1) There is nothing "wrong" with a MkIV 2-bolt main block. But with a 2-bolt block, you are limited to 500hp and 5,300 rpm for continuous duty, even with good cranks and rods. A stroker (496ci) crank will lower your continuous rpm to around 5,000. BUT replacing the main cap bolts with an alloy stud set will bump those numbers probably another 300 rpm. Machining for 4-bolt caps is another option. But you need to know that any time you add cap bolts, you SHOULD have the block align honed. Many guys ignore that when swapping from bolts to studs, but adding bolts involves machining on the webs and a different stress pattern when torqued properly.

A Gen 5 block is a tiny bit less desirable than a MkIV block due to some of the weight-saving changes in the bulkhead and web thicknesses - but at your power goals, a Gen5 is fine. One thing that is changed in a Gen5 is a little MORE thickness in the internal water jacket by the lifter gallery. They are a LITTLE less prone to crack inside when allowed to freeze (not necessarily a big plus since you should still winterize properly).

A 454/365 did come with aluminum intake, steel crank, steel rods with 7/16 bolts, forged pistons, and rectangle port heads - a much better "starting point".

But you don't know WHAT you currently have, and you WON'T know what block and heads until you find and run the numbers. You won't know what crank, rods, pistons until you open it up and look. (If you can stick a camera into the cylinder and find a part number on the piston crown, THAT can also help.

Sure, a 502 is a wonderful choice but is costly.

Keep us informed as to WHAT you find when you know more.

Mcollinstn---I was kinda hoping and wondering when you were going to chime in with your experience and knowledge---and I agree with the same things you said in your previous post.

I also agree with what Mcollinstn said about the block work, etc. If you have a 2-bolt main block(s) then you still may be able to use them. I don't know how deep you want to get on this subject of what kind of power you are looking to make, the application and your own personal goals, but anyway---Let the games begin!---

That being said, keep in mind for the most part, we only had Factory GM parts to work with back then 40+ years ago (especially the stock/factory GM cast iron heads, etc)....today there are a ton of aftermarket go-fast parts options available. So, back in 1984/85 when we were modifying our Merc 454cid/330hp engines that came stock back then with 2-bolt main blocks--- I remember how we were really concerned with those 2-bolt mains and thought we absolutely needed blocks with 4-bolt mains, BIG GM rectangle port heads and all forged parts----but were able to find and talk to a shop in the Detroit area back then who did lots of marine upgrade mods for offshore boat customers with the various Merc 454cid engine packages...especially the Merc 454"/330hp engine----And those Merc 454/330's had all CAST rotating assemblies back then, cranks, rods, pistons. We took this particular shop's advice and we kept the 2-bolt main blocks, but we removed the bolts and replaced them with ARP studs (about twice the hold power over bolts) and had the main saddles/caps align-honed to make sure the crank was sitting and spinning well.

If you want to make sure you have the beginnings of a good engine, then it all begins with good main saddle alignment of the block with the crank--- all other measurements of the block depend on the main saddle alignment such as squaring the decks perpendicular to the crankshaft center-line from side to side/end to end, etc----all start from/off the measurement of the main saddles of the block.

Some guys may argue that if it is within a certain accepted tolerance, then an align hone is not needed, but if you are removing the main saddle bolts and installing studs, then the block is most likely going to need an align-hone of the mains----especially for a marine hi-perf boating application.

Bolts or studs on the mains, I would have the machine shop do a"run through" align-hone on the mains just in case. For the most part, if the main saddles are off, then it throws all the other measurements off as well and the will not be as accurate or "square"

When it comes to a 2-bolt main block and what you're doing, I would absolutely have an ARP stud kit installed on the mainsand tell the shop you absolutely want it align-honed even if the shop tells you that you don't really need it.

Years ago, there was once a local shop in my area that told us that an align-hone wasn't really necessary and that we didn't need it, but we were intent on having our blocks align-honed so we took it to another shop another 30 minutes away and we found out that the blocks we brought to them told us we were indeed in need of an align-hone procedure.

We had found out that the other local shop did not even have align-hone machine equipment and it had a reputation for cutting corners on their machine work. Just make sure the shop you take it to is hell bent on accurate machine work with good equipment.

If your budget is 10k, sell your engine and buy a short block 454 and build it up. I bought a long block 454 (496 stroker), swapped some parts, built it up, and should have over 600hp and around 600ft/lbs. did it for about 8k. Still sitting in my garage as I’ve been heavily delayed dealing with health issues of my wife. Get used exhaust manifolds, Gil or Stainless Marine as new ones are super spendy and can easily cost you 3-5k.
just my opinion though.

