  Hanging out with the Mustang crowd has opened my eyes to a whole new way of thinking (kind of) researching the 4.6L 3-valve SOHC engines (https://en.m.wikipedia.org/wiki/Variable_Cam_Timing).  For example, Mustang guys are really a plug-and-play mentality.  There are not a lot of cam selections.  Also, being that the intake and exhaust lobes are on same shaft, overlap (and lift) is fixed.

CompCams describes their Mutha Thumpr cams as "...Great power above 4900 rpm. Compatible with stock valve springs, benefits from converter & gears, requires cam phaser upgrade & custom tuning" (http://www.compcams.com/Company/CC/cam-specs/Details.aspx?csid=1434&sb=2).

The CompCams cam phaser limiter kit "restrict the range of cam timing movement to only 20 degrees" compared to the OE cam phasers "...ability to retard the cams up to 60 crank degrees" (http://www.compperformancegroupstor...c?Store_Code=CC&Screen=PROD&Product_Code=5449).

Ho Lee Fuk!  Did that just say the OEM cam phasers retard the cams 60-degrees?!  And the CompCams phasers restrict that to only 20-degrees?!  In comparison, my Milodon gear drive 7-bolt vernier cam gear hub is only about +/-12-degrees in about 2-degree increments.  Maybe, I am not thinking broad enough.

The reason the Mustang crowd has to use the cam phasers is piston-to-valve clearance on the intake valves.  Advancing a cam reduces clearance on the intake side and increases clearance on the exhaust side.  Retarding has the opposite effect.  I have not actually measured cams for these applications, nor has the vast majority of the Mustang crowd.  I have been told that the reduction in variable timing is taken off the advance side.

Then there are the hard core Mustang builders.  They lock out the variable cam timing and 'degree' the cams, just like us.  The result is, the ECU can not advance the cam.  A little bit of power is sacrificed down low for more peak power.

I hate to end it like this, it's like watching a movie where the plot is totally predictable.  Being that a jet boat has such a limited power band, about the last 20% (or less) of the RPM range, it should be pretty easy to find a camshaft.  The closer you get to the ideal camshaft, for the intended application, the less need there is to advance or retard cam timing.

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.