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

["Jim Blair" <carnuck@xxxxxxxxxxx>]

A: The lower the manifold vacuum (lower than the base peak power RPM with 
extra throttle to keep moving = lower vacuum) with fuel injection means the 
MAP sensor thinks you require far more fuel than needed, so extra is dumped 
into the chambers, further reducing the power because the spark is partially 
snuffed (or at least slowed down) PING also becomes a factor (especially on 
rigs with a knock sensor) because timing drops back to account for the 
pre-ignition, further compounding the problem.


From: farna@xxxxxxx
Subject: Re: AMC 4-cylinder in a Spirit (Revisited)
To: mail@xxxxxxxxxxxx
Message-ID: <ADVANCES62ZbLAX3cf300000054@xxxxxxxxxxxxxxxxxxxxxxx>

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).