Camshaft selection

[COLD]

That's what I thought too! :lol: Thats why I threw it out there! :wink:

 

[Waterthunder]

Now if I can talk you into a gearbox your almost there. Here is my last shot. When you are done I promise you, one day you will say I wish I started out with a gearbox! Everybody that I have ever known who owned a DD and switched to a gearbox has made this statement. It's like the old Fram Filter commercial "you can pay me now or pay me later". HE-HE.

 

[warrior-p]

Does anyone know the results from John Brown or patloco and what they chose?

 

[filetnrelease]

Don't know the answer to your question Warrior, but, thanks for bringing up this old thread. I sure liked reading Mike's posts... and although dated, still a good read!

 

[cntry141iq]

There is somebody on this site that I had Comp Cams grind a custom 4/7 swap cam for. The only reason he installed it is because he wanted his motor to smooth out at idle he had some serious problems. I will ask him if it solved any problems. I believe there can not be any HP gain from a firing order swap cam in an airboat application because of the RPM’s we run. With Iron heads on pump gas I like to run no more than 10.25 to one and with aluminumheads I try to stay around 11.25 to 11.50. However goofy cam grinds and different type pistons and deck heights change all of this so you need to have all of your information on a matching combination. If you want a good combo PURCHASE THE CAM COMP RECOMENDS spend the extra hundred bucks for a custom grind. If you buy a shelf cam it will not be the best cam for your application.

That person was me ... and yes it did smooth things out a little and no there was no extra hp just from firing order but we changed lift and duration and went full rollers with beehive springs, stud girdles later were added ... all in all it worked out good ... I would never buy a off the shelf cam for anything except my lawnmower ... and not my briggs airboats either .. I mean my lawnmower LOL .. spend the money ... tell your cam grinder the truth about what you are using and what you want out of it .. and they will hook ya up ... always has worked for me .. they usually understand cams ... just a thought

 

[Barrett .502]

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

Would this work well with a similar setup except 2:38 gear box? Looking for more snap and some lift

Airboat is Gen V zz502 15 ft deck over, SS cage, 3/8 poly, 4 blade water walker

 

[SWAMPHUNTER45]

I would look for a little less duration and a wider LSA on an engine that is using a 2:38

If you had a 2.55 or 2.68 it would match well but with your ratio you need to get your power in sooner.

The higher the duration you run typically moves the peak power up higher in the rpm scale.

The narrower the LSA the more peaky it will act. The narrow LSA with a large duration will have the engine struggling to make vacuum down low. The wider LSA smooths thing out a bit building vacuum better.

With airboats it’s better to have a power curve that is linear. Coming in early and building steady rather that peaks and valleys.

 

[John Fenner]

I would look for a little less duration and a wider LSA on an engine that is using a 2:38

If you had a 2.55 or 2.68 it would match well but with your ratio you need to get your power in sooner.

The higher the duration you run typically moves the peak power up higher in the rpm scale.

The narrower the LSA the more peaky it will act. The narrow LSA with a large duration will have the engine struggling to make vacuum down low. The wider LSA smooths thing out a bit building vacuum better.

With airboats it’s better to have a power curve that is linear. Coming in early and building steady rather that peaks and valleys.

Great advice here, I would like to chime in, do not run any type of single plane intake or a huge carb thinking that it is a race car engine, this is still a "tow truck/Rv engine" just at a higher RPM, so the math or say blueprinting needs to be done, plus tuning, and jetting, had a big block tour boat come in, engine had been built and Dynoed with an 850 Demon carb, sluggish out of the hole, installed a 750 Holley out of the box, an air/fuel gauge, ran it up, added a bit more secondary jet size, woke it up, burned less fuel and had much more power at the torque curve, added pitch to prop as well, there are many factors involved other than cam profiles.

 

[Barrett .502]

Great advice here, I would like to chime in, do not run any type of single plane intake or a huge carb thinking that it is a race car engine, this is still a "tow truck/Rv engine" just at a higher RPM, so the math or say blueprinting needs to be done, plus tuning, and jetting, had a big block tour boat come in, engine had been built and Dynoed with an 850 Demon carb, sluggish out of the hole, installed a 750 Holley out of the box, an air/fuel gauge, ran it up, added a bit more secondary jet size, woke it up, burned less fuel and had much more power at the torque curve, added pitch to prop as well, there are many factors involved other than cam profiles.

Thanks for that - I have my 750 cfm 4160 dialed in with the factory GM duel pane cast aluminum manifold. I will consider playing with the prop pitch as well.

 

[Barrett .502]

I would look for a little less duration and a wider LSA on an engine that is using a 2:38

If you had a 2.55 or 2.68 it would match well but with your ratio you need to get your power in sooner.

The higher the duration you run typically moves the peak power up higher in the rpm scale.

The narrower the LSA the more peaky it will act. The narrow LSA with a large duration will have the engine struggling to make vacuum down low. The wider LSA smooths thing out a bit building vacuum better.

With airboats it’s better to have a power curve that is linear. Coming in early and building steady rather that peaks and valleys.

Thanks for the information, I’ll post the other grinds I’ve researched too.

Any other tips for making the cam snappy and having a linear power curve?

I cruise around 3500-4K and want a bit more punch/snap when hitting the gas. Lighter distributor spring helped a bit.

As waterthunder has said, I’ll take 10 less HP for more snap.

