[urq] EFI + MAC-14 maxing ataround 5k RPM

Ben Swann benswann at verizon.net
Fri Oct 26 06:09:02 PDT 2007

I am working on ruling out spark as the culprit.  


I know I can do spark with Megasquirt, however, it is not likely I’ll be able to map the
timing as well as was done with the factory – that I can tweak with a Hex editor.
Also, the single window distributor and crank-sensors give a degree of accuracy that the
5 window sensor method won’t provide.  Audi did a great job with their ignition and I
can modify it to accomodate the higher boost levels.  At some point I may move over to
ignition too, but there is a big learning curve with just the fuel portion.


I think the problem I’m having is that injector pulsewidth is spiking as RPM hits around
5K.  I’ll provide more info on what I found yesterday, but basically pulling plugs
showed indications of excessively rich burn.


You can see graphs or the problem I posted on MSExtra forum:
http://www.msextra.com/viewtopic.php?p=172326#172326  At first glance it looks like RPM
is dropping before recorded megasquirt events.  I did another run and it became clearer
that the RPM would start to drop as pulsewidth is just in the middle of spiking.  You
can see PW jump from around 8ms at 5K to over 14ms as RPM jumps to over 5500.  It all
happens in 3 records of the log.  


So I am going to make some adjustments to the VE table today and see if that solves the





From: Stephen Redford [mailto:shr42 at msn.com] 
Sent: Friday, October 26, 2007 5:21 AM
To: Ben Swann
Cc: URQ; quattro at audifans.com
Subject: Re: [urq] EFI + MAC-14 maxing ataround 5k RPM





------------------ Ben , you can get Megasquirt with Spark,  Why the hybred combo?  It
sounds like spark might be the issue and maybe the Mac 14 is the reason .    Just a
guess.       Stephen Redford  '85Urq with Megasquirt and still in the shop !!!     :)))


----- Original Message ----- 

From: Ben Swann <mailto:benswann at verizon.net>  

To: urq at audifans.com ; quattro at audifans.com 

Cc: 'Ben Swann' <mailto:benswann at verizon.net>  

Sent: Tuesday, October 23, 2007 6:41 AM

Subject: [urq] EFI + MAC-14 maxing ataround 5k RPM


I think it is safe to rule out fuel delivery as a problem with my EFI'd engine now that
I have replaced fuel pump and assured getting good fuel delivery at all times.  So back
to the problem.  I think the ignition is dropping out at around 5000RPM, but at a loss
on how to verify it and how to fix it.  To review, the engine is completely rebuilt MC-1
with NF head being fed by Megasuirt V2.2 fuel injection through 42 lb injectors - need
to verify as these are Lucas Bluetops, and might be 36#.  

For ignition I am using the MAC-14 with dual knock sensors.  I have the car tuned fairly
well by now in the below 5000 RPM range, and the engine runs great and pulls hard until
it hits about 5000 RPM.  It doesn't matter what load and engine can be free-revving and
still reaches about a maximum of 5000 RPM.  If I gun the engine briefly, the tach will
hit 6K or so, but gradually increasing RPM it will go to about 5300 RPM and seem like it
is hitting a rev limiter.  Holding the throttle open, the engine bounces - the RPM
oscillates between like 4800 and 5400.  Occasionally I have seen 6000 RPM and holding,
but not able to maintain that result.

Things that can be automatically ruled out:
Fuel pump relay - this does not apply, as I am providing continuous power to the fuel
pump with a direct feed of #10 wire direct to the pump - relayed of course, but always
on with ignition.  Once I get engine tuned, I'll enable cutout relay circuit - that can
actually be done through MS.

Fuel delivery - I have set at 45 PSI and it maintains good pressure with regulated flow.
There is a pressure port on the regulator that can vary fuel pressure about +/- 5 PSI
depending upon load/manifold pressure.  I see not reason to suspect there is inadequate
fuel flow or pressure in the rail.

There are no codes from the MAC-14.  Most of the fuel injection portion has been
disabled and only providing power to O2 Heater at this time.  MAC-14 is being used
mainly for ignition only.

I swapped coils and it made no difference.

I pulled wires one by one and no one cylinder deviates differently from the others  I
will check plugs and do compression test - again.  When I installed the engine, I had
even compression on all cylinders - consistant with a new engine.  Cap and Rotor are
new.  Engine now has about 2000 miles on it and still running on the break-in oil.

It does not make sense that it is EFI since the cutout happens regardless of how the
engine is being driven or not - the cutout occurs at around 5000 RPM even just opening
the throttle without any engine load.  If it were fuel, I'd think it would happen only
under load.  Possibly the injectors are maxing out, but this does not make sense, as it
should not take much fuel to rev the engine without a load. I also don't notice any lean
condition when the problem occurs - seeing like a 12:1 on the O2 guage.   Still I won't
rule out fuel injectors, but ..

I think there is some reason the ignition is dropping out - like not enough current to
the coil when the engine hits around 5000 RPM.  There is of course the long run from the
battery located under the seat.  I ran an additiona #6 wire to a jumper post located
near the coil and then runs down to the starter.  Although the engine cranks faster when
starting, the RPM limit problem is still there.  

I'm wondering if the coil is not saturating, and not enough current is available at
higher RPMs.  Why this would happen on this car and not on a 200?  

It doesn't make sense - this balanced engine should be good for well over 7000 RPM.  I'm
going to post on some other forums - Megasuirt and MotorGeek, but this seems more like
an ignition problem where the MAC-14 is  providing the ignition.  Maybe installing a
capacitor or battery up front to test.I'm looking for ideas.

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