bowtie89

I have a 1989 206 formula so my 350 magnum is pretty good it gets me up to about 55mph with the 260-270 hp that is has. I would like to upgrade it a little bit without going to crazy!! I want to see 60 65 so maybe 300-350hp. Where should I start heads?? I don't know to much about engine internals or what mine has so alot of information would help!!
Thanks alot!!!
Harry

Iggy

First thing to do is pick up a copy of Dennis Moore's book "Small-Block Chevy Marine Performance", HP Books ISBN 1-55788317-3.
His book will tell you everything you need to know about what your engine has and what you can and cannot do.

Your 350 Mag is the same as mine, 270 hp.
Depending on how many hours you have on it will determine just how good your modifications work.
I'd start with a leak-down test to see what shape the cylinders are in.
Then look into a new camshaft, lifters and valve springs.
In Dennis' book he states the stock heads will do fine with larger valves installed and new springs. Maybe a little grinding in the valve pockets to help airflow. And screw in rocker studs. I'd also go with roller rockers myself.
A change of intake manifold will be needed too. An aluminum Edelbrock RPM Air Gap is what I'd go with but if you boat in salt water there will be a corrosion problem. Dennis also states that if you already have the correct high rise iron manifold it be fine as is. His book will explain how to tell which is which.
Carb? Go with Holley of find someone that knows how to modify your stock Rochester carb.
Don't forget to install new exhaust manifolds too.

With these modifications I'll bet you could see a gain of at least 50 hp.

What would I do?
What parts would I install?

I'll make that another post as soon as I can.

Liberator21

I agree with what Iggy said. If salt water is an issue, stick with the Iron intake manifold. If you have the magnum, you probably have the high rise intake. Also, as far as the exhaust goes, again if you're in salt water, it's either stainless (Big bucks), or a good quality iron manifold. I've had good results with the GLM manifold. It's made with the lost foam casting process so they were able to design in small runners which help to isolate adjacent ports, and reduce reversion. Magazine tests on a 350 chevy in a 21' boat showed a 2 -3mph improvement. They're a little lighter than stock manifolds too. I bought a pair last year, and so far so good. Head work is a good thing. Small blocks respond well to some mild porting. I did a pocket port, and gasket match to mine, and the engine really came alive.
A mild cam can show quite an improvement as well. But remember, unless you want to spin this thing up real high, don't go crazy with the porting, and the cam. Too much port work, and cam can turn your 5000 RPM cruiser into a 6000+ RPM screamer. But then again, it all depends on what you want to do. It sounds like you just want to beef up your stock engine a little. That's the first step (Hee,Hee,Hee). Like Iggy said though, do the compression, and leak down tests first. Also make sure you've got good oil pressure as well. Beefing up a tired engine can almost guarantee you a rebuild in the near future. I've been there, I know. How many hours on that engine? One other thing, check out the chevy magazines, and their web sites. There's a lot of good info, and tech articles there. I'm a Mopar guy, so I did a lot of research before I built my engine, and I'm real happy with how it turned out.

Paul

Anyone have an old Mercruiser or Aquapower riser they want to get rid of cheap?

Iggy

Ok, Here's what I'd do if I really wanted to do it:

Start with a complete rebuild of the bottom end and new pistons, quench type, with a "D" shaped dish and rings of course.

Heads
Stock Iron heads, look for a set of late 90's high compression type with the 64cc combustion chamber and 2.02/1.60 diameter valves. Add screw in studs, proper springs to match the cam and Comp Cams 1.5 or 1.6 roller rockers. Pushrod guide plates too.

Cam
Chevrolet P/N 24502586 roller cam. matching springs and lifters.

Intake
For salt water I'd stick with the iron high rise. I'ts a direct copy of the old Z-28 aluminum high rise.

Carb
A Holley, 650 cfm, should work right out of the box. If I can learn enough I'd attempt to tune the Rochester Q-Jet.

Fuel Pump
I'd love to go with an electric pump, regulator and a return line to the tank, But a good high volume/pressure mechanical and a regulator will work just fine.

Ignition
Stock Merc Thunderbolt or switch to a Mallory unit.

Exhaust
GLM manifolds and risers.


Take this same combination but use Vortec heads, 1.6 rockers, 750 Holley, Edelbrock Vortec RPM manifold, a 383 stroker kit and you'd pump out 400 or so hp.

Liberator21

Hey, Who makes a good high volume mechanical pump for the small block Chevy? I'm suspecting my fuel pump might be starving my engine.

Iggy

Holley has two mechanical marine fuel pumps.
One has 3/8" ports and flows 110 gph, the other has 1/2" ports and flows 130 gph.
Don't forget the pressure regulator. It doesn't just maintain an even pressure, it also evens out the pressure pulses.

Check out www.holley.com

bowtie89

That is alot of helpful information! I just ordered the Moore book!!!It will be on my front porch Thursday!! I am very excited to start my wintertime projects!!! thanks again

Cord

iTrader: (2)

Or, you could bolt on a 144 supercharger and a new carb.

jafo

Yeah, what they said!!

I had a Formula 242 with twin SBC's that I built - although I didn't go with a stroker kit, I did install forged components (crank and pistons) and used Milodon four-bolt caps.
Be wary of spending much $$ on the stock heads on these engines. They are the "light cast" heads, and I'll bet that any good machinist is going to find a crack or two in them. They also do not lend themselves to planing or machining well. You will be much farther ahead with the newer Vortec heads, or World Products.
I used WP Sportsmans.
The high rise cast-iron is a good choice- stick with the Q-Jet- very tuneable for an SBC application. The Thunderbolt IV ignition is hard to improve on as well.
CompCams has several good grinds; my personal preference is Isky.