Sender: <marv@lancaironline.net> (Marvin Kaye)
To: flyrotary
Date: Tue, 29 Oct 2002 18:37:24 -0500
Message-ID: <redirect-1848007@logan.com>
From: "Marc Wiese" <cardmarc@midsouth.rr.com>
Subject: interesting discussion on intake temp and fuel delivery.
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Does anyone know if TC's EC2 makes adjustments for intake temp in the
fuel maps for boosted engines? I seem to remember it does.
BTW, this guy below is a very astute rotary engine aficionado. His
posting are always spot on.
Marc Wiese
---------------------

Date: Mon, 28 Oct 2002 22:10:47 -0800
From: "David Lane" <dlane@peabody.jhu.edu>
Subject: (3) My Dyno Results...Nice...but at what temps?

I have conflicting information on this subject, and I am 
wondering if anyone can straighten me out.

At one time, my car had an added injector controller on it.  The 
AIC did not know or care what the ambient temperatures were.  At 
one point I was thinking this kind of thing might have something 
to do with general inconsistency in how my car reacted to the 
throttle from day to day.  I mentioned it to Corky Bell, who 
said that within average temperatures, it would not make that 
big a difference.  I figured since he was in Texas, maybe he was 
thinking average Texas temps.  Or possibly he was thinking that 
the added injectors were only doing part of the work, and the 
other part (from the stock ECU) did respond to ambient temps.  
Anyway, since I was headed toward a TEC-II system, I didn't 
think much more about it.

Then Tony K. said, in part (temp conversions are mine):

    I'd be happy with 12.0 AFR in 10C (50F), but terffied with 
it at 30C (86F)! Why does
    everyone ignore ambient temperature correction maps when 
tuning with the
    PFC?  

He also said:

    I run 850s/1300s with stock twins running 14.7psi of boost. 
At ambient 80F
    (during a tuning session) my maximum injector duty cylce was 
under
    72%...tuned varying targets of AFR, based on RPM and boost. 
When I tested
    the car (and tune) at ambient 55F, I was on average 1.2 AFR 
LEANER. After
    making the appropriate adjustments, I was back on/near 
target AFR but
    injector duty cycle shot up to 90%.  

Ashraf echoed the importance of the concept, and suggests that 
the general idea of ambient air temp and density is covered in 
Maximum Boost (Corky's book).  I did a quick check, and saw 
graphs about air temps vs. air density (vs. altitude) but didn't 
see anything obvious that directly addressed required fuel 
enrichment as a function of ambient air temps.  It was a very 
quick check, so I may have missed something.

Certainly it makes sense.  However, for only a 25 F drop in 
ambient temps (80F to 55F), the 18% increase in duty cycle Tony 
reported as necessary to maintain a given A/F ratio is 
startling.  Mind you, it is observed data, so I don't doubt it.  
And, if I am understanding correctly, Tony is saying that his 
A/F ratio changed by 1.2--which I take to mean that if he was 
(for instance) at 12:1 at 80 degrees ambient, the same program 
would only provide enough fuel for 13.2:1 at 55 degrees ambient.

YIKES!  If that is universally true, I suspect we would be 
losing a bunch of engines every time the weather turned cool.

Further confusion comes from the TEC3 tuning manual.
They state that an engine running at -40F needs 25% more fuel 
than one running at 70F.  That's a temperature range of 110 
degrees, which seems to my untrained eye to be out of line with 
Tony's observation of 18% more fuel to compensate for just a 25F 
degree drop in temps.

Now, the same TEC3 manual goes on to talk about manifold air 
temps (which is really the issue since we are all running 
intercoolers).  They make the interesting point that if you have 
done your tuning at 68F, you would actually want to add a little 
fuel (I suppose to offset the tendency to detonate with less 
dense air) as manifold air temp increases--although the amount 
is only about 4% at 176F (after the intercooler temp), so it 
should not be a major concern if you have some headroom in your 
fuel maps.

As manifold air temps decrease, the curve hits a 25% increase in 
fuel by -40F, but at typical temps when most of us are likely to 
stress our engines--say down to freezing, the suggested 
enrichment relative to that 68F zero point looks to be more like 
8%.  Except that unless we have a perfect intercooler, the air 
ain't gonna be entering the engine at 32 degrees F on a 32 
degree day under full boost.  If it heats back up to 50F by the 
time it gets swallowed into a combustion chamber, it would only 
require about 4% more fuel (acording to the chart).

So, if 4% more fuel can get you from a zero point at 68F to an 
intake temp of 176F, and the same 4% additional fuel can 
accommodate an ambient temperature of 32F (with a guestimated 
intake temp of 50F), I would guess that the kind of headroom 
most of us program in for safety would be very near to 
accomplishing the task.

Which may be what Corky was trying to tell me in the first 
place.

Bottom line for me is that for most of us who do not have their 
engines tuned to the ragged edge, and who do not push their 
engines to maximum boost on ice cold tracks, the ambient air 
temp issue should be less of a panic concern than is implied by 
this thread.

Casual conversations with a couple of experienced tuners lead me 
to believe that most are now looking for average A/F ratios in 
the mid 11s, and some don't even mind seeing 10s if the boost is 
high enough.  Granted this is less than ideal for someone "going 
for it," but the tuning community would rather have someone 
complain that the car is a bit richer than optimum as opposed to 
dealing with a blown engine.

Under these circumstances, it seems to me that there would be 
enough slop in the tuning to accommodate less than extreme 
ambient temperature issues.

Still, none of this explains what Tony observed.  These tunable 
engine management systems are complex, though, and I can't help 
but wonder if maybe some other setting (either in the 
aftermarket unit, or the stock ECU to which it is attached) was 
having an unobserved effect.  There is so much I don't know 
about this stuff.

Anyway, I think you can see why I am questioning all of this.  
As some of you know, I do not have a technical education, so I 
can only present what I am seeing, and hope that someone can 
account for the conflicts in relatively simple terms.  I won't 
be able to engage in an engineering discussion.

It would help if there is some published or theoretical data 
(other than the TEC3 manual I have been looking at) which would 
confirm or disparage what I have written--or at least something 
that might explain why Tony's observed results appear extreme.

Note:  I have a TEC-II on my car, but the TEC3 manual (available 
on their web site) is far more complete, so I have read it as a 
reference.

Best wishes,
David Lane
dlane@peabody.jhu.edu
'85 GSL-SE (Cartech Turbo)
Info on the car at:
http://www.wankel.net/DavidLane/

Marc Wiese