dennyp123

I'm looking real close at a used boat with 10 hrs on rebuilds. They appear to be filled with all the good stuff but I have two (2) concerns:

1.) The new pistons have a bore of 4.53" and stroke of 4" which makes it a 502 CID bored .060 over. The engines have "block only" fresh water cooling (which makes sense to protect those thin walls). The CR is 10:1 and the cam has a advertised cruising range of 4,000 rpm. Provided nothing else breaks, how many hours can I expect. I know I can't rebuild them again (which sucks) but I'm worried the thin walls now become the "weak link"

2.) The engines have CraneCams #139651 which based on the numbers on Cranes website appears to be aggressive. The exhaust is gil with inline silencers inside the boat (just behind the transom) and the water and gases meet at the silencers. Should I be concerned with reversion? If I go on a sea trial, can reversion be masked with a recent oil change, or will idling for 5 to 10 minutes produce the tell tale signs with milky white oil?

Thanks for all your help

SB

iTrader: (1)

Doesn't seem to be a well thought out engine package.

OEM 502 blocks don't have a whole lot of 'meat' on the cylinder walls. Recommended max overbore is .030" and .060" is really it. Probably no room for any more rebuilds.

'651 cam is seemingly used a good amt by builders in 502's - however, I cannot understand why. Usually makes for a very peaky engine, having max torque real close to a typical valvetrain limited peak hp.

I'm not a fan of the compression for a typical heavier boat either. Maybe a little small weight catamaran or something, but not typical V hull.

You asked for opinions and I gave you my honest one.

Nordicflame

iTrader: (4)

We have a Gen VI 502 block bored to that same 4.530 bore with a 4.25 crank (548ci) and 7-8 lbs of boost that has run flawless for three years pushing 900+ hp.
The block is not an issue and can go a little more when needed. There are some that are taking them much further but I would have the block sonic tested before going any more than 4.560 just to make sure you have a good casting.

Cam seems too big and likely to cause reversion with that exhaust setup.

Dave

PatriYacht

Gen V blocks are at their limit at 4.530. Maybe if you had it sonic tested it would be able to go larger but not likely. Gen VI blocks are thicker. I agree that the cam is too big. You would be happier with the next size smaller cam, the 741. However the compression probably makes the cam tolerable and vice versa. It will make the engine peaky and probably makes peak power around 6000 rpm.

cstraub

.060" over on those blocks is the max. Drag racers will "short fill them" and run them semi dry and go 4.560". I would not recommend this in a marine application.

10 to 1 and if they are iron heads could be a problem. You will only be able to run 93 octane. If aluminum heads then its a little more detonation safe but you still should run good gas.

The cam doesn't seem like a good fit. I will bleed off some DCR and help with detonation but I would say something smaller would be a better over all fit.

I will offer a program I have on Excell. It will figure overlap for you when you plug in the duration and you can compare it to other cams overlaps to see if it is reversion prone. It will also allow you to figure Dynamic Compression to see if it will be pump gas friendly. I can e-mail it to you if interested.

dennyp123

Thanks for all the replies...they are helpful. I'll have to find out if the blocks are Gen V or VI, but that won't solve the big cam issue. Correct me if i'm wrong, but engines are most efficient at max torque. If that's correct, I do not want to run at 6000 all day long.

With regard to reversion, is there any opinion as to surefire ways to determine if it's occurring?

Rebel_Heart

The engine builder in my recent .30 over in my Gen VI 502 block also used the same cam. I had the dyno sheets with 1.7 rockers on the intake and the curve was pretty flat, however when I went to 1.8 rockers I did pick up 10 more hp. The tradeoff was that it was at 5800, rather than at 5400-5800 with the 1.7s.