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Thread: SBC 5.0 to 5.7 Change

  1. #1
    A question for the experts... Blew the 5.0 at the end of last season and replaced it with a 5.7. The 5.0 had TBI and was in a 1998 deck boat. I replaced the long block with a fresh 5.7 and installed all the 5.0 electronics with exception to the knock sensor which I installed a 5.7 knock. Now, the engine will run but cannot hold a steady idle. The timing light shows the mark moving about 15 degrees each direction. The injectors are for a high output 5.7 so should be OK. Dealer is now telling me that I need a 5.7 ECU at a cost of $1100. I was under the impression that the 5.0 ECU would work fine on the 5.7 and have a hard time believing that a 5.7 ECU will solve the problem. Not to mention $1100!!! Is there that big a difference between a 5.0 and 5.7 ECU, is there a big difference between an automotive and marine application? Summer is coming quick so any input would be greatly appreciated!

  2. #2
    What 5.7 motor did you use as a replacement? Often the vacuum signal will cause the 'rough/ uneven idle'. Please list the EXACT changes you have made. I am currently working on a "dial your fuel curve" system for Mercury (GM) EFI's. If the distributor is not ECM controled, it will 'seek' a level idle. If the distributor is ECM controled, it will 'seek the best mean idle'. ECM's that control the distibutor often drop the timing to -4 or -7 degrees. This is being done to prevent reversion (water into cyls) and to create a "hot/ rich" smog legal idle. The other area of 'rough idle' may be caused by too large of replacement injectors. Let me know.

  3. #3
    i would think the ECU would be differant for the two engines as it also triggers the EFI timing aswell as the ignition timing. i can tell you this, when i installed the edelbrock multi point EFI on my trucks 502 crate motor, i had to make sure i used the 502 chip and not the 454 chip in the ECU. im thinking your situation is not much differant. id look more into the question of the ECU being the same as an automotive one. you can pick one of those up from a kreagan or chiefs or something for pretty cheap. i know mine was about $100 exchange. hopefully someone on here will have some good info for you. good luck..

  4. #4
    sounds like bigkatboat is the guy to help you out on this one.

  5. #5
    Everything on the motor is the same, the only things changed were the long block, intake manifold (from the stock cast iron to an edelbrock performer) and the knock sensor (from a 5.0 to 5.7). The TBI and injectors are the same but I looked up the color code and they are for a high output 5.7 (came from Volvo that way). The distributor is ECU controlled (no change). All the electronics, TBI, computer, sensors (except knock), exhaust, distributor are all from the 5.0 original equipment. 5.7 motor, heads and intake manifold are new.

  6. #6
    Sorry, forgot the motor stuff. The 5.0 was a 1998 Vortec motor and the 5.7 is a 1995 (non-vortec). That's why we had to go with the Edlebrock since the original cast intake wouldn't bolt to the vortec heads. The cam was ground by Steve Long Racing cams and has a marine grind on it. I don't have the specs with me right now but it's fairly mild.

  7. #7
    Ultra, Those TBI's can act funny when the cam is changed, due to a different vacuum signal to the MAP sensor. The factory MAP is calibrated to a particular signal and will change timing and pulse width if it sees different, especially at idle. It may be as simple as changing your MAP sensor. It doesn't necessarily need to be a "big" cam to muck up the signal, sometimes the smallest changes can fool the ECM. Someone like AZ speed and marine, or others can probly provide you with the updated part. They will ask a battery of questions, so make sure all your info is correct, i.e. cam specs, injectors, exhaust, rocker ratio, compression ratio, etc.

  8. #8
    Don't you still have to manually set the timing at idle??? If so, depending on what chip you're running the timing at idle probly needs to be retarded.
    I have a 95 5.7 and with the stage 2 chip the timing at idle is 12 degrees before tdc as apposed to 0 degrees called for in the stock configuration.
    I might be way off base on this.....just giving you my experience with the 5.7.
    mirvin :clover:

  9. #9
    Sorry I haven't been on but I've been talking to a couple of shops and just keep getting more and more confused. First of all, why would I have to change the computer from a 5.0 to a 5.7? What information are the monitors relaying back that the computer would be able to tell the difference between the two motors when all the parts are the same? Even the injectors haven't changed. They came with the 5.0 but the color codes says they are for a high output 5.7 so I didn't bother changing them. The 5.7 is stock, as was the 5.0. I read some posts on and lots of street rodders have made this same change without any problems. Is there a difference between the automotive applications and marine? Does anyone have a number for someone at Volvo Penta that might be able to clear some things up?

  10. #10
    Not an expert on the issue, but the marine computer runs "open loop" because it does not have an O2 sensor (would never live in the wet marine exhaust). This means that the injector map or program is pre set for a certain volume of air (based on the 5.0 engine) and it does not have any way to compensate for the larger volume of air the 5.7 engine pumps on each stroke. You need to reprogram the computer so it feeds the correct amount of fuel for the larger engine.
    The car computer runs closed loop, taking air/fuel ratio measurements from the O2 sensor so it can compensate for the larger displacement engine.

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