OPINIONS WANTED: Carb vs. Fuel Injection
 

I have a `98 7.4 MPI (310hp). I am seriously considering swapping to a carburetor. Here`s my reasoning: I have added a good exhaust already and would like to add a cam and other items soon. I just feel like I would get more return from the mods if engine was carbureted. Plus it will be easier to tune and won`t have to mess with ECM. I want to take this a step at a time. My concern is that my gas mileage will go to crap. I am very satisfied with it right now. I would probably install an Edelbrock RPM dual-plane with a 750-850 cfm Holley.
I would like everyone`s opinion on this and relay any experiences they had.
Did fuel consumption increase?
What intake / carb would you use?

All opinions are greatly appreciated and respected.
Thanks!

Don't know if this helps but I went the other way
330hp carb to 502 mag efi

fuel consumption was similar and I believe a little better with the 502, maybe 3-5% if at the same speed, changed prop size to tho so more than one variable but a cruise speeds there was not a big difference,

My Opinion
 

Just my opinion, but Fuel Injection is great. But Carbuerated is easier. If I had the money I would use fuel injection. I know everytime I put a new cam in my boat I don't have to worry about getting the ECU remapped. It is probably the most efficient way, but for someone just tinkering around, Carb's are much easier.

Have you guys ever seen the fuel injection set up on a 7.4 MPI?
It looks very restrictive. I believe that is where my problem is, I am using all the CFM this system can flow and not making any more power. THe system on a 502 has got to be better, but you still have to recalibrate the ECM ($350) each time you change something. My throttle body is a 1 barrel!!!

If fuel consumption was similar, I could live with that. I would hate for there to be a noticable difference.

What`s the difference between your 330hp carb engine and my 310hp injected engine?

Someone once told me that I would gain 20hp by swapping to a carb.....

Quote:

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What`s the difference between your 330hp carb engine and my 310hp injected engine?

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none I think merc rated the 330hp 454 Mk IV was at the crank versus 310hp efi at the prop.

BTW I burn a lot more gas this year because altho the new engine gets similar milage at the same speed for some reason I drive faster more often this year, :D new toy syndrome I guess:cool:

I've had both (have carbs now). With EFI, everything is pretty much trouble free, good fuel consumption, automatic air/fuel ratio, smooth running, good cold startups, good throttle response. The minus' that I can see is that not everybody can work on them, and they can get pretty expensive.

I think its easier and cheaper to make more power with a carb, as well as much easier to work on.

Also, don't forget the "cool" factor. IMO you can't beat the sight of a couple of Holleys under the hatch.:cool: :cool: :cool:

Maybe its my age, but I'll stick with carbs.:D :D

I agree the carbs have that "cool" factor, but you know, I think women are impressed when you mention fuel "injection".:D :D :D

Here`s an idea:
I pull my fuel injection off (complete) and put it on Ebay (How much can I get $ ??). Then use the money and buy a carb / intake / distributor.

BTW, what carb / intake are you guys running? / recommend for a near stock 7.4 ?

carb/efi
 

I would assume you are talking about fuel economy when the boat is operating at cruise. If you have the same air to fuel ratio with the carb as the EFI of course you would use the exact same amount of fuel.

A marine engine doesn't go into a closed loop mode like a car engine. It doesn't have an oxygen sensor to constantly be monitoring the amount of oxygen in the exhaust. A marine engine has fewer input sensors and mainly relys on the correct air/fuel ratio from the info programed into the computer at the factory. Switching to a carburetor on a marine engine would not lose that much fuel economy (not like a car)

Most fuel economy improvements with MEFI come from cold start mode (no choke), the enrichment circuit (no accelerator pump) and variable ignition timing (available with Thunderbolt V ignition).

If you are interested in fuel economy do not use a Holley (especially one with dual acelerator pumps). Use either a Quadrajet or a Edelbrock 750 Marine. These carburetors have power valve circuits that are much more adjustable than a Holley (a Holley power valve can only be adjusted for on or off, not the amount of fuel that it flows).

My suggestion would be to find a used high rise cast iron oval port intake manifold (1970's Chevelle style) and a marine Quadrajet.

I remember when the mechanical fuel injection was discontinued on the Corvette in the mid 60's. The Quadrajet was introduced at about that time and people where heard to be saying that the Quadrajet was better than the fuel injection! It is the best carb to use on a general purpose marine engine for performance and fuel economy. The only people that argue the point are the people that stand to gain by selling you a Holley or have never seriously understood a Quadrajet! To bad new marine Quadrajets are unavailable!

Sincerely
Dennis Moore
FAMILY AND PERFORMANCE BOATING MAGAZINE

DonMan,

Can't get better response than that!:)

Re: carb/efi
 

I`ve got to tell you Dennis, you always come through with good, sound advice! You have told me exactly what I have wanted to hear, but no one else could explain. I agree with the Q-jet choice, just a matter of finding one. I will most likely go with the Edelbrock carb for simplicity. I believe I would rather buy a Performer or RPM intake, just for the weight savings (ever lifted a cast iron big block intake?). You have cleared my head of many questions I was concerned with.
Now, I wonder how much a street rodder will give for my MPI?

DONT!
DONT!
DONT!

