SoCal Jet Boats
Tech Talk => Engine Mechanical / Electrical => Topic started by: 19ftCheyenne on July 07, 2008, 02:22:45 PM
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Whats the perfect size cam for our jet boats? and how fast well our pumps max out at? 5500? 6000?
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Whats the perfect size cam for our jet boats? and how fast well our pumps max out at? 5500? 6000?
First off, what are you going to put it in ???
2sav
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Whats the perfect size cam for our jet boats? and how fast well our pumps max out at? 5500? 6000?
Need more infor as 2Sav stated. Big Block Ford or Chevy or other, Boat ?(I assume 19 Ft Dana). What impeller are you running. What mods are done to the engine, heads. Are you running headers or logs. Etc.
T.J.
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yes, 19 foot dana, 454 bored 60 over, all roller parats, aluminum bow tie heads, headers, single dominator 1150, berkeley jet done by jack. skid plate, droop snoop, jetovator, ruffly 11 1/2 to 1 compression.
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yes, 19 foot dana, 454 bored 60 over, all roller parats, aluminum bow tie heads, headers, single dominator 1150, berkeley jet done by jack. skid plate, droop snoop, jetovator, ruffly 11 1/2 to 1 compression.
i dont know chevys but tom at jet boat performance does so does jim brock at cyclone speed and marine
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What is the impeller cut. I am running a Comp Cams. Hyd Roller CCA-11-456-8.
It runs good. Spins to 5950 RPMs with a tight pump with B-cut impeller.
T.J.
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Any more feedback?
I'm finishing off the build of the new motor for my kids 19' Spectra and have the gut feeling I've gone too conservative in the cam choice (still in the box at the moment though). 468" BBC, 8.75:1 forged K-B, Dart Iron Eagle heads, Performer RPM manifold, log exhaust manifolds for the moment and an "A" impeller. Staying with the existing 780 Holley for a carb.
The cam I just bought from Summit was the Comp Cams XM 270 (11-236-4). It's a flat tappet hydraulic with 270/286 and .544/.547" lift.
Does anyone out there run this cam and are they happy with it? Do you feel you could have used a bit/lot more?
Thanks, Mark
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IMO the XM270H is a fairly mild cam, especially with the compression ratio you're running. I believe you could get away with a little more duration and lift, however with the manifolds you will find yourself limited do to possible reversion (water being pulled into the engine via the exhaust pulse) you did say something about "for now" on the exhaust, I do like the Exteme Marine line from Comp, good stuff at a fair price (been using them for 15+years). I like that cam if you want it to be mild, the limiting factor for me is the exhaust manifolds.
This combo will make good power to probably about 5,000 rpm, which will get after it and live forever, it also keeps valve components cheap, which brings me to point two, stock ratio rockers? you could replace the rockers and get that little bit more lift which will help wake up the big end, but for that matter go up one cam size and stay with stock ratio's (you can always change rockers later).
My 468 has the XM278H and I really like it, but I'm running Bassetts and roller rockers and a 10.5:1 CR
But it turns a stainless "A" to 5200 RPM like its hitting a wall.
Hope I didn't confuse the matter...I tend to get verbal diarreah. :P
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BTW its very easy to overcam IMO :-\
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The duration at .050 is what matters, that sets the rpm range of the engine.
This is why duration to impeller is important, as well as limitations such as logs to the engine.
If you have a 280 degree @ .050 cam that sets the rpm range to 4500-8000 rpm, and then you use logs, and a AA impeller - the engine will never spin high enough to make the rpm range. That is over camming.
From what I've experianced a 225-235 degree duration cam is probably as much as you want to go on a stock engine (lower duration for taller impellor A4, A3, etc) , and up to 250 for a modified engine (better heads and such with a looser impellor). Start getting into headers, loose impellors, big heads, etc, and the duration can go up even further into the big HP numbers - BUT you have to be able to spin it up there.
I personally am running 246 duration with logs and a 4.5 inch exhaust with an A.
While my engine could physically handle a much bigger cam (duration), the logs will keep it from spinning in the sweet zone.
Duration @ .050 = rpm range
Adv duration - duration @ .050 = ramp velocity (smaller number makes more power)
Lift = HP
I look for the highest lift in the duration I want with the steepest ramp.
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Nice post Speedy. Getting the engine in the "sweet spot" is the key. The impeller cut is what allows this to do this. An impeller lets say a "AA" with a wild cam in it (realativly small Cube BBC) will never hit the sweet spot and RPM desired. I am old school and have read many BBC chevy books and even the good ol' How TO HOT ROD BIG Block Chevy. I got out of that book that if a BBC is built right, then spin it up. That is why I spin mine to about 6K. Some think it is too high but I think otherwise. I know people are running "AA" that only spin to 4800 RPMs and the engine was built to make power at a higher RPM. I have a friend who has one of these boats and I keep telling him to cut it down to an "A" impeller.
When I put the blower EFI on next year I will put an "A" impeller or maybe a "AA". I do not know yet until I get it dyno'd.
T.J.
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Thanks guys!
Sorry about the duration. I stated the "advertised" in the first post, at 0.050" it is a 226/236 degree cam. It's very close to the Crane cam I run in my boat (an old man's cruiser ;D)
Basic long slot 1.72:1 rockers are being used.
As far as max RPM the lower end is built to take a bit. Forged steel crank, "H" beam rods, recip assembly fully balanced. I don't want to have to worry about the abuse an 18 and 20 year old will give it. It should be perfectly safe at 5,500-6,000 RPM.
Log exhaust for now but we do have a set of Bassetts hanging from the rafters, perhaps later on. These logs reduce down to 2.75" ID hoses so they're a bit more restrictive than the ones on my boat.
Cheers, Mark
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I figured you were talking advertised, but just wanted to clarify.
With those small logs and exhaust, you did the right thing. You will never spin it to 6000 RPM - the fact that mechanically the engine can take it means the engine will last a long time. My Mopar with the HV pump is mechanically good to a sustained 6500 RPM - but I won't even get close to that. It too should last quite a long time.
Your RPM range with that cam really is 1000-5000 RPM give or take.
Run it with a performer / Perf RPM (or a tunnel ram with small carbs), and an AA impellor should move it pretty good.