Stroke/Bore 502

[Pismo]

I have a stock 1996 Mercruiser 502MPI with I believe a stock 9.8 deck height 502 block. How far can I bore and stroke this engine? Now it's 4.47x4.00. If I go to 4.25 stroke can I use stock pinned pistons? What size rods would I need? Bore to 4.5 for now, how far can I go evetually without blowing head gaskets, 4.53, 4.56, I read somewhere 4.625 but don't believe it.
Looking for a very reliable 550-600hp setup. Will be scraping the EFI.
Thanks

[Foggerjet]

I have a stock 1996 Mercruiser 502MPI with I believe a stock 9.8 deck height 502 block. How far can I bore and stroke this engine? Now it's 4.47x4.00. If I go to 4.25 stroke can I use stock pinned pistons? What size rods would I need? Bore to 4.5 for now, how far can I go evetually without blowing head gaskets, 4.53, 4.56, I read somewhere 4.625 but don't believe it.
Looking for a very reliable 550-600hp setup. Will be scraping the EFI.
Thanks
Jus' wondering, why would you want to ditch the EFI? Also, you will need new pistons if you stroke it. Is there something wrong with the 502?
fog

[Party Cat]

I put together a 509, (4.50x4.00), Dart Pro 1 heads, Crane 741 cam, RPM AirGap with a reworked holley 830, MSD 6m2 w/Pro Billet dist....
Dyno'd 628/652...Ran Great all last year....running great so far this year...

[Oldsquirt]

GM's maximum recommended bore on the 502 is 4.500".
Hey PC, what rpm are those numbers at?

[Party Cat]

GM's maximum recommended bore on the 502 is 4.500".
Hey PC, what rpm are those numbers at?
RPM TQ HP
3000 637.4 364.1
3500 649.1 432.6
4000 652.0 496.6
4500 645.1 552.7
5000 626.5 596.4
5500 600.0 628.3

[Pismo]

Party Cat,
That sounds like a great setup. How's the idle quality? What exhaust are you running? Full roller setup, rockers etc? Did you use the same rods and crank or upgrade? So many questions.................
Thanks
Pismo

[Party Cat]

Party Cat,
That sounds like a great setup. How's the idle quality? What exhaust are you running? Full roller setup, rockers etc? Did you use the same rods and crank or upgrade? So many questions.................
Thanks
Pismo
Idle id great...a slight surge while cruising through the channel...cam is slightly noticable. The cam is a Hydraulic Roller with Crane Gold rockers.
The Base motor was a 502 crate motor, no upgrades on the crank or rods. I'm running a 10 qt pan and a decent oil cooler. The original motor I had was Gen V so the Raw water pump has the mechanical fuel pump on it and I just put a high volume mech pump on it. Exhaust is the Dana Cast Manifold with Stainless Risers...I dunno which ones cause I had a major reversion problem with their cast ones (which they said would work), they replaced them with a set of Stainless ones...nice step up...but it still cost me $3k to R&R the motor and have it cleaned up... The MSD Dist is locked out at 36 degrees (I think).
The motor was assembled by Tom at Pro Marine in Havasu...
With my stock Bravo III 28 props (non cupped) I got 57.1 mph @5500 rpm (at Havasu) pushing about 5000-5500 lbs of Party Cat....I then had the props blueprinted and cupped by Kevin of R&R Prop Shop in Vegas....got a GPS of 62.5 mph, also at 5500 rpm (at Elsinore)....Depending on how it runs at Havasu next weekend, (warmer air temp, lower elevation and the addition of Gibsom Muffs) I may have some more cup put in the props
For the past year...this thing has been turn-key....For insurance purposes...I do keep a brand new MSD 6M2 box in the boat....for whatever reason...those things like to die without any notice....

[Havasu47]

4.250 stroke, 4.500 bore (I wouldnt go any more) == 540 CI
Use .250 longer rod and get a piston with the pin moved up. JE / SRP has a shelf piston for this application. We have built many of these and they are very good running engines. We prefer the 10.200 block becaue you will have to run a rail support for the oil ring, but this wont cause you any problems.
This was a very popular upgrade for the old 502 carbureted Mag's.

[Pismo]

PC,
Do you know what size Dart Pro 1 you have, are they the 325cc? Did they put inconel exhaust valve in them?Thanks

[kojac]

I put together a 509, (4.50x4.00), Dart Pro 1 heads, Crane 741 cam, RPM AirGap with a reworked holley 830, MSD 6m2 w/Pro Billet dist....
Dyno'd 628/652...Ran Great all last year....running great so far this year...
Party cat,
What compression did you use? I have a 509 currently getting 780 hp with a solid roller, and 13.1 compression completely ported aluminum chevy heads. Dart single plane manifold. topped with a 1050 dominator just sitting on the engine stand.Turns pump 6400rpm's.
Am thinking of lowering the compression to accomodate lake pump gas(93octane) and going to smaller hydaulic roller for a get in boat and drive application.
sounds like you have a good combination.
Thanks Kojac