BadDog

Stock 1999 GM 502 Gen VI crate motor with several hundred hours on it. No problems to date. Dart single plane intake with Holley 800. Lightning Headers with Teague tips and Crane ignition. Stock cam is probably not allowing the engine to breathe well enough in combination with the Lightning’s. I know it has been said many times before that the large oval ports are a better choice than the GM rectangular port heads. New heads are not in the budget though.

What would be the best cam selection with the headers and rectangular port heads? Would something in the range of a 226/234 or 230/236 be a good choice? Other recommendation? I plan on using the Crane Gold Rockers and leaving the single plane Dart on. I tried the GM hi-rise dual plane and did not see much improvement in mid-range, boat is fairly light for a 502. The Dart did seem to even up the plugs a little better. How much can I change the compression ratio by going to a thinner head gasket and would this be a good thing to do with the larger cam? Would there be much benefit to having some minor port work done? I never really did get any good plug reading from this engine, most likely the fuel in this area. Ended up jetting in the neighborhood of what the HP 500 called for. Any thoughts on jetting for this combination?

Thanks,
Tim T.

Dennis Moore

Tim,
The 500 HP carb engine has a roller cam with 222 intake and 230 exhaust duration with .576 intake and .598 exhaust valve lift on 110 degree Lobe Separation Angle. I feel that this is a VERY good camshaft for a 502. The GM # 12366543 camshaft has specs very close to this with 224/234 duration, 110 degree Lobe Separation Angle. The difference is that the GM cam has less valve lift .527/.544. This means that the GM camshaft can be used with the stock GM dual valvesprings (higher than this requires valve spring replacement, I am sure that GM designed the camshaft with this amount of lift so it could be used with the stock dual springs).
Many people say that a 110 LSA is to tight for a marine engine and that there will be water entering into the exhaust port at idle speeds. I think that any duration higher than 224/234 (for example 230/236) DOES need a 112 degree lobe separation angle but the tighter LSA is so important for engine power that there is very little difference between a bigger cam with 112 LSA and a slightly smaller cam with 110 LSA. One last thing, never go with a camshaft with less than 110 LSA (only the old timers and car racers do that!) or more than 112 (unless you have a VERY large cam).
Sounds like the jetting you are working with (the 500HP jetting) would be very close, particularly with the above mentioned camshaft.


Hope this helps!
Sincerely
Dennis Moore

BadDog

Dennis,
Yes this helps. I had your book but need to order another copy, lent it out and it was lost. If I retain the stock valve springs I would still probably consider the roller tip rockers just to keep the valve train in good order. This engine is not the HP500, plain stock 502.

Would there be any power gains by going with the HP 500 camshaft? What if I used a Crane cam with a little more lift and did switch my springs? Would the Crane 139731 be a good choice? I currently have a prop that I would like to use but can only turn it 4500 rpm at about 70mph. I was hopping to get another 400 or 500 rpm with a cam change, upping the cr to about 9 to 1 and maybe a little porting. What type of head gasket will get me to 9 to 1 and is that a good idea with this cam?

Thanks,
Tim T.

Dennis Moore

I understand that you have a standard 502, you should definetly use the Crane cam if you don't mind changing valve springs (but I believe you want #168731 for a Gen VI).
I just did some research on head gaskts for BB Chevys and wrote an answer to a question and printed it in the Sept. 2002 FAMILY AND PERFORMANCE BOATING MAGAZINE. It seems that there are no good, reliable, thin marine head gaskets made for the 502 (a couple for the 454 though). I don't want to alarm anyone but some improvements have been made to the 502 marine head gaskets (although I have never seen one fail I have been asked a few times by Mercruiser Tech people if I have!). I would buy the latest and greatest GM/Mercruiser head gaskets if I where you just to be sure I got the best!
Sincerely
Dennis Moore

BadDog

Dennis,

Thanks for the reply; it is good to have your input on the board. I hope we all don't wear you out! I guess this leaves me with the other option of milling the head slightly. If I decide to have any port work done on the heads I will talk to the machinist about how much to take off. Seems I heard something about milling the rectangular port heads and using the Dart intake with intake sealing problems. Anyone have any experience on this?

Tim T.

Lee

It is also a good idea to stagger jet the carb when using a Dart unmodified single plane intake because of the A/f distribution, Also check cyl #8 when jetting since it tends to be the leanest one.

86 those non adjustable rockers, The conversion kit to adjustables is fairly inexpensive, You will have to replace your current pushrods and rockers so get some good aluninum rollers and some quality pushrods

And make sure that you degree the new cam in. Replace the springs to match the cam and verify that their intalled height is correct.

Welcome aboard Dennis, I'm sure that your wealth of knowledge will be a great asset to this board.

BadDog

Lee,

Are these the correct procedures to degree in the cam or do you know of a better way.

http://64.90.9.168/cranecams/pdf/803.pdf

http://www.cranecams.com/master/degree.htm

How common is it to have to change the cam timing?

After reading through these it looks like unless you have done this before you could lead yourself down the wrong path.

Tim T.

Dennis Moore

Tim, if the procedure looks a little daunting to you I would just install the camshaft. I have degreed many camshafts and they were all perfect. No, the valve intake centerline position (timing) very seldom needs to be changed.
Sincerely
Dennis Moore

BadDog

OK, cool. I have some dial indicators and other assorted gauges form setting up rear ends. I think before I yank the old cam out I will check the existing setup. If it comes out OK I can assume (big ole hole to step in) that either the cam is ground correctly as well as the keyways cut correct, that the differences offset one another or I am not doing it correctly. Either way I will probably seek professional help less it turn into a "project". Best to pay a little up front than a lot of grief latter.
Tim T.

rjcardinal

Man its great having Dennis around. I followed his advice on the last 3 big blocks I built and he was right on. The last one was a 502 with the Chevy ZZ502 cam that Dennis recommended. Man did that thing run in a 22 Scarab. I had EMI exhaust with the cheaper aluminum risers and did not have a reversion problem. The only thing I did different was go to a better valve spring because I knew I was going to see sustained high rpm.

In my research Chevy uses different valve springs for the ZZ cam than the valve springs that the stock Merc engine has. I was going to buy the "special" (thats how they are listed in the GM performance catalog) GM springs but my head guy talked me into going with a Comp spring. I forget the number but we called Comp and gave them the cam card and told them it was a marine engine and they recommended a spring.

Hope this helps,

Ron