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smokediver
04-24-2006, 06:06 PM
I had a laser2 prop that I was running on the boat and wasn't quite getting the rpm's i wanted . 27 pitch 1.62 drive ratio 4600 rpm . I had a 25 that went right into the rev. limiter after the intake / carb / offshore risers instal.
Anyway , i ran up on a deal , 27 laser2 labbed by mazco propeller . between 4900 and 5000 rpm 67 mph. i think that works out to around 12% slip which isn't too bad on our hulls . the midrange drivability really improved and I think I finally got it all dialed in . about 1200 bucks for close to 5 miles an hour . My cruise speed at 3800 rpm is right at 50 mph .

CrackerJack
04-24-2006, 06:18 PM
Sounds good, what drive ratio did you have before the repower? What HP are you running now? Where did you get the lower drive ratio? We will probably go
thru some of the same issues.

smokediver
04-24-2006, 06:36 PM
I had a 1.81 drive ratio w/ the V-6 . I kinda wish I would have went with the 1.47 ( it may be 1.5 as I think merc. changed things around a little bit) I bought the new upper for 695 bucks at a place in fort myers . 2 year warranty even came with zincs ! I can dig up the name if you want it . good place to deal with . horsepower is unknown but probably around 300 . vortec 5.7 bored .040 over 9.5:1 compression , edelbrock intake/ 600 cfm carb . the gear change is worth about 2 inches in pitch between each gear set . can't wait to see your new setup ! I may try a prop with a little more diameter and a little less pitch to try and bring down the slip ratio a little bit .

CrackerJack
04-24-2006, 06:52 PM
We started off with a 25 pitch mirage plus and don't seem to have any problem getting it to 4000. Haven't gone over that yet until the next time out.
If we can spin that to 5k plus, will try a 27 pitch mirage. I believe the ratio you have is probably the best and we would probably go with the same. The HP on ours is unknown. Does the guy you purchased the upper from take trades on the old unit?

mrfixxall
04-24-2006, 11:08 PM
I had a 1.81 drive ratio w/ the V-6 . I kinda wish I would have went with the 1.47 ( it may be 1.5 as I think merc. changed things around a little bit) I bought the new upper for 695 bucks at a place in fort myers . 2 year warranty even came with zincs ! I can dig up the name if you want it . good place to deal with . horsepower is unknown but probably around 300 . vortec 5.7 bored .040 over 9.5:1 compression , edelbrock intake/ 600 cfm carb . the gear change is worth about 2 inches in pitch between each gear set . can't wait to see your new setup ! I may try a prop with a little more diameter and a little less pitch to try and bring down the slip ratio a little bit .


I bet you'll get alot better performance with a 750 cfm vr a 600 cfm and better fuel milage...

smokediver
04-25-2006, 05:00 AM
jack , the 695.00 is with a trade-in. I looked at keeping my case and doing a gear change but this was a simple way to go ... and with a warranty . the upper was like brand spanking new , yoke , u-joints , everything . be careful of the 27 mirage ... i think it is a lot different in that paticular pitch ..I thought 600 cfm was a good choice for a 350 ? am I wrong ?

The Hedgehog
04-25-2006, 06:10 AM
Throttle-up told me that the 27" Mirage Plus had some real squirrely tendancies on some boats. It sort of made me want to try it but I was getting so much slip on a 25" with my heavy boat that it seemed pointless.

I would be interested in hearing some stories about the 27" Mirage. It is supposed to be an entirely different prop.

Sam
04-25-2006, 09:35 AM
mrfixall, can you elaborate a bit on your carb comments, just curious.

THX
Sam

mikev
04-25-2006, 10:58 AM
Calculating exact engine air-flow: You can easily calculate the actual engine air-flow directly. For normally-aspirated engines (ie. not turbo- or super-charged):

CID * RPM / 3456 * 0.85 = CFM required
CID is engine size, in cubic inches (CID = cubic centimeters (cc) * 0.06102), RPM is maximum engine speed. 0.85 is a close approximation of Volumetric Efficiency.

For turbocharged or supercharged engines:

CID * RPM / 3456 * %boost + 1.00 = CFM required
Normal inlet air pressure is 14.7PSI; supercharging merely increases inlet pressure. For example, 6PSI boost means 20.7PSI pressure, or 140% boost.

so if you take 350CID*5000rpm / 3456 * 0.85= 430CFM even at 100% volumetric efficiency =506 cfm so a 600 should be fine. if you went with a 750 CFM carb you would loose alot of your low end power and gain nothing on the top end.

Sam
04-25-2006, 11:55 AM
Say what ? :boggled: :boggled: :boggled: :boggled: :boggled: :D

CrackerJack
04-25-2006, 03:05 PM
More or bigger isn't always better.
Thanks for the real technical answer.

blackhawk
04-25-2006, 03:16 PM
Throttle-up told me that the 27" Mirage Plus had some real squirrely tendancies on some boats. It sort of made me want to try it but I was getting so much slip on a 25" with my heavy boat that it seemed pointless.
I would be interested in hearing some stories about the 27" Mirage. It is supposed to be an entirely different prop.

I heard that too but it makes no sense at all on paper. The Mirage series gets smaller as the pitch gets larger. Every size larger in pitch gets .125" smaller in diameter(except going from the 27 to the 29) and the 27 follows that pattern. :confused:

Sam
04-26-2006, 08:13 AM
Jamie, your comments are very timley. I picked up a vacum gauge late last fall and plan to use it very soon. I'll post the results in a week or two :biggrin: .

mikev
04-26-2006, 12:26 PM
Thanks for the info rootsy I pulled the info I posted off of one of the carb companys sites several years ago but did not think of the difference in the load between a car and a boat.

Rob
04-27-2006, 07:07 PM
That's why this site is so helpful. My hotrod roots are land based and I would have pulled out the same calculation that MikeV quoted and said that 600cfm for 350ci is more than enough. However Rootsy's vaccum approach and analysis of the difference between applications makes a lot of sense.

Anyone know what CFM theyrun on non-restricted NASCAR tracks? I would think the aerodynamic load with stock cars running at 180-190 MPH has to be significant.