Big Block Question

tinman565

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I'm hoping Don will take the reigns on this question..but I'd appreciate any info from all you guys. I have a Merc 454 in my boat now (26' Chris-Craft limited). I just purchased a 1991 502 Mag from a good friend of mine, that is in perfect shape, which I plan on doing a few horsepower upgrades to (cam, intake, carb). I could use some help with stock horespower numbers for this engine..and any other pertinent info about it (good,bad things). I know you guys are all gonna roll your eyes when I tell you this..but I'm gonna make 500 hp with it..stick it in my boat..and be the happiest guy around...lol. What do you think ?
 

Bondo

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Re: Big Block Question

Are these motors Carbed,..??

The 7.4s were the 330hp motors, with the 4bbl. Merc/ Weber...
 

tinman565

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Re: Big Block Question

This 502 Mag has a Q-jet on it. I was just curious as to what kinda power it makes now. It was in a 25' Scarab. I think the Bravo-1's were rated to 500 hp, so I think I'll be fine. I may not make exactly 500..maybe 480 or so. Gonna call a few cam companies, and get their advice as to an upgrade. I'm also not gonna drive it like a tard on M&M's and Pepsi...lol.
 

Don S

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Re: Big Block Question

Be careful with your cam choices, or you may have the horsepower on paper, but not in a boat. It could also suck water back in and kill the engine with the wrong type cam.
These are boats, not street rods. Completely different power band.
The '91, 502 Mags were rated at 400 hp.
 

tinman565

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Re: Big Block Question

Thanks Don..your a gentleman and a scholar. I would LOVE to be careful on my cam choice. Is there a reputable company (boat oriented) that you'd suggest I call ? I'd like to talk to the most knowledgable boat motor power builders around. C'mon sir...point the light in the right direction for me.
 

Faztbullet

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Re: Big Block Question

This is the people I use for Marine HP parts and link to web site.
http://www.cpperformance.com/products_comp_cams.aspx

Chevrolet 396-502 CID
XTREME MARINE Gen 4&5
Note: " CL" Indicates Cam & Lifter Kit Part Number
Hydraulic - biggest for inboard/outboard, good for ski, economy and performance, 1000-5000 RPM 295-112323 295-112323CL 262 268 218 224 .505 .515 112

Hydraulic - good for inboard/outboard, pleasure use, 1600-5400 RPM Gen 4&5 295-114458 295-114458K 270 276 218 224 .510 .510 112?
Gen 6 295-014458 295-014458K

Hydraulic Roller - perfect for a customer seeking HP 500 style performance with a silent choice style exhaust system. Gen 4&5 295-111008 295-111008K 284 284 224 224 .566 .566 112?
Gen 6 295-011008 295-011008K
 

Bondo

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Re: Big Block Question

Is there a reputable company (boat oriented) that you'd suggest I call ? I'd like to talk to the most knowledgable boat motor power builders around.

Ayuh,...

Are you boating in Sweetwater, or Saltwater,..??

If you're boating in Sweetwater,...
Swapping on a Hi-rise dual-plane Intake,+ Carb will probably give you nearly the hp you're lookin' for, without openning up the motor...
An Edlebrock Performer RPM Intake, with the Edlebrock Performer 750cfm Carb would be a Sweet Choice...

For More Go Fast Ideas,...
You'll do Better if you search out some Hot Rod Boating forums...
 

tinman565

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Re: Big Block Question

I'm boating in "freshwater" sir (Lake Ontario)..although it aint always to fresh...lol. I appreciate all the help from yooooze guys. My boat went 58 mph on the GPS..and I'll be happy if I can get it to run 65 mph. Any other suggestions ? Props maybe ? Thanks.
 

Bondo

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Re: Big Block Question

and I'll be happy if I can get it to run 65 mph.

Ayuh,....

An 8 to 10mph Increase is going to be Very Expensive....

Breaking 60mph is Realistic....

To see 65mph, or more,... You'll need nearly Double the Horsepower....

