tcelano

When you go to the lake, it's usually the elevation that gets you, not the water. While the hull does produce slightly less lift due to the decreased water density, the prop also gets a little less bite. This can be good or bad depending on your current setup. Altitude aside, you might be able to turn a few more RPM. For your setup, calcs show a 1 mph loss on fresh water at 1000' elevation, and about 3 mph loss by 2000'.

Sean H

he figured it with a 24 prop and you are running a 19... the speeds won't be right... if your not on the limiter with the 19 going to the 24 probably won't do anything for top speed...

Dave M

As far as I know his motor is a stock 500HP w/carb. He's running a 24 pitch prop. It was just a driver in the boat on a hot & humid day. I was showing 72-73 on my GPS and he had me by a MPH or two.

I haven't ran into him since I put a 496HO in my boat. I thnk he's avoiding me because he knows what the outcome will be.

sleeper_dave

I had the marina check with the builder. I couldn't get a build sheet, but the builder said they are flat-top pistons and the compression should be stock, 8:1 or so.

I'm still trying to ID this carb. For all i know the guy who put the motor in the boat used a carb off of a 350. Weber carb, #8867

Pat McPherson

Flat top pistons and 8:1 compression is NOT stock in a 502, and will not likely make good power with rectangular ports heads.
The engine should have dome pistons with about 8.7:1 compression.
Merc did use a 750cfm Weber carb on a 1" open spacer for a couple of years on a 502.
I believe the engine was rated at 390HP.

sleeper_dave

Well i guess i need a blower then.

the shop that installed the motor said it has oval port heads

but if it's lower compression than stock i guess my options are blower or new pistons. what's a good blower setup? not too outrageous, preferebly somewhat inexpensive?

sleeper_dave

Spent the night thinking about this. I determined that i really have 3 options, if in fact those flat tops are "blower" pistons.

My main problem here, is that the boat needs to go 40-45 to get up on top of the waves and ride nicely. With the current prop, which seems to work well with the current, apparently too low, power level, that means i'm turning 4000-4200 RPM for a nice cruise. If i back it down to 3000 or so RPM, which should be a nice cruising speed for the motor (i figure), i'm only going 32-35 mph, and the ride sucks, even in light chop typical of your average day on the lake. Beats you up. So, my main goal should be to optimize cruising speed.

That leaves me with 3 options.

1. Add a supercharger. Makes sense, if i have a desire for performance and a motor built for a blower. Problem is, based on my (decidedly limited) research, that'll cost about $6100 for a basic, new "done right" setup. I figured procharger 1MA222-MAG SC (M1SC for a carb setup) - 4500, plus fuel system upgrades - 300?, plus carb - 500, plus intake - 200, plus prop - 600. Would i have to worry about burning an exhaust valve with 6-8 psi on stock exhaust manifolds?

2. Change the pistons. Not something i want to do, and it will probably cost me nearly as much as the blower. So this is probably out. I will be talking to the shop again today or monday and expressing my displeasure at the performance of my motor, and i'll see what they say, but even if they can cut me a deal on labor this is still probably not the way to go.

3. Change the prop. I would need to do this with option 1 or 2 anyway, but if i just change the prop i can make the boat go 45 at 3000 RPM, i just might not get past 4000 RPM at WOT. And if i'm going 5 mph slower at WOT, it's just more incentive to just cruise, and lie and tell people the boat goes 75 then segway into a rant about high gas prices. If they're really stupid i can point at the boost gauge then point at the alternator and say "see, that's the blower". Plus, a prop change now makes the blower conversion $600 cheaper later, if i guess right on the pitch. I can do the intake now too and then the blower conversion is only $5300 or so next year.

Does anybody else have any suggestions, or input on one of my options? $$ is a big factor here. I can spend some money, but i really shouldn't spend $6000 this winter. More like $2000, maybe, and i need to redo the dash and have some upholstery fixed too.

Also, i am an engineer, but i was horrible at thermo. Can anyone tell me what kind of PSI would equate to 8:1 compression, and what PSI would correlate to 8.7:1 compression? I'd like to try to ballpark my compression ratio with a compression check. I realize cam timing will affect this and i will really be checking "dynamic" compression, but it should be a reasonable ballpark. Although i could loosen the rockers on one of the cylinders at BDC and take the pressure at TDC to get a better representation of static compression.

Thanks for the help so far folks.

sleeper_dave

what is the chamber size on a non-decked oval port head?

the pistons, from the bottom:


Says "JE700"... any way i can correlate that to dome volume?

Pat McPherson

I'm sure someone can confirm that those JE Pistons are Flat tops. If they are, I would change them to something that will raise the compression to around 9:1.
What are the heads? World Products, Dart? Most after market heads have 119cc combustions chambers and bigger than stock valves.
If I were you, I would change the pistons to something that gives you around 9:1 compression with the 119 chamber.
Then choose a cam that with make power to at least 5200rpm. On top use an Eddelbrock RPM dual plane if your goal is a real strong cruise. Then a 850 Holley or Bary Grant carb.
That engine can be rebuilt to make 500 HP for around $2000 in parts. Labor $$ depend upon how much you can do your self.
You don't need a blower unless you want to go 80+ in your boat...

sleeper_dave

I just had the motor rebuilt. It has maybe 30 hours on it. That cost 7 grand, someone else doing the work. It's not coming back out, if i can help it. I can do intake, carb, exhaust maybe, but kinda pricey, and maybe a cam if i can get it out in the boat. Overall it seems like slapping a blower on it is the best way to go, if they're blower pistons.

Searching around on this site, the only mentions of JE700 pistons are as stock pistons. And i can't find anything in the catalog.

The block is a 94 502 MPI. Presume heads are as well. Shop said oval port. Heads are stock, as far as i can tell.

Running gear, i don't know what it is. Looks older, like '91. At least the thermostat housing setup matched a 1991 diagram. Might be stock with the boat. Might also be stock 454 mag. What size carb did a 454 mag come with?

If they are in fact blower pistons, i'm thinking option 3 for now. Just over prop it until it cruises nice at 3000 RPM. Just won't be as fast or as efficient as with more compression. I just don't have the money to do the rest this year.

Oh, and who doesn't want to go 80 mph