Large inch NA build
 

First of all....let's not turn this into a p!ssing match that gets locked or deleted...

Thought it would be fun to see what guys would put together for combos and share ideas/thoughts to build a larger inch NA motor.

Let's start with say a 502 mag MPI box stock no mods with 700 hours. What would you do if you were shooting for say turn key 700? Is it feasible out of a gm 502 block?

How big would you go to keep as much reliability as possible? 540, 548, 555, bigger/smaller?

Obviously brand and preference are highly subjective but let's keep this simple and as fun as possible....what do you think for hard parts? Ported aluminum heads? Ported stock heads? Ported/cleaned cast iron heads?

Same for rotating assembly...What's all needed? Crank, rods, pistons, cam, lifters?

And a big question...can it be done at all? Let's throw budget out the window as that's subjective too but to do it right and reliable can you squeeze 700 out of a NA gm 502 block?

Let's have some fun with it...what's everyone's thoughts?

So long as the block passes porosity tests, .030 502 with a 4.25 crank - 540 cubic inch. Factory crank, good rods, AFR 315 or 335 heads, cam to match the flow capability, good single plane with a 1050 Dominator. Should get you there.

I built a pair of 548's that made 700hp. Stock block, Big Al heads and a custom Lunati cam. Topped them with Holley EFI. I really like 555ci or 565ci though in a std deck block.

I have read a few older threads and articles saying boring out to 555 or 565 on a gm 502 block is getting dangerously thin on the cylinder walls? Mentioned something about having to sonic test the block or something? Has this practice changed and not relevant anymore?

Good info so far guys! Hope this turns into a very informative thread that can stand the test of time!

Here you go, you won't have to rev it over 5000 RPM, ZZ632 Crate Engine | Chevy Performance Parts (chevrolet.com)

I'm going a route most wouldn't consider. Looking for max torque all in by 5-5200rpm. Just haven't decided if going 540 or 565. 10:1cr, roller cam, probably solid since I have lifters already. Intake from a Merc HP500. Holley 950 or 1000cfm 4150 with annular boosters. Floteck Oval port heads, 290cc intake ports that peak flow at over 340cfm at .600 lift. Should have a high flat torque curve from 2500-5000rpm or a little longer.

we went the other way with a 540 10-1 solid roller ported heads we wind it up pretty tight up to 7000 running an xr standard length 1.5 with a 26p bravo one prop it sits in a 25 active thunder its been the most reliable and most fun boat i have ever run ......building a lot of torque in the lower rpms is very hard on bravo gears letting it wind up reduces the strain but you need really good guts in it to do it reliably

I had 900 hours on mine. It’s a carbed 540 now. 712 horsepower at 5600 rpm same torque at 4700. Dart cnc 335 heads , dart intake , 850 qft carb. Turning all accys but on dyno headers. Running stainless Gen 3 exhaust. Closed cooling.
you may have to run a larger diameter fuel line and pickup. I went to 1/2 inch. Also , there is no mech fuel pump provision so an electric fuel pump is a must. If you are going carb .. I am using a Weldon A 600 going through the carb to a Weldon return regulator and back to the tank..And you will have to ditch the stock mercury oil filter mount , cooler and lines and go with bigger stuff as well.

Coming from the drag racing world... I know without a doubt... cubic inches rule in the marine world.

I started out with small blocks, and immediately jumped to big blocks. First was a 427... then that ended up as a 511.
Ran a 555 and a 565 in two different cars. They ran great... but the ultimate beast was the 632 I had in the last one. That engine was an absolute animal. Any speed, any gear, any RPM it just pulled like a freight train.

Thing is... if you have to buy a block/crank/rods/etc... it really isnt much more money to go bigger. A long stroke, tall deck combo in my (new to marine stuff) opinion is absolutely the way to go.
To play it safe... keeping the bore @ 4.500" and stroke at 4.500" leaves some meat in the cylinder walls and keeps rod/stroke ratio reasonable... so that said... my vote is for a 572 (4.500x4.500) or 598 (4.600x4.500).

My current boat has a 496/8.1 Gen7 in it... so I found a 535/8.8 PSI engine (also gen7) to build and swap in. The longer 4.5" stroke and +.100 bore will make a huge difference.

Thats just my two cents.

