88 Engine swap 350-454?

[DukeBoy]

I have a 88 GMC Jimmy fullsize with a TBI 350 and TH700R4. I've located a 88 TBI 454 I'd like to rebuild and swap. I want to know if my 350 ecm would control the 454 with a different chip. I'd like to retain the ecm to control my tranny. Painless wiring lists both harnesses as the same. didn't know if there were differences with the computers themselves. The 454 doesn't include harness or computer, $500. Thanks in advance.

88 GMC 350-700R4, 10bolt front, 14boltrear, 4.10, 35" BFG, 6" Skyjacker, soft top, custom metalwork

 

[azblazor]

Yes your computer and harness will work. You will need a new (454) chip or your existing chip reprogrammed.

<font color=blue>azblazor</font color=blue>
<font color=orange> 79 K5 454-FI / 4L80E, NP205, D60, FF14, 4" lift, 9K Warn </font color=orange>

 

[HarryH3]

I'm doing a similar swap on a '90 Suburban. I'm guessing that the larger injectors used on the 454 will let the computer at least run the 454, even with the 350 chip in the computer. It certainly won't be optimal, but I think it will be in the ballpark. The stock 350 pulse widths to the injectors don't have any clue that the engine is larger or that the injectors are larger. My guess is that the larger injectors will come pretty close to making up for the larger displacement of the engine. Once I have it running, then I can connect the scanner and see what mods need to be made to optimize the fuel curve.

I don't think that a 454 chip will work correclty, since GM never offered a 700R4 behind the 454. With the 454 chip installed, the computer will ignore any input from the tranny and won't ever tell the torque converter to lock up.

I'm not planning to change anything in the wiring harness. It looks like I'll need an adapter to go between the 350 harness and the Idle Air Control motor and possibly the throttle position sensor. I'm using most of the parts from the 350 on the 454. (Distributor, MAP sensor, etc).

<font color=black>HarryH3 - '75 K5</font color=black>
<a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
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[4X4HIGH]

You are partly correct. The computer chip has a grid of fuel curves programmed into it. What is going to happen here is that the oxygen sensor is going to detect that you are running lean and then it will try to compensate for it by looking for part of the grid that would be correct for the current engine requirements. What this is going to do is pull a code because your computer is going to be constantly trying to compensate for the lack of fuel. Built into the computer is what's called an integrator, this responds to signals from the oxygen sensor to control fuel delivery through the injectors. A reading of 128 +/- 10 would be normal. There is also something called BLM, block learn multiplier. This reacts according to what the integrator is doing. Unlike the integrator BLM is stored in memory and can adjust to compensate for a lean air fuel ratio. Basically because the computer is constanly trying to richen the mixture it will pull a code.

 

[HarryH3]

Why do you think it will be running lean? The larger 454 injectors will deliver more fuel for a given pulse width than the 350 injectors would deliver. Is the extra amount of fuel going to be enough to make the 454 run OK? I think it will be pretty close, at least well within the integrators ability to compensate. Now if one were trying to use the 350 throttle body and injectors on top of the 454, with the stock 350 chip, then yes, I could see where it would run lean.

I don't have the specs for the injectors, but I'm guessing that since the 454 has about 30% more displacement than a 350, that the injectors for the 454 also deliver about 30% more fuel for a given pulse width. I'd be surprised if it runs perfectly, but I think it will be pretty close.

<font color=black>HarryH3 - '75 K5</font color=black>
<a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
It's a great day to be alive...

 

[4X4HIGH]

It will run lean because the pulse width for the 454 is greater than for the 350. Pulse width is something that is built into the chip and not the computer itself. Just because that computer can compensate for some descrepencies doesn't mean it will work ok. You have to remember that the computer has limitations. It can only compensate to a given degree and after that it will start pulling codes. You go ahead and try it and let me know how it goes, my bet is that you end up having someone burn you a new chip. Why would you not want to have a chip burnt anyway? They aren't that much money and it will only make your swap that much sweeter. Not sure what state you live in but i would almost bet that it will not pass a smog check.

1989 blazer, 383 stroker, 4.56 geared and detroit locked D44 front and 14BFF rear

 

[HarryH3]

Is the pulse width really longer for a given point on a 454 than on a 350? Or is it the same (or very close) and the larger injectors just spew more fuel during the on time? That's what I want to find out.

For example, let's pick some random numbers (since we don't have the actuals).

Let's say that an injector from a 350 will spray 1 "unit" of fuel, when turned on for 20 milliseconds. How much fuel will a 454 injector spray when turned on for that same 20 milliseconds? I'm guessing it's pretty close to 1.3 "units". I don't think that GM could have just used longer pulse widths on the 454, or they would have run into injector saturation issues at wide open throttle, under heavy loads. (I.e., as you approach somewhere around an 80% duty cycle, you can't make the pulses any wider or the injector starts to flutter). So you need bigger injectors to deal effectively with the top end engine demands. My guess is still that the larger injectors will come pretty close to making up for the larger displacement (and thus larger fuel demands) of the engine.

I didn't say anything about not getting a custom chip. In an earlier post I said "Once I have it running, then I can connect the scanner and see what mods need to be made to optimize the fuel curve. " I'm curious to see just how it works. I'm guessing that it will require a custom chip to run perfectly, but I also think that it will be pretty close with the 350 chip and the 454 injectors. Check out my first reply again to get a better idea of what I plan to experiment with.

<font color=black>HarryH3 - '75 K5</font color=black>
<a target="_blank" href=http://www.angelfire.com/super/ThunderTruck>www.angelfire.com/super/ThunderTruck</a>
It's a great day to be alive...

 

[4X4HIGH]

Yes the pulse width is longer than with a 350. I don't have an answer as to how much longer it is. Why do you think there is a different chip for a 454 versus a 350? One of the things that is burnt into the chip is the fuel curve, another is the timing curve. Once you have the correct chip made then you can fine tune the system for optimum efficiency. When it comes to fuel i know that i don't want to experiment and have a catastrophic failure just because i wanted to see if it would work. The factory has done years of R&amp;D to discover what works and what doesn't. I have said enough on this subject now i think. Go for what you want and let us know what your experiment findings are. I am sure everyone would like to know. I am not trying to be a smart ass or anything, just trying to give you the facts.

1989 blazer, 383 stroker, 4.56 geared and detroit locked D44 front and 14BFF rear

 

[Overkill]

HarryH3, I see your point. I "think" if the 454 injectors do actually flow more fuel than the 350 injectors than the pulse width should match enough to get er running. It makes sense to me that the same pulse width will flow more fuel in a larger injector. Might also want to check up on fuel pressures, making sure you have enough for the added flow. I am definitely curious as to whether it will work out or not. Keep us posted. I am only speaking from theory (and we all know how that goes haha) so don't take my work for experience.

Good luck,
Jamie

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