articfriends

iTrader: (1)

Since this comes up over and over again ,here is what I have to add since there are VERY few things you can "bolt" on a 502 mag while in your boat besides a supercharger that will give you much of a gain, I get this question OFTEN, as guys want a "magic" bullet , ie,get there cc out and buy some 200$ piece to find that 6 or 8 mph gain they want.
1. CHECK FUEL PRESSURE, so often the cool fuel regulator is no where near holding pressure where its supposed 2, actually CHECK base timing and set it to 8 dgrees so tables are correct, often chain stretch, gear wear have retarded total timing.
2. Tune up, new plugs, fuel filters, cap, wires, rotor and while changing rotor, check for slop in distributor gear from wear
3.. On a "healthy" 502, I have flowed/cleaned injectors or sold guys new injectors to ensure putting hotter tune in wont melt a piston, re-tuned ecm with more timing 9stock 454/502 mages have LESS than 30 degrees total even on the top., slightly different fuel tables and saw 1-2 mph as long as its NOT over propped so much its straining 500 rpms below rev limiter
3. LABBED prop, try SEVERAL labbed props, often the biggest gain in speed on stock boats, especially singles that are under-rpming and not breaking 60mph is someone randomly sticks wrong prop on, once in a while but not as often, someone sticks too small of prop on and their banging the rev limiter.
4. No way around it, normally aspirated boats are susceptible to power loss on hot, humid days and/or those times when you got a full tank and 8 people in boat. There's a reason dynos have correction factors, on a 55/60 degree day with low humidity, your 430 hp motor makes its 430, on a "typical" summer day in michigan when Im dynoing, its not uncommon at all to see a 6-7% correction factor, that means your 430 hp boats gonna make 400 hp. A HOT, super humid day, 12% easy, means your boats gonna make 380 hp. IF your serious about squeezing every mph out, you WILL have a "fast" labbed prop for those days you run with a 1/3rd tank fuel and your hot gf or wife, not your whole family , a "tall" fast labbed prop for those good air days and a small prop for those hot, humid days with a fiull boat.
5. Prop slip, most over rated, over discussed , beat a dead horse subject there is. Your boat will like what it likes, unless your prop slip at cruise is awful or your boats acting evil, there's not nearly as much gain from lowering it as people fantasize about. IE, your boat runs 80 at 20 % with a stock 28 pitch, it may run 82 with the right labbed 29, 30 but if you could lower slip to say 10% its not going to pull same rpms and run 88 (unless your way off to start), a more likely scenario is if you lowered slip 10%, your peak rpm would go DOWN 10%, possibly more as it gets further away from peak power. I had a 272 Baja, ran 99.5 gps with 22/24% prop slip, all my attempts to lower it slowed boat down.
6. REAL mods for 454/502 MPI (every example would also include tuning ecm optimum): fresh 502 short block, new valve springs, good 3 angle valve job, will make 450 to 460 on most dynos in dyno trim at J607CF, 10-15 hp gain
7. Cam change along with above, 454/502 mags, easily add 30 to 50 hp, the factory cam is TERRIBLY small at [email protected] and .480 lift, would see 480 to 500 on most dynos
8. Aggressive cam change, supported by a exhaust change to lower chance of reversion, some bowl work and chamber work on the stock heads, a little porting, will get you into the 520 to 540 range, maybe higher depending how talented the head guy is
9. Above PLUS cut down runners in lower intake, port match to heads, cut dividers down, hand blend all lower plenum entrys, bore out TB, will get you past 550 in dyno trim, I made 560+ from a iron headed/modded intake 502 w a used crane 731 cam. I made 558/560 from a pair of .030 over 454s with ported iron heads, chambers, extensive intake work , Bob M cams, both pulled to 5800 plus
10. Cylinder heads, add a pair of good afr, dart cylinder heads to THIS along with proper camming, 600 + in dyno trim isn't out of the question. Jeff H made 611 w promax heads on my dyno w a 509 with my previous listed mods. ran 81 at Loto shootout with a 280 Powerquest, Jason Scott made in the 630/640 range with his 509 w dart 325s, extensive intake mods including extrude honing, ran 85 at Loto this August w a 280 PQ
Theres power to be made with mpis IF your willing to do real work, spend money, there is NO magic bullet though without tearing into it, Smitty

articfriends

iTrader: (1)

