Griff

The newest cam will be fine and you should have no problems with reversion. You could even go bigger. I definately would.

My cam is a Crane 168731. Its .587/.610, 226/236, 112 lob sep and its in 454. Idles at 700rpms in gear and no problems with reversion. I would not go any smaller than this for 490ci.

klaw

when we went through my 500hps we used 850 cfm marine demons I am prsently at 565hp

bcarpman

I tried getting those numbers for the first cam and they didn't have them. I forgot to ask for the second cam. I'll ask when I talk to them again. Sorry

SB

Here's a good comparison of 3 Mercruiser hydraulic roller cams. One is for low hp 454's, next is for Magnum 454/502 motors, and third is for the 502 HP500 (carb) . Take a good look at the .004" , .006" seat #'s , .050" #'s, lift, etc,etc

If we just went by .050" #'s we'd probably make incorrect assumptions about all the rest of each cams spec's.

Also, since most manufacturers don't come out and tell you what the seat °was taken at (.004", .006" or what have you) imagine if you where told cam #1 had 280°,299° advertised duration and cam #2 had 288°, 288° seat duration. We'd be screwed !!! This is why all of us, when comparing cams, we need to know where the seat ° durations are taken at. Impossible to compare two cams without EXACT info.

Merc Std BB Roller
.004” 280,299
.006” 266*/284*
.050” 204*/209*
.200” 113*/114*
Lift .480”/. 483”
ICL 115.5*
ECL 120.5
LSA 118*

………………
Merc Mag Roller
.004” 300*/ 300*
.006” 288*/288*
.050” 225*/225*
.200” 125*/125*
Lift .483”/. 483”
ICL 109*
LSA 115.5*

……………………..
Merc HP500 Carb
Crane Gen VI#169611 Grind#HR-284-2S-10IG
Adv 284, 292 at .004” Lift with 1.7 .576 / .598
At .050” 222, 230
ICL 105, ECL 115
LSA 110

bcarpman

Ok, so maybe I'm back to this (168731) or
Merc 500EFI
Gen VI #169621 Grind#HR-292-2S-14 IG
292,298 at .004” Lift with 1.7 rocker .598” , .610”
230, 236 at .050”
ICL 109 , ECL 119
LSA 114

It appears to be similar except for a little more intake and lobe separation. Since I wouldn't mind sacrificing a little top end for more assurance of a better idle and no reversion, maybe this is my best bet. Of course that's where I started in the first place. With the HP500EFI cam, just not the specs I thought that cam had.

thanks again everyone

SB

Similar - Let's look closer.

Hp500.....731.....500EFI
#19611...#168731...#169621

Note: figures below include cams ground or installed 5°advanced

IVO...33BTDC....33BTDC.....33BTDC
IVC...71ABDC....75ABDC.....79ABDC
EVO...77BBDC....82 BBDC....84BBDC
EVC...35ATDC....36ATDC....34ATDC

68°......69°.....67° overlap measured from .004"

105°....107°....109° intake centerline
115°....117°....119° exhaust centerline

284°.....288°....292° intake seat duration at .004"
292°.....298°....298° exhaust seat duration at .004"

222°.....226° ...230° intake duration at .050"
230°.....236° ...236° exhaust duration at .050"

SB

In case one hasn't logged in I edited info to include the 731 as was being compared. Sorry. So please log in to see the 3 cams side by side with each other. Kind of interesting anyway when looking at them this way.

SkiDoc

Bc, Talk to Tyler about Quickfuel Carbs. They know how to build them. They engine test each one. They will be cheaper than a stock Holley or Demon. Mine on my previous boat felt like fuel injection. Tyler can order one set up perfect for your app.

bcarpman

Thanks for the direct comparison! My educated guess would still be that the 731 and 500EFI would be pretty similar. They're pretty close except for intake duration (+4 deg) and overlap (+2 deg)

Actual power output would probably depend on the In/Exh flow balance of the heads, but the 500EFI would probably also idle a tad better and make a little more torque down low. The 731 might pull it at the top end if the heads were intake dominant.

Then again that might just be mental M@$terbation given that who really knows what a cam will do in an engine.

bcarpman

Thanks, I'll do that. Up until know I had settled on a 850 Barry Grant, but I'm still open to suggestions, especially if Tyler feels more comfortable tuning them.