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Old 05-21-2020, 01:13 PM   #1 (permalink)
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OEM MPFI vs modern TBI conversions

Hi guys,

Some background, this is a 1970 Nova that i am building to be a daily driver. The car is currently lowered and has been swapped to a 1998 Tahoe L31 engine hooked to a T5 5 speed manual. The engine is currently being fed by a dual plane intake manifold with an Edelbrock carb. I still have most of the original EFI gear from the Tahoe. (no computer or harness.) The intake has injector type spider replacing the ghastly poppet type injector spider

I am looking to move the car to EFI with economy/drivability as the goal... all on a sub LS swap budget. 270 or so horsepower is plenty for a car with 1970 brakes and handling.

My question is what would be the best route, the TBI carburetor replacement efi conversions like FItech or Holley Sniper? or the stock GM MPFI intake with a flashed 411 pcm?

It feels like a weird step backwards to move to TBI, but everything i have ever read about the Vortec 5.7 intake just calls it awful. Perhaps its only bad for power which is what most folks are aiming for with these v8s. The idea of all those fuel hoses inside the intake plenum doesn't seem like a good plan for anything. Unfortunately the only intakes with conventional fuel rails are so expensive I may as well kick the L31 to the curb and go LS.

I feel like the TBI units might have some edge here with a more modern computer and a wideband. I considered megasquirt on the original intake, but that would require an MS3 with the expansion to get full sequential injection on a v8.

Thoughts?

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Old 05-21-2020, 02:40 PM   #2 (permalink)
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I think that depends on how comfortable you are with scavenging all the appropriate harnesses ECM and modifying the wire harnesses to work with what you have. If you're trying to do throttle body injection on a cheaper budget, many people are happy with the throttle body found on the pre 95, especially the versions found behind the sporty cars versus the pickups. I don't think there are going to be massive fuel economy differences between the setups being considered.
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Old 05-21-2020, 05:19 PM   #3 (permalink)
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Go LS for the win...

Find a wrecked or beat up car or truck with a good LS engine and scavenge everything needed for the swap. Then part out what you can and scrap the rest.

Itís hard to beat the LS platform.



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Old 05-21-2020, 08:20 PM   #4 (permalink)
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Unless the MPFI is sequential instead of simultaneous, I'd take a look at the TBI. Otherwise, an LS swap might not be a bad option.
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Old 05-21-2020, 08:59 PM   #5 (permalink)
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Thanks for the responses.

I would do an LS if i wasn't stuck on keeping my manual transmission, suddenly the engine becomes the cheapest part when you start adding in the cost for a conversion flywheel, new bell housing, new clutch, etc.... I am aware they are better engines in every way. I suppose it would increase the value of the car a bunch though. I'll recalculate the cost to convert it.

The stock MPFI is supposedly sequential injection. That said it only has a 4 tooth crank position sensor with the distributor for cam position... So i don't know how accurate that can possibly be.

GM TBI would be for sure the cheapest route if I junkyard all of the parts and maybe a megasquirt, that said i am seeing used FITechs now for about $700 or so, and some even have the timing control. I figure the extra cost might be worth it for the simplicity. I'd wager i can get the original MPFI up and running for around $200 or so, but a lot more time learning the software and how to flash the GM PCMs. I have a decent enough background in automotive wiring that the electrical doesn't scare me, software does.
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Old 05-21-2020, 09:15 PM   #6 (permalink)
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First ensure if the stock MPFI is either simultaneous or sequential, and once you get it all figured out it will become easier to decide what is going to bring you more peace of mind. However the 4-tooth crank sensor and resorting to the distributor for cam position makes me believe it's simultaneous. I remember some 4-cylinder Opel engines from the same vintage resorting to a similar approach, even though the distributorless wasted-spark ignition relied on a different pick-up for cam position.
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Old 05-21-2020, 10:58 PM   #7 (permalink)
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I am pretty sure it is sequential rather than batch, the original name for that particular system is CSFI, central sequential fuel injection. It replaced the TBI systems before it. That said i think it gets away with the simpler crank and cam triggers because it is still a spinning distributor with one coil as opposed to the coil on plug that replaced it.


For those that are not familiar i am talking about this intake system i am talking about with the injectors inside the plenum.
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Old 05-21-2020, 11:15 PM   #8 (permalink)
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I would think that the stock intake and injector rail is much easier to do, how much money would you save with a throttle body injector?
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Old 05-22-2020, 12:34 AM   #9 (permalink)
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I wouldn't save anything but a lot of software head scratching and complexity with TBI. I am ok with spending some money, I just don't want to take a step backwards in terms of efficiency.
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Old 05-22-2020, 07:59 PM   #10 (permalink)
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Quote:
Originally Posted by foppert View Post
I am pretty sure it is sequential rather than batch, the original name for that particular system is CSFI, central sequential fuel injection.
It seems to be worth keeping this system, as long as replacements are still easily found. Well, usually it's simpler to deal with stock setups than looking for miscelaneous parts for an adaptation.

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