I'll be pulling my motor this fall to remove "Y" pipe to get rid of silent choice exhaust. I will also be painting motor as well as repainting bilge and doing some re-rigging. What I am wondering if while the motor is out, what kind of performance gains would I get out of adding the right combination of roller cam, roller lifters and rockers. Intake manifold and carb. Motor is a 1990 454 Magnum 365hp with Imco Thumper Power Exhaust. Estimated to be 385hp with exhaust. Motor only has 100hrs. on it since it was freshened.

Call Bob, "RMbuilder" of Marine Kinetics. He will set you up with all the good parts and it will run. Do a search of the forum and you will find a lot of good feedback from all of us that used him.

X2

You should be able to get 450hp+ out of it with a cam, intake carb combo. If you have silent choice with the short risers, then your cam options will be more limited.

Thanks for some of the info. Not really sure what I want to do. Just trying to weigh out some of my options. Doesn't seem like much of a performance gain to be worth it right now to me. I may just wait another season and either stroke the 454 and add heads or look for a 502 block and build over the next year.

OK, help me out here. I called Mercury today and gave them the serial number from my motor and was told it was 350hp at the prop. Also asked what it would be at the crank and was told it would be approx. 10 to 15 percent more. That puts it somewhere between 385 and 400hp stock. Also talked to Imco about the Thumper Power Exhaust I have on my motor and they told me that they would improve HP by about 10 percent. That would put it roughly at 420 to 440hp. I quess my question is, Am I going to see a significant performance gain to make it worth what I figure is about $1000.00 investment I guess, with cam, roller lifters, push rods, roller rockers, intake and carb. I guess I'll also need valve springs too? Is it all worth the cost for minimal improvement in performance?

The carbed 454mag is 365hp and rated at 350 at the prop. The Imco exhaust will only add about 20hp at most. You are not anywhere near 420-440hp.

Its a LOT more than $1000 to do what you are talking about. More like 3k in just parts. Not to mention you may need a bigger prop.
You would probably gain 5-7mph once its dialed in.

Roller cam $450
Roller lifters $500
Timing chain set $150
Roller Rockers $400
Valve Springs $200
Pushrods $150
Intake $300
Carb $600

Ditto what Griff just said.

I had a stock 1989 454 mag engine dyno tested with the original cast iron merc exhaust, it made 385 hp.Changed the cam,bowl cleanup on the heads,dart single plane intake,Imco Powerflow manifolds and a Nickerson holley.Made 465 hp at 5100rpm.the boat went from 62 mph to 70 mph after I went up 2 inches in pitch.

I would be interested in the Y pipe and silent choice if your selling? Thanks, Jeremy [email protected]

PS. Forget the "upgrades" and slap a Procharger on it....Magnums like that......

Thanks for all the great info everyone. Griff can you recommend some parts to go with, to set-up this application? What cam, intake, carb size. I guess for the other parts, whoever I buy the cam from will match me up. Also found out that the stock cam is .224 Duration/ .300 lift. I quess you know too that it has rectangular port heads on this motor. As for the exhaust, it doesn't have the stainless risers that dump water at the tips. I guess I have to watch for water reversion with the duration of the cam. Thanks again.