525 SC build questions
 

Hey there...I have a 28 Nordic heat with a 525 sc(1997) and im going to do some building on it this winter...I have a couple questions as I am in the process of part buying...

1. For a cam I'm planning to go with one from Bob at marine kinetics...a custom grind I guess?? He seems to know exactly what I need lol
2. He's trying to sell me on AFR heads however I'm having a hard time justifying the money so I'm considering sticking with the stock 088's and do a little porting myself, clean the valves up, throw a new set of springs on them, and mill them to up the compression a little...I've been told I can safely be in the 8.4-8.8 range...does anyone know how to figure out how much to mill off the heads to get in this ballpark?? I'm going to do the milling myself at work just not sure how much to take off??
3. Going with a refurbed 6-71 from Dyers blowers as I had an old GMC blower laying around from a swap meet so for the money I cant pass it up...plant to run it at 7-8 lbs of boost at the most
4. Plan to pull the pistons, rering it, hone it, and put in new rod and main bearings while I'm there so that leads me to another question...would I be in better shape to replace the pistons vs. milling the heads?? Keep in mind I can mill them for free just need to figure out how much to take off..but I also want this built as "right" as possible for the money I'm willing to spend
5. What kind of power difference can I expect with my stock heads vs the AFR's??
6. I'm planning to run twin holley carbs off of an older 8 or 900 sc cant remember which it was...planned on jetting them to 700's...would that be sufficient for my setup?? also if anyone has jet size suggestions I'm all ears

Sorry for the long post just looking for as much help as possible...I'm a mechanic for a living however this is my first go at building a blower motor honestly so I want to make sure I don't make a pile of scrap metal hahaha

Thanks in advance!!

Stewart

I have never used Bob M nor ever had any contact with him. With that said, there are quite a few threads here about him. Some for, some against. Do some searching and make up your own mind.

Good luck

Oh really?? I've read nothing but good about him and that's why I was gonna go with him...I'll have to do some more searching I guess!!

Thanks!!!

Stewart

How much power are you trying to make?? what drive do you have??

No real power goal in mind...boat does 75 now stock...I'd love to see 80-85....according to that bob fella this combo should make a reliable 700 hp roughly...that was with the heads tho but I'm pretty confident I can get there on stick heads and still be "safe and efficient"

Drive is a bravo 1 with xz gear "upgrade" from cp performance...has a nose coned lower with a drive shower...boat had external steering as well....1.50 gears 28 p bravo 1 4 blade prop

stewart

no matter what you do to the gm heads they will never make power like a set of afr,s will.i built a pair of 454s for another oso member that made 691 hp with dyno headers.custom pistons at 9 to 1 c/r,a set of afr 305 heads with cnc chaimbers and a cam from bob madera.i was out in the boat with the owner last month and it runs very good.ps,the blowers are weiand 177s with singel holley 1025 carbs and mercury marine gen4 ignitions.with a 6.71 i would expect even better performance.

700 hp is going to eat bravos for breakfast.

Instead of milling heads, I would go with better pistons.. well I did go with better pistons lol....

You have a lot more blower than the 177. The combo Im putting together with a 731 roller cam, Al's heads, 8.5 compression and the 177 blower at 5 psi is supposed to come out over 600hp.

So I suppose the real question is are the heads worth the 2500-3000 right now...I totally understand yes they would be better hands down but are they gonna make the boat 750 hp and run 85 vs 600 hp and run 76?? I guess I'm really wondering as a best guess what the difference would be

thanks!!

Yea I'm kinda worried about the drive too however I'm hoping I can nurse it next summer and possibly do at a minimum an xr upgrade or better next winter...that's why I'm thinking maybe skip the heads for now and do the drive and heads next winter?? Idk thoughts?? Trust me I've been a boat mechanic for a looong time I understand what power does to bravos hahaha

stewart

Not offering any advice here, only info. I had a pair of bone stock 525 SC's years back, put the Superchillers on, changed out the pulleys to up the boost, little carb adjustments. All else left as Merc built them, never even took a valve cover off. Ran at 10 psi boost +/- and made 638 on a dyno in full in the boat trim. I ran them for many seasons with no issue before selling.

P.S. Was a Bravo killer.

IMO, you'd be better off selling your 525SC complete and then look for a 502 that needs a rebuild.

You can't mill the heads more than .030 or you'll have issues with the intake manifold fitting.
If I remember correctly, every .005 taken off reduces chamber size by 1cc. You might be able to get about a 7.9 CR.

I agree overall that may be the best thing..however what I'm looking at is if I were to do that then I'm pretty much re rigging the boat with power steering, alt, etc which I don't want to do..I believe I can get a couple grand at least for my intercooled 174 setup so that means I'd have maybe another 2 grandish to be close to 700 with what I already have...truth be told I would expect in the next 4-5 years I'll be upgrading to a bigger boat so if I can do this build with what I have and have 4-5 years of trouble free 80+ mph boating(minus outdrive issues of course) then I'll be happy....I love the reliability of the 525 as I can run that thing damn near wide open for as long as I want with no issues and I don't want to lose that entirely....I know seems silly to think performance boating will be trouble free but I'd like to try with the motor at least lol!!

