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Well DragBoatDad and I are kicking around putting a new cam in the sled. Our current motor is a 468 with TRW d-cupped pistons and cast rectangle port 990 heads giving us a static compression ratio of less than 7.5:1. We spin a stripped 671 13% over making 14lbs of boost and draw fuel through twin 850's lit off by a vertex mag. The only thing I know about our current cam is it's a roller cam and the net left is roughly .615 on both the intake and exhaust side. Originally the motor was set-up by Norm Grimes to be a mild pump gas motor, but we want to wake it up a little. What kind of cam would you guys recomend, keeping in mind that we don't want to take the heads off to open up the spring seat for larger springs. The installed height on the springs is 1.90 and the diameter is 1.55. Also we are running our timing locked out at 31 degrees and were considering advancing it a little more but we don't want to hurt the motor. Already I can see some heat in the plugs and I fear with more timing comes more heat
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Much more cam and the springs will have to go to 1.625 diameter at 2 inches installed height. Remember that the 14lbs of boost creates a negative effect on spring pressure by the square inches of area of the intake valve. In other words, you don't have the seat pressure you think you do in actual operations under boost.
Unless you want to step up to the big springs and machine the heads, I suggest you leave it alone and watch your spring pressure carefully. Otherwise I'd say close to .750 lift and about 284 degrees duration at .050 ground at 112/114.
I wouldn't run any less advance. Creates exhaust heat. 31 is bare minimum. Run a 9 range plug.
PS: My powdercoat work is finally done but nowhere near as nice a job as yours turned out. If you have time, would you take a picture of your quadrant for me as it will help me route the steering cable. Been apart so long, I forgot how it goes back together! :rolleyes:
[ May 09, 2003, 07:28 PM: Message edited by: Fiat48 ]
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as always thanks for the reply, I guess we'll have to step up and pull the heads again cry because we defiantely need more power. I think this time we're going to ditch the o-rings all together. Here's the quadrant
[img]http://www.***boat.com/image_center/data/500/110106-0682_IMG_2-med.JPG[/img]
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I am thinking about stepping up on cam. Maybe you guys can use mine. It is a Crane 138791 636 lift 254 262 @ .050 114. Good to 7200 RPM. It works great for me. 1 1/2 seasons young.
I am upping the compression this year.
[ May 09, 2003, 07:55 PM: Message edited by: Infomaniac ]
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That is an idea Infomaniac. Do you think it would work as well in a gas deal as it has in your alcohol deal. Plus do you think we would see a marked improvement in performance. Lastly is their a way for us to check our duration and lob centers without removing the timing chain cover.
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It is not alcohol specific.
Just degree the cam. Intake and exhaust. It will tell you everything you need to know.
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Thanks for the picture! Fast service! And please put a hose clamp on that key just in case it someday walks up the rudder. I've seen it happen.
Also: Remember most cams talk lobe lift and valve lash must be subtracted for actual lift at the valve. Most decent Chevy heads like lots of lift (up to .750) intake, .725 exhaust. Head flow slows down drastic above that. Blown motors get away with lots of duration without losing low end power. You can degree the cam by just removing the valve cover and a rocker arm and indicating off a pushrod. Example= Intake opens 32 BTDC. Intake closes 72 abdc. Add 180 degrees. Duration is 284 @ .050.
Here's a link to Info's cam card so you can compare notes with your cam. http://dab7.cranecams.com/SpecCard/D...1=Display+Card
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It's funny you should mention putting a clamp on that key. The first trip out after putting it all together the key worked itself out, luckily I lost steering right when I got on it and not at the big end. Needless to say I have two clamps on it now.
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P.S. that was a picture of the boat before we redid everything, it's much cleaner now.
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When I received my "merlin motor" ready to go, it had a .714 lift cam and 268 deg duration @.50. I went to a smaller, Better Builder,He changed the cam and corrected the valve train geometry. Installed larger seats to match the 2.25 valves already installed and pocket port and port matched the heads to the intake.The cam was smaller,roller,and had a cast gear so I didn't need a bronze distributor gear. I picked up 7-10 mph. I would consider a nice port job to match the intake, and a good pocket port to see your cam change make a real world difference.
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The heads have been completely ported and polished and matched to the intake.
