Thoughts on 502 roller cams?
 

I had a 502 built up last year. It dynoed at 540hp. Since then I've made some minor mods which required more fuel (about 4 jet sizes), so I wouldn't be surprised if it was making around 550-560hp now. It has the following specs:

Crate 502 (all forged parts)
9:1 comp
Dart Pro1 310 Rec alum heads
Dart single plane manifold
reworked 850 carb
Crane 268761 cam
(this is a 18726543 firing order cam, .598 .610 230/[email protected] 114LSA)
1.75 rockers

I've managed to get this combination to Idle VERY smoothly (adjusting four corner idle, various combinations of carb spacers, plug gaps, Crane 6 marine spark box w/ 24deg at idle)

In fact the idle is almost too smooth! Not that I'm complaining, but I could easily sacrifice a little idle quality for more power. I did a quick "Desktop Dyno" sym on this combo with the current cam and a 741. The 741 showed a 50hp gain.

I'm taking the engine out for some hull work this winter, so it would be a good time to make the change. Any thoughts? I'm thinking more along the lines of the HP525 cam which I'm told is a 741 on 114LSA.

Anyone run the HP525 cam in a carb motor? How's the idle?

Anyone got one sitting around? Anyone want to trade for what I have now (about 25 hours)? It's got the ford firing order which I'm told actually makes a couple extra HP. Not sure about that, but it has a very cool sound to it. Kind of raspy like it's got a lot of compression.

If you want a cam that works with what you have PM Rmbuilder he can help. :drink:

What is your WOT RPM?....also what exhaust do you have?
I don't think your going to feel much going from the 761 to the 741.
Plug the 771 in your desk top and have a lookie at those numbers...:cool:

I put the HP525 cams in my carb'ed 500HP's with M1 Prochargers. I haven't gotten the boat in the water yet so I can't tell you about idle, but they made 700 hp at 5700 rpm at around 5-6 lbs of boost on Nickerson's dyno.

Your cam is a 731 with the 4/7 swap. I highly doubt you will see anywhere near a 50hp gain going to a 741 or 525EFI cam. It all depends on your head flow numbers. Chances are, you will need to spin the 741 a few hundred rpm faster than you are spinning now to make its max hp.

Keep in mind, the 741 cam was designed for use with the Chevy iron head and it's crappy exhaust port. That's the reason for the much longer exhaust duration. The Pro 1's have a much better exhaust port so you should go for a custom cam that better matches the heads breathing ability.

From the dyno results I have seen, the 741 makes about 25HP more than a 731 in a 502.
What kind of exhaust are you running?
If you have wet with short risers, don't go bigger.

Ok, this is embarassing, but I can't remember the name of my exhaust system. Brain dead this morning I guess. Stainless Marine sticks in my head, but they don't look identical to the current Stainless Marine stuff.

It's a cast, big, short tube header, dry all the way to the back where I put just a little water into the pipe just before the flappers. Tyler Crockett sold them to me used with the assurance that they would be good for as much power as this motor was ever going to make.

Currently makes max HP at 5300. Wouldn't mind moving it up a couple hundred rpm.

Yikes! the 771 looks like a pretty big jump in duration. Got any experience with how it idles. As the others have been kind enough to point out, reversion is one concern I had forgotten about.

I have a friend that has a Comp Cams XM296HR in a 502 built similar to yours.
It made just shy of 620HP@6000 on the dyno. Probably 50HP less in the boat with wet exhaust and accessories.

Longer duration, but a lot lower lift than my present cam, and way more overlap. My numbers were with wet, shorty marine exhaust, but no power steering, so I would guess this cam is only making about 20-30hp more.

Sounds like I'm not going to get as much from a mild cam upgrade as DD had predicted. Probably best to hold off. The engine runs great right now, and nothing leaks. Be my luck, I'd change cams and end up spinning a lifter next June. I want to keep my eye out for a HP525 manifold and Procharger for NEXT winter anyway :)

bcarpman:

Only thing I see that seems a little off is the 114LSA on this cam. I know we are always worried about reversion, but carburated N/A engine tend to like a 110-112 LSA to make bigger power up in the higher rpms. The duration numbers and lift numbers seem good , but the 114 LSA cam is more like what a fuel injected engine would want for idle quality.
Did you degree in the cam and if so at what numbers?
If you dumping your water at the tips you should not see a real reversion problem unless there's little or no drop to the tips from the manifold elbows.
On your buildup and combinations listed here I should think you will be seeing about 550-570HP if everything is working correctly, but getting your peak power closer to 5400 rpms should give you some more prop speed and hull speed.
Good Luck,

Best Regards,
Ray @ Raylar

The cam is installed straight up.

I am confused by the comment regarding EFI engines wanting less overlap. I've always found EFI to be easier to tune for good idle at a given overlap, not worse. I know you know your stuff, so I'm just curious as to where that comes from.

thanks

Wider LSA is less overlap not more. FI cams usually need wider LSA's or less overlap to keep the manifold vacume higher to better control the MAP signal especially at idle. You might be able to retard the cam about 2-4 degrees and improve the topend power peak , but be careful of the reversion issues!

Best Regards,
Ray @ Raylar

Yep, that's what I was saying, less overlap (larger LSA) for EFI wasn't making sense to me.

I get your point about the MAP signal though. EFI systems I've been working with lately have Hybrid MAP/TPS cal tables for radical cams. Makes tunning at idle doable with big cams.

More cam duration will make it pull more rpm and create more power.

If the exhaust you describe above are Eickerts, definatley call RMBuilder as he has done many 502's and larger with these exhausts. Done a bunch with other exhausts too, of course.

Just imagine a short SS tube of this end of this:
http://www.keitheickert.com/images/755-8920.jpg

I do not run a 525EFI cam, but I have heard a few 525 cams running in other guy's boats and I must say, they have some pretty good lope sound going on. I like the sound of the 525, but I was surprised at how choppy they sound----I guess in designing the 525, Merc was trying to get a lot of that power out of the cam profile in a 502cid engine.

I should say that I did just the opposite with my NA 565cid engines---my goal was to get more of the power out of the cubic inches, high flowing heads and exhaust system vs trying to get it from the cam. Heck, my engines run a smaller cam than the Merc 525!!! I can get away with a smaller cam because of the flow quality in the design of my heads. A smooth idle was my first priority---not HP, so I did this at the sacrifice of some top end power. My 565cid engines almost sound like the family van aka "grocery getter" compared to the 525/502cid. My 32' AT runs in the mid 90's GPS with full fuel and flat water.

Do yourself a big favor, get ahold of RMbuilder. If you really want a cam for your specific build, he is the man. And yes, he has done this one with the KE exhaust.

HERE!!! I totally agree---RMBuilder/Bob Madara of Marine Kinetics is the cam with the cam! 585-654-8583 in N.Y.