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Any 253 Convincer with 454mag experts wanta led a helping hand
- Thread starter SED
- Start date
here it goes...97...253 Convincer...454 mag...bravo 1..with a nose cone...c-mate says to run a 23 mirage plus and spin the motor at 4900...rev limiter is set at 5080 from the factory...ok...i put on a 23 mirage plus and am in the rev limiter and have barely even started to trim..so i go to a 25 mirage plus and great ...out of the rev limiter but am only turning 4100 rpms..not good....so i put on my bravo 1 24/4blade and am now turning 4500 rpms..and am only getting 56.4 mph on GPS...Something is not right here...According to c-mate this boat should run 65-68 mph...what are everones suggestions on what to do now to get RPM's and MPH up to where they should be....Again..454 mag(110 hrs)...bravo 1 with imco nose cone and all stock...Please help
mkhammer
Well-known member
I would suggest a bravo1 22. it has less slippage than a 3-blade if the bravo1 22" hits the rev limit you can have it lab finished and that will bring your r's down and bring top speed up. personally I would never use a 3-blade prop on a 253 or larger convincor. a four blade thats dialed in will excelarate quicker,better mid & top end your boat will handle and track better. the reason you may be in the rev limiter with the 23 before you trim is that prop is slipping.
vinny p.
Active member
I have a '99 259 with a mostly stock 454 magnum. I spin a labbed 26" Bravo 1 to 4900 r.p.m. and about 67 mph on GPS with 1.50 gears in the drive. The only thing you have that I don't is a nose cone. I can't be sure whether that will play a role in your problems but, Imco claims that they don't help you until you hit about 80 mph.
Dave's ZT260
Member
800rpm loss from a 23 to 25 is not right. Maybe one of these props is marked wrong or not identical models. I would have them checked. Nose cones are made for selling for most boat owners. If you can't hit 90mph don't waste your time. It will slow you down and make the boat less stable. What were the gps numbers for the mirage plus 23 and 25? Your slip is very high, so I would think you might need a spacer.
Dave's ZT260
Member
A drive spacer, it lowers the drive height. I would start with 1 inch. I reduced my slip 5% on my old setup.
mkhammer
Well-known member
SED
It sounds like you have a 1.36 gear ratio thats what I had in my old boat when I had simular problems there is no way you can spin a 25-3 blade 5000 rpm. with your power. The way your engine is running sounds right to me if you have a 1.36 its a taller gear and puts you right where your at with your power and with the props you have tried. A nose cone won't bennifit you at your speed. I'm putting a Imco lower case on my boat soon I just got back from the lake today water was glass I hit 92.6 mph on a hand held gps. I personally think a nose cone will not hurt you either. If you have a 1.36 ratio and you are sure your engine is running correct (witch it sounds like it is) get a prop that works. Or buy some headers they will give you the most h.p. gain for a bolt on item then try your bravo1 24" again.
It sounds like you have a 1.36 gear ratio thats what I had in my old boat when I had simular problems there is no way you can spin a 25-3 blade 5000 rpm. with your power. The way your engine is running sounds right to me if you have a 1.36 its a taller gear and puts you right where your at with your power and with the props you have tried. A nose cone won't bennifit you at your speed. I'm putting a Imco lower case on my boat soon I just got back from the lake today water was glass I hit 92.6 mph on a hand held gps. I personally think a nose cone will not hurt you either. If you have a 1.36 ratio and you are sure your engine is running correct (witch it sounds like it is) get a prop that works. Or buy some headers they will give you the most h.p. gain for a bolt on item then try your bravo1 24" again.
Dave's ZT260
Member
SED-
No way you’re running a 1.36, maybe a 1.65 but most likely a 1.5 ratio. I can also guarantee speed loss on your setup with a nose cone. I have tested the idea several times. Now we are not talking about 10mph, but your losing 2-3mph for sure. That being said, maybe you could swap your lower with someone needing a nose cone. If not no big deal- you have bigger issues to diagnose.
Check that ratio it should be on the upper next to the serial #'s.
