454 in 2550sx
11-07-2008 | 09:53 AM
#12
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Joined: Nov 2005
Posts: 4,554
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From: Fredericksburg, Va
would not change the intake....loose low to mid that you need, I would go to the holley 750-800 even though the q-jet rated the same, the holley will run faster, also watch the flame arrester as some hurt performance as well..Rob
11-10-2008 | 07:41 PM
#13
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Posts: 113
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From: Edmond, OK
I agree the single plane intake isn't going to help the mild, small cube motor. just my opinion but with 7.5:1 compression you aren't going to be happy. Are you sure about the compression ratio. I would definitely cc the chambers at tdc to find out exactly before you proceed.
I remember having a sim. setup with a 454 mag, flattop pistons and stock heads. I was closer to around 9:1. With that setup, an aluminum intake, 850 double pumper, revolution marine exhaust and msd i was making 450 hp easily. It pushed a pretty heavy 25ft v to 63mph.
it sounds like something isn't right. You have to assume that the person doing the rebuild before you bought it didn't have everything as it should until you do your own investigation.
I remember having a sim. setup with a 454 mag, flattop pistons and stock heads. I was closer to around 9:1. With that setup, an aluminum intake, 850 double pumper, revolution marine exhaust and msd i was making 450 hp easily. It pushed a pretty heavy 25ft v to 63mph.
it sounds like something isn't right. You have to assume that the person doing the rebuild before you bought it didn't have everything as it should until you do your own investigation.
11-24-2008 | 07:15 PM
#14
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From: Austin, TX
[QUOTE=KAAMA;2684762]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.
I swapped out exhaust for mercury 496 alum manifolds. picked up 2 mph on the top end, better acceleration overall. Next I'm going to try Holley 800.
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.
I swapped out exhaust for mercury 496 alum manifolds. picked up 2 mph on the top end, better acceleration overall. Next I'm going to try Holley 800.
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