496 mag to 496 mag HO conversion
 

I know the only differnce is the ecm program and camshaft.

has anybody done it? worth it?
for less then 2k I could add 100hp to my twin engine boat and go from 75mph gps to 81 mph gps.

I like to hear back from someone that has done it

thanks

I have the cam and ecu if you want to do it. PM me. Both are brand new

I don't think I would mess with just a cam. I would go ahead a Raylar it up and you will be more happy. My thinking is you won't be happy with a 100 hp, it will make you want more.
http://www.raylarengine.com/packages.html

how much for the cam and ecu?

do a search raylar has a kit , more aggressive cam and ecu reprog. that will bring you up to 460hp instead of 425hp.

A gain of 50hp to each engine will not get you a 6 mph gain. More like 2-4 mph.
You should gain about 300rpms.

all that work on twins for 2 mph gain will suck,but if I gain 4mph it would be ok,well I will be doing the cam and ecu switch Saturday,then will have to wait for the 1st nice weekend to see if it was worth it or not.

Throttle body has some mod also I recall...

Just plug all your numbers into the propslip calculator and then increase the rpm by 300 and see what you get. That should be pretty close.
http://go-fast.com/Prop_Slip_Calculator.htm

Problem is that it's a ton of work for very little gain.
You'll need parts obviously.

Even the balancer has to come off. Lot of work but hey I remember all that.

was told I had to drill out the throtle plate hole to 1/4 .

Well thats what I do for a living been a GM certified tech for 25 years, hoping I could wrap up both engines in a weekend,
I see from reading the merc manual I dont have to drop the oil pan to remove and reinstall the timimg cover and all gaskets are reuseable,is that correct?

Well, I'd love to hear you could do that in a weekend! If you're not taking them out of the boat which I highly recommend, I think it will take most of the weekend for you do completely finish one of the two.

Not all gaskets are reusable. Best I remember, you'll need some extra valve cover bolt seals - those little rubber grommet type pieces.... They usually break apart on you if you don't go real slow. I go through a couple here and there.

You'll need some exhaust manifold gaskets - I bought the stainless ones from GM.

You'll need head gaskets.

Intake, valve covers, timing cover are all o-ring type and can be reused no problem.

You may need to buy the metal-backed seal from Merc that sits between your circ pump and the bottom of the cross-over. They take shape and it's not worth trying to get to it later if it leaks. Check your heat exchanger seals carefully. May want to replace them if they are not nice and soft etc..

A 25 year GM tech should not have many problems. We've done it more than once and I can tell you that it's a mess to try to do it in a "nice" boat. If the boat is a "beater" or you don't care then it will be easier.

I take it you have excellent access all around the engine itself in your rig...

If you can finish two engines in a weekend that'd be a record. Good luck and have fun!

:coolcowboy:

Also, you have torque-to-yield head bolts. You can argue with someone about whether or not you want to replace them.

Merc specifically says not to reuse them for what that's worth to you. Don't over torque on reinstall. You'll see not much meat on that deck.

Do yo have DTS on the boat??

Heads will stay on the engine for a cam change

I have about 3 feet in front of the engines and the back seat so engines are staying in the boat.

I know on the 5.3 gm engines we have to remove the heads to get the lifters out but the 8.1 on the trucks we dont have to remove the heads,so I was not planning on messing with cyl heads or exhaust manifolds,

something different in the marine 8.1 that I dont know?

and its cable shift and cable throtle no DTS

Naw I just figured you might slap a nice set of heads on there while you're that far. I only asked bout the DTS cause of the computer flash

What I wouldn't give for that.

Not if he gets the bug. I was gonna do that cam change too remember??

I found in a Service Book from MerCruiser that said, The throttle body has a 75 mm (2.95 in) bore ( the 496/8.1 also has a 0.100" bypass hole for base model engines and a 0.200" bypass hole for HO [high output] models).

thanks:drink:

so what you are saying is drill out the by pass hole when converting to the ho? if you don't what will happen ? starting problems etc.?

Just the idle. I wound up plugging mine but it's a 90 mil tb.

YOUR IDLE SHOULD BE AROUND SIX HUNDRED RPM,,,i would start small and drill until your idle reaches six hundred

Wasn't sure who you were talking to...

The engine in stock form idles at 650. Certainly no reason to go lower as you increase your chances of reversion.

The bigger the cam, the higher it really needs to be. I try to keep mine at 750 or just below 800 mark.

This is specific to the 600 though.

boatnt

I thought 496's were supposed to idle at 750-800?

Get Raylar in here, he'll know.

All older stock MerCruiser EFI engines 496/8.1 and below idle at 590- 610ish RPM's

As of somewhere around 2007ish the idle speed has been dropped down to 540-550 RPM's on small blocks V6 & V8,

Here is the list for non emissions engines.
Vazer engine is at 600 rpm
4.3 mpi is at 550 rpm
5.0 mpi is at 550 rpm
350 mag is at 550 rpm
377 mag is at 550 rpm
496 mag and HO is at 600 rpm if it has the mechanical shift
496 mag with DTS is 575 rpm (including Axius)
496 mag HO with DTS is at 625 rpm (including Axius)

And here is a list of engines with emissions.
Vazer engine is at 650 rpm
3.0 MPI with emissions is at 700 rpm
5.0 mpi is at 550 rpm
350 mag is at 550 rpm
350 mag is at 550 rpm
377 mag is at 550 rpm
496 mag and HO is at 650 rpm (ALL MODELS)

All the V6 & V8 carburetor engines should be set at 640- 650 RPM's, And the 3.0 TKS should be set at 700 RPM's.

I don't see any reason that you couldn't do them both in a weekend if you have enough room to work in front of the engines.

actually the idle is controled by the IAC,the size of the vacoum/bleed hole has to do with the pintle position of the IAC and the feed back it gives to the ECU.

example would be, if the bleed/vacoum hole was to big the ecu would command the IAC pintle all the way out to bring the idle down which that could trigger a false code,making the ECU thinking the engine has a vacoum leak or running lean.

if the bleed/vacoum hole was to small then the ecu would command the IAC pintle out so it could keep a idle at a certain rpm.

Thats what Im thinking,
I know its going to take some time but if I gain the speed others have with the same boat and engines as mine and make it a 80mph it will be worth it.
when someone asks how fast is your boat 80 sound so much faster then 75,also not that I want to sell the boat but resale value is much higher with 496 mag HOs then 496 mags,

only time will tell,thanks again for all the input

so for the guys that have done this before,what kind of harmanic puller did you guys use?I beleive merc uses a different harmanic balancer then GM so I dont know if the puller I have at the shop is the same.

how about a harmanic installer?pleese dont say use a hammer.

We used CJ2001 to remove and the YA188G installer (Snappy).

Don't for get to pull those heads and replace!! :evilb:

But I ahve a Raylar cam and big heads.... Still 650?

Well Port side is a 496 mag Ho and the Starboard side is a 496 mag.

Got a later start yesterday then I had hoped for,
started around 11:00am and finished around 5:30 pm
the Port engine is complete and put back together,filled with antifreese.

today, as soon as my back is up to it I will go and do the Starboard engine.

thanks for the tool info!
worked out good

all DONE!!!!!!!
just have to wait a couple weeks and I will give a spin.

new stickers as well

Numbers?

How did she do? I'm kinda curious