PatriYacht

Because you're using iron heads, I'd advise using the 9.1 pistons. If you change to aluminum heads, the 9.5 would work because the aluminum tends to dissapate heat faster, avoiding hot spots.

You can find info on the Crane cam at www.cranecams.com. Look under marine cams. They have all of the parts required listed there. I have 2 sets of the recommended pushrods very low hours I'll sell for half price.

A bunch of us are going to Key West. Look for us, Ian and Donna.

Cig35Mistress

Okay guys . . . bear with me here

Can you explain in layman's terms the difference between the two cams we are discussing - the Crane 741 and the Comp XM298H.

Crane 741 = 234 @ .050 intake duration and 242 @ .050 exhaust duration and .610 intake lift and .632 exhaust lift

Comp XM298H = 254 @ .050 intake duration and 264 @ .050 exhaust duration with .575 intake lift and .578 exhaust lift

I am just trying to get a handle on what I am doing and why I am doing it, so I can go to the machine shop educated!

Also, do you know off hand if the GM rectangular port 990 heads are open chamber or closed chamber? Either way, could I safely run the JE closed chamber dome piston?

So many questions, so little time . . . and worst of all, NO boat!

Har

PatriYacht

The duration numbers at .050 are the thing to look at on these cams, although the Crane has a little more lift. The Comp cam has 20 degrees more duration. A cam with that much duration will put your power peak in a 496 at about 6500 rpm. It will cause a very poor idle at about 1000 rpm and possible stalling when you put it in gear. Also it will cause reversion with almost any wet exhaust system. The 741 has enough duration to put your power peak in the high 5000's. The idle will be choppy and loud but you should be able to idle around 800. Depending on the exhaust, the 741 may revert water.

I looked up your pistons at JE/SRP. They are a SRP number. The website says they are suitable for open or closed heads. Any heads made after about 1970 are open chamber except for the Vortech and the new 8.1.

Cig35Mistress

I've done a little searching and found what I think would be a close match to the Crane 741 but in a complete Comp Cams kit.

Competition Cams Kit #K11-244-4 (244/254 & .570/.575) a.k.a. XM288H
Competition Cams Kit #K11-240-4 (234/244 & .564/.567) a.k.a. XM278H

I like the idea of these because they are relatively reasonbly priced and they are complete kits, rather than having to source out the individual parts. Is the XM288H still too big? What's the disadvantage to the lesser amount of lift as compared to the Crane? Can I run a slightly more aggressive cam and increase my idle RPM speed to increase driveability around the docks?

Hoping to figure this out so I can get the boat ready for Key West . . . November 1st is my target!

HSL

PatriYacht

The Comp Cams you listed are hydraulic flat tappet cams while the Crane is a hyd. roller cam. The difference is probably 50+ horsepower. The hyd.roller costs about $800 more per engine after you buy all of the req'd. pieces. Speed costs money, how fast do you want to go?

cstraub69@comcast

I argree with Laz and RMbuilder. With iron heads you need to stay around 9 to 1 with pump gas. Flat tops promote better flame travel and will make more horsepower when equal to compression of a dome piston engine. Milling the heads some will increase the compression.

Camshafts. Even for 496's everything you have posted is to large. You have a heavy boat, you need TORQUE. The runner volume on the 990's is quite large, even for 496CID's. I would suggest smaller shafts on duration, and larger lift, perferable around 600 intake and 580is on exhaust. Think of lift as what allows the engine to accerate to any given rpm and duration is the ability to sustain power at that rpm.
Duration split between intake and exhaust is determined by flow ratio of exhaust to intake. When this ratio is known, the proper amount of split can be determined for the mill. Joe Petelle of HVH and I have discussed this at length and then proven it at the track. The flow ratio is key.

Chris

cstraub69@comcast

Forgot. When comparing a roller lobe to a flat tappet lobe, this is just a rule of thumb not exact, a roller lobe that is 250 degrees at .050" is equal to a hyd. that is 270 degrees at .050".

Chris

PatriYacht

I think a bunch of us jumped to the conclusion that the cam in your first post was a hyd. roller. They're so common now. They do offer more power and better drivability, that's why.

delsol

If you're going to buy all new rotating assembly anyway why not just build a 572 right off. Sell the 454 short blocks to offset the cost of a new dart/Merlin block. You have to totally machine your old blocks anyway, and the price for a 4.25 crank is the same for a 4.5. This will help with the torque that you need. You could still bolt on your GM heads if they're in good shape or if they need alot of work try to fing a set of new/used aftermarket -- depending on your budget---
A friend of mine is running 572's in a 86 mistress and seeing 75 mph with a trs set up. You can always pull the advance back a little to ward off detonation.
I've been where you're at, do it once and make sure the person assembling knows marine My 1.5 cents
Ps. you'll have to buy bigger aftermarket oil pans though, this I learned

KAAMA

Cig35,

cstraub69, is right about the cams. I appologize---for some reason I kept thinking you were building 540 cubic inch engines---therefore I believe the Crane cams 741 are a bit too large for a 496. Remember, you need to build your engines for torque. Torque is what moves the mass---not horsepower! Those Cigs are HEAVY boats.

Also, if I can give you some friendly advice---not to bust your bubble, but don't be too disappointed if you don't make it by Nov 1. Building a pair of engines, putting them in the boat and getting them dialed in correctly usually takes longer than you think. There can be lots of little things that pop up that you may not be expecting, therefore I wouldn't rush it if I were you. Just take your time, do it right the first time and things will usually go a lot smoother. I am quite sure the others here would agree.

rmbuilder/Bob and/or cstraub69/Chris can probably help you get dialed in with parts/combinations and they seem to offer pretty good advice.

I do have a set of used (fresh water use only) Stellings (size/fit code color "green") full length tubular headers with approx 175-200 hours on them for sale ($2800 per set) if you're interested. There are no leaks or repair welds or anything wrong with them, and are in excellent condition. They sit a little too high in my engine compartment, but may fit/work in your Cig. You would probably have to call the Cig factory to find out if a Stellings "green" will work in your particular Cig. They are more suitable for a boat with a transmission set-up like yours vs. my Bravo One drives. So they are available.

Mark/KAAMA