horsepower

I have installed twin 425hp 454`s into my scarab 31 excell replacing 365 bravo`s and didnt gain any speed at all 62mph max at 4500 rpm. I am now installing stainless tube manifolds from custom marine. Will that help to increase my speed or will i have to resort to drastic measures like supercharging? Any Ideas anyone?
Fred

[blue thunder]

Have you weighed the boat to see what you have? Sounds like you are low on power for some reason if the weight is not too high. I have a 30' scarab panther with modified 330hps. I guess to have about 420hp engines now running through bravo 1 drives (1.5). I run stock manifolds with imco ss risers. With 25p mirage props I get 68.1 (5k rpm). With 27p mirages plus' I get 66 or so (gps at 4.5k rpm). You should be even steven with me. With the exhaust change, (assuming runnered exhaust manifolds) you should be seeing low seventies. Check the simple stuff like carb not opening all the way, timing, plug readings, etc. Let me know how you make out. Sumtin aint right!

BT

[SledgeHammer]

Not sure what prop you are running, but if you are only able to pull 4,500 with that motor, it would seem to me like you are over propped. Try a little less pitch and you may find some more speed.

Hello B T ,I checked all the usual stuff timing 36 deg carbs ok, i am running 25p mirage & bravo ones .The carbs are original carter 4bbl and the plug color ok. My last guess had to be the exhaust as it looks very restricted compared to the runnered manifolds that i am going to put on. Maybe a bigger carb is needed too. The motors are brand new gen 6 chevy roller cam 454
Is the panther a similar hull?
Fred

[blue thunder]

The panther is a flat deck performance type hull. Loves the heavy seas. I believe the deadrise of the hull would slow it down over the excel hull (in flat water), but am just guessing. I am running rochester quadrajets with the fattest a/f mix I can get. I think your problem may be carb. I would look into some 800cfm dp holleys before you do the exhaust, or at least not at the same time. Do those two things and your boat should fly. What are you installing for exhaust, headers or manifolds?

BT

[scarabchick34]

sry i cant help...but scarabs rock!

Sorry B T, i got quadrajets not carter. The exhaust is Custom Marine Inc tubular headers with straight thru tails and no muffler.
Fred

[KAAMA]

Those 425hp engines you have are going to want to BREATH and installing those CMI's should wake those things up and increase your RPM's by roughly 300-500rpm---but that is just a guess on my part.----But it should be interesting because if you are now only running the stock Merc cast iron exhaust system it is pretty restrictive and very crude! And the more horsepower you have the more restrictive it can be on an engine. I'm not sure how much weight you'll save with the CMI's, but they are going to dress up your engines much nicer and be more eye appealing when you lift the hatches for your friends to see---plus they are going to have a MUCH better sound and rumble to them.

If you are going with a Holley carb a 800cfm would be okay, but a 750cfm is all you need on a 454 and will give very crisp throttle response and that's what you want in a marine engine.

Keep us posted as to what the out come is with the performance of your boat with these mods you are doing. My guess is that it will be a real eye opener once you bolt on those CMI exhausts!---- Have fun!

[blue thunder]

The headers will certainly help, but I am convinced you should be doing better now than you are. Sumtin else just ain't right. With headers you will only be catching up to me, and I think you should be even steven with me now. I would do the 800cfms over the 750s because I find they are more marketable later if you chose to sell them.

BT

[Ratchet]

With 425hp, you should be going faster.......

I have a 31 scarab with twin 425-450 Hp (355 small blocks) and I run 70-72

Another with ~ 500-525 509's (77-80)
Another with ~ ???-??? 540's (77-80) still in progress
Another with ~ 385 502's (70-71)
Another with ~ 700 555's (over 90)

I say you need to do some tweaking, or the engines aren't making that much power ?

Just a thought

RJ