Give this 502/509 Combo Some Thought
 

I have basically a marinized 502/502 and I suspect the heads are my biggest bottleneck. With that being said, the current combo runs fine, might make a little too much heat with small combustion chambers, but otherwise a lot of bang for the buck. I'd guess 525HP as is and only has 15-20 hours on it.

I am looking for 300-500 More RPM before I start working on the drive, prop and bottom.

Current Spec:
502ci
9.6:1
Isky 238/248 .578/.608 on 114

290cc Runner 110cc Comb Chamber 2.25/1.88 Stock Alum GM Heads w/Stainless and Inconel and Comp 1.7 RA's

GM Dual Plane Intake
QFT Marine 850
Thunderbolt w/ HP Module
Dry Stainless Marine

It seems I can spend some good money on heads, cam and intake for maybe another 75HP or I can work on these heads a little, bump the cam up to the famous 741 and try a single plane. While I want to run a little faster, I don't want to sacrifice 3500 rpm cruise to get it. Based on my research, a well put together 502/509 is going to make 625 with some significant cylinder heads. I hate to cut a good set of cylinder heads down to make the compression necessary with my flat tops. I do have a spare block and crank to build a 509 out of but again, that's a lot of coin for 50-75 more HP than the current combo might make with work.

Am I right to think $1500 might get me what I want vs. $5-8k to start over and make a little more?

This is my light weight 27 that's running a best of 80 as it is. 90 would be fun but 60-65 MPH at 4K would be just as cool.

BM/MK had a dream about my current cam once. My lifters made a noise when when they were installed. With a dual plane, it has the torque of an RV cam but the sound of a super custom, computer software proven, 20 hr valve train cam.

What, not interesting enough for this crowd without drama?:food-smiley-007:

FWIW, Endeavor1 drove it this week and said I should leave it alone!

I have a very similar setup with 9.6:1, ZZ502 cam, and Eddy Performer heads and manifold. I don't know how the Eddys compare to yours, but when I spoke with MK he said that 600 should be easy and maybe 650.

Edit: 600+ with a NEW cam. The ZZ502 is pushing 500 undressed at best.

Do you know the basic specs on those heads?

I know that boat of yours is turn key and super fun to drive:wizard::noevil:

Those afr 300 cc oval ports would be awesome on that with 454-0 intake.

600-650 with zz502 cam? I'd love to see the dyno proof of that. Lol

For the sake of argument I'm hunting 600+. 502s. EQ heads. Off the shelf cams. (I know crazy right?) Merlin intakes and quick fuel carbs. Looking around 9:1.

600 HP isn't hard to make from scratch. I am just trying to figure out if I can get close without starting over.

I am tempted to just try a little more came and a single plane intake but again, I don't want to worry about heat when leaning on it.

I'm gonna have larger cams than you and I'm single plane.

No, as it is it's probably barely 500 undressed. The 600+ was with a new cam.

Txhawk, whats this heat you speak of with the small chamber? I ran 107cc chambers, with a blower for years, never had an issue.

Thats a huge chamber compared to a big chief head, no issues there at some extreme power levels. I wouldnt worry about that .

Those heads can be improved, if you wanted to sink some money into them. One of the best areas to focus on , for the low lifts we run in marine, is the chamber, throat/bowl and valve job. Im sure those heads could benefit from a little unshrouding in the chamber. Typically, doing that, will knock out 4-5cc of material. You'll lose a bit of compression, but done right, the airflow gains and burn quality will more than offset that.

The 741 cam wouldnt do much for you over your current cam. Theres better options for an off the shelf cam, that will be easy on parts and last a long time.

Id lose the dual plane intake, and go to a single plane like you said.

As far as a 100hp gain from just a cam swap in a combo like that, is well, a pretty tall order. Ive heard before how swapping out a crane 651 cam with a "custom" modern hyd roller, will net 100hp gain in a 540. If that was true, the 675hp 540s with a set of 290 afrs , and 651 crane cams, would have made 775hp with a "custom" hyd roller? Where those builds at lol. Seems like the 651 crane did just fine when compared to some of the recent 540 builds around here. Even if its an old grind. And it does it without hammering the pushrod tips , breaking rockers , or pounding out the valve seats.

Have you considered some 55mm cams, 903 lifters, 600lb spring pressures, 1/2" pushrods, and steel shaft rockers for a 5800rpm combo?

{ Have you considered some 55mm cams, 903 lifters, 600lb spring pressures, 1/2" pushrods, and steel shaft rockers for a 5800rpm combo? }

Calm down, deep breath, its Friday :party-smiley-020:

Not even close with that cam...bin said so it just be true lol

The heat concern comes from the builder I had go through this engine. He builds mostly SC power locally for the marine crowd. He likes the combo for what it is, but had concerns about heat during long runs.

No, I haven't considered getting deep into this shortblock, it only has 15 hours on it. I freshened it up to basically get the boat on the water after major restoration. I haven't changed a thing other than up 2 jet sizes since this boat came off the trailer. 28P LH at 5000 is 79-80 MPH and 9-10% Slip. If I can turn it another 200-300 RPM's I would be thrilled for the current money invested in power and would likely see 5 MPH based on slip.

85 in this boat will be a handful. Its entertaining at 75.

I was looking at Dart BBC Oval Port Single Planes and noticed they state to mill .060" to fit GM heads. Is this a Dart thing or something I should expect in general.

Is the consensus to run a Dominator flange and step back to 4150 with spacer to run my 850?

Pretty sure your heads were made by edelbrock, the GM aluminum ovals? Cant remember

I think thats right. Some 502/502's had a GM stamped head and others edelbrock. These are the GM Perf Parts stamped oval ports.