454 in 2550sx
 

New to this forum, but not new to boating. Just bought an 89 Chaparral 2550 sx, 5000 lb boat with 454 bravo, prev owner upgraded to gm rect heads (don't know casting # yet), and matching gm intake off a 72 camaro, dual plane high rise, isky cam 280/592 with 565 lift. Stock thru hull exhaust, stock Quad carb.

no speedo or gps, but boat was only hitting 4000 rpm wot with 25p mirage, so switched to 20p turbo prop just to see what would happen, 5000 rpm, better holeshot and feels like better top end. now 22p mich wheel, 4700 wot, ok holeshot, top end feels ok. before I switch props again, what do you suggest I do with the motor. My buddy has a merlin single plane with brass jackets in his garage that he would let go cheap... is the qjet acting like a restrictor plate on this thing?

You need better exhaust and switch to a holley 800 carb. The cam also sounds too big unless its a roller cam.

You need to get a gps to check your speeds ! Had a 91 2550 with a stock 365 HP and ran a 23p prop . Just a little trick I fond was to run with the sun pad up 5 or 6 inch's . It was good for 3 mph on gps telling me the engine needed more air. Good luck :ernaehrung004:

Do you remember what your speed was at WOT with that 23
pitch? Mirage Prop?

If it'sthe 750cfm Qjet from a 454 mag, it's not restricting you abit. Your exhaust still is though. If you have the stock454/330 bottom end 4700rpm is about all you want to turn.
You should check to make sure you don't have a hook in your hull. Use a 4 foot or longer straight edge. If you can see a 1/4" of daylight anywhere, it's like dragging your tabs.

Sounds like the engine once was a Merc 330hp. I take it those are the advertised duration numbers on the cams specs you gave???

You'll need to start with a better breathing aftermarket exhaust system like Stainless Marine. If you switch to the better exhaust, then you can switch to a single plane intake manifold---especially using the big rectangle port head on a 454cid engine. Add a Holley 750cfm or 800cfm carb.

This isn't neccessary, but if you have the time and want to have a little fun and do a little experimenting I would try running and testing the boat after the exhaust modification without any other mods/upgrades other than carb jetting-----then I would try the single plane intakes to see what happens. It should run much better on top with the single plane intakes.

FYI, If you change to a single plane intake manifold and still retain the stock Merc exhaust system your engine will most likely run worse---especially with the larger cam you now have in there.

Anyway, the exhaust system is really where you need to start if you want to see a true improvement in performance gains on a naturally aspirated engine. Have fun!

thanks for the suggestions. I'm going to upgrade the exhaust and then go from there. This is not really pure perf boat, so if I can get it to 55-60 mph without a supercharger I'll be happy.

just picked up a pair of Kodiak exhaust manifolds with IMCO risers. Will try those out first, then go to 800 cfm holley. Then after that will try the Merlin single plane.

I finally pulled valve cvrs to reveal I have 188 rect heads. I started thinking that those heads with the bigger cam, didn't' the compression go down since the prev owner just rebuilt with flat-tops pistons? I would have gone with some domes to bump the comp up a little or at least keep it the same. Tough to make power out of a 454 when comp is 7.5:1 !!!

Good numbers to add a blower to !!!!!!!!!!!!!! 7.5:1 My 91 would run 58 on gps bone stock so with a little work 60 should not be hard to reach . Just remember more hp and less weight = more speed So if you are out of hp just leave the old lady at home and take a smaller cooler !!!!!!!!! Good luck , Jeff :boat:

58, wow...what stock motor did you have. 502 magnum or something. My 454 mag clone will only hit 48 on the speedo...Who knows what a gps is going to show.