alecsammy

Hello All,

I installed a crane cam 731 into a 502 gen-6 it has a stock mercruiser timing chain set. My question is should I install it straight up or use a degree wheel and retard the degree of the cam. I talked with crane and they told me to install it straight up. I know I gained around 100 HP with all the upgrades put still can turn more than 5k rpm same prop as before and same results. I can't believe that I gain nothing with new dart heads new probe pistons can ,intake , carb exhaust, and rods. I was told by a boat racer that you should never run a cam straight up in a boat and crane said it does not matter if you have 500 or 1000 HP you will still turn 5000 rpm with out changing the prop, I don't buy that. But something is not right I know people with the same boat ,same gears and less HP which run larger props and turn more rpm. any suggestions and if I need to degree this cam how much should I with valve clearance. Thanks Alec

SB

Your cam would have to be way off.

Check fuel system. Most people don't upgrade this (tank pick-up, anti-siphon valve, all lines and fittings, fuel pump, etc,etc) when building their engines.

Then carb calibration.

Always put aside 20% or more of engine build cost for all the extra misc's you need to properly rig it.

What boat ? What prop/rpm/mph did you get on which original engine ?

Griff

Crane is wrong about the rpm staying the same. You should have picked up about 500rpms with a 100hp increase. lSomething is wrong with your combo. Have you taken plug readings??? How much timing are you running???
I would install the cam straight up as well. I installed my 731 when I put one in my 525SC and picked up about 100rpms just with a cam change.

H2Xmark

most of the time straight up is best but on a gen 6 with a stock timing chain they stretch pretty quick, some people degree them in to make up for the slack

800XCR

If you are running T-Bolt V, it has a rev limiter.

Kidnova

Did you have the engine dyno'd ?

alecsammy

I had a stock 502 gen6 changed over to a RPM Air Gap intake zz502 cam 496 exhaust, holley 800 carb with 7 psi of fuel pressure stock mpi ignition 25 pitch prop spinning at 5000 rpm . The tack is new and reading correct. I change to a 731 cam , iron eagle 308 121cc heads probe flat top pistons eagle rods , same carb and intake and fuel system thunderbolt 1V iginition without the rev limiter, dana torque flow exhaust.The plugs look great right on. I gain a little speed running the same 25 mirage plus prop and still spinning 5000 rpm can't spin any more. Guys with the same boat donzi 22 classic with stock 502 or 496 are spinning bigger props and more RPM is the mercruiser timing set different or the timing degree the same as the 731. The motor runs very strong , I talked with a fellow that say don't run it straight up you will gain on the bottom and lost on the top. Is the distributor holding be back . I have not dyno it but I was told that I should have gained about 100 HP with these upgrades. It seems the I gain nothing.

PatriYacht

That Crane cam has 5 deg. advance built into it. Retarding it should move the hp up in the rpm range. While I don't have the specs in front of me, I don't think the 731 is much bigger than the ZZ502 cam. Also in lower rpm range the Iron Eagles are not going to help much. You need to get the rev's up to take advantage of their superior breathing. The rev limiter thing is the first to check though. The old 502magum carb engines were limited to 5050 rpms.

mjb

I'm not sure if cam grinders still do this but I understand that there were(are??) some cam manufacturers that would shift the pin (that aligns the chain gear to the cam) so that when you installed the cam "straight up", it would really be 4* advanced. The grinders knew their cams ran better at 4* advanced and they figured most guys were using stock(non adjustable) gears or just didn't have enough experience to advance it on their own. I think this was only done with lower HP cams geared towards the driveway mechanic market. So if you used one of these cams and actually did advance it 4*, you really had 8* advance. I'm not sure if this practice is still used. I think Comp cams use to do it??
Maybe Crane does it and thats why they told you to install it straight up??

PatriYacht

I just reread that you used flat top pistons. Unless the block was decked, you're compression ratio is about 8.5 to 1. Compensate for the lack of cylinder pressure by setting the ignition timing to 36 deg full advance and let the idle advance fall where it may.