Pat McPherson

I did a basically stock rebuild on my 1997 Merc. 502MagMPI last winter and it has been running great.
Since then I have been working toward acquiring the parts to turn it into a HP500EFI.
I do have the AZ speed intake, throttle body, and cam shaft, but I have not bought the fuel injectors and a few other goodies that I would also need.
My question is:
Would the the engine in its stock configuration make more power if I was to just swap the Merc intake for the AZ Speed intake?
I am assuming that the AZ Speed intake flows better than the Merc intake since Merc uses it on the 500EFI and 525EFI...

Pat McPherson

Hey Guys,
Should I leave it alone or swap to the AZ Speed?

800XCR

If it were me, I'd sell the AZ parts and use the money to upgrade what you have. You'll make the same hp for less money and quite a bit less hassle.

The stock intake, throttle body and stock injectors will support 500+ hp.

Put bigger intake valves in, clean the heads up a touch, purchase some good exhaust and get your ecm's done.

Pat McPherson

The AZ Speed stuff I have did not come off of a "Blue Engine" so it does not seem to be worth as much...

The only real hassle will be pulling my back seat out to gain better access to my engine. Working on the engine is part of my boating hobby, but I don't want to create work for nothing...

When I rebuilt the engine, I did have heads bowl blended but I did not increase the intake valve size. Do the HP500s have larger valves?
I do run Stainless Marine exhaust...

I'm just wondering if you fellows think that the AZ Speed intake will help make more power on my engine with the stock cam?
Or should I wait to make the swap when I change to the HP500 cam and valve train?

800XCR

500's have 2.25 Intake valves. AZ's intake is the same as Mercs.

I personally believe that once done, you wouldn't do it again. I say no gain or benefit with stock cam and heads.

Linster

I agree with 800, You will need to do more internal to realize any gain.

I would put in the 500 EFI Cam. 230/236
Install Crane lifters in place of the GM.
Install some Inconnel Exhaust Valves
Install SS Severe Duty Intake Valves
Comp 929 Springs
Mill The heads .020
Install Cometic .027 Gaskets.
This will Bring the compression up to 9.3
Have your ECU programmed to a 500EFI
Should Make 535HP

100 HP Gain.

Strip has some cams.

Pat McPherson

Thanks Guys,
You are right, I just needed to here it from someone else I guess. The engine is just running too good to go pulling it apart again just yet.
I did buy the cam and some other goodies from Strip...

MAG502NUM

Pat- Volvo used the ASM intatke on their 502 420hp for several years before the 496's. That would be the same long block (cam & all) as the 502 mag which was maybe a little overated @ 415 hp. Inquiring with Cobalt boats a few years back, I was told that the Volvo was a tick or two faster and was better out of the hole with the DP outdrive! I think the DP outdrive was probably responsible for the holeshot, but what if the Volvo really did make its rated 420hp while the Merc only made 400hp as many have claimed?...Just somthing to ponder.
More evidence can be found in the old Gary Knox articles "502 Buildup" (HotBoat magazine appx. 1998) Where he dynoed a built 502 MAG mpi with a modified (shortened runner) stock intake, then switched it with ASM's then new manifold and made an extra 10-15 hp with no other changes.
Bottom line IMO, is that the smaller plenum ASM manifold does hold an advantage over the stock mpi manifold....I've always wondered why as the runners are "amost" the same length, shape,& size. The ASM runner does have slightly more area at the port.
If you have the ASM intake, I say use it.
Good Topic, Chuck

800XCR

Mag...I would agree if the engine was apart. To do all the work it takes to get there. 10-15 hp on a boat that runs 70 mph means notta.

Pat McPherson

Thanks for the reply Chuck!
I do rember reading an artical about the stock 502MPI Merc only making 396 or there about...
If the AZ Speed is worth 20HP then I would probably make the swap...