 

[SWAMPHUNTER45]

With your engine, gear ratio and prop it is a little hard to get the sweet spot. The ZZ502 is a sort of unique build. I suspect it was really designed for a 1970 Chevelle driven to a car show.

Your engine has a fairly short stroke for its displacement. Going with the factory spec it is only a 4 inch stroke. A shorter stroke typically requires an engine to turn more RPM to make “horsepower.” The ratio you run in my experience is more suited for an engine with a longer stroke which generally produces more “torque” at lower rpm ranges. Yet GM opted to tame it with a cam that actually is close to perfect for a airboat.

Shop your options and compare to the factory specs. I would stick with a cam offering a similar lift profile or just stay with the factory cam.

If a financial option a 2.55 or 2.68 will offer more snap and allow you to get up in the rpm range. That said more noise and possibly fuel use, wear and tear.

Years ago in the big block Chevy world the beast boats ran 540 inch builds with a 2.55 drive ratio and 4 big blades. The 540 has a 4.25 stroke and allows it to make about 75 to 85 more pounds of torque when all things are equal. That 1/4 inch longer stroke allows the engine more power as the piston is traveling under burn a further distance in its power stroke.

My Cadillacs run a variety of stroke lengths starting at 4.3 inches and going as deep as 4.6 inches. This allows us to use a lower ratio like 2.37 and spin super wide blades.
Can you imagine the usable power an additional 1/2 inch would yield! This is why fuel and timing is so critical in a Cadillac.

Your ZZ502 engine was built with a lot of though at GM and they matched up all the components well. I would explore pitch, tune and gear ratios before tearing into a healthy engine. If it is meeting your need then just enjoy the ride and keep it serviced.

 

[Barrett .502]

With your engine, gear ratio and prop it is a little hard to get the sweet spot. The ZZ502 is a sort of unique build. I suspect it was really designed for a 1970 Chevelle driven to a car show.

Your engine has a fairly short stroke for its displacement. Going with the factory spec it is only a 4 inch stroke. A shorter stroke typically requires an engine to turn more RPM to make “horsepower.” The ratio you run in my experience is more suited for an engine with a longer stroke which generally produces more “torque” at lower rpm ranges. Yet GM opted to tame it with a cam that actually is close to perfect for a airboat.

Shop your options and compare to the factory specs. I would stick with a cam offering a similar lift profile or just stay with the factory cam.

If a financial option a 2.55 or 2.68 will offer more snap and allow you to get up in the rpm range. That said more noise and possibly fuel use, wear and tear.

Years ago in the big block Chevy world the beast boats ran 540 inch builds with a 2.55 drive ratio and 4 big blades. The 540 has a 4.25 stroke and allows it to make about 75 to 85 more pounds of torque when all things are equal. That 1/4 inch longer stroke allows the engine more power as the piston is traveling under burn a further distance in its power stroke.

My Cadillacs run a variety of stroke lengths starting at 4.3 inches and going as deep as 4.6 inches. This allows us to use a lower ratio like 2.37 and spin super wide blades.
Can you imagine the usable power an additional 1/2 inch would yield! This is why fuel and timing is so critical in a Cadillac.

Your ZZ502 engine was built with a lot of though at GM and they matched up all the components well. I would explore pitch, tune and gear ratios before tearing into a healthy engine. If it is meeting your need then just enjoy the ride and keep it serviced.

Thanks for that.

I’ll likely do the 540 when it blows next (I just got done rebuilding) and it’s good to know that the cam is good for airboats.

That’s interesting about the change in gear ratio to get more snap and get “up in the rpm range”. By that, are you speaking to the fact that the higher gear ratio will spin the prop with at a higher RPM? (I don’t necessarily want to run more engine RPM so just wanted to clarify)

Thanks for the advice and I’ll be playing with the prop pitch next and nailing down the tuning but the plugs look good and it’s running great.

Just want more power/snap but that’s always the case!

 

[Slidin Gator]

Thanks for that.

I’ll likely do the 540 when it blows next (I just got done rebuilding) and it’s good to know that the cam is good for airboats.

That’s interesting about the change in gear ratio to get more snap and get “up in the rpm range”. By that, are you speaking to the fact that the higher gear ratio will spin the prop with at a higher RPM? (I don’t necessarily want to run more engine RPM so just wanted to clarify)

Thanks for the advice and I’ll be playing with the prop pitch next and nailing down the tuning but the plugs look good and it’s running great.

Just want more power/snap but that’s always the case!

Adding gear in an Airboat is similar to adding gear in a hot rod, Acceleration is increased. At top end, yes it takes more engine RPM to achieve the same prop RPM, but the added gear allows for more pitch in the prop. So you can compensate to keep the engine in your preferred power band.

Given the 4 blade WW prop you have and the present 2.38 ratio, I imagine you have pitch left in the prop. Moving to a 2.55 and adding pitch is probably the single most effective change you can make to the rig to achieve your goal of more snap, but you will also find lower cruise RPM.

 

[Barrett .502]

Would the 2.55 be more effective than my 2.38 in windy conditions? Boat RPM is about 4K in a 10 mph headwind and I ride in the Gulf near Steinhatchee so it’s common.

Getting the gearbox rebuilt into a 2.55 most likely especially it it will help in the wind.

Other than that - any prop suggestions?

16 ft aluminum deckover with 2.38 gearbox and zz502 , 4 blade 80 inch water walker propeller