What are the casting numbers off the heads. Those should be L-29 vortecs, large ovals and small chambers.

 

No, the L29 vortecs had the traditional size oval ports. These are the round port heads, similar to the 236 peanut port castings they put on them up until 1990 or so, except they have the non-adjustable rocker arms. There is a stud kit that allows these to be converted to accept adjustable rockers - I think they are good up to around 420 lbs of spring pressure.

 

It sounds like you have already done alot of homework and know what to expect from the results from similar builds. You are looking at forged crank kits right?
I was thinking about asking if there is such a thing as higher ratio rockers that you could run with the cam you mentioned too. But I am starting to wonder if that cost along with opening your heads for larger intake valves isn't going to become costlier than a different set of heads all together...?hmmm?
I am o expert but thought I would throw out those thoughts...
Good Luck, post your decision and results!

 

Thanks for the reply, RSCHAP. My 454 had the stock cast crank in it, and there were no problems with it, so I think the cast steel Scat or Eagle crank with the good rods should hold up OK. Still mulling the cast hyper versus forged piston decision. Plenty of normally aspirated 7.4's running around out with cast pistons there that are surviving. I am going to make sure that compression stays at 9.0 or less - really shooting for around 8.75:1 like the stockers run. Usual failure mode with these big blocks seems to be dropped valves and other valvetrain issues from the heavy valves. That is one of the reasons I want to go with a sane lift on the cam. That being said, my 454 did meet its demise from broken piston ringlands, but I think that was a function of too high compression combined with ringlands that were VERY thin where the valve reliefs had been flycut. Add to that the early-70's 820 casting heads, which has smaller 112 cc chambers and detonation-prone poor quench area, and you can see my recipe for disaster with the other engine.

The 781's would definitely flow better, and the ideal cam would come in around 224/232 @ 112, .540/.540, but that would probably add $500 to my build. I have to wonder if I would be better served by putting that towards a set of better exhaust, although I would prefer to limit my spending on this particular boat. I'm hoping that a 30-ish Formula will be in my future down the road.

 

Again from everything I have heard and read all the Gen VI blocks all came with the Vortec heads. Never heard of peanut ports on that engine.After reading the post over again you say you have dogbone rollers, so the block must be Gen VI.
I use the 820's with 2.19, 1.88 valves with the heads milled a little. The chambers CC'd at 110 and I have not had one problem with them.

 

I am a little confused on what you have posted. You say you have a Gen VI block with peanut port heads and that they came that way. Are you sure you do not have a Mark IV block and heads?

Nope - it's a 99 model GEN VI, came factory with the peanut port 156 casting heads, flat tappet cam, although the block has the provisions for the roller lifter dogbone arrangement.

Not all of the GEN VI engines were set up with the roller cam and the L29 Vortec heads.

 

We were both posting at the same time. LOL, I started editing my post at the same time you were quoting mine. Never heard of that setup. Even the pickup trucks were set up as Vortecs. Maybe this was a crate replacement engine for somthing.
The 156 castings were for Gen V engines.

 

I'm still trying to figure out what it came out of. It didn't look like it had many hours on it - clean inside like a new one, and I think I could even see assembly lube on the cam lobes. Whatever it was in, it looks like they were using an external oil cooler. I suspect someone let it overheat, because the pistons were way too tight on the wrist pins like they had galled up. There was some scuffing on the cylinder walls too. I bought the thing from an engine remanufacturer that had closed up shop. He had dozens of big block cores sitting around. He had a bunch of factory GM 5.3 LS longblocks that had never been run too. Still has them if you know someone doing a pro touring project.

 

Jeff, are you running the 820's on a 454? Are you running a dished piston to keep the compression down, or flat tops? Part of my problem was the fact that I was running domed pistons on mine. I had the domes cut down to try to get the compression down around 9:1, but the engine builder thought (mistakenly) that the 820's had the larger 120 cc chambers. My actual compression ratio was probably up around 9.6 - 9.8:1. Way too much for a marine engine.

It might still be running to this day if I had spent an extra $400 and bought some flat top pistons. :banghead:

I am just going by what I have heard/read concerning the lack of quench area on the 820 heads. Chances are you will be fine if you keep the compression to a sane level.

 

My guess is that engine had been pieced together with that combo. I am not a engine builder so I cannot say for sure.
I have a 489 stroker with flat tops (forged), when you figure in all the variables I am running 8.9 to 1 comp. I use 89 oct fuel and run 36 deg of timing.

 

Which engine are you talking about - the Gen VI or my original 454? I'm pretty sure the Gen VI was never apart from GM - it still had all of the inspection paint dabs on the con rods and main bearing caps. He also had dozens of other Gen VI motors that were set up the same way.

Sounds like you are running a similar setup to what I am planning to run. What cam are you running with your setup? Are you happy with the performance?

 

Just another thing I thought about. Be sure you remove the center plug out of the oil filter housing and install the 30psi bypass in the one to the side. It is often overlooked and will cause the oil not to be filtered or cooled correctlly.

 

Wot?

I thought the old 454 and 502 blocks were to use the 30 lb. bypass or
plug it off?



The new 496's use a 17 lb. bypass?

Good call, I worded that wrong, need to replace with 30psi. I think 17psi is what is installed if set up for auto.

 

The first 454 I swapped in my Chaparral was a gen 6 with , roller cam and peanut ports.
Merc rated it at 310hp, I removed the efi and put a carb on it to make the swap easier.
Very reliable and torquey setup but not much HP past 4500rpm..