And FLAME arrestors , IF you mod your 502 and buy one of these FA kits that changes shape from rectangle to a round camp on, in all my dyno testing, they made LESS hp on top then stock stuff, especially the mefi1/gen V FA that everyone likes to take off!

bajaman

Yep...amazing that so many people don't seem to understand the OEMs put millions of dollars into R&D to get the best compromise of power and longevity...so there are VERY few aftermarket parts that do anything more than drain your wallet.

F-2 Speedy

iTrader: (6)

Nice write up Mr. Smitty, I built a pair of 540's with AFR 305, BM double swap cams and they made 628 hp through stock 500 EFI intakes full dressed with CMI sweepers,

xlint89

Thanks for posting this. I was looking for this type of info last year. Ended up buying a HP 500 instead.

Do you feel the oil system need to be addressed when boosting the HP on a stock 454/502 mag?

articfriends

iTrader: (1)

Yes, the stock oiling system on a 454/502 mag efi is marginal stock.years ago i posted a giant thread on the deffieciencys. Can you mod a 454/502 and use stock lines and cooler, if you keep your wot burst fairly short, it has been done. But heres a good example of how restrictive it actually is. I recently tuned a 380 pq with 502 mags for " gimmefuel", he has owned boat fir 5+years. 960 hours, it has had 38 to 45 lbs of oil pressure HOT since hes owned it. Motors are virtually stock with addition of bored out tbs and exhaust this year. Over the winter he bought actual 9 plate oil coolers, -12hoses, 1/2" npt fittings etc and re-rigged oil system. Same long blocks, gauges, sending units, pans, oil pumps he's had forever. Magically the boat has 55 to 60 lbs hot. That tells me the internal relief in oil pumps and possibly block bypasses have been opening forever on top of the tiny oil coolers barely cooling. I exoerienced same thing years ago when i upgraded same stuff too. Bigger lines and adapters cant MAKE oil pressure but small ones can restrict it.

bencini231

Great write up there to help with the winter upgrade questions we will be seeing very soon. For that first time boat owner that reads this post and decides to go for more power, please go through the old bravo drive before it becomes the next weakest link after upgrades.

articfriends

iTrader: (1)

Heres another bit of "info", been asked a thousand times: The STOCK mercruiser inline efi pump used in cool fuel is a 210 LPH, the capacity of a 210 LPH is calculated by multiplying it by 1.63, THE MAX LBS HR YOU'LL EVER GET FROM STOCK PUMP, MINT CONDITION, PERFECT VOLTAGE is 342 lbs hr.
So IF you see a 650, 675 hp modded mercruiser with stock pump, they are LYING. the average bsfc for most these modded is about .58, so come about 580 hp , the pressure on top will start to drop and tune will go lean. Come 600, its a serious problem unless you got a bunch of compression and extraordinary bsfc numbers. I got Jeff H 509 to make 611 on dyno.by boosting his factory pump with pump that forces fuel thru my bsfc meter. His pressure still dropped from.the 40/41 on top to about 36 lbs. When we called the headers w mufflers, he made in the 590's, when i leaned tune out slightly, drop wasn't as bad.
With his EMI manifolds he was in 570/580 range, i leaned out tune again, pressure drop was almost ok but we were still force feeding the factory pump.
In boat, we took out a little more fuel in tune to hit our AFR targets, press held at 38/39 at wot, was using EVERYTHING it could put out.

articfriends

iTrader: (1)

And a "better, bigger " replacement pump, to.my.knowledge there is absolutely no "drop in, upgrade" pump that will easily install in.place of stock with stock hoses, pieces. If someone knows of one, post it up!

SB

iTrader: (1)

How about upping the voltage to the pump ?