;)

Go with an Edelbrock Marine 750 with an electric choke. That combo will give you good performance and fuel mileage. The Edelbrocks are easy to tune. The standard jetting should be very close to what you will need, maybe even a little too rich but, that is a good place to start. My '93 7.4L with a Weber (Edelbrock) 750 runs exellent and gets about 2.5mpg average. Good luck.

timucin

Don't, don't don't what?

Don Man
 

dont know what your fuel econamy is now but I run a 1995 23 foot wellcraft that is about 4500-5000 pounds loaded I only run it WOT and have it a little over proped for that reson and get over 2 mpg with it and it has the 650 weber (like edlebrock or carter) hope this helps? Charlie

I`ve never actually checked my MPG, but comparing gallons burned per running time (hours), this Baja uses about the same amount of fuel as my last boat, a 19' with 350-4 bbl.
I just feel like I have a more efficient hull along with a under-utilized engine (at cruise) than my previous boat.
The carb idea came about after realizing all the tinkering needed to add any power to this fuel injected engine. I believe I would be making a good decision to swap. If I can sell the MPI for a fair price, the swap would practically be free!

Don`t what? Swap to a carb?
Give me a good arguement why I shouldn`t and I`ll listen.....

Unless you have closed cooling or only boat in fresh water don't use an aluminum manifold or make sure you have it coated before installing it.

Dennis, the power valve circuit in a Holley can be adjusted for timing and amount. The bad part is that the power valves eventually blow even with the check valve in them.

Actually your reasoning should be the other way around.

What is the really good reason you want to change and what do you hope to gain, can you gain that and what is the cost and is it worth it. Im not saying efi is better I would give you same advice if you were thinking of switching from carb to efi.

I bot a 502 long block to replace the tired 330hp I had and stumbled accross a complete efi from a 1999 502mag included: plennum,manifold,fuelrail/injectors/distributer/coil/ecu/fuelpump/sensors and wires for about $700, you can find efi systems for around this price and less depending on completeness. This was the mag efi system and used one season pulled off by someone who went to a blower/carb msd ignition.

I wouldn't have bothered with the 7.4 mpi but a hot roder might be interested and who knows on ebay . I was originally planning to go carb but I couldn't have bot a manifold/carb fuel pump and ignition for that price and it was bolt on and turn the key.

P.S. and I did, bolt on turn key and spent every weekend and several weeks on the water this summer without problems/headaches, setting up, tinkering etc etc and thats worth a lot to me, more than 3 mph

An afterthot, the increased fuel economy I saw was more likely due to the change in comp ratio, 8:1 with the 330 to 8.8:1 with the 502 than the carb/efi

The MAIN reason I would like to swap to a carb is, I am planning on doing several different upgrades, both large and small over the winter and perhaps on into next season. With the EFI you must recalibrate the ECM each time you change something. I am very familiar with carbs, not that educated on EFI. My ultimate goal is to build a much larger, more powerful engine when this 7.4 gets tired. So I will definately be switching to carb at that time anyway.
As far as costs, I have estimated that I will need about $600 to swap from injection to carb, all new parts. I am hopeful that I can get somewhere near that for my MPI. So the actual cost would be minimal. THis is my initial plan, I think it`s going to work out......

EFI goes bad and so does a piston or worse/. Carb goes bad and you can usually diagnose yourself and fix as well as limp home.

I was also thinking about going to a carb on my 7.4 mpi, can any of the ignition system be reused? or is it taking info from the fuel injection?

Have you guys ever thought about throttle body FI. Seems to me its the best of both worlds if you aquire one thats adjustable enough.

The efi ignition is run by the putr which needs all the sensors, map,mat, knock, water temp etc, if you go to carb you will need a tbolt 4 or 5 ignition distributor/coil/ign amp/wireing harness

My .02 is to run what you have right now. I guarantee you will be very disappointed in the gains by changing to a carb. If your ultimate goal is a much more powerful engine in the future, why not run and enjoy your very reliable 310 HP fuelie, and take the money that you would spend on the carb conversion and start building the big, powerful replacement engine. Why not just go through this one time. You can get good performance out of the base 454's but it takes some effort and careful selection of components. My suggestion is buy a 502 long block, put it in your garage and build what you want. Leave the injection on the 310.
Good luck which ever way you go.

Re: OPINIONS WANTED: Carb vs. Fuel Injection
 

:rolleyes: Does anyone have a cutler mpfi unit for a BBC with rectangular port heads for sale out there? Thanks RC

Re: OPINIONS WANTED: Carb vs. Fuel Injection
 

turbojack had one last week,Smitty

Re: OPINIONS WANTED: Carb vs. Fuel Injection
 

I have a complete new cutler efi rest port 1000cfmTB set-up and another cutler manifold and fuel rails and 1000cfm TB available. I finally brought them home and can post pictures soon. Marty.

I like to start my boat the same way I start my mornings, 2 pumps and a shoulder roll. Works every time. :)

Take a look in this technical sections - tons of current efi related issues.

EFI advantage is not much in warm weather non emission, very inefficient to move, things we call boats. :)
An exception is something like Merc;s 1100+HP twin turbo engines. :)

BTW: How do you expect to repair fuel injection issues when you or mechanic can't fix a carburetor issue ?

BTW#2: No biggie, but in case you missed it, thread is 15yrs old