Re: AMC 4-cylinder in a Spirit (Revisited)

[farna@xxxxxxx]

I understand your thinking completely Andrew! I have a 258 cam in my 4.6L stroker. Great off-idle torque. I didn't use a radical cam because no one really knew what to use when I built mine about six years ago. I was one of the first dozen or so to build a stroker. My machinist and I looked at the engine and decided a "slightly larger" cam would be better than the stock 4.0L. So we looked for something with just a bit more lift and/or duration. I have the exact specs at home -- the only thing I can say right now is it was in the NAPA books as a 258 "econo-power" cam. 

Now it DOES pull the 3.07 gears with no problems. Engine doesn't bog or anything. You can tell it's working a little when pulling hills at speed or with a load in OD, but it doesn't strain in the least bit. What it DOESN'T do is get good gas mileage with those high gears! Or I should say it didn't when in OD. It would run 1900-2000 rpm @ 70 mph in OD with the 3.07 gears. With 3.55 gears at the same speed/gear it runs 2200-2300 rpm. You wouldn't think 300 rpm would make much difference, but apparently 2200-2400 rpm is the Renix EFI setup and/or that cams "sweet spot". Those 300 extra rpm ADDS 2-3 mpg on the highway instead of taking it away! This was phonomena discussed at length a year or two ago here on the list -- mileage seems to be best at the an engine's torque peak. That's the only explanation I can come up with.  

With the Renix (and at least 91-95 HO systems) the computer richens the mixture somewhere over 2400 rpm, so you don't want to cruise above that engine speed -- mileage goes down noticeably. I can't help but think the four computer system is set up in a similar manner. Above a certain rpm it should enrichen the mixture for added power, and there is likely a "sweet spot" for cruising. Of course it should be higher rpm than the 4.0L six models, somewhere around the torque peak (rated torque rpm). 

On September 28, 2005 andrew hay wrote:

> but if you're willing to change cams...
> 
> ecms have a measure of latitude to cope with changes.  most of the
> investigation with the 'stroker' [4.0 with 258 crank] crowd has been
> with more radical cams to cope with the higher compression since there
> isn't a good cheap stroker piston.  but the mopar 258 mpi kit was
> based on a stock '93 4.0 ecm, and at least one stroker is running a
> 258 cam, with great off-idle torque just like a 258.  it doesn't even
> give up much power - mopar claims 160hp for the mpi 258; one mag
> measured 170.  with the 4.0's better head...
> 
> that would pull those 3.07s easily, with good mileage.  eagles with
> similar tires had 2.73s.  i would expect, naively perhaps, that a
> 2.5 would respond similarly to a cam change.
> 
> it would be interesting to see what changes, if any, the 2.5's cam
> went through in its lifetime.  afaik the 4.0 had only two: a fairly
> radical grind used '87-'95 and a less radical split pattern '96-up.
> ________________________________________________________________________
> Andrew Hay                                  the genius nature
> internet rambler                            is to see what all have seen
> 
> adh@xxxxxxxxxxxxxxxxx                       and think what none thought


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