Just to point out, a faulty tach can break up ignition in the higher RPM ranges. Did you ever sort that out before tearing into the engine? Hate to see you do all that work and have a bug carryover with her. Simply disconnected from the coil to rule it out.

BLOCKS: There are some other smaller things they may need to do to your block(s), but these are the major things that you will need to do....Install main studs (avoid using bolts on a 2-bolt main block)
Align-hone main saddles
Figure deck height and square decks perpendicular to crankshaft centerline (using a deck jig/fixture)
Bore cylinders .030" (you may need to do this depending on the condition of your cylinder bores)
Bore notch cylinders Intake/exhaust (this is an option worth about 20-30hp)
Hone cylinders with torque plate
Rotating Assembly: ---Complete balance

RODS---some other mods we did to uprade our Merc 454"/330hp engines was that we used the same stock/factory cast rods with the 3/8" rod bolts and had them checked for straightness, magnafluxed, did a little de-burring, then had them shot peened/stress relieved and reconditioned them with the much better ARP rod bolts----with all that labor you might be into it for $500-$600 so by that time you would probably be better off buying a new set of much better/improved aftermarket rods like Eagle H-beam rods. Back in the 80's we didn't have the advantage of aftermarket parts options with less expensive rods like we have these days and we had no other recourse other than improving/modifying the stock/factory stuff we already had.

PISTONS---You will be better off if you do NOT use CAST pistons---we did not use the cast pistons like the ones that came stock in those Merc 330's ...we bored the blocks .030"/over and you may have to as well, but we went with some Speed Pro or TRW forged 8.8:1 compression pistons---although some of my friends have used hypereutectic pistons and made 505hp @5500rpm or so on the dyno doing these types of Merc 330hp upgrade mods from back in 2007 or so and still running today.

Heads----Lots of options these days, but back in the early 1980's not so much and hardly at all. Depending on the year of your Merc 330hp engine(s) you may have the older/earlier GM factory cast iron "049" oval port casting heads...or you may have the smaller GM oval port castings "peanut port" heads (about 201cc size Intake runners)---both heads came with 2.06"/Intake and 1.72" exhaust valves, but the "049" (about 234cc size Intake runners) castings make more power and have the potential to make more power because of their larger intake runner ports. We had the "049" heads and thought they were too small for application. We always thought we needed the larger GM rectangle port heads, but with all of our research on BBC hi-perf boat engines back we were thrilled to discover that we had the right size heads for our combination and did NOT need the bigger rectangle heads like we thought we would. We just had them modified with the larger 2.19" Intake valves and 1.88" Exhaust valves and then bowl/pocket ported--

----But these days it just might be easier to buy some aftermarket oval port heads with most of these attributes from the factory less the bowl/pocket porting. I think World Products/Merlin makes these in cast iron form with large (about 265cc) oval ports and the larger valve sizes vs having to rebuild the heads and do all the machine work especially if you have the smaller "peanut port" heads.

It all depends on your budget and how much power you want to make--- money related. The "peanut port" heads might only make 450-465hp @5000rpm with bowl/pocket porting and larger valves---and something like a Comp Cams 268 hydraulic roller cam, Edelbrock RPM dual-plane Air-Gap intake manifold, and something like a Holley 750-800cfm mechanical double pumper carb. We didn't have enough money to buy the Holley carbs at that point, so we just still used the old Quadrajet spread-bore carbs

The larger "049" castings might make you about 475-500hp or so.

I once had a pair of 468cid BBC's in one of my boats with aftermarket Merlin oval port heads with the larger valves and some bowl porting, Dart single plane intake manifolds and if I remember correctly the cylinders were "bore notched", sollid rollermechanical camsand it made about 620hp @5600rpm which was not too bad for 30+ years ago back in 1992.

Intake Manifold---- Edelbrock RPM Air-Gap dual-plane intake manifold

Exhaust System...a decent free breathing DRY exhaust system is the first thing you're going to need---if you want to make power with all those upgraded induction parts, then you need to let it breath well out the exhaust.

Find a good reputable machine shop in your area or someone you know you can trust... I am sure there are others on this website that can do it ("articfriends/Steve Smitty Smith" would be an excellent choice)...you will need accurate machine work to take all the slop out of a factory mass produced engine and try to bring to the best clearances and tolerances as possible--- basically a well machined "blue-printed and balanced" engine so that it lives in a marine environment/application.

Anyway, that's my .02---I am not an expert--- I am an engine enthusiast and offshore boater since the 70's. Like I may have said before, these are only my experiences since about 40 years ago upgrading these BBC 454cid engines---but not saying I am right.

