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5.7 TBI build questions

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Old 10-03-2017, 08:24 PM
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Default 5.7 TBI build questions

I picked up a very well maintained 1997 Cobalt 20' with 180 hours on everything. Its in very good shape and everything works as it should. Right now with 4 passengers, half tank of gas and all its running gear it will top out just above 50mph at 5000rpm. The motor is a 220hp non vortec 5.7, outdrive is Alpha 1 with a 19p prop.

I would like to do some motor work on it, but the TBI has me a bit leary. Initial plans were to do a set of aluminum heads (looking at BluePrint 195cc), Comp cam xm270 cam and a dual plane intake. I'm worried about tuning TBI and it not flowing enough for the build. I spoke with Ben at MefiTuning today and was told to do a full tune in the boat would cost 1700+ or 495 for a canned tune. I contemplated tuning it myself, but I would prefer a professional to do it.

1. Am I pissing in the wind building a TBI motor?
2. Would this motor combo work well? 195cc aluminum heads, Comp Cam XM270, and a dual plane intake.
3. How are people using a TBI on an aftermarket intake, would an adapter plate work to mate the TBI to a 4brl intake?
4, Should I ditch the TBI for a Holley Sniper self tuning kit?

I realize the alpha 1 is only good to 330ish hp. It will have a drive shower and high quality oils. The boat is used on small lakes in Nebraska with the occasional river run.
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Old 10-03-2017, 09:04 PM
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I'm old school and would ditch the tbi in a second. Keep it simple....
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Old 10-03-2017, 09:39 PM
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I like the two pumps and a shoulder roll technique. Ask my wife.
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FYI: carbs usually take to quick small pumps and a shoulder roll to start cold.
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Old 10-03-2017, 09:49 PM
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Been there done that. I switched my carbs to big block TBI units with a Holley adapter. I used a TPI ecu.
It works fine and has good fuel economy. Even with the bigger throttle bodies, I would say they are flow limited to about 330hp.
If you want to make more than that, I would go with a carb or port injection.

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Old 10-04-2017, 12:00 AM
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The e TBI is your stumbling block. It will potentially limit airflow and injector selection will be problematic. With a TBI I'd swap out to a Holley system. IF you had a MP Merc EFI setup I'd tune it myself. You are going to have to spin the motor faster for more power and thats where the TBI will hamstring you.
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Old 10-04-2017, 05:45 AM
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Tbi is good for emissions and gas mileage. I have 2 big 2 inch 454 tbi units. Complete with injectors and the big water filled OEM adaptor to a q-jet base. If your TBI is working, you could swap throttle pos sensors and IACS and mebbe give one of these a try. I have no idea what the injectors are like. The adaptors alone sell for 50 bucks. I will do a whole tbi with adaptor for 50 plus 25 to ship it. And I will probly profit about a buck fifty on ahipping. THEN you can get hold of harris tbi chips on ebay who still does a lot of tbi Eprom work. (These ecu are really old school. But they work.) But your best bet is run it as-is till you get a better setup. Now, an alpha with power. The lighter the boat, and the longer pitch the prop, the more likely the drive will hold up. You put a 454 in a old alpha sundancer and hit the gas with a big 17 inch prop, that drive will last about 5 seconds. You got a hot 383 in that cobalt with a 25 inch prop, it doesn't have the low gear bite to snap shafts like a short pitch prop does.



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Old 10-04-2017, 07:02 AM
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Question. Does the ecu look like a car ecu? Or sealed like marine stuff?
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Old 10-04-2017, 07:37 AM
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Toss the TBI,. Go with an Edelbrock air gap dual plane and a 4bbl. carb.
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Old 10-04-2017, 08:57 AM
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stock OEM back then used MEFI 1 for 1997.

Your set up is not up for making more than 300 hp Not worth spending 1000 s on. Out dated fuel delivery system and a really outdate ECM. . Stock exhaust very poor as well. The risers and the riser gasket are the exact same used on 180 / 190 hp V 6.

I would play with props and weight and or take weight out of the boat. - Stock Cobalts are fairly heavy built boats.

I would leave it stock

And when one goes to sell a Cobalt runabout - they are worth more stock than one that's moded. Most Cobalt owners would buy anther Cobalt.
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Old 10-04-2017, 11:17 AM
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Quote:
Originally Posted by BUP View Post
stock OEM back then used MEFI 1 for 1997.

Your set up is not up for making more than 300 hp Not worth spending 1000 s on. Out dated fuel delivery system and a really outdate ECM. . Stock exhaust very poor as well. The risers and the riser gasket are the exact same used on 180 / 190 hp V 6.

I would play with props and weight and or take weight out of the boat. - Stock Cobalts are fairly heavy built boats.

I would leave it stock

And when one goes to sell a Cobalt runabout - they are worth more stock than one that's moded. Most Cobalt owners would buy anther Cobalt.
His issue is more of the TBI and intake vs the MEFI 1. The MEFI 1 is perfectly capable of what he's trying to do and would be the most cost effective should he decide to tune it himself.
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