502 water reversion
 

I have seen a ton of posts on this but have not found one describing my problem so ill start a new one I guess, I have a gen VI 502 with comp cam xtreme marine roller 11-461-8, 360 cc square port heads and 1150 dominator, mercruiser exhaust manifolds with through hull exhaust below water level, angle tips, I am getting enough water reversion to drop cylinders and run rough anytime boat is not moving, no matter if rpm increases over 2000 rpm, I noticed last night there are no flapper valves in the tips, a straight vertical bar is located inside the tips but no flapper, but I do not see how that is my problem, even with a flapper they will leak and have some water in the exhaust pipe, especially if you were anchored for a while, is this camshaft to large for this setup? cam is .575 lift 112 degree lobe center that I did degree, 254 and 260 @.050, Thanks for your help!

I hope you are not using factory iron manifolds, that's just asking for trouble.

Heads, cam, carb are way too big for a 502. The torque curve is going to be terrible and it won't have any low or midrange power.
Stock Merc exhaust is choking everything. You can't use stock Merc cast iron exhaust with any performance cam or it will revert water.

Even stock camshafts in these engines have some revision.. Dry is the only way to go with tighter lsa cam shafts.

Don't take this the wrong way, especially since you where smart enough to ask, but you almost couldn't have a worse combination for a 502 powered I/O with wet exhaust...factory at that.

Lol! I will certainly not take it the wrong way! I have Drag raced for many years and this boat thing is a whole another ball game, I had this engine in the corner, a spare for my super gas car, and I thought I could detune it enough to make it go, sounds like even if I put a stock camshaft in it, I will still have the problem? I know the heads and intake and carb will not make any low end power, changing the exhaust around will take some fab time, it is june and I would at least like to go chase some bikinis, I do appreciate the responses.

OMG.

Yes, I agree. A 360CC runner head is way big for a 502, ESPECIALLY a naturally aspirated one. Even blown 360 would be way big.

That cam you have, is also too much duration, and not enough lift.

With stock exhaust, and that camshaft, the only way I see reversion not being an issue, is more than likely with an aftermarket exhaust, and full dry tails. Even so, its still gonna be way to much duration for a marine 502 turning 6000RPM or less, in my opinion.

It sounds like you just want a useable boat right now. The only way I see that happening anytime soon, is ditching that cam. While the heads are too big, and the carb, at least you'd be able to run the boat. While it still wouldn't be ideal, Right now its useless unless you find a dry exhaust system. The stock merc iron exhaust is worthless above about 400HP. Best bet to get going, would be find a better exhaust, and downsize the cam a bunch.

To answer your last question, I think changing the cam out for something with a lot less duration than you currently have, and a wider LSA, would help fight reversion, and help crutch the monster port size in the heads. I'd like to tell you to call Bob Madera at Marine kinetics to grind a custom cam for you, but not sure he would want to until you're ready to put a solid package together. Like, the proper size head, carb/intake/exhaust, etc. You might consider something like a stock 500EFI cam or something like that for time being. That cam is 230/236 .598/610 114lsa. Even that might be borderline with iron exhaust. Merc used performance exhaust on the 500EFI. Only thing they used iron manifolds on 502 wise, was the 502/415HP mag engine. Which was like 224/224 115lsa I believe.

i 100% agree,and if you decide to go with the hp500 efi cam,i have 2 forsale.

I don't agree.

I don't agree with the word crutch.

The person who invented the word crutch was a disaster in the offshore forums.

That's how well 'crutching' worked.

Don't tease this guy. Many parts need changing.

i dont think anyone is teasing him,he stated that he is a car guy and new to the marine thing.i think most of the posts were to school him and not meant to tease or belittle ,jmo.

I know a 502 with 360cc heads is a bad choice. But, is it really that much worse than a 454 with 325 cc heads that flow like crap? Because that's what GM/merc used for decades. Ideal ? No.

Dominator carb, yea a bit too much. But really any worse than a 800cfm quadrajet on a 330hp 454 ?

I guess what I was trying to get at, was even though the parts he has are far from ideal, I wouldn't throw the engine in the trash bin and leave the boat in storage until he can perform an ideal build. I think by shrinking his cam choice and changing exhaust he could be tooling around the lake for the interim. Just my opinion.

I'm guessing the "360" heads he has are the Dart "race" series castings. Pretty nice heads, and if he wanted to keep them he can have a guy like valako wedge the ports like he used to do with the gm square ports on marine setups.

My experience. When someone mentions 320 or 360 with no reference to the company they are usually chinese Procomps.

If someone has Brodix, AFR, Dart, or even Edelbrock's, they mention it.

Anyway, we all agree, the exhaust is first, then cam, then heads, then carb.

Ah screw it, put it in a v drive or jet boat and start over for the I/O.

MT - I totally understand what you are saying. I've tried to do the same you are with many people over the years. It doesn't really help them. In fact, it usually hurts them. Even thoy it feels like you are trying to help.

I
That big carb will suck with a small cam.

General rule of thumb I've come up with. 650 or more HP, then it's Do time. Engine will like it. Under that, the carb will be out of it's elemnt and pretty much suck azz.

The one part that will get him back on the water - dry exhaust with thru transom water dumps.

Thanks to all the responses, Mild thunder I appreciate your input and advice, seems like you understand my situation, and nice avatar pic!! I think if I was to listen to SB I should not go anywhere near the water, lol! I realize this combination is probably not the best suited for this boat, my goal was to do some boating, my last boat had zero cool factor and was quite boring, I knew if I listed the head size and carb it would open a can of worms, I mentioned that in hopes of getting across that with larger port sizes port velocity would be way down and should help with reversion, these are brodix heads with jesel rockers, way overkill, but I have them, and they are nice pieces, carb size also was mentioned, yes its a big carb, but what is worse? a 800 cfm on a 330 hp engine? I put this carb on my s/g car, its a 377 small block, it ran within a tenth of the best 4150 style carb, it idles great, and has great throttle response, would a smaller carb work better under 4000rpm, most likely, but I have this one and it was expensive, I do not think anyone has lifted their hatch and someone said "Hey nice Quadrajet" although I ran one on my NHRA stock eliminator car and if you spend about 1k on them they work real well, I ran 2 1150 dominators on a 458 cu in engine in my comp eliminator car, it made 1185 hp on gas, so I am aware of proper combinations and buying good parts, but my question was about water in the exhaust, I did not realize these manifolds were that inefficient, thanks for the education, but this boat has 290 hrs on it with this setup with a big block in it so it worked for some time, so ill change the camshaft, the 502 cu in really shouldn't be an issue, it will start and get me around, we only have a couple months of possible boating time around here so I will change exhaust around this winter, mike tkach, I would be interested in one of those cams :-)

I have to answer it that way, because if I don't, the person ends up blaming me and others when things don't work the way they dreamed.

That's why.

You know the deal.

try advancing your cam 4* and see what that does for you.. or to be honest theirs a set of stelling headers for under a grand on the swap shop and it might be cheaper to put the exhaust on instead of changing the cam which is a roller and you will need retrofit lifters..sounds like it could be a mean mid and high rpm cam..

i can fix you up with the hp500 efi cam and a set of retrofit hyd roller lifters for a fair price and i garantee the parts are good.pm me if interested.i really think a stock 415 horse 502 hyd roller cam would be the way to go with the stock merc manifolds,no chance of reversion and you will be boating!im sure someone on oso will have that cam for 100 bucks or less.