So we're actually talking "large inch SHORT 9.8 deck , STOCK 502 gen 6 block build" to narrow things down a little? There's been quite alot of debate/claims/information thru the years of 'how far you can bore a production gen 6. I will say this as far as boring a stock block to its "end", only do it IF it needs done because of bore wear/damage. doing it to gain 5 cubic inches isnt a great plan.
I had plans on building a pair of maxxed out 548s for my 33 scarab AVS, 13-1 or so compression, try a bunch of different heads and intakes while on my dyno on engine one, etc. Actually built engine one most the way, I had the cncd afr heads and ported promaxx heads to try. Then after dynoing several other E85 engines, saw how atrocious the BSFCS actually were even with the BSFC gain from the compression and abandoned the idea because of fuel tank size/fuel range. Sold that 548 short block to a local mud bogger who had me put his USED AFR 290 heads on it with proform intake/modded 4150 carb. It made about 750 hp on dyno and about 680 torque on E85 with cam that would never work in a boat. Those top end parts were chosen strictly because he already owned them off engine he blew up.
So a ideaL big cu inch engine w stock gen 6 block to me is a optimized 540/548, a 555 to 565 with 4.375 stroke is pushing it. , with 6.385 rod. CH is around 1.270. Yes, a oil ring rail supports needed but skirts are just long enough to get a few hundred hrs out between rebuilds. At 4.375 stroke, your into a 1.190/1.20 ch and piston is getting mighty short, rings are starting to get squeezed into quite a small area. So as far as "practical limit" , keep in mind as stroke goes up, ring life goes DOWN.
Maximum bore, IDK what to believe, a well known engine builder in Tennessee told me in conversation that he doesn't even bother sonic checking gen 6 502s anymore as they all pass and can go to 4.600 , has blown Whippled 4.600 bore gen 6 engines out there making 1100/1200 hp at 14 lbs boost that have hundreds of hrs on them but in different conversation told me you can go 4.600 once in a while but 2 out of 3 blocks wont sonic check good enough to do it. I typically order custom pistons for gen 5/6 stuff I build that has to go past 4.530, as in I did a 4.540 bore 502 whipple engine, it was at 4.530, so I special ordered pistons .010 bigger as to keep bore rigidity//cylinder distortion down under boost. A local shop bored their customers gen 6 502 block to 4.560 or 4.580, was a compression engine. When they were dynoing it, they had rings seated, etc, were doing power pulls. They more they pulled it, the less power it made. Started getting blowby. They tore it down and looked it over, the cylinders were distorting and moving around. I dont know if they saw this while re honing it or with a dial bore gauge but they said they scrapped that block.
The next limiting factor is exhaust BUDGET. Im at that cross roads with my own current builds for my boat. I have seen now that I have dynoed countless engines here that you can make just about ANY bbc make a whole ton of hp, but usable torque, thats a whole other story. A 496 spun hard with big cam/heads can make 800+ hp but torque will be aweful. So as far as exhaust, lets say you pump a 540 up to 750 hp with big daddy cam, unless you have dry, big tube headers in boat, the dismal tq it made is gonna make boat hard to prop and hurt all around drivability and if you slap on a set of imco power flows or stainless marine 1s, gils, you can probably take 50 or 60 hp off that glorious hp number. So now you've shifted your torque peak way up as far as rpm wise . gave up usable tq and now killed off the top end hp with a set of manifolds. This is point where alot of my customers in engine build plans get frustrated. Im at that point myself. Im not gonna put used , matched up headers on my boat but Im not gonna spend 20,000+ on 2 pairs of big tubes cmis new either so.
I was having a conversation with a west Michigan engine builder/boat guy who came to my shop to buy parts last summer. A interesting point he brought up was "square tq/hp", basically if your engine makes 720 hp but makes 620 tq, its gonna be a touchy, turd with tiny prop, real sensitive to load in boat etc. would be fine in a competition boat where you have a trailer full of props, boats light, runs on fumes, etc. Would suck in a typical pleasure boat. So if you have pretty square tq/hp, say 680 tq/680 hp, your boat is going to be better all the way around, the average power under curve will make the 720 hp engine look like a joke. When your wife or Gf says oh yeah, I invited more people to go out on the boat your not going to be wondering if it will get on plane. Ill add more later, Smitty