I hope it helps anyone of you guys looking to get more power and do a budget upgrade/mods on some of the early Merc 454's out there...especially the ol' low dog 330hp version----

If any of you feels like I have overlooked anything or am in error anywhere and you feel I need correction on anything I said, or want to add anything further, then please say something---just go easy on me as I am old man now. Sorry if I sounded repetitious----In the meantime, I think I need a break with all this typing in the past few days my fingers feel like they are getting tied in a knot. God bless you all.

It’s not that I’m looking for a different sized hull. I like the 24x7 hulls and that size is perfect for the archipelago where I’m boating. Could go a little bit bigger. Maybe 28-30 feet max but that’s it. Already getting a bit big then.

The change I’m thinking of doing is to a more well-proven hull. Maybe scarab or something along those lines. Since mine is a ”no name” hull that hasn’t been proven to run those big speeds.

And yes! I’ve thought about the tach theory! I was sure that was it becuase I have a broken tach. But sure enough, the problem was still there when I unhooked it.

Today I went and tried to get the block numbers from the boat. Was very hard to make out what they say. Maybe you guys can read them better.

These ones are from the back of the engine:

https://cimg9.ibsrv.net/gimg/www.off...c6e86198a.jpeg
https://cimg0.ibsrv.net/gimg/www.off...1bc3460af.jpeg
https://cimg1.ibsrv.net/gimg/www.off...04e36e66b.jpeg

and this is from the front

https://cimg3.ibsrv.net/gimg/www.off...cdc657b0d.jpeg
https://cimg4.ibsrv.net/gimg/www.off...e11d966d1.jpeg

Thanks a lot for the great tips and stories kaama!

Pull one valvecover and check the head casting numbers. Might be a 360 casting small oval heads.

I'll do that once the engine is out of the boat soon! I guess then I don't need the numbers to tell though..

You may end up having one of Merc's old 365hp Mag engines----if it is, then you will have a good base engine to work with. The old 454"/365hp Mag engine has/had all forged steel rotating assemblies and the bigger Rectangle Port heads. However, you will still need to have everything machined/blue printed the way I described for the Merc 330hp version, but modifying the Merc 365hp Mag would be much cheaper to upgrade than the Merc 330,

A couple of things I forgot to mention in my previous post is that with the Merc 330's and their oval port heads (whether they are the bigger "049" castings (234cc size) or the smaller "peanut port" (201cc size),---depending on the cam duration and lift...you MAY need to have your valve spring pockets machined for larger diameter valve springs (MAYBE)....AND ALSO the valve guides machined/cut down depending on the LIFT of the cam. Back in 1983/84, when we used the Merc/440hp cam (hydraulic flat tappet) it was a 231*/239* @.050" Lift with something like .550"/.558" Lift. we had to do these machining steps on our "049" castings.

...and by installing larger valves it required us to install larger seats---I cannot remember if we did ALL the seats on both Intakes and exhausts, but it was A LOT of work and expense to have all of that done-- including installing bronze wall valve guide liners, and resurfacing the heads. You may not need cams as big as what we used and having to have all that extra head work done, you'd be much better off just buying a new or used set of Merlin BBC oval ports that already come from the factory with the bigger valves, hardened valve seats, larger valve spring pockets, longer valve guides, etc and just maybe do a little bowl porting/short-side radius work done to them and blend it in the valve job.

As I have said before, we did NOT have much of a choice back then with GM heads mainly being the only type available to work with and the rectangle ports were very expensive back then as well so we couldn't afford those at that time. But with all the work and money expense we poured into those cast iron oval port "049" castings, we had some very badassery heads and they made a HUGE difference for those old 454"s we had back then.

Aha! What exhaust did you run back then? Was exhaust options also limited?

If you go back and read my previous posts, then you will see what I have stated about exhausts systems---even from back then in the early 80's. These days you have even more options for aftermarket exhaust systems. You absolutely DO NOT want to reuse the stock Merc heavy cast iron exhaust manifolds...

---in a nutshell, they're too crude and choke the exhaust flow and will kill any potential power gains that you are trying to achieve.

But if you really want to make good power for these BBC marine engines, then you need a decent open breathing dry aftermarket water jacketed exhaust system--- one good example would be the Stainless Marine makes a decent manifold system for the BBC ...

I’m a bit afraid that the completely dry systems will be TOO loud. If that’s even possible. My cousin had a ski boat with a dry system and the people in the marina where he kept his boat made a petition for him to not be able to run it after 10pm. It’s the same one where I keep my boat as well.

