Converting to carburetor from mpi 350 mag

[ROY WILLIAMS]

Ok guys after going back and forth and talking to few pros. Even one who is in the ECM business it seemed the most agree that to this … if you’re having problem with your ECM, EFI etc it is cheaper to go with the carb, the gain is far less headaches far easier and cheaper to fix and potential to gain few horse power… one of pros told me that NESCAR when they had to switch to ECM or EFI. They weren’t pleased they lost 100 hp and after long struggle they barely gained it back the down side is that the carb you have to warm the engine up before taking off you lose efficiency on fuel and boat value in case you’re selling … if the ECM is fixable and you’re not having much problems is better to keep it.
So in my situation it made more sense to switch to carb especially since they don’t make the ECM 16236999 MEFI3 anymore much cheaper far less headaches I can maintain it myself without any special tools and digital gadgets.
I’m going with 800cfm carb I was told this might be a little too much if I’m running low RPM but I will gain up to 60 hp pet engine if I everything is set right.
Your thoughts is well appreciated
Thank you very much.

the electrical issues have a lot of damage ! lightning ..

 

[jimmbo]

You should always have the engine up to Operating Temp before "Taking Off", One of the best ways to have an engine wear out soon, is to slam the throttles on a cold/warming up engine.

 

[Scott Danforth]

I run a carb without a choke and only wait about 30 seconds from startup at the dock to backing into the channel and mashing the throttle

 

[jimmbo]

Not the wisest thing to do to the engine, but ...

 

[Scott06]

Your thoughts is well appreciated
Thank you very much.

My thoughts- not sure who is giving you this advice of 100 hp gains and 60 hp gains doesn't seem to now what rpm a marine carb runs at. Seems like salesmanship more so than sound mechanical advice...

What 800 cfm carb is recommended - Jimbo has a 650 on his 5.7 that would essentially be the same as yours (long block).

Are you doing this conversion or do you have a mechanic who is on board to do it?

 

[Scott Danforth]

Agreed. NASCAR runs 830 CFM annual discharge Holley's. However their motors are spinning 9000 RPM

I'm running a HP830 on my 468 that sees 5500

For a stock 350, 377, or 383, 650-715 CFM would be the carb size recommendation

 

[Lou C]

I don't see a carb adding any significant amount of power vs EFI under most circumstances....
As far as the warm up it depends partly on your climate, here in the North East our summer temps don't require that much warm up time (but my raw water cooled I/O with the 160 stat always takes longer to warm up than either my Jeep inline 6 or V8 Hemi) but in the fall you need a choke for starting, and it will not run that smoothly till the temp gauge is at least at about 120* or so. To me that's the main advantage of EFI, faster and better adaptation to air temp and other operating conditions, especially in cold climates. In a warm climate like Fla, no real advantage and a lot of cost advantages of a carb. In cooler climates, if you boat early and late in the season, EFI does have some practical advantages. To me though for those with older boats the parts cost and repair complexity are issues, as is NLA parts....

 

[stresspoint]

i have 500 hp 5.7 that spins to 6700 rpm and only run a 600 cfm Holley performance carb ,
i could be wrong but IMO just a carb conversion will never give 100 hp to a motor , i think the "mechanic " is telling you lies.
in saying that ,a carb conversion from EFI may give a little more , like maybe a hardly noticeable 10 or 15 because of the ECU playing it safe with fuel mixtures ,
a 600 cfm square bore Holley "or" as i have recently found thanks to Lou a quadrajet is a really good option.

 

[Scott Danforth]

Agreed, Carb vs EFI both equal in power on a given engine architecture.

Carburetor vs. Fuel Injection. We Put Both to the Test on an LS Engine

We do back-to-back testing on a carburetor and fuel injection to see which one performs best on a 6.0L LS engine.

www.motortrend.com

EFI shines with variable valve timing and in high boost applications

 

[jimmbo]

A 600 is overkill for a 5.7 going 5000 rpm, as it is using less than 430 CFM.
When I have the 2bbl 500cfm Holley on my 5.7, the engine has about 4.5-5 inches of Manifold Vacuum at WOT(4800 rpm), which indicated that the 2bbl(which when tested like a 4bbl, flows about 375 CFM) was a bit undersized. When I put the Holley 650 spreadbore on, I gained about 550 rpm, and had to reprop, to bring the rpms down to 5000. In this configuration, the Manifold Vacuum was at 1.5", the Secondaries were far from fully open. Even though the engine was making more HP at the top end, I lost Holeshot because of the higher pitched prop. The tiny Primaries on the Spreadbore, may have helped in reducing the loss of Hole Shot.

