Exhaust facts to make build decision
 

I have searched for information about exhaust without finding the facts I need to make decisions for my 454 mag build. Here is my dilema and questions that I would greatly appreciate any help or opinions with. Thanks in advance ! So I have a 454 mag at my engine builder that has very low time on that was sunk and started, no real damage, but was turned in as a core and I have spoken for it to build in place of my spun bearing 330hp 454. He will be rebuilding this engine. I am trying to use all resources to fund this build. It has the center rise manifolds. What are the stock manifolds capable of flowing for hp ? I think I found it would be at about a 425hp limit ? Are some better, 90 deg. risers worth buying for under $400 ? I dont't ever see spending the money on the high end exhaust. Are the 496ho manifolds worth the purchase for my build ? I have decided that a flat tappet cam is ok to use since I wont need the capabilities of roller to push the lift limits for an under 500hp build. I may be able to use the stock cam but don't know that at this time. I also like the idea of just purchasing another flat tappet cam with newer technology. Also, would it be wort asking Bob Madura to help with a cam grind ? My exhaust decision will decide on my cam !

A set of take-off 496 manifolds and risers would be a nice upgrade from the stock cast iron manifolds/risers. Early 496 manifolds are aluminum. They made the switch to cast iron around 2004-2005. The manifolds on my 496 built 9/03 are aluminum. All the risers are stainless. Also, there is no difference between 496 MAG and 496 MAG HO exhaust. Sets usually go in the $400 - $500 range.

The aftermarket riser that fits stock center dumps is pretty. That is its only improvement over the stock iron riser. No real extra dry length, no improvement on breathing. AND it is about 800 a set. The stock manifolds are maybe good to 400 hp or so. Get the best exhaust you can. It is difficult to get exhaust that is too big for your needs...

I don't believe that the 496 manifolds will work on the pre Gen 7 engines. Different port and stud size and spacing.

Dan

I have 496 manifolds on a Mark IV w/ 781 heads. No issues, had to change some brackets, different bolts & such, but fit the heads just fine. Ports match great. Nice upgrade over stock for not much money.

Good to know. Thanks for the correction and information.

Forgot to add the risers do not line up with old style Merc manifold riser locations, so changes to the exhaust might be necessary. I had enough room the 1" or so of extra height could be taken up by the exhaust hose. But this is a potential issue with any exhaust change.

Not sure if the 496 manifolds make much of an improvement on performance, but the fact that they went to a dry riser / manifold joint should make them less prone to internal leakage and more reliable. Thousands of engines have been ruined when the stock Merc wet joint manifolds start leaking water back into the engine internally. Hot exhaust valves definitely do not like being quenched with cold lake water!

Wasnt that gil manifold also used on 454-420 hp engines? Anybody know how much power they support?

Thanks to all that replied ! I think I have been on the right page after all I have tried to learn and decide on for my build. So,what do the 496 manifolds and risers support for hp ? I have also seen sets on ebay of different popular brand for under a $1000. anything in particular to look for ? Any flat tappet cam recommendations ?

According to Raylar who has more 496 experience than most, something over 525hp http://www.offshoreonly.com/forums/g...ml#post1580344

They sure work well for my 450 hp build, and you can't beat the $ value - I paid $350 shipped for 20 hours takeoffs from the swap shop.

Big improvement on performance (on modified engines) - individual runners cast into the manifold vs a log type manifold, and smooth sweep risers vs the crappy sharp bends on the old style cast risers.

The 496 risers have a smoother bend and larger inside then cast.
Risers are SS so you can have them cut and shortened or lengthened if needed.
Dry joints.
Aluminum ...lighter by 36lbs a manifold.
Risers.... stainless, don't rot and lighter then cast by about 10 lbs each
Exhaust runners/dividers are built into manifold (cast are just open inside)
They are smaller over all and not as bulky as the cast.
Fits most BBC. I have them on 3 Gen V 454's and a Mark IV 454/496 stroker now.
Easy to drain with a plastic wingnut on the fill elbow
And I could go on and on over the benefit over stock exhaust but I hate doing it. I started buying them as new take-offs years ago. People would buy a brand new boat and take them off and buy new then sell them for nothing. The deal is, is this. They are a great midgrade exhaust system. Not a super set from a famous builder but if Merc could have picked up any HP by casting it better they would have. IMHO a lot of us have exhaust systems we should never have. 5 grand in exhaust on a $5 grand motor. Crazy.

Merc looks to be using the 496 setup on the newer HP 520. Check out the dyno sheets Full Throttle Marine did on the 496 exhaust vs Dana Marine. The Dana setup was only worth 16 hp over the 496 w turbulators removed at peak hp. Thru the curve they were dead even until the mid 4k range. If you can find the alum editions there is a nice 70lb weight savings.

They are good to about 600hp before they start choking down power.

So when do the 496 manifolds start to choke ?

Sorry ! I now see where this was discussed !

It looks like the Isky 270/280 mega cam will be a good choice for my build. (454 mag stock) This is a .542 int./.565 ex., dur. @ .05 - 221int./232ex. 114 lsa. I think this should allow me to run the stock exhaust now and upgrade later to the 496 manifolds. This is probably as big a cam as I would want for my boat from what I understand. Another option is this comp marine extreme cam, .544int/.547ex., dur,@.050 is 226int./236ex., 112 lsa. Another option, Howards cams .527int./.533ex., [email protected] is 225/235, 114 lsa. What do you think ?

I run the ISKY MEGA 270/280-114. Ran fine with stock manifolds. The 496 manifolds allowed it to rev a couple hundred more after tuning. It's a great cam for a mild build.

If I was to do it over again, I'd put the 496 manifolds on and run the ISKY 280/290 mega or a similar Crane.

Make sure to run the ISKY lifters and springs to go with their cam. ISKY's stuff is high quality, never had one go flat - proper break in and it will run a long time.

How about some "made in china" headers? Anyone on here ever use these?

http://i1283.photobucket.com/albums/...psse3vw0ah.jpg