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Camshaft selection

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Getting it done
John Brown

John Brown

Member
I'm looking at changing cams in a ZZ502/502 and not sure what's going to work best. Any specific advice on duration, lobe separation and lift? I have a grasp of the function of each, but no experience putting together the right combination for an airboat.

The motor is in a 16' Diamondback deckover with a stock cam, mildly modified heads, and a 2:1 belt drive turning a 4 blade Powershift.

Right now 5100 is about max engine RPM with the props reference marks lined up on the parting plane of the hub. I think I may be overpropped, I would like to see it max around 5600-5700 and pick up more torque in a broader power band below that without taking pitch out of the prop.

I was hoping somebody might have some experience choosing a cam for this motor, or maybe one of the desk top dyno programs they could run some numbers for me on. I have been told a cam change can really wake this motor up.


The cam I'm running now is stock, it looks like this-
.050 Duration Int: 224°
.050 Duration Exh: 234°
Intake Lift: .527�
Exhaust Lift: 544�
Lobe Sep Angle: 110

The cam I am considering looks like this-
.050 Duration Int: 221°
.050 Duration Exh: 229°
Intake Lift: .575�
Exhaust Lift: .575�
Lobe Sep Angle: 112

Does anybody have an opinion on this grind or a favorite one for a 502 on an airboat?

If anybody has a dyno program here are my best guesses for the other numbers that will be needed for input.

Engine Make: Marinized BBC ZZ502/502
Engine Year: '01
CID: 502
Compression Ratio: 9.6 to 1
Bore: 4.47"
Stroke: 4"
Rod Type: Forged steel - shot peened, 7/16" bolts
Head Type: Cast Aluminum Oval Port
Cylinder Head Year: '01
Head Mfgr: GM
Combustion Chamber cc:: 110cc
Rocker Ratio: 1.7
Int. Valve Diam: 2.25�
Exh. Valve Diam: 1.88�
Ported/Stock: mild porting & work in the bowl area. Never been on a flow bench
Carb Type: Holley 850
Intake Type: Dual Plane Aluminum GM P/N 12363407
Piston Brand: GM Forged Aluminum P/N 12533507
Exhaust: Headers - 2" primaries to 3.5"collectors to 3.66" dia. stainless flex pipe to Magnaflow mufflers, 5' from head to rear of muffler.
Current Cam Type: Hydraulic Roller
Cam Brand: GM P/N 12366543
Current Engine RPM Low: 400
Current Engine RPM Hi: 5100

Thanks, John
 
John, you have a good topic here. I run a similar setup myself with a 502/450hp and am about to make a cam change to go after more low end and mid-range. Plans are to slow my engine down and add more pitch. The cam manufactures are great to work with. I am most likley going with a Lunati's Voodoo Series roller cam for GenVI. I would recommend giving some of the cam manufactures tech dept. a call.
 
I would not run the cam your thinking of if you’re concerned about power. That cam makes its power in the wrong area however it would be more fuel efficient but it’s the wrong cam for an airboat. Out of curiosity how did you come up with that grind is it a G.M. grind? The best lobe center would be a 108 at the most a 110 however a 108 will work the best. Anything over a 110 IS NOT a good airboat cam. Some newer grinds are going to a 112 or 114 centerline for fuel economy reasons. If your motor is a 502 it has a 4.5 bore not a 4.7. If you want I can get you a part number for the cam you need. You need an aggressive lobe profile with a 108 especially since your running a 2to1 belt drive. You will have to check your seat PSI to make sure you have enough seat pressure for the larger cam you should also check for coil bind you may need new springs.
 
Thunder is so correct here. Centerline playes a very important role as to when the motor will make power/torque., and how broad the rpm range is.....do not forget to deg. the cam as well. Im my 68 camaro with a 468ci b.block, i run a comp cam,,,230 dur. 520 lift. 108 center line..put it in 1 deg. advanced.....with a 2500 stall,,the engine comes on right now, and pulls to 5500. Rocker ratio/alinement pushrod leanth important as well.
 
I have some grind numbers if you’re interested. However expecting a 600RPM difference from only a cam change on a four blade wide blade while only running a 2to1 belt drive is allot to expect. The difference between 5,100RPM to 5,700RPM is much harder and a higher percentage than 5,800RPM to 6,400RPM. However this cam will give you more snap and make more horsepower everywhere thru your RPM range if someone talks you into a cam while only considering your max RPM you will end up with a none revving stone that will never reach it’s intended RPM range.
 