 

[Lou C]

For engines the size of a 5.0-5.7 spread bore carbs are a good compromise, better than a 2bbl & better than a square bore 4bbl. Good low speed performance from the small primaries but still has the kick from the secondaries that marine engines need...
GM engineers knew what they were doing when they designed the Quadrajet way back in the mid 60s...

 

[jimmbo]

The Q-jet is probably the most sophisticated Carb ever designed. It certainly could make Power, and also could deliver good Economy and Low Emissions. It was produced in 2 sizes, 750 CFM and some made for certain Buick 455's were 800 CFM, the difference being the size of the Primary Venturi

 

[ROY WILLIAMS]

Hello All,
Ok short and sweet:
I have a Larson Cabrio 330 year 2000
Engines are 350 mag mpi vortec serial number L365398
Struck by a lightening fried both ECMs
Sent over to test them the tech said they’re fried you can’t refurbish them, he has a refurbished ones each for $2500 plus tax plus shipping plus labor and I’m sure the bill will go way beyond that…ouch
So I’m considering converting to carburetors.
Please any advise/info on a kit to convert them where do I go to or order them from online sites what should I buy or look for or cheaper mefi 3 your thoughts/advise is well appreciated, God bless.

look up the ECM part on your 2000 motor .. ebay ?
I did not change the carbs a lot of $$ and a motor damage ...

 

[Ignatios]

At this point I’m not sure if I’m doing it or a mechanic, depends on how complicated it is.
My mistake I meant to say 60hp out of both engines or up to 30hp per engine but after asking around I find your info fellows to be more accurate so I stepped down to 650cfm .
Thank you

Agreed. NASCAR runs 830 CFM annual discharge Holley's. However their motors are spinning 9000 RPM

I'm running a HP830 on my 468 that sees 5500

For a stock 350, 377, or 383, 650-715 CFM would be the carb size recommendation

i wound up going with 650 thanks

 

[Ignatios]

I don't see a carb adding any significant amount of power vs EFI under most circumstances....
As far as the warm up it depends partly on your climate, here in the North East our summer temps don't require that much warm up time (but my raw water cooled I/O with the 160 stat always takes longer to warm up than either my Jeep inline 6 or V8 Hemi) but in the fall you need a choke for starting, and it will not run that smoothly till the temp gauge is at least at about 120* or so. To me that's the main advantage of EFI, faster and better adaptation to air temp and other operating conditions, especially in cold climates. In a warm climate like Fla, no real advantage and a lot of cost advantages of a carb. In cooler climates, if you boat early and late in the season, EFI does have some practical advantages. To me though for those with older boats the parts cost and repair complexity are issues, as is NLA parts....

Moved to Florida recently from CT so I’m good now thanks

 

[Ignatios]

i have 500 hp 5.7 that spins to 6700 rpm and only run a 600 cfm Holley performance carb ,
i could be wrong but IMO just a carb conversion will never give 100 hp to a motor , i think the "mechanic " is telling you lies.
in saying that ,a carb conversion from EFI may give a little more , like maybe a hardly noticeable 10 or 15 because of the ECU playing it safe with fuel mixtures ,
a 600 cfm square bore Holley "or" as i have recently found thanks to Lou a quadrajet is a really good option.

I ordered the Holley 650 4 bbl
They suggested up to 15 hp more
Thanks

 

[Ignatios]

Another challenging question to me anyhow:
What would I do for harness, distributor and coil pack cup style didn’t have to change, fuel electric pump stays except installing a low pressure regulator … but a harnes I still have to connect starter distributor coil pack oil pressure and temperature knock sensor(?) even the intermediate peep when I turn the ignatios on … should I leave the same harness but remove the sensors and ecm also the injectors wiring out and leave the rest or there is a harness that I can but for that?
Please advise thank you all and God bless.

 

[jimmbo]

I ordered the Holley 650 4 bbl
They suggested up to 15 hp more
Thanks

You did order the Marine Model?

 

[Ignatios]

You did order the Marine Model?

Yes I did

 

[Ignatios]

Jeez. Never heard of that happening. What was the circumstances and details, out of interest? Where was the boat etc ? Marina ? Was it running when it happened? You’d think earthing would avoid this.

Boat was on rack hooked to a land power.