Dwayne-
What prop you running?
No way you’re running a 1.36, maybe a 1.65 but most likely a 1.5 ratio. I can also guarantee speed loss on your setup with a nose cone. I have tested the idea several times. Now we are not talking about 10mph, but your losing 2-3mph for sure. That being said, maybe you could swap your lower with someone needing a nose cone. If not no big deal- you have bigger issues to diagnose.
Check that ratio it should be on the upper next to the serial #'s.
Dwayne-
What prop you running?
mkhammer
Well-known member
Dave ZT 26 I can tell you first hand I went trough the same thing with a 25' larson. it came with a 502 mag carb rated @ 390 h.p. and came stock w/ a 1.36 gear ratio and a 23 mirage. it blew that prop out out of the hole untill I got on plane and then when I nailed it it blew it out when I turned it blew it out. that prop flat didn't work on that boat. it also was a 4000 lb. boat it even had a stepped bottom with a delta pad. I tried a 25 mirage and it wouldn't spin it past 4600 rpm.I finally put headers on it and bought a hydramotive quad4 24 then it ran 65mph so beleave it. Sed is dealing with less h.p. than I had and I don't know what ratio he has. but if he has a 1.50 ratio I would say he's got a differant problem than a nose cone. I asked hydramotive about thier nose cones, they told me it wouldn't hurt my performance but it would't help either so I didn't put one on. SED has to tell us his ratio other wise we are all guessing
miller time
Member
hey sed still need help? i have a 95 253 convincor with an inj 454 385 hp with less than 200hrs.the boat came w/25marige prop i could not give the boat any trim or accelerate hard with out blowing the prop.i switched to a marige 24 4 blade added a stainless marine ex system and went 64mph(gps)with me and a full tank of fuel at 4600-4700rpm
Dave's ZT260
Member
<BLOCKQUOTE class="ip-ubbcode-quote"><font size="-1">quote:</font><HR>Originally posted by Dwayne:
I can get 62 out of the 23 mirage at 4600 and if I use the cabellas 4 blade 24 I get 64 or 65 at 4800 to 4900 and that is with half trim and the props blow out quickly if I turn and not much of a turn and the cabellas gets some chine walk at wot. <HR></BLOCKQUOTE>
I think you maybe off on the numbers here. If not, the mirage is running 7% slip and the Cabellas is at 12%. If the mirage is truly at 7% (unlikely, so you need to check again) then have it labbed by Houston prop and you will be doing great. A bravo 1 22" would also be good to try. What’s the diameter of the Cabellas 4 blade? Also whats your drive ratio.
I can get 62 out of the 23 mirage at 4600 and if I use the cabellas 4 blade 24 I get 64 or 65 at 4800 to 4900 and that is with half trim and the props blow out quickly if I turn and not much of a turn and the cabellas gets some chine walk at wot. <HR></BLOCKQUOTE>
I think you maybe off on the numbers here. If not, the mirage is running 7% slip and the Cabellas is at 12%. If the mirage is truly at 7% (unlikely, so you need to check again) then have it labbed by Houston prop and you will be doing great. A bravo 1 22" would also be good to try. What’s the diameter of the Cabellas 4 blade? Also whats your drive ratio.
mkhammer
Well-known member
As published in hot boat July 2003.
prop slippage formula. PROP PITCH X RPM X
.000947 DIVIDED BY GEAR RATIO MINUS SLIPPAGE = MPH. Turning a 22 4 blade 5000 rpm at 10% slippage would put you right about 63 mph. you can't tell your real slippage untill you try the setup against g.p.s speed as with any setup. This is the formula used as a starting point results may very but not much.
prop slippage formula. PROP PITCH X RPM X
.000947 DIVIDED BY GEAR RATIO MINUS SLIPPAGE = MPH. Turning a 22 4 blade 5000 rpm at 10% slippage would put you right about 63 mph. you can't tell your real slippage untill you try the setup against g.p.s speed as with any setup. This is the formula used as a starting point results may very but not much.