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DonMan 09-18-2002 01:44 PM

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.

ursus 09-18-2002 01:54 PM

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,

Panther 09-18-2002 02:08 PM

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.

DonMan 09-18-2002 02:18 PM

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!!!

DonMan 09-18-2002 02:23 PM

Originally posted by ursus
I went the other way
330hp carb to 502 mag efi

fuel consumption was similar

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.....

ursus 09-18-2002 02:35 PM

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

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:

bajadude 09-18-2002 02:37 PM

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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

DonMan 09-18-2002 02:48 PM

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

DonMan 09-18-2002 02:51 PM

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 ?

Dennis Moore 09-18-2002 02:58 PM

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!

Dennis Moore

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