alecsammy

I have a post on crane cam731. several things happen ,first I had revesion with the dana torque flow they added about 8 inches to stop it. The 800 holley carb had stock jets with the old 502 mpi distributor, I changed the dist to a thunderbolt1V and when to 84 and 92 jets . These are different two different problems that are corrected but what would cause this to happen is it possible the cam is ground wrong to cause reversion and can the change of the dist cause the carb jets to change . The stock jets with the 502 MPI dist made the plugs look great I was unable to increase the jets wuthout running too rich. This fall I will be taking the engine out and degreeing the cam is there anything else I should look at . I know the old engine discussed had to have less hp than the new one.but shows very lkittle results.

PatriYacht

It is possible the cam caused reversion due to the short risers. The original HP 500's had a bit of reversion with smaller cams than yours with Gil manifolds and short risers. Degreeing the cams is always a good idea. An ignition change should not have anything to do with your carb mixture. Was this done at the same time as your cam change? Going to a larger cam many times requires larger jetting.

alecsammy

when I removed the mpi to carb I used the mpi iginition. The motor at this time had just finished rebuilding ,I tried running bigger jets on the marine holley 800 but was unable ,running very rich and showing sutty on the plugs . Then I installed a 1988 Thunderbolt1V ignition and the engine starting running lean that when I increased the jets. I was hoping to get over 500HP and would like to get well over that HP any suggestions. I'm against a blower or supercharger.

PatriYacht

Changing ignitions will not change the carb mixture. Maybe the first ignition system wasn't working properly resulting in sooty plugs. Or maybe the advance curve is different. Re the hp question, your exhaust manifolds may revert if you go to a larger cam. Are you prepared to change to dry tailpipes? Also, your low compression is likely to cause drivability problems with a larger cam. Remember all parts of an engine have to match. A good running engine is a well designed system.

Right now, based on the parts in your other post, I would guess that you have about 475 hp. Raising the compression to 9.7 would be about a 15% increase and probably a 40 hp increase. Changing to the Crane 741 cam would probably be good for another 40 however a good single plane intake and at least a Holley 800 dp is required at this point. Max hp will move up to 5500-5600 rpm. At this point changing to a Crane 651 cam is good for about another 25 hp. Max hp moves up to 5600-5800 rpm and dry tailpipes are required.

If all of this stuff had been done during the build, it wouldn't been too expensive. But to go back now and do it over again for 100 hp is hard to justify. Putting a small Procharger on the engine would cost about the same and produce more hp. It would also avoid the need to pull the engine and tear it apart. Good luck with whatever you end up doing.

SB

iTrader: (1)

I'm going to suspect the issue mayu lie with how/when you read the plugs.

The MPI ignition has a small advance curve and thus will use much more initial timing, and of course thus have more timing per rpm until full advance. I'm assuming your setting full advance to same spec on both systems.

More timing at idle/low speed/etc can give you more intake vacuum and less intake reversion thus pulling more fuel out of the carburetor with the same jet and idle settings.

If you read the plugs above the total timing rpm spec with both ignitions, then the plugs should read the same.

BTW: I really couldn't understand what you where asking there in your first post so my answers above are assuming your questions where how they appear interpreted by the others first responding to your questions.

Raylar

Lots of carbed marine engine performance camshafts sold usually have LSA'a around 108 to 110 degrees. This makes for a lot more overlap of exhaust valve and intake valves open at the same time. This is generaly what causes reversion in normally aspirated motors.
I would talk to a good cam grinder like Cam Motion, ETC. or such and come up with a spec. grind that has LSA's somewhere around 112 to 114 degrees. This change may loose just a little top end on the motor, but reversion with these kind of 741 durations should disappear. This is what Mercury Racing ran on the cams in the HP500's late on when they cured most of the reversion issues on these engines. Fuel injected engines are spoiled, they can run bigger durations with the LSA's of 116-119 degrees needed for EFI motors and still not see any reversion!

Watch those LSA's!

Best Regards,
Ray @ Raylar

Pat McPherson

Will you tell me how you did this please?
http://www.offshoreonly.com/forums/s...d.php?t=195633

Thank you,