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post #1 of (permalink) Old 12-14-2002, 01:06 PM
brendan
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questions re: mpi on 258/4.0 head

I have decided that mpi is the way to go for the j10. I had been planning to go with gm tbi, but I am worried about the totally different character of the 4.0 head on the 258,(comp. ratio, flow rates of head) and I don't want to have to have a custom eprom burned for this. If the engine was in a cj, I'd be more willing to confer with people about having a custom chip burned, but where it is in an fsj, I run with way more load on the engine and wouldn't really trust a curve designed for a cj.

I feel safer with the idea of MPI, mostly because i know I can eliminate ping with larger injectors. I could also use the extra horses out of the 1 and 6 cans.

One thing that has been bothering me though, is the operating temp of the engine. The 4.0 head article says to use plugs of one range colder than stock, and a 180* tstat. 4.0 MPI goes closed loop @ 195* though, correct? What I am thinking is to find a switch that closes at 180* instead of 195*. I'd rather have the engine run at 195*, but obviously can't if it will cause detonation.

My questions are:
1. does the 180* switch idea seem feasible?

2. would using larger injectors (such as mustang 24 lb/hr injectors as opposed to stock which I believe are 19 lb/hr) eliminate the question of operating temperature vs.detonation by richening the fuel mixture?
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post #2 of (permalink) Old 12-14-2002, 02:34 PM
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Re: questions re: mpi on 258/4.0 head

</font><blockquote><font class="small">In reply to:</font><hr />
1. does the 180* switch idea seem feasible?

[/ QUOTE ] No, the computer will always see the engine as under temp and enrich the mix. </font><blockquote><font class="small">In reply to:</font><hr />
would using larger injectors (such as mustang 24 lb/hr injectors as opposed to stock which I believe are 19 lb/hr) eliminate the question of operating temperature vs.detonation by richening the fuel mixture?

[/ QUOTE ] Possibly, as the larger surface area of the pintel - seat would let a larger amount of gas through given the same pressure and on-time.

The problem here is that if you're in closed loop, would the larger injector now still be within the operating limitations of the ECM? If so then the ECM will still trim back the Fuel Ratio. If not then you my throw codes.
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post #3 of (permalink) Old 12-14-2002, 09:39 PM
brendan
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Re: questions re: mpi on 258/4.0 head

Thanks for the reply, LEVE.

you wrote:
No, the computer will always see the engine as under temp and enrich the mix.

How will it always see the engine as being under temp? It's temperature sensing capabilities are binary, right? If I use a 180* switch, how will it know that it is not at 195* ?
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post #4 of (permalink) Old 12-14-2002, 09:59 PM
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Re: questions re: mpi on 258/4.0 head

I'm not sure on the SMPI, but often it will use a seperate sensor to trip the computer into Closed Loop. If the temp for the trip is under 180* the you'd be OK, but if it's in the 180* to 195* window.. then the computer will stay in Open Loop and run rich. In Closed Loop all the sensors are providing input. In Open Loop the computer enriches the mix to help the engine run while warming up. This is usually a set time limit from the factory.
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