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350 meg not performing up to par ?

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350 meg not performing up to par ?

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Old 07-10-2019, 03:33 PM
  #21
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He also stated above he is still using THRU PROP exhaust setup. That is the major restriction. I bet once you install the through-transom exhaust it will perk up quite a bit.
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Old 07-10-2019, 05:11 PM
  #22
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Originally Posted by oconnor marine View Post
nice boat. That set up is probably around 300 hp at the prop. To get that to 450 hp you would probably be better off supercharging it.
218/224 at .050" hyd roller / AFR 180 heads - 300hp ?
I work on a boat with near identical set up, 68mph with 21pitch cleaver stock SEI Alpha, no mufflers /65 with super restrictive mufflers / 73mph with Alpha SS with the mufflers. Police being tough so have not had chance to remove restrictive mufflers with this drive on.
Merc 260hp 350 with Brodix 180 IK's, 218/224 Comp HR / Weiand Air Strike / BG 650 vac sec Sea Demon / 20ft boat

Again, the other week boat wouldn't run over 50mph. Engine ran fine but seemed to lug planing and wouldn't reach top rpm and speed. Had same 21 cleaver it's run for years. Installed the other 21 Cleaver and Bamm !!!! Just over 65mph with SEI drive and super restrictive muffs...with a few people and mucho fuel. Never would have believed it but I was there for both runs. Prop looked fine but something happened obviously.
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Old 07-10-2019, 06:59 PM
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Originally Posted by SB View Post
218/224 at .050" hyd roller / AFR 180 heads - 300hp ?
I work on a boat with near identical set up, 68mph with 21pitch cleaver stock SEI Alpha, no mufflers /65 with super restrictive mufflers / 73mph with Alpha SS with the mufflers. Police being tough so have not had chance to remove restrictive mufflers with this drive on.
Merc 260hp 350 with Brodix 180 IK's, 218/224 Comp HR / Weiand Air Strike / BG 650 vac sec Sea Demon / 20ft boat

Again, the other week boat wouldn't run over 50mph. Engine ran fine but seemed to lug planing and wouldn't reach top rpm and speed. Had same 21 cleaver it's run for years. Installed the other 21 Cleaver and Bamm !!!! Just over 65mph with SEI drive and super restrictive muffs...with a few people and mucho fuel. Never would have believed it but I was there for both runs. Prop looked fine but something happened obviously.
SB, sounds like a lot of work to put the SS drive back on, you should just sell that old thing....
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Old 07-10-2019, 10:21 PM
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Takes 10-20 minutes depending on how old I feel.

Main reason for taking it off at times is lake level. My ramp is low sand/muck bottom with only few feet water level. The SS has low water pickups and easy to pack closed. I could use other ramps, but this one is 200yds behind the house.
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Old 07-11-2019, 06:14 AM
  #25
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Originally Posted by Trash View Post
As others have stated you are currently overpropped. With SBC head/cam/intake upgrades you are better off keeping the same prop and simply spinning it faster. You might go up a little in pitch but not a lot.

Your thru prop exhaust is not limiting the motor to 4300 rpm. Your prop is. Thru hull will allow your motor to breath better above 5000 rpm.

Don't waste your money on exhaust manifolds over what your have. Ryan, myself and others have spun strokers well north of 5000 rpm with GLM style manifolds.

Don't waste your time port matching. There will be zero difference.

A prop is a simplest fix. Go back to your original prop see how things go. You may not need a new prop.
Originally Posted by SB View Post
218/224 at .050" hyd roller / AFR 180 heads - 300hp ?
I work on a boat with near identical set up, 68mph with 21pitch cleaver stock SEI Alpha, no mufflers /65 with super restrictive mufflers / 73mph with Alpha SS with the mufflers. Police being tough so have not had chance to remove restrictive mufflers with this drive on.
Merc 260hp 350 with Brodix 180 IK's, 218/224 Comp HR / Weiand Air Strike / BG 650 vac sec Sea Demon / 20ft boat

Again, the other week boat wouldn't run over 50mph. Engine ran fine but seemed to lug planing and wouldn't reach top rpm and speed. Had same 21 cleaver it's run for years. Installed the other 21 Cleaver and Bamm !!!! Just over 65mph with SEI drive and super restrictive muffs...with a few people and mucho fuel. Never would have believed it but I was there for both runs. Prop looked fine but something happened obviously.
what horse power do you think?
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Old 07-11-2019, 07:33 AM
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run a 23 stainless on a lil 350 , no prob . on a cabin cruiser
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Old 07-11-2019, 08:01 AM
  #27
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Originally Posted by oconnor marine View Post
what horse power do you think?
With 9.5:1 compression those cams make 375-400hp fly hp and 305-320 rear wheel hp religiously with GM Vortec heads. So.....we will call it 375hp. Obviously the tune needs to be on target.
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Old 07-11-2019, 09:52 AM
  #28
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Originally Posted by SB View Post
With 9.5:1 compression those cams make 375-400hp fly hp and 305-320 rear wheel hp religiously with GM Vortec heads. So.....we will call it 375hp. Obviously the tune needs to be on target.
since 1988 output rated at prop. I could be off a year but if what you say is correct its 300 +or-. 10 % margin of error. I see on single engine boats like that you need 100 hp for 5 mph
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Old 07-11-2019, 10:47 AM
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Sorry. Your data is flawed.

Many 260hp 5.7 mags have been upgraded with just Vortec heads, RPM style intake, and Comp XM270HR or GM Hot Cam. Both have 218 .050" intake. 112LSA.

Most boats pick up 8-12mph. There are some pigs out there I guess, but not many people modify those boats.

going from 260 to "300+ or -10" won't do that. And 5mph for 100hp doesn't work either. With your data that means these boats picked up 200hp, but you say that these motors will only make 300hp + or - 10.

Furthermore, speaking of hp and small blocks, 450hp 383's have brought 56-58mph 260hp boats to 76-78. One even 80 !
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Old 07-11-2019, 11:09 AM
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Originally Posted by SB View Post
Sorry. Your data is flawed.

Many 260hp 5.7 mags have been upgraded with just Vortec heads, RPM style intake, and Comp XM270HR or GM Hot Cam. Both have 218 .050" intake. 112LSA.

Most boats pick up 8-12mph. There are some pigs out there I guess, but not many people modify those boats.

going from 260 to "300+ or -10" won't do that. And 5mph for 100hp doesn't work either. With your data that means these boats picked up 200hp, but you say that these motors will only make 300hp + or - 10.

Furthermore, speaking of hp and small blocks, 450hp 383's have brought 56-58mph 260hp boats to 76-78. One even 80 !
Lol good luck
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