JIM V

I am running one of the 502/502 partial kits from G.M. in a 25ft switzer. It has the large oval aluminum heads (225 intake / 188 exaust.) 9.6:1 comp. If i understand what you are saying, the heads I have are good but some work could really wake the motor up. (we ran 79.1 on gps @5250+- rpm )with an 850 double pumper and a victor jr intake.

what is the weight of your boat? Mine is a 24ft Magnum approx 4500lbs with min 390hp and I can only get a true 61 mph max after 15 hrs at wot or the full length of lake Huron(just kidding) is your hull that much better or can 110 hp realy give you 18mph

Yes, it is true, so I've read, that Merc basically had to use what it got it's hands on when it came to using the heads discussed. Oval's weren't prevalent in those years.

With extensive work done to ovals, they can out perform rectangles to 6000 r's on the 454's and 502's. When you get into the 540's and above, I would use the rectangles.

Jim basically reinforced what I said, with a much better explaination. That's why he's the expert though, huh?

Velocity is actually the key. Like Jim said, the air gets lazy without it.

As I've said 100 times, you have to match your parts with your application. Ya just cant through a bunch of parts together and expect them to be optimal.

DAVE

hpbigblock,

Yes, by modifying your heads you can increase power. The heads are capable of flowing as much or more than the rectangular ports. Your heads in particular, are superior in design and would be a great starting point. Your 502 would benefit changing to a 2.300 intake valve. If the airflow numbers coinside with the valve sizes you will make more power. Depending on where the numbers are now you can make another 40 hp.






Dave has been making this point since I have been around and I agree with him 100%

JimV im wondering if you have done any wedges and cam changes to a other wise stock 500hp carb motor and what results have you gotten. i see on your web page that you are doing a 502 with 650hp but ther hasnt been much updates. were do us guys with 500's go beside supercharging. im interested in what kind of numbers your getting with the wedge modified heads. im assuming the reason for the wedges is for velocity because the rectangle port head the port are to large.
thank you in advance

Andy

I dont know much about magnums. when i bought the switzer I knew nothing about them and still know verry little because the company went out of buisness in th early 90's and they seem to be somewhat rare (at least there is no info on the web about them that i can find.) but are a light boat. I did find an article in an issue of Power Boat from the 1991 year that rated them stern drive performer of the year.(I didn't know this untill after i bought the boat for a song and dance. Idont think the owner knew what he had in tearms of performance) The 79.1 was done on flat water and this boat lives for 2ft chop. So on good water who knows what it will do.

Great thread we've got going here !

Blue Thunder, There is a point when air is flowing thru a head that if you have too much volume ( rec heads ) the air stalls and you loose some of the velocity needed to move the air faster thru the port. The water hose reference was for velocity, volume demonstration purposes only. For best performance the intake port should be no bigger than needed to supply the cylinder with all the air & fuel it can use at the highest RPM it is to be run at, any more & performance will decrease.

I've never heard of the wedges put in the rectangle port head but it makes a lot of sense. How is this done & how much does it cost ?

The reason the rectangle port heads are used instead of heads that would be more marine friendly is because GM never really paid much attention to the marine industry, when building marine high performance engines they just grabbed what was on the shelf that they always used for automotive high performance engines. Unfortunately its not the most ideal choice. The GEN VI engines got cylinder heads that, although the're not perfect, are a lot better for marine use than the earlier rectangle port heads.

Bo

Yes, like Dave F said, matching the correct components together is crucial to max performance. It is also the single biggest challenge I have with my current project boat. It is so mismatched its not even funny. Next winter I'll have to do some sprucing up in that area. Thanks for all the great info.

Excaleagle42... the bravo is progressing, but very slow. The SB engine is re-assembled and back on board ready for fire. A little warm weather would help... Just installed my newly fabricated dash plate with all new gaffrigs and switches... pretty happy with that. Then I need to reinstall all the upholstrey and clean everything... then theres the trailer brakes. Wow, good thing I love doing this stuff . It should be in respectable shape by Canal Days. We will have to hook up and have a cold one. Your in a black eclipse 42'? See ya later.

BT

Ahhh, the C&D canal.

God, I can't wait to see you guys down there this summer. If you see me first wave me down to say hello.
The upper Bay and canal area is where I spend my time.

DAVE
242 ls Formula
"Blue Bayou"

Ah yes.... the c & d indeed. Abosultely my favorite performance boat adventure. Blue bayou... cool name, I can remember that. See ya therre

BT