How does a mercruiser factory efi (1999 5.7) respond to engine modifications?

silvert

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Thinking of buying a boat that needs an engine. I did this three years ago and made out pretty good on the deal. Sold that boat, now going bigger. Anyways, the boat has a mercruiser 5.7 efi with a bravo 3. The block is junk. Thinking about doing a new 350 but porting the heads and adding compression, and possibly different cam. I'm from the Drag racing world and have built many engines, mainly efi stuff but with aftermarket systems in which I would also do the tuning. I know if the efi is speed density (no mass air flow sensor) that usually the tune needs to be adjusted for any big changes to the engine. I know nothing about mercruisers efi. Is it speed density? Is there companies that program these? Thanks in advanced
 

Fun Times

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Hi there, yes it is a speed density system and a lot of your programing questions should be found with contacting the company linked below that specializes in the MEFI, ECM's

http://mefiburn.com/

If you'd like to learn more about the MEFI, ECM's, here is a long blog about them that you might find interesting. http://www.michaelstractors.com/forum/topic.asp?TOPIC_ID=199

Let us know how it all turns out, good luck.:)
 

Scott Danforth

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Regarding a cam, boats respond well to a cam to a point, however you have to reprogram. you're limited on a boat with wet exhaust to about 270 degrees of duration, otherwise you will suck water back up thru the exhaust.

Also, unlike drag racing where everything is high RPM, a boat motor is more like a tow truck - built for torque up to 5000 RPM. Porting the heads does the exact opposite of what you want to do. you would want to have smaller ports for higher torque at low RPM.

your best bet is build a 383 stroker motor with proper D-shaped pistons, zero decked with a 0.041" quench area and about a 12cc dish. Use a 264-270 duration cam, and reprogram the EFI. that will give you about 420 hp at the crank, or about 360-370 at the prop. the big difference is that you are adding a bunch of torque.
 

tpenfield

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The folks at mefiburn.com can do some custom tuning. As stated, boat engines are a different animal than automotive, particularly performance automotive. What works there tends not to work here.

Think truck engine with low-end torque. That is what you should tune to.
 

Tail_Gunner

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Responding directly to your question it work's but mefi is really old hat so to speak and is speed density. You will find mefiburn actually a fairly good software for mefi system but your burning eprom's read 80's tech it's long and boring as a process and expensive make no mistake.

I can tell you with almost 100% certainlty you could build a efi engine using efi..... a fairly moderate build for say 3500/4000 or build a carbed engine for 3500 using forged internals top shelf valve train for the same money...make more horsepower and be more reliable....Now if your just going to go efi and that's that take a close look at the below


Opps you would need something like this below....to complete the system. I m sure when yu map out the entire cost vs performance that holley system is very intresting.

326323d1203356605-o2-sensor-installation-05-12-06_2011.jpg



Then there is this.....http://www.uscgboating.org/assets/1/AssetManager/ABYC.1002.01.pdf

I dont believe there is a standard for the efi system itself but the fueling is highly regulated.
 
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silvert

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Thanks for the replies everyone. I bought the boat last night. It's in very good shape minus the engine. What stinks is I just sold a holley hp efi system I had laying around the garage. This woukd have been perfect. I have an older accel gen7 I think I'm going to use. Appears this engine is based off the 96-99 vortec truck engines. I guess my next step is to find a block. Looks like I can get brand new heads from jegs for $350 each, not too bad. Need to figure out what to do with the intake now. Thinking of a edelbrock and then weld in injector bungs.i have a couple fuel pumps here I may use.
 

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Bondo

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Thanks for the replies everyone. I bought the boat last night. It's in very good shape minus the engine. What stinks is I just sold a holley hp efi system I had laying around the garage. This woukd have been perfect. I have an older accel gen7 I think I'm going to use. Appears this engine is based off the 96-99 vortec truck engines. I guess my next step is to find a block. Looks like I can get brand new heads from jegs for $350 each, not too bad. Need to figure out what to do with the intake now. Thinking of a edelbrock and then weld in injector bungs.i have a couple fuel pumps here I may use.

Ayuh,.... Sounds like an interestin' build,.....

Just be sure to keep yer electrical, 'n fuel systems, Marine Safe,......
 
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