I am not trying to stir the pot, just trying to learn.
This is the information I was talking about when refering to Raylar.
I thought I might chime in here to kind of even out the questions of horsepower and torque gains from exhaust upgrades on 496's. First, keep in mind that for any motor to gain horsepower or torque it must basically get more efficient. Obviously any things changed on a motor that increase the volumetric efficiency can increase power. There are basically three types of marine exhaust manifolds,
1. The tradtional old mercruiser log manofolds that came on the 350 thru 502 carburated and MPI motors from 60's thru mid 90's. these manifolds because of their log (all exhaust ports dump close to the head into a long square chamber and then exit a back end up thru a small vertical riser entrance) are very innefficient, heavy (iron) and were poor at letting the motor develop hp and torque.
2. The aftermarket and new Mercury (Gil) short runner, center riser manifolds. These are aluminum units like Eddie,, Revolution Imco, Gil, CMI E-tops and 496 Merc., etc. they all have similar short runners 8" - 10" that merge into center riser openings and definitly flow better than the old log type most boaters have been use to. They will definitly make more power and torque than the old log manifolds.
3. The tubular stainless and special cast units with long 18" to 24" semi-equal length with good merge collectors, like CMI, IMCO Super Powerflo, Stellings, Eichert Bassett, Dana, etc. will generally flow exhaust more efficiently than either of the two other types.
The real question here is the 496 engine. The stock 496 Merc (GIL) aluminum manifolds (type 2) are really pretty good units when compared to other (type 2) manifolds. Their individual runners and merge into the stainless steel round big radius risers with merge collectors can do a very good job of helping the 496 develop its great torque and good power. ESPECIALLY WHEN THEY ARE GIVEN EVER SO SLIGHT MODIFICATONS! If you were to take the turbulator plates alone out of the 496HO and dyno the motor you will see about a 15HP increase alone! So if you compare certain aftermarket manifolds (not type 3 tube headers) you will in most cases see no significant power increases from stock 496 manifolds.
If you are looking for more Bling, polish or paint the Mercury 496 manifolds, polish the stainless risers and you have some good look'in aluminum manifolds that work well and you will save yourself about $1500 minimum.
If your 496 is stock and you choose to step up to a good (type 3) header system you will probably see about 25-40HP net increase over the stock exhaust manifolds and risers with the "turbulators" removed, depending on your boats riser to transom configuration and muffler use if applicable.
Trust me when I say that Raylar has probably made more dyno pulls on stock and modified 496's with many combinations of exhaust than probably anybody in the industry short of GM and I have the dyno sheets to prove it! We have made over 525HP on a normally aspirated 496 with stock Mercury 496 exhaust manifolds (turbulators removed and manifold riser openings lightly ported.) This is about the limit, normally aspirated, for these stock manifolds.
If your plans are to go above 525HP with a 496 either normally aspirated or supercharged 496, then invest in a good set of (type 3) real headers or manifolds, you won't get there without them.
I am not trying to endorse or critisize any aftermarket exhaust manufacturers here, I am just trying to get the real world results of their use on stock 496's where I have my expertise.
Remember engines increase power from increased breathing and efficent use of fuel. The 496's need to breathe better on the intake side to make more power above 450HP and this will only come from improvements in heads, cams and intake manifolds, supercharger or turbo charger addtions . You are not going to get a 475HP from a stock 496 from just installing any set of headers and especially not from a set of $2000 manifolds. If you do, let me know how you did it.
Just some real world facts!
Ray @ Raylar