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Thread: Tuning Help

  1. #1
    Jet Junk
    Alright, got the boat semi-assembled and have decided to dial-in my motor before the local lake becomes crowded. My motor is a 383 ci. small block chevy. Dart block, Scat crank, Eagle rods, forged 2 valve relief flat tops. The heads are S/R torquers, 2.02/1.60, 72 cc. Compression works out to around 10-1. Last year this combo pulled 5000 rpm with a B impeller. The cam was a hydraulic flat tappet .487/.490 228/236 and I had 1.6 full roller rockers. This year I switched to a solid roller, .600/.600 260/260 with 1.5 rockers, k-900 springs, and did some bowl work. I installed the cam 4 deg. advanced and ran .005 more lash on the intake. The carb is a BG 650 dp and the intake is a Dart dual plane. I use a Hei, gutted and a MSD 6al and a MSD blaster 3 coil. Fuel pump is a Mallory comp 140 using all -8 line and a Mallory filter. This year the only thing changed was the bowl work and the cam and I gained 0 rpm. I switched to a victor Jr. and lost 100 rpm. What's it going to take to make this combo work. Exhaust is out of the question for this year at least, as the engine compartment is enclosed and everything jacketed is out of my budget. Sorry for the long post, an help is appreciated.

  2. #2
    Havasu Hangin'
    In my unedumacated opinion....
    That's a big cam...probably pulling max HP around 6,000RPMs? Definately will lose some low-end torque...do those heads have 180cc intakes?
    How big is the boat?

  3. #3
    jim@pj
    jet junk. Well you went to a bigger cam to move more air thru the engine but you still have the same carb and exhaust. I prefer the 750 dp. High flowing exhaust is a must. Do you have the small 2 7/8 risers? Sometimes you can open these up to 3.5 inches and run bigger exhaust outlets. Jim www.performancejet.com

  4. #4
    DUCKY
    #1 I think you need more carb. #2, you had a cam that was at it's happiest at around 5500-6000 rpm, and now you have a cam with a much higher powerband. You are probably making less power now at 5000 than at the same rpm before. Your current cam probably makes it's best power at about 6800-7200 rpm. I would add a 750 carb, and then think about having your impeller cut down to a "C" or "B-C" I would have left in the other cam, or found one with more lift, but the same duration.
    Have fun....
    [ March 14, 2003, 07:56 PM: Message edited by: DUCKY ]

  5. #5
    Jet Junk
    OK, HH: Yes, these heads have 180cc intakes, The boat is a Glastron-Carlson Cvx-20, basically an 18 footer
    Jim: Plan on trying a 750 dp tomorrow, will post results, the risers are 2 7/8, will see about opening them up.
    Ducky: I was happy with the other cam, but I knocked a lobe off it, so I planned on having this cam re-ground, but the money ran out after I got a piece of metal in my eye, causing me to go to the ER with a bill of $180 from the hospital and a bill of $200 from the doctor who only sent me to an Optomologist who charged me $380 to remove the metal. Kinda big chunk out of the boat budget when you make $275 on a good week. Anyway I scavenged this cam from my used race car parts so I figured just use it and see what it'll do.

  6. #6
    Havasu Hangin'
    Well...I agree with these guys.
    I have an Holley 800 on my 383, and I only turn it to 5,200RPMs.
    I think that cam is very big for those heads, exhaust, and intake. Those race guys use higher flowing parts to go with those big cams. Also, those big cams bleed off alot of cylinder pressure.
    Jungle Boy made 500+HP with his SBC, and his 21' squirt boat...maybe he has some setup suggestions.
    [ March 14, 2003, 08:58 PM: Message edited by: Havasu Hangin' ]

  7. #7
    Duane HTP
    We see your question and perdiciment over and over year after year. Race car cams don't work very well in jet boats. Ducky gave you some good advice on the lift and duration thing.

  8. #8
    Jet Junk
    HH and Duane I see your point on the exhaust and carb, but I just can't see why its not working. The motor this cam came out of was a basic 350 flat top motor with factory chevy cast iron 2.02/1.60 heads with some decent port work, the same victor Jr. and a 650 dp. This combo ran 11.40's in a 3400lb monte carlo on 9in street tires and a single 3in exhaust. It was a killer combo that hooked me on what a sbc could do, whup some big blocks! Anyway I guess I'll try the carb and probably end up sending another of my roller cams off to be re-ground. Can anyone recommend a grind for this app. Thanks....Tommy
    btw, HH can you share the specs on your motor. Thanks
    [ March 14, 2003, 09:32 PM: Message edited by: Jet Junk ]

  9. #9
    Havasu Hangin'
    Jet Junk:
    HH can you share the specs on your motor. Thanks Well...those race guys don't need a lot of low-end torque (with a clutch or stall converter)...but boats love torque.
    Anyway, my motor has a Chevy race-prepped block (4 bolt; decked and line-bored), Scat crank, Eagle 5.7" rods, TRW 10-1 pistons, Edelbrock Performer RPM heads (w/mild porting and bowl-blending), a Crane Hydraulic roller cam (222/230; 509/528) with Crane gold roller rockers (1.5), MSD box, coil, dist, rev limiter, and start-retard, Holley 750 (coverted to an 800) with a Holley Street Dominator intake. I'm running EMI Thunder exhaust, and 2 Holley pumps.
    With dry headers, it pulled 457HP on the dyno. More important to me was it has over 400ft-lbs at 2,500+, with a max of over 460ft-lbs. My boat is very heavy, and needs all the bottom-end it can get.
    Once again, I'm not an expert, but by switching to the bigger cam, you probably moved your powerband up significantly. In other words, you may have lost some torque at 5,000RPMs with the new cam. Add the restrictive pieces, and your probably not using that cam to it's (race) potential.
    Also, like I mentioned, in my (non-expert) opinion, that cam will probably bleed off quite a bit of cylinder pressure, taking away from your lower-end torque.
    These guys are always beating up onb us SBC guys, so we gotta stick together. I don't have a jet, but I'd say that you might need to change your setup to get that motor into it's powerband...
    As for cam specs, with those heads, if it were me (and my non-expert opinion), I'd probably take Ducky's advice and don't go over 240 on the duration, and as much lift as is safe.
    Jungle Boy may chime in...he's been down this road before with his SBC powered beer-can boats...

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