NordicVenture

First timer to the forum but have been impressed with the content so I joined. My questions are in reference to my '93 Nordic Venture (thus the original user name) . The boat is powered by a Gen V 502 Mag w/ 200 hours. The boat runs a solid 75 mph (by the speedo) in good conditions. I have gotten the bug to perform some slight upgrades to hopefully hit the 80 mph mark. To date the only upgrade has been a set of EMI Thunder manifolds. Eventually I would like to upgrade to a roller valvetrain and aftermarket aluminum heads. The motor has been well maintained and for now i am only looking for moderate upgrades that will integrate with my future plans. My current plan is to upgrade the Intake and Carb. Does anyone have any experience/input with the Edelbrock RPM Airgap and the Barry Grant Carb? I know there are better intakes out there but I'm looking for reliability and keeping everything tucked under the hatch. My hopes are that these upgrades might help me upgrade to a 23p Mirage Plus from the current 21p. Anybody have any thoughts/ideas? Sorry for the long winded post I have to learn to get to the point. Thanks in advance for the input.

Griff

I hate to burst your bubble, but there is no way you are close to 75mph with a 21" prop. Even if you have a 1.36 gear drive with 10% prop slip at 5200rpms you're going 68mph. These numbers are even very generous. Chances are you've got 1.5 gears and 14% slip which would put you right at 60mph.

As for adding HP, you need to build an entire package at once to see any gains. The stock magnum intake is pretty good. You might see a 15hp gain with the air gap and new carb. That = 1mph.

vandeano

I would have to agree with Griff on that. I have the same engine you do GEN V 502. I put a cam and Revolution exhaust and convert it to a roller motor. And I can barely turn a 25p mirage plus at 4600 rpm. But it is also is a 27' Carrera. I can get to about 63-65 mph (GPS) When I had a 21p at 5000 rpm it did 60mph, but major slippage. If you could get a hold of the Performance book by Dennis Moore, its a good read book on 502's. He actually recommends the stock manifold over anything else. He basically says mercruiser did there homework on that.

NordicVenture

Hey guys thank for the responses. Don't worry about bursting my bubble, I have heard alot of people 'overestimate' their boats speed. While I haven't verified the speed w/ GPS I have run against a buddy with a Magic Cat 525 SCI that GPS'd @ 80 and he barely pulls me. This boat is also only 23' which is a big contibutor to it's performance. If I heard the same story I'd have to question it's legitamacy too. Any opinions on the carb selection over the stock weber? Thanks for the info on the Dennis Moore book I'll have to find a copy.

Griff

For what you are doing, a standard marine Holley 800cfm will work fine.

vandeano

You really need to get a hold of that book by Dennis Moore. It will explain in great detail on the stock 502. It mentions all of the 3 or 4 Carbs that came with the 502 and basically says if your running it stock changing the stock carb could rob you of performance plus bad fuel economy. It really changed my mind on wasting money on changing little things here and there. "If ain't broke don't fix it". Mercruiser did a find job putting all components to work together. But I can tell you have the itch to spend money on something..... :

Like Griff said do everything together. Find out on the forum from someone that has already proven what works together. It will save you lots of headaches.
There are many guys on here that have there sh*t together when it comes to building HP. Unfortunately Im not the one....Just read and read on all the past and present posts on here and you'll learn plenty. You'll even find yourself not being able to sleep after reading some posts, because you'll be thinking "ohh sh*t what was I thinking when I did that".........sweet dreams

CayFomula

I agree with the others it is very difficult to pickup 5mph by making small performance enhancements on the motor.

If you are only looking for say 5mph your best bet if you don't already have it would probably be a labbed prop and either a shorty lower unit or a 3" lift extension box. Typically you can get 2-4mph from a labbed prop and about the same or more from a shorty lower or a raised extension box.

On the exhaust I noticed in the thread on the Dana's some of the guys to their surprise picked up a few mph but you already added the EMI.

NordicVenture

Well, I had a chance to run the boat w/ a GPS and needless to say Griff my bubble was burst a bit. I also found out that you can't always take people's input @ face value (including friend's w/ supposed GPS results on their own boat). So, with my tail between my legs I have to revise my previous performance results. I found that my speedo is about 9-10 mph fast, meaning that my 75 mph boat is really 65.6 mph. I guess the good news is that I think I'm up against the rev limiter (5250 rpm) assuming that the tach is more accurate than the speedo. Any input on the rev limit for the 502 or an easy way to check the tach accuracy?

PatriYacht

It's easy to get a 150 hp gain out of a 502 mag. It has a good bottom end and good heads. Cam, carb and intake will do it. However, you have to look at an engine as a system. The new engine will require an upgraded fuel system to supply the new horses, the stock exhaust will revert water so that will have to be modified and the cooling system will have to be modified to handle the extra heat produced. It can work out well if you do it right. A lot of us here have done it. Do more research. Search some threads here.

sandcraft

Hello fellow Nordic guy! I would like to see some pics of that boat. Im new to boats but not to motors. Im still learning about props/slippage/pitch and everything else concerning getting the power from the engine to the water. Mine is a 32' offshore, (twins) so my advice concerning power would cost you half as much as what its going to cost me. I still have to buy a pair of stock 496 mags as mine came with big inch high horse motors.

Freshen up that 502 to original specs, prep it for storage and set it on an engine cradle in the corner. Or freshen it later when you have time. (I would'nt use a gen V as my base) If you have the ability or know someone to help, build at least a 572 ci or bigger engine. (stay away from the GM crate engines unless you blueprint it yourself or have someone do it) I'm going through World Products for my hardware. A dependable 600hp plus engine can be built for less than you think. The classified section in this site is full of them. if you decide to sell, it can be sold with a fresh repower (the 502) and you'll still have the nice hardware you built.

Vandeano and griff are spot on in regards to not doing "a little thing here and a little thing there" approach. IE...Change the cam figure out all the prop stuff for that change etc,etc...You'll wind up with more headaches more down time and possibly more spent. After the engine is built and dynoed (in my opinion the easy part) start figuring out the best way to get that power into the water. Thats where I would have to leave you as Im still pretty clueless on that point. This forum has been the single most effective asset I have found since becoming a new boater and theres lots of cool people to talk to. Good luck and dont forget the pics....Casey