I have twin 502 mags. 1 mark IV and 1 gen V
replacement engine. I thinking on bumping them
up to about 500hp, Want to do it as resonable
as possible. Can it be done with cam,carbs & exhaust,
or do I need to get into the heads?

ttt

Come on! Nobody has built up their 502 mags ?
Some help would be much appreciated

with a cam change plus some good exhaust so you do not get reversion from the cam shaft's bigger overlap, a bigger carb and intake you can get more than 500 hp.
www.crockettmarineengines.com might give Tyler a call also he has kits you can buy

Stroke them to 540, cam and a little head work. Make sure you have good exhaust. Will be more than enough if you just want 500.

Dont you dare, you bastard, You promised to go slow with me !!!!

Rogers speed shop in garwood NJ is really great for stuff like that. They did one of my 502's in the past, and are building a custom engine for me right now. They are very reasonable. They can get 575hp out of a 502 w/o s/cing.

Cams (Crane 731),
Intake (Dart or Merlin Single OR Airgap dual plane)
850 Holley with tall flame arrestor
good set of rockers (Harland Sharp)
pocket port and 3 angle valve job on stock heads.
Any performance xhaust manifold.

530hp give or take.

Mcollinstn. Now thats the kind of info I'm looking
for. How about the 741 compared to the 731?
As far as exhaust, are Gil or Eddie marine manifolds
as good in this h-p application as CMI or Stellings?
Whats the advantage to useing a single plane
manifold over a duel plane? Is the head work
nessecary, or can you get 500 H-P without it?
Thanks,Rick

731 is real close to a 500EFI cam with a little more lift.
741 is a notch more cam.

Stock untouched heads show max flow at around .545" lift. The Crane cam spends more time at or above that lift figure and as such flows a bit better than the 500EFI cam. Pocket porting and 3 angle valve job will raise the flow peak to around .565"

741 will net you around 12hp over the 731 at 5500rpm. Torque at 2000 will be down around 15 ft lbs.

Advantage to single plane is that you can use Tomcat's new intercooler with it. And it might give you 5 hp at 5200. If you run a light boat, go with the single plane. AirGap is adang good dual plane, though. 850 Holley either way.

As far as exhaust, any of the onew you mentioned are within 10hp of each other on this motor

You can reach 500 with the 850 holley, 741, single plane, and Gils. No headwork.

741 versus 731 is more of an idle quality issue on a 502. 741 has a rumplier sound by just a touch.

Neither one is what I would call a radical cam. Spring life will be 150 hours with either one, though.