Thunder, question ....

Davie mentioned that he had installed his cam with 1 deg. advance. In my pitiful little stock block 350 (flat tappet, 112 deg. seperation), would there be any advantage in advancing the cam a little? It has roller rockers, and a 2:1 belt drive.

BF
 
Thunder, yes, please tell me the cam you recommend. It’s surprising how much difference there is in what the manufacturers recommend.
And I stand corrected on the bore, it was a typo, I was trying to type 4.47.

I did exactly what PatoLoco suggested and contacted all the major cam makers last week, I told them what the what my application was, what the motor was doing now and that my priorities were as follows -
1. Wider power band
2. No decrease in torque
3. Another 500-600 RPM on top.

I got replies from Crower, Crane, Lunati and Lazer, their recommendations were as follows -

Crower -
Said send them the cam I had and they would grind 10-12 deg. of duration out of it.

Crane- #139001
.050 Duration Int: 214°
.050 Duration Exh: 222°
Intake Lift: .533�
Exhaust Lift: .576�
Lobe Sep Angle: not given
Crane says this grind had an RPM range of 1200-5000 with valve float at 5600 with the springs they recommend.

Lunati- #60621
.050 Duration Int: 221°
.050 Duration Exh: 229°
Intake Lift: .575�
Exhaust Lift: .575�
Lobe Sep Angle: 112

Lazer-
.050 Duration Int: 234°
.050 Duration Exh: 240°
Intake Lift: .617�
Exhaust Lift: .608�
Lobe Sep Angle: 106


The grind I was considering is the Lunati #60621. It seemed to be the best compromise from what was recommended by the manufacturers combining an increased separation angle and a moderate decrease in duration accompanied by an significant increase in lift.

What I thought I knew was that a longer separation angle made the torque peak later in the power band and produced a flatter torque curve where a shorter angle produced peaky torque earlier in the power band. Duration moves the power band up or down the RPM range, and that more lift was needed if duration was shortened to accomplish the same amount of flow, therefore the more aggressive the profile the more overall power.

What I don’t know is how to make all of that work together to produce the results I am looking for. Apparently there are some pretty different opinions or these guys aren't really listening to me, and the only way to know is either trial and error or possibly using one of the dyno programs.

Thunder, if you’ve already got it worked out lay it on me man.
 
What I have learned is that a lot of car folks do not undestand airboat. Airboat engines have different needs-so to speak. They load different and work harder.
 
In all honesty if you didn’t post who recommended what I could tell you by their recommendations. 1 Crane is always conservative, 2 Lanati is close and Laser is the newer more aggressive but without doing that many airboat grinds. Comp Cams is by far the most aggressive and forward thinking of all the cam companies right now. I only use Comp Cams in my airboat motors we have worked together on some crazy grinds and they seam to always hit the nail on the head. I quit using Brand X because after 10 years they were still recommending the same grind. I would recommend a 236/242@ .050 with a .520/.540 lift on a 108 the grind # would be 3346-3347-HR108 I don’t know if you have a gen IV V or IV and that info is needed for the part#. Also anything bigger would probably require a valve spring change. I would call Comp and see what they recommend however no matter what do not change the 108.

As for the 112 centerline 350 cam this is not a good cam for an airboat so I would change the cam because to degree it your already 1/2 way there on changing it. P.S. on a airboat motor using a timing chain I always degree the cam in with 2 deg advance to compensate for the chain stretch that will occur in the 1st 50 hours of running.
 
Thanks Thunder that's interesting, not what I would have guessed.

The motor is an '01 GenVI. I was planning on going to a stiffer spring, I am running roller rockers and have read they may cause valve float for some reason, and I experienced it when I broke a drive belt, the motor immediately went into valve float when it revved up in the high 5's.

What you are saying with the cam is increase duration about 10-12 deg. which should lower the power band in the RPM range. Decrease LSA 2 deg., which should lower and narrow the power band. Decrease the lift around .005. Right? I can see where I would have gobs of low end, and I can see how that could help push the RPM up, any idea where the max RPM would be? I'm not disputing you, there is no substitute for experience. You are probably saving me some grief if I'm smart enough to take your advice.