I'm boating in "freshwater" sir (Lake Ontario)..although it aint always to fresh...lol.

I'm up here at the Outlet,...
The water don't get much Sweeter than the Great Lakes...
 

tinman565

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Re: Big Block Question

Well, in this case sir..I hope your wrong (kidding)...lol. The motor in my boat now is rated at 330 hp...and it went 58 on GPS. If we can make (and it should be easy) 500 hp with this new 502 mag, that would be a 170 hp increase with no weight added. I think I can hit the 65 mph mark, or be reeeeeal close. The wife may hafta stay home for the speed run, and I'll only put a gallon of gas in the tank, but I'll get that number sir...lol.
 

Bondo

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Re: Big Block Question

Well, in this case sir..I hope your wrong

Ayuh,.... It wouldn't be the 1st time,....

Btw,...
What's with this Sir, Crap,....
We're All here on the Same Level....;)

2 things,...
1),.. You didn't say the 502 was a Magnum,... It already has a respectable Cam,+ the Hi-rise Intake.... Probably in the 440hp range...
2),.. When you get to the point this boat is at,...
The General rule of thumb is,...
It takes an Additional 100hp, for each 1mph increase in WOT speeds....
 

Don S

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Re: Big Block Question

Like I said earlier, the '91 502 Mag is rated from Merc at 400 Hp.
I wonder if the Bravo I gear case design will work at 65 mph without causing cavitation of any kind of prop you try. It's just not shaped right for high speed.
After 60 mph, every mph is a LOT harder to come by.
 

tinman565

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Re: Big Block Question

Well, I'm gonna try to hit that number. I cant say in words how much I really appreciate you guys helping me out. I can make big power with a Pontiac..but a boat is a whole new world for me, but I'm learnin !!!

By the way...the "sir" thing is just a sign of respect. I call everbody sir, so dont take it like you guys are special or something..HAHAHAHAHAHAHAHAHAHA !!!!!!!!!!

Thanks for your help. ;)
 

QC

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Re: Big Block Question

I agree 65 will take some ponies, but there are a lot of 70, 80 and 90 MPH Bravo 1s where I boat . . . Mainly Cats, and a lot of twins etc. Most test results show 1s as slightly faster than Xs and XRs, but that's due to the difference in the gears, not the shape. There are also a lot of guys running 1s with 600+ bhp too. They won't live forever for sure, but they also can go a long time if you can control your right hand . . . There were also a lot of guys making money fixing Bravos, but not as much this year. Broken boats sit now . . . :(
 

Mischief Managed

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Re: Big Block Question

Drag is a function of the square of speed. Assuming equal prop efficiency, if your boat can go 58 MPH GPS with 330 HP, it will go 65 MPH GPS with 387 HP. 500 HP should yield 71 MPH GPS.

If 65 is your goal, and both engines are rated at the crank (and are in good shape), you don't need to do anything to the 502 to get 65. You'll need to increase the prop pitch a little though.
 

tinman565

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Re: Big Block Question

You may be on the right track with your calculations sir. The 502 Mag I bought was in my buddies 25' Scarab, and it went around 73 mph. My boat is heavier than his, so it automatically will go slower (and the hull design is a little different), so with the slight increase in power I plan to make with the 502, I believe I can hit the 65 mph mark. We will see in the spring. I look outside right now, and see nothing but snow...sigh....o well...gotta think positive. Thanks for the math lesson.
 

Mischief Managed

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Re: Big Block Question

It's just simple physics. The key to an accurate prediction is the validity of the data. If your engine is not really making 330 HP or 58 MPH was measured wrong (like while going downstream with a 5 MPH current...) the numbers will not work.

That said, I musta been tired last night because I made a mistake in my math.

To go 65 you need 414HP, not 387. 500HP will yield 71 though

The math:

(top speed x top speed)/HP = Z (58x58)/330 = 10.19
(desired top speed x desired top speed)/Z=required HP (65x65)/10.19= 414 HP
expected HP x Z = predicted top speed squared 500x10.19 = 5095
The square root of 5095 is 71.4
 
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