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Discussion Starter #1 (Edited)
Junkyard MPFI VS TBI 4.2L

I just did a 4.0 head swap with a clifford intake that was laying around and a pacesetter header on my 87 4.2 and I'm daily-ing it with a Webber 38/38 carb.
When I bought the head, I also got the entire MPFI system off a 94 wrangler.

MY QUESTION IS, is the MPFI really that much better than a GM TBI? I can get an entire TBI system out of a Astro van locally for under $300 and it will be much easier to convert to that vs a stock 4.0 MPFI. Will I be able to swap the TBI to a small v8 set up? Can I run it with a more aggressive camshaft? TIA! I'm really torn between the two.
 

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Others may disagree but my understanding is that TBI is actually better for high rpm max power and MPFI is better for everyday drivability.

Keep in mind that multi port has one injector per cylinder, and if you so desire, with some aftermarket systems, you are able to really tune in each cylinder to get the most power and economy possible.

With chevy TBI you have 2 big injectors that work very similar to a carb.

You are depending on the intake manifold having even and balanced air flow and being properly designed.

Which brings me to my second thought.

A TBI/Carb intake is designed with the idea of flowing fuel /air mix. So that means no sharp bends and a gnenerally "downward" design.

An MPFI manifold is much more forgiving since air can flow up just as easily if there is no fuel in it.

Best example would be tunnel ram vs GM TPI or LS1 intake.

The ls1 actually has air flowing up and over. This would be terrible for TBI. The runners on a TPI are something like 17" from end to end, and certainly would not work well for flowing fuel.




So if your intake is designed for a carb, TBI will probably work great. MPFI will require drilling / welding fuel injection bungs, which may significantly disrupt airflow.

If your intake is designed for MPFI, it will have the proper cross section around the bungs, but may not be well designed for flowing atomized fuel.
 

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Discussion Starter #3
Others may disagree but my understanding is that TBI is actually better for high rpm max power and MPFI is better for everyday drivability.

Keep in mind that multi port has one injector per cylinder, and if you so desire, with some aftermarket systems, you are able to really tune in each cylinder to get the most power and economy possible.

With chevy TBI you have 2 big injectors that work very similar to a carb.

You are depending on the intake manifold having even and balanced air flow and being properly designed.

Which brings me to my second thought.

A TBI/Carb intake is designed with the idea of flowing fuel /air mix. So that means no sharp bends and a gnenerally "downward" design.

An MPFI manifold is much more forgiving since air can flow up just as easily if there is no fuel in it.

Best example would be tunnel ram vs GM TPI or LS1 intake.

The ls1 actually has air flowing up and over. This would be terrible for TBI. The runners on a TPI are something like 17" from end to end, and certainly would not work well for flowing fuel.




So if your intake is designed for a carb, TBI will probably work great. MPFI will require drilling / welding fuel injection bungs, which may significantly disrupt airflow.

If your intake is designed for MPFI, it will have the proper cross section around the bungs, but may not be well designed for flowing atomized fuel.
That's the thing, I have the entire MPFI system including intake. It's pretty much ready to install I just have to put on my now trans to make a spot for the CPS.
Or I could sell it all and pick up the TBI and just run that on my Clifford intake I have now. Im really trying to figure out if the TBI will do just as good of a job.
 

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That's the thing, I have the entire MPFI system including intake. It's pretty much ready to install I just have to put on my now trans to make a spot for the CPS.
Or I could sell it all and pick up the TBI and just run that on my Clifford intake I have now. Im really trying to figure out if the TBI will do just as good of a job.
You shouldve sold/never bought the clifford intake and weber. Taken those funds and purchased a 4.0L stand alone harness and used the rest of the jeep parts like a factory install. Plug n play and been rolling along.

All your aftermarket FI systems are a glorified TBI. However, youre talking about hacking in 30 year old parts.
 

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Discussion Starter #5
You shouldve sold/never bought the clifford intake and weber. Taken those funds and purchased a 4.0L stand alone harness and used the rest of the jeep parts like a factory install. Plug n play and been rolling along.

All your aftermarket FI systems are a glorified TBI. However, youre talking about hacking in 30 year old parts.
Ok, I paid $40 for the Clifford and got the 38/38 for free so I'm not really worried about that. Are you saying that the 4.0 MPFI will work better on the 4.2 than TBI? I know it will "be more efficient" than TBI, but in the end I'll end up with roughly the same power and fuel economy right? Either way if I go with a junkyard TBI or my junkyard MPFI I have laying around, I'll be cutting and hacking wires. Buying an aftermarket kit is not an option according to my current income as a student.
 

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'if' you have all of the electronics to run the MPFI setup....it should run a little better. The TBI is kind of a 'once size, fits all' solution. The MPFI can tweak the fuel a little better for better emissions and economy.

But for us jeep guys, TBI is a pretty reliable way to go. You can get parts anywhere and there is less to break.

But if you have the MPFI and all the electronics.....go for it.
 

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It's extremely easy to cut down an XJ harness to make the MPFI work standalone.

I would ditch that overflowing toilet bowl and move straight to that option.
I hate to even ask this... But do you know of a link to help walk a person through this? Otherwise I have schematics and can figure it out, but the more info the better.
 

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Buying an aftermarket kit is not an option according to my current income as a student.
I hate that excuse. Plenty of us built all sorts of rigs while paying for our own education. You hustle and prioritize to make it happen.

You know what costs more? Doing it twice because you didnt do it right the first time. MPFI all the way.
 

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Discussion Starter #11

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I hate that excuse. Plenty of us built all sorts of rigs while paying for our own education. You hustle and prioritize to make it happen.

You know what costs more? Doing it twice because you didnt do it right the first time. MPFI all the way.
Doing it twice? That's one reason I was even considering TBI. You can put gm TBI on a v8 unlike the MPFI. Either way I might just keep the MPFI and see where it takes me. I've already invested too much into it so might as well finish the project. I could've spent less than half on the TBI and got my jeep on fi months ago, but screw it. I believe y'all that it will be a giant improvement over TBI.
 
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