Sometimes it just like pop used to tell me, son, if you're gonna be stupid you gotta be tough.
 
That cam profile would give you a max between 5,600 and 6,000RPM. Here is the trick if your motor can make it there. There is one detail I have learned with camshafts in airboats I can't divulge and Comp has been the only company to put that into consideration most cam company's think airboat and go back to the old redneck or 3/4 race cam or the dreaded RV cam them day's are gone if there is one thing I strive for in an airboat motor is snap and I feel anybody who has ever bought an airboat motor from me will say IT SNAPS. In an airboat that is worth much more than some peak power number I will gladely give up 10HP for instant snap and torque. I would call Comp and give them all your info and tell them your willing to run more spring and they will get you the right cam. It may be a tad bigger then what I told you since your willing to change your springs.
 
I called my buddy at comp and he recomended a 236/242@.050 with a 547/566 lift on a 108 Grind number 3355-3356-HR108
 
Thunder,

Thanks again, making long distance calls was more than you had to do.

If you notice that's getting pretty close to what the guy at Lazer said, he spent more time answering than anybody besides you.

Interesting that the 2 guys who put the most thought into it came up with similar answers.
 
I’m doing some serious dyno testing with of one of my LS6’s for a new engine package. I may call up Lazer and have them grind me a grind just to see what they come up with. I always try to deal with hungry people who try to do things better. Because if you do what has already been done, at best you will only accomplish what has already been done.
 
Interesting stuff Waterthunder! What would you recomend for a DD 496? Probably end up with a reduction unit in the future but not right away! 8)
 
I won't go into details as far as specifying a cam. "General" cam grinds are just that, and the only cams I use are specific ground to my cylinder head flow numbers, and every other detail of the combination.

But some things I would like to clarify to those with a relatively elementary education on camshafts, including some of the people who work for cam companies that reccomend them to you.

# 1....Wide lobe (Smaller numerically I.E. 114) seperation angles decrease the amount of overlap (period the intake and exhaust valves are open at the same time), resulting in higher cranking (low RPM) pressure. They also tend to have later intake events, that tend to increase high RPM performance, but also have earlier exhaust events which depending on application can work in either direction.

# 2 Narrow separation angles (higher numerically I.E. 106) tend to open the intake sooner, but have more overlap, and generally later exhaust events, given the same lobe profile/duration. The earlier intake event tends to signal better at low RPM, but the overlap bleeds off cylinder pressure, and contributes less to VE% at higher RPM....

# 3 This seems to be a catch 22...It is....somewhat. Depending on duration used, and the lobe profile, mixed in with a little bit of static compression as well as 100 other variables, the undeniable answer becomes....there is NO magic cam, or anyone who can tell you so, other than if they ran the same engine, with the same heads, on the same boat with the same gearbox, with the same prop at the same pitch. The only "GENERIC" thing anyone can tell you about a cam is that wide lobe seperations idle better, and "tend" to get a second wind at higher RPM. A narrow lobe seperation idles rough, and falls off short, but makes a ton of meat in the middle.

Now why a 114 lobe seperation cam intalled (advanced) on a 100 intake lobe centerline on a short track circle track engine will work well, and why a nitrous engine that would work with a 109 seperation cam installed on a 112 are some things you should consider when asking for a "magic" cam. Of course these are examples, and nothing to go by if you didn't get the point of this post (But don't think for one minute that they aren't true successful examples)

As for me...I design a cylinder head that has flow curves to suit cams that run seperation angles, and centerlines that best suit the torque curve that the application requires. AKA get maximum volumetric efficiency with the least amount of valve event timing (duration) and use a camshaft lobe design, that is the proper balance between high lift velocities, and durability.

I know I just made a mess here...but I just want anyone looking for a cam, to provide ALL the data to a good cam company, and throw in some points to consider about the torque curve, that you have learned.

What you want is high lift velocity but the degree of this will be durabilty and application, and relatively narrow lobe seperation with consideration to all the crap I just wrote...And if the person selling you a cam pays no attn or doesn't understand it...Move on to someone that does..JMO

Felber
 
One more question! The dual idler cam gear setup's are reasonable in price now. Are there more negatives to running them, (Vibration and noise) versus the positive of no chain strech? :?
 
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