Thanks!!

Stewart

all the accessories from the 454 will bolt on to a 502.

not in all cases, ran over 700hp with a std bravo(non XR) for 5 seasons with no failures, was in a 25 checkmate and were careful but ran mid 80's, on the 525, I agree with the sell it and just do a 502, you are gaining nothing from starting from it. If you insist on keeping it I would change pistons, as they are not available anymore and are odd balls, so if you ever need one its tuff...ask me how I know..

What kind of money do you think it would be worth with no accessories tho??? just the carb to oil pan with no ignition or accessories?? it has all msd ignition that I would like to keep and I don't have the stock stuff

525 sc with intercooler 520 hours

Also...would stock 502 pistons and rods etc. be adequate for my build with above blower etc??

thanks

Stewart

i don,t know what your sc525 is worth.some people have used the origional 502 pistons and added blowers but that is not the best way to go,if starting from scratch bying a piston designed for a boosted engine is the best way to go.

So I guess if I would have to buy pistons anyway...what would I be gaining besides a little bit of displacement?? not discounting your suggestion just trying to understand it fully

Stewart

Isn't a 525 sc nothing more than a Gen 6 502 mag long block with a blower ??

525SC is a 454, not a 502.

525sc is a 454 with a 177 blower.

What heads are on the 454 sc........Mercury has an odd way of naming their power

from what ive read they are gm cast 088 heads

Stewart

we ran 6psi on stock style 502 pistons and stock rods for 5 years

They are forged, essentially flat pistons and good rods. People put blowers on cast 3/8 bolt rod 330 hp engines...

So to recap...if I get a stock 502 put a 671 on it with roller cam roller rockers at 7 psi vs my 454 with same mods...what kind of power differences we talkin?? Don't know if I can swing heads at this point so lets just say stock heads on both

thanks

Stewart

10% displacement. 10% more output. maybe a bit more cuz you can run a little more cam...

So if I find one cheap enough...Cam suggestions?? also would the supercharger intake be the same for a 502 vs my 454?? not sure what heads are on a stock 502

also what compression ratio would I be looking at for a stock 502??

thanks

Stewart

also...any specific 502 I should look for?? would a truck motor from a boneyard work??

there are no boneyard 502's. they are boat and counter engines only. same heads as 454. same piston, rod, crank. all forged rotating assy.

I gotcha...I just ran across a place that is willing to sell me a useable rebuildable core for pretty reasonable and theyre only a couple hours away from me...so if I buy this with no heads would the AFR 454 heads that I was quoted for be the same as the 502 or would I need 502 specific heads??

thanks

stewart

Make sure it's gen 6 502

How can I tell that by looking at it??

Why is that? They do not have fuel pump provision.

All heads interchange, except for the 8.1, gen 7, 8100 vortec block. That design added the 2 missing head bolts that would hafta go through the intake ports. Gen 5 and 6 both have no fuel pump mount. Gen 5 has 10 bolt timing cover. Gen 6 has 6 bolt. Gen 6 is machined for factory roller lifters and dogbones. Gen 5 is flat tappett. Roller conversion is very little problem. If it is a cheap 502, it'll be a gen 5 ...

I gotcha...think I have it figured out...502 long block AFR heads 6-71 rebuilt GMC blower 7 lbs of boost..should be interesting!!

:riding:

Stewart

The amount of boost you run safely is dependent on the static comp ratio and fuel octane you intend to run.
With an 8.5 CR and 6# of boost, you're about maxed for 92 octane.
http://documents.holley.com/techlibr..._tech_info.pdf

I run premium 93 octane in it already so that's what ill continue to run...I don't think you can even buy 92 around here lol..I know its only one point but I see what you're saying....thanks for that tidbit!! something I would have overlooked for sure

Stewart

Don't the Afr heads have steel valve seats that suck ass. I remember reading that somewhere and seats should be changed. Anyone?

They do in fact come with a standard ductile iron seat. Darts marine heads, GM/merc heads, engine quest heads, and Afr's bought from teague marine with their service performed, have powdered metal, sintered, upgraded seats to hold up to the abuse of marine environment . The very early merc 565s were recalled , as they found the standard seats they used werent holding up. Some guys say the standard seat is fine. Guess it depends what you want out of them. I know of quite a few oso members who pulled their engines down with low hours and had excessive exhaust valve seat erosion with the afr heads.

I switched directions again as far as where I'm getting heads...going with af racing...talked to Alan there and I'm pretty sure he addresses the seat issue...he's been following this thread so he can correct me if I'm wrong

good into tho guys thanks!!

stewart