Merc HP500 Exhaust
 

I am looking to swap my gen V 454mag with 600 hours for a rebuilt merc 500hp from a local marine mechanic with a solid reputation. The engine comes without exhaust and we talked about either A.) Keeping the 454 mag exhaust setup with corsa silent choice or B.) buying a set of new or used cmi headers. Buying new exhaust will be better but it will also run a few thousand dollars that I dont have right now. Can I keep the stock 454 mag exhaust on this engine and buy headers after a season or two? I posted the engine build info below. Thanks.

Bored .030 over, from 502 c.i.d. to current 509 c.i.d
589 Dyno proven horsepower @ 5600 R.P.M
*Engine is completely rebuilt, all machine shop work and rotating assembly work performed by K-S Machine Shop.
-Cylinders bored & honed.
-New cam bearings.
-New crank bearings.
-Balance Crankshaft.
-New roller lifters/rockers.
-New ARP rod bolts.
-New JE pistons.
-7/16” Bolt forged rods.
-Crane cam/Keith Eickert camshaft

I guess not

Hp500 cam with 454 mag manifolds? You're gonna revert water. Not a good idea using those iron manifolds with a 110LSA cam.

The stock cam was replaced, although I am not sure of the specific LSA or lift. When I talked to the builder last he said it is specifcly cam'ed so there is no overlap to avoid reversion. If this is true can keeping the iron manifolds alone lead to water reversion?

with a stock type manifold like that ,it will hurt you in two ways.like mildthunder said.they will " drink water" and will not flow well.a cheaper choise would be a stainless marine manifold set up.this system will outlast headers hands down and will almost flow as well as headers.

Post the cam specs if you can.

ok thanks for advice guys

I have a set of mint low hour freshwater use only cmi headers with bravo tails email me if interested
I can make a pretty good deal on them.
[email protected]

No way an HP500 bumped to 589hp will be able run 454 mag stock exhaust and not revert water.

I thought the HP 500 came from the factory with the Gil manifolds? Is that incorrect?

x2!!!

The early ones did. Almost 100 hp over stock will have a pretty healthy cam.

All carb HP 500 had Gil to my knowledge, mine is the last version with tall risers and it's Gil/ merc risers and manifolds

Can you give more details about what was done to the engine
 

100 horsepower seems like a lot from 7 cubic inches and a cam shaft. I have to redo mine one day and am interested in what was done.

IMO never buy used headers period !!!!! I would rather see you buy less expensive new headers. Too much riding on the line and then you will blame the guy that sold you the headers when you pop your pride and joy (motor). I would suggest people buy there
headers first and then build your motor second. Once the money is gone on upgrades on the motor then there is the feeling of (I cant afford another $4500 on new headers). Worst mistake you can make IMO

Someone recently posted a set of Stainless Marine manifolds and risers for $900. The SM exhaust would be a good choice as long as wet exhaust is good with the cam. Next choice for the price would be a set of Gil's...

Correct.
Merc even redesigned the risers to make them taller because of reversion on early HP500's

i like the way builder said special cam with no overlap to avoid reversion. 260 mercs have overlap. my toyota prius has overlap...

If you cant swing the manifolds and risers that makes the hp500 complete may want to take it down a notch and do a 502 mag long block with a holley carb

And Merc came out with a Service Bulletin to increase the idle speed to help against water reversion for those apps using Merc / Gil exhaust

He doesn't have HP500 exhaust, he has Stock 454 mag. I would get a set of EMI or Stainless with long risers. 1/2 the price of CMI and you might lose 10HP. Also there are some good deals on used EMI & Stainless around.

i have heard of people altering stock merc risers by welding an extension tube to exhaust outlet that runs the length of the rubber exhaust hose. seems kinds half azz but if he has to run these exhausts with this motor...

i am going back to this post. if this statement is true, this is no 589 horsepower engine. we need the cam specs...

If it WAS dynoed, mayhaps it was done with the engine builders headers. I have seen many Big blocks on a Dyno with car headers. Or engine builder is guessing @ HP.
True, cam specs would be helpful.