Originally Posted by articfriends (Post 4862007)

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

Originally Posted by Brad Christy (Post 4862009)

Smitty,

So, I've got this hair-brained idea (not that I have any immediate plans) of starting with a BigM tall deck, 4.25" stroke, bored to 4.6 (565ci) with 6.7" rods. My math puts CH at 1.575". Too much? 4.375" stroke and 4.54" bore gets you to about the same displacement (566ci), and a CR of 1.3125". It would seem Molnar sells rods of just about any length, that would allow the "fixing" of any CR stack-up. Is it that simple?

Anybody ever done this? Morbid curiosity.....

Thanks. Brad.
(937)545-8991

Originally Posted by articfriends (Post 4862010)

Yes, Im building a 4.375/4.600 tall deck right now for a non boating application and its as simple as getting pistons with right Ch. If I was going to put a 4.25 stroke tall deck together, Id probably go 6.800 rod length to get some of the mass out of the piston, Smitty

Originally Posted by KWright (Post 4862014)

Building NA engines for boats can be a little tricky. While trying to achieve a certain HP goal, sometimes the torque band is overlooked. The most important piece is cylinder heads, and cam. I will post our performance sheet on our 42 fountain, with 790 hp 565's. If you notice we never reach peak hp. You can also see how hard the boat accelerates through the torque curve. We collect our data through smartcraft, garmin that are linked together. Then apply the torque and hp figures from our dyno sheet.
https://cimg1.ibsrv.net/gimg/www.off...fb91a99828.jpg

Originally Posted by boostbros (Post 4861977)

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

Originally Posted by ICDEDPPL (Post 4862070)

The first engine builder who was a nervous nelly and wanted me to buy the best of the best was having a stroke when we had to go 4.530 on a Big M block.thought there wasn`t eanought meat on the bone
As far as I know 4.600 is the limit ..... Dart sais



If you go 4.600 and it gets hurt you`re out of meat on the bone and you have to buy a new block no ?
Thats the issue I see with your plan Brad.