Feels bad to buy a system with mufflers though…

Alright. I’m back with the verdict;


https://cimg4.ibsrv.net/gimg/www.off...266d18958.jpeg


https://cimg0.ibsrv.net/gimg/www.off...b6d3f5113.jpeg
https://cimg8.ibsrv.net/gimg/www.off...7990d1dbc.jpeg

A 2-bolt block. Not sure what these heads are though. With this knowledge what I’m looking for changed a bit. I think only a modest build with stock exhaust is enough. If there’s any idea of doing that. Since finding someone that’s capable of a line hone is a bit harder where I live. Also feels like it’s not worth it to invest as much in a 2 bolt block

I'm back and after giving it some thought I decided to go the budget route. It'll be more of a refresher with some higher compression and a new cam than a full blown performance build. The main reason for this is because the uncertainty of the performance of my hull and because my block was a 2-bolt.

This is my order from summit:

https://cimg6.ibsrv.net/gimg/www.off...f2f234d149.png
https://cimg7.ibsrv.net/gimg/www.off...e9b02c0d20.png

The block is being bored as of this post and I started disassembling and sand blasting my heads. 1 out of 16 valve seals wasn't cracked. :D I was also planning on porting the heads to fit the oval port gasket but it seems I don't have enough material to remove that much. Anyone with any experience porting the peanut heads? I'll try to update as I go

No need to enlarge the port entry of the heads, dont waste your time! the restriction is deeper in the port, atleast it was in my 360 casting peanut heads.

Thanks for the reply! Not even worth it to try to port match the intake?

Anyone know what bore headgasket i should get? Seems I bought the wrong one and the bore of the gasket is always bigger than the bore of the cylinders it seems

I use 4.370" for my 454's
17046 or 1037's

If you have aftermarket heads, be aware you may have to use a bigger bore headgasket than you’d think.

I have a pair of 1996 454s that I had sitting around after I built fresh power. We bore notched, bigger cams, swapped to AFR aluminum heads that also added a lil compression, aluminum intakes. Seen this done this to several 454s and it's tough to beat the bang for the buck. You will need AFR for a day to add the required fuel but it's not that big of a deal, and not hard to hit the 500HP with nice torque as well. Much more than that you'd be pulling it all apart to change crank so then the sky is the limit... just get the cash ready. And obviously not running stock exhaust.. Stainless marine or many other options are available usually find stuff used for the 5-600HP quite reasonable.

It appears that you have the old Merc 454cid/330hp engines with the 2-bolt main block and "Peanut Port" Oval port design.
And yes, you can do a port match when mating the head Intake runners to the Intake Manifold runners. The gaskets you're going to use may have some effect on this and don't let any silicone sealant squish it's way into the ports as the intake manifold is being installed on the heads as this can disrupt/disturb critical airflow effect power... not much, but it still will.
As long as you got the heads off I would also do some slight bowl porting work and short side radius work and blend it in nicely with the valve job

I had prematurely sent my previous post by accident before I was finished and this post is to finish what I was saying...

Add an accurate 3-angle or 5-angle valve job. Any porting should only be minor in the bowl area and around the short side radiuses under the valve---have it professionally done by someone who knows what they are doing----(not sure maybe $300/pair for what you need)----- and needs a good accurate valve job and also "Lap-In" the valves for a good tight seal and the head porter should blend it all in with the valve job.

Something else you could do as an option is to have the cylinder bores all "bore notched" to unshroud the intake and exhaust valves for better breathing around the valves. This is usually good for about 20 to 25 horsepower or so. However you need to do the math with your pistons because it will lower your compression. If you already have your Pistons with the compression ratio that you want then it may be too late to do the bore notching procedure---- But if the compression ratio and math works out in your favor and you are able to proceed with the bore notching procedure, then make absolutely sure you have somebody do it that absolutely KNOWS what the hay they're doing‼️

I would suggest you replace the valve guides with a bronze wall valve guide liner or bronze guides and make sure the guides are properly honed for the right clearances and tolerances for an engine destined for marine use.
I hope this helps you out--- have fun!

Thanks for the reply! Sure helpful

Rectangular port mag motor 4 bolt main Inconel valves etc! 330 maybe peanut round heads not as easy to soup up.

You have a good point, but he's on a pretty tight budget so I think he is wanting to stick with the simple stuff upgrades/mods. There's a couple of simple things he can do that I have mentioned to be able to give his Merc 330hp engine a little more kick to possibly a 350-400hp engine.

Anyone have a service manual for mark iv 